Maximum Lasing Temperature Research Articles

Theoretical Study of Quasi One-Well Terahertz Quantum Cascade Laser

Developing a high-temperature terahertz (THz) quantum cascade laser (QCL) has been one of the major challenges in the THz QCL field over recent decades. The maximum lasing temperature of THz QCLs has gradually been increased, arguably by shortening the length of repeating periods of the quantum structure in the device’s active region from 7 wells/14 layers to 2 wells/4 layers per period. The current highest operating temperature of 250 K was achieved in a two-well direct-phonon design. In this paper, we propose a potential and promising novel quantum design scheme named the quasi one-well (Q1W) design, in which each quantum cascade period consists of only three semiconductor layers. This design is the narrowest of all existing THz QCL structures to date. We explore a series of the Q1W designs using the non-equilibrium green function (NEGF) and rate-equation (RE) models. Both models show that the Q1W designs exhibit the potential to achieve sufficient optical gain with low-temperature sensitivity. Our simulation results suggest that this novel Q1W scheme may potentially lead to relatively less temperature-sensitive THz QCLs. The thickness of the Q1W scheme is less than 20 nm per period, which is the narrowest of the reported THz QCL schemes.

Progress in Germanium Tin (GeSn) Photonic Crystal Lasers

We report on lasing in two types of Germanium-Tin (GeSn) photonic crystal lasers, with band-edge and H4 hexagonal cavities. A GeSn 16% step-graded structure was used as the optical gain material. The strong out-of-plane emission observed in the band-edge cavity was attributed to the thick optical gain layer, in addition with the presence of leaky band-edge modes above the light cones and a high photonic density of states. The maximum lasing temperature in both types of photonic crystal lasers, 180 K, was lower than in a micro-disk laser fabricated from the same stack, i.e., 230 K. Meanwhile, the lasing thresholds at low temperature (15 K) were very similar (120 kW/cm² for a band-edge cavity and 140 kW/cm² for a H4 cavity, to be compared with 134 kW/cm² for a micro-disk cavity). Since the injected carrier density is governed by the pump power, we suggest the strong variation of optical gain at low temperature, for a small variation of injected carrier density, as a possible cause of such a phenomenon.

Температурные характеристики кольцевых лазеров с активной областью на основе InAs/InGaAs/GaAs-квантовых точек оптического диапазона 1.3 μm

The temperature characteristics of ring lasers with a diameter of 480 μm of an original design with an active region based on 10 layers of InAs/InGaAs/GaAs quantum dots are studied. The lasers demonstrated a low threshold current density (200 A/cm2 at 20°C CW), the characteristic temperature of the threshold current in the range of 20–100°C was 68 K, the maximum lasing temperature was as high as 130oC. These values are only slightly inferior to the parameters of the edge-emitting lasers fabricated from the same epitaxial wafer.

Temperature-dependent characteristics of 1.3 μm InAs/InGaAs/GaAs quantum dot ring lasers

The temperature characteristics of ring lasers with a diameter of 480 μm of an original design with an active region based on 10 layers of InAs/InGaAs/GaAs quantum dots are studied. The lasers demonstrated a low threshold current density (200 A/cm2 at 20oC), the characteristic temperature of the threshold current in the range of 20-100oC was 68 K, the maximum lasing temperature was as high as 130oC. These values are only slightly inferior to the parameters of the edge-emitting lasers fabricated from the same epitaxial wafer. Keywords: semiconductor ring lasers, InAs/GaAs quantum dots, optical waveguide, temperature characteristics.

Lasing up to 380 K in a sublimated GaN nanowire

We report on GaN nanowire lasers fabricated by selective-area sublimation, and we show that sublimated GaN nanowires can exhibit ultraviolet lasing action under optical pumping beyond room temperature, up to 380 K. We study by microphotoluminescence the temperature-dependent behavior of single nanowire lasers between 7 K and 380 K and extract a characteristic temperature of T = 126 K. We finally present a statistical study of the maximum lasing temperature in individual sublimated GaN nanowires and use it to assess the performance of the selective-area sublimation method for nanowire-based lasing applications.

