GaAs-based Quantum Cascade Lasers Research Articles

Doping study of two-well resonant-phonon terahertz quantum cascade lasers part I: doping profile dependence

The influence of impurity doping on GaAs-based two-well resonant-phonon terahertz quantum cascade lasers is investigated theoretically, and efficient doping schemes are discussed. By using the rate equation model, the impacts of dopant amount, position, and distribution on the performance of a high-performance device is simulated focusing on a single module. The calculated optical gain is found to have a peak over the range of sheet doping density from 1.0 × 1010 to 1.0 × 1012 cm−2 in all eight doping conditions examined in this work. Among these patterns, the devices with the undoped condition and homogeneous-doping in phonon-wells mark high optical gain, and the latter is also resistant against the detuning of subband alignment due to band-bending under the high doping conditions. Furthermore, based on the simulation results, a modulation doping scheme whose active cores include both doped and undoped modules is suggested and discussed.

Increasing the output power of a heavily doped terahertz quantum cascade laser by avoiding the subband misalignment

A significant enhancement in the output power of a GaAs-based terahertz quantum cascade laser (THz QCL) was achieved by implementing a relatively high impurity doping concentration. The QC structure was precisely designed using the nonequilibrium green function method by considering the band bending effect caused by a higher doping concentration. This enabled us to avoid the subband misalignment induced by the strong band bending effect, to provide efficient carrier transport even at high doping concentrations, and to improve the output power. The maximum output power of a GaAs/Al0.16Ga0.84As 3.5 THz-band QCL at 10 K was enhanced from 390 to 600 mW by increasing the doping concentration at the depopulation layers from 6 × 1016 to 1.2 × 1017 cm−3. The average power of 21 mW was achieved by implementing the appropriate pulse drive conditions.

Lateral waveguide scanner integration on surface-emitting mid-infrared lasers.

In this paper, a novel, to the best of our knowledge, monolithic non-mechanical semiconductor laser scanner in the mid-infrared (MIR) spectrum is proposed. A deflector above the active region at the substrate side is used for coupling the vertical light into a lateral substrate waveguide, which creates a chain of coherent emitters such as optical phased arrays (OPAs) for beam steering. The numerical simulation reveals that GaSb-based surface-emitting interband cascade lasers (SE-ICLs) are an excellent platform for waveguide scanner integration. Due to the hundreds of micrometers of optical path difference and the narrow gap between each emitter, an extremely high angle tuning coefficient of 0.84°/nm covering the whole 28.6° steering range is obtained. This work theoretically verifies the feasibility of integrating an OPA scanner into the GaSb-based SE-ICLs, providing a practical solution to fabricate compact steerable MIR laser sources. Note that this substrate OPA concept has strong adaptation potential to extend to even longer wavelength devices such as InP and GaAs-based quantum cascade lasers.

High-power portable terahertz laser systems

Terahertz (THz) frequencies remain among the least utilized in the electromagnetic spectrum, largely due to the lack of powerful and compact sources. The invention of THz quantum cascade lasers (QCLs) was a major breakthrough to bridge the so-called ‘THz gap’ between semiconductor electronic and photonic sources. However, their demanding cooling requirement has confined the technology to a laboratory environment. A portable and high-power THz laser system will have a qualitative impact on applications in medical imaging, communications, quality control, security and biochemistry. Here, by adopting a design strategy that achieves a clean three-level system, we have developed THz QCLs (at ~4 THz) with a maximum operating temperature of 250 K. The high operating temperature enables portable THz systems to perform real-time imaging with a room-temperature THz camera, as well as fast spectral measurements with a room-temperature detector. GaAs-based terahertz quantum cascade lasers emitting around 4 THz are demonstrated up to 250 K without a magnetic field. To elevate the operation temperature, carrier leakage channels are reduced by carefully designing the quantum well structures.

Numerical simulation of GaAs-based mid-infrared one-phonon resonance quantum cascade laser

Non-equilibrium Green’s function method is used to analyze electronic transport in a mid-infrared quantum cascade laser on microscopic level. Basing on the excellent agreement found between calculated and experimental data, the conclusions are derived that the carrier distribution in the lower laser subband is non-thermal, and the carriers are extracted from active region both in cold and hot state. An estimate tau _{3}=0.66 ps for upper lifetime at 77 K was found which considerably differs from the value 1.4 ps evaluated from the form factors.

