11 May 2007
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. (Physical review 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.