Emitting Quantum Cascade Lasers Research Articles

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 Lasers Grown by Metalorganic Chemical Vapor Deposition on Foreign Substrates with Large Surface Roughness

The surface morphology of a buffer template is an important factor in the heteroepitaxial integration of optoelectronic devices with a significant lattice mismatch. In this work, InP-based long-wave infrared (~8 µm) emitting quantum cascade lasers with active region designs lattice-matched to InP were grown on GaAs and Si substrates employing InAlGaAs step-graded metamorphic buffer layers, as a means to assess the impact of surface roughness on device performance. A room-temperature pulsed-operation lasing with a relatively good device performance was obtained on a Si template, even with a large RMS roughness of 17.1 nm over 100 µm2. Such results demonstrate that intersubband-operating devices are highly tolerant to large RMS surface roughness, even in the presence of a high residual dislocation density.

Surface Emitting Quantum-Cascade Lasers with a Second-Order Grating and Elevated Coefficient of Coupling

A study is performed of features of surface-emitting quantum-cascade lasers with a ring resonator and second-order diffraction grating. The coefficient of coupling can be raised to 12 cm−1 by selecting the modes for direct ion lithography etching. Lasing is done near 7.6 μm with a threshold current density of only 0.3 kA/cm2.

Super-resolution infrared microspectroscopy reveals heterogeneous distribution of photosensitive lipids in human hair medulla

Human hair medulla chemical composition appears mostly homogenous when mapped by FTIR microspectroscopy even when using a synchrotron radiation source (SR-μFTIR) but it is expected to be heterogeneous. We performed sub-micron chemical mapping of hair cortex and medullas using Optical Photothermal Infrared microspectroscopy (OPTIR) and a mid-infrared Quantum Cascade Laser (QCL) source covering the fingerprint and the CH stretching region. Photodamages were observed in the hair cortex at mild laser power and occurred in the hair medulla even at the lowest power settings of the IR QCL pulsed at 100 kHz rate (4 μW/μm2 average power density) and visible probe laser (200 μw/μm2 average power density). Photoconversion of calcium carboxylates in other molecules, possibly sodium carboxylates, was observed. Attenuation of the IR QCL power by 40% using ZnSe filter and/or high-speed measurements (1000 cm−1/s) succeeded in almost completely eliminating the photodamages and photoconversion. OPTIR maps and images showed that the medullas were highly heterogeneous at the submicron scale. We found calcium carboxylates, aliphatic lipids and wax esters in small units, hundreds of nanometers in size. The 1470 cm−1 CO sym stretching peak of calcium carboxylates and the CH2asym stretching peak from aliphatic lipids proved to be the most efficient peaks to track the distribution of these molecules. OPTIR had enough sensitivity to map accurately only the strongest peaks from lipids and calcium carboxylates, weaker peaks such as the ester CO and sulfoxide SO bands were not accurately detected by OPTIR even when they were shown to be present by SR-μFTIR. Quantification of the medulla components by OPTIR is difficult due to several factors: discontinuous QCL emission, and noise. The weaker peaks such as CH3, CO, SO are often underestimated or not detected. We demonstrate here that OPTIR can be used to measure, map and image dark, photosensitive samples using very low IR power.

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.

Quantum Cascade Lasers: Controlling and Phase‐Locking a THz Quantum Cascade Laser Frequency Comb by Small Optical Frequency Tuning (Laser Photonics Rev. 15(6)/2021)

In article number 2000417, Luigi Consolino and co-workers demonstrate a novel degree of freedom for controlling the emission of quantum cascade lasers (QCLs). It is based on small optical frequency tuning (SOFT) by means of a white LED. The slightly perturbative regime accessible in these conditions allows tweaking the semiconductor complex refractive index. The resulting additional tuning knob is capable of phase stabilizing the modes spacing of a QCL comb.

Terahertz Frequency Combs: Chip‐Scale Terahertz Frequency Combs through Integrated Intersubband Polariton Bleaching (Laser Photonics Rev. 15(6)/2021)

In article number 2000575, Miriam S. Vitiello and co-workers report on an integrated architecture for the generation of high power terahertz (THz) frequency combs (FCs) comprising an ultrafast THz polaritonic reflector, exploiting intersubband cavity polaritons, and a broad bandwidth heterogeneous THz quantum cascade lasers (QCL). It is shown that the polaritonic reflector induces spectral reshaping of the QCL emission and stable FC operation over 38% of the laser operational range.

Mid-infrared homodyne balanced detector for quantum light characterization.

We present the characterization of a novel balanced homodyne detector operating in the mid-infrared. The challenging task of revealing non-classicality in mid-infrared light, e. g. in quantum cascade lasers emission, requires a high-performance detection system. Through the intensity noise power spectral density analysis of the differential signal coming from the incident radiation, we show that our setup is shot-noise limited. We discuss the experimental results with a view to possible applications to quantum technologies, such as free-space quantum communication.

