Mid-infrared Quantum Cascade Lasers Research Articles

High Frequency Mid-Infrared Quantum Cascade Laser Integrated With Grounded Coplanar Waveguide Transmission Line

High Frequency Mid-Infrared Quantum Cascade Laser Integrated With Grounded Coplanar Waveguide Transmission Line

High-Power Mid-Infrared Quantum Cascade Laser with Large Emitter Width

High-power quantum cascade lasers (QCLs) have a wide application prospect. In this paper, a high-power high-beam-quality device with a large ridge width is demonstrated. The effect of different ridge widths on mode loss was studied, and the results showed that the mode loss decreased as the ridge width increased. Furthermore, as the width of the ridge increased, the temperature of the active region rose. In the experiment, the wafers were grown by metal–organic chemical vapor deposition (MOCVD), and the ridge width of the device was controlled by wet etching. A laser with a ridge width of 15 µm and a length of 5 mm achieved an output of 2.2 W under 288 K continuous wave (CW) operation, with a maximum slow-axis divergence angle of 27.2° and a device wavelength of 5 μm. The research results of this article promote the industrial production of base transverse mode QCL.

The study on application of high-order tilted asymmetric Bragg gratings in quantum cascade lasers

High-order gratings eliminate the need for high-resolution lithography in the definition process. However, they are not widely adopted due to inability to provide sufficient coupling coefficients in long-wave mid-infrared quantum cascade lasers (QCLs). Fifth order tilted asymmetric gratings (TAS-gratings) distributed feedback (DFB) QCLs with stable single longitudinal mode are firstly reported in this paper. TAS-gratings exhibit longitudinal mode selection capability across a broad range of asymmetric distances around 1.18 μm and duty cycles around 0.5, which is attributed to their larger effective duty cycles and phase differences caused by an 18° tilt angle of the grating in the middle and two asymmetrical distributions of gratings on both sides. Further experimental results indicate that DFB QCLs with TAS-gratings exhibit stable excitation of single longitudinal modes within the duty cycle range of 0.48–0.54 due to the large coupling coefficients, the temperature drift coefficient of 0.29 nm/K of TAS-grating device is reduced by 64% compared with symmetric gratings (S-gratings) devices, the maximum side-mode suppression ratio (SMSR) of the devices exceeds 25 dB. TAS-gratings offer greater process tolerance, design flexibility and lower preparation difficulty. This study provides a solution to achieve sufficiently large coupling coefficients for high-order gratings applied to long-wave mid-infrared QCLs.

Modeling with graded interfaces: Tool for understanding and designing record-high power and efficiency mid-infrared quantum cascade lasers

Abstract By employing a graded-interfaces model based on a generalized formalism for interface-roughness (IFR) scattering that was modified for mid-infrared emitting quantum cascade lasers (QCLs), we have accurately reproduced the electro-optical characteristics of published record-performance 4.9 µm- and 8.3 µm-emitting QCLs. The IFR-scattering parameters at various interfaces were obtained from measured values and trends found via atom-probe tomography analysis of one of our 4.6 μm-emitting QCL structures with variable barrier heights. Those values and trends, when used for designing a graded-interface, 4.6 μm-emitting QCL, led to experimental device characteristics in very good agreement with calculated ones. We find that the published record-high performance values are mainly due to both injection from a prior-stage low-energy (active-region) state directly into the upper-laser (ul) level, thus at low field-strength values, as well as to strong photon-induced carrier transport. However, the normalized leakage-current density J leak /J is found to be quite high: 26–28 % and 23.3 %, respectively, mainly because of IFR-triggered shunt-type leakage through high-energy active-region states, in the presence of high average electron temperatures in the ul laser level and an energy state adjacent to it: 1060 K and 466 K for 4.9 µm- and 8.3 µm-emitting QCLs, respectively. Then, modeling with graded interfaces becomes a tool for designing devices of performances superior to the best reported to date, thus closing in on fundamental limits. The model is employed to design a graded-interface 8.1 µm-emitting QCL with suppressed carrier leakage via conduction-band engineering, which reaches a maximum front-facet wall-plug efficiency value of 22.2 %, significantly higher than the current record (17 %); thus, a value close to the fundamental front-facet, upper limit (i.e., 25 %) for ∼8 µm-emitting QCLs.

