Appl. Phys. Lett. 92, 071101 (2008); DOI:10.1063/1.2840164
Published 19 February 2008
Alireza Hassani, Alexandre Dupuis, and Maksim Skorobogatiy
Engineering Physics Department, Ecole Polytechnique de Montréal, C.P. 6079, succursale Centre-Ville Montreal, Québec H3C3A7, Canada
We propose a porous polymer terahertz fiber with a core composed of a hexagonal array of subwavelength air holes. Numerical simulations show that the larger part of guided power propagates inside the air holes within the fiber core, resulting in suppression of the bulk absorption losses of the core material by a factor of ~10–20. Confinement of terahertz power in the subwavelength holes greatly reduces effective refractive index of the guided mode but not as much as to considerably increase modal radiation losses due to bending. As a result, tight bends of several centimeter bending radii can be tolerated
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Wednesday, March 12, 2008
Low loss porous terahertz fibers containing multiple subwavelength holes
Wednesday, May 30, 2007
Silicon could open the way for new terahertz technology
Surface plasmon resonance is used for a variety of purposes including detecting protein or DNA and enhancing the sensitivity of spectroscopy. However, surface plasmon resonance requires a metal. Gold and silver are among the metals that best support surface plasmons. Unfortunately, Weili Zhang, a professor at Oklahoma State University, tells PhysOrg.com, “Silver isn’t always long lasting and gold can be too expensive.” The solution? Zhang and his colleagues suggest that silicon can be used for surface plasmon resonances. But first it needs to become something metallic. Along with colleagues Abul Azad and Jiaguang Han from Oklahoma State and Jngzhou Xu, Jian Chen and X.-C. Zhang from Rensselaer Polytechnic Institute in Troy, New York, Zhang has shown how the use of laser pulses can create a surface plasmon resonance from a photonic crystal effect. “This is the first time anyone has reported seeing this transition. This is a very interesting change,” he says.
Zhang and his coauthors report their findings in “Direct Observation of a Transition of a Surface Plasmon Resonance from a Photonic Crystal Effect,” published in Physical Review Letters.
Surface plasmons can only exist in a metal/dielectric interface. They are electromagnetic waves that run along the surface of this interface. “What we wanted to do,” explains Zhang, “is start with a non-conductive material to see if we could excite surface plasmons in the terahertz region.” For their attempt, Zhang and his colleagues use silicon because of its properties as a semiconductor. “We used ultra-fast laser pulses that resulted in photodoping.”
Zhang explains that initially the signature of the microstructured silicon is that of a photonic crystal resonance. But as the laser pulses are introduced, the resonance changes. “We see the photonic crystal signature disappear because the permittivity changes, the silicon becomes metallic, and the condition for surface plasmons is satisfied, thus the resonance changes.”
This work is likely to result in a variety of applications across different fields, Zhang explains. Terahertz systems, which are used for spectroscopy and imaging, can be modified more efficiently with this new way of generating surface plasmon resonance, which Zhang describes as “tunable.”
“Terahertz systems always need some kind of filters to control operating frequencies and wavelengths,” Zhang points out. “But with regular metals, once the structure is fixed, the operating frequencies are fixed. With this silicon process, these things can be changed. Both the frequencies and intensity can be controlled. This new way is more flexible and efficient.”
Biomedicine is a field especially where terahertz systems can find good use. Terahertz radiation can be used to “look” deep inside organic materials, and they do it without causing the damage that X-rays do. Additionally terahertz radiation is being considered for use in screening airport passengers.
Zhang also points out that surface plasmon resonance to direct terahertz systems can also be used to enhance space communication: “This would be ideal for making tunable switches.” Indeed, astronomers are interested in using terahertz technology to study the particles that fall into the category of “far-infrared.”
“Because silicon is cheap, rigid, and tunable,” concludes Zhang, “this is an important and exciting finding. The applications for technology are just beginning.”
Source: PhysOrg.com
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Gold gratings give off terahertz pulses
A new method of generating terahertz pulses could be a step closer to producing safe radiation for medical imaging, biological research and homeland security.
Firing ultrashort pulses at gold-coated nanostructured gratings is a convenient way to produce terahertz pulses, say researchers from the University of Strathclyde, UK. The team claims that the new method generates just as much power as the best inorganic crystals and can be optimized to produce substantially more. (Physicalreview Letters 98 026803). “It is the sharp acceleration of the electrons that produce the terahertz emission,” Klaas Wynne from the University of Strathclyde told optics.org. “The nanostructured surface of the terahertz emitter allows the femtosecond laser to rapidly “push” electrons out of the metal resulting in a nanometer scale free electron laser.”
The team hopes to further increase the terahertz power by optimizing the plasmons on the surface of the structure to increase the acceleration of the electrons. “We think there are ways to increase the fields by about 1000 times, which would produce 6 orders more power,” said Wynne.
This new approach exploits surface-plasmon excitation to generate terahertz pulses on a gold surface. “Due to circumstances and luck, we noticed the connection between terahertz emission from nominally flat metal surfaces and the ultrafast nonlinear photoelectric effect associated with surface plasmons.” said Wynne. “Previous methods have used either optical rectification in fragile and expensive inorganic crystals or photoconductive antennas. In our new technique, electrons are accelerated by a ponderomotive potential associated with surface plasmons.”
Wynne and colleagues used an 800 nm laser emitting 1 mJ pulses with 100 fs pulse duration at a 1 kHz repetition rate. The laser was incident on a UV-grade fused silica grating coated with 30 nm of gold and measuring 10x10 mm2. The grating had a 40 nm etch depth and a high section of 340 nm in every 500 nm grating period.
The next challenge is to optimize the nanostructured surface. “We are now starting to use electron beam lithography to gain more control over our nanostructures. The key problem is to be able to make large aspect ratio (tall and narrow) nanostructures in a reliable way,” concluded Wynne.
Source: Optics.org
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Saturday, March 31, 2007
Transmission resonances through aperiodic arrays of subwavelength apertures
Nature 446, 517-521 (29 March 2007) | doi:10.1038/nature05620; Received 14 June 2006; Accepted 19 January 2007
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Wednesday, January 24, 2007
Nanotubes light up solar cells
From Optics.org
Researchers in the UK have made a new type of hybrid electrode from multiwalled carbon nanotubes and indium-tin oxide that could be used for solar cell applications.
The nanotubes are directly grown on indium-tin oxide coated glass, a transparent electrode commonly employed in organic optoelectronic devices, such as solar cells and light-emitting diodes. The nanotube electrodes are highly transparent at longer wavelengths, making them ideal for harnessing light from the Sun.
Organic solar cells are cheaper and easier to make than their inorganic counterparts but their efficiencies and lifetimes remain an issue. This is because charge carrier mobilities - especially those of holes - are low, which limits the current flowing through the device. One way of overcoming this problem is to reduce the thickness of the active layer, but doing this also reduces the solar cell's ability to absorb light.
An alternative approach, adopted by Ravi Silva and colleagues at the University of Surrey, is to use an electrode that penetrates into the organic layer and which extracts charge carriers so that they are free to move through the device.
Silva's team has shown that it can grow multiwalled carbon nanotubes directly onto indium-tin oxide (ITO) coated glass using chemical vapour deposition (CVD) techniques. Multiwalled carbon nanotubes are very good electrical and thermal conductors and can therefore function as part of the underlying electrode. The scientists demonstrated that the nanotubes act as interpenetrating electrodes over large surface areas and efficiently help to extract positive charge carriers, or holes, from the active layer in the organic devices. "Crucially, these electrodes have high transparency, particularly at longer wavelengths, making them well matched to the solar spectrum," Silva told nanotechweb.org.
The researchers also showed that they were able to carefully control the growth of the nanotubes on the ITO substrate. Controlling the growth of the nanotubes is critical if nanotextured electrodes of this kind are to be used in thin film optoelectronics devices because nanotubes that are too long will short a device. Silva and co-workers discovered that the carbon nanotubes grow much more slowly on ITO than on other materials, such as silicon and glass, which allows the growth rate to be controlled. Moreover, the nanotubes are directly bonded to the ITO substrate, which improves the mechanical strength of the device and ensures that all the nanotubes are in direct contact with the substrate.