Controlling and Phase‐Locking a THz Quantum Cascade Laser Frequency Comb by Small Optical Frequency Tuning

AbstractFull phase control of terahertz (THz)‐emitting quantum cascade laser (QCL) combs has recently been demonstrated, opening new perspectives for even the most demanding applications. In this framework, simplifying the set‐ups for control of these devices will help to accelerate their spreading in many fields. This study reports a new way to control the emission frequencies of a THz QCL comb by small optical frequency tuning (SOFT), using a very simple experimental setup, exploiting the incoherent emission of an ordinary white light‐emitting diode. The slightly perturbative regime accessible in these conditions allows tweaking the complex refractive index of the semiconductor without destabilizing the broadband laser gain. The SOFT actuator is characterized and compared to another actuator, the QCL driving current. The suitability of this additional degree of freedom for frequency and phase stabilization of a THz QCL comb is shown and perspectives are discussed.

Beam stability of buried-heterostructure quantum cascade lasers employing HVPE regrowth

Measurements of beam stability for mid-infrared (IR)-emitting quantum cascade lasers (QCLs) are important for applications that require the beam to travel through air to remote targets, such as free-space communication links. We report beam-quality measurement results of narrow-ridge, 4.6 µm-emitting buried-heterostructure (BH) QCLs fabricated using ICP etching and HVPE regrowth. Beam-quality measurements under QCW operation exhibit M2 < 1.2 up to 1 W for ∼5 µm-wide ridges. 5 µm-wide devices display some small degree of centroid motion with increasing output power (< 0.125 mrad), which corresponds to a targeting error of ∼1.25 cm over a distance of 100 m.

External cavity terahertz quantum cascade laser with a metamaterial/graphene optoelectronic mirror

Photonic engineering of the terahertz emission from a quantum cascade laser (QCL) is fundamental for the exploitation of this unique source in a myriad of applications where it can be implemented, such as spectroscopy, imaging, and sensing. Active control of the frequency, power, polarization, and beam profile has been achieved through a variety of approaches. In particular, the active control of the emitted frequency, which is difficult to determine a priori, has been achieved through the integration of a photonic structure and/or by using external cavity arrangements. In this work, an external cavity arrangement, which implements a metamaterial/graphene optoelectronic mirror as an external feedback element, is proposed and demonstrated. The reflectivity and dispersion properties of the external active mirror were tuned via electrostatically gating graphene. It was possible to electronically reproduce the mode-switch occurring in a QCL emitting ∼2.8 THz by mechanically changing the external cavity length formed by an Au mirror. The external cavity arrangement was investigated and described in the framework of the self-mixing theory. These results open a way for the all-electronic engineering of the QCL emission by the use of a fast reconfigurable external mirror. This approach can uniquely address both power and frequency control, with ∼100 MHz reconfiguration speeds, using an integrated external element. Furthermore, the metamaterial/graphene mirror's strong dispersive properties might be implemented for the active mode locking of THz QCLs. Finally, this approach offers a unique opportunity to study the laser dynamics and mode competition in THz QCLs in the self-mixing feedback regime.

Spectral Shift of Quantum-Cascade Laser Emission under the Action of Control Voltage

Spectral redistribution of the intensity of short- and long-wavelength emission components within gain bandwidth of a 7- to 8-μm quantum-cascade laser under the action of control voltage is demonstrated. As the voltage was increased from 10.5 to 18.2 V, the wavelength of maximum laser emission intensity shifted by approximately 200 nm. The maximum bandwidth of laser gain was about 300 nm (at a temperature of 80 K). The quantum-cascade laser heterostructure was grown by molecular beam epitaxy. The laser active region design was based on double-phonon depopulation of the lower level as implemented on In0.53Ga0.47As/In0.52Al0.48As heteropair of solid alloys lattice-matched with an InP substrate.

Dual-Gas Quartz-Enhanced Photoacoustic Sensor for Simultaneous Detection of Methane/Nitrous Oxide and Water Vapor.

The development of a dual-gas quartz-enhanced photoacoustic (QEPAS) sensor capable of simultaneous detection of water vapor and alternatively methane or nitrous oxide is reported. A diode laser and a quantum cascade laser (QCL) excited independently and simultaneously both the fundamental and the first overtone flexural mode of the quartz tuning fork (QTF), respectively. The diode laser targeted a water absorption line located at 7181.16 cm-1 (1.392 μm), while the QCL emission wavelength is centered at 7.71 μm and was tuned to target two strong absorption lines of methane and nitrous oxide, located at 1297.47 and 1297.05 cm-1, respectively. Two sets of microresonator tubes were positioned, respectively, at the antinode points of the fundamental and the first overtone flexural modes of the QTF to enhance the QEPAS signal-to-noise ratio. Detection limits of 18 ppb for methane, 5 ppb for nitrous oxide and 20 ppm for water vapor have been achieved at a lock-in integration time of 100 ms.