Quantum cascade laser absorption sensor for in-situ, real-time and sensitive measurement of high-temperature SO2 and SO3

We report a mid-infrared quantum cascade laser absorption sensor capable of measuring SO2 and SO3 simultaneously and sensitively at elevated temperatures. In the sensor development, the intense transitions of SO2 and SO3 in the mid-infrared region of 1129 cm−1 and 1398 cm−1 were exploited by two quantum cascade lasers. A high-temperature multipass cell was adopted to increase the absorption path length to 10 m. The quantitative concentrations of SOx were directly obtained from the calibration-free wavelength modulation spectroscopic method, which was validated at varied temperature and pressure conditions. From Allan deviation analysis, we achieved a minimum detection limit of 8 parts per billion (ppb) for SO2 and 3 ppb for SO3, with an average time of 100 s. Lastly, we successfully demonstrated the real-time and sensitive measurement of SO2 and SO3 during the oxidation reaction of SO2 by O3 at 460 K. Our laser sensor shows great potential for in-situ and real-time monitoring of SOx from combustion emissions.

Controlled generation of harmonic states in mid-infrared quantum cascade laser frequency combs by external cavity optical feedback.

We demonstrate the implementation of external cavity optical feedback to improve coherence and promote generation of harmonic states by a mid-infrared quantum cascade laser frequency comb. In particular, we present a Vernier-like scheme to realize harmonic comb states that increase the repetition rate of the comb by a factor of up to 6 and broaden spectral coverages from 46 cm-1 to 92 cm-1. Intermode beatnote and dual comb characterization indicate that the coherence of the comb has greatly improved for sub-optimal devices when the comb is operated in these harmonic states. This approach to control the generation of harmonic states and improve comb performance can be readily incorporated to various sensing systems and has great potential in spectroscopic measurements that require high repetition rates and/or broad optical bandwidth.

Re-injection off-axis integrated cavity output spectroscopy for the simultaneous detection of N2O, H2O and CO with a mid-infrared QCL laser.

Off-axis integrated cavity output spectroscopy (OA-ICOS) has attracted much interest because it potentially allows highly sensitive field measurements with robust optical alignment. In this paper, a novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of multi-component gases N2O, H2O and CO in the atmosphere based on a mid-infrared quantum cascade laser (QCL) and OA-ICOS. In order to improve the energy utilization and increase the signal-to-noise ratio (SNR) of the signal, a new type of optical path structure of the laser re-injection method is adopted. Furthermore, the system performance can be effectively improved by using a new intervention method of injecting radio frequency (RF) white noise into a laser driver to suppress cavity mode noise and combining the wavelength modulation method (WMS). We compared the sensitivity of the second harmonic signal demodulation between the re-injection method and the standard OA-ICOS, and the SNR increased by 2.68 times compared to the latter. Analysis of the spectral measurements with Allan variance indicates that within an integration time of 1 s, the measurement accuracy of N2O, H2O, and CO is 6.71 ppb, 13.945 ppm, and 1.81 ppb, respectively, and within an integration time of 820 s, the measurement accuracy of N2O, H2O, and CO can be further improved to 1.26 ppb, 2.089 ppm, and 172 ppt, respectively. Our approach represents an underlying analytical method that provides guidelines for monitoring of representative gases in the atmosphere, industrial processes, emergency safety, etc.

Mid-infrared High-power Quantum Cascade Laser Grown by MOCVD

Mid-infrared High-power Quantum Cascade Laser Grown by MOCVD

Photoacoustic Heterodyne CO Sensor for Rapid Detection of CO Impurities in Hydrogen.

