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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