Complete rate equation modelling of quantum cascade lasers for the analysis of temperature effects

The effect of temperature on the dynamics of a GaAs-based quantum cascade laser (QCL) is analysed using a complete rate equation model. The analytical expressions for the threshold current density and the output power are derived using the model and the thermal behaviour of these parameters is examined. A better conformity of the threshold current density with experiment at higher temperatures is achieved. The effect of temperature on the 3dB optical bandwidth is further investigated using the same model. A comparative analysis of the model is performed with the recently reported rate equation models. It is observed that the 3dB optical bandwidth falls more rapidly at higher operating temperatures that highlight the effects of leakage and backscattering processes present in the device.

Room temperature negative differential resistance in terahertz quantum cascade laser structures

The mechanisms that limit the temperature performance of GaAs/Al0.15GaAs-based terahertz quantum cascade lasers (THz-QCLs) have been identified as thermally activated LO-phonon scattering and leakage of charge carriers into the continuum. Consequently, the combination of highly diagonal optical transition and higher barriers should significantly reduce the adverse effects of both mechanisms and lead to improved temperature performance. Here, we study the temperature performance of highly diagonal THz-QCLs with high barriers. Our analysis uncovers an additional leakage channel which is the thermal excitation of carriers into bounded higher energy levels, rather than the escape into the continuum. Based on this understanding, we have designed a structure with an increased intersubband spacing between the upper lasing level and excited states in a highly diagonal THz-QCL, which exhibits negative differential resistance even at room temperature. This result is a strong evidence for the effective suppression of the aforementioned leakage channel.

Recent progress of tunable terahertz sources based on difference frequency generation

Terahertz technology has been developed rapidly in the past 30 years. Numerous applications in medicine, biology, agriculture, materials, security, communication and astronomy have been demonstrated. Terahertz sources can be divided into narrowband (monochromatic) source and broadband source according to their spectral characteristics. From a spectral perspective, coherent broadband and narrowband terahertz sources are mutually complementary, each having its own characteristics and scope of applications. Broadband terahertz sources can be used for quick access to the hybrid spectra of rotational and vibrational molecular fingerprints or imaging in a wider spectral range. Narrowband terahertz source with good spectral resolution and sensitivity, is suitable for pump-probe, fine structure resolution of molecular fingerprints and terahertz remote detection and imaging. Therefore, developing the tunable high peak power and narrowband terahertz sources is very important for the applications in the detection and identification of molecular fingerprints. The difference frequency generation is one of the most important techniques for obtaining widely tunable, high power and narrowband terahertz sources. In this review, the recent progress of tunable terahertz sources based on the difference frequency generation in the last five years is reviewed, including the two fields of optical laser-based difference frequency sources and quantum cascade laser-based difference frequency sources. For the former class, the experimental results from reports with different difference frequency sources and several typical nonlinear crystals are classified, and the corresponding experimental techniques and results are introduced. For terahertz wave generation, different optical difference frequency sources by a dual-wavelength laser, double laser, a laser and an optical parametric oscillator (OPO), the signal and idler waves of an OPO, and double OPOs are demonstrated in increasing their tunabilities. Significant progress has been made in the nonlinear crystals used to generate terahertz wave by the difference frequency process, for example, by improving the property of inorganic crystals with ion doping, taking advantage of waveguide and PPLN structures, and especially developing novel nonlinear organic crystals. For the quantum cascade laser-based difference frequency sources, the latest advances in the techniques of difference frequency generation and wavelength tunability are presented. GaAs-based terahertz quantum cascade lasers are powerful semiconductor THz sources but cryogenic cooling is still a necessity. Recently, difference frequency generation was combined with the mid-infrared quantum cascade laser technology, thus becoming a leading room temperature semiconductor source in the terahertz range. To improve the frequency tuning range in the difference frequency terahertz quantum cascade laser, wavelength tuning techniques of the inner cavity and the external cavity have been developed. The difference frequency generation quantum cascade terahertz laser source has been the only technique workable at room temperature for the quantum cascade laser so far, which opens the door for developing the compact and widely tunable room temperature terahertz sources.

Nonequilibrium phonon effects in midinfrared quantum cascade lasers

We investigate the effects of nonequilibrium phonon dynamics on the operation of a GaAs-based midinfrared quantum cascade laser over a range of temperatures (77–300 K) via a coupled ensemble Monte Carlo simulation of electron and optical-phonon systems. Nonequilibrium phonon effects are shown to be important below 200 K. At low temperatures, nonequilibrium phonons enhance injection selectivity and efficiency by drastically increasing the rate of interstage electron scattering from the lowest injector state to the next-stage upper lasing level via optical-phonon absorption. As a result, the current density and modal gain at a given field are higher and the threshold current density lower and considerably closer to experiment than results obtained with thermal phonons. By amplifying phonon absorption, nonequilibrium phonons also hinder electron energy relaxation and lead to elevated electronic temperatures.