Hydrogen (H2) fuel cells have been developed as an environmentally benign, low-carbon, and efficient energy option in the current period of promoting low-carbon activities, which offer a compelling means to reduce carbon emissions. However, the presence of carbon monoxide (CO) impurities in H2 may potentially damage the fuel cell's anode. As a result, monitoring of the CO levels in fuel cells has become a significant area of research. In this paper, a novel photoacoustic sensor is developed based on photoacoustic heterodyne technology. The sensor combines a 4.61 μm mid-infrared quantum cascade laser with a low-noise differential photoacoustic cell. This combination enables fast, real-time online detection of CO impurity concentrations in H2. Notably, the sensor requires no wavelength locking to monitor CO online in real-time and produces a single effective signal with a period of only 15 ms. Furthermore, the sensor's performance was thoroughly evaluated in terms of detection sensitivity, linearity, and long-term stability. The minimum detection limit of 11 ppb was obtained at an optimal time constant of 1 s.

Terahertz molecular water laser using quantum cascade laser pumping

Molecular lasers pumped by quantum cascade laser (QCL) open new possibilities for THz generation and its numerous applications, in particular, for high resolution molecular spectroscopy. In this article, a THz water laser pumped by a mid-infrared QCL was demonstrated using the broad tunability of the pump laser. Twenty D2O laser lines were measured under a continuous wave pumping regime, in a spectral range expending from 63 to 177 cm−1 (1.9–5.3 THz), and with an output power ranging from tens to hundreds of μW. This letter contains a description of the experimental setup used to produce the THz laser radiation and a comparison of the measured output power with a molecular gain factor used to sort out the most favorable laser lines. In addition to the measured laser transitions, a complete list of laser frequencies together with their corresponding molecular gain is given in the supplementary material, for both H2O and D2O isotopologues excited in their bending and stretching vibrational states.

NH3 absorption line study and application near 1084.6 cm−1

Ammonia (NH3) holds significant importance as an industrial product, ambient trace pollutant, and a promising zero-carbon energy carrier, underscoring the critical need for accurate quantification in various applications. In this study, a mid-infrared quantum cascade laser (Mid-IR QCL) based tunable diode laser absorption spectroscopy (TDLAS) spectrometer was developed. It was coupled with two reactors, namely a gas cell and a shock tube and operated in three modes: low- and high-frequency (140 Hz and 10 kHz) scan modes and fixed-wavelength mode, tailored to the specific needs of each experimental scenario. The primary objects were NH3 line parameters measurement and absolute NH3 quantification. With the 140 Hz low-frequency scan mode, NH3-Ar pressure broadening coefficients of six transition lines near 1084.6 cm−1 were measured in a gas cell at 295 K and 1–11 mbar for the first time. These coefficients were subsequently employed to quantify the initial NH3 mole fraction in shock tube experiments, considering the substantial adsorption effect of NH3. Using the high-frequency scan mode (10 kHz), spectrally resolved NH3 absorption cross-sections were obtained in a shock tube across a pressure range of 1.46–3.25 bar and a temperature range of 1000–1800 K. This data, acquired for the first time, significantly contributes to the understanding of NH3 behavior in high-temperature environments. Additionally, potential interference from high-temperature water absorption near 1084.6 cm−1 was thoroughly examined. The high-temperature water absorption cross-section was measured over a pressure range of 0.99–3.02 bar and a temperature range of 1322–2905 K, utilizing a near-infrared water laser. Leveraging these comprehensive datasets, the study quantified the NH3 mole fraction during the NH3 oxidation process in the shock tube, employing the fixed-wavelength mode to achieve high time resolution. The quantified NH3 mole fraction was then compared to predictions from eleven recent chemical kinetic mechanisms, demonstrating the utility of the developed spectrometer in validating these mechanisms.

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.

Urban open-air chemical sensing using a mobile quantum cascade laser dual-comb spectrometer

Detection of airborne chemical releases in densely populated urban environments requires precise sensors with high temporal and spatial resolution capable of covering large areas. For this purpose, we present a mobile mid-infrared quantum cascade laser dual-comb spectrometer for identification and quantification of chemical plumes. Field tests with the remote sensor were conducted during daytime in the downtown Boston area over a five day period during which chemical releases were simulated by intermittently emitting non-toxic substances. Open-air sensing was performed with retroreflectors positioned at up to 230 m distance and with sensitivities in the ppm m range for one second of averaging time. The field campaign demonstrates a step toward a semiconductor dual-comb spectroscopic sensor in the mid-infrared fingerprint region, suitable for long-term deployments. These types of sensors will be valuable complements to existing optical sensors for urban hazardous gas leak monitoring, air quality assessments, and localization of clandestine chemical production.