The Surrey team tested its solar cells using a "solar simulator" that provides light with a power of 100 mW/cm2. It found that the same number of photogenerated electrons are extracted, despite around a 30% reduction in the amount of light entering the cell, with an overall efficiency as high as around 1%. "Since the CVD growth method is scalable and relatively low-cost, we envisage the widespread utility of multiwalled carbon nanotube electrodes of this type in organic solar cells," said Silva. "The electrodes will facilitate efficient charge carrier extraction upon improved matching of the photoactive matrix with the solar spectrum".
The team reported its work in Appl. Phys. Lett.
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High density optical storage - on a single photon
From Optics.org
New technique developed at the University of Rochester stores and retrieves an entire digital image from a single photon.
Researchers at the University of Rochester, NY, US, say they have made an optics breakthrough that allows them to encode an entire image's worth of data into a single photon, slow the image down for storage, and then retrieve the image intact. While the initial test image consists of only a few hundred pixels, a tremendous amount of information can be stored with the new technique.
The image - "UR" for the University of Rochester - was made using a single pulse of light and the team can fit as many as 100 of these pulses at once into a tiny, 100 mm cell. The researchers say that squeezing that much information into so small a space and retrieving it intact opens the door to optical buffering - storing information as light.
"It almost sounds impossible, but instead of storing just ones and zeros, we're storing an entire image," said John Howell, associate professor of physics and leader of the team that created the device, which is revealed in Physical Review Letters. "It's analogous to the difference between snapping a picture with a single pixel and doing it with a camera--this is like a 6-megapixel camera."
"You can have a tremendous amount of information in a pulse of light, but normally if you try to buffer it, you can lose much of that information," said Ryan Camacho, Howell's student and lead author on the PRL article. "We're showing it's possible to pull out an enormous amount of information with an extremely high signal-to-noise ratio even at low light levels."
Optical buffering is currently a particularly hot field of research because engineers are trying to speed up computer processing and network speeds using light, but their systemsare slowed down when they have to convert light signals to electronic signals to store information.
Howell's group employed a new approach that preserves all the properties of the pulse. The buffered pulse is essentially a perfect original; there is almost no distortion, no additional diffraction, and the phase and amplitude of the original signal are all preserved. Howell is also working to demonstrate that quantum entanglement remains unscathed.
To produce the UR image, Howell shone a beam of light through a stencil with the U and R etched out. Anyone who has made shadow puppets knows how this works, but Howell turned down the light so much that a single photon was all that passed through the stencil.
Quantum mechanics dictates some strange things at that scale, so that bit of light could be thought of as both a particle and a wave. As a wave, it passed through all parts of the stencil at once, carrying the "shadow" of the UR logo with it. The pulse of light then entered a 100 mm cell of cesium gas at a warm 100°C, where it was slowed and compressed, allowing many pulses to fit inside the small tube at the same time.
"The parallel amount of information Howell has sent all at once in an image is enormous in comparison to what anyone else has done before," said Alan Willner, professor of electrical engineering at the University of Southern California and president of the IEEE Lasers and Optical Society. "To do that and be able to maintain the integrity of the signal is a wonderful achievement."
Howell has so far been able to delay light pulses by 100 ns and compress them to 1% of their original length. He is now working toward delaying dozens of pulses for as long as "several milliseconds", and as many as 10,000 pulses for up to 1 ns.
"Now I want to see if we can delay something almost permanently, even at the single photon level," said Howell. "If we can do that, we're looking at storing incredible amounts of information in just a few photons."
About the author
Matthew Peach is a contributing editor to optics.org and Optics & Laser Europe.
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Saturday, January 20, 2007
Superradiant terahertz Smith-Purcell radiation from surface plasmon excited by counterstreaming electron beams
Young-Min Shin, Jin-Kyu So, Kyu-Ha Jang, Jong-Hyo Won, Anurag Srivastava, and Gun-Sik Park
School of Physics and Astronomy, National Research Laboratory for Micro and Nano Vacuum Electrophysics, Seoul National University, Seoul 151-747, Korea
(Received 4 September 2006; accepted 13 December 2006; published online 17 January 2007)
The authors show that evanescent tunneling transmission of effective surface plasmon polaritons between two counterstreaming electron beams noticeably increases Smith-Purcell radiation (SPR) intensity by about two orders of magnitude as well as lower its transition threshold from a spontaneous emission to a stimulated one. An emission mechanism of the superradiant SPR is theoretically analyzed by the dielectric conversion of the structured metal surface and the boundary matching condition of Maxwell's equations in comparison with numerical simulations. ©2007 American Institute of Physics
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Saturday, January 13, 2007
Light squeezes through nano coax
Physicists in the US have created the first nanoscale coaxial cables for the transmission of light.
Operating much like the coaxial cables used to distribute television and radio signals, "nanoscale" cables can transmit light with wavelengths nearly four times their 200 nm diameter.
coax
Coaxial cables comprise an inner and outer conductor separated by an insulating dielectric layer and are used to transmit all manner of electromagnetic waves from radio to microwave. They are extremely useful because they can transmit waves with wavelengths much greater than their diameter, making cable television and other technologies possible.
Light is an electromagnetic wave so there is no reason why it cannot be transmitted in a similar manner via a coaxial cable - but conventional wisdom had held that light could not travel through a cable of diameter less than its wavelength. Now, Boston College's Jakub Rybczynski, Mike Naughton and colleagues realized that a coaxial could carry sub-wavelength light waves if it were miniaturized.
Their coaxial cable is based around a carbon nanotube, which forms the central conductor (see "nano-coax" diagram, above). The nanotube is surrounded by a concentric ring of transparent aluminium oxide - which acts as the dielectric layer - and finally a concentric metal ring that acts as the outer conductor. The separation between the inner and outer conductors is about 100 nm.
Some of the central conductor protrudes from the cable and acts like an "antenna", gathering light and sending it down the cable. The cable works exactly like a conventional coax, constraining the transverse electric and magnetic fields of the light wave between the two conductors, thereby guiding the light along the cable for distances of up to 50 µm. While this is not very far, it could allow the structures to be exploited in a number of ways.
Naughton and Rybczynski said that the cable's ability to control light over sub-wavelength distances could be exploited to solve a wide range of technological problems. For example, the dielectric material could be replaced by a photovoltaic material like silicon, which would convert the light to electricity. This could be used to create better solar cells that exploit the cable's ability to constrain the light wave into an area smaller than its wavelength, thereby boosting the efficiency of the conversion process.
This ability to constrain light could also be exploited in new optical microscopes and optical techniques for processing computer chips that can resolve features smaller than about half the wavelength of light - something that conventional optics cannot do. The researchers also believe that the technology could someday be used to fabricate components for optical communications including switches that stop the flow of light by applying an electrical signal across the inner and outer electrodes.
About the author
Hamish Johnston is editor of Physics Web.
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Terahertz-Pulse Emission Through Laser Excitation of Surface Plasmons in a Metal Grating
Phys. Rev. Lett. 98, 026803 (2007)
ulse Emission Through Laser Excitation of Surface Plasmons in a Metal Grating
Gregor H. Welsh, Neil T. Hunt, and Klaas Wynne
Department of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, Scotland, UK
(Received 28 July 2006; revised 26 September 2006; published 10 January 2007)
The second-order processes of optical-rectification and photoconduction are well known and widely used to produce ultrafast electromagnetic pulses in the terahertz frequency domain. We present a new form of rectification that relies on the excitation of surface plasmons in metal films deposited on a shallow grating. Multiphoton ionization and ponderomotive acceleration of electrons in the enhanced evanescent field of the surface plasmons results in a femtosecond current surge and emission of terahertz electromagnetic radiation. Using gold, this rectification process is third or higher-order in the incident field.
