Screening of Deep Ultraviolet Plasmonic Materials for Enhanced High-Order Harmonic Generation in the Extreme Ultraviolet

High-harmonic generation (HHG) driven by intense deep ultraviolet (DUV) pulsed lasers provides a bright, narrow-line width source of vacuum ultraviolet (VUV), extreme ultraviolet (EUV), and soft X-ray light. Its spectral and temporal properties are distinct and complementary to the radiation produced by infrared-driven HHG. Plasmonic nanostructures that can confine electromagnetic energy tightly enable the enhancement of weak nonlinear optical effects and are therefore promising for boosting the efficiency of HHG in solid-state nonlinear media. However, the DUV plasmonic nanostructures for enhanced HHG are rarely explored. We identified DUV plasmonic materials through dielectric function analysis of 19 elemental candidates and proposed a DUV plasmonic nanostructure-enhanced HHG strategy to efficiently produce coherent EUV light. Our finite-difference time-domain (FDTD) simulations on nanodisks and nanodisk dimers revealed that Al, Mo, and Si nanostructures are superior for the confinement of DUV optical fields. Further calculations demonstrated that employing a dual-step HHG approach, involving the conversion of 810 nm light to 270 nm using a commercially accessible nonlinear crystal, followed by further conversion to 90 nm EUV through DUV plasmonic nanostructure-enhanced HHG, could result in a theoretical EUV HHG conversion enhancement of up to 107 times compared to direct ninth-harmonic generation at the surface of the solid medium. Moreover, we evaluated the thermal stability of various DUV plasmonic nanostructures under the illumination of DUV laser light. We found that Al nanobowties exhibit the strongest local electric field enhancement but suffer from the lowest thermal stability, while their Mo and Si counterparts with high material melting points are able to maintain structural integrity at the same laser intensity. We believe that our work will provide an accessible methodology to boost the development of coherent EUV light sources with improved brightness and narrow line width, which will facilitate their applications in coherent diffractive imaging as well as advanced EUV lithography.