Operation Of Quantum Cascade Lasers Research Articles

Index-coupled surface porous grating distributed feedback quantum cascade laser

We report design of special low loss, index-coupled surface porous grating distributed feedback profile by using holographic lithography combined with electro-chemical etching technology. Room temperature continuous-wave (cw) operation of single-mode quantum cascade lasers emitting at λ ∼ 7.6 μm has been achieved. Due to attenuate interplay between the surface plasmon waves and the waveguide waves, the waveguide loss of the lasers is significantly reduced, when compared to conventional surface metal/semiconductor grating devices. This resulted in an improved overall laser performance, such as a reduction of the threshold current density and an increase in cw output power.

Single-mode quantum cascade lasers employing asymmetric Mach-Zehnder interferometer type cavities

We employ properly designed asymmetric Mach-Zehnder interferometer structures as effective wavelength filters and monolithically integrate them in conventional Fabry-Perot cavities to facilitate single-mode operation of the lasers. With such asymmetric Mach-Zehnder interferometer type laser cavities, continuously tunable single-mode operation of quantum cascade (QC) lasers is achieved in pulsed mode from 80 K up to room temperature and in continuous-wave mode with side-mode suppression ratio up to ∼35 dB. These devices are fabricated with the same process as simple ridge lasers, therefore providing a promising solution to achieving more cost-effective single-mode QC lasers.

On the active conductivity of a three-barrier resonant-tunneling structure and optimization of quantum cascade laser operation

Within the context of the model of effective masses and rectangular potentials, the theory of active electronic conductivity is proposed for a three-barrier resonant-tunneling structure in a dc electric field, as the active element of a quantum cascade laser. It is shown that the chosen geometrical parameters of the active regions in the first experimentally fabricated quantum cascade lasers are most optimal and the experimental and theoretical values of the radiation energies correlate with each other with an accuracy of up to a few percent.

Resonant-phonon depopulation terahertz quantum cascade lasers and their application in spectroscopic imaging

The terahertz (THz) frequency quantum cascade laser (QCL) is a semiconductor heterostructure laser that has attracted much research interest over the past decade. We report on the high performance of THz QCLs based on a three-well resonant-phonon (RP) depopulation active region (AR) and operating in the frequency range 2.7 THz to 4.0 THz. Devices, processed into surface-plasmon waveguides, lased up to 116 K in pulsed mode with threshold current densities as low as 840 Acm−2. The effects of the design frequency and laser cavity length on performance are discussed. We also report on the operation of QCLs with reduced AR thicknesses, and show, for the first time, that the AR thickness of RP QCLs processed in a surface plasmon waveguide can be reduced to as little as 5 µm. Finally, we demonstrate the use of an electrically tuneable THz QCL, based on a heterogeneous AR, for spectroscopic imaging of the high-explosive pentaerythritol tetranitrate.

High-Power High-Temperature Continuous-Wave Operation of Quantum Cascade Laser at λ∼4.6 μm without Lateral Regrowth

High-power quantum cascade lasers (λ = 4.6 μm) working in continuous wave (cw) up to 90°C are presented. The material was grown by solid-source molecular beam epitaxy and processed into narrow conventional ridge geometry without lateral regrowth. High cw output power of 850mW at 10°C and more than 200mW at 90°C were obtained with threshold current densities of 1.34 and 2.47 kA/cm2, respectively, for a high-reflectivity-coated 12-μm-wide and 3-mm-long laser.

Improved performance of quantum cascade laser with porous waveguide structure

Quantum cascade laser (QCL) with porous structure is reported in this work. The porous structure is fabricated onto the wafer top surface towards the sides of the QCL waveguide by electro-chemical etching. The pore size of the porous structure is in hundreds of nanometers. Experimental results show that QCL with porous structure exhibits better beam quality. The far field of the typical 60 μm-ridge-width QCL is modulated from double-lobed shape to single-lobed shape due to the utilizing of porous structure. The full width at half maximum is 5.26°, which agrees very well with that of fundamental transverse mode. An interesting spectra behavior is also observed. The QCL with porous structure represents a temperature tuning rate of 0.49 nm/K, which is comparable to distributed feedback QCL.

