C. Stellmach

Gate-tunable THz detector based on a quantum Hall device

Recent investigations showed that the THz photoresponse of quantum Hall systems (QHS) is a complex combination of different mechanisms. In this work, we separate these parts by varying the electron concentration and the source–drain current. For our photoconductivity measurements, we used a pulsed p-Ge cyclotron laser as a monochromatic THz radiation source and changed the magnetic field, the source–drain current and the electron concentration (via a backgate) of the QH sample. The spectral resolution of the QHS was found to improve with increasing values of the source–drain current up to the breakdown of the QH effect.

Terahertz Photovoltaic Effect in Bilayer Two‐ Dimensional Systems

We report the observation of THz photovoltage in double quantum wells in tilted magnetic field. The signal appears at the normal component of the magnetic field corresponding to the electron cyclotron resonance conditions and directly related to electron CR absorption. The phenomenon is accounted for by a photovoltaic effect in both electron‐electron and electron‐hole bilayer systems.

Short decay times of the THz photoresponse in quantum Hall Corbino detectors with spectral tunability

The THz photoconductivity of quantum Hall systems (QHS) around filling factor v=2 has been investigated. As radiation source a pulsed p‐Ge Laser tunable in the frequency range form 1.7 THz to 2.5 THz is used. Via variation of the magnetic field, the carrier concentration (tuned by a gate) and the source drain voltage the spectral resolution of QHS as THz detector is determined. The spectral resolution is found best at voltages close to the breakdown value. In addition we report time resolved measurements with relaxations times from 20 ns to 150 ns.

Relaxation oscillations in a bistable quantum Hall system

Quantum Hall systems can show a bistability in the hysteresis of the breakdown. We realized a relaxation oscillator based on a quantum-Hall device with Corbino geometry. Investigations of the performance of such an oscillator revealed an increase of the hysteresis of the current–voltage curve of the device in comparison with the width of hysteresis at dc voltages. By direct measurements of the hysteresis of the current–voltage curve at different frequencies, we found a marked increase of the breakdown hysteresis at low frequencies of some Hz. We explain the observed dynamical enhancement of the breakdown hysteresis applying an electron heating model with a background (delocalization-related) component of conductivity, which decreases with increasing frequency, indicating a more effective localization already at low frequencies.

Edge‐Channel Photo‐Effect In Asymmetric Two‐Dimensional Systems

We report on the photo‐effect in an asymmetric two dimensional electron system where edge currents are induced by homogeneous terahertz (THz) radiation in tilted magnetic field without any external bias. We discuss the features of the observed effect and its mechanism taking into account properties of edge states in magnetic fields. The process is understood as the quantum Hall effect associated with electron‐hole pairs generated in Landau levels by THz radiation under CR conditions.

Calculations of the THz Conductivity of Quantum Hall Systems

We present calculations of the dynamic conductivity of the two‐dimensional electron systems (2DES) in GaAs/AlGaAs heterostructures of the power absorption (Joule heating) and the change of the electron temperature for different magnetic fields and photon energies for better understanding of our optical and electrical excitation measurements performed on quantum Hall (QH) systems with Corbino geometry.

Fast and Tunable Photodetectors for Terahertz Radiation based on the Landau Quantization at HgTe/HgCdTe Heterojunctions

We measured the photoresponse of samples with HgTe quantum wells (Hall bars and Corbino samples). A fast THz detector (τrel = 50 ns) with spectral tunability (ΔE = 4.6 meV at photon energies of Ephot = 8 meV) was realized, operating at low values of the magnetic field (B < 2 T).

Time- and Spectrally Resolved THz Photoconductivity in Quantum Hall Devices

The Terahertz photoconductivity is measured on GaAs/AlGaAs quantum Hall systems. Time resolved investigations show relaxation times form 10 to over 200ns and a dependence on the sample mobility and source-drain voltage. The latter is explained with a heating effect. In addition, spectrally resolved measurements are discussed. The spectral resolution is a function on the sample mobility. Finally, similar measurements on a HgTe/HgCdTe quantum well are presented. All results are important in respect to a possible THz detector application.