Rolf Szedlak

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 surface operation mode combined with the low divergence emission of the ring quantum cascade laser enables for long analyte interaction regions spatially separated from the sample surface. The device enables for sensing of gaseous analytes which requires a relatively long interaction region. Our design is suitable for 2D array integration with multiple emission and detection frequencies. Proof of principle measurements with isobutane (2-methylpropane) and propane as gaseous analytes were conducted. Detectable concentration values of 0–70% for propane and 0–90% for isobutane were reached at a laser operation wavelength of 6.5 μm utilizing a 10 cm gas cell in double pass configuration.

Linearly polarized light from substrate emitting ring cavity quantum cascade lasers

The authors report on substrate emitting ring cavity quantum cascade lasers that feature linearly polarized emission beams at an emission wavelength of 8.8 μm. A central lobed far field is achieved by a modification of the distributed feedback grating, at which two π phase-shifts at an angular distance of 180° are applied. In this central lobe, 80% linear polarization is measured. In order to extend this polarization property to the whole far field, an on-chip wire grid polarizer is used. These devices show linearly polarized substrate emission with an extinction ratio higher than 1:16.

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.

On-chip focusing in the mid-infrared: Demonstrated with ring quantum cascade lasers

We report on collimated emission beams from substrate emitting ring quantum cascade lasers with an on-chip focusing element fabricated into the bottom side of the device. It is formed by a gradient index metamaterial layer, realized by etching subwavelength holes into the substrate. The generated optical path length difference for rays emitted under different angles from the ring waveguide flattens the wavefront and focuses the light. Our far field measurements show an increased peak intensity corresponding to 617% of the initial value without the focusing element. Far field calculations, based on a Fourier transformation of the metamaterial area, are in good agreement with our experimental data.

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. We find that this rotation can be connected to the location of the mode relative to the grating. Furthermore, the direction of rotation depends on the radial order of the whispering gallery mode. This enables a distinct identification and characterization of the mode by simple analysis of the emission beam.

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. The differentiation between emitter and detector is eliminated, which enables mutual commutation. Proof-of-principle gas measurements with a limit of detection below 400 ppm are demonstrated. This concept enables a crucial miniaturization of sensing devices.

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., 1144 and 1170 cm-1). Hence, multianalyte gas sensing via a unique mid-infrared (MIR) sensor concept is demonstrated.

Surface emitting ring quantum cascade lasers for chemical sensing

Abstract. We review recent advances in chemical sensing applications based on surface emitting ring quantum cascade lasers (QCLs). Such lasers can be implemented in monolithically integrated on-chip laser/detector devices forming compact gas sensors, which are based on direct absorption spectroscopy according to the Beer–Lambert law. Furthermore, we present experimental results on radio frequency modulation up to 150 MHz of surface emitting ring QCLs. This technique provides detailed insight into the modulation characteristics of such lasers. The gained knowledge facilitates the utilization of ring QCLs in combination with spectroscopic techniques, such as heterodyne phase-sensitive dispersion spectroscopy for gas detection and analysis.

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. Our design principle provides an interesting alternative to recent implementations of microlasers operating at an exceptional point.

Substrate-emitting ring interband cascade lasers

Interband cascade lasers (ICLs) [1, 2] combine two concepts for semiconductor lasers, in particular the long carrier lifetime of photodiodes together with the voltage-efficient cascading of the quantum cascade laser. The low threshold power makes them very attractive for mobile systems used for spectroscopy, process control or medical applications. To date no suitable overgrowth process exists for GaSb-based ICLs, which prohibits the usage of a top-grating etched into the cladding layer. Single-mode emission from a ridge facet has been achieved via sidewall gratings [3], patterning of germanium on the surface [4] or lateral metal gratings [6]. Light outcoupling towards the surface of the ICL has been demonstrated with vertical-cavity surface-emitting lasers [6].