Ingrid Wilke

Terahertz emission by InN

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.

Convertible transmission-reflection time-domain terahertz spectrometer

The creation of reliable instrumentation for performing complex reflectance and transmittance measurements of dielectrics, metals, and superconductors in the frequency range from 60 GHz to 1.5 THz is reported. The system allows continuous variation of the THz radiation incidence angle from 25° to 80° without major realignment of the optics and provides the signal-to-noise ratio of 1000:1.

Ultrafast recombination in Si-doped InN

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.

Terahertz surface resistance of high temperature superconducting thin films

We report on measurements of the surface resistance of YBa2Cu3Ox thin films at frequencies between 0.087 and 2 THz and temperatures between 50 and 120 K by time-domain terahertz-transmission spectroscopy (TDTTS) and resonant microwave spectroscopy. The determination of the surface resistance of superconducting thin films by TDTTS is extended to higher frequencies and thicker films than previously by numerically solving the complex transmission coefficient. The numerical solution also provides the dielectric function of the YBa2Cu3Ox thin films. The temperature and frequency dependence of the surface resistance of YBa2Cu3Ox thin films in the THz range is successfully explained by a weak coupling model of d-wave superconductivity which incorporates inelastic and elastic scattering. The surface resistance of YBa2Cu3Ox thin films at THz frequencies is compared to the surface resistance of gold and niobium. The advantages of YBa2Cu3Ox thin films for superconducting THz electronic devices are discussed.

Terahertz emission from silicon and magnesium doped indium nitride

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.

Radiative decay of optically excited coherent plasmons in a two-dimensional electron gas

We report on the observation of coherent submillimeter-wave emission from optically excited plasmons in a two-dimensional electron gas. Phase-synchronous plasma oscillations are induced by femtosecond optical pulses generating electron–hole pairs in the accumulation channel of an AlGaAs/GaAs heterostructure. The radiative decay of the grating-coupled plasmons is traced in time by terahertz-emission spectroscopy. An analysis of the data suggests that ultrafast thermalization and current surges are the plasmon-driving mechanism.

Terahertz detection using on chip patch and dipole antenna-coupled GaAs High Electron Mobility Transistors

This paper presents high responsivity plasmonic terahertz (THz) power detectors operating at 0.3 THz. The detectors are implemented using 130 nm depletion mode InGaAs/GaAs pseudomorphic High Electron Mobility Transistors (pHEMT) with on chip patch and dipole antennas connected to the gate terminal. The measured absolute responsivity at room temperature is 7V/W, while the normalized responsivity with respect to the THz beam and physical antenna area is 8kV/W, with a noise equivalent power (NEP) of 9.1 pW/√(Hz). The paper also addresses the bias dependency of the signal to noise ratio (SNR), THz detector input impedance and the matching requirement for the connection between the antenna and input to the gate of the THz detector.

Characterization of nanometer As-clusters in low-temperature grown GaAs by transient reflectivity measurements

We report a systematic experimental study of the transient reflectivity of low-temperature grown GaAs as a function of excitation wavelength, excitation density, and lattice temperature. We observe that the reflectivity decay is temperature independent for excitation energies between 20 and 70 meV above the band gap of GaAs. Under this condition the reflectivity increases linearly with excitation density and is in very good quantitative agreement with the Drude model. Subsequently, we present a model which allows the extraction of the diameter and density of As clusters in low-temperature grown GaAs from the reflectivity decay.

Time-domain Terahertz spectroscopy as a diagnostic tool for the electrodynamic properties of high temperature superconductors

Abstract Time-domain Terahertz transmission spectroscopy (TDTTS) has proved to be a unique method for the experimental study of electromagnetic properties of high temperature superconducting thin-films at THz-frequencies in basic and applied research. We have improved the analysis of TDTTS-measurements by numerically extracting the complex index of refraction from the measured complex transmission of the superconducting thin film. Based on this other important figures as surface resistance and dielectric function are derived. We have applied this method to study the surface resistance and the dielectric function of YBa2Cu3Ox thin films between 10K and 120K in the frequency range between 0.15THz and 2.4THz. The temperature and frequency dependence of our surface resistance data is successfully explained by a weak coupling model of d-wave superconductivity which incorporates inelastic and elastic scattering at temperatures above 50K. At lower temperatures a deviation of the experimental data from theory is observed which is due to a resonance of the dielectric function.

Surface impedance of Tl-2212 thin films at THz-frequencies

We report on first measurements of the surface resistance and the London penetration depth of Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ thin films in the frequency range from 200 GHz to 1.0 THz at temperatures from 25 to 268 K carried out by time-domain Terahertz transmission spectroscopy. We observe in the superconducting state an increase of the surface resistance with the square of the frequency up to 0.9 THz at 77 K and above, At 78 K and 0.9 THz the surface resistance is 1.6 /spl Omega/. Below 77 K the surface resistance increases with the cube of the frequency. Finally, the surface resistance of Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ is compared to that of YBa/sub 2/Cu/sub 3/O/sub 6.95/ and gold.