URL: http://link.aps.org/abstract/PRL/v98/e026803
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Phonon-Wave-Induced Resonance Fluorescence in Semiconductor Nanostructures: Acoustoluminescence in the Terahertz Range
Phys. Rev. Lett. 98, 027401 (2007)
K. J. Ahn, F. Milde, and A. Knorr
Institut für Theoretische Physik, Nichtlineare Optik und Quantenelektronik, Technische Universität Berlin, Hardenbergstraße 36, PN 7-1 10623 Berlin, Germany
(Received 10 November 2005; published 8 January 2007)
Acoustic wave excitation of semiconductor quantum dots generates resonance fluorescence of electronic intersublevel excitations. Our theoretical analysis predicts acoustoluminescence, in particular, a conversion of acoustic into electromagnetic THz waves over a broad spectral range.
URL: http://link.aps.org/abstract/PRL/v98/e027401
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Sunday, December 17, 2006
Active terahertz metamaterial devices
Hou-Tong Chen1,4, Willie J. Padilla1,4,3, Joshua M. O. Zide2, Arthur C. Gossard2, Antoinette J. Taylor1 and Richard D. Averitt1,3
Center for Integrated Nanotechnologies, Materials Physics & Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Materials Department, University of California, Santa Barbara, California 93106, USA
Present addresses: Department of Physics, Boston College, 140 Commonwealth Avenue, Chestnut Hill, Massachusetts 02467, USA (W.J.P.); Department of Physics, Boston University, 590 Commonwealth Avenue, Boston, Massachusetts 02215, USA (R.D.A.).
These authors contributed equally to this work.
Correspondence to: Hou-Tong Chen1,4 Correspondence and requests for materials should be addressed to H.-T.C. (Email: chenht@lanl.gov).
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The development of artificially structured electromagnetic materials, termed metamaterials, has led to the realization of phenomena that cannot be obtained with natural materials1. This is especially important for the technologically relevant terahertz (1 THz = 1012 Hz) frequency regime; many materials inherently do not respond to THz radiation, and the tools that are necessary to construct devices operating within this range—sources, lenses, switches, modulators and detectors—largely do not exist. Considerable efforts are underway to fill this 'THz gap' in view of the useful potential applications of THz radiation2, 3, 4, 5, 6, 7. Moderate progress has been made in THz generation and detection8; THz quantum cascade lasers are a recent example9. However, techniques to control and manipulate THz waves are lagging behind. Here we demonstrate an active metamaterial device capable of efficient real-time control and manipulation of THz radiation. The device consists of an array of gold electric resonator elements (the metamaterial) fabricated on a semiconductor substrate. The metamaterial array and substrate together effectively form a Schottky diode, which enables modulation of THz transmission by 50 per cent, an order of magnitude improvement over existing devices10.
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Sunday, November 05, 2006
Terahertz radiation: applications and sources
Until recently, researchers did not extensively explore the material interactions occurring in the terahertz spectral region—the wavelengths that lie between 30 µm and 1 mm—in part because they lacked reliable sources of terahertz radiation. However, pressure to develop new terahertz sources arose from two dramatically different groups—ultrafast timedomain spectroscopists who wanted to work with longer wavelengths, and longwavelength radio astronomers who wanted to work with shorter wavelengths. Today, with continuous-wave (CW) and pulsed sources readily available, investigators are pursuing potential terahertz-wavelength applications in many fields.
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| Figure 1. An electromagnetic wave is produced by this broadband short-pulse terahertz source when a dc bias is placed across the antenna and an ultrashort pump-laser pulse is focused in the gap. |
Much of the recent interest in terahertz radiation stems from its ability to penetrate deep into many organic materials without the damage associated with ionizing radiation such as X-rays (albeit without the spatial resolution). Also, because terahertz radiation is readily absorbed by water, it can be used to distinguish between materials with varying water content—for example, fat versus lean meat. These properties lend themselves to applications in process and quality control as well as biomedical imaging. Tests are currently under way to determine whether terahertz tomographic imaging can augment or replace mammography, and some people have proposed terahertz imaging as a method of screening passengers for explosives at airports. All of these applications are still in the research phase, although TeraView (Cambridge, England), which is partially owned by Toshiba, has developed a technique for detecting the presence of cancerous cells that is currently in human trials.
Terahertz radiation can also help scientists understand the complex dynamics involved in condensed-matter physics and processes such as molecular recognition and protein folding.
CW terahertz technology has long interested astronomers because “approximately one-half of the total luminosity and 98% of the photons emitted since the Big Bang fall into the submillimeter and far-infrared,” says Peter Siegel of the Jet Propulsion Laboratory (Pasadena, CA), and CW THz sources can be used to help study these photons.
One type of CW terahertz source is the optically pumped terahertz laser (OPTL). OPTL lasers are in use around the world, primarily for astronomy, environmental monitoring, and plasma diagnostics. A system installed at the Antarctic Submillimeter Telescope and Remote Observatory at the South Pole is the local oscillator for a THz receiver, which will be used to measure interstellar singly ionized nitrogen, H2D+, and carbon monoxide during the polar winter. Another system is slated for sub-Doppler terahertz astronomy use on the National Aeronautics and Space Administration’s SOFIA airborne astronomical platform.
In 2004, a 2.5-THz laser will ride a Delta rocket into space aboard NASA’s AURA satellite to measure the concentration and distribution of the hydroxyl radical (OH–) in the stratosphere, a critical component in the ozone cycle. (Currently there are no global data for OH– concentrations; only two spot measurements have been made using OPTL systems carried aboard high-altitude balloons.) The AURA system is less than 0.2 m3, weighs less than 22 kg, and consumes 120 W of prime power. It works autonomously and is designed to operate in orbit for more than five years.
The emerging field of time domain spectroscopy (TDS) typically relies on a broadband short-pulse terahertz source (Figure 1). A split antenna is fabricated on a semiconductor substrate to create a switch. A dc bias is placed across the antenna, and an ultrashort pump-laser pulse (<100>
This TDS switch puts out a train of pulses, whose repetition frequency is the same as that of the femtosecond pump laser. Pulse widths are on the order of 100 fs, with average powers of a few microwatts and a frequency spread of >500 GHz. The pulse bandwidth is typically centered at about 1 to 2 THz. The details of the spectrum can vary significantly, however, depending on the design of the switch and pump-laser power, pulse width, and configuration.
Figure 2a shows a typical TDS setup. The terahertz pulse is distorted by selective absorption as it passes through a sample, causing delays in its arrival time at the detector. The transmitted beam is then focused onto a detector, which is essentially identical to the emitter except that it is unbiased. By varying the time at which the sample pump pulse arrives at the detector, successive portions of the terahertz pulses can be detected and built into a complete image of the pulse in terms of its delay time, or time domain. The data are then processed by fast Fourier transform analysis in order to convert the delay time into the frequency of the terahertz signal that arrives at the detector.
The absorption characteristics of terahertz radiation vary greatly from material to material, and this property can be used to create images. In 1995, Binbin Hu and Martin Nuss at Lucent Technologies’ Bell Laboratories created a terahertz imaging system using TDS and coined the term T-ray for these short, broadband terahertz pulses. The T-ray pulse is measured as it reflects from a sample. Because the pulse is so short, distance can be resolved by looking at the time of flight and then used to create a three-dimensional transparent reconstruction of various objects by measuring the time lapse between pulses reflected from different areas within the object (Figure 2b).
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| Figure 2. In time domain spectroscopy, an image of the sample is built up based on selective absorption, which causes delays in arrival time at the detector (a). A typical result is this three-dimensional tomograph of a tooth, showing areas of decay (b). (SPIE/Teraview, Ltd.) |
Optically pumped lasers
In its simplest embodiment, an OPTL system consists of a grating-tuned carbon dioxide pump laser and a far-infrared (FIR) gas cell mounted in a laser resonator. The pump beam enters the cell through an aperture in the high-reflecting resonator mirror. The pump laser is tuned to the appropriate absorption band, and lasing occurs. For several reasons, this is not as easy as it sounds. Both the absorption bandwidth of the vibrational energy state and the lasing bandwidth of its excited rotational states are quite narrow. Moreover, slight changes in the OPTL’s pumping wavelength or changes in the cavity length itself can inhibit lasing, and feedback interaction between the pump laser and the terahertz laser can affect stability. Therefore, designers must pay careful attention to all of these things to achieve reliable performance.
