High-performance Quantum Cascade Lasers Research Articles

Millimeter-wave generation with a room-temperature nonlinear quantum cascade laser

Millimeter-wave difference frequency generation is reported for a dual-wavelength mid-infrared quantum cascade laser operating at room temperature. To overcome a low mid-infrared-to-terahertz conversion efficiency below 1 THz, a long-wavelength, high-performance mid-infrared quantum cascade laser structure with higher nonlinear susceptivity is adopted. By designing the efficient allocation of mid-infrared pumps to two sections of fabricated distributed feedback grating, a closely separated dual-wavelength (λ1 ∼ 13.53 μm and λ2 ∼ 13.39 μm) laser oscillation was obtained. Consequently, a millimeter-wave emission at a frequency of 231 GHz was successfully observed at room temperature.

Quantum cascade lasers monolithically integrated on germanium.

Silicon (Si) photonics can have a major impact on the development of mid-IR photonics by leveraging on the reliable and high-volume fabrication technologies already developed for microelectronic integrated circuits. Germanium (Ge), already used in Si photonics, is a prime candidate to extend the operating wavelength of Group IV-based photonic integrated circuits beyond 8 µm, and potentially up to 15 µm. High performance quantum cascade lasers (QCLs) and interband cascade lasers grown on Si have been demonstrated, whereas no QCLs monolithically integrated on Ge have been reported yet. In this work, we present InAs-based QCLs directly grown on Ge by molecular beam epitaxy. The lasers emitting near 14 µm exhibited threshold current densities as low as 0.8-0.85 kA/cm2 at room temperature.

High-performance quantum cascade lasers at λ ∼ 9 µm grown by MOCVD.

We demonstrate a high power InP-based quantum cascade laser (QCL) (λ ∼ 9 µm) with high characteristic temperature grown by metalorganic chemical vapor deposition (MOCVD) in this article. A 4-mm-long cavity length, 10.5-µm-wide ridge QCL with high-reflection (HR) coating demonstrates a maximum pulsed peak power of 1.55 W and continuous-wave (CW) output power of 1.02W at 293 K. The pulsed threshold current density of the device is as low as 1.52 kA/cm2. The active region adopted a dual-upper-state (DAU) and multiple-lower-state (MS) design and it shows a wide electroluminescence (EL) spectrum with 466 cm-1 wide full-width at half maximum (FWHM). In addition, the device performance is insensitive to the temperature change since the threshold-current characteristic temperature coefficient, T0, is as high as 228 K, and slope-efficiency characteristic temperature coefficient, T1, is as high as 680 K, over the heatsink-temperature range of 293 K to 353 K.

Thermal-Dynamics Optimization of Terahertz Quantum Cascade Lasers with Different Barrier Compositions

The interplay of high operating temperatures and good heat dissipation is crucial for high-performance terahertz quantum cascade lasers. We therefore study the influence on the cross-plane thermal conductivity of different aluminum concentrations in the barrier of ${\mathrm{Ga}\mathrm{As}/\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ active regions. The thermal conductivity is decreasing from $30\phantom{\rule{0.1em}{0ex}}\mathrm{W}\phantom{\rule{0.1em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ to $12\phantom{\rule{0.1em}{0ex}}\mathrm{W}\phantom{\rule{0.1em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ if the aluminum concentration is increased from 15% to 24%. The temperature during pulsed-laser operation is obtained by measuring the variation of the emission frequency for different laser pulse lengths. This shows, that besides the thermal conductivity, the amount of electric input power has a strong influence on the temperature reached internally during laser operation and is critical for creating high-power devices operating at high temperatures. We show that active regions with thin but high ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ barriers fulfill this need and are well suited for high-temperature operation. A thermal model of the devices allows prediction of the active-region temperature increase for very short pulse durations. For the structure with 24% Al barriers and a starting temperature of 10 K, the model shows an increase by 24 K for a pulse length of only 300 ns.

High performance quantum cascade laser frequency combs at λ ∼ 6 μm based on plasmon-enhanced dispersion compensation

We demonstrate quantum cascade laser (QCL) optical frequency combs emitting at λ ∼ 6 μm. A 5.5 μm-wide, 4.5 mm-long laser exhibits comb operation from -20 °C up to 50 °C. A maximum output power of 300 mW is achieved at 50 °C showing a robustness of the system. The laser output spectrum is ∼80 cm-1 wide at the maximum current, with a mode spacing of 0.334 cm-1, resulting in a total of 240 modes with an average power of 0.8 mW per mode. To achieve frequency comb operation, a plasmonic-waveguide approach is utilized. A thin, highly-doped indium phosphide (InP) layer is inserted in the top cladding design to compensate the positive dispersion of the system (material and waveguide). This approach can be further exploited to design QCL combs at even shorter wavelengths, down to 4 μm. Different ridge widths between 2.8 and 5.5 μm have been fabricated and characterized. All of the devices exhibit frequency comb operation. These observations demonstrate that the plasmonic-waveguide is a robust and reliable method for dispersion compensation of a semiconductor laser systems to achieve frequency comb operation.

