In many precision sensing applications, the final detection sensitivity is tightly related to the intensity noise of the laser source, which might represent the ultimate limit to the sensor performance. In this framework, we present here the intensity noise characterization of three different mid-infrared semiconductor devices (two quantum cascade lasers and one interband cascade laser). A fast homemade balanced detection system is used to measure the intensity noise of the emitted radiation over a broad Fourier-frequency range, facilitating the observation of shot-noise-limited radiation under specific measurement conditions and detection efficiency. This study allows for a direct performance comparison of the most widespread laser sources in mid-infrared sensing systems.
Interband Cascade Lasers are becoming more and more attractive sources for sensing applications due to their direct mid-IR emission and low power consumption. In many demanding applications of precision spectroscopy and imaging, the laser frequency and intensity noise represent the ultimate limiting factor for the final sensitivity. Here, we first characterize the response of a DFB ICL to a frequency modulation and measure its tuning coefficients. A frequency noise analysis of the ICL is then provided, through experimental investigation of the frequency noise power spectral density, with a particular attention to the technical noise contribution. A possible scheme for frequency stabilization and linewidth narrowing is presented through frequency locking to a high-Q factor microresonator.
Mid-infrared liquid sensing on the chip-scale is a newly emerging field of research, especially with respect to fully monolithic integrated devices. They enable addressing applications scenarios in chemical reaction monitoring and real-time sensing, which were so far prevented by the existing much more bulky technology (e.g. FTIR-based systems). In this work we present a quantum cascade laser (QCL), QC detector (QCD) and novel type of midinfrared plasmonic waveguide that are integrated into one substrate and which we use in real-time protein sensing and residual water in solvent measurements. Furthermore, we present how this rather simple linear geometry can be further improved by implementing other (more spectrally broadband) materials such as Germanium and integrating surface-passivation and -functionalization for improving sensing capabilities. In the last part we will demonstrate two pathways for introducing plasmonic mode-guiding along the chip-surface, which is the key to realizing much more complex geometries including integrating more active and passive elements into one PIC.
The mid-IR band recently attracted great interest for future wireless communication due to its low attenuation and high tolerance against atmospheric perturbations. Recent advances in monolithic integration of same-wavelength quantum cascade lasers (QCLs) and detectors (QCDs) paved the way for a new generation of functional photonic integrated circuits. In this context, integrating novel mid-IR plasmonic waveguides has been highly suitable for realizing efficient chip-scale optical links between different active components. Here we report on developing a mid-IR on-chip heterodyne receiver. This includes the first demonstration of plasmonic waveguides for on-chip beam-guiding in the long-wave infrared and novel high-performance QCLs and QCDs.
Mid-infrared chemical sensors based on quantum cascade (QC) devices offer improved sensitivity, portability and costs compared to FTIR-based spectrometers. In this work, we combine for the first time a broadband external-cavity QC laser (EC-QCL) with a spectrally tailored QC detector (QCD) for broadband detection of bovine milk proteins including β-lactoglobulin, α-lactalbumin and casein. We analyze concentrations between 0.25-15 g/L in a 12.5-µm transmission flow cell in the amide-I and -II band (~1730-1470 cm-1) and obtain: a RMS noise-level of 0.067 mAU, a limit-of-detection of ~0.09 g/L, excellent agreement with FTIR absorbance-spectra and similar performance as much more bulky high-end FTIR-spectrometers.
We present the first realization of substrate-emitting double-ring Interband Cascade Lasers (ICLs) in Continuous-Wave (CW) mode. The devices are realized in the GaSb material system and emit at around 2.77 µm wavelength. Through the implementation of second-order distributed feedback gratings, single-mode and simultaneous vertical emission through the GaSb substrate are realized. By implementing a concentric double-ring arrangement (diameters of approximately 700µm and 900 µm), two-wavelengths emission on the same optical axis can be achieved, which is desirable for spectroscopic applications. For improved thermal management, the devices are mounted epitaxial side down on custom-made AuSn/AlN heatsinks, enabling individual laser operation.
