K. Unterrainer

GaAs/AlGaAs superlattice quantum cascade lasers at λ≈13 μm

We report the realization of an injection laser based on intraband transitions in a finite AlGaAs/GaAs superlattice. The active material is a 30 period sequence of injectors/active regions made from AlGaAs/GaAs quantum wells. By an applied electric field, electrons are injected into the second miniband of a chirped superlattice and relax radiative to the lowest miniband. At a heat-sink temperature of 10 K, the laser emission wavelength is 12.9 μm with peak optical powers exceeding 100 mW and a threshold current density of 9.8 kA/cm2. The maximum operating temperature is 50 K. For this device, a waveguide consisting of heavily doped GaAs cladding and low doped core layers has been used as a plasma-enhanced confinement.

Magnetic-field-enhanced quantum-cascade emission

We have observed an enhancement of terahertz intersubband electroluminescence in a quantum cascade structure in the presence of a magnetic field applied normal to the epitaxial layers. At a field of B=7.2 T the emission efficiency doubles. This effect is attributed to the suppression of nonradiative Auger–intersubband transitions caused by Landau-quantization of the in-plane electron motion. The magnetic field dependence of the luminescence intensity shows strong oscillations. These magnetointersubband oscillations are caused by the modulation of the transition rate via resonant inter-Landau-level transfer.

GaAs/AlGaAs-based microcylinder lasers emitting at 10 μm

The realization of electrically pumped GaAs/AlGaAs quantum cascade microcylinder lasers is reported. Design and fabrication of special resonator shapes (microcylinder and ridge waveguide) are presented. Threshold characteristics and optical output of different resonators of the same quantum cascade laser material emitting at 10 μm are investigated. A low threshold current of 318 mA is obtained for a microcylinder resonator with circular cross section. The maximum working temperature of the microcylinder lasers is 165 K. Single mode emission is detected with a side mode suppression ratio better than 20 dB.

Spectroscopy in the gas phase with GaAs/AlGaAs quantum-cascade lasers.

We demonstrate what we believe is the first application of the recently developed electrically pumped GaAs/AlGaAs quantum-cascade lasers in a spectroscopic gas-sensing system by use of hollow waveguides. Laser light with an emission maximum at 10.009 microm is used to investigate the mid-infrared absorption of ethene at atmospheric pressure. We used a 434-mm-long silver-coated silica hollow waveguide as a sensing element, which served as a gas absorption cell. Different mixtures of helium and ethene with known concentrations are flushed through the waveguide while the laser radiation that passes through the waveguide is analyzed with a Fourier-transform infrared spectrometer. The experimentally obtained discrete ethene spectrum agrees well with the calculated spectrum. A detection threshold of 250 parts per million is achieved with the current setup.

Energy level engineering in InAs quantum dot nanostructures

We present an advanced method to tailor the optical and electrical properties of semiconductor quantum dot structures. By embedding vertically stacked quantum dots in a two-dimensional superlattice, the advantages of self-organized growth and of band structure engineering can be combined. The transition energies between the dot levels and the extended states of the superlattice can be adjusted by the period of the superlattice. We apply this scheme for photodetectors made of InAs quantum dots embedded in an AlAs/GaAs superlattice. The dark current of these devices is reduced by more than one order of magnitude compared to the devices without a superlattice.

Ballistic electron spectroscopy of vertical superlattice minibands

We present a study of ballistic electron transport in GaAs/GaAlAs superlattices with different well widths. A three terminal device is used to inject an energy tunable electron beam via a tunneling barrier into a field free superlattice and to collect the transmitted current as a function of the injector energy. A significant increase of the collector current is observed due to miniband conduction in the superlattice. The transfer ratio α=IC/IE can be used to probe miniband positions and the miniband widths in field free superlattices. Longitudinal optical phonon replicas of the eigenstate structure are presented. The tunneling spectroscopy data agree well with self-consistent Poisson–Schrodinger calculations.

FAR-INFRARED EMISSION FROM PARABOLICALLY GRADED QUANTUM WELLS

We have observed grating coupled far infrared (FIR) emission from parabolically graded quantum wells (PQWs) by the application of an in‐plane electric field. The peak emission frequency from different wells matches the designed harmonic oscillator frequency for each well, as determined by the curvature of the PQWs. This is a confirmation that the generalized Kohn theorem applies for emission of FIR radiation.

Mode structure of the p-germanium far-infrared laser with and without external mirrors: Single line operation

The mode structure of p‐Ge far‐infrared lasers with and without mirrors is investigated. Without external mirrors a multimode spectrum is observed and quantitatively explained in terms of waveguidelike modes. An external resonator drastically reduces the number of oscillating modes. For the first time single line operation is demonstrated in this configuration.

Temperature dependence of far-infrared electroluminescence in parabolic quantum wells

We have measured the far-infrared emission from parabolically graded quantum wells driven by an in-plane electric field in the temperature range from 20 to 240 K. The peak emission corresponds to the intersubband plasmon in the parabolic potential. Its photon energy (6.6/9.8 meV) remains rather unaffected by temperature variations, the full-width at half-maximum ranges from 1 (T=20 K) to 2 meV (T=240 K). The reduction of emission efficiency with increasing temperature is attributed to the change in the nonradiative lifetime.

Resonant metamaterial detectors based on THz quantum-cascade structures

We present the design, fabrication and characterisation of an intersubband detector employing a resonant metamaterial coupling structure. The semiconductor heterostructure relies on a conventional THz quantum-cascade laser design and is operated at zero bias for the detector operation. The same active region can be used to generate or detect light depending on the bias conditions and the vertical confinement. The metamaterial is processed directly into the top metal contact and is used to couple normal incidence radiation resonantly to the intersubband transitions. The device is capable of detecting light below and above the reststrahlenband of gallium-arsenide corresponding to the mid-infrared and THz spectral region.