In the past, research groups often built their own OPTLs, which were typically large and extremely difficult to use and maintain. Today, OPTL laser systems are smaller and more reliable turnkey systems. These improved systems stem from several developments, including permanently sealed, single- mode, frequency-stabilized, folded-cavity, radio-frequency-excited waveguide CO2 lasers; sealed FIR gas cells that eliminate gas transport issues; and exquisitely stable passive resonator structures. The integration of these various improved laser technologies into a truly operator-friendly system has ensured ease of use.
Indeed, OPTLs can operate at many discrete frequencies, ranging from less than 300 GHz (1,000 µm) to more than 10 THz (30 µm). Different molecular gases each have their own spectrum of available lines. Sideband generation technology can add instantaneous tunability to any of the available OPTL laser lines.
Other terahertz sources
Many other terahertz source technologies have been investigated in the past four decades. Numerous groups worldwide are producing tunable CW terahertz radiation using photomixing of near-IR lasers. For example, Gerald Fraser’s group at the National Institute of Standards and Technology is frequency mixing the output of a near-IR, fixed-frequency diode laser with that of a tunable Ti:sapphire laser in a lowtemperature- grown gallium arsenide photomixer fabricated with the appropriate antenna pattern. This approach yields tens of nanowatts of tunable output with a spectral content governed by the spectral content of the near-IR laser.
Backward-wave oscillators (BWOs) are electron tubes that can be used to generate tunable output at the long-wavelength end of the terahertz spectrum. To operate, however, they require a highly homogeneous magnetic field of approximately 10 kG.
Direct multiplied (DM) sources, such as those marketed by Virginia Diodes, Inc. (Charlottesville, VA), take millimeter-wave sources and directly multiply their output up to terahertz frequencies. DM sources with frequencies up to a little more than 1 THz and approximately 1 µW of output have been used as local oscillators for heterodyne receivers in select applications, most of which are in radio astronomy. However, they can produce substantially more output power at lower frequencies, and they are often well suited to applications requiring frequencies of less than 500 GHz.
In addition, physicists in Italy, Switzerland, the United States, and the United Kingdom have recently demonstrated quantum-cascade semiconductor lasers operating at wavelengths in the 4.4-THz regime. These lasers are made from 1,500 alternating layers (or stages) of gallium arsenide and aluminum gallium arsenide and have produced 2 mW of peak power (20 nW average power), and advances in output power and operating wavelength continue at a rapid pace. Applying a potential across the device causes electrons to cascade through each stage, emitting photons along the way. The photon wavelength is determined by the thickness of the stages. These lasers currently work best at only a few kelvins, but in the future they could become an important source of commercial terahertz systems.
Table 1 compares some of the techniques for generating terahertz radiation. At present, only the OPTL, TDS, and DM systems are commercially available as turnkey systems. However, many researchers assemble TDS systems in the laboratory using readily available laser sources, and DM sources are often procured from a number of research organizations and at least one commercial source. The availability and operation of BWOs at terahertz frequencies are somewhat problematic, but several groups use lower-frequency (<500-ghz)>
The choice of a terahertz source will determine the type of detection scheme required. Sources with submilliwatt output power complicate detection and often necessitate the use of liquid-helium-cooled bolometers or similar devices. Short-pulse terahertz devices often need gated detection using a TDS switch.
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For time-domain spectroscopy, or where an overall snapshot of the spectral characteristics of a sample in the terahertz region is important, TDS technology may be the optimal choice. For a more precise, higher-resolution look, consider the OPTL system, using either discrete frequencies or tunable sideband generation technology. Many applications do not need the complete terahertz spectrum of a sample but merely need to identify one or two characteristic features. In these cases, the OPTL system may be preferable to the TDS system because of its operational simplicity, high signal-to-noise ratio, and ability to use conventional, roomtemperature detectors.
Although the practical application of terahertz radiation is in its infancy, the recent availability of reliable sources in the 0.3- to 5-THz range may have a wide-ranging impact on science, industry, and medicine. Short-pulse terahertz systems are used in time-domain spectroscopy to understand biological processes and to create two- and three-dimensional images. CW OPTL systems have been used extensively in aerospace and astronomical applications, primarily for remote sensing, and may find new uses as terahertz applications mature.
Further reading
Arnone, D. D.; et al. Application of terahertz (THz) technology to medical imaging. In Proc. SPIE Terahertz Spectroscopy Applications II; International Society for Optical Engineering: Bellingham, WA, 1999; pp. 209–219.
Köhler, R.; Tredicucci, A.; Beltram, F.; Beere, H. E.; Linfield, E. H.; Davies, A. G.; Ritchie, D. A.; Iotti, R. C.; Rossi, F. Terahertz semiconductor-heterostructure laser. Nature 2002, 417, 156.
Mueller, E. R.; Fontanella, J.; Henschke, R. Stabilized, Integrated, Far-Infrared Laser System for NASA/Goddard Space Flight Center. 11th International Symposium on Space Terahertz Technology, Ann Arbor, MI, May 1–3, 2000. Available here.
Mueller, E. R.; Waldman, J. Power and Spatial Mode Measurements of Sideband Generated, Spatially Filtered, Submillimeter Radiation. IEEE MTT 1994, 42 (10), 1891.
Rochat, M.; Ajili, L.; Willenberg, H.; Faist, J.; Beere, H.; Davies, G.; Linfield, E.; Ritchie, D. Low-threshold terahertz quantum- cascade lasers. Appl. Phys. Lett. 2002, 81, 1381.
Siegel, P. H. Terahertz Technology. IEEE MTT 2002, 50 (3), 910.
Williams, B. S.; Callebaut, H.; Kumar, S.; Hu, Q.; Reno, J. L. 3.4-THz quantum cascade laser based on LO-phonon scattering for depopulation. Appl. Phys. Lett. 2003, 82, 1015.
Biography
Eric R. Mueller is manager of engineering and specialty products at Coherent-DEOS in Bloomfield, Connecticut
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Continuous-Wave Frequency-Tunable Terahertz-Wave Generation From GaP
Nishizawa, J. Tanabe, T. Suto, K. Watanabe, Y. Sasaki, T. Oyama, Y.
This paper appears in: Photonics Technology Letters, IEEE
Publication Date: Oct.1, 2006
Volume: 18, Issue: 19
On page(s): 2008- 2010
ISSN: 1041-1135
Digital Object Identifier: 10.1109/LPT.2006.882278
Posted online: 2006-10-23 09:26:27.0
Abstract
Continuous-wave (CW) single-frequency terahertz (THz) waves were generated using difference-frequency generation via excitation of phonon–polaritons in GaP. The two pump sources were an external cavity laser diode (LD) and an LD-pumped Nd : YAG laser. The power density of the latter beam was enhanced by using a ytterbium-doped fiber amplifier. The two incident beams were focused to near the wavelength of THz waves. This optical alignment enabled us to generate frequency-tunable CW THz waves in the 0.69–2.74 THz range. With a fixed angle between the pump beams, we obtained a frequency bandwidth as large as 600 GHz.
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Wednesday, October 18, 2006
Terahertz technology safer, more effective than X-ray
In conjunction with researchers at Queens College (City University of New York), the University of California at Santa Barbara and the Institute of Physical and Chemical Research in Japan, professors and graduate students at UB are making strides to develop a technology that could revolutionize current military, medical and airline technology.
Terahertz frequencies, which are higher than microwaves but lower than potentially harmful infrared rays, are being studied for their implications in investigative searches.
Andrea Markelz, Ph.D., principal investigator for the project, said that there are significant potential benefits for the technology, especially in things like airport security.
“Using terahertz frequencies, you can definitely see if someone has metal on them,” Markelz said. “It’s a way of seeing metal objects, packages and powders through clothing, without the potential biological damage of an X-ray.”
Using an example of anthrax in an envelope, Markelz said that it is almost impossible to search every envelope that goes through current airport security. However, using the terahertz frequency, one would be able to see that powder was inside of an envelope. They would then be able to determine whether or not the substance had the potential for harm.
Markelz began working with the idea over two years ago, when an event overseas prompted her to action.
“It was Christmas Eve, and I heard about a bombing in a U.S. military cafeteria in Baghdad. I just thought, ‘this should not be happening,’” Markelz said. “I think the technology will have a big impact in a military setting.”
