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/cm2) 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.
Highlights
Illumination of a metal surface with ultrashort laser pulses can result in the generation of terahertz radiation[1]
In order to verify this assumption, we investigate in this paper terahertz generation from an array of Au nanorods illuminated by femtosecond optical pulses of different central wavelengths
The Au nanorod sample was fabricated on a Au-coated glass substrate by electroplating with use of a liquid crystalline block copolymer template PEO-b-PMA(Az), which is consisted of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(methacrylate) bearing an azobenzene mesogen on the side chain (PMA(Az))[15,16]
Summary
Illumination of a metal surface with ultrashort laser pulses can result in the generation of terahertz radiation[1]. The generation efficiency can be enhanced by using metallic nanostructures[2], such as gratings[3], percolated films[4,5,6,7], randomly arranged nanoparticles[8], nanohole and nanoparticle ordered arrays[6,9], and metasurfaces[10,11] This is explained by the plasmonic enhancement of the optical fields. Since the metal work function is typically several times larger than the photon energy of near-infrared pump lasers, the latter model invokes multiphoton ionization and, predicts a high-order dependence of the terahertz yield on the optical intensity. If multiphoton ionization contributes considerably to the electron emission and by that to terahertz generation, one can expect a wavelength dependence of the nonlinear order of the generation process. The optical pump intensity was varied in a wide range ~1–100 GW/cm[2]
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