Phase-matched generation of coherent euv light by propagating localized field in plasmonic waveguide

Abstract

The objective of this research is to design and analyze plasmonic structure which can generate a large amount of coherent EUV (extreme-ultraviolet) light by near-field amplification of the incident femtosecond laser. Previously, it was reported that plasmonic nanostructures can induce localized field enhancement at a certain region, thereby creating high harmonics at a wavelength shorter than 100 nm. This new method of HHG (high harmonic generation) has a strong potential to be compact EUV light source which do not require complex optical configuration. However, harmonic conversion efficiency is quite low compared to that of conventional HHG techniques. In this report, we presented a plasmonic waveguide structure which exploits SPP (surface plasmon polariton) enhancement. Proposed plasmonic waveguide structure is composed of tapered conical waveguide part and sub-wavelength cylindrical waveguide part. Because SPP can propagate inside a sub-wavelength-scale waveguide, possible volume range of HHG could be considerably enlarged. The material for plasmonic waveguide structure is silver due to its smallest loss in the visible range of the optical spectrum. To get the optimum geometric parameters, we analyzed electro-magnetic behavior of plasmonic waveguide structure by both solving dispersion equation analytically and investigating XFDTD simulation results. Because of waveguide dispersion, coherence lengths for high harmonics are too short to generate high harmonic efficiently by constructive interference. To solve this problem, we introduced quasi-phase-matching method of dielectric thin film deposition. With 80 nm-thick $SiO_2$ layer at the inner wall of cylindrical waveguide, we could achieve spatial modulation period similar to fundamental wavelength of 800 nm, thereby increasing coherence length remarkably. Increase in coherence length implies huge gain of EUV signal amplitude. Theoretically, we can improve harmonic conversion efficiency at least by a fact...