Dynamic Analysis of GaAs-based Quantum Cascade Laser

The quantum cascade lasers are such unipolarity lasers that have the emissivity from mid infrared to far infrared range. In this paper the dynamic effects of quantum cascade lasers (QCL) GaAs/AlGaAs characteristics in the mid-infrared range of λ=9μm is investigated by solving the numerical equations. Calculating of the spontaneous emission rate changes, its effects on the number of carriers, the number of photons, and the output power are the objectives of the presented study. Finally, the output power changes, by increasing the number of stages in the quantum cascade laser, have been studied. The obtained results showed that, the increase of spontaneous emission rate parameter causes delay in the laser output. In addition, there is a direct relationship between the number of stages and the output power.

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.

Strained Active Regions in GaAs-based Quantum Cascade Lasers

This paper explores the effects of low indium concentration in deep-well AlGaAs/GaAs quantum cascade laser structures. Photon emission from these strained active regions involves nonlinear optical processes, which reduce the emission wavelength. Designs that incorporate strain in one and two wells of the active region, respectively, on (111) GaAs are compared. Results demonstrate wavelength extendibility in compressively strained deep-well GaAs-based devices.

Switching operation of lasing wavelength in mid-infrared ridge-waveguide quantum cascade lasers coupled with microcylindrical cavity

We demonstrated switching operation of lasing wavelength in GaAs-based mid-infrared coupled-cavity quantum cascade lasers. Our cavity structures consist of a ridge-waveguide Fabry–Perot cavity coupled with a microcylindrical cavity. Individual control of currents injected into both cavities enabled single-mode operation and facilitated intentional mode switching between coupled-cavity modes with spacing over 120 nm. The integrated microcylindrical cavity expanded the separation of coupled-cavity modes due to the small size of the cavity and enabled switching the lasing wavelength over a wide spectral range.

Mid-Infrared GaAs/AlGaAs Quantum Cascade Lasers Technology

The quantum cascade lasers (QCLs) are unipolar devices based on intersubband transitions and tunnelling transport [1, 2]. The GaAs-based QCLs have proved to be an effective source of laser radiation in mid-infrared (MIR) as well as far-infrared (FIR) regions. They find application in the gas sensing systems (e.g., for detecting CO2, NO, CH4) [3], medical diagnostics [4] and environment monitoring [5]. The complex design and operating principle require the technology of quantum cascade lasers to ensure the highest accuracy, uniformity and repeatability of both epitaxy and device processing [6]. High requirements for epitaxy are connected with ultrathin layers of periodic active region, where the wave functions are to be properly engineered [7–9]. The thickness accuracy of the structure should in general be better than 3%. Another crucial issue is the optimisation of injector doping, which is necessary to assure a large enough dynamic range of laser operation [10, 11]. In order to restrict the lateral current spreading in the device it is necessary to form a good quality deep mesa structures. Since the available gain is usually limited, the device design should minimize the radiation losses. The mesa is etched through active region and reduction of the optical losses requires a smooth waveguide sidewalls [12]. There is a number of QCLs designs reported in the literature [6, 13]. Sirtori et al. [6]. have presented the ridge structure and double trench structure, in which ion implantation was used for electrical isolation instead of dielectric layers. The optimum one seems to be the double trench construction which insures a good overlapping of the lateral mode with the gain region and enables stable epi-down mounting necessary for efficient cooling of the device [14]. The construction in which the extended proton-implanted regions were applied in the area defined by the two trenches, was the another presented solution. In this case the channel for current injection was narrower than the ridge width, which minimizes absorption of generated radiation in dielectric layers insulating mesa sidewalls [6, 15]. As a rule the relatively high voltages as well as current densities are necessary to polarize QCLs. That indicates demand for low resistance ohmic contacts in order to reduce the device serial resistance. These contacts should be characterized by thermal stability, low depth of metal diffusion into semiconductor layers and lateral uniformity of metal–semiconductor interfaces. In this paper we discuss some issues fabrication technology of Al0.45Ga0.55As/GaAs QCL operating at λ ≈ 9.4 μm. The devices lased up to 262 K (−11 ◦C) with optical powers over 1 W at 77 K, threshold current density values of 7 kA cm−2 and differential efficiencies above 0.6 W/A. Since lasers were uncoated, the quoted values should be understood.

