Quantum Cascade Laser Devices Research Articles

Enhancement of longitudinal mode characteristic of quantum cascade laser by high-order tilted asymmetric surface grating

The high-order tilted asymmetric surface grating (TASG) is firstly investigated and applied to quantum cascade lasers (QCLs) here. Due to the tilted grating of 16°in the middle of ridge, the longitudinal mode loss difference of TASG is about 4% higher than that of the uniform grating (UNG), which results in better single mode selectivity in reflectance spectrum. Further experimental results indicate that the TASG distributed feedback (DFB) QCL maintains more stable single longitudinal mode performance with increasing the current and temperature because that TASG have a larger longitudinal mode loss difference. While the UNG QCL with the equivalent coupling coefficient exhibits mode jumping. Hence, the drift coefficient of 0.311 nm/K of TASG QCL is 60% lower than that of the UNG device and the maximum side-mode suppression ratio (SMSR) achieves 30 dB. The characteristic temperature of TASG QCL device is 36K, which is higher than 32K of UNG device. This study showcases the feasibility and superiority of the TASG in mid-infrared active devices.

Driving up the quality of metal organic chemical vapor deposition-grown epitaxial wafers for highly innovative photonic applications

Quantum cascade lasers (QCL) and extended InGaAs photodetectors (ex-PDs) (operating in regime 1.9–2.6 µm wavelength) are very attractive materials for use in sophisticated photonics and microelectronics such as optical recording, scanners, laser spectroscopy, thermal or gas sensing. Once a laboratory curiosity, QCLs and ex-PDs are now becoming commercially available products, shaping exciting applications. In this work, we present the intricacies and challenges of the development and industrial production of Metal Organic Chemical Vapor Deposition (MOCVD)-grown QCL and ex-PD structures as well as demonstrate their viability for large-scale manufacturing.Different types of epitaxial structures were grown using a multi-wafer production MOCVD system, in 12 × 3″ wafer configuration per growth run. Structural and electrical properties of epitaxial layers were carefully examined.MOCVD growing of QCL and ex-PD structures is a very demanding process. MOCVD production reactor enables development of highly complicated chemical reactions, however, as gas residence is much longer than in small R&D systems, the hetero-interfaces of InGaAs/InAlAs are inevitably graded, which negatively impacts optical and electrical parameters of QCL devices. In addition, QCL structure requires the exact layer thickness as well as absolute precision and repeatability when developing strained InAlAs and InGaAs epi‑layers. In the case of ex-PDs fabrication, the crucial thing is to develop a specially designed and graded buffer layer, and to avoid numerous misfit defects and anomalies in dark current. We obtained high crystallographic quality and repeatability of the epi‑layers, which resulted in QCL high-performance continuous-wave operation and fabrication of ex-PDs with low dark current density (less than 1 × 10−4 A/cm2 at -100 mV/room temperature (RT)) achieving results consistent with Rule07 and higher quantum efficiency.

Температурное затухание генерации квантово-каскадных лазеров с частотами 2.3, 3.2, 4.1 ТГц

In this work, we conduct research of spectral and power characteristics of quantum cascade lasers (QCLs) based on a GaAs/Al0.15Ga0.85As active region emitting at 2.3 (A), 3.2 (B) and 4.1 (C) THz. The QCL devices had a double-metal Au waveguide and operated in pulsed mode with 1.5–9 us pulses at 20 Hz repetition rate. Using the integral output power curves measured with different pulse durations, we consider the potential mechanisms of QCL temperature degradation using Arrhenius plots. Moreover, we present the spectra of the lasers measured at fixed operating points for devices A, С and with current scanning for device B in a wide temperature range from 5 to 120 K. We hope that our results will prove useful for research concerning QCL maximum operating temperatures.

