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Quantum cascade detectors (QCDs) are mid-infrared and far-infrared, low-noise, photovoltaic detectors utilizing intersubband transitions. This Letter presents an InAs/AlAs0.16Sb0.84 based QCD lattice matched to an InAs substrate. This material system exhibits properties like a low effective electron mass of the well material of 0.023 m(0), beneficial for higher optical absorption strength, and a high conduction band offset of 2.1 eV, allowing the design of QCDs in the mid-infrared and near-infrared region. The presented QCD has a peak spectral response at 2.7 mu m (0.459 eV), the center of a CO2 absorption band. To enable top side illumination, a grating was implemented. This additionally bypasses absorption by the narrow bandgap 0.345 eV (3.54 mu m) InAs substrate material. The QCD has a peak responsivity at a room temperature of 5.63 mA/W and a peak specific detectivity of 1.14 x 10(8) Jones. (c) 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Ring quantum cascade lasers generate self-starting frequency combs and dissipative cavity solitons. We present a reengineered ring resonator with an evanescent wave directional coupler that allows coherent control over laser state and enhances power extraction.

We present the design, growth, and optical and electrical characterization of an InAs/AlAs0.16Sb0.84 quantum cascade detector at 2.7 μm designed for top-side illumination assisted by a diffraction grating with a room-temperature responsivity of 5.63 mA/W.

Quantum cascade lasers (QCLs) facilitate compact optical frequency comb sources that operate in the mid-infrared and terahertz spectral regions, where many molecules have their fundamental absorption lines. Enhancing the optical bandwidth of these chip-sized lasers is of paramount importance to address their application in broadband high-precision spectroscopy. In this work, we provide a numerical and experimental investigation of the comb spectral width and show how it can be optimized to obtain its maximum value defined by the laser gain bandwidth. The interplay of nonoptimal values of the resonant Kerr nonlinearity and cavity dispersion can lead to significant narrowing of the comb spectrum and reveals the best approach for dispersion compensation. The implementation of high mirror losses is shown to be favorable and results in proliferation of the comb sidemodes. Ultimately, injection locking of QCLs by modulating the laser bias around the round trip frequency provides a stable external knob to control the frequency-modulated comb state and recover the maximum spectral width of the unlocked laser state.

Mid-infrared (mid-IR) interband cascade detectors (ICDs) combine interband optical transitions with fast intraband transport to achieve high-frequency and broad-wavelength operation at room temperature. To understand the photo-response and saturation characteristics of these detectors, we measured the bias-dependent impulse response to femtosecond pulses for the first time. To probe the ICD, we use an optical parametric oscillator emitting 104 fs pulses. The ICD, which is embedded in a ridge waveguide, contains four active stages consisting of a W -shaped quantum well, an electron and a hole extractor. We measured the photo-response as a function of reverse bias and average incident power. These measurements show that a reverse bias increases the peak voltage for a given input power as well as the saturation power. Viewing the dynamic response on a log scale reveals a faster exponential decay (around 40 ps, likely associated with electrons) followed by a slower second exponential decay (around 150 ps, likely associated with holes). We observed that both of these characteristic decay times reduce with increasing reverse bias. To determine the frequency response, we performed a direct measurement on a microwave spectrum analyzer and calculated the Fourier transform of the temporal response. We see that the 3-dB power bandwidth of the ICD increases from 1.24 GHz at zero bias to 1.67 GHz at -3.0 V of reverse bias. In conclusion, we have measured the saturation behavior and impulse response of an ICD for the first time. Our results provide insights into the ultrafast dynamics of ICDs, and will help enable the development and characterization of infrared light sources, especially mode-locked lasers.

In recent years, optical frequency comb sources have been realized in the mid-infrared and terahertz region by using quantum cascade lasers (QCLs) [1] , [2] . These electrically driven, on-chip sources provide high output power and a compact design. Commonly used waveguide designs include ridge type resonators supporting Fabry-Perot modes. The appearing effect of spatial hole burning in this resonator type prevents the exploitation of the whole gain bandwidth of the active region [3] . This limitation can be overcome using ring-shaped resonators, which exhibit either a standing-wave pattern formed by two counterpropagating waves, or support a traveling wave, which does not cause spatial hole burning.

Amplifiers of Terahertz (THz) radiation are quite useful, because in this spectral range radiation is normally weak and sensitive detectors are hard to get. Monolithically integrated master-oscillator power-amplifier (MOPA) [1] , [2] as well as free-space amplifiers [3] have been demonstrated by using Quantum Cascade laser (QCL) structures with amplifications in the range of 3 – 12 dB. Although these devices are very promising, they all provide a linear amplification. However, for most telecommunication applications an amplification only above a certain threshold is advantageous, since this avoids the amplification of noise.

