Operation Of Quantum Cascade Lasers Research Articles

Phase Locking of Quantum-Cascade Lasers Operating Around 3.5 and 4.7 THz With a Schottky-Diode Harmonic Mixer

Phase Locking of Quantum-Cascade Lasers Operating Around 3.5 and 4.7 THz With a Schottky-Diode Harmonic Mixer

Broadband long-wavelength upconversion in ultra-short nonlinear crystals.

Since the inception of the second-order nonlinear frequency conversion in 1961, enhancing the inherent low conversion efficiency has been a primary objective. This goal has been successfully accomplished through the utilization of cm-long nonlinear crystals characterized by high quality and nonlinearity, coupled with versatile phase-matching strategies and high-power mixing lasers. However, the reliance on lengthy nonlinear crystals and the necessity for precise phase-matching introduce stringent tolerances on acceptance angles and spectral bandwidths for the interacting fields, thereby constraining its widespread applicability in scientific and industrial domains. This challenge is addressed by combining a broadly tunable ∼5 mW quantum cascade laser operating in the 9.5-12.5 µm range with upconversion detection in ∼100 µm long AGS crystals. Using a tightly focused continuous wave Nd:YVO4 laser with 20 mW output power and spatial filtering of the upconverted beam lead to a SNR of 55 for 50 µs averaging time sufficient for many applications.

Time-Efficient SNR Optimization of WMS-Based Gas Sensor Using a Genetic Algorithm.

This paper presents the description of the wavelength modulation spectroscopy (WMS) experiment, the parameters of which were established by use of the Artificial Intelligence (AI) algorithm. As a result, a significant improvement in the signal power to noise power ratio (SNR) was achieved, ranging from 1.6 to 6.5 times, depending on the harmonic. Typically, optimizing the operation conditions of WMS-based gas sensors is based on long-term simulations, complex mathematical model analysis, and iterative experimental trials. An innovative approach based on a biological-inspired genetic algorithm (GA) and custom-made electronics for laser control is proposed. The experimental setup was equipped with a 31.23 m Heriott multipass cell, software lock-in, and algorithms to control the modulation process of the quantum cascade laser (QCL) operating in the long-wavelength-infrared (LWIR) spectral range. The research results show that the applied evolutionary approach can efficiently and precisely explore a wide range of WMS parameter combinations, enabling researchers to dramatically reduce the time needed to identify optimal settings. It took only 300 s to test approximately 1.39 × 1032 combinations of parameters for key system components. Moreover, because the system is able to check all possible component settings, it is possible to unquestionably determine the operating conditions of WMS-based gas sensors for which the limit of detection (LOD) is the most favorable.

Resonant Tunnelling and Intersubband Optical Properties of ZnO/ZnMgO Semiconductor Heterostructures: Impact of Doping and Layer Structure Variation.

ZnO-based heterostructures are up-and-coming candidates for terahertz (THz) optoelectronic devices, largely owing to their innate material attributes. The significant ZnO LO-phonon energy plays a pivotal role in mitigating thermally induced LO-phonon scattering, potentially significantly elevating the temperature performance of quantum cascade lasers (QCLs). In this work, we calculate the electronic structure and absorption of ZnO/ZnMgO multiple semiconductor quantum wells (MQWs) and the current density-voltage characteristics of nonpolar m-plane ZnO/ZnMgO double-barrier resonant tunnelling diodes (RTDs). Both MQWs and RTDs are considered here as two building blocks of a QCL. We show how the doping, Mg percentage and layer thickness affect the absorption of MQWs at room temperature. We confirm that in the high doping concentrations regime, a full quantum treatment that includes the depolarisation shift effect must be considered, as it shifts mid-infrared absorption peak energy for several tens of meV. Furthermore, we also focus on the performance of RTDs for various parameter changes and conclude that, to maximise the peak-to-valley ratio (PVR), the optimal doping density of the analysed ZnO/Zn88Mg12O double-barrier RTD should be approximately 1018 cm-3, whilst the optimal barrier thickness should be 1.3 nm, with a Mg mole fraction of ~9%.

