Anomalous Temperature Effect in Weakly Coupled Superlattices: Carrier Transport in a THz Quantum Cascade Laser.

We present an investigation into the vertical transport through 13 different superlattice structures, where the well and barrier widths, doping concentration, dopant position, and contact layers were varied. Although superlattices have been extensively studied since 1970, there is a lack of publications on transport through superlattices similarly low doped as THz quantum cascade lasers (QCLs), for which the doping is in the 3-5×10^{10}  cm^{-2} range. The superlattices presented are doped in the same range as THz QCLs, with contact layers and fabrication comparable to high-temperature THz QCLs.

Shot noise in a strange metal.

Strange-metal behavior has been observed in materials ranging from high-temperature superconductors to heavy fermion metals. In conventional metals, current is carried by quasiparticles; although it has been suggested that quasiparticles are absent in strange metals, direct experimental evidence is lacking. We measured shot noise to probe the granularity of the current-carrying excitations in nanowires of the heavy fermion strange metal YbRhSi.

Thermoelectric-cooled terahertz quantum cascade lasers.

We demonstrate the first lasing emission of a thermo-electrically cooled terahertz quantum cascade laser (THz QCL). A high temperature three-well THz QCL emitting at 3.8 THz is mounted to a novel five-stage thermoelectric cooler reaching a temperature difference of ΔT = 124 K.

Suppression of axial growth by boron incorporation in GaAs nanowires grown by self-catalyzed molecular beam epitaxy.

The addition of boron to GaAs nanowires grown by self-catalyzed molecular beam epitaxy was found to have a strong effect on the nanowire morphology, with axial growth greatly reduced as the nominal boron concentration was increased. Transmission electron microscopy measurements show that the Ga catalyst droplet was unintentionally consumed during growth. Concurrent radial growth, a rough surface morphology and tapering of nanowires grown under boron flux suggest that this droplet consumption is due to reduced Ga adatom diffusion on the nanowire sidewalls in the presence of boron.

Ring quantum cascade lasers with twisted wavefronts.

We demonstrate the on-chip generation of twisted light beams from ring quantum cascade lasers. A monolithic gradient index metamaterial is fabricated directly into the substrate side of the semiconductor chip and induces a twist of the light's wavefront. This significantly influences the obtained beam pattern, which changes from a central intensity minimum to a maximum depending on the discontinuity count of the metamaterial.

High-Power Growth-Robust InGaAs/InAlAs Terahertz Quantum Cascade Lasers.

We report on high-power terahertz quantum cascade lasers based on low effective electron mass InGaAs/InAlAs semiconductor heterostructures with excellent reproducibility. Growth-related asymmetries in the form of interface roughness and dopant migration play a crucial role in this material system. These bias polarity dependent phenomena are studied using a nominally symmetric active region resulting in a preferential electron transport in the growth direction.

Advanced gas sensors based on substrate-integrated hollow waveguides and dual-color ring quantum cascade lasers.

This study shows the first combination of a ring-shaped vertically emitting quantum cascade laser (riQCL) providing two distinct emission wavelengths combined with a substrate-integrated hollow waveguide (iHWG). This ultra-compact riQCL-iHWG gas sensing device enables the simultaneous detection of two vapor phase species - here, furan and 2-methoxyethanol - providing distinctive absorption features at the emission wavelengths of the riQCL (i.e.

Remote Sensing with Commutable Monolithic Laser and Detector.

The ubiquitous trend toward miniaturized sensing systems demands novel concepts for compact and versatile spectroscopic tools. Conventional optical sensing setups include a light source, an analyte interaction region, and a separate external detector. We present a compact sensor providing room-temperature operation of monolithic surface-active lasers and detectors integrated on the same chip.

Measurement of bound states in the continuum by a detector embedded in a photonic crystal.

We directly measure optical bound states in the continuum (BICs) by embedding a photodetector into a photonic crystal slab. The BICs observed in our experiment are the result of accidental phase matching between incident, reflected and in-plane waves at seemingly random wave vectors in the photonic band structure. Our measurements were confirmed through a rigorously coupled-wave analysis simulation in conjunction with temporal coupled mode theory.

Mid-infrared surface transmitting and detecting quantum cascade device for gas-sensing.

We present a bi-functional surface emitting and surface detecting mid-infrared device applicable for gas-sensing. A distributed feedback ring quantum cascade laser is monolithically integrated with a detector structured from a bi-functional material for same frequency lasing and detection. The emitted single mode radiation is collimated, back reflected by a flat mirror and detected by the detector element of the sensor.

The influence of whispering gallery modes on the far field of ring lasers.

