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A model is constructed for extrinsic photoconductivity in GaAs heavily doped with Er and driven around 1550 nm. The model is based on internal photoionization of ErAs nanoparticles in the diameter range 2.0 to 2.5 nm, which occurs via a resonant, bound-to-continuum electron transition between the Γ-point in the atomic-like ErAs and the Γ-valley in the GaAs. The model is consistent with all experimental...
The conductivity of graphene and its change induced by electromagnetic waves is crucial for many applications. Here we show that both the photoconductivity and high-field conductivity of highly doped graphene can be described by a simple thermodynamic balance maintained within the free carrier population of graphene.
Terahertz (THz) streaking at individual sharp metallic tips allows for the control of photoelectron pulses in nanoscale volumes. The time-varying acceleration in the enhanced THz near-field facilitates a reshaping of electron energy spectra, corresponding to a manipulation of the phase space distribution. Here, we discuss our recent experimental and theoretical work on THz near-field streaking, with...
We report thermal effects on carrier dynamics in graphene revealed by terahertz spectroscopy. We attribute our observations to extreme sensitivity of carrier density and mobility to environmental changes.
The Quasi-Optical Photoconductivity Decay (QO-PCD) technique is developed by UNIST for precise measurement of minority carrier life time. This technique is suggested to an alternative method for a measurement technique of minority carrier lifetime using millimeter & THz waves. This paper presents a comparison of 2D mapping results between micro-PCD technique and QO-PCD technique. From the measurement...
We have used optical-pump-terahertz-probe spectroscopy (OPTPS) to study a range of novel of semiconductors including III-V nanowires and metal halide perovskites. We show that OPTPs allows key figures of merit to be extracted in a non-contact manner, including charge mobility, surface recombination velocity, and doping density. Furthermore, the technique allows charge recombination dynamics to be...
Reliable doping in GaAs nanowires is essential for the development of novel optoelectronic devices. Previously, GaAs nanowires have been shown to exhibit extremely short photoconductivity lifetimes of a few picoseconds due to their high surface recombination velocity, which is detrimental for nanowire devices, such as solar cells and nanowire lasers. Here, we show that, by exploiting engineered band-bending...
We study nonlinear THz effects in photoexcited gated graphene, using optical-pump/Zwtewse-THz-probe spectroscopy. A crossover from negative to positive differential THz transmission has been observed at the lowest doping concentration when the THz field is increased. Using a simple Drude model of the graphene photoconductivity, we attributed the observed behavior to a transition from a domain in which...
We investigate optical resonant cavities using metallic mirror in order to increase the absorption in low-temperature-grown GaAs (LT-GaAs) based photoconductors operating at telecom wavelength. Two different semi-transparent front mirrors are compared: the first one is a thin gold layer whereas the second one consists of a gold periodic array. We show that the generated photocurrent is 3 times higher...
We report on terahertz radiation from topological insulator (TI) Sb2Te3 thin film under ultrafast optical excitation with different helicity. Polarity-reversals of the emitted THz radiation were observed as the helicity of optical pulses reversed. The observed phenomena are coincident with the characteristics of the helicity-dependent photocurrent on TIs. Our work demonstrates the potential applications...
Continuous-wave (CW) terahertz (THz) photomixers are fabricated using 1-D nanocontacts on low-temperature-grown (LTG) GaAs and InGaAs, for operation wavelengths of 850 nm and 1550 nm, respectively. Silver (Ag) nanowire (NW) used as the nanocontact is aligned between the antennas patterned onto these photoconductive material by dielectrophoresis (DEP) technique. Reliable photocurrent measurements for...
Monte Carlo simulations of photoexcited carrier dynamics combined with time domain electromagnetic modeling of the radiated field reveal different roles of the photocurrent vector components in the process of THz pulse generation upon ultrashort optical excitation of semiconductor surfaces. We find that only low frequency components (below 0.3 THz) are generated mainly by the initial transient current...
We developed photothermoelectric devices based on graphene transistors with tunable p-n junctions achieved through partial polymer gating. This approach presents the advantage of combining broadband photoresponse with transparent gates. After demonstrating that the photocurrent mechanism is consistent with photothermoelectric effects, we studied different device designs in order to determine the best...
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