We report an experimental study of femtosecond optically excited emission of terahertz frequency electromagnetic radiation from as-grown n-type InN, silicon doped InN, and magnesium doped InN. We have measured the terahertz emission from these materials as function of dc Hall mobility and carrier concentrations. Terahertz emission from InN:Si and native n-type InN increases with mobility as expected for transient photocurrents as primary mechanism of terahertz emission from InN. InN:Mg exhibits enhanced terahertz emission compared to InN:Si. This is experimental evidence for Mg being electrically active as an acceptor in InN. Terahertz emission from InN:Si is less strong than terahertz emission from native n-type InN because of an increased electron concentration due to silicon being an electrically active donor in InN.

We report an experimental study of femtosecond near-infrared optically excited THz-emission from InN thin films grown by molecular beam epitaxy (MBE) on sapphire substrates. THz-emission was investigated as a function of structural as well as electronic properties such as types of buffer layer, film thickness, electron mobilities, electron concentrations and doping of the InN with Si and Mg atoms. The THz-emission mechanism in InN has been analyzed. Ultrafast transient currents are identified as dominant THz-emission mechanisms. Ultrafast carrier recombination is identified as a limiting factor of THz-emission from n-type InN: Si.

We report an experimental study on terahertz (THz) emission from Ga1-xInxSb with 0 <= x <= 1. THz emission is excited by femtosecond near-infrared laser pulses. For this material system THz emission is maximized for an In mole fraction x approximate to 0.5. The maximum in THz emission occurs as a result of carrier compensation (N-A approximate to N-B) for this specific material composition. The THz emission from n-type InSb is twice as large than that from p-type GaSb. The THz emission from Ga1-xInxSb is explained according to the photo-Dember model. The Ga1-xInxSb material system enabled the study of the influence of carrier concentrations on the THz emission process in narrow band gap semiconductors. Our study demonstrates the existence of a compromise between the positive effect of high electron temperature provided by narrow band gap materials and the negative effect of a high intrinsic carrier concentration. This compromise dictates the extent to which the band gap in a semiconductor can be reduced in order to enhance the THz emission. This same analysis can be extended to explain why the THz emission from InSb is lower than that of InAs.

We report femtosecond near-infrared transient photoreflection measurements of native n-type indium nitride and silicon-doped indium nitride thin films. The overall time dependence of the ultrafast reflectivity transient is characterized by the different time scales of carrier cooling and carrier recombination. Experimental analysis demonstrates nonradiative recombination in the picosecond and subpicosecond range as the dominant recombination mechanism at room temperature even at very high carrier concentrations. Silicon-doped InN films exhibit carrier lifetimes as short as 680fs.

Time-domain Terahertz (THz) spectroscopy and imaging is currently evaluated as a novel tool for medical imaging and diagnostics. The application of THz-pulse imaging of human skin tissues and related cancers has been demonstrated recently in-vitro and in-vivo. With this in mind, we present a time-domain THz-transmission study of artificial skin. The skin samples consist of a monolayer of porous matrix of fibers of cross-linked bovine tendon collagen and a glycosaminoglycan (chondroitin-6-sulfate) that is manufactured with a controlled porosity and defined degradation rate. Another set of samples consists of the collagen monolayer covered with a silicone layer. We have measured the THz-transmission and determined the index of refraction and absorption of our samples between 0.1 and 3 THz for various states of hydration in distilled water and saline solutions. The transmission of the THz-radiation through the artificial skin samples is modeled by electromagnetic wave theory. Moreover, the THz-optical properties of the artificial skin layers are compared to the THz-optical properties of freshly excised human skin samples. Based on this comparison the potential use of artificial skin samples as photo-medical phantoms for human skin is discussed.

We report femtosecond optically excited terahertz (THz) emission from tellurium doped GaSb at room temperature. The influence of the majority and minority carrier type and concentrations on the strength of the THz emission is investigated. Strong enhancement of THz emission in GaSb is observed as a result of compensation of native acceptors by tellurium donors. Surface field acceleration and the photo-Dember effect are identified as THz emission mechanisms in GaSb and modeled in dependence of the majority and minority carrier type and concentrations in our GaSb samples. THz emission from $p$-type GaSb is dominated by the photo-Dember effect whereas THz emission from $n$-type GaSb is dominated by surface field acceleration. The doping conditions under which THz emission is maximized are identified.

Strong enhancement of optically excited THz emission from GaSb due to compensation of native acceptors by tellurium donors has been observed. THz emission of compensated GaSb is an order of magnitude stronger than undoped GaSb.

Strong optically excited terahertz emission has been observed from InN thin films. The emission mechanism has been determined to be photocarrier acceleration. This observation implies that InN has a bandgap smaller than 1.5 eV.

We report on optically excited terahertz (THz) emission by indium nitride (InN) thin films. We have used 70 fs titanium–sapphire laser pulses with wavelengths at 800 nm to generate THz-radiation pulses. The InN thin films are deposited on sapphire substrates with GaN buffer layer by molecular-beam epitaxy. The THz-radiation emitted from the InN surface is significantly stronger than that of the GaN/InN interface. The origin of the THz emission are transient photocarrier currents. These results are in agreement with recent experimental results of InN which show that this material is a small band-gap semiconductor. The magnitude of the THz emission from the InN is strong compared to THz emission from previously investigated semiconductors.

We describe a method to determine the radiation spectrum of terahertz sources using a transmitted Fabry-Perot interferometer and a bolometer detector. Our novel Fabry-Perot spectrometer can measure the spectrum of an unknown broadband sub-terahertz and terahertz source. The spectrometer does not need to be pre-tuned. We develop a new algorithm to support the measurements using this spectrometer. Our technique allows us to measure the spectrum over more than an octave, and our numerical algorithm is very stable and robust, providing for an accurate spectrum determination. Two Gunn oscillation sources with the main frequencies at 200 GHz and 600 GHz are used to test the proposed method. The spectrum extracted from the measured data shows that this method is accurate and reliable.

We report on millimeter wave electromagnetic radiation from a GaN high electron mobility transistor with the gate length of 1.5 μm at 8 K. The emission takes place at gate and drain voltages in the linear regime of operation but close to the saturation voltage with the principal emission peak at approximately 75 GHz, which is much higher than the device cut-off frequency. An explanation of this effect involves the “shallow water” plasma wave instability, with the frequency of the plasma waves decreased by the ungated regions of the device.

We report for the first time femtosecond optically excited terahertz pulse emission from indium nitride (InN). MBE-grown InN is a novel electronic material. The THz emission from InN is strong compared to previously investigated semiconductors

The dephasing of intersubband transitions in semiconductor heterostructures was investigated by time-resolved THz spectroscopy. Single quantum structures show dephasing rates, which are nearly identical to scattering rates obtained by conventional Hall measurements and allow insight into the dephasing dynamics. In multiple quantum wells, inhomogeneous broadening of the density of states is the main dephasing mechanism.