Short Barriers for Lowering Current-Density in Terahertz Quantum Cascade Lasers

Scattering due to interface-roughness (IR) and longitudinal-optical (LO) phonons are primary transport mechanisms in terahertz quantum-cascade lasers (QCLs). By choosing GaAs/Al0.10Ga0.90As heterostructures with short-barriers, the effect of IR scattering is mitigated, leading to low operating current-densities. A series of resonant-phonon terahertz QCLs developed over time, achieving some of the lowest threshold and peak current-densities among published terahertz QCLs with maximum operating temperatures above 100 K. The best result is obtained for a three-well 3.1 THz QCL with threshold and peak current-densities of 134 A/cm2 and 208 A/cm2 respectively at 53 K, and a maximum lasing temperature of 135 K. Another three-well QCL designed for broadband bidirectional operation achieved lasing in a combined frequency range of 3.1–3.7 THz operating under both positive and negative polarities, with an operating current-density range of 167–322 A/cm2 at 53 K and maximum lasing temperature of 141 K or 121 K depending on the polarity of the applied bias. By showing results from QCLs developed over a period of time, here we show conclusively that short-barrier terahertz QCLs are effective in achieving low current-density operation at the cost of a reduction in peak temperature performance.

High-Performance Growth of Terahertz Quantum Cascade Laser Structures by Solid Source MBE

High performance growth of terahertz quantum cascade lasers (THz QCLs) based on hybrid bound-to-continuum transition and resonant phonon extraction is presented by solid source molecular beam epitaxy. The corresponding accurate control of layer thickness and alloy composition during growth, with precise calibration of the growth rate of Ga and Al by reflection high energy electron diffractometry and high resolution XRD techniques are studied in detail. By utilizing surface plasmon ridge waveguide device structures, the THz QCLs lasing at 3.3 THz with maximum powers of 426 mW at 10 K and 213 mW at 80 K are realized in pulsed mode and the maximum lasing temperature achieves 110 K.

All group-IV SiGeSn/GeSn/SiGeSn QW laser on Si operating up to 90 K

In this work, all group-IV band-to-band lasers based on SiGeSn/GeSn/SiGeSn multi-quantum-well structures were demonstrated. Lasing performance was investigated via two 4-well samples. The thinner GeSn well sample exhibits a maximum lasing temperature of 20 K and a threshold of 55 kW/cm2 at 10 K, while the thicker well sample features a higher maximum operating temperature of 90 K and lower lasing thresholds of 25 and 62 kW/cm2 at 10 and 77 K, respectively. The distinct results were tentatively interpreted mainly by the difference of gain volume. This result provides guidance for the future GeSn quantum well laser optimization for higher performance.

Optically pumped lasing at 3 μm from compositionally graded GeSn with tin up to 22.3.

The recent demonstration of the GeSn laser opened a promising route towards the monolithic integration of light sources on the Si platform. A GeSn laser with higher Sn content is highly desirable to enhance the emission efficiency and to cover longer wavelength. This Letter reports optically pumped edge-emitting GeSn lasers operating at 3μm, whose device structure featured Sn compositionally graded with a maximum Sn content of 22.3%. By using a 1950-nm laser pumping in comparison with a 1064-nm pumping, the local heating and quantum defect were effectively reduced, which improved laser performance in terms of higher maximum lasing temperature and lower threshold.

Dual-lasing channel quantum cascade laser based on scattering-assisted injection design.

A dual lasing channel Terahertz Quantum Cascade laser (THz QCL) based on GaAs/Al0.17Ga0.83As material system is demonstrated. The device shows the lowest reported threshold current density (550A/cm2 at 50K) of GaAs/AlxGa1-xAs material system based scattering-assisted (SA) structures and operates up to a maximum lasing temperature of 144K. Dual lasing channel operation is investigated theoretically and experimentally. The combination of low frequency emission, dual lasing channel operation, low lasing threshold current density and high temperature performance make such devices ideal candidates for low frequency applications, and initiates the design strategy for achieving high-temperature performance terahertz quantum cascade laser with wide frequency coverage at low frequency.