Above 20 GHz high frequency operation of mid-infrared doughnut-shaped microcavity quantum cascade lasers

Microresonator-based high-speed single-mode quantum cascade lasers are ideal candidates for on-chip optical data interconnection and high sensitivity gas sensing in the mid-infrared spectral range. In this paper, we propose a high frequency operation of single-mode doughnut-shaped microcavity quantum cascade laser at ∼4.6 µm. By leveraging compact micro-ring resonators and integrating with grounded coplanar waveguide transmission lines, we have greatly reduced the parasitics originating from both the device and wire bonding. In addition, a selective heat dissipation scheme was introduced to improve the thermal characteristics of the device by semi-insulating InP infill regrowth. The highest continuous wave operating temperature of the device reaches 288 K. A maximum −3 dB bandwidth of 11 GHz and a cut-off frequency exceeding 20 GHz in a microwave rectification technique are obtained. Benefiting from the notch at the short axis of the microcavity resonator, a highly customized far-field profile with an in-plane beam divergence angle of 2.4° is achieved.

Interdiffusion limiting on self-consistent optical gain in terahertz quantum cascade lasers

In this study, the physical picture of imperfect interfaces is characterized by two terms of in-plane roughness and out-plane interdiffusion. The corresponding scattering is quantified using the nonequilibrium Green’s function model by setting an additional self-energy, and then the impact of scattering strength on GaAs/Al0.3Ga0.7As 2-well terahertz quantum cascade laser performance is investigated. It shows that the interdiffusion scattering term can lead to a maximum reduction of approximately 30% in self-consistent optical gain at a temperature of 300 K. The decrease of population inversion, rather than the broadening linewidth of radiation transition, is the primary factor contributing to the gain degradation. By excluding the effect of band structure deformation caused by interdiffusion in advance, the main cause for the reduction in population inversion is identified as the decoherence of resonant-tunneling injection resulting from interdiffusion scattering.

Electrically-pumped compact topological bulk lasers driven by band-inverted bound states in the continuum

One of the most exciting breakthroughs in physics is the concept of topology that was recently introduced to photonics, achieving robust functionalities, as manifested in the recently demonstrated topological lasers. However, so far almost all attention was focused on lasing from topological edge states. Bulk bands that reflect the topological bulk-edge correspondence have been largely missed. Here, we demonstrate an electrically pumped topological bulk quantum cascade laser (QCL) operating in the terahertz (THz) frequency range. In addition to the band-inversion induced in-plane reflection due to topological nontrivial cavity surrounded by a trivial domain, we further illustrate the band edges of such topological bulk lasers are recognized as the bound states in the continuum (BICs) due to their nonradiative characteristics and robust topological polarization charges in the momentum space. Therefore, the lasing modes show both in-plane and out-of-plane tight confinements in a compact laser cavity (lateral size ~3λlaser). Experimentally, we realize a miniaturized THz QCL that shows single-mode lasing with a side-mode suppression ratio (SMSR) around 20 dB. We also observe a cylindrical vector beam for the far-field emission, which is evidence for topological bulk BIC lasers. Our demonstration on miniaturization of single-mode beam-engineered THz lasers is promising for many applications including imaging, sensing, and communications.

Terahertz Semiconductor Dual‐Comb Sources with Relative Offset Frequency Cancellation

AbstractA dual‐comb source, employing two frequency combs with a slight difference in repetition frequencies, shows unique advantages in high precision spectroscopy, imaging, ranging, communications, and so on. In the terahertz (THz) frequency range, the electrically pumped quantum cascade laser (QCL) offers the possibility of realizing a compact dual‐comb source due to its semiconductor‐based chip‐scale configuration. Although dual‐comb operation of THz QCLs has been experimentally demonstrated, a full stabilization of THz dual‐comb sources is still challenging. Here, a self‐reference method is proposed for a pure THz QCL system. Without using any external locking components, one dual‐comb line is filtered out and it is beaten with the whole dual‐comb signal, which eliminates the common carrier offset frequency noise and reduces the dual‐comb repetition frequency noise. It is demonstrated that the self‐reference technique can significantly improve the long‐term stability of the dual‐comb signal. A record “maxhold” linewidth of 14.8 kHz (60 s) is obtained by implementing the self‐reference technique, while without the self‐reference, the dual‐comb lines show a “maxhold” linewidth of 2 MHz (15 s). The method provides the simplest way to improve the long‐term stability of THz QCL dual‐comb sources, which can be further adopted for various measurements.