Temperature degradation of 2.3, 3.2 and 4.1 THz quantum cascade lasers

In this work, we conduct research of spectral and power characteristics of quantum cascade lasers (QCLs) based on a GaAs/Al0.15Ga0.85As active region emitting at 2.3 (A), 3.2 (B) and 4.1 (C) THz. The QCL devices had a double-metal Au waveguide and operated in pulsed mode with 1.5-9 μs pulses at 20 Hz repetition rate. Using the integral output power curves measured with different pulse durations, we consider the potential mechanisms of QCL temperature degradation using Arrhenius plots. Moreover, we present the spectra of the lasers measured at fixed operating points for devices A, C and with current scanning for device B in a wide temperature range from 5 to 120 K. We hope that our results will prove useful for research concerning QCL maximum operating temperatures. Keywords: quantum cascade laser, terahertz range, quantum well, molecular beam epitaxy, activation energy, temperature degradation.

Performance of open-path GasFinder3 devices for CH<sub>4</sub> concentration measurements close to ambient levels

Abstract. Open-path measurements of methane (CH4) with the use of GasFinder systems (Boreal Laser Inc, Edmonton Canada) have been frequently used for emission estimation with the inverse dispersion method (IDM), particularly from agricultural sources. It is common to many IDM applications that the concentration enhancement related to CH4 sources is small, typically between 0.05 and 0.5 ppm, and accurate measurements of CH4 concentrations are needed at concentrations close to ambient levels. The GasFinder3-OP (GF3) device for open-path CH4 measurements is the latest version of the commercial GasFinder systems by Boreal Laser Inc. We investigated the uncertainty of six GF3 devices from side-by-side intercomparison measurements and comparisons to a closed-path quantum cascade laser device. The comparisons were made at near-ambient levels of CH4 (85 % of measurements below 2.5 ppm) with occasional phases of elevated concentrations (max. 8.3 ppm). Relative biases as high as 8.3 % were found, and a precision for half-hourly data between 2.1 and 10.6 ppm-m (half width of the 95 % confidence interval) was estimated. These results deviate from the respective manufacturer specifications of 2 % and 0.5 ppm-m. Intercalibration of the GF3 devices by linear regression to remove measurement bias was shown to be of limited value due to drifts and step changes in the recorded GF3 concentrations.

High-speed modulation of a terahertz quantum cascade laser by coherent acoustic phonon pulses

The fast modulation of lasers is a fundamental requirement for applications in optical communications, high-resolution spectroscopy and metrology. In the terahertz-frequency range, the quantum-cascade laser (QCL) is a high-power source with the potential for high-frequency modulation. However, conventional electronic modulation is limited fundamentally by parasitic device impedance, and so alternative physical processes must be exploited to modulate the QCL gain on ultrafast timescales. Here, we demonstrate an alternative mechanism to modulate the emission from a QCL device, whereby optically-generated acoustic phonon pulses are used to perturb the QCL bandstructure, enabling fast amplitude modulation that can be controlled using the QCL drive current or strain pulse amplitude, to a maximum modulation depth of 6% in our experiment. We show that this modulation can be explained using perturbation theory analysis. While the modulation rise-time was limited to ~800 ps by our measurement system, theoretical considerations suggest considerably faster modulation could be possible.

Thermal analysis and heat dissipation improvement for quantum cascade lasers through experiments, simulations, and structure function

We quantified the thermal resistance of quantum cascade lasers (QCLs) using their structure functions and increased the laser output by employing ridge structures in which thermal resistance was reduced. To improve heat properties, three different QCL devices were prepared as follows. One was a device whose ridge was covered with SiO2, another was a device whose ridge was embedded with Au, and the other was a device whose ridge was embedded with Cu. The temperature distributions were measured with a thermoviewer and were analyzed with three-dimensional simulations. From the results, improved heat dissipation by embedding the ridge was clarified. Furthermore, the structure functions obtained by static mode measurement suggested that the thermal resistance was improved from 9.3 to 6.5 K W−1 by embedding the ridge. As a result of the improvement, the QCL with the Au-embedded ridge had a 1.5-fold higher laser power than the QCL with the SiO2-covered ridge.