Frequency combs induced by optical feedback and harmonic order tunability in quantum cascade lasers

This study investigates the interaction between frequency combs and optical feedback effects in Quantum Cascade Lasers (QCLs). The theoretical analysis reveals new phenomena arising from the interplay between comb generation and feedback. By considering the bias current corresponding to free-running single mode emission, the introduction of optical feedback can trigger the generation of frequency combs, including both fundamental and harmonic combs. This presents opportunities to extend the comb region and generate harmonic frequency combs with different orders through optimization of external cavity parameters, such as losses and length. Furthermore, this study demonstrates that optical feedback can selectively tune the harmonic order of a pre-existing free-running comb by adjusting the external cavity length, particularly for feedback ratios around 1%, which are readily achievable in experimental setups. Under strong feedback conditions (Acket parameter C > 4.6), mixed states emerge, displaying the features of both laser and external cavity dynamics. While this study is predominantly centered on terahertz QCLs, we have also confirmed that the described phenomena occur when utilizing mid-infrared QCL parameters. This work establishes a connection between comb technology and the utilization of optical feedback, providing new avenues for exploration and advancement in the field. In fact, the novel reported phenomena open a pathway toward new methodologies across various domains, such as the design of tunable comb sources, hyperspectral imaging, multi-mode coherent sensing, and multi-channel communication.

Mid-infrared photocurrent nano-spectroscopy exploiting the thermoelectric effect in graphene

We develop a conductive-atomic force microscope coupled to a mid-infrared (IR) quantum cascade laser that allows to measure mid-IR photocurrent maps and spectra with nanometric spatial resolution. Here, we exploit the photo-induced thermoelectric effect of graphene to obtain the mid-IR local absorption spectra of nanoscale devices and materials. By scanning the probe in AFM-contact mode, the thermoelectric photocurrent can be mapped throughout the graphene layer, which acts as a semitransparent transducer. At zero voltage bias applied to the probe tip, we observe very low noise (about 25 pA/Hz), which makes our technique competitive with others in measuring the local IR absorption in nanomaterials with insufficient thermal expansion coefficient, or to characterize nanoscale devices that significantly absorb IR radiation, such as mid-infrared photodetectors.

Ppb-level NH3 photoacoustic sensor combining a hammer-shaped tuning fork and a 9.55 µm quantum cascade laser

We present a quartz enhanced photoacoustic spectroscopy (QEPAS) gas sensor designed for precise monitoring of ammonia (NH3) at ppb-level concentrations. The sensor is based on a novel custom quartz tuning fork (QTF) with a mid-infrared quantum cascade laser emitting at 9.55 µm. The custom QTF with a hammer-shaped prong geometry which is also modified by surface grooves is designed as the acoustic transducer, providing a low resonance frequency of 9.5 kHz and a high-quality factor of 10263 at atmospheric pressure. In addition, a temperature of 50 °C and a large gas flow rate of 260 standard cubic centimeters per minute (sccm) are applied to mitigate the adsorption and desorption effect arising from the polarized molecular of NH3. With 80-mW optical power and 300-ms lock-in integration time, the detection limit is achieved to be 2.2 ppb which is the best value reported in the literature so far for NH3 QEPAS sensors, corresponding to a normalized noise equivalent absorption coefficient of 1.4 × 10−8 W cm−1 Hz−1/2. A five-day continuous monitoring for atmospheric NH3 is performed, verifying the stability and robustness of the presented QEPAS-based NH3 sensor.

