APPLICATIONS OF TECHNOLOGY:
- Plasmonic lens for ultrashort laser pulses
- Achieves high field enhancement in flat areas with very small roughness
- Conserves the high initial brightness
There has been a recent proliferation of the applications of high brightness ultrafast-electron beams. As many applications require ultra-relativistic high-charge pulses or would greatly benefit from an improved degree of coherence, researchers at Berkeley Lab have developed a photocathode that utilizes novel physics to enhance the field produced by an incident electron. This is a gold resonant nanostructure that effectively acts as a plasmonic lens for ultrashort laser pulses.
The invention involves the fabrication of nanostructures with vacuum-compatible techniques. A “template stripping” procedure was applied to fabricate the structure with the Electron Beam Lithography (EBL) technique. After the researchers used HSQ resist deposited on atomically flat silicon waters, they evaporated a thin film of gold on top. The substrate of choice could be glued to the gold surface and the entire structure could be removed from the silicon water via thermal stress. Two nanofabrication techniques, EBL and Focused Ion Beam (FIB), were compared in this technology. Analysis of the surface through Atomic-Force-Microscopy (AFM) indicated the difference in roughness between the two cases, with the EBL approaching atomic flatness. Such a nanostructure -referred to as a Bull’s eye – was extensively simulated using the code LUMERICAL to extract the field distribution and enhancement upon illumination with a radially polarized ultrafast laser at 800 nm. Since photoemission from gold occurred through a three-photon absorption process, the photoemission area could be decreased to below 100 nm through careful tuning of the impinging laser fluence. Plasmon coupling and interference was tested by performing hyperspectral Cathodo-Luminescence measurements. This revealed the presence of enhancement in the flat central area of the structure around 800 nm, which is characteristic of surface-plasmon-polariton (SPP) interference.
Although photo-emission attempts have already been made previously using nano-cavities or nano-holes, they faced a disadvantage because the highest enhancement is achieved at the cavity edges for conventional geometries. This resulted in large transverse momentum spread and thus emittance degradation. The present invention overcomes this limitation, as it involves interference between SPP waves launched on the surface from different source points. It achieves high field enhancement in flat areas with very small roughness as well as directs the extracting field toward the direction of acceleration, conserving the high initial brightness.
DEVELOPMENT STAGE: Proven principle
STATUS: Patent pending. Available for licensing or collaborative research.
SEE THESE OTHER BERKELEY LAB TECHNOLOGIES IN THIS FIELD:
Sensor for Low-Noise Force Detection in Liquids 2017-184
Low Damping Force Sensor for Operation in Liquids IB-3051