Terahertz emission from gold nanorods irradiated by ultrashort laser pulses of different wavelengths

Abstract

Electron photoemission and ponderomotive acceleration by surface enhanced optical fields is considered as a plausible mechanism of terahertz radiation from metallic nanostructures under ultrafast laser excitation. To verify this mechanism, we studied experimentally terahertz emission from an array of gold nanorods illuminated by intense (~10–100 GW/cm 2 ) femtosecond pulses of different central wavelengths (600, 720, 800, and 1500 nm). We found for the first time that the order of the dependence of the terahertz fluence on the laser intensity is, unexpectedly, almost the same (~4.5–4.8) for 720, 800, and 1500 nm and somewhat higher (~6.6) for 600 nm. The results are explained by tunneling currents driven by plasmonically enhanced laser field. In particular, the pump-intensity dependence of the terahertz fluence is more consistent with terahertz emission from the sub-cycle bursts of the tunneling current rather than with the ponderomotive mechanism.

Terahertz Pulse Generation from GaAs Metasurfaces

“…Metasurfaces composed of plasmonic resonators were recently developed to generate THz pulses through enhancement of a weak process of optical rectification at metallic surfaces. 12 − 17 This process was further enhanced by integration of an epsilon-near-zero material underneath the resonators. 18 , 19 However, optical rectification in common metals occurs only at the surface because of the inversion symmetry of the crystal lattice.…”

“…However, material limitations can be mitigated with optical metasurfaces. Metasurfaces composed of plasmonic resonators were recently developed to generate THz pulses through enhancement of a weak process of optical rectification at metallic surfaces. − This process was further enhanced by integration of an epsilon-near-zero material underneath the resonators. , However, optical rectification in common metals occurs only at the surface because of the inversion symmetry of the crystal lattice. As a result, metallic metasurfaces still suffer from low THz generation efficiencies in comparison with the established schemes involving nonlinear dielectrics and semiconductors, such as optical rectification in phase-matched ZnTe crystals , and transient photocurrents in low-band-gap InAs. ,,, …”

Metallic and dielectric metasurfaces in photoconductive terahertz devices: a review

“…131,132 Concerning THz emission, different designs of the metallic metasurfaces can be used as a standalone THz emitter with no antenna metallization. [133][134][135][136] In this case, the THz generation occurs due to nonlinear optical processes 134 as well as the heat redistribution inside metal due to electron heating by an optical excitation. 137,138 However, the emitted THz power of the standalone metal metasurface is relatively low compared with the plasmonic PCA 69 and even to the surface photo-Dember THz emitter 136 and amounts, on average, to 0.15 mW, corresponding to an optical-to-THz conversion efficiency of up to 0.01% at 5.8 THz.…”

“…[133][134][135][136] In this case, the THz generation occurs due to nonlinear optical processes 134 as well as the heat redistribution inside metal due to electron heating by an optical excitation. 137,138 However, the emitted THz power of the standalone metal metasurface is relatively low compared with the plasmonic PCA 69 and even to the surface photo-Dember THz emitter 136 and amounts, on average, to 0.15 mW, corresponding to an optical-to-THz conversion efficiency of up to 0.01% at 5.8 THz. 135 Furthermore, both metallic and dielectric metasurface configurations can be designed to effectively interact with THz radiation and thus can be used in ultrasensitive THz sensors, THz absorbers, highly selective THz detectors, tunable THz field modulators, [139][140][141][142] and THz mirrors, 143 as well as to control a wavefront at THz frequency, 144 etc.…”

Laser Intensity Profile Based Terahertz Field Enhancement from a Mixture of Nano-Particles Embedded in a Gas

“…Heating of the electrons in the NPs array allows coherent THz pulse generation that can be tuned under the variation of the geometries of the metallic NPs (Fadeev et al, 2018). Electron photoemission and ponderomotive acceleration through surface enhanced optical fields can produce THz radiation from gold nano-rods when illuminated by intense laser pulses with different central wavelength (Takano et al, 2019). THz pulses are generated through the acceleration of the ejected electrons under the influence of ponderomotive force that arises from the inhomogeneous plasmon field when femtosecond laser pulses interact with the array of silver nanoparticles.…”