Mid-infrared optical sensors integrating plasmonic waveguides and quantum cascade optoelectronics are an emerging field of research leading to promising results in chemical sensing, environmental monitoring, and biomedical diagnosis. In this work, we investigate TiO2 as waveguiding material for mid-infrared surface plasmon polariton waveguides and show its potential for integrated sensors. Simulations reveal suitable TiO2 dimensions and diffraction grating couplers for ~4.3 µm light. Following these theoretical considerations, we fabricated such devices monolithically integrated with quantum cascade detectors (QCDs) and present their characterization. We further discuss their application in innovative biosensing experiments including glucose detection.
In this work we monolithically integrate a quantum cascade laser (QCL) and detector (QCD) addressing the same wavelengths lambda=1550-1650 cm-1 for liquid spectroscopy. QCL and QCD are combined using a 50-100 µm-long dielectric-loaded surface-plasmon-polariton (DLSPP) waveguide, which typically guides >>90% of the mode outside of the cavity. We show the analysis of the protein bovine serum albumin (BSA) and its denaturation process between 25°C-90°C in real time in a microfluidic cell (60 µl) for 20-60 mg/ml BSA-concentrations. To further test the sensor-robustness, we directly submerge it into a beaker and detect H2O up to 35%-40%, solved in isopropyl alcohol.
Semiconductor-loaded plasmonic (SLSPP)-waveguides are a very efficient link for optoelectronic devices, facilitating miniaturized photonic integrated circuits. However, for long-wave infrared applications (8-12 µm), the material selection is challenging as most commonly used mid-IR materials absorb in this region. Therefore, we selected and investigated the properties of germanium in a hybrid semiconductor-metal-configuration to overcome these limitations. The experimental characterization of Si(substrate)-Au-Ge fabricated SLSPP-waveguides show very good agreement with FEM-simulations. Moreover, the realized devices offer low losses between 8.8 and 22 dB/mm (single device) and even within 8.8-15 dB/mm (multiple devices), respectively, for the entire investigated octave-spanning 5.6 – 11.2 µm range.
We present a novel InGaAs/InAlAs/InP quantum cascade detector (QCD) operating in the long wave infrared (LWIR) range, crucial for the exploitation of new free-space optical telecommunication channels at wavelengths between 8-12 µm. The comparison of differently sized detector ridges, processed on substrates with a 15-period as well as a single-period design, allows a characterization of the spectral photocurrent and a comparison of their performance in terms of sensitivity, spectral responsivity, detector noise etc. The goal is to distinguish design guidelines for the best candidate to establish a monolithic-integrated heterodyne detection system, able to secure high-speed and low-noise free-space data transmission.
Zincoxide is a rather new material system and promising candidate for mid-infrared (mir) and THz optoelectronic devices like quantum cascade lasers (QCLs) and detectors (QCDs) due to its twice as high LO-phonon energy as GaAs. The non-polar m-plane orientation allows designing and realizing such complex devices without internal electrical fields.
We present the full fabrication scheme of such QCL/QCD devices including novel optimized etching techniques, surface leakage current suppression by multiple orders of magnitude and low resistance Ohmic contacts (~10^(-5) Ohm x cm^2). Optimized fabrication schemes resulted in fabrication yielding up to more than 80% of operational devices.
We have demonstrated that both ring quantum cascade (QCLs) and interband cascade lasers (ICLs) are excellent platforms for vertical light emission. Of these two lasers ICLs typically show lower power consumption and lasing threshold, qualifying them especially for miniaturized and battery-powered applications. With our work on ring ICLs we are aiming to build a compact portable sensing device, employing interferometric cavity-assisted photothermal spectroscopy. Here, we present our current work on interband cascade devices, as well as an overview of previous studies on ring QCLs. These devices rely on the light outcoupling via a second-order distributed-feedback grating from a ring cavity.
