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Tuesday, October 18, 2011

Funnel Infrared Light To Convert Become Ultraviolet

metallic waveguide
Ultrashort pulses of extreme-ultraviolet is the key in time-resolved spectroscopy to investigate the motion of electrons in atoms, molecules, solids. High-harmonic generation is an established process in producing ultrashort pulses of extreme-ultraviolet through directly converting the frequency of femtosecond near-infrared. However, elaborate pump–probe experiments performed on the attosecond timescale require continuous efforts to improve the spatiotemporal coherence and also the repetition rate of the generated pulses. Scientists from the Korea Advanced Institute of Science and Technology (KAIST) in Daejeon (South Korea), the Max Planck Institute of Quantum Optics (MPQ) in Garching (Germany), and the Georgia State University (GSU) in Atlanta (USA) has made a tool of metallic waveguides (silver) for the plasmonic generation of ultrashort pulses of extreme-ultraviolet. The trick is, by using the field enhancement of surface-plasmon polaritons.

Basically, the light can be converted. A wavelength of light can change when it interacts with matter. Shape and type of materials greatly affect it. Nanotechnology is one tool that can generate conversions as needed. With these fields electrons can be extracted from the atoms and accelerated with full force back onto the atoms. Upon impact highly energetic radiation in the extreme-ultraviolet is generated. To reach the necessary strong electric fields for the production of extreme-ultraviolet light, the team of scientists has now combined this scheme with a nano funnel (from silver) in order to concentrate the electric field of the light.

The core of the experiment was a small, only a few micrometers long, slightly elliptical funnel made out of silver and filled with xenon gas. The tip of the funnel was only ca. 100 nanometers wide. The infrared light pulses were sent into the funnel entrance where they travel through towards the small exit. The electromagnetic forces of the light result in density fluctuations of the electrons on the inside of the funnel. Here, a small patch of the metal surface was positively charged, the next one negative and so on, resulting in new electromagnetic fields on the inside of the funnel, which are called surface plasmon polaritons. The surface plasmon polaritons travel towards the tip of the funnel, where the conical shape of the funnel results in a concentration of their fields. “The field on the inside of the funnel can become a few hundred times stronger than the field of the incident infrared light. This enhanced field results in the generation of extreme-ultraviolet light in the Xe gas.”, explains Prof. Mark Stockman from GSU.

“Due to their short wavelength and potentially short pulse duration reaching into the attosecond domain, extreme ultraviolet light pulses are an important tool for the exploration of electron dynamics in atoms, molecules and solids”, explains Seung-Woo Kim. Electrons are extremely fast, moving on attosecond timescales (an attosecond is a billionth of a billionth of a second). In order to capture a moving electron, light flashes are needed, which are shorter than the timescale of the motion. Attosecond light flashes have become a familiar tool in the exploration of electron motion. With the conventional techniques, they can only be repeated a few thousand times per second. This can change with the nano funnel. “We assume that the few femtosecond light flashes consist of trains of attosecond pulses”, argues Matthias Kling, group leader at MPQ. “With such pulse trains, we should be able to conduct experiments with attosecond time resolution at very high repetition rate.”

source: sciencedaily, nature.com

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