Low dissipation integrated frequency combs are ideal candidates to realize miniaturized spectrometers without moving parts and hence are of great interest for integrated photonics.
After reviewing frequency comb generation in interband cascade lasers (ICLs), the nonlinear dynamics and performance limiting mechanisms of ICLs will be discussed. A newly developed k-space resolved non-equilibrium carrier transport model combined with experimental studies enables us to explore different loss mechanisms, as well as to explore the reasons, why passive mode-locking of ICLs for short pulse generation remains challenging.
We present GaSb based interband cascade lasers with a center emission wavelength of 6170 nm that emit in cw operation up to a temperature of 40°C. While the pulsed threshold current density for a broad area laser is as low as 500 A/cm² at a temperature of 20°C a ridge waveguide laser with 23µm ridge width reaches more than 25mW of output power, the results were achieved by careful optimizations of the active region design, the waveguide design and epi down mounting of the laser chips.
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.
Quantum cascade detectors (QCD) are photovoltaic mid-infrared detectors based on intersubband transitions. The sub-picosecond carrier transport between subbands and the absence of a bias voltage make QCDs ideally suited for high-speed and room temperature operation. Interband cascade detectors (ICDs) combine interband optical transitions with fast intraband transport to achieve high-frequency and broad-wavelength operation at room temperature.
We report the design, fabrication, and characterization of QCDs optimized for large electrical bandwidth. Femtosecond pulses generated by a mid-infrared optical parametric oscillator are used to demonstrate QCDs with a 3-dB bandwidth of more than 20 GHz and measure the saturation characteristics of ICDs.
Interband cascade lasers (ICLs) are attracting more and more attention, being established as reliable mid-infrared laser sources. In particular, their low threshold current densities and low power consumption qualify them for a multitude of applications aiming for portable, miniaturized sensing systems. ICLs show a performance sweet spot around 3-4 μm and continuous-wave (cw) operation at room temperature has been shown at wavelengths from 2.8-5.6 μm in the GaSb material system. When trying to extend this range towards even longer wavelengths, several difficulties are inevitably faced. Some can partly be traced back to a still insufficient understanding of the internal device physics.
Here, we report on our latest findings showing the impact of intersubband transitions in the valence band band of ICL active regions on the performance of these devices.
Using a numerical model employing the eight-band k·p method, we calculate the electronic band structure of the active W-quantum well (QW) in an ICL. We then use a generalized momentum matrix element model to determine the wavelength-dependent absorption between subbands in the valence band. This model can explain all contributions to the absorption in the W-QW, regardless of the nature of the transitions, whether they are interband or intersubband.
We experimentally observe a clear dependence of performance metrics on the thickness and composition of the GaInSb hole-QW. Specifically, the threshold current density Jth, and its dependence on the operating temperature, described by the characteristic temperature T0, are influenced. This is in good agreement with our model. By carefully adjusting the design of the active W-QW the intersubband absorption in the valence band can be modified and even sufficiently avoided, allowing us to enhance the ICL performance outside of the sweet spot 3-4 μm region.
We will report on the high-frequency operation of QCDs, ICDs, and QWIPs connected to a monolithic coplanar transmission line without air-bridge. A single-period QCD with more than 0.8 A/W responsivity at 300K paves the way towards sensitive multiheterodyne detection at room-temperature.
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.
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