Performance optimization for terahertz quantum cascade laser at higher temperature using genetic algorithm

The aim of this work is to establish an approach for obtaining improved design parameters for high temperature operation of terahertz quantum cascade lasers using a multi-objective evolutionary algorithm. For studying the lasing conditions of a quantum cascade laser, a self-consistent model is adopted. This model uses standard wave function approximation and effective mass approximation with relevant scattering mechanisms to solve Schrodinger’s equation for the cascaded quantum wells. Fermi’s Golden Rule is then used to calculate the corresponding lifetime of each eigen states. To describe the coherent evolution of wave functions and phase breaking, density matrix formalism is employed. Subsequently, laser rate equations are used for calculating the parameters related to electronic transport in the device. These parameters are then utilized for investigating the temperature dependence of existing terahertz quantum cascade lasers. Finally, using an optimization technique based on Genetic algorithm, design parameters for resonant-phonon quantum cascade laser are obtained within the terahertz frequency range. The results illustrate that this optimization process can offer improvement in the performance of quantum cascade lasers in terahertz region at an elevated temperature. Moreover, the results also reveal that significant increase in operating temperature of a resonant phonon terahertz QCL is unlikely and hence novel design approaches should be considered for operating THz QCLs at room temperature.

High temperature operation of short wavelength InAs-based quantum cascade lasers

InAs/AlSb quantum cascade lasers emitting at 3.06 and 3.22 μm at room temperature has been studied. The lasers with high reflection coating on back facets operated in pulse mode up to 400 and 423 K, respectively. The obtained results showed no dramatic performance degradation of the InAs-based QCLs with decreasing emission wavelength down to 3 μm.

Linewidth of a quantum-cascade laser assessed from its frequency noise spectrum and impact of the current driver

We report on the measurement of the frequency noise properties of a 4.6-μm distributed-feedback quantum-cascade laser (QCL) operating in continuous wave near room temperature using a spectroscopic set-up. The flank of the R(14) ro-vibrational absorption line of carbon monoxide at 2196.6 cm−1 is used to convert the frequency fluctuations of the laser into intensity fluctuations that are spectrally analyzed. We evaluate the influence of the laser driver on the observed QCL frequency noise and show how only a low-noise driver with a current noise density below ${\approx} 1~\mbox{nA/}\sqrt{}\mbox{Hz}$ allows observing the frequency noise of the laser itself, without any degradation induced by the current source. We also show how the laser FWHM linewidth, extracted from the frequency noise spectrum using a simple formula, can be drastically broadened at a rate of ${\approx} 1.6~\mbox{MHz/}(\mbox{nA/}\sqrt{}\mbox{Hz})$ for higher current noise densities of the driver. The current noise of commercial QCL drivers can reach several $\mbox{nA/}\sqrt{}\mbox{Hz}$ , leading to a broadening of the linewidth of our QCL of up to several megahertz. To remedy this limitation, we present a low-noise QCL driver with only $350~\mbox{pA/}\sqrt{}\mbox{Hz}$ current noise, which is suitable to observe the ≈550 kHz linewidth of our QCL.

