Laser Active Region Research Articles

Strain-balanced InGaAs/AlInAs/InP quantum cascade laser grown on GaAs by MOVPE

A systematic study was performed to elucidate the properties of strained-layer InGaAs/AlInAs superlattices (SL) on metamorphic buffer layers grown by MOVPE on GaAs substrates. Differences observed in surface morphology indicate relatively tight control over the SL average net-strain is required for the growth of micron-thick SL structures, as needed for quantum cascade laser (QCL) active regions. Using such conditions, 5.7 μm-emitting, strain-balanced InP-based QCLs grown on (001) GaAs were demonstrated with comparable device performance to their counterparts grown on native InP substrate. 3 mm-long and 25 μm-wide uncoated-facets devices, grown on GaAs and having epi-side lateral contacts, provide peak-pulsed output powers of 2.65 W per facet at room temperature. The corresponding threshold-current density and maximum wall-plug efficiency is 1.61 kA/cm2 and 6.0% respectively. HR-XRD measurements of the strain-balanced QCL grown on GaAs show broadened satellite peaks which are well aligned with the peaks from their counterparts grown on InP.

Reduction of nonradiative recombination for high-power 808 nm laser diode adopting InGaAsP/InGaAsP/GaAsP active region

Reducing nonradiative recombination is a key problem to achieving high-power laser diode. InGaAsP/InGaAsP/GaAsP active region was designed so as to increase injection efficiency and decrease leakage currents which corresponds to a lower nonradiative recombination. The simulation results showed that the output power of 15.59 W and the wall-plug efficiency of 78.95% were obtained for the InGaAsP/InGaAsP/GaAsP active region laser at injection current density 6 kA/cm2 at room temperature of 25 °C by improved carrier injection efficiency. Meanwhile, the temperature drift coefficient of 0.217 nm/∘C was obtained for the peak wavelength of InGaAsP/InGaAsP/GaAsP active region laser at the temperature range from 5 to 65 ∘C. The designed InGaAsP/InGaAsP/GaAsP active region structure is an effective way to reduce the nonradiative recombination loss for achieving high-power laser diode.

Interface tomography of GaInAs/AlInAs quantum cascade laser active regions

We present a high-resolution electron tomography study of buried ultra-thin layers and their interfaces from the active region of a (Ga,In)As/(Al,In)As quantum cascade laser (QCL) test structure. Using a high-angle annular dark-field scanning transmission electron microscopy image series, a three-dimensional (3D) reconstruction of the complex layer structure is obtained. From this 3D information, we determine quantitative values for the chemical width and, simultaneously and independently, the root mean square roughness (rms) and lateral correlation length of the individual interfaces of a cascade using topographic height maps. The interfacial widths are comparably small for all interfaces within a cascade and the layer thicknesses show only a small standard deviation of less than one monolayer. The rms roughness is systematically lower at the direct (Ga,In)As-on-(Al,In)As interface compared to its inverse. In addition, using the one-dimensional height–height correlation function, different lateral correlation lengths along the two perpendicular in-plane directions are detected indicating a distinct anisotropy of the interface morphology. Our accurate and comprehensive results on the QCL test structure will serve as feedback to evaluate the growth process and help to assess the performance of corresponding future laser devices in detail.

Active Modelocking Improvement of MIR-QCL by Integrating Graphene as a Saturable Absorber

The active mode-locking technique of quantum cascade lasers is highly affected by the bias current level and the magnitude of the modulated signal because of the spatial hole burning effect. The laser generates a stable pulse train near the threshold current and for a high modulation magnitude. We show in this paper an improvement of these conditions with an integration of a single-layer graphene which works as a saturable absorber. The light-matter interaction in the laser active region is described by the two-level Maxwell-Bloch equations and in graphene layer by the Maxwell-Ampere equation and Maxwell-Bloch equations. These system equations are solved using the Finite-Difference Time-Domain (FDTD) method which has the main advantage of not involving any approximation. The weakly coupled splitting scheme is adopted for the time discretization of Maxwell and Bloch equations. The graphene saturable absorption is modelled by a saturable conductivity in Maxwell-Ampere equation and with the dipole moment and carrier density in Maxwell-Bloch equations. For a QCL structure with a gain recovery time of 50 ps, simulation results show better stability of the mode-locking over a broad range of injection current and for smaller magnitude of the modulated signal. QCL structure with gain recovery time close to 1 ps reveals the generation of two pulses per roundtrip linked to spatial hole burning. The carrier grating strength can be reduced by changing the boundary conditions of the cavity such as with a high reflectivity mirror in front of graphene layer.

