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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Wednesday, May 30, 2007
Silicon could open the way for new terahertz technology
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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