M. Austerer

Single-mode surface-emitting quantum-cascade lasers

We present high-power surface-emitting second-order distributed feedback quantum-cascade lasers in GaAs and InP material systems. The GaAs device, grown by molecular-beam epitaxy, showed single-mode peak output powers of 3 W at 78 K in pulsed operation. With the InP-based devices, which are grown by metalorganic vapor phase epitaxy, we obtained single-mode peak output powers of 1 W at room temperature. These are the highest output powers for surface emission of quantum-cascade lasers reported so far. The InP-based distributed feedback lasers also have very low threshold current densities and are working well above room temperature.

Impact of doping on the performance of short-wavelength InP-based quantum-cascade lasers

The effect of doping concentration on the performance of short-wavelength quantum-cascade lasers based on the strain-compensated InGaAs/InAlAs/AlAs heterostructure on InP, emitting at 3.8 μm, is investigated for average doping concentrations between 0.3 and 3.9×1017 cm−3 (sheet densities between 1.6 and 20.9×1011 cm−2). Although the threshold current density is rather independent of doping concentration, the maximum current density increases with doping and exhibits a saturation for the highest doping level. Other important performance characteristics such as differential quantum efficiency, peak optical emission power, slope efficiency, and maximum operating temperature are observed to be maximized for structures with an average doping of 2−3×1017 cm−3, corresponding to a sheet density of about 1.5×1012 cm−2.

Hybrid integration of GaAs quantum cascade lasers with Si substrates by thermocompression bonding

A hybrid GaAs quantum cascade laser system obtained by Au–Au thermocompression bonding epilayer down onto gold coated silicon substrates is presented in this paper. The performance of the hybrid laser in low-duty-cycle pulsed operation in comparison to an unbonded one was not deteriorated. The lasers run with a threshold of 4.6kA∕cm2, emit around 12μm, and with a maximum optical output power of 550mW at cryogenic temperatures. The key advantage of such hybrid chips is the possibility of integrating III-V cascade lasers with established silicon photonics technology, such as silicon-on-insulator waveguides, V-groove fiber coupling and microfluidics.

Ultrafast phase-resolved pump-probe measurements on a quantum cascade laser

The dynamics of optical gain in an InGaAs/AlInAs quantum cascade laser is studied in midinfrared pump-probe measurements. Pump and probe pulses of identical wavelength, polarization, and propagation direction through the laser waveguide are detected separately by electro-optic sampling. For injection currents below the lasing threshold, we observe an absorption decrease in the laser transition with a ≈3 ps recovery time. Above threshold, the gain strongly saturates and shows a fast recovery time decreasing with current. Such kinetics is superimposed by oscillations with a frequency of 0.8 THz originating from coherent electron tunneling through the injection barrier.

Surface emission from episide-down short distributed-feedback quantum cascade lasers

Increased coupling is observed in distributed-feedback quantum cascade lasers when placing a shallow second order grating between a continuous surface-plasmon layer and the active region. The combined effect of an air cladding and a metallic layer on the opposite sides of the waveguide increases the overlap with the grating region resulting in calculated coupling coefficients up to 100 cm(-1). The waveguide design was implemented by Au thermo-compression bonding after grating formation and subsequent backside processing of ridges with air claddings. Lasers as short as 176 microm show single-mode behavior with a side-mode-suppression-ratio of 20 dB and thresholds (10 kA/cm(2)) as well as output powers (> 150 mW) close to Fabry-Pérot device performances are reached for 360 microm long devices.

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.

GaAs/AlGaAs quantum cascade lasers with dry etched semiconductor–air Bragg reflectors

Electrically pumped GaAs/AlGaAs quantum cascade rib lasers with a wavelength of 9.7 µm utilizing deep etched photonic band gap mirrors are discussed. The smallest operating device was 150 µm long and 8 µm wide. Further, a description of the fabrication process of such high aspect ratio devices is given.

Monolithic Mach–Zehnder-type quantum cascade laser

A midinfrared quantum cascade laser with Mach–Zehnder cavity and split contacts is investigated with respect to interference effects. By increasing the temperature in one of the two coupled active waveguides, the value of the effective refractive index is varied and the modal phase is shifted. As a result, destructive interference is observed within the resonator, which manifests itself in a minimum of the modulated output power. The dissipated heat is controlled by locally adding a continuous current to the drive current pulses. In the first step, thermal properties, threshold values, and far fields are analyzed and compared to a Fabry–Perot resonator to gain insight into the physical principles of the monolithic interferometer. Based on these findings, the temperature distribution is calculated in a two-dimensional heat transfer simulation, which leads to a match between the thermal change of the effective refractive index and the condition for destructive interference; a phase shift of π between the tw...

Čerenkov-type phase-matched second-harmonic emission from GaAs∕AlGaAs quantum-cascade lasers

Second-harmonic (SH) emission in the form of Cerenkov-type phase-matched radiation has been measured for quantum-cascade lasers (QCLs) with built-in nonlinearities. The QCLs operate at 10.9μm wavelength and, due to material dispersion, show a high collinear phase mismatch, resulting in low SH external conversion efficiencies on the order of 2μW∕W2. Due to our waveguide design, we were able to couple out and measure the generated SH light in the form of Cerenkov-type phase-matched radiation from the substrate of the device. The SH power collected from the Cerenkov beam leads to an increased external conversion efficiency of ∼50μW∕W2.

Reversible switching of quantum cascade laser-modes using a pH-responsive polymeric cladding as transducer

We present a novel approach for the reversible switching of the emission wavelength of a quantum cascade laser (QCL) using a halochromic cladding. An air-waveguide laser ridge is coated with a thin layer of polyacrylic acid. This cladding introduces losses corresponding to the absorption spectrum of the polymer. By changing the state of the polymer, the absorption spectrum and losses change, inducing a shift of 7 cm(-1) in the emission wavelength. This change is induced by exposure to acidic or alkaline vapors under ambient conditions and is fully reversible. Such lasers can be used as multi-color light source and as sensor for atmospheric pH.