Experimental investigation of terahertz quantum cascade laser with variable barrier heights

We report an experimental study of terahertz quantum cascade lasers with variable barrier heights based on the AlxGa1–xAs/GaAs material system. Two new designs are developed based on semiclassical ensemble Monte Carlo simulations using state-of-the-art Al0.15Ga0.85As/GaAs three-quantum-well resonant phonon depopulation active region design as a reference. The new designs achieved maximum lasing temperatures of 188 K and 172 K, as compared to the maximum lasing temperature of 191 K for the reference structure. These results demonstrate that terahertz quantum cascade laser designs with variable barrier heights provide a viable alternative to the traditional active region designs with fixed barrier composition. Additional design space offered by using variable barriers may lead to future improvements in the terahertz quantum cascade laser performance.

Effect of oscillator strength and intermediate resonance on the performance of resonant phonon-based terahertz quantum cascade lasers

We experimentally investigated the effect of oscillator strength (radiative transition diagonality) on the performance of resonant phonon-based terahertz quantum cascade lasers that have been optimized using a simplified density matrix formalism. Our results show that the maximum lasing temperature (Tmax) is roughly independent of laser transition diagonality within the lasing frequency range of the devices under test (3.2–3.7 THz) when cavity loss is kept low. Furthermore, the threshold current can be lowered by employing more diagonal transition designs, which can effectively suppress parasitic leakage caused by intermediate resonance between the injection and the downstream extraction levels. Nevertheless, the current carrying capacity through the designed lasing channel in more diagonal designs may sacrifice even more, leading to electrical instability and, potentially, complete inhibition of the device's lasing operation. We propose a hypothesis based on electric-field domain formation and competition/switching of different current-carrying channels to explain observed electrical instability in devices with lower oscillator strengths. The study indicates that not only should designers maximize Tmax during device optimization but also they should always consider the risk of electrical instability in device operation.

On metal contacts of terahertz quantum cascade lasers with a metal–metal waveguide

This paper reports an experimental study of the effects of different metal claddings on the performance of terahertz quantum cascade lasers. The experimental results show that by using a metal cladding made of Ta/Cu/Au to replace that of Pd/Ge/Ti/Pt/Au, the maximum lasing temperature of the devices is increased from 132 to 172 K, and the threshold current density of the devices at 10 K can be reduced from 0.74 to 0.68 kA cm−2. The improvement of the device performance is attributed to lower optical losses associated with the metal cladding layers. The different effects of the metal contacts on device optical properties and electrical properties are also discussed.

MBE growth and characterization of 5-μm quantum-cascade lasers

Quantum-cascade lasers, which are obtained by molecular-beam epitaxy, emit at wavelengths of 5.0 μm at 77 K and 5.2 μm at 300 K and are based on a design with four quantum wells in the active region with vertical transitions and strain-compensated superlattices with high-efficiency injection and a short lifetime of the ground state are fabricated. The typical thresholds for lasing at 300 K were in the range 4–10 kA/cm2. The maximum emission power was as high as ∼1 W, the maximum lasing temperature was ∼450 K, and the maximum characteristic temperature T 0 ≈ 200 K. The use of a modified process of postgrowth treatment made it possible to reproducibly obtain high-quality devices.

High-Performance $\lambda \sim 8.6~\mu {\rm m}$ Quantum Cascade Lasers With Single Phonon-Continuum Depopulation Structures

We report high performance λ~8.6 μm m quantum cascade lasers based on 5 quantum well single phonon-continuum (SPC) depopulation scheme, grown by metal organic vapor-phase epitaxy. Devices with SPC depopulation structures exhibit high device performances in pulsed and continuous wave (CW) operation due to a large dipole matrix element (3.0 nm) and short depopulation time (0.22 ps). A 4 mm-long, 8 μm-wide, high reflective (HR)-coated buried-hetero-structure laser with 33 cascade stages, for active region, demonstrates a high output power of 313 mW, a threshold current density of 1.35 kA/cm2, and slope efficiency of 547 mW/A at 30°C in CW operation. The maximum CW lasing temperature of the device is observed to be 120°C. Furthermore, by using a higher number of cascade stages (N=70) in a 4 mm-long, 22 m-wide, HR-coated ridge laser, a high peak power of 6.3 W is obtained together with a high wall-plug efficiency (WPE) of 10.8%, a slope efficiency of 2.7 W/A, and a threshold current density of 0.76 kA/cm2 in pulsed operation at 300 K. The measured WPE of over 10% at room temperature is close to the theoretical values.