The project has been in the works for nearly two years. It started with interdepartmental effort and a seed grant given by the Office of the Vice President of the Department of Research, according to Markelz.
Dr. Jorge Jose, vice president for research, also helped to secure a four-year, $1.2 million grant from the National Science Foundation (NSF), for which there was a great deal of competition.
“Of the six who applied for seed grants at UB, two were selected to be sent to the National Science Foundation,” said Jose. “They were selected among the 10 out of over 400 applications which received funding.”
Markelz, who specializes in physics, said that after being paired with Dr. Jonathan Bird, professor of electrical engineering, the combined brainpower allowed the project to take shape.
“(We) were working on completely different projects,” Markelz said. “Which is how I think a lot of ideas come to be.”
One of the most exciting aspects of the research for Markelz and Jose is the interdisciplinary and multinational effort. The collaboration begins in Japan, where a UB graduate student will go for research.
Jose said that the international effort between universities is one that shows a global connection.
“Science and physics are international endeavors,” Jose said. “This is one of many sources of pride (for UB).”
The ultimate goal of the project is to provide the technology, which can then be used to create products easily accessible to any security worker.
“A person in airport security wants a button,” Markelz said. “What government does is package and format your device so the button can be pushed.”
Though the grant is for four years, the first leg of the project is expected to be complete before the end of the spring semester.
Source: The Spectrum, University of Buffalo.
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Sunday, October 15, 2006
Pump it up: new proposals for X-ray lasers
Scientists at Berlin's Max Born Institute have devised a new type of pump laser for X-ray lasers that for the first time produces a continuous, repetitive regime at 100Hz and energies exceeding 1J.
At the 10th International Conference on X-Ray Lasers in Berlin, organized by the Max Born Institute (MBI), novel designs for X-ray lasers and the associated pump lasers led many presentations.
MBI itself proposed a significant new solution to the challenge. Scientists worldwide are working on lasers with ever shorter wavelengths; the shorter the wavelength applied, the smaller the structures one can inspect, investigate and develop.
optics.org interviewed Peter Nickles, project coordinator at MBI, about the conference he chaired and about MBI's work on a novel DPSS pump laser.
optics.org: What was the purpose of the conference in Berlin?
Peter Nickles: MBI's X-Ray Laser group organized the 10th International Conference on X-Ray Lasers, to which we welcomed 115 mainly academic participants from 14 countries. The event was also supported by measurement and materials companies. We presented details of MBI's new pump laser, developed without input from industry.
o.o: What is your definition of an X-ray laser?
PN: The X-ray laser generates short-wavelength radiation by amplified spontaneous emission in a highly ionized active medium. The device is mirrorless, so no resonator is used to increase the interaction length between signal and medium. The latter is caused by an extremely short-lived (10-15 ps) gain.
The term X-ray laser is confusing because it was created at least 40 years ago and is still used by tradition, but current X-ray lasers generally work in the XUV or soft X-ray spectral range.
o.o: Why do we need new designs of pump laser to drive X-ray lasers?
PN: The majority of laser-plasma-based X-ray lasers until the development of the GRIP (grazing incidence pumping) geometry worked as single-shot devices. They delivered a high number of photons in a single shot. However, many applications require repetitive irradiation with a reasonable average photon flux. A typical repetition rate of 10 Hz (with GRIP and conventional Ti:sapphire laser) is too low and a new laser driver with a higher repetition rate and stability, compared with the common 10 Hz Ti:Sa laser, is needed. This is possible only with a diode-laser-pumped system.
o.o: How does MBI's design of pump laser operate?
PN: Our DPSS laser driver is the first and only pump laser that permits a continuous, repetitive regime at 100 Hz and energies exceeding 1 J. It could be a breakthrough in the pump technique for X-ray lasers.
o.o: What is the specification of the new MBI-pumped X-ray laser?
PN: We expect an average power of the new X-ray laser to be around 100 µW with an average brightness of 1016-1017 photon s-1mm-2mrad-2/0.1%Bw. at a wavelength of 13.9 nm. The new laser should have a conversion efficiency of ~30% and proportionally should increase the total conversion efficiency, achieving a value of about 10-5.
o.o: Did the conference discuss other models for pumping and designing X-ray lasers?
PN: New X-ray laser schemes, especially the so-called injector-amplifier arrangement, were discussed vigorously. These could offer a reduction in the pulse length of X-ray lasers to less than 1 ps. The conference also saw the first demonstration of a portable capillary discharge-based X-ray laser (by the Colorado State University group). This could find laboratory applications such as experiments on chemical catalysis.
o.o: What are the likely applications of the new MBI design of DPSS-pumped X-ray laser?
PN: Microlithography is less probable at the moment due to severe constraints put on the source by industry. However, such an application cannot be excluded with further development of the pump laser technology.
The X-ray laser in the form of a beamline could have broad application in short-wavelength metrology and inspection techniques, now that imaging with a resolution better than 38 nm has been demonstrated with an X-ray laser working around 13 nm. Many possible applications that are analogues to the visible and IR spectral regions (such as speckle-technique and diffraction microscopy) have not yet been explored.
Generally, conventional pumping lasers are not stable enough to ensure accurate scientific measurements. Particularly in sequences of measurements, when we average signals, we can get "fuzzy" results. Diode lasers are far more stable and thus more suitable for the pumping process. They lead to more exact results and also allow high repetition rates, i.e. fast repeating pulses.
o.o: What is the commercialization potential of the MBI pump, who are likely customers, and what could be the market value?
PN: This pulsed pump laser would be a less repetitive but more energetic variant of an industrial laser. This is a rare combination of the output parameters and some specific material processing by energetic picosecond or nanosecond pulses could be applicable.
Our laser does not match the industrial standards known to us. However, it could be used for surface processing and as an efficient and more stable pump laser in specific high power lasers - such as Ti:Sa lasers - as well as for incoherent X-ray sources.
The MBI X-ray laser, which should be ready for use by the end of 2007, marks a milestone in the development of such lasers. The neighboring Ferdinand Braun Institute (FBH) is also involved in this research project, providing the special diode lasers. These sources are based on new designs of laminar structures and lateral structuring. The highly brilliant and efficient laser diodes emit at wavelengths around 935 nm and allow simple and reliable beam formation at low production costs.
o.o:What are the advantages of the new pump/X-ray laser design?
PN: One of the great advantages of such an X-ray laser is its comparatively small size. Furthermore, the diode-based pumping lasers require less energy than solid-state pumping lasers. A couple of standard-sized desks would be sufficient space to build such an X-ray laser. Thus, an intense short-wave light source can easily be moved - a feature that is especially interesting for industrial applications.
Their flexibility and easy transport would make the new design an interesting source of short-wave pulses complementary to short-wave free electrons lasers (FEL) which work as individual large-scale facilities based on particle accelerators.
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Tuesday, September 12, 2006
Screening for Terrorism - Scientific American
Be they trains, planes or automobiles, the world’s transportation network needs better protection. In the U.S. alone, Government Accountability Office investigators snuck bomb components through more than 20 airports and the recent roll-up of a terror plot in the U.K. highlighted the inability of current technologies to detect items such as liquid explosives or their precursors. But a slew of new devices could help fill this security gap, including millimeter-wave cameras.
Developed by companies ranging from industry heavyweights General Electric (GE) and L-3 Communications to smaller firms such as QinetiQ, millimeter wave detectors work in one of two ways: active or passive. Active devices bombard people with millimeter waves to reveal what may be hidden inside clothing, whereas passive devices rely on collecting the ambient waves in the environment. “Millimeter wave covers a broad range of frequencies–from 30 to 300 GHz. At some, the sky illuminates the object, the image being collected in much the same way as an optical camera,” explains John Salkeld, QinetiQ’s director of optronics. “At others, you can pick up emission from the human body.”
Whether active or passive, millimeter waves’ real attraction lies in what it is not: overly revealing. But revealing is exactly what security experts desire for passenger and luggage screening. And for that, x-ray remains the best probing wave. Already, x-ray machines form the core of checked baggage security, peering inside suitcases much as doctors peer inside bodies using CAT scans. Carry-on baggage screening also enhances x-ray imagery by overlaying color-specific highlights that identify the type of material.