Room temperature continuous-wave operation of a dual-wavelength quantum cascade laser

We report on the design and fabrication of a dual-wavelength switchable quantum cascade laser (QCL) by optimizing the design of a homogeneous active region and combining superposed distributed feedback gratings. Coaxial, single-mode emissions at two different wavelengths were achieved only through adjusting the bias voltage. Room temperature continuous-wave operation with output powers of above 30 mW and 75 mW was realized for single-mode emission at 7.61 µm and 7.06 µm, respectively. The simplified fabrication process and easy wavelength control of our designed dual-wavelength QCL make it very attractive for developing miniature multi-species gas sensing systems.

MBE growth and optimization of the InGaAs/InAlAs materials system for quantum cascade laser

High material quality is the foundation for the excellent performance of quantum cascade lasers. This paper investigates the growth conditions for InGaAs/InAlAs/InP superlattices combined solid-source molecular beam epitaxy with metal-organic chemical vapor deposition. Based on the optimization of growth conditions of InGaAs buffer epilayers, the effects of the growth temperature, interruption time, and V/III flux ratio on the quality of InGaAs/InAlAs/InP superlattices were systematically investigated. High-resolution x-ray diffraction and atomic force microscopy prove that this optimized InP-based superlattice has excellent periodicity and sharp interfaces.

Fast-sweeping continuous wave quantum cascade laser operating in an external cavity with polygon mirror.

We report a continuous wave room temperature quantum cascade laser operating in an external cavity in the Littrow configuration with a 10-facet polygon mirror rotating at 24,000 RPM. The quantum cascade laser emission is swept across ∼1520 - 1625 cm-1 wavenumber range in less than ∼45 µs with a sweep repetition rate of 4 kHz. The measured maximum output power at the laser gain maximum, 15°C and 0.86 A driving current is ∼90 mW; the estimated average output power across the 45 µs wavenumber sweep is ∼50 mW. Through its sweep, the laser emits on the sequential Fabry-Perot longitudinal modes of the laser chip cavity with the mode separation of ∼0.5 cm-1. The linewidth of the emitting modes is less than ∼0.05 cm-1. Spectral measurements of the infrared absorption features of a 10 µm thick layer of acetophenone and water vapor in the air have demonstrated the capability of obtaining spectral data in less than 45 µs.

Application of High-Tc Josephson Detectors for Spectral Characterization of THz Quantum Cascade Lasers

We have demonstrated the potential of high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_c</i> Josephson junctions (JJ) for broadband spectral characterization of electromagnetic radiation from terahertz (THz) quantum cascade lasers (QCLs). We have used frequency-selective Josephson detection with Hilbert-transform spectral analysis (HTSA) and classical detection in YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-x</sub> bicrystal junctions to measure and interpret the spectra of QCL radiation from a few tens of GHz to 2.5 THz. The dominant part in the measured spectra concentrates near the designed frequency of 2.2 THz and includes several lines with the frequencies of the spatial modes in the QCL. We have found that, in addition to expected 2.2 THz radiation, QCL spectra demonstrate a low-frequency part with the frequencies <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> < 400 GHz and an intermediate region with increasing intensity for the frequencies from 500 GHz to 2 THz. We have found that intensities and spectral contents of three parts non-monotonically depend on the injection current of QCLs. We interpret the recovered spectra as an interplay of designed QCL operation with nonlinear effects of 2nd and 3rd orders in the active volume of QCLs. Compared to conventional techniques, the advantages of high- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_c</i> JJs with HTSA are in frequency-domain operation and extended ranges, including THz, sub-THz, and GHz frequencies of QCL nonlinear dynamics.

