Light at the End of the Funnel: IR Becomes EUV

The energy of IR light pulses has been concentrated with a nanofunnel and used to generate extreme-ultraviolet (EUV) light flashes. These femtosecond flashes are thought to contain trains of attosecond pulses. Attosecond EUV light pulses are an important tool for exploring electron dynamics in atoms, molecules and solids.

Scientists at the Max Planck Institute of Quantum Optics (MPQ), along with colleagues at the Korea Advanced Institute of Science and Technology (KAIST) and Georgia State University, said the new technology could help measure the movement of electrons with the highest spatial and temporal resolution.

IR light can be converted to the EUV by a process known as high-harmonic generation, whereby the atoms are exposed to a strong electric field from the IR laser pulses. These fields have to be as strong as the fields holding the atom together. 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 EUV is generated.

Scheme of the generation of EUV light by the 3-D nanofunnel. The infrared light (shown in red) is incident at the entrance of the xenon (green-depicted particles) filled nanofunnel (shown as a half-cut). The surface plasmon polariton fields (wave pattern) concentrate near the tip of the structure. EUV light (shown in purple) is generated in the enhanced fields in xenon and exits the funnel through the small opening, while the infrared light cannot penetrate the small opening and is back-reflected. (Image: Christian Hackenberger)

The core of the experiment was a small (only a few microns long), slightly elliptical funnel made out of silver and filled with xenon gas. The tip of the funnel was only about 100 nm wide. The infrared light pulses were sent into the funnel entrance, where they travel through toward 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 toward 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 EUV light in the xenon gas,” said Mark Stockman, a professor at Georgia State.

The nanofunnel has yet another function. Its small opening at the exit acts as “doorman” for light wavelengths. Not every opening is passable for light. If the opening is smaller than half of a wavelength, the other side remains dark. The 100-nm-large opening of the funnel did not allow the infrared light at 800 nm to pass. The generated EUV pulses with wavelengths down to 20 nm passed, however, without problems.

“The funnel acts as an efficient wavelength filter: at the small opening, only EUV light comes out,” said Seung-Woo, a professor at KAIST. “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.”

Electrons are extremely fast, moving on attosecond timescales. 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 be repeated only a few thousand times per second. This can change with the nanofunnel.

“We assume that the few femtosecond light flashes consist of trains of attosecond pulses,” said 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.”

The repetition rate is important for the application of EUV pulses in electron spectroscopy on surfaces, for example. Electrons repel each other by Coulomb forces, which is why it may be necessary to restrict the experimental conditions such that only a single electron is generated per laser shot. With low repetition rates, long data acquisition times would be required to achieve sufficient experimental resolution.

“In order to conduct experiments with high spatial and temporal resolution within a sufficiently short time, a high-repetition-rate EUV source is needed,” Kling said.

The research team said the novel combination of laser technology and nanotechnology could help in the future to record movies of ultrafast electron motion on surfaces with temporal and spatial resolution that so far have been unreached in the attosecond-nanometer domain.

For more information, visit: www.mpq.mpg.de