Theoretical analysis of free carrier absorption in the cavity of a quantum cascade laser

AbstractIn this work we analyze free carrier absorption (FCA) and polarization ratio (transversality degree) for eigenmodes of a quantum cascade laser (QCL) waveguide. We consider the dielectric function and conductivity of the waveguide core and cladding layers within the Drude–Lorentz approximation. We show that the entire spectrum of a QCL cavity consists of three kinds of eigenmodes: volume, surface, and Langmuir modes. We perform an analytical analysis and numerical calculations of FCA and polarization ratio for each type of the eigenmodes within a wide frequency range from the microwave up to the ultraviolet spectrum. We make a comparative analysis of FCA in the cladding layers and waveguide core. We specify frequency intervals where absorption in the core or in the cladding layers is dominant. Identification of the most favorable modes for lasing is carried out for each part of the spectrum. So, we identify that the main Langmuir mode is the most favorable mode for the lasing at the long‐wave edge of the terahertz (THz) region: (i) it has no frequency cutoff and can be excited at arbitrarily low frequency, (ii) it is nearly transversal that is very favorable for the QCL operation, and (iii) it is almost totally confined within the waveguide core. The model analyzed is directly related to one‐dimensional photonic crystals and metamaterials consisting of alternating anisotropic layers.

A Circuit Model of Quantum Cascade Laser Based on a Numerical Analysis

Based on the dynamic analysis and numerical computation of quantum cascade laser(QCL), a simple equivalent circuit model of QCL was established, in which we considered the spontaneous emission effect for QCL's start delay. The direct current(DC), transient and frequency response characteristics of QCL were obtained by means of the circuit simulation of PSPICE program, and some relevant parameters that may affect the QCL performance have been analyzed. The results of simulation are consistent with that of numerical calculation in previous literatures.

Self-consistent thermal simulation of GaAs/Al0.45Ga0.55As quantum cascade lasers

This paper presents a self-consistent thermal model for quantum cascade lasers (QCLs) that takes into account the nonuniform heat generation distribution in the active region as well as the temperature dependences of the heat generation rate and thermal conductivity. The model extracts the heat generation rate from the electron-optical phonon scattering recorded during the ensemble Monte Carlo (EMC) simulation of electron transport in a single QCL stage at different temperatures. The extracted heat generation rate, in conjunction with temperature-dependent thermal conductivities, enables us to solve the nonlinear heat diffusion equation in a self-consistent manner. The model is used to investigate the cross-plane temperature distribution throughout a 9.4 μm infrared GaAs-based QCL. The nonlinear effects stemming from the temperature dependence of thermal conductivity and the heat generation rate are studied. Finally, the accuracy of using the equivalent uniform heat source with the total power obtained from experiments to model the thermal performance of QCLs is evaluated and discussed.

Broad gain in a bound-to-continuum quantum cascade laser with heterogeneous active region

We demonstrate the operation of heterogeneous terahertz quantum cascade lasers with broadened gain by optimising the sub-stacks to align at the same field. In single plasmon waveguides, we find two-colour operation for nearly the entire dynamic range of the lasers with similar performance to homogeneous lasers. Time domain spectroscopy measurements confirm that a flat gain spectrum is present and the sub-stacks align at the same time. When incorporated into metal-metal waveguides, we find that performance is consistent with the constituent sub-stacks and there is broadband operation over 380 GHz.

Third-order photonic-crystal distributed-feedback quantum cascade lasers

We demonstrate room-temperature operation of broad-area edge-emitting photonic-crystal distributed-feedback quantum cascade lasers at λ∼4.6μm. The lasers use a weak-index perturbed third-order photonic-crystal lattice to control the optical mode in the wafer plane. Utilizing this coupling mechanism, the near-diffraction-limited beam quality with a far-field profile normal to the facet can be obtained. Single-mode operation with a signal-to-noise ratio of about 20dB is achieved in the temperature range of 85–290K. The single-facet output power is above 1W for a 55μm×2.5mm laser bar at 85K in pulsed mode.

Low-Threshold Continuous-Wave Operation of Distributed-Feedback Quantum Cascade Laser at $\lambda \sim 4.6\ \mu$m

Room-temperature continuous-wave (CW) operation of distributed-feedback quantum cascade laser (DFB-QCL) emitting at 4.6 μm is presented. Strain-compensation combined with two-phonon resonance in active region design promises low-threshold current density. A record low-threshold current density of 1 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for DFB-QCL operated in CW mode at 30°C is realized without a high-reflectivity coating on the rear facet. Reliable dynamic single-mode emission with a sidemode suppression ratio of about 30 dB is achieved from 10 to 50°C.