Continuous-Wave Operation of Microcavity Quantum Cascade Lasers in Whispering-Gallery Mode

Continuous-Wave Operation of Microcavity Quantum Cascade Lasers in Whispering-Gallery Mode

The impact of additional injected signals on the stability and bandwidth of the quantum cascade lasers with external cavities

- In recent years, special attention has been paid to the injection lock of quantum cascade lasers, which leads to the improvement of their small signal properties and output power. In this paper, the operation of an injection-locked quantum cascade laser including the output characteristics and modulation response has been investigated. By using one external cavity for optical lock, the resulted structure has complexities may cause instability of a laser that limit the use of this system. In this paper, for the first time, to solve this problem, an optical self-locking system was proposed and the properties of laser under these conditions were investigated. By adding a second external cavity to a single external cavity laser, output power will reach a constant value. The output power of the single cavity laser in free-running mode was about 400 mw while the output power of the laser with two external cavities has been increased to 700 mw. In addition to stabilizing the output power, we also see an increase in bandwidth. We found that for different lengths and reflectivity, the output power of the proposed one external cavity laser in free-running mode does not reach a fixed value and even in some cases, it becomes unstable. But by adding a second cavity to the laser in the free-running mode, it will be able to produce a stable laser with constant output power. Adding external cavities can be used as an effective tool to improve frequency stability or increase bandwidth.

Real-time multimode dynamics of terahertz quantum cascade lasers via intracavity self-detection: observation of self mode-locked population pulsations.

Mode-locking operation and multimode instabilities in Terahertz (THz) quantum cascade lasers (QCLs) have been intensively investigated during the last decade. These studies have unveiled a rich phenomenology, owing to the unique properties of these lasers, in particular their ultrafast gain medium. Thanks to this, in QCLs a modulation of the intracavity field intensity gives rise to a strong modulation of the population inversion, directly affecting the laser current. In this work we show that this property can be used to study in real-time the dynamics of multimode THz QCLs, using a self-detection technique combined with a 60GHz real-time oscilloscope. To demonstrate the potential of this technique we investigate a 4.2THz QCL operating in free-running, and observe a self-starting periodic modulation of the laser current, producing trains of regularly spaced, ∼100ps-long pulses. Depending on the drive current we find two distinct regimes of oscillation with dramatically different properties: a first regime at the fundamental cavity repetition rate, characterised by large amplitude and phase noise, with coherence times of a few tens of periods; a much more regular second-harmonic-comb regime, with typical coherence times of ∼105 oscillation periods. We interpret these measurements using a set of effective semiconductor Maxwell-Bloch equations that qualitatively reproduce the fundamental features of the laser dynamics, indicating that the observed carrier-density and optical pulses are in antiphase, and appear as a rather shallow modulation on top of a continuous wave background. Thanks to its simple implementation and versatility, the demonstrated broadband self-detection technique is a powerful tool for the study of ultrafast dynamics in THz QCLs.

Quantum Simulating the Electron Transport in Quantum Cascade Laser Structures

AbstractUltracold fermionic atoms are proposed to be used in 1D optical lattices to quantum simulate the electronic transport in quantum cascade laser (QCL) structures. The competition between the coherent tunneling among (and within) the wells and the dissipative decay at the basis of lasing is discussed. In order to validate the proposed simulation scheme, such competition is quantitatively addressed in a simplified 1D model. The existence of optimal relationships between the model parameters is shown, maximizing the particle current, the population inversion (or their product), and the stimulated emission rate. This substantiates the concept of emulating the QCL operation mechanisms in cold‐atom optical lattice simulators, laying the groundwork for addressing open questions, such as the impact of electron–electron scattering and the origin of transport‐induced noise, in the design of new‐generation QCLs.

Terahertz imaging with self-pulsations in quantum cascade lasers under optical feedback

The phenomenon of self-pulsation (SP) in terahertz (THz) quantum cascade lasers (QCLs) due to optical feedback was reported recently. In this Letter, we propose a THz imaging modality using the SP phenomenon in a THz QCL. We explore the theoretical oscillation properties of the SP scheme and demonstrate its suitability to perform imaging experimentally. The SP imaging scheme operates in self-detection mode, eliminating the need for an external detector. Moreover, the scheme requires only a fixed current, meaning that one can avoid many of the pitfalls associated with high temperature operation of THz QCLs, including frequency chirp and mode hops caused by sweeping the laser current. This also means that one is free to locate the operating point at the maximum power, to produce the desired beam profile or for highest spectral purity, depending on the application. The SP imaging modality proposed in this work can be translated directly to high operating temperature THz QCLs.