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| Millimeter wave provides an image that can reveal concealed items. |
Besides seeing what passengers bring on board, security officials also want to sniff them. The most common “smelling” devices are trace detection portals, known as puffers, which work by loosening particles on a passenger’s clothing with blasts of air and then analyzing them for traces of explosives or other suspicious chemicals. But the machines have proved susceptible to malfunction, prompting a halt in their installation at airports, and some experts question their effectiveness. “Airplane security stops the sloppy and the stupid,” argues Bruce Schneier, chief technology officer of Counterpane Internet Security. Puffer makers, though, are quick to defend their technology. “It doesn’t take much to be sloppy,” counters Jay Hill, chief technology officer for GE Security. “We’re talking about picogram concentrations.”
Regardless of whether terrorists are sloppy or stupid, they do have a wide array of explosive tools at their disposal: from highly volatile chemical bombs manufactured from relatively common ingredients, such as the hexamethylene triperoxide diamine (HMTD) suspected in the foiled London terror plot, to the military grade plastic explosive Semtex. Because of the problems with existing puffers, some security experts are looking at alternatives, such as quadrupole resonance, terahertz detectors and neutron bombardment machines.
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| Color coding reveals materials in a traditional x-ray image but backscatter pierces the clutter to highlight a hidden bottle. |
Determining how many bags–or passengers–should qualify for more screening is the difficult balance of any security procedure: screen too many, and the utility of the transportation is compromised; screen too few, and its security is. “They have done the analysis on what is a minimum threat quantity that would be the minimum amount necessary if placed strategically to cause a catastrophic event,” explains Peter Kant, vice president for global government affairs for detection device maker Rapiscan Systems. “All the machines are tested towards that threat quantity.” In other words, if the machine is too lenient it will not make it into use, but if it is too stringent it will not find application either.
And critics charge that much of transportation security development is targeted at past threats, such as shoe bombs, not at future, untried tactics. It may also be providing a perverse incentive. “If you look at the history of aviation security, starting with the Cuban hijackings in the ’60’s, each security measure put in place may have deterred things for a while but ended up raising the bar,” offers R. John Hansman, director of M.I.T.’s International Center for Air Transportation. “It would be much more effective to have more random screenings with more types of devices. From a deterrence standpoint, an attacker does not know what screen will be used.”
Advanced terror detection technologies remain expensive and sometimes difficult to use, challenges that will have to be overcome before they can find widespread deployment, but they are useful in liberating the most important part of any transportation security scheme: security officers themselves. “You might want to be getting more technology out there to free up the people to do other things like behavioral pattern recognition, interfacing with passengers,” notes Craig Coy, president and chief operating officer of the Homeland Security Group at L-3 Communications. Technology may be able to help here as well, via biometric devices that read a person’s physical signals for signs of agitation or other warnings.
“In this world, our technology development cycle has to be faster than the bad guy’s learning cycle,” adds Randy Null, chief technology officer at the Transportation Security Administration. “[But] we clearly would like to find people before we have to find items.” Technology can detect bombs or other terrorist devices but it is the people behind the machines–and behind the intelligence rendering this last line of defense redundant–that truly protect.
By David Biello
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Smart fibres measure optical and thermal signals
They may look unconventional but the photosensitive fibre structures being developed at MIT promise a new way to measure the amplitude and phase of an optical signal. Rob van den Berg talks to the team to find out more.
Using a combination of lenses, filters, beam splitters and detectors to measure an optical field could be a thing of the past thanks to Yoel Fink and his group at Massachusetts Institute of Technology, US. The team has developed a geometric approach to obtaining both the amplitude and phase of an optical field using tough photosensitive fibres woven into lightweight two- and three-dimensional structures (Nature Materials 25 June).
Fink and colleagues say that these fibres do not suffer from the same constraints as their classic glass counterparts and enable access to optical information on unprecedented length and volume scales. What's more, the group has shown that by changing the chemical composition of the fibres, they can be made to detect heat, vibrations and even specific chemical components.
Ayman Abouraddy is a research scientist in Fink's group that has been pioneering the smart fibre work over the last few years. "The basic principle behind these fibres is very simple," Abouraddy told OLE. "The core consists of a light-sensitive semiconductor chalcogenide glass. Along the full length and in intimate contact with this semiconductor material are four thin strips of metal, usually tin. When light or heat impinges on the fibre, photons are absorbed and electron-hole pairs may be generated. These are collected by the electrodes producing an electrical response."
In order to protect the fibres from the environment, a resilient polymer insulator, such as polyethersulphone, covers the semiconductor core and metal electrodes. Combining these three different materials – a semiconductor, a metal and a polymer – is not as difficult as it sounds. Just like their standard glass counterparts, these "smart" fibres are produced from a macroscopic preform, approximately 30 cm long and 3–4 cm in diameter.
Smart fibre fabrication
The fabrication process begins by preparing cylindrical rods of the glassy semiconductor material. A cylindrical shell of polymer having an inner diameter equal to that of the glass rod is prepared with four slits removed from the walls for the metal electrodes.
The glass rod is inserted into the polymer shell and a polymer sheet is then rolled around the resulting cylinder to provide a protective cladding. This is then consolidated into an integrated structure by heating under vacuum. Finally, the cylinder is put in a standard drawing tower producing hundreds of metres of fibre. This maintains the geometry and structure of the macroscopic preform and contacts are formed at the glass/metal interfaces.
"It is very important that the thermo-mechanical properties, such as the melting temperatures of the different materials, are properly matched otherwise the fibre drawing process fails," explained Abouraddy. "People have assumed that it would be impossible to integrate materials with highly different electrical and optical properties into the same fibre because they would have different thermo-mechanical properties. We have shown that this is not necessarily the case."
The fibres are mechanically tough, yet flexible, lightweight and protected (both electrically and chemically) from environmental effects. Arranging them into a closed-surface sphere creates an omnidirectional light-detection system capable of discerning the direction of illumination over 4πsr.
A more sophisticated detection scheme results from using two-dimensional arrays or webs. With a single fibre web, Abouraddy explains that it is possible to reconstruct the intensity distribution of an arbitrary optical field using an algorithm similar to that used in computerized axial tomography (CAT) scans.
"We illuminate a 32 × 32 fibre web with a simple image using a white-light lamp and each fibre records the total intensity of the light along its entire length," said Abouraddy. "In order to reconstruct an estimate of the optical intensity distribution that impinges on the web, we record a set of rotated projections and use a back-projection algorithm."
In the case of fibre webs, these projections can be obtained by rotating the web or alternatively, rotating the object that is being imaged. The image reconstruction improves as more projections are taken into account.
A unique advantage of this detector is the fact that no lens is needed because of the large dimensions used (relative to the wavelength of the light). And, as Abouraddy explains, with two parallel fibre webs it even becomes possible to reconstruct both the amplitude and the phase distributions of an incoming field. "Once the amplitude of a field is known in two different planes, the phase can be obtained using an iterative algorithm," he said.
Abouraddy is convinced that this approach will eventually lead to non-interferometric, lensless imaging, when a larger number of fibres are included in the web to form images of objects in more detail. "The system has an infinite depth of focus," he said. "An image of the object is formed regardless of the distance of the object from the webs, provided that the diffracted field at the locations of the two webs is intercepted."
According to Abouraddy, the image reproduces the object with its physical dimensions and also determines its physical distance from the webs. "Instead of choosing and positioning lenses and detector arrays to perform an optical field measurement, you now only have to design the proper geometrical constructions of polymeric, light-sensitive fibres," he added.
Changing the chemical composition
Abouraddy is keen to point out that the method is by no means limited to measurements in the optical domain. Changing the chemical composition allows the team to tune the electronic bandgap of the semiconducting material. For example, by including germanium, the material becomes sensitive to slight changes in temperature.
The team believes that there are already numerous potential applications for the thermally sensitive fibres. By weaving them into large arrays, for example, he says that thermal information over areas as large as tens of square metres can be obtained with cm2 resolution.