It is a well-established truth that spatial hole burning (SHB) in a standing-wave cavity is an essential single-mode instability mechanism for multimode operation of quantum cascade lasers (QCLs). We discovered recently that another instability mechanism–phase turbulence–is capable of triggering an onset of previously unseen types of frequency combs in traveling-wave ring cavity QCLs in absence of SHB. This new regime of laser operation reveals a connection with Kerr combs and paves the way to manipulation and engineering of comb states in QCLs.

Frequency combs are ideal candidates to realize miniaturized spectrometers without moving parts and hence are of great interest for integrated photonics. Here, an overview on the generation electrically pumped optical frequency combs on integrated platforms using semiconductor lasers. This includes self-starting generation of frequency modulated combs in quantum cascade laser in the 8um and interband cascade lasers in the 3-4um wavelength region, respectively. Furthermore, we will discuss how to integrate efficient high-speed modulators in these devices in order to facilitate the generation of picosecond pulses.

We present comb formation in ring-shaped THz quantum cascade lasers. Devices are spontaneously operating in a harmonic state transiting to a dense comb regime exhibiting over 30 equidistant modes covering a bandwidth of 622 GHz.

A Terahertz optical amplifier based on a Quantum Cascade laser structure with a lossy double-metal cavity is demonstrated. Amplification appears only above a certain threshold and an amplification of ~17 dB is achieved.

Interband cascade infrared photodetectors (ICIPs) combine interband optical transitions with fast intraband transport to achieve high-frequency and broad-wavelength operation at room temperature. Here we study the bias-dependent electronic impulse response of ICIPs with a mid-infrared synchronously pumped optical parametric oscillator (OPO). Since the OPO produces ultrashort 104-fs pulses, it is possible to probe the impulse response of the ICIP. From this impulse response, we identify two characteristic decay times, indicating the contribution of electron as well as hole carriers. A reverse bias voltage applied to the ICIP reduces both time scales and leads to an increased electrical cut-off frequency. The OPO emits up to 500 mW average power, of which up to 10 mW is directed to the ICIP in order to test its saturation characteristics under short-pulse illumination. The peak of the impulse response profile as well as the average photocurrent experience a gradual saturation behavior, and we determine the corresponding saturation powers by measuring the photo-response as a function of average power directed to the ICIP. We demonstrate that an increasing reverse bias increases the saturation power as well as the responsivity of the ICIP.

The recent study of ring quantum cascade lasers [1] , [2] (QCLs, Fig. 1a ) revealed a new laser instability. It is triggered by phase turbulence akin to the wave instabilities that occur in other nonlinear systems such as fluids, superconductors and Bose-Einstein condensates. The choice of the ring geometry took inspiration from Kerr combs [3] , that are commonly generated in passive ring microresonators and have attracted great attention within the photonics community in the last years thanks to their rich physics.

A monochromatic wave that circulates in a nonlinear and dispersive optical cavity can become unstable and form a structured waveform. This phenomenon, known as modulation instability, was encountered in fiber lasers, optically pumped Kerr microresonators and, most recently, in monolithic ring quantum cascade lasers (QCLs). In ring QCLs, the instability led to generation of fundamental frequency combs—optical fields that repeat themselves once per cavity round trip. Here we show that the same instability may also result in self-starting harmonic frequency combs—waveforms that repeat themselves multiple times per round trip, akin to perfect soliton crystals in ring Kerr microresonators. We can tailor the intermode spacing of harmonic frequency combs by placing two minute defects with a well-defined separation between them along the ring waveguide. On-demand excitation of frequency comb states with few powerful modes spaced by hundreds of gigahertz may find their use in future sub-terahertz generators.

We demonstrate that ring lasers can attain multimode operation at a very low pumping level, contrary to common belief. The instability is explained via the Ginzburg-Landau theory, establishing a connection between laser and microresonator combs.

Semiconductor ring lasers are miniaturized devices that operate on clockwise and counterclockwise modes. These modes are not coupled in the absence of intracavity reflectors, which prevents the formation of a standing wave in the cavity and, consequently, of a population inversion grating. This should inhibit the onset of multimode emission driven by spatial hole burning. Here we show that, despite this notion, ring quantum cascade lasers inherently operate in phase-locked multimode states, that switch on and off as the pumping level is progressively increased. By rewriting the master equation of lasers with fast gain media in the form of the complex Ginzburg-Landau equation, we show that ring frequency combs stem from a phase instability---a phenomenon also known in superconductors and Bose-Einstein condensates. The instability is due to coupling of the amplitude and phase modulation of the optical field in a semiconductor laser, which plays the role of a Kerr nonlinearity, highlighting a connection between laser and microresonator frequency combs.