Selectively-Grown Quantum Dot Active Region Lasers

Nanopatterns formed by dense nanoscale (20–30nm diameter) block copolymer lithography and subsequent selective area MOVPE were utilized to realize room temperature operation of quantum dot (QD) active region laser diodes on InP substrates, by employing a carrier-injection layer adjacent to the QDs. Initial studies focused on InGaAs QDs grown on GaAs substrates, which exhibit relatively broad photoluminescence (PL) emission linewidths (FWHM~93meV at RT). Atomic force microscopy measurements reveal that a wide distribution of QD thickness variations correlates well to the observed broad PL linewidths, based on the simulated range of QD transitions. The fabricated edge-emitting devices on InP substrate exhibit lasing emission near <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.57\sim 1.67~\mu \text{m}$ </tex-math></inline-formula> at room temperature with a threshold current density as low as 1.6 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 4mm-long devices. The observed dependency of device characteristics, such as threshold current density and lasing wavelength, on the cavity length and on the heat sink temperature suggests the dominant lasing transition energy corresponds to the QD excited state, except for the longest cavity length devices (L = 4mm) which operate on the QD ground state transition near room temperature. We believe these data represent the only reports of lasers in this wavelength region employing QDs formed by diblock co-polymer lithography and selective area MOVPE growth. These data also suggest that QD height variations need to be further reduced, through MOVPE growth optimization, to achieve higher optical gain from the QD ground state transition.

Analysis of interface roughness in strained InGaAs/AlInAs quantum cascade laser structures (λ ∼ 4.6 μm) by atom probe tomography

In this study, in-plane interface roughness (IFR) values are extracted via atomic probe tomography (APT) for a few key interfaces within a quantum cascade laser (QCL) active region composed of compressively-strained InGaAs quantum wells (QWs) and tensilely-strained AlInAs barriers. The structure was grown by organometallic vapor phase epitaxy (OMVPE) at 605 °C, 100 rpm, 100 torr, with a 5 s interruption time between layers. The full-stage thickness measured via high-resolution x-ray diffraction (HR-XRD) is used to calibrate the reconstruction to within 5%. From the APT results, it is found that interfaces to two barriers of different tensile-strain value (i.e., of different Al concentration) have different RMS-roughness amplitude. Specifically, barriers of higher Al concentration have ~50% larger RMS-roughness amplitude, as the strain differential increases by a factor of 2.2. To the best of our knowledge, this is the first experimental data showing that roughness parameters within a QCL active region are different for different interfaces within the same grown structure.

Towards Interband Cascade lasers on InP Substrate

In this study, we propose designs of an interband cascade laser (ICL) active region able to emit in the application-relevant mid infrared (MIR) spectral range and to be grown on an InP substrate. This is a long-sought solution as it promises a combination of ICL advantages with mature and cost-effective epitaxial technology of fabricating materials and devices with high structural and optical quality, when compared to standard approaches of growing ICLs on GaSb or InAs substrates. Therefore, we theoretically investigate a family of type II, “W”-shaped quantum wells made of InGaAs/InAs/GaAsSb with different barriers, for a range of compositions assuring the strain levels acceptable from the growth point of view. The calculated band structure within the 8-band k·p approximation showed that the inclusion of a thin InAs layer into such a type II system brings a useful additional tuning knob to tailor the electronic confined states, optical transitions’ energy and their intensity. Eventually, it allows achieving the emission wavelengths from below 3 to at least 4.6 μm, while still keeping reasonably high gain when compared to the state-of-the-art ICLs. We demonstrate a good tunability of both the emission wavelength and the optical transitions’ oscillator strength, which are competitive with other approaches in the MIR. This is an original solution which has not been demonstrated so far experimentally. Such InP-based interband cascade lasers are of crucial application importance, particularly for the optical gas sensing.