Spatially resolved thermal sensing enables failure detection in systems where the failure mechanism is linked to a change in temperature, such as chemical reactors or car tyres. An intriguing application involves the thermal monitoring of the body of large aircraft or measuring the skin temperature of the space shuttle beneath its thermal tiles.
The method could also be used for thermal monitoring of battlefield soldiers by medical staff. "By weaving these fibres into the clothing of soldiers we can allow them to thermally sense both the environment and their own body," said Abouraddy. "Our optically sensitive fibres may detect the tiny dots of laser light used by snipers for aiming. If a soldier is hit by a bullet, blood will rush to the wound leading to a local increase in temperature, which can be monitored."
Fibres used for infrared laser beam delivery, regardless of the guiding mechanism or materials used, must transport significant power densities through their core. This leads to another important application: self-monitoring of the fibres' condition.
Defects in fibres tend to be highly localized but even a small defect within such a high-power optical transmission line can result in an unintentional energy release with potentially catastrophic consequences. High-power infrared light travelling through the fibre will accumulate at the defect site, heating up the region and eventually leading to failure. The research team has demonstrated that it is possible to localize these defects with high precision (Nature Materials November 2005).
The Fink group was fast in coming up with a promising application of such a local temperature probe. In 2002, the group unveiled a photonic bandgap (PBG) fibre to efficiently guide high-power infrared radiation at 10.6 µm from a CO2 laser (Nature 12 December 2002). Today, these have been incorporated into a device (recently approved by the FDA for use in patients) that enables surgeons to efficiently remove cancerous tissue from the lungs using infrared laser light.
Structural perturbations such as fibre bends also tend to increase the overall losses through coupling to both higher-order propagating modes and to localized defects. "This may happen, for instance, when the fibre enters the throat," explained Abouraddy. "My colleague Mehmet Bayindir came up with the idea to surround these PBG fibres with extra layers just like those used for thermal sensing. This allows us to sense the escape of light via the heat generated as soon as it occurs and switch off the treatment laser immediately."
These applications highlight the value of combining various functionalities into a single smart fibre. And there are yet more promising prospects by going beyond the optical and thermal regimes.
"This is a very flexible process. We have found many more combinations of materials that are compatible and can be drawn into fibres," concluded Abouraddy. "You could think of adding completely different functionalities to the fibres, such as pressure sensitivity, or the ability to detect specific chemicals, just by tuning the chemical composition of the chalcogenide glass and the polymer and designing a suitable fibre structure."
About the author
Rob van den Berg is a freelance science journalist based in the Netherlands.
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Saturday, September 09, 2006
Micromachined waveguide antennas for 1.6 THz
S.T.G. Wootton, N.J. Cronin, S.R. Davies, C.E. McIntosh,
J.M. Chamberlain, R.E. Miles and R.D. Pollard
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Diamagnetic Response of Metallic Photonic Crystals at Infrared and Visible Frequencies
2Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
3Surface Physics Laboratory (National Key Lab), Fudan University, Shanghai 200433,
We show analytically and numerically that diamagnetic response (effective magnetic permeability miu_e <>
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Friday, September 08, 2006
On-chip gratings improve stability of laser diodes
Quintessence Photonics has written gratings into its infrared laser diodes that narrow the emission spectra and reduce temperature sensitivity. As Paul Rudy explains, this makes the diodes ideal for medical applications and could lead to cheaper diode-pumped systems.
The combination of compactness, low running cost and excellent electrical-to-optical efficiency has enabled high-power edge-emitting laser diodes to serve many applications in industrial, medical and defence markets. A growing number of these lasers are directly addressing “thermal” applications such as printing, medical and plastics welding, but the majority have a well-defined emission spectra and are used as sources to pump solid-state and fibre-laser systems.
The advantages of diode pumping over lamp pumping are well known, and include increased system efficiency, greater reliability and lower cost of ownership. However, these systems cannot deliver the temperature-independent performance of lamp-pumped designs because of the laser’s lack of stability. Instead, thermal management and temperature control of the diode are needed to precisely tune its emission wavelength. But even with this control, the linewidths produced are insufficiently narrow for some applications.
It is critical to improve the stability and spectral narrowing of high-power laser diodes so that they can simultaneously deliver the efficiency associated with diode pumping and temperature stability provided by lamp pumping. If these objectives are met at a well-defined wavelength, then laser system design- ers can improve the decvice’s compactness, efficiency, power and beam quality while reducing its thermal-management cost.
The improvements would also mean that these lasers could be used directly for scientific and medical pumping applications, such as Raman spectroscopy and enhanced magnetic resonance imaging, which require precise tuning of narrow emission wavelengths to hit atomic or molecular absorption spectra.
Various methods have already been used to improve the spectral brightness, stability and accuracy of laser diodes. These approaches include various external techniques using either volume Bragg gratings, external lenses and bulk gratings, or seed lasers in master oscillator power amplifiers. However, all of these approaches require sensitive and high-precision alignment, costly additional lasers and/or optics and specially designed coatings. On-chip solutions are possible with internal distributed feedback gratings similar to those that are used in singlemode telecom lasers. However, it is difficult to transfer this technology to high-power multimode lasers because multimode devices require more complex grating designs to capture and lock the large number of transverse modes.
Recently, Quintessence Photonics Corporation (QPC) has overcome these challenges and demonstrated a range of high-power lasers operating at 808, 976, 1470, 1535 and 1550 nm, which are fabricated at our headquarters in Sylmar, CA. These MOCVD-grown InP-based and GaAs-based lasers feature internal gratings that narrow the spectral linewidth, reduce wavelength-temperature sensitivity, and ensure that the device operates at the required wavelength.
High-power laser diodes are usually constructed by inserting a gain-producing active stripe into the device’s resonant Fabry-Pérot cavity. Aside from defining a periodic “comb” of resonant frequencies, the cavity provides no wavelength control. The emission wavelength is controlled by the active layer’s gain spectrum. Unfortunately, this gain spectrum is “flat”, has a characteristic width of typically 20 nm, and is strongly temperature dependent. This makes for a spectrally broad laser output, particularly at high power fluxes, which is highly dependent on the operating temperature. The emission wavelength can typically vary by 0.3 nm/°C.
However, when the on-chip grating is added to select the longitudinal mode, temperature sensitivity is governed by the changes in refractive index of the grating region, and is reduced to 0.1 nm/°C or less.
These devices are fabricated in a similar way to conventional laser diodes, with the gratings defined by optical lithography into a photoresist, followed by etching, or formed during a growth and re-growth process.
The InP and GaAs lasers have different grating geometries that are designed through extensive modelling, but use similar processes to write the gratings. After the design has been optimized, the total processing time for the grating-based lasers is only slightly longer than that for conventional emitters. Our development has led us to believe that high-power grating-based lasers promise excellent manufacturing yields through improved targeting of the wavelength, which leads to reduced yield loss compared with conventional laser diodes.
When 808 nm pump lasers are sold, it is typically with a 3 nm centre wavelength tolerance, a spectral width of less than 2-4 nm and a 0.3 nm/°C temperature tuning coefficient. However, for common gain media, such as neodymium-based crystals, absorption peaks can be as narrow as 1 nm. This means that system manufacturers have to control the operating temperature to within 0.1 °C to correctly tune and maintain the appropriate emission wavelength. Unfortunately, the diode red-shifts as it ages, and to maintain efficient lasing the diode has to be increasingly cooled, often until it reaches the dew point. Once this point is reached, catastrophic damage to the laser’s mirrors can occur.
QPC released 808 nm lasers in June with 100 μm wide stripes that avoid these issues by using internal gratings to deliver the performance described in the table above. These lasers have much narrower laser emission widths than their Fabry-Pérot cousins (see figure 1), and have great promise for Raman spectroscopy, pumping alkali vapours for medical imaging and atomic vapour lasers, and simplifying neodymium-based diode pumped systems.