3.5 THz quantum-cascade laser emission from dual diagonal feedhorns

AbstractAntenna-pattern measurements obtained from a double-metal supra-terahertz-frequency (supra-THz) quantum cascade laser (QCL) are presented. The QCL is mounted within a mechanically micro-machined waveguide cavity containing dual diagonal feedhorns. Operating in continuous-wave mode at 3.5 THz, and at an ambient temperature of ~60 K, QCL emission has been directed via the feedhorns to a supra-THz detector mounted on a multi-axis linear scanner. Comparison of simulated and measured far-field antenna patterns shows an excellent degree of correlation between beamwidth (full-width-half-maximum) and sidelobe content and a very substantial improvement when compared with unmounted devices. Additionally, a single output has been used to successfully illuminate and demonstrate an optical breadboard arrangement associated with a future supra-THz Earth observation space-borne payload. Our novel device has therefore provided a valuable demonstration of the effectiveness of supra-THz diagonal feedhorns and QCL devices for future space-borne ultra-high-frequency Earth-observing heterodyne radiometers.

Thermal imaging of buried heterostructure quantum cascade lasers (QCLs) and QCL arrays using CCD-based thermoreflectance microscopy

Thermal profiles of a single-element quantum cascade laser (QCL) and a five-element QCL array at different bias currents, under quasi-continuous-wave (QCW) conditions, are obtained using the charge-coupled device (CCD)-based thermoreflectance imaging technique. Peak temperature changes of 55 K and 105 K are measured on the single-element QCL (operating at 1.2 A) and the central element of the QCL array (operating at 4.2 A), respectively. The average facet temperature of the single QCL device shows a linear relationship with the dissipated power, indicating an effective thermal resistance of Rth = 3.0 ± 0.2 K/W (7%) for the device. The thermal transient behavior of the single QCL device, in response to a 35 μs-wide heating pulse, is also measured. From the transient curve, an effective thermal time constant of τth = 9.5 ± 0.4 μs (4%) is obtained. Experimental results are compared to the results obtained from heat-transfer models for both the single-element and array devices. Thermal profiles show a thermal lensing effect at the facet of the single-element QCL. In the array device, a more pronounced heating is observed at the center of the device while the temperature gradually decreases away from the central element.

Interfacial Mixing Analysis for Strained Layer Superlattices by Atom Probe Tomography

Quantum wells and barriers with precise thicknesses and abrupt composition changes at their interfaces are critical for obtaining the desired emission wavelength from quantum cascade laser devices. High-resolution X-ray diffraction and transmission electron microscopy are commonly used to calibrate and characterize the layers’ thicknesses and compositions. A complementary technique, atom probe tomography, was employed here to obtain a direct measurement of the 3-dimensional spatially-resolved compositional profile in two InxGa1−xAs/InyAl1−yAs III-V strained-layer superlattice structures, both grown at 605 °C. Fitting the measured composition profiles to solutions to Fick’s Second Law yielded an average interdiffusion coefficient of 3.5 × 10−23 m2 s−1 at 605 °C. The extent of interdiffusion into each layer determined for these specific superlattices was 0.55 nm on average. The results suggest that quaternary active layers will form, rather than the intended ternary compounds, in structures with thicknesses and growth protocols that are typically designed for quantum cascade laser devices.

Laser driving and data processing concept for mobile trace gas sensing: Design and implementation.

High precision mobile sensing of multi-species gases is greatly demanded in a wide range of applications. Although quantum cascade laser absorption spectroscopy demonstrates excellent field-deployment capabilities for gas sensing, the implementation of this measurement technique into sensor-like portable instrumentation still remains challenging. In this paper, two crucial elements, the laser driving and data acquisition electronics, are addressed. Therefore, we exploit the benefits of the time-division multiplexed intermittent continuous wave driving concept and the real-time signal pre-processing capabilities of a commercial System-on-Chip (SoC, Red Pitaya). We describe a re-designed current driver that offers a universal solution for operating a wide range of multi-wavelength quantum cascade laser device types and allows stacking for the purpose of multiple laser configurations. Its adaptation to the various driving situations is enabled by numerous field programmable gate array (FPGA) functionalities that were developed on the SoC, such as flexible generation of a large variety of synchronized trigger signals and digital inputs/outputs (DIOs). The same SoC is used to sample the spectroscopic signal at rates up to 125 MS/s with 14-bit resolution. Additional FPGA functionalities were implemented to enable on-board averaging of consecutive spectral scans in real-time, resulting in optimized memory bandwidth and hardware resource utilisation and autonomous system operation. Thus, we demonstrate how a cost-effective, compact, and commercial SoC can successfully be adapted to obtain a fully operational research-grade laser spectrometer. The overall system performance was examined in a spectroscopic setup by analyzing low pressure absorption features of CO2 at 4.3 μm.