Improved comb and dual-comb operation of terahertz quantum cascade lasers utilizing a symmetric thermal dissipation.

In the terahertz frequency range, the quantum cascade laser (QCL) is a suitable platform for the frequency comb and dual-comb operation. Improved comb performances have been always much in demand. In this work, by employing a symmetric thermal dissipation scheme, we report an improved frequency comb and dual-comb operation of terahertz QCLs. Two configurations of cold fingers, i.e., type A and B with asymmetric and symmetric thermal dissipation schemes, respectively, are investigated here. A finite-element thermal analysis is carried out to study the parametric effects on the thermal management of the terahertz QCL. The modeling reveals that the symmetric thermal dissipation (type B) results in a more uniform thermal conduction and lower maximum temperature in the active region of the laser, compared to the traditional asymmetric thermal dissipation scheme (type A). To verify the simulation, experiments are further performed by measuring laser performance and comb characteristics of terahertz QCLs emitting around 4.2 THz mounted on type A and type B cold fingers. The experimental results show that the symmetric thermal dissipation approach (type B) is effective for improving the comb and dual-comb operation of terahertz QCLs, which can be further widely adopted for spectroscopy, imaging, and near-field applications.

Data Link with a High-Power Pulsed Quantum Cascade Laser Operating at the Wavelength of 4.5 µm.

This article is a short study of the application of high-power quantum cascade lasers and photodetectors in medium-infrared optical wireless communications (OWC). The link range is mainly determined by the transmitted beam parameters and the performance of the light sensor. The light power and the photodetector noise directly determine the signal-to-noise power ratio. This ratio could be maximized in the case of minimizing the radiation losses caused by atmospheric attenuation. It can be obtained by applying both radiation sources and sensors operated in the medium infrared range decreasing the effects of absorption, scattering or scintillation, beam spreading, and beam wandering. The development of a new class of laser sources based on quantum cascade structures becomes a prospective alternative. Regarding the literature, there are descriptions of some preliminary research applying these lasers in data transmission. To provide a high data transfer rate, continuous wave (cw) lasers are commonly used. However, they are characterized by low power (a few tens of mWatts) limiting their link range. Also, only a few high-power pulsed lasers (a few hundreds of mWatts) were tested. Due to their limited pulse duty cycle, the obtained modulation bandwidth was lower than 1 MHz. The main goal of this study is to experimentally determine the capabilities of the currently developed state-of-the-art high-power pulsed quantum cascade (QC) lasers and photodetectors in OWC systems. Finally, the data link range using optical pulses of a QC laser of ~2 W, operated at the wavelength of ~4.5 µm, is discussed.

Broad gain, continuous-wave operation of InP-based quantum cascade laser at λ ∼ 11.8 μm* *Project supported by the National Basic Research Program of China (Grant No. 2018YFA0209103), the National Natural Science Foundation of China (Grant Nos. 61991430, 61774146, 61790583, 61734006, 61835011, 61674144, 61774150, and 61805168), Beijing Municipal Science & Technology Commission, China (Grant No. Z201100004020006), and the Key Projects of

We demonstrate a broad gain, continuous-wave (CW) operation InP-based quantum cascade laser (QCL) emitting at 11.8 μm with a modified dual-upper-state (DAU) and diagonal transition active region design. A 3 mm cavity length, 16.5 μm average ridge wide QCL with high-reflection (HR) coatings demonstrates a maximum peak power of 1.07 W at 283 K and CW output power of 60 mW at 293 K. The device also shows a broad and dual-frequency lasing spectrum in pulsed mode and a maximum average power of 258.6 mW at 283 K. Moreover, the full width at half maximum (FWHM) of the electroluminescent spectrum measured at subthreshold current is 2.37 μm, which indicates a broad gain spectrum of the materials. The tuning range of 1.38 μm is obtained by a grating-coupled external cavity (EC) Littrow configuration, which is beneficial for gas detection.