L. K. Hoffmann

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.

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.

Tree array quantum cascade laser.

A monolithic coupling scheme for mid-infrared quantum cascade laser arrays is investigated with respect to brightness enhancement. The tree-shaped resonator enables parallel coupling of six laser elements into a single element by means of several Y-junctions. Phase-locking is observed on the basis of far field analysis, and leads to in-phase emission on both sides of the device. The experimental results match calculated far field profiles and demonstrate a high level of modal control when driven far above threshold. Whereas optical power measurements confirm negligible coupling losses, the slope efficiency is below the theoretically expected value, which is attributed to modal competition. Additional evaluation of near fields and spectral characteristics provides background on the modal dynamics of the sophisticated cavity and reveals limitations to coherent beam combining. The findings pave the way to improved coupling efficiency and brightness scaling of a single facet emitting compact quantum cascade laser array.

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.

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...

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.

Phase‐locking in Y‐junction Quantum Cascade Lasers

We present a coherent coupling scheme for mid‐infrared quantum cascade lasers. Based on far field analysis, fundamental lateral modes are realized in Y‐coupled resonators, which are locked in phase. The concept enhances the output power of a single facet coherent emitter, while maintaining good beam quality due to modal control. High flexibility in the choice of material system, emission wavelength and waveguide design combined with mature processing techniques make quantum cascade lasers an ideal candidate to gain insight into coherent spacial beam combining for high power laser applications.

Photonic crystal band edge and defect states in the spectral response of intersubband detectors

We fabricated photonic crystal intersubband detectors and measured the spectral photocurrent upon angle and polarization resolved illumination. Light is coupled into the cavity whenever phase matching occurs between a cavity mode and the incident wave. The method allows us to map out the photonic band structure including its polarization dependence, which is shown to be highly correlated with the symmetry properties. Furthermore we discuss our investigations of non‐bandgap defect states concerning dispersion, linewidth and geometrical tunability.

Grating‐Induced Beam‐Tuning in Quantum Cascade Ring Lasers

We describe 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 spectroscopic applications.