Optical properties of a triple AlGaAs/GaAs quantum well purported for quantum cascade laser active region

A theoretical investigation on the conduction electron states in a triple inverse parabolic AlGaAs/GaAs quantum well, designed in the spirit of the active region for a quantum cascade laser, is performed. The study includes the influence of externally applied static electric and magnetic fields. The energy values and corresponding eigenfunctions are determined with the use of the effective mass approximation within the parabolic band description. The information on the allowed states is, then, used to evaluate some coefficients that represent the intraband nonlinear optical response of the structure. Numerical results are presented for the optical absorption and relative refractive index change coefficients as well as for those of nonlinear optical rectification and second harmonic generation. It is discussed that, in general, the amplitude and position of resonant peaks in the calculated quantities can be noticeably affected by the variation in the strength of the applied probe fields. Accordingly, the features of the investigated properties could suitably determine the application of this kind of structures for device proposals, as long as they can affect lasing regime in them.

Effects of Feedstocks and Laser Remelting on Microstructural Characteristics of ZrO&lt;sub&gt;2&lt;/sub&gt;-7wt.%Y&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Thermal Barrier Coatings Prepared by Plasma Spraying

Conventional and nanometer aggregate ZrO2-7wt.%Y2O3 ceramic powders taken as raw materials, plasma spraying and plasma spraying-laser remelting compound technology was used to prepare conventional and nanostructured thermal barrier coatings on the TiAl alloy surface. Effects of powder structure (feedstock) and laser remelting on organizational structure and phase of the coatings were analyzed using scanning electron microscope (SEM) and X-ray diffractometer (XRD). Results indicate that: conventional plasma sprayed ceramic coating presents typical lamellar stacking features; plasma sprayed nanostructured coating consists of fully melted region and partially melted region, presenting a two-phase structure. Under the comprehensive impacts of laser power, energy density, temperature field distribution in the laser action region, ceramic heat conductivity coefficient and coating thickness and other factors, the coating presents obvious lamellar structural features after laser remelting; the upper part is compact columnar crystal remelting region and the lower part is residual plasma spraying region. Due to toughening effect of residual nanoparticles in the remelting region of laser remelted nanostructured coating, grain-boundary strength is high and there are a considerable number of transgranular fractures, but the fractures in the remelting region of laser remelted conventional coating are basically intergranular fractures. Conventional plasma sprayed ceramic coating is mainly of tetragonal phase together with a small quantity of monoclinic phases, but nanometer plasma sprayed ceramic coating only has non-equilibrium tetragonal phases. After laser remelting, both conventional coating and nanometer coating mainly have non-equilibrium tetragonal phases with a small quantity of cubic phases.

On quantum-dot lasing at gain peak with linewidth enhancement factor αH = 0

This paper describes an investigation of the linewidth enhancement factor αH in a semiconductor quantum-dot laser. Results are presented for active region parameters and laser configurations important for minimizing αH. In particular, the feasibility of lasing at the gain peak with αH = 0 is explored. The study uses a many-body theory with dephasing effects from carrier scattering treated at the level of quantum-kinetic equations. InAs quantum-dot lasers with different p-modulation doping densities are fabricated and measured to verify the calculated criteria on laser cavity design and epitaxial growth conditions.This paper describes an investigation of the linewidth enhancement factor αH in a semiconductor quantum-dot laser. Results are presented for active region parameters and laser configurations important for minimizing αH. In particular, the feasibility of lasing at the gain peak with αH = 0 is explored. The study uses a many-body theory with dephasing effects from carrier scattering treated at the level of quantum-kinetic equations. InAs quantum-dot lasers with different p-modulation doping densities are fabricated and measured to verify the calculated criteria on laser cavity design and epitaxial growth conditions.