The active region of a terahertz quantum cascade laser with two optical transitions is studied. The population of upper laser states, which correspond to 3.4 and 3.8 THz, are investigated at different operating temperatures and in a strong magnetic field.

Interband and quantum cascade lasers are ideal sources to realize compact mid-infrared sensing instruments. Here, we present recent findings on the generation of optical frequency combs. Combined with their inherent detection capabilities, this technology paves the way towards monolithically integrated dual-comb spectrometers.

Funding: Austrian Science Fund (F4909-N23, P28914-N27,W1243); Österreichische Forschungsförderungsgesellschaft (ATMO-SENSE, ERANet Photonic Sensing program, COMTERA-FFG849614); European Science Foundation (ESF) (CZ.02.2.69/0.0/0.0/16_027/0008371); Air Force Office of Scientific Research(AFOSR FA9550-17-1-0340). The generation of mid-infrared pulses in monolithic and electrically pumped devices is of great interest for mobile spectroscopic instruments. The gain dynamics of interband cascade lasers (ICL) are promising for mode-locked operation at low threshold currents. Here, we present conclusive evidence for the generation of picosecond pulses in ICLs via active mode-locking. At small modulation power, the ICL operates in a linearly chirped frequency comb regime characterized by strong frequency modulation. Upon increasing the modulation amplitude, the chirp decreases until broad pulses are formed. Careful tuning of the modulation frequency minimizes the remaining chirp and leads to the generation of 3.2 ps pulses. Publisher PDF

Optical frequency combs are coherent sources that emit a series of evenly spaced lines. In the mid-infrared, comb based spectroscopy is of particular interest and, without the need of any movable parts, will potentially lead to a breakthrough in miniaturization. Interband cascade lasers, with their low power consumption and inherent detection functionality, are an ideal candidate for practical implementations. Here, we present the generation of low-dissipation optical frequency comb utilizing interband cascade lasers. Other than one might have expected, the long lifetime of the interband transition does not automatically lead to slow gain dynamics that would favor in-phase mode-locking. We discuss why ICLs should be considered as fast gain media and why passive mode-locking is difficult or even impossible to be achieved. We applying shifted-wave interference Fourier transform spectroscopy to show that ICL frequency combs naturally favor repulsive intermode beat synchronization with the same chirped FM character recently found in QCL combs. Furthermore, we show first evidence of multiple normal modes of the intermodal beats in frequency combs and picosecond pulse generation from ICLs.

Following the goals of single-chip integrated dual comb spectrometers, we report on recent results on mid-infrared frequency combs. We demonstrate frequency comb operation with a bi-functional quantum cascade material, which allows the integration of lasers and detectors on one chip. With this device, we hold the power and efficiency record of QCL frequency combs. In the second part, we will present first evidence of frequency comb generation using mode-locked interband cascade lasers. With the demonstration of picosecond pulse generation in the mid-infrared, we open a new path towards battery driven sensitive high-resolution spectrometers miniaturized to chip-scale dimensions.

Funding: Austrian Science Fund (FWF) within the projects "NanoPlas" (P28914-N27), "Building Solidsfor Function" (Project W1243), "NextLite" (F4909-N23), as well as by the Austrian Research Promotion Agency through the ERA-Net Photonic Sensing program, project "ATMO-SENSE" (FFG:861581). H.D. was supported by the ESF project CZ.02.2.69/0.00.0/16_027/0008371. A.M.A was supported by the projects COMTERA - FFG 849614 and AFOSR FA9550-17-1-0340. New insights into the laser dynamics of interband cascade lasers reveal the possibility to generatefrequency modulated combs by utilizing their inherent gain nonlinearity. The resultingcomb state is characterized by chirped instantaneous frequency, which appears to be universalto frequency combs based on gain induced four-wave mixing. The fast dynamics in the injectorsfurther allow the realization of exceptionally sensitive and high-speed photodetectors,operating at room-temperature, using the very same epilayer structure. With the capability ofintegrating frequency combs and ultra-fast detectors on a single chip consuming less than awatt of electric power, interband cascade laser technology provides a complete and unmatchedplatform for future monolithic and battery-driven dual-comb spectrometers. Publisher PDF

Quantum cascade lasers are known to spontaneously skip modes and generate widely-spaced harmonic frequency combs. We show that engineering defects in a ring waveguide allows for deterministic control of the comb intermode spacing.