Electrical isolation of GaAs and AlGaAs/GaAs Quantum Cascade Lasers by deep hydrogen implantation

Ion implantation can be applied to form the electrical isolation in AlGaAs/GaAs Quantum Cascade Laser (QCL) instead of mesa etching. In this paper, we present in detail the designing of hydrogen implant isolation scheme, alongside with its verification and study of thermal stability by structural and electrical characterization techniques. Our scheme employed 4µm thick metallic mask made mainly of gold which also served as contact layer, and 640keV hydrogen implantation to a fluency of 1 × 1015cm−2. We obtained the sheet resistivity RSH of (1.5 ± 0.9) × 109Ω/□. The critical temperature for the hydrogen implant isolation fabrication of the AlGaAs/GaAs QCL was determined to be 310℃. Defect density dropped to residual levels after 1min annealing at 300℃ and 400℃, while the material was still resistive with RSH above 108Ω/□. Based on our simulated vacancy maps we concluded that the minimum width of the masking structure should be at least 5µm to avoid the effects of lateral isolation for the given implantation conditions. The QCL device fabricated with this isolation scheme operated with threshold current densities of 6kA/cm2 at the temperature of 77K. Ultimately, we confirmed the applicability of hydrogen implant isolation for the manufacturing of optical devices.

Achieving comb formation over the entire lasing range of quantum cascade lasers.

Frequency combs based on quantum cascade lasers (QCLs) are finding promising applications in high-speed broadband spectroscopy in the terahertz regime, where many molecules have their "fingerprints." To form stable combs in QCLs, an effective control of group velocity dispersion plays a critical role. The dispersion of the QCL cavity has two main parts: a static part from the material and a dynamic part from the intersubband transitions. Unlike the gain, which is clamped to a fixed value above the lasing threshold, dispersion associated with the intersubband transitions changes with bias, even above the threshold, and this reduces the dynamic range of comb formation. Here, by incorporating tunability into the dispersion compensator, we demonstrate a QCL device exhibiting comb operation from Ith to Imax, which greatly expands the operation range of the frequency combs.

Analysis of Operating Regimes of Terahertz Quantum Cascade Laser Frequency Combs

In recent years, quantum cascade lasers (QCLs) have shown tremendous potential for the generation of frequency combs in the mid-infrared and terahertz portions of the electromagnetic spectrum. The research community has experienced success both in the theoretical understanding and experimental realization of QCL devices, capable of generating stable and broadband frequency combs. Specifically, it has been pointed out that four wave mixing (FWM) is the main comb formation process and group velocity dispersion (GVD) is the main comb-degradation mechanism. As a consequence, special dispersion compensation techniques have been employed, in order to suppress the latter and simultaneously enhance the former processes. Here, we perform a detailed computational analysis of FWM, GVD, and spatial hole burning (SHB), all known to play a role in QCLs, and show that SHB has a considerable impact on whether the device will operate as a comb or not. We therefore conclude that for a successful implementation of a quantum cascade laser frequency comb, one would need to address this effect as well.