Unintentional As incorporation into AlSb and interfacial layers within InAs/AlSb superlattices

The InAs/AlSb material system has proven to be an excellent choice for high-performance mid-infrared quantum cascade lasers. In this work, an unintentional displacement of Sb by residual As flux incident on the wafer was studied by direct monitoring of such flux during a simulated molecular beam epitaxy (MBE) growth sequence and dynamical simulations of high-resolution x-ray diffraction data collected on InAs/AlSb periodic structures grown under similar conditions. The results revealed that predominantly Al–As bonds detected at the InAs/AlSb interfaces, which were reported earlier, can be attributed to a residual bypass As flux on the wafer after closing the As shutter. Moreover, the experiments revealed that under typical growth conditions, AlSb binary barriers are converted into AlAsSb ternary layers with appreciable As content. The exact As content in the barriers is proportional to the effective As flux bypassing the closed shutter and thus depends on the particulars of the MBE system design and the exact As flux used for the growth of InAs wells.

Active mode-locking of mid-infrared quantum cascade lasers with short gain recovery time.

We investigate the dynamics of actively modulated mid-infrared quantum cascade lasers (QCLs) using space- and time-domain simulations of coupled density matrix and Maxwell equations with resonant tunneling current taken into account. We show that it is possible to achieve active mode locking and stable generation of picosecond pulses in high performance QCLs with a vertical laser transition and a short gain recovery time by bias modulation of a short section of a monolithic Fabry-Perot cavity. In fact, active mode locking in QCLs with a short gain recovery time turns out to be more robust to the variation of parameters as compared to previously studied lasers with a long gain recovery time. We investigate the effects of spatial hole burning and phase locking on the laser output.

Low‐strain, quantum‐cascade‐laser active regions grown on metamorphic buffer layers for emission in the 3.0–4.0 μm wavelength region

We have investigated metamorphic buffer layers (MBLs), as so-called virtual substrates, for accessing a compositional range of In x Ga 1-x As/Al y In 1-y As superlattice (SL) materials which would otherwise be prohibited due to excessive strain when grown on conventional substrates. Such materials have application in the realisation of high-performance Quantum Cascade Lasers (QCLs) of short emission wavelengths (i.e., ≤4.0 μm). Simulation studies suggest that significant enhancement of performance in terms of reduced device temperature sensitivity and reduced thermal resistance is possible over conventional InP-substrate devices by employing MBL-based QCL designs on a GaAs substrate. Furthermore, such devices would exhibit significantly lower strain compared to conventional QCLs on InP emitting within the 3.0-4.0 μm wavelength region. To improve the planarity of MBL top surfaces, we employ chemical mechanical polishing (CMP) prior to the growth of the QCL SL structures. 20-period In x Ga 1-x As (wells)/Al y In 1-y As (barriers) SLs are grown by metalorganic vapour phase epitaxy (MOVPE) on an InGaAs step-graded, hydride vapour phase epitaxy (HVPE)-grown MBL. Employing CMP on the top of the MBL, prior to the SL growth, results in significantly improved X-ray-diffraction SL fringes. Electroluminescent devices, incorporating a single stage of QCL-SL active-region material grown on an MBL subjected to CMP, demonstrate intersubband emission near 3.6 μm.

Design for high-power, single-lobe, grating-surface-emitting quantum cascade lasers enabled by plasmon-enhanced absorption of antisymmetric modes