Photonic molecule based on coupled ring quantum cascade lasers

Photonic molecules—particular systems composed of coupled optical resonators—emulate the behavior of complex physical systems exhibiting discrete energy levels. In this work, we present a photonic molecule composed of two strongly coupled, mid-infrared ring quantum cascade lasers. We explore both experimentally and numerically the key features of the photonic molecule such as the energy level splitting of bonding and antibonding supermodes. Due to the large size of the resonators, the energy splitting results in bands containing tens of modes. Each of these modes is furthermore doubly degenerate with respect to the direction of propagation, namely, clockwise and counterclockwise. We explore several methods to carefully break these symmetries of the system in a controlled manner by introducing spatial and temporal asymmetries in the pumping scheme of the ring lasers. By employing these techniques, we achieve a high degree of precision in the dynamic control of the photonic molecule. Owing to their inherent suitability for on-chip integration, this class of devices may enable applications as varied as mid-infrared sensors or a rich playground for studying non-Hermitian photonics and quantum optics with quantum cascade lasers.

Efficient strain-modified improved nonparabolic-band energy dispersion model that considers the effect of conduction band nonparabolicity in mid-infrared quantum cascade lasers.

Intersubband polar-optical-phonon (POP) scattering plays an important role in determining the population inversion and optical gain of mid-infrared (mid-IR) quantum cascade lasers (QCLs). In particular, the nonparabolicity of the conduction band (CB) significantly affects the energy dispersion relation and intersubband POP scattering time. However, the currently used parabolic-band (PB) and nonparabolic-band (NPB) energy dispersion models are not appropriate for mid-IR QCLs because they are unsuitable for high electron wave vectors and do not consider the effect of applied strain on the energy dispersion relation of the CB. The eight-band k·p method can provide a relatively accurate nonparabolic energy dispersion relation for high electron wave vectors but has the disadvantages of high computational complexity and spurious solutions to be discarded. Consequently, we propose a strain-modified improved nonparabolic-band (INPB) energy dispersion model that has no spurious solution and acceptable accuracy, compared to the eight-band k·p method. To demonstrate the accuracy and efficiency of our proposed INPB model compared with those of the PB, NPB, and eight-band k·p models, we calculate the energy dispersion relations and intersubband POP scattering times in a strain-compensated QCL with a lasing wavelength of 3.58 µm. Calculation results reveal that our proposed model is almost as accurate as the eight-band k·p model; however, it enables much faster calculations and is free from spurious solutions.

Phase-matching in terahertz quantum cascade laser sources based on Cherenkov difference-frequency mixing

Terahertz quantum cascade laser sources based on intra-cavity Cherenkov difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers are currently the only monolithic semiconductor laser technology that can deliver continuous-wave coherent terahertz output at room temperature. Because the Cherenkov difference-frequency generation process enables terahertz radiation generation and extraction across a wide range of frequencies, it is often assumed that phase-matching conditions for this process are automatically fulfilled. We theoretically analyze and experimentally demonstrate that phase-matching plays an important role in these devices, and significant improvements in terahertz power output can be achieved by adjusting the waveguide configuration of the quantum cascade lasers to provide better phase-matching.

Analytical Method Development Using Quantum Laser Cascade Spectroscopy with Diffuse and Attenuated Total Reflectance for Determining Low Concentrations of Active Pharmaceutical Ingredients

Quantum Cascade Laser Spectroscopy (QCLS) will quantify acetaminophen as an active pharmaceutical ingredient in different low concentrations formulations in tablet presentation. Tablets contain acetaminophen in nine blends ranging from 0.0% to 3.0% w/w, with mannitol, croscarmellose, cellulose, and magnesium stearate, as excipients. The tablets were analyzed in non-contact mode by mid-infrared attenuated total reflectance and diffuse reflectance backscattering. Measurements were conducted covering the spectral range 770–1890 cm-1. Calibrations were generated by applying multivariate analysis using principal component analysis. The high power of the quantum cascade laser-based spectroscopic system attached to attenuated total reflectance and diffuse reflectance backscattering resulted in the design of discrimination methodologies for pharmaceutical applications with acetaminophen as an active pharmaceutical ingredient in the formulation. The main conclusion is that attenuated total reflectance is better for other analyses. For tablet analysis using mid-infrared quantum cascade lasers, diffuse reflectance backscattering is more accurate for predicting the API content. QCLS is gaining even more acceptance as a valuable tool in Process Analytical Technology.