THE STUDY OF OPTICAL GAIN FOR TERAHERTZ QUANTUM CASCADE LASER USING DENSITY MATRIX METHOD

Terahertz (THz) quantum cascade lasers (QCL) are currently increasing in popularity. It is expected to become the main source of emerging terahertz radiation technology and applications. However to produce the device within the application specification is costly and time consuming. This is because the manufacturing process of the superlattice growth and the device processing and testing are long and expensive processes. Thus a prediction tool is needed to overcome the problems in designing and producing THz QCL within the needed optical expectation. The density matrix method is used to calculate the performance of this device electronically and optically. The result obtained was compared to the experimental result conducted by previous researchers. The calculation result showed that the gain is 20 when the population inversion occurs at threshold current density of 400 A cm-2. Meanwhile a negative gain or loss occurs below 350 A cm-2. As a conclusion, it is demonstrated that this method has a capability to explain the transport phenomena as well as to predict the performance of the THz QCL device design.

Detection and measurement of electroreflectance on quantum cascade laser device using Fourier transform infrared microscope

We demonstrate the use of a Fourier Transform Infrared microscope system to detect and measure electroreflectance (ER) from mid-infrared quantum cascade laser (QCL) device. To characterize intersubband transition (ISBT) energies in a functioning QCL device, a microscope is used to focus the probe on the QCL cleaved mirror. The measured ER spectra exhibit resonance features associated to ISBTs under applied electric field in agreement with the numerical calculations and comparable to observed photocurrent, and emission peaks. The method demonstrates the potential as a characterization tool for QCL devices.

Nonequilibrium Green’s Function Model for Simulation of Quantum Cascade Laser Devices Under Operating Conditions

A simulation scheme based on nonequilibrium Green's functions for biased periodic semiconductor heterostructure devices is presented in detail. The implementation can determine current and optical gain both for small and large optical fields. Specific results for superlattices, quantum cascade lasers, and quantum cascade detectors are shown which demonstrate the capabilities of the approach.

Equivalent Circuit-Level Model of Quantum Cascade Lasers: Influence of Doping Density on Steady State and Dynamic Responses

An equivalent circuit model of quantum cascade lasers (QCLs) is introduced by virtue of revised three-level rate equations. This model accounts for the influence of injector doping on electron dynamics of QCLs. Both the photon gain coefficient and the injection current efficiency depend on the injector doping density in this model. The nonradiative scattering times, radiative spontaneous relaxation time, and electron escape time are obtained by a fully nonequilibrium self-consistent Schrodinger-Poisson analysis of the scattering rate and energy balance equations. A general diode subcircuit is adopted to model the current-voltage relationship. Based on this new model, the steady and dynamic characteristics of devices with injector sheet doping densities in the range of $4\times 10^{11}\sim 6.5\times 10^{11}~{\rm cm}^{-2}$ are investigated by using a circuit simulator. Results indicate that doping density variations play an important role on threshold current and delay time of QCL devices.

Two‐dimensional profiling of carriers in terahertz quantum cascade lasers using calibrated scanning spreading resistance microscopy and scanning capacitance microscopy

The distribution of charge carriers inside the active region of a terahertz (THz) quantum cascade laser (QCL) has been measured with scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM). Individual quantum well-barrier modules with a 35.7-nm single module thickness in the active region of the device have been resolved for the first time using high-resolution SSRM and SCM techniques at room temperature. SSRM and SCM measurements on the quantum well-barrier structure were calibrated utilizing known GaAs dopant staircase samples. Doping concentrations derived from SSRM and SCM measurements were found to be in quantitative agreement with the designed average doping values of the n-type active region in the terahertz quantum cascade laser. The secondary ion mass spectroscopy provides a partial picture of internal device parameters, and we have demonstrated with our results the efficacy of uniting calibrated SSRM and SCM to delineate quantitatively the transverse cross-sectional structure of complex two-dimensional terahertz quantum cascade laser devices.

Photocurrent Measurements on a Quantum Cascade Laser Device by Fourier Transform Infrared Microscope

Intersubband photocurrent in the InAs/AlSb mid-infrared quantum cascade laser (QCL) device is measured. To characterize subband energies in a fabricated and functioning QCL laser device, Fourier-transform infrared (FTIR) microscope is used to focus the probe light on the cleaved mirror of QCL. Photocurrent associated with intersubband transitions in the active layer are observed up to room temperature and the origin of peaks is identified by numerical simulation, which demonstrates the potential as a characterization tool for QCL devices.