T. Roch

Terahertz microcavity quantum-cascade lasers

We demonstrate circular-shaped microcavity quantum-cascade lasers emitting in the THz region between 3.0 and 3.8 THz. The band structure design of the GaAs∕Al0.15Ga0.85As heterostructure is based on longitudinal-optical phonon scattering for depopulation of the lower radiative state. A double metal waveguide is used to confine the whispering gallery modes in the gain medium. The threshold current density is 900A∕cm2 at 5 K. Lasing takes place in pulsed-mode operation up to a heat-sink temperature of 140 K.

Influence of doping on the performance of terahertz quantum-cascade lasers

The authors present the effects of the doping concentration on the performance of a set of terahertz quantum-cascade lasers emitting around 2.75THz. The chosen design is based on the longitudinal-optical-phonon depopulation of the lower laser state. An identical structure is regrown varying the sheet density from 5.4×109to1.9×1010cm−2. A linear dependency of the threshold current density on the doping is observed. The applied field where lasing takes place is independent of the doping. The field is responsible for the alignment of the cascades and therefore the transport of the electrons through the structure.

Hydrogen gas sensors based on nanocrystalline TiO2 thin films

Titanium dioxide thin films are extensively studied for applications in solid state gas sensor devices. Their gas sensing properties are strongly dependent on deposition technique, annealing temperature, film thickness and consequent properties like crystalline structure, grain size or amount of defects and impurities. In this work we report the gas sensing properties of TiO2 thin films prepared by reactive magnetron sputtering technique and subsequently annealed at temperatures 600°C and 900°C. The films were exposed to different concentrations of H2 gas up to 10 000 ppm. Their sensitivity to gas at various operating temperatures, ranging from 250°C to 450°C, was obtained by measuring their resistance.

Band structure mapping of photonic crystal intersubband detectors

The authors report on a quantum well infrared detector embedded in a surface-plasmon waveguide and processed into a deeply etched photonic crystal structure. The device was characterized by collecting the polarization dependent response spectra at different angles of incidence. With this method it is possible to map the photonic band structure by directly detecting the modes of the photonic crystal. It therefore represents a new and direct characterization procedure for photonic crystals. The device shows a strong mixing between TE and TM polarized modes, which is caused by the asymmetric vertical waveguide design.

Subwavelength Microdisk and Microring Terahertz Quantum-Cascade Lasers

We report on the emission characteristics of microcavity quantum-cascade lasers emitting in the terahertz frequency range based on circular-shaped microresonators. Strong mode confinement in the growth and in-plane directions are provided by a double-plasmon waveguide and due to the strong impedance mismatch between the gain material and air. This allows laser emission from devices with overall dimensions much smaller than the free-air emission wavelength (lambda > 100 mum). Hence, for the smallest microdisks we achieved a threshold current as low as 13.5 mA (350 A/cm2) in pulsed-mode operation at 5 K and stable single-mode emission up to 95 K in continuous-wave mode operation. We have observed dynamical frequency pulling of the resonator mode on the gigahertz scale, as a consequence of the gain shift due to the quantum-confined Stark effect. Thus, we were able to estimate the peak gain of the material to 27 cm-1. The smallest microcavities exhibited a strong dependence on the exact placement of the bond wire which resulted in single- as well as double-mode emission. Finite-difference time-domain simulations were performed in order to identify the modes of the recorded spectra. They confirm that most of the observed spectral features can be attributed to the lasing emission of whispering-gallery modes.

Growth of branched single-crystalline GaAs whiskers on Si nanowire trunks

In this paper we present the hetero-epitaxial growth of single-crystalline GaAs whiskers on Si(111)-nanowire trunks forming hierarchical star-like structures with a six-fold symmetry. These hierarchical nanostructures have been successfully formed utilizing both vapor?liquid?solid (VLS) growth by low-pressure chemical vapor deposition (LPCVD) and molecular-beam epitaxy (MBE) techniques. High-resolution transmission electron microscopy (HRTEM) studies revealed the [111] growth direction of the core Si nanowires (Si-NWs) with six {112} facet planes. The sequentially grown branches are single-crystalline hexagonal GaAs nanowhiskers which grow preferably in the [0001] direction and are perpendicular to the {112} facets of the Si-NW backbone. Photoluminescence (PL) measurements confirm the good crystalline quality of the GaAs nanowhiskers and a blueshift of about 30?meV compared to bulk zinc blende-type GaAs. The ability to prepare rotationally branched NW structures should open new opportunities for both fundamental research and applications including monolithic three-dimensional nanoelectronics and nanophotonics.

Studies of resistance switching effects in metal/YBa2Cu3O7−x interface junctions

Current-voltage characteristics of planar junctions formed by an epitaxial c-axis oriented YBa2Cu3O7-x thin film micro-bridge and Ag counter-electrode were measured in the temperature range from 4.2 K to 300 K. A hysteretic behavior related to switching of the junction resistance from a high-resistive to a low-resistive state and vice-versa was observed and analyzed in terms of the maximal current bias and temperature dependence. The same effects were observed on a sub-micrometer scale YBa2Cu3O7-x thin film - PtIr point contact junctions using Scanning Tunneling Microscope. These phenomena are discussed within a diffusion model, describing an oxygen vacancy drift in YBa2Cu3O7-x films in the nano-scale vicinity of the junction interface under applied electrical fields.

Resonant enhancement of second order sideband generation for intraexcitonic transitions in GaAs/AlGaAs multiple quantum wells

We present an experimental study on efficient second order sideband generation in symmetric undoped GaAs/AlGaAs multiple quantum wells. A near-infrared laser tuned to excitonic interband transitions is mixed with an in-plane polarized terahertz beam from a free-electron laser. The terahertz beam is tuned either to the intraexcitonic heavy-hole 1s-2p transition or to the interexcitonic heavy-hole light-hole transition. We find strong evidence that the intraexcitonic transition is of paramount influence on n=±2 sideband generation, leading to an order-of-magnitude resonant enhancement of the conversion efficiency up to 0.1% at low temperature. At room temperature, the efficiency drops only by a factor of 7 for low terahertz powers.

Coherent 5.35μm surface emission from a GaAs-based distributed feedback quantum-cascade laser

We present second-harmonic surface emission from distributed feedback GaAs∕AlGaAs quantum-cascade lasers with integrated intersubband nonlinearities. The devices show single mode fundamental and second-harmonic emission at wavelengths of 10.7 and 5.35μm, respectively. Only 5.35μm light is emitted from the surface, as the fundamental light is not coupling to radiating modes for the grating of our choice. The second-harmonic peak optical power via the surface at 78K is 150μW for a fundamental peak power of 1.1W.

Effect of crystallographic anisotropy on the resistance switching phenomenon in perovskites

Resistance switching effects in metal/perovskite contacts based on epitaxial c-axis oriented YBa2Cu3O6+c (YBCO) thin films with different crystallographic orientation have been studied. Three types of Ag/YBCO junctions with the contact restricted to (i) c-axis direction, (ii) ab-plane direction, and (iii) both were designed and fabricated, and their current-voltage characteristics have been measured. The type (i) junctions exhibited conventional bipolar resistance switching behavior, whereas in other two types the low-resistance state was unsteady and their resistance quickly relaxed to the initial high-resistance state. Physical mechanism based on the oxygen diffusion scenario, explaining such behavior, is discussed.