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.

Experimental study of stimulated emission of quantumcascade lasers on an InP substrate

The results of the experimental study of stimulated emission of a quantum-cascade laser with a wavelength of 7.6 μm are presented in this paper. The dependence of the intensity of laser radiation on the duty cycle was obtained. The linearity of this dependence was maintained up to a duty cycle of 40%, thus achieving the quasi-continuous mode of operation of the quantum cascade laser at 110 K.

Versatile mid-infrared frequency-comb referenced sub-Doppler spectrometer

We present a mid-IR high-precision spectrometer capable of performing accurate Doppler-free measurements with absolute calibration of the optical axis and high signal-to-noise ratio. The system is based on a widely tunable mid-IR offset-free frequency comb and a Quantum-Cascade-Laser (QCL). The QCL emission frequency is offset locked to one of the comb teeth to provide absolute-frequency calibration, spectral-narrowing, and accurate fine frequency tuning. Both the comb repetition frequency and QCL-comb offset frequency can be modulated to provide, respectively, slow- and fast-frequency-calibrated scanning capabilities. The characterisation of the spectrometer is demonstrated by recording sub-Doppler saturated absorption features of the CHF3 molecule at around 8.6 μm with a maximum signal-to-noise ratio of ∼7 × 103 in 10 s integration time, frequency-resolution of 160 kHz, and accuracy of less than 10 kHz.

Multi-Terahertz Sideband Generation on an Optical Telecom Carrier with a Quantum Cascade Laser

Quantum cascade lasers (QCLs) are the principal semiconductor light sources for the mid-infrared (MIR) and terahertz (THz) spectral regions. However, up-converting their emission to the technologically mature telecom spectral region (1.3 to 1.6 μm) remains an important goal for applications. This would permit new QCL functionalities to be realized, such as stabilization and injection locking of the QCL emission to precise and low-cost telecom frequency combs for spectroscopy. In this work, we demonstrate the up-conversion of the QCL emission to the telecom band at room temperature, using the QCLs’ inherent resonant optical nonlinearities that reduce the constraints of phase matching. This is based on the generation of multi-THz sidebands on a telecom carrier, where a low power telecom beam at 1.55 μm is injected into the cavity of an InP-based MIR QCL (9 μm, 33 THz), giving rise to the nonlinear sum frequency at 1.3 μm. The results are supported by a theoretical model that highlights the giant enhancement...

Coherent detection of THz laser signals in optical fiber systems

Terahertz (THz) coherent detectors are crucial for the stabilization and measurement of the properties of quantum cascade lasers (QCLs). This paper describes the exploitation of intra-cavity sum frequency generation to up-convert the emission of a THz QCL to the near infrared for detection with fiber optic coupled components alone. Specifically, a low cost infrared photodiode is used to detect a radio frequency (RF) signal with a signal-to-noise ratio of approximately 20dB, generated by beating the up-converted THz wave and a near infrared local oscillator. This RF beat note allows direct analysis of the THz QCL emission in time and frequency domains. The application of this technique for QCL characterization is demonstrated by analyzing the continuous tuning of the RF signal over 2 GHz, which arises from mode tuning across the QCL's operational current range.

Spectrally resolved modal characteristics of leaky-wave-coupled quantum cascade phase-locked laser arrays

The modal characteristics of nonresonant five-element phase-locked arrays of 4.7-μm emitting quantum cascade lasers (QCLs) have been studied using spectrally resolved near- and far-field measurements and correlated with results of device simulation. Devices are fabricated by a two-step metal-organic chemical vapor deposition process and operate predominantly in an in-phase array mode near threshold, although become multimode at higher drive levels. The wide spectral bandwidth of the QCL’s core region is found to be a factor in promoting multispatial-mode operation at high drive levels above threshold. An optimized resonant-array design is identified to allow sole in-phase array-mode operation to high drive levels above threshold, and indicates that for phase-locked laser arrays full spatial coherence to high output powers does not require full temporal coherence.

Nonlinear dynamics investigation in few-cycle laser seeding of quantum cascade lasers: role of permanent dipole moment

The ultrafast dynamics in the few-cycle laser seeding of quantum cascade laser (QCL) is numerically investigated via the exact solution of the full-wave Maxwell-Bloch equations. It is found that, with or without taking permanent dipole moment (PDM) into account, the QCL emission is quite different: beyond the fundamental frequency band, additional high and low bands occur for that with PDM, which forms an ultra-broad quasi-comb. The origin for this is closely related to the generation of second order harmonic and direct-current components as a result of PDM breaking down the parity symmetry. Moreover, the carrier-envelope-phase (CEP) of laser seed is locked to the QCL output, no matter with or without PDM, and this phase controlled QCL maybe has more wide and convenient applications in related fields.