S. Schartner

Low divergence single-mode surface emitting quantum cascade ring lasers

We describe the fabrication and operation of surface emitting second-order distributed feedback quantum cascade ring lasers. The devices exhibit single-mode emission at a wavelength of 3.95μm with a side mode suppression ratio of 25dB. A linear tuning coefficient of 0.13cm−1∕K is observed. A single longitudinal mode in the ring shaped resonator results in a highly symmetric far-field pattern and a low beam divergence, represented by a full width at half maximum of ∼3°. Based on these characteristics the presented compact coherent light source may find its way into today’s midinfrared spectroscopy applications.

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.

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.

Grating-coupled surface emitting quantum cascade ring lasers

We report on the fabrication and operation of quantum cascade ring lasers providing grating-coupled surface emission. The devices exhibit tunable far fields, ranging from spot- to ring-shaped symmetric beam cross sections, depending on the grating period. This—along with threshold current densities as low as for comparable Fabry–Perot lasers—demonstrates the compatibility of reduced beam divergence and two-dimensional integrability, resulting in an attractive light source for applications in midinfrared spectroscopy and imaging.

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.

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.

Wavelength dependent phase locking in quantum cascade laser Y-junctions

Midinfrared quantum cascade lasers with monolithically integrated Y-junctions are investigated. Two different emission wavelengths of 10.5 and 4.2μm were realized in two different material systems, lattice-matched GaAs∕AlGaAs on GaAs and strain-compensated InGaAs∕InAlAs∕AlAs on InP. With identical Y-junction dimensions, phase locking is observed in both structures. In GaAs based devices, fundamental lateral modes are present in the coupled waveguides, which are coherently synchronized at the Y-junction. In InP based devices, modes of higher order are excited, which originate from coupling. The generation of multiple modes yields an out-of-phase fraction which reduces the level of coherence.

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.

Photocurrent response from photonic crystal defect modes

The authors use a quantum well intersubband photodetector fabricated into a two dimensional photonic crystal to investigate the optical defect modes of a single missing hole defect. The modes appear as a local enhancement in spectral photocurrent due to an increased in-coupling of surface incident light when a defect mode is present. The frequencies of these localized modes are tracked as they are varied by the defect geometry and compared to simulations.