Resonant coupling of the transverse-magnetic polarized (guided) optical mode of a quantum-cascade laser (QCL) to the antisymmetric surface-plasmon modes of 2nd-order distributed-feedback (DFB) metal/semiconductor gratings results in strong antisymmetric-mode absorption. In turn, lasing in the symmetric mode, that is, surface emission in a single-lobe far-field beam pattern, is strongly favored over controllable ranges in grating duty cycle and tooth height. By using core-region characteristics of a published 4.6 μm-emitting QCL, grating-coupled surface-emitting (SE) QCLs are analyzed and optimized for highly efficient single-lobe operation. For infinite-length devices, it is found that when the antisymmetric mode is resonantly absorbed, the symmetric mode has negligible absorption loss (∼0.1 cm−1) while still being efficiently outcoupled, through the substrate, by the DFB grating. For finite-length devices, 2nd-order distributed Bragg reflector (DBR) gratings are used on both sides of the DFB grating to prevent uncontrolled reflections from cleaved facets. Equations for the threshold-current density and the differential quantum efficiency of SE DFB/DBR QCLs are derived. For 7 mm-long, 8.0 μm-wide, 4.6 μm-emitting devices, with an Ag/InP grating of ∼39% duty cycle, and ∼0.22 μm tooth height, threshold currents as low as 0.45 A are projected. Based on experimentally obtained internal efficiency values from high-performance QCLs, slope efficiencies as high as 3.4 W/A are projected; thus, offering a solution for watt-range, single-lobe CW operation from SE, mid-infrared QCLs.

Thermally activated leakage current in high-performance short-wavelength quantum cascade lasers

The threshold condition for a 4-level quantum cascade laser (QCL)-active region is formulated to include thermally activated leakage of charge carriers from active region confined states into states with higher energy. A method is described and demonstrated to extract the associated thermal escape current density from measurements at laser threshold. This current is modeled by including both the temperature dependent subband-distribution of charge carriers and longitudinal optical-phonon probability. The method is used to analyze the thermally activated leakage of charge carriers in two short-wavelength strain-compensated InGaAs/InAlAs QCL-structures. The energies of the higher-lying states extracted from the model are in good agreement with the values calculated numerically within the effective-mass approximation. The estimated scattering time for the thermal activation process agrees with the expected value as well. Our approach offers a straightforward and accurate method to analyze and troubleshoot thermally activated leakage in new QCL-active region designs.

High-performance uncooled distributed-feedback quantum cascade laser without lateral regrowth

We demonstrate, uncooled, room-temperature continuous-wave (cw) operation of single-mode distributed-feedback (DFB) quantum cascade lasers (QCLs) emitting around 4.6 μm without lateral regrowth. The effects of cavity length on device performance are studied. A record low threshold electrical power consumption of 2.3 W for the entire laser system with a 1.5-mm-long cavity is realized. For the 2-mm-long laser, high cw output power of 125 mW and very low threshold current density of 0.86 kA/cm2 are obtained. Our devices represent an important step towards using uncooled DFB QCLs in mid-infrared spectral range for practical applications.

High-performance quantum cascade lasers with wide electroluminescence (∼600 cm−1), operating in continuous-wave above 100 °C

The authors report high temperature continuous-wave (cw) operations of broad-gain quantum cascade lasers based on the anticrossed dual-upper-state to multiple-lower-state design. The devices exhibit extremely wide electroluminescence (>600 cm−1) and subthreshold amplified spontaneous emission (∼570 cm−1) spectra at room temperature. Despite showing such broad electroluminescence spectra, the high-reflection coated, buried heterostructure lasers operating at 6.8 μm demonstrate a low threshold current density of ∼1.5 kA/cm2 and a high power of >500 mW with a high slope efficiency of ∼1.6 W/A in cw mode at 300 K. The maximum cw operating temperature of above 100 °C is achieved.

High average power uncooled mid-wave infrared quantum cascade lasers

High-performance uncooled quantum cascade lasers emitting at wavelengths of 4.6 and 4.0 µm are reported. The 4.6 µm lasers show an average optical power in excess of 2 W at a heatsink temperature of 300K, wall-plug efficiencies of 13 and 10% at output power levels of 1 and 2 W, respectively, and an average power in excess of 1.2 W at 340K. The 4.0 µm lasers show an average optical power in excess of 1.5 W at 300K and wall-plug efficiencies of 9 and 7% at output power levels of 1 and 1.5 W, respectively.

Progress in high-performance quantum cascade lasers

Because of their compact size, reliability, tunability, and convenience of direct electrical pumping, quantum cascade lasers have found a number of important civilian and defense applications in the midwave infrared and long-wave-infrared spectral range. Most of these applications would benefit from higher laser optical power and higher wall-plug efficiency. We describe some of the most important features of high-efficiency quantum cascade laser design and realization of high-power quantum cascade laser systems. Specifically, optimization of the active region and waveguide, thermal management on the chip level, and impact of the laser facet coating on laser efficiency and scaling of optical power with cavity length are discussed. Also, we present experimental results demonstrating multiwatt operation with reliability of at least several thousands of hours on a system level.