We introduce ring lasers with continuous π-phase shifts in the second order distributed feedback grating. This configuration facilitates insights into the nature of the modal outcoupling in an optical cavity. The grating exploits the asymmetry of whispering gallery modes and induces a rotation of the far field pattern.

Coupled cavity terahertz quantum cascade lasers with integrated emission monitoring.

We demonstrate the on-chip generation and detection of terahertz radiation in coupled cavity systems using a single semiconductor heterostructure. Multiple sections of a terahertz quantum cascade laser structure in a double-metal waveguide are optically coupled and operate either as a laser or an integrated emission monitor. A detailed analysis of the photon-assisted carrier transport in the active region below threshold reveals the detection mechanism for photons emitted by the very same structure above threshold.

Quantum cascade lasers with a tilted facet utilizing the inherent polarization purity.

We report on quantum cascade lasers (QCLs) with a tilted facet utilizing their polarization property. Contrary to diode lasers, QCLs generate purely TM polarized light due to the intersubband selection rules. This property enables the utilization of reflectivity in terms of only TM polarized light (TM reflectivity).

Grating-based far field modifications of ring quantum cascade lasers.

We present methods for beam modifications of ring quantum cascade lasers emitting around λ = 9μm, which are based on novel distributed feedback grating designs. This includes the creation of a rotationally symmetric far field with a central intensity maximum using an off-center grating as well as the generation of partial radially polarized emission beams induced by a rotation of the grating slits.

Monolithically integrated mid-infrared lab-on-a-chip using plasmonics and quantum cascade structures.

The increasing demand of rapid sensing and diagnosis in remote areas requires the development of compact and cost-effective mid-infrared sensing devices. So far, all miniaturization concepts have been demonstrated with discrete optical components. Here we present a monolithically integrated sensor based on mid-infrared absorption spectroscopy.

Subwavelength micropillar array terahertz lasers.

We report on micropillar-based terahertz lasers with active pillars that are much smaller than the emission wavelength. These micropillar array lasers correspond to scaled-down band-edge photonic crystal lasers forming an active photonic metamaterial. In contrast to photonic crystal lasers which use significantly larger pillar structures, lasing emission is not observed close to high-symmetry points in the photonic band diagram, but in the effective medium regime.

Enhanced light output power of quantum cascade lasers from a tilted front facet.

We present a technique for enhancing the light output power of quantum cascade lasers (QCLs) by tilting of the front facet, which leads to a change of the modal reflectivity, resulting in an asymmetric light intensity distribution along the laser cavity. This asymmetry provides most of the light being emitted through one facet of the laser. An experimental study of threshold current, slope efficiency and light output power as a function of the front facet angles were performed and compared to conventional QCLs.

Probing scattering mechanisms with symmetric quantum cascade lasers.

A characteristic feature of quantum cascade lasers is their unipolar carrier transport. We exploit this feature and realize nominally symmetric active regions for terahertz quantum cascade lasers, which should yield equal performance with either bias polarity. However, symmetric devices exhibit a strongly bias polarity dependent performance due to growth direction asymmetries, making them an ideal tool to study the related scattering mechanisms.

Monolithically integrated mid-infrared quantum cascade laser and detector.

We demonstrate the monolithic integration of a mid-infrared laser and detector utilizing a bi-functional quantum cascade active region. When biased, this active region provides optical gain, while it can be used as a detector at zero bias. With our novel approach we can measure the light intensity of the laser on the same chip without the need of external lenses or detectors.

Microcavity-integrated graphene photodetector.

There is an increasing interest in using graphene (1, 2) for optoelectronic applications. (3-19) However, because graphene is an inherently weak optical absorber (only ≈2.3% absorption), novel concepts need to be developed to increase the absorption and take full advantage of its unique optical properties.

Terahertz active photonic crystals for condensed gas sensing.

The terahertz (THz) spectral region, covering frequencies from 1 to 10 THz, is highly interesting for chemical sensing. The energy of rotational and vibrational transitions of molecules lies within this frequency range. Therefore, chemical fingerprints can be derived, allowing for a simple detection scheme.

Higher order modes in photonic crystal slabs.

We present a detailed investigation of higher order modes in photonic crystal slabs. In such structures the resonances exhibit a blue-shift compared to an ideal two-dimensional photonic crystal, which depends on the order of the slab mode and the polarization. By fabricating a series of photonic crystal slab photo detecting devices, with varying ratios of slab thickness to photonic crystal lattice constant, we are able to distinguish between 0th and 1st order slab modes as well as the polarization from the shift of resonances in the photocurrent spectra.

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.

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.