Second-order optical effects in seed-mediated grown palladium nanoparticles

Palladium nanoparticles deposited on SnO2-doped In2O3 substrate show substantial optical second harmonic generation (SHG) in the spectral range (lambda = 120-160 nm) which is a part of the vacuum ultraviolet (VUV) spectrum. A single crystalline Li2B4O7 optical parametric generator (OPG) pumped by nanosecond xenon-fluorine excimer laser (EMG 500/218 (Lambda Physics)) with the wavelength 218 nm, pulse duration about 6-8 ns; pulse rate about 80 hz, average pulse power about 0.2 MW and beam diameter varying within the 1.3-7.5nm was used to form the fundamental beam. The OPG Li2B4O7 single crystal was cut in the XZ optical plane. We have tuned the fundamental wavelengths within the 250-320nm spectral range varying the angle of the plane with respect to the incident pumping beam. Maximal SHG output (in the reflected SHG geometry) is observed for the incident angles 75 degrees-80 degrees with respect to the surface normal and p-polarized incident fundamental ultraviolet beams. Spectral separation between the vacuum ultraviolet (VUV) SHG intensities and the fundamental beams was performed using a VUV Seya-Numioka vacuum monochromator with spectral resolution 6 nm in the investigated spectral range. We have found that decreasing mean average palladium nanoparticle sizes favour substantial enhancement of the output SHG within the 120-160nm spectral range. A layer of platinum nanoparticles coated on a layer of palladium nanoparticles suppresses the SHG effect indicating a quenching of the surface plasmon excitation originating from the palladium nanoparticles. The observed effect allows utilizing the palladium nanoparticles as an efficient material for frequency transformation of the UV nanosecond pulses (spectral range 240-310 nm) into the nanosecond laser pulses with wavelengths 120-160 nm.