Dual wavelength operation of the GaSb-based Y-branch distributed Bragg reflector lasers near 2.1 μm

The GaSb-based diode lasers operating simultaneously at two wavelengths near 2.1 μm have been designed and fabricated. The Y-branch devices used the 6th order distributed Bragg reflectors (DBR) to provide spectrally selective feedback. The laser active region contained two asymmetric quantum wells with allowed optical transitions between two lowest electron (e1 and e2) and top hole (hh1) subbands. The laser emission lines loosely matched the e1-hh1 and e2-hh1 optical transitions with separation corresponding to ∼3.1 THz. The 10 μm wide deeply etched straight ridge DBR lasers demonstrated stable single mode operation with more than 50 mW of output power.

Small-Signal Equivalent Circuit Model of Photonic Crystal Fano Laser

We develop a new small-signal equivalent circuit model for photonic crystal Fano laser based on standard linearized equations. The results are compared with those obtained by the numerical simulation approach. This novel circuit model being a cost-effective fast tool is advantageous over solving the rate equations for designing and extending the photonic crystal Fano laser. Using this circuit model, we have investigated the laser current modulation via the conventional method and via modulating the Fano mirror. The results show that the bandwidths of the amplitude modulation of the laser through-port and the frequency modulation of its cross port are both in the range of THz. It is also shown that the larger the nanocavity detuning, the greater the relaxation oscillation frequency, increasing the possibility of widening the laser bandwidth, through which the modulation efficiency may decrease. Moreover, we show that the longer the laser active region, the narrower the modulation response bandwidth. The results show that the circuit model accurately explains the photonic crystal Fano laser small-signal modulation characteristics.

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.

Line-defect photonic crystal terahertz quantum cascade laser

The terahertz (THz) quantum cascade laser (QCL) provides a versatile tool in a plethora of applications ranging from spectroscopy to astronomy and communications. In many of these fields, compactness, single mode frequency emission, and low threshold are highly desirable. The proposed approach, based on line defects in a photonic crystal (PhC) matrix, addresses all these features while offering unprecedented capabilities in terms of flexibility, light waveguiding, and emission directionality. Nine line-defect QCLs were realized in a triangular lattice of pillars fabricated in the laser active region (AR), centered around ∼2 THz by tuning the photonic design. A maximal 36% threshold reduction was recorded for these ultraflat dispersion line-defect QCLs in comparison to standard metal-metal QCL. The mode selectivity is an intrinsic property of the chosen fabrication design and has been achieved by lithographically scaling the dimension of the defect pillars and by acting on the PhC parameters in order to match the AR emission bandwidth. The measured line-defect QCLs emitted preferentially in the single frequency mode in the propagation direction throughout the entire dynamic range. An integrated active platform with multiple directional outputs was also fabricated as proof-of-principle to demonstrate the potential of this approach. The presented results pave the way for integrated circuitry operating in the THz regime and for fundamental studies on microcavity lasers.

Advanced AlGaAs/GaAs Heterostructures Grown by MOVPE

AlGaAs/GaAs heterostructures are the base of many semiconductor devices. The fabrication of new types of devices demands heterostructures with special features, such as large total thickness (~20 μm), ultrathin layers (~1 nm), high repeatability (up to 1000 periods) and uniformity, for which a conventional approach of growing such heterostructures is insufficient and the development of new growth procedures is needed. This article summarizes our work on the metalorganic vapour-phase epitaxy (MOVPE) growth of AlGaAs/GaAs heterostructures for modern infrared devices. The growth approaches presented allow for the improved output characteristics of different emitting devices such as multi active region lasers, epitaxially integrated via highly doped tunnel junctions (emission wavelength λ ~ 1 μm), quantum cascade lasers (λ ~10 μm) and THz laser (λ ~100 μm), based on short-period superlattice with 500–2000 layers.

Active region of a functionally integrated laser – modulator

A method for the functional integration of injection lasers and modulators of generated light in a single heterostructure with spatially displaced quantum wells in the conduction and valence bands, formed by the second-type heterojunctions, is proposed. The structures and energy band diagrams of the active regions of functionally integrated lasers – modulators with amplitude and frequency modulation are considered. A physical – topological model and a technique for numerical simulation of lasers – modulators is proposed. It is shown that the maximum modulation frequencies for the proposed heterostructures, in contrast to the same frequencies implemented under pump current modulation, are not limited by the duration of transients in the laser power supply circuit and the carrier lifetime in the laser active region but are determined by the photon lifetime in the cavity.