Comparison of SiO2, Si3N4, As2S3, and Ge0.25Se0.75 dielectric layers for InP- and GaAs-based material systems for midinfrared quantum cascade laser waveguides

We present in this paper a direct comparison of the optical properties of InP- and GaAs-based quantum cascade laser waveguides operating in the midinfrared wavelength range. The absorption loss and optical confinement were calculated using a two-dimensional electromagnetic finite-element method. The effect on the waveguide properties of SiO2, Si3N4, As2S3, and Ge0.25Se0.75 used as electrical insulation layers is investigated. The results indicate that a careful choice of this particular layer according to the geometrical structure and the emission wavelength should enhance the laser performance.

Quantum cascade laser: Applications in chemical detection and environmental monitoring

In this paper we consider the structural parameter optimization of the active region of a GaAs-based quantum cascade laser in order to maximize the optical gain of the laser at the characteristic wavelengths, which are best suited for detection of pollutant gasses, such as SO2, HNO3, CH4, and NH3, in the ambient air by means of direct absorption. The procedure relies on applying elaborate tools for global optimization, such as the genetic algorithm. One of the important goals is to extend the applicability of a single active region design to the detection of several compounds absorbing at close wave-lengths, and this is achieved by introducing a strong external magnetic field perpendicularly to the epitaxial layers. The field causes two-dimensional continuous energy subbands to split into the series of discrete Landau levels. Since the arrangement of Landau levels depends strongly on the magnitude of the magnetic field, this enables one to control the population inversion in the active region, and hence the optical gain. Furthermore, strong effects of band non-parabolicity result in subtle changes of the lasing wavelength at magnetic fields which maximize the gain, thus providing a path for fine-tuning of the output radiation properties and changing the target compound for detection. The numerical results are presented for quantum cascade laser structures designed to emit at specified wavelengths in the mid-infrared part of the spectrum.

Intervalley scattering in GaAs/AlGaAs quantum wells and quantum cascade lasers

The results of simulations of Γ−X scattering in GaAs/AlGaAs quantum wells are presented, discussing the importance of the mole fraction, doping density, and lattice and electron temperatures in determining the scattering rates. A systematic study of Γ−X scattering in GaAs/Al x Ga 1− x As heterostructures, using a single quantum well to determine the importance of well width, molar concentration x, lattice temperature, and doping density, has been performed. After this we consider a double quantum well to determine the role of intervalley scattering in the transport through single-layer heterostructures, i.e. Γ−X− Γ scattering compared with Γ− Γ scattering. Finally, we estimate the relative importance of intervalley scattering in a GaAs-based quantum-cascade laser device and compare it with other relevant scattering mechanisms important to describe carrier dynamics in the structure. Our simulations suggest that Γ−X scattering can be significant at room temperature but falls off rapidly at lower temperatures.

Design and optimization of a GaAs-based sub-7-μm quantum cascade laser based on multivalley Monte Carlo simulation

We present the design and optimization of a deep-well GaAs-based quantum cascade laser (QCL) emitting at 6.7 μm, the shortest wavelength in GaAs-based QCLs, using a multivalley Monte Carlo simulation. Simulation results provide direct insights into optimizing the layer sequence design. The optimized structure exhibits sufficient gain for lasing at both 77 and 300 K. The calculated threshold-current densities are 5 kA/cm2 at 77 K and 14 kA/cm2 at 300 K.

Design and simulation of deep-well GaAs-based quantum cascade lasers for 6.7μm room-temperature operation

We present the design and simulation of a GaAs-based quantum cascade laser (QCL) emitting at 6.7 μm, the shortest room-temperature lasing wavelength projected to date for GaAs-based QCLs. This is achieved by introducing compressive strain only in the active quantum wells, where the optical transition occurs. A Monte Carlo simulation including both Γ- and X-valley transport demonstrates that the proposed QCL achieves room-temperature lasing at a threshold-current density of 14 kA/cm2, lower than that of the conventional 9.4 μm QCL (16.7 kA/cm2). Furthermore, the electron temperature at 300 K lattice temperature is similar to that of the 9.4 μm device.

Comparative analysis of resonant phonon THz quantum cascade lasers

We present a comparative analysis of a set of GaAs-based THz quantum cascade lasers, based on longitudinal-optical phonon scattering depopulation, by using an ensemble Monte Carlo simulation, including both carrier-carrier and carrier-phonon scattering. The simulation shows that the parasitic injection into the states below the upper laser level limits the injection efficiency and thus the device performance at the lasing threshold. Additional detrimental effects playing an important role are identified. The simulation results are in reasonable agreement with the experimental findings.