High-performance quantum cascade lasers in the 7.3- to 7.8-μm wavelength band using strained active regions

We consider the use of strained quantum cascade (QC) laser designs in the 7.3- to 7.8-µm wavelength region to improve the cw operating characteristics of these lasers at room temperature. We compare the performance of a QC laser with a strain-balanced active region with that of two similarly designed QC lasers with lattice-matched active-region layers, and we show that the strain-balanced design significantly outperforms the unstrained-layer designs in terms of threshold-current density, power slope efficiency, and cw output power at room temperature. A epi-side-down-mounted, double-channel ridge-geometry laser that was fabricated from the strained design with a ridge width of 13 µm and a cavity length of 3 mm produced cw output power in excess of 500 mW at room temperature. The characteristic temperature T0 for the strained-layer design as determined from pulsed measurements between 25 and 80°C is 221.5 K, compared with a value of 194.0 K for one of the unstrained designs.

Electron leakage and its suppression via deep-well structures in 4.5- to 5.0-μm-emitting quantum cascade lasers

The equations for threshold-current density Jth and external differential quantum efficiency d of quantum cascade lasers (QCLs) are modified to include electron leakage and the electron-backfilling term corrected to take into account hot electrons in the injector. We show that by introducing both deep quantum wells and tall barriers in the active regions of 4.8-µm-emitting QCLs, and by tapering the conduction-band edge of both injector and extractor regions, one can significantly reduce electron leakage. The characteristic temperatures for Jth and d, denoted by T0 and T1, respectively, are found to reach values as high as 278 and 285 K over the 20 to 90°C temperature range, which means that Jth and d display 2.3 slower variation than conventional 4.5- to 5.0-µm-emitting, high-performance QCLs over the same temperature range. A model for the thermal excitation of hot injected electrons from the upper laser level to the upper active-region energy states, wherefrom some relax to the lower active-region states and some are scattered to the upper miniband, is used to estimate the leakage current. Estimated T0 values are in good agreement with experiment for both conventional QCLs and deep-well QCLs. The T1 values are justified by increases in both electron leakage and waveguide loss with temperature

High-performance InP-based midinfrared quantum cascade lasers at Northwestern University

We present recent performance highlights of midinfrared quan- tum cascade lasers (QCLs) based on an InP material system. At a repre- sentative wavelength around 4.7 μm, a number of breakthroughs have been achieved with concentrated effort. These breakthroughs include watt-level continuous wave operation at room temperature, greater than 50% peak wall plug efficiency at low temperatures, 100-W-level pulsed mode operation at room temperature, and 10-W-level pulsed mode oper- ation of photonic crystal distributed feedback quantum cascade lasers at room temperature. Since the QCL technology is wavelength adaptive in nature, these demonstrations promise significant room for improvement across a wide range of mid-IR wavelengths. C 2010 Society of Photo-Optical

Electrically tunable, high performance quantum cascade laser

A quantum cascade laser design for wide voltage-tuning, emitting at ∼8.5 μm, is presented based on a diagonal bound-to-continuum design. The relatively short period length and the diagonal nature of the laser transition guarantees a wide tuning of the emission due to the linear Stark shift effect. Tuning of both the spontaneous and stimulated emission is presented over almost 100 cm−1. In spite of the large tuning, laser performance are comparable with the best results present in literature in this spectral range. In particular, continuous wave operation up to 450 mW and pulsed wall plug efficiencies up to 11.5% were measured at 300 K. A transport model, based on the density matrix formalism, was used to simulate spontaneous and stimulated emission as function of the applied field. Same model was also used to predict light-current-voltage characteristics of the lasers.

Corrections to “High Performance Quantum Cascade Lasers Grown by Metal-Organic Vapor Phase Epitaxy and Their Applications to Trace Gas Sensing” [Nov 08 3534-3555

For the above titled paper (ibid., vol. 26, no. 21, pp. 3534-3555, Nov. 08), there are some additional comments that are presented here.

High performance quantum cascade lasers based on three-phonon-resonance design

A quantum cascade laser structure based on three-phonon-resonance design is proposed and demonstrated. Devices, emitting at a wavelength of 9 μm, processed into buried ridge waveguide structures with a 3 mm long, 16 μm wide cavity and a high-reflection (HR) coating have shown peak output powers of 1.2 W, slope efficiencies of 1 W/A, threshold current densities of 1.1 kA/cm2, and high wall-plug efficiency of 6% at 300 K. A 3 mm long, 12 μm wide buried-heterostructure device without a HR coating exhibited continuous wave output power of as high as 65 mW from a single facet at 300 K.