High-power, continuous-wave, phase-locked quantum cascade laser arrays emitting at 8 µm.

We report a room-temperature eight-element phase-locked quantum cascade laser array emitting at 8 µm with a high continuous-wave power of 8.2 W and wall plug efficiency of 9.5%. The laser array operates primarily via the in-phase supermode and has single-mode emission with a side-mode suppression ratio of ~20 dB. The quantum cascade laser active region is based on a high differential gain (8.7 cm/kA) and low voltage defect (90 meV) design. A record high wall plug efficiency of 20.4% is achieved from a low loss buried ridge type single-element Fabry-Perot laser operating in pulsed mode at 20 °C.

Impact of InGaAs carrier collection quantum well on the performance of InAs QD active region lasers fabricated by diblock copolymer lithography and selective area epitaxy

Quantum dot (QD) laser diodes, where the active region consists of wetting-layer-free InAs QDs, were demonstrated by block copolymer (BCP) lithography and subsequent selective area metal-organic vapor phase epitaxy, which results in the QD density of ∼4 × 1010 cm2. In this work, we show that an In0.1Ga0.9As QW located in close proximity to the wetting-layer-free InAs QDs leads to an enhanced carrier injection into the QDs, allowing for lasing at room temperature (RT). Devices employing InAs QDs grown with and without In0.1Ga0.9As QW carrier collection layer exhibit lasing at 80 K, while only the lasers grown with the In0.1Ga0.9As QW exhibits lasing at RT. Driving current-dependent electroluminescence measurements reveals a low carrier injection into the QDs in the device grown without In0.1Ga0.9As QW carrier collection layer. In addition, the appearance of lasing emission, corresponding to high energy transitions, indicates a relatively flat gain profile for this InAs QD active region grown by SA-MOVPE. In addition, EL measurements at 80 K on devices employing varying thickness (dInAs) QDs indicate the peak emission wavelength varies from 920 nm (dInAs ∼ 1.5 nm) to 974 nm (dInAs = 3 nm), although the devices employing dInAs ∼ 3 nm did not exhibit lasing, possibly because of a significant degree of strain relaxation for this QD thickness.

Numerical design and investigation of an optically pumped 1.55 μm single quantum dot photonic crystal-based laser

Quantum dot microlasers are promising for application in photonic integrated circuits and lab-on-a-chip systems. In this article, we sought to design an optically pumped single quantum dot photonic crystal microlaser for 1.55 μm telecom-band operation. In the proposed scheme, an InAs quantum dot placed in the point defect of an InP photonic crystal slab acts as a laser active region. The significance of the proposed laser is its cavity design that supports two high-quality resonant modes at the pump and lasing wavelengths. Hence, the pump resonantly excites the active region. Simulations demonstrate satisfactory lasing characteristics such as a lasing threshold power of 55 nW and a spectral linewidth of ∼0.019 nm for the designed quantum dot photonic crystal-based laser.

Robust Density Matrix Simulation of Terahertz Quantum Cascade Lasers

A common setback to electron transport models for quantum cascade laser active regions is the inability to freely simulate widely varying designs. One solution to this problem is to use a density matrix formalism with a generalized treatment of scattering, wherein the well-defined energy eigenbasis is used, and the relative simplicity of the density matrix can be taken advantage of for rapid simulations. We have developed such a model from first principles in the past, and now built a simulator for terahertz quantum cascade lasers that calculates a fully self-consistent solution to the coupled problem of bandstructure, lasing field strength, and space charge. This level of depth enables us to examine the model's performance across much of the design space and operating temperatures, for which we find generally good agreement. Areas for future improvement of the model are discussed, particularly the treatment of electron–electron scattering and continuum leakage. The model also enables us to make qualitative insights into the microscopic workings of the active regions, such as the nonequilibrium subband distributions and their response to the optical field, and the possibility for using two sequential optical transitions.