We report on the first demonstration of quantum cascade detectors based on ZnO/ZnMgO quantum wells grown by molecular beam epitaxy on an m-plane ZnO substrate. The sample is processed in the form of square mesas with special attention paid to the passivation of the side facets. Photocurrent spectroscopy reveals a resonance at 2.8 μm wavelength slightly blue-shifted with respect to the intersubband absorption peak at 3 μm wavelength. The photocurrent persists up to room temperature. The peak responsivity amounts to 0.15 mA/W under irradiation at Brewster’s angle of incidence of the top surface of the mesas.
Mid-infrared (MIR) spectroscopy is a reliable tool for the identification of gaseous and liquid mixtures due to their unique and inherent absorption spectra. Quantum Cascade (QC) Lasers and Interband Cascade Lasers are modern reliable sources to penetrate the MIR spectral range.
To increase the functionality of QC devices we designed and optimized a QC material that can be used as a QC laser and as a QC detector for the very same MIR wavelength, respectively. Switching from laser to detector is achieved by biasing the semiconductor (lasing mode) or operate it without any electric field applied (detecting mode), respectively. Due to this functionality increase the on-chip integration of a designable QC light source, an interaction zone and a QC detector is now feasible and has been demonstrated recently.
In this talk we present improved bi-functional QC material for the integration and further development of sensor systems, as well as different cavity concepts for gas and liquid sensing scenarios. Proof of concept sensing examples to demonstrate the integrated sensor systems will be given. Multi mode and single mode lasers made from bi-functional materials show comparable performance to regular state of the art QC lasers and no performance drop due to the additional detection functionality.
While QC lasers are already accepted within the scientific community, QC detectors still need to be further promoted. Thus, in addition to the improvement of the bi-functional QC material, we demonstrated a single period quantum cascade photo-detector with a responsivity of up to 1.3 A/W.
Rolf Szedlak, Jakob Hayden, Pedro Martín-Mateos, Martin Holzbauer, Andreas Harrer, Benedikt Schwarz, Borislav Hinkov, Donald MacFarland, Tobias Zederbauer, Hermann Detz, Aaron Maxwell Andrews, Werner Schrenk, Pablo Acedo, Bernhard Lendl, Gottfried Strasser
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.
The development of Zinc Oxide (ZnO)-based applications have been strongly limited due to the lack of reproducible p-type doping. Here we present novel opportunities in the field of unipolar oxide wide band gap semiconductors. First we have developed the growth of nonpolar ZnO/ZnMgO multiple quantum wells (MQWs) and then we demonstrate that the structural and optical properties of the MQWs are reaching the required level for intersubband devices in terms of defects, surface and interface roughness and doping. We will show and discuss the most recent results as, for instance, intersubband transitions which have been observed in such structures.
This "Zoterac" project has received funding from the European Union’s Horizon 2020 research and
innovation programme under grant agreement No 665107
The terahertz (THz) spectral range (lambda ~ 30µm – 300µm) is also known as the “THz-gap” because of the lack of compact semiconductor devices. Various real-world applications would strongly benefit from such sources like trace-gas spectroscopy or security-screening. A crucial step is the operation of THz-emitting lasers at room temperature. But this seems out of reach with current devices, of which GaAs-based quantum cascade lasers (QCLs) seem to be the most promising ones. They are limited by the parasitic, non-optical LO-phonon transitions (36meV in GaAs), being on the same order as the thermal energy at room temperature (kT = 26meV). This can be solved by using larger LO-phonon materials like ZnO (E_LO = 72meV). But to master the fabrication of ZnO-based QC structures, a high quality epitaxial growth is crucial followed by a well-controlled fabrication process including ZnO/ZnMgO etching.