In the 915-976 nm regime, high-power laser diodes are used to pump fibre lasers that have a typical centre wavelength tolerance of 5 nm, a spectral width of less than 5 nm and a temperature tuning coefficient of 0.3 nm/°C. The fibre laser’s absorption spectrum has a relatively weak broad peak of 915-960 nm, and a peak that is three to four times a strong at 976 nm. Using this shorter wavelength peak is not ideal for a growing number of pulsed fibre laser applications, because longer lengths of fibre increase nonlinear losses. Until now, the choice has been between using an uncooled diode to pump the broad but weak absorption peak, or a temperature-controlled laser to excite the stronger and narrower 976 nm peak. However, our 976 nm single-emitting device shows that it is possible to enjoy the benefits of pumping strong but narrow peaks without the need for high precision temperature controls.
Laser diodes emitting between 1.4 and 1.6 μm are used for various applications, including pumping Er:YAG lasers that are used for range finding, materials processing and aesthetic medical treatments. Er:YAG sources, which emit in the eye-safe regime, are also becoming widely used to reduce the impact of potentially hazardous unintended scattered radiation from either laser sources, optical delivery systems and targets. Applications are plentiful in the industrial, defence and medical markets.
For Er:YAG pumping, lasers operating at 0.9-1.0 μm can be used, but optical conversion is more efficient at 1532 nm where there is a 1 nm-wide absorption peak. This peak can be pumped using typical high-power temperature-controlled InP lasers that have a 10 nm spectral width and 0.35 nm/°C temperature tuning, but it can also be excited with increased efficiency with our grating-based laser bars.
Fibre laser sources
High-power fibre lasers often use several expensive amplifying stages, but this could be avoided by using 1550 nm single frequency, single transverse mode diodes that can deliver sufficient power. At higher powers, singlemode operation has been demonstrated in tapered devices. However, producing more power while maintaining a near diffraction-limited performance and narrow linewidth is challenging, because of yield losses owing to beam quality deterioration at high powers, and filamentation at relatively low powers. These issues have been addressed with QPC’s high-power 1550 nm laser, which contains a buried heterostructure singlemode waveguide and a tapered gain region. The waveguide acts as a mode filter, but once the beam is fed into the tapered gain region the mode can freely diffract and be amplified by a tapered electrical contact. These lasers can deliver more than 1.5 W at 28% wall-plug efficiency, using a 5 A drive current. Spectral linewidth is limited by the test equipment, but was measured at less than 6 MHz, and suppression of the sidemodes is more than 50 dB.
The combination of our range of diodes’ spectral brightness, stability and spatial brightness opens the door to deployment in tasks such as the seeding and core pumping of fibre systems, as well as providing the source for second harmonic generation of light for biotech and display applications. And even higher output powers could be reached while maintaining diffraction-limited performance if emitters can be coherently combined. Our motivation is to expand the number of pumping and direct diode applications with enhanced performance, increased temperature stability and reduced system complexity, while maintaining the device’s compactness, low running cost and excellent efficiency.
Acknowledgments
Part of this work was supported by the Naval Air Warfare Center Weapons Division and by the US Army.
• This article originally appeared in the August issue of Compound Semiconductor magazine.
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Thursday, September 07, 2006
Photonic crystals go magnetic
Physicists in Germany have made a new type of photonic crystal by fine-tuning the magnetic, rather than the electric, properties of a material.
Stefan Linden of the Karlsruhe Research Institute and colleagues at Karlsruhe University made a novel type of photonic crystals from pairs of gold wires, which act as artificial magnetic atoms. The discovery has opened up new ways to manipulate light on the nanoscale, the scientists claim (Phys.Rev.Lett. 97 083902).
Photonic crystals are nanostructured materials in which periodic variations of some property - usually, the material's electric permittivity - produce a "photonic band gap". This affects how photons propagate through the material. This effect is similar to how a periodic potential in semiconductors affects the flow of electrons by defining allowed and forbidden energy bands. In particular, photons with wavelengths or energies in the photonic band gap cannot travel through the crystal, which allows scientists to control and manipulate the flow of light by introducing carefully selected defects.
Until now, all photonic crystals operating with visible light have worked by modifying a material's electric permittivity - a measure of the extent to which a material concentrates electrostatic lines of flux. Although the same effects are expected for periodic modulations of the magnetic permeability ("mu") - which is a measure of how a material responds to a magnetic field - all known natural substances have a ("mu") of 1 for visible light. This means that researchers have not been able to make photonic crystals that operate through variations in the magnetic permeability.
Now, however, Linden and colleagues have found a way round this problem by using "metamaterials". These are composite structures made from tiny rods, ensembles of metal rings and the like, in which the individual components act as "artificial atoms". Metamaterials therefore have very different properties from their component parts, including values of ("mu") not equal to 1.
In the current work, the researchers used pairs of gold wires a mere 220 nm wide and 100 micrometres long, separated by a 50 nm thick layer of magnesium fluoride, to create a one-dimensional periodic lattice of artificial "magnetic atoms". This was then placed on a quartz-based slab, which acts as a waveguide to channel light along certain paths, to create a 1D "magnetic" photonic crystal.
"Our findings are a proof of principle for the concept of a magnetic photonic crystal," says Linden. "However, there still is a long way till we can utilize it as a real-world application." The ability to use both electrical permittivity and magnetic permeability will give physicists more design freedom. It could even lead to new effects such as three-dimensional photonic bands - a prerequisite if photonic crystals are to fulfil their potential - made of stacks of one-dimensional magnetic photonic crystals. The team is now trying to fabricate 3D metamaterials based on its 1D structures.
About the author
Bob Swarup is a science writer for physicsweb.org
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Generation of coherent terahertz pulses in ruby at room temperature
(Received 8 May 2006; published 29 August 2006)
We have shown that a coherently driven solid state medium can potentially produce strong controllable short pulses of THz radiation. The high efficiency of the technique is based on excitation of maximal THz coherence by applying resonant optical pulses to the medium. The excited coherence in the medium is connected to macroscopic polarization coupled to THz radiation. We have performed detailed simulations by solving the coupled density matrix and Maxwell equations. By using a simple V-type energy scheme for ruby, we have demonstrated that the energy of generated THz pulses ranges from hundreds of pico-Joules to nano-Joules at room temperature and micro-Joules at liquid helium temperature, with pulse durations from picoseconds to tens of nanoseconds. We have also suggested a coherent ruby source that lases on two optical wavelengths and simultaneously generates THz radiation. We discussed also possibilities of extension of the technique to different solid-state materials.
©2006 The American Physical Society
URL: http://link.aps.org/abstract/PRA/v74/e023819
doi:10.1103/PhysRevA.74.023819
PACS: 42.50.Gy, 42.65.Ky
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Wednesday, September 06, 2006
Theory of the ultrafast nonlinear response of terahertz quantum cascade laser structures
- C. Weber, F. Banit, S. Butscher, and A. Knorr
- Institut für Theoretische Physik, Nichtlineare Optik und Quantenelektronik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
- A. Wacker
- Fysiska Institutionen, Lunds Universitet, Box 118, 22100 Lund, Sweden
(Received 9 March 2006; accepted 10 July 2006; published online 30 August 2006)
Using density matrix theory, the linear and ultrafast nonlinear optical properties of a recently developed terahertz quantum cascade laser are investigated. All relevant excitation regimes, from coherent Rabi flopping up to the scattering dominated stationary response, are covered by the theory. It is shown that the coherence transfer between different periods is important to describe optical effects. ©2006 American Institute of Physics
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Saturday, September 02, 2006
Gap structures and wave functions of classical waves in large-sized two-dimensional quasiperiodic structures
(Received 8 May 2006; published 30 August 2006)
By using the sparse-matrix canonical-grid method, we performed large-scale multiple-scattering calculations to study the gap structures and wave functions of classical waves in two-dimensional quasiperiodic structures. We observed many interesting phenomena arising from the quasiperiodic long-range order. In particular, a self-similar wave function with resonant structures was observed at a band edge. Our findings indicate that two-dimensional quasiperiodic systems exhibit a universal behavior that applies to both electrons (or phonons) in discrete lattices and classical waves in continuous media.
©2006 The American Physical Society
URL: http://link.aps.org/abstract/PRB/v74/e054305
doi:10.1103/PhysRevB.74.054305
PACS: 71.23.Ft, 43.40.+s, 42.70.Qs, 61.44.Br
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