High-power THz to IR emission by femtosecond laser irradiation of random 2D metallic nanostructures

The split-off terahertz radiating dipoles on thermally reduced α-V2O5 (001) surface

The trapped electron states on a pliable lattice have different localization and physical chemistry characteristics. Here, terahertz time-domain measurements suggest that the formation of vanadyl oxygen defect, in the presence of the surface potential traps and mobile charge carriers, leads to a transient charge distribution that forms terahertz radiating dipoles in V2O5. The emergence of radiating dipoles is evidenced by terahertz responses with a two-valley feature of the thermally reduced α-V2O5 (001) thin films in the temperature range of 300-700 K. The two photoconductance valleys on a several millielectron volts interval are related to two emergent split-off traps, which originate from the VO6 octahedra distortion upon the vanadyl oxygen desorption on the surface. The pliable surface lattices plays a decisive role. So long as the α-V2O5 (001) thin films are covered by a 30 nm-thick Al2O3 capping layer, the distinct two-valley feature disappears completely in the full temperature range. The terahertz radiating dipoles with a fine energy structure is potentially a new measure for charge dynamics on the α-V2O5 (001) surface.

Terahertz and Photoelectron Emission from Nanoporous Gold Films on Semiconductors

Efficient terahertz and photoelectron emission were observed from nano-porous gold (NPG) films deposited on an intrinsic gallium arsenide (GaAs) semiconductor substrate stimulated by femtosecond laser with pulse width of 60 fs. Time-domain THz emission and reflection spectroscopy confirmed that the free charges accelerated by irradiated femtosecond laser pulses transferred from the NPG films into the GaAs substrates. Accordingly, charges accumulation was reduced in the NPG films, resulting in a stronger emission of THz pulse than that from NPG films deposited on SiO₂ substrate. Charges injected into the GaAs substrate enforced an observable decrease of the THz refractive index proportional to the intensity of incident light. In comparison, for NPG deposited on glass substrates, laser induced free charges were accumulated in the NPG films, and femtosecond laser pulses irradiating on the NPG films made no changes of the THz refractive index of the glass substrates.

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

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.

Silicon Field Effect Transistor as the Nonlinear Detector for Terahertz Autocorellators

We demonstrate that the rectifying field effect transistor, biased to the subthreshold regime, in a large signal regime exhibits a super-linear response to the incident terahertz (THz) power. This phenomenon can be exploited in a variety of experiments which exploit a nonlinear response, such as nonlinear autocorrelation measurements, for direct assessment of intrinsic response time using a pump-probe configuration or for indirect calibration of the oscillating voltage amplitude, which is delivered to the device. For these purposes, we employ a broadband bow-tie antenna coupled Si CMOS field-effect-transistor-based THz detector (TeraFET) in a nonlinear autocorrelation experiment performed with picoseconds-scale pulsed THz radiation. We have found that, in a wide range of gate bias (above the threshold voltage Vth=445 mV), the detected signal follows linearly to the emitted THz power. For gate bias below the threshold voltage (at 350 mV and below), the detected signal increases in a super-linear manner. A combination of these response regimes allows for performing nonlinear autocorrelation measurements with a single device and avoiding cryogenic cooling.