G. Fasching

Imaging with a Terahertz quantum cascade laser.

We demonstrate bio-medical imaging using a Terahertz quantum cascade laser. This new optoelectronic source of coherent Terahertz radiation allows building a compact imaging system with a large dynamic range and high spatial resolution. We obtain images of a rat brain section at 3.4 THz. Distinct regions of brain tissue rich in fat, proteins, and fluid-filled cavities are resolved showing the high contrast of Terahertz radiation for biological tissue. These results suggest that continuous-wave Terahertz imaging with a carefully chosen wavelength can provide valuable data on samples of biological origin; these data appear complementary to those obtained from white-light images.

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.

Active photonic crystal terahertz laser

We present the design and the realization of active photonic crystal (PhC) semiconductor lasers. The PhC consists of semiconductor nanostructure pillars which provide gain at a quantized transition energy. The vertical layer sequence is that of a terahertz quantum cascade laser. Thereby, the artificial crystal itself provides the optical gain and the lateral confinement. The cavities do not rely on a central defect, the lasing is observed in flat-band regions at high symmetry points. The experimental results are in excellent agreement with the finite-difference time-domain simulations. For the vertical confinement a double-metal waveguide is used. The lasers are showing a stable single-mode emission under all driving conditions. Varying the period of the PhC allows to tune the frequency by 400 GHz.

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.

Terahertz photonic crystal resonators in double-metal waveguides

We present the design and the fabrication of photonic crystals with a complete bandgap for TM-modes used as a resonator for terahertz quantum-cascade lasers (QCL), which are lasing around 2.7 THz. The emission of the devices with and without a photonic crystal shows a shift in the emission from the gain maximum to the bandgap of the crystal. The devices are built up by a core, which provides the optical gain, and by a surrounding photonic crystal, which acts as a frequency selective mirror. The whole device is processed into a double-metal waveguide.

Electrically controllable photonic molecule laser

We have studied the coherent intercavity coupling of the evanescent fields of two microdisk terahertz quantum-cascade lasers. The electrically controllable optical coupling of the single-mode operating lasers has been observed for cavity spacings up to 30 mum. The strongest coupled photonic molecule with 2 mum intercavity spacing allows to conditionally switch the optical emission by the electrical modulation of only one microdisk. The lasing threshold characteristics demonstrate the linear dependence of the gain of a quantum-cascade laser on the applied electric field.

Modulated reflectance study of InAs quantum dot stacks embedded in GaAs/AlAs superlattice

Optical transitions in vertically stacked InAs quantum dot (QD) superlattice (SL) with and without AlAs barriers were examined by photo- and electroreflectance techniques. The interband transitions corresponding to the QD, wetting layer (WL), and InAs/GaAs/AlAs SL have been identified. Experimental data and numerical calculations show that blueshifts and enhancement in the intensity of WL-related optical transitions in an InAs/GaAs/AlAs SL originate mainly due to off-center position of the QD layers in the quantum wells. The appearance of multiple WL-related features in the modulated reflectance spectra was revealed and discussed.

Optimization of MBE Growth Parameters for GaAs‐based THz Quantum Cascade Lasers

A 15 μm thick active region terahertz (2.86 THz) quantum cascade laser was optimized for maximum continuous wave temperature operation. Through the use of high‐resolution x‐ray diffraction, improved growth rate and doping calibrations, minimization of flux transients, a double‐metal waveguide, and optimization of doping the threshold current was reduced from 1.88 to 0.20 kA/cm2 and a maximum continuous wave operating temperature of 72K was achieved. The tolerable thickness variation in one cascade was <1.1%. Details of the optimizations are discussed in detail.

Photonic crystal mode terahertz lasers

We present the design and fabrication of photonic crystal (PhC) based resonators for terahertz quantum-cascade laser with a gain maximum at 2.7 THz. The PhC provides the confinement in lateral direction, for the vertical confinement a double-metal waveguide is used. We show theoretical and experimental analyses of devices based on a central gain region, which is surrounded by a PhC mirror. The devices are lasing in the bandgaps or at high-symmetry points of the PhC, depending on the period. These concepts allow us to tune the emission of the lasers in the range of a few hundred gigahertz.