Room-Temperature GaAs/AlGaAs Quantum Cascade Lasers Grown by Metal–Organic Vapor Phase Epitaxy

We demonstrate λ ~ 9 μm GaAs/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.45</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.55</sub> As quantum cascade lasers (QCLs) operating up to 320 K. Metal- organic vapor phase epitaxy has been used throughout for the growth of the devices. Detailed comparison has been carried out for the QCLs with various waveguides and grown on (100) GaAs substrates with different miscut angles towards (111)A. Introduction of InGaP cladding layers into the optical waveguide significantly improves the QCL performance due to a better optical confinement and lower waveguide losses compared with the GaAs-based waveguide. A 20- μm-wide 4-mm-long device with high reflectivity coating on the laser back facet demonstrates room-temperature pulsed output power of 220 mW and a lowered threshold current density of 6.5 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

A CO 2 sensor based upon a continuous-wave thermoelectrically-cooled quantum cascade laser

A CO 2 sensor based upon a continuous-wave thermoelectrically-cooled distributed feedback quantum cascade laser operating between 2305 and 2310 cm −1 and a 54.2 cm long optical cell has been developed. Two approaches for direct absorption spectroscopy have been evaluated and applied for monitoring of the CO 2 concentration in gas lines and ambient laboratory air. In the first approach optical transmittance was derived from the single channel laser intensity, whilst in the second approach a ratio of signal and reference laser intensities (balanced detection) was used. The optimum residual absorption standard deviation was estimated to be 1.9 × 10 −4 for 100 averages of 1 ms duration and 0.1 cm −1 scans over the P(46) CO 2 absorption line of the ν 3 vibrational band at 2306.926 cm −1. A CO 2 detection limit (1 standard deviation) of 36 ppb was estimated for 0.1 s average and balanced detection.

Extremely sensitive detection of NO_2 employing off-axis integrated cavity output spectroscopy coupled with multiple-line integrated absorption spectroscopy

We report on the development of a new sensor for NO₂ with ultrahigh sensitivity of detection. This has been accomplished by combining off-axis integrated cavity output spectroscopy (OA-ICOS) (which can provide large path lengths of the order of several kilometers in a small volume cell) with multiple-line integrated absorption spectroscopy (MLIAS) (where we integrate the absorption spectra over a large number of rotational-vibrational transitions of the molecular species to further improve the sensitivity). Employing an external cavity quantum cascade laser operating in the 1601-1670 cm⁻¹ range and a high-finesse optical cavity, the absorption spectra of NO₂ over 100 transitions in the R band have been recorded. From the observed linear relationship between the integrated absorption versus concentration of NO₂ and the standard deviation of the integrated absorption signal, we report an effective sensitivity of detection of approximately 28 ppt (parts in 10¹²) for NO₂ To the best of our knowledge, this is among the most sensitive levels of detection of NO₂ to date.

Coherent sampling of active mode-locked terahertz quantum cascade lasers and frequency synthesis

Researchers demonstrate a terahertz quantum cascade laser operating in a regime of active mode-locking by modulating its bias current with a radiofrequency synthesizer. This technique allows coherent sampling of the terahertz electric field as well as control over the laser's carrier–envelope phase shift.

Modelling of optical Kerr effects on the static and dynamic behaviors of quantum cascade laser

The effect of optical Kerr nonlinearity on the static and dynamic behaviors of quantum cascade laser operating in the mid-infrared is theoretically investigated. Our model is based on three-level rate equations including the dependence of the loss on photon number in the cavity. The optical stability domain that allows for the determination of current injection is derived within the premises of our model. The analytical solutions of eigenvalue equation allowing the investigation of stability analysis are obtained. Furthermore, nonlinear effects influence significantly the dynamics of photons in cavity and electrons in the upper laser level.