We use devices grown on m-plane ZnO-substrate by molecular beam epitaxy. They are patterned by reactive ion etching in a CH4-based chemistry (CH4:H2:Ar/30:3:3 sccm) into 50μm to 150μm square mesas. Resonant tunneling diode structures are investigated in this geometry and are presented including different barrier- and well-configurations. We extract contact resistances of 8e-5 Omega cm^2 for un-annealed Ti/Au contacts and an electron mobility of above 130cm^2/Vs, both in good agreement with literature. Proving that resonant electron tunneling can be achieved in ZnO is one of the crucial building blocks of a QCL.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 665107.
The equations for the threshold-current density Jth, differential quantum efficiency ηd and maximum wallplug efficiency
ηwp,max for quantum-cascade lasers (QCLs) have been modified for electron leakage and backfilling. We used a thermalexcitation
model of "hot" injected electrons from the upper laser state to upper active-region energy states to calculate
leakage currents. Then the calculated characteristic temperature T0 for Jth was found to agree well with experiment for
both conventional and deep-well QCLs. The characteristic temperature T1 for ηd was deduced to be due to both electron
leakage and an increase in the waveguide-loss coefficient. For conventional mid-infrared QCLs ηwp,max is found to be
strongly temperature dependent which explains experimental data. By using a new concept: tapered active-region (TA),
deep-well QCLs have been optimized for virtual suppression of the electron-leakage currents. In turn, at room
temperature, for continuous-wave (CW)-operating, 4.5-5.0 μm-emitting TA QCLs we estimate the threshold current to
decrease by ~ 25 %, the active-region temperature rise at the ηwp,max point to decrease by ~ 30 %, and the single-ended,
ηwp,max value to become at least 22 %. Preliminary results from TA QCLs include T1 values as high as 454 K, over the
20-60 oC heatsink-temperature range.
We report large beam steering effects, observed in the far-field pattern of InP-based mid-infrared quantum-cascade lasers
along the slow axis. Changing the temperature by a few degrees around room temperature or varying the drive current
strongly affects the lateral direction of the output beam. The position of maximum intensity in the far-field-distribution
changes by more than 20°. This beam steering effect is correlated to changes in the lateral mode distribution, as revealed by
time-resolved spectroscopy of the lasing spectrum.
In this contribution we present the results of an imaging stand-off detection system based on a mid-IR external-cavity
quantum cascade laser (EC-QCL) with a broad tunable range of 200 cm-1. Traces of TNT (trinitrotoluene) and PETN
(pentaerythritol tetranitrate) as well as various non-hazardous substances such as flour or skin cream on different
substrate-materials were investigated by illuminating them with the EC-QC laser and collecting the diffusely
backscattered light. By tuning the EC-QCL across the significant absorption spectra we were able to detect the
explosives
Results on the detection of traces of trinitrotoluene (TNT) on different substrate-materials like Aluminum and
standard car paint are presented. We investigated different samples with a movable imaging standoff detection
system at angles of incidence far away from specular reflection. The samples were illuminated with a tunable
mid-infrared external-cavity quantum cascade laser. For collection of the diffusely backscattered light a highperformance
infrared imager was used. Trace concentrations of TNT corresponding to fingerprints on realworld-
substrates were detected, while false alarms of cross-contaminations were successfully suppressed.
We present experimental results on a Quantum cascade laser (QC laser) embedded in an external cavity. These results were obtained with a broadly tunable laser exceeding 80 cm-1 covering a characteristic absorption band of trinitrotoluene
(TNT). By combining the laser source with a high performance IR imager a stand-off detection setup based on multi-
spectral MIR backscattering spectroscopy has been realized. With this technique TNT surface-contaminations of as low
as 10 μg/cm2 could be detected on surfaces such as an aluminum-sheet and standard car paint. The contrast of the
detection technique depends on the reflectance of the surface. A surface leading to mirror-like reflectance of the IR laser radiation leads to absorbance-like signatures of the TNT contamination, while surfaces showing high absorbance of the laser light may induce a contrast-reversal in the resulting image of the TNT coverage. This effect can be explained by a theoretical model for thin film coated substrates taking into account differences in the reflectance. Limitations and
further work needed to explore the full potential of the IR backscattering technique are also discussed.
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