Effects of extraction barrier width on performance of terahertz quantum-cascade lasers

Experimental results are presented on the effects of extraction barrier on the performance of resonant phonon terahertz quantum-cascade lasers. The extraction barrier is varied from 48 to 39 A in an otherwise identical 3.4 THz laser structure. It is found that, when the extraction barrier width decreases from 48 to 39 A, the threshold current density at 10K increases monotonously from 900 A/cm2 to 1.4 kA/cm2, and the maximum lasing temperature increases from 95 to 110K. No optimum extraction barrier width was found for maximum lasing temperature.

Resonant-Phonon Terahertz Quantum-Cascade Lasers and Video-Rate Terahertz Imaging

We review the development of terahertz quantum-cascade lasers (QCLs) that can be uniquely qualified based on a resonant-phonon depopulation scheme. Record performances in terms of operating temperature and optical power output are reported. The best temperature performance is achieved in the metal-metal (MM) waveguides, which provide near-unity mode confinement and low waveguiding loss at terahertz (THz) frequencies even for cavities with subwavelength dimensions. A pulsed operation up to a heat-sink temperature of 169 K at v ~ 2.7 THz and a continuous-wave (CW) operation up to 117 K at v ~ 3 THz are demonstrated with a five-level design that has a two-well injector region. Some of the key temperature degradation mechanisms for this design are discussed. For operation at lower frequencies (v < 2 THz), a one-well injector design is developed that reduces intersubband absorption losses in the injector region. A QCL operating at v = 1.59 THz (lambda = 188.5 mum) up to a heat-sink temperature of 71 K in cw mode is demonstrated with that design. To obtain high-power output and low beam divergence from the MM waveguides, a lens-coupled scheme is demonstrated. A peak power output of 145 mW, a beam width of 4.8deg, and a maximum lasing temperature of 160 K are obtained from a 4.1 THz QCL in this configuration. In the latter part of the paper, we report on the demonstration of video-rate (20 frames/s) terahertz imaging with QCLs as the source for illumination and a 320 times 240 element room-temperature microbolometer focal plane array as the detector. The QCLs for the imaging system are processed into semiinsulating surface-plasmon waveguides, and are operated in a cryogen-free thermomechanical cooler in quasi-CW mode at a heat-sink temperature of ~30 K. Real-time imaging in transmission mode is demonstrated at a standoff distance of 25 m with a v ~ 4.9 THz QCL in this setup.

Reduced free carrier absorption loss in midinfrared double heterostructure diode lasers grown by liquid phase epitaxy

An improved InAsSb∕InAsSbP double heterojunction ridge laser was designed and grown by liquid phase epitaxy. The cladding layer absorption loss was minimized by the introduction of two undoped quaternary layers on either side of the active region to form a five layer epitaxial structure. The inserted layers also helped alleviate interdiffusion of unwanted dopants into the active region and reduced current leakage in the device. The resulting diode lasers operate readily in pulsed mode near 3.5μm at elevated temperatures and with a threshold current density as low as 118Acm−2 at 85K. Compared to the conventional three-layer double heterostructure laser, the modified structure with reduced optical loss increased the maximum lasing temperature by 95–210K.

Effects of injector barrier on performance of terahertz quantum-cascade lasers

An experimental study of the effects of injector barrier thickness on the performance of terahertz quantum-cascade lasers is reported. The injector barrier is varied from 54 to 38 Å in an otherwise identical 3.4 THz laser structure. It is shown that the threshold current density at 10 K increases monotonically from 445 A/cm2 to 1.29 kA/cm2 with reducing barrier thickness. On the other hand, an optimum injector barrier thickness of 44 Å is found to give rise to a maximum lasing temperature of 141 K.

Terahertz quantum cascade lasers: Fabrication, characterization, and doping effect

The terahertz gap, lying roughly between 300GHz (0.3THz) and 30THz in the electromagnetic spectrum, exists because the frequencies generated by semiconductor devices based on transistors and lasers do not overlap. Generation of coherent terahertz radiation has traditionally involved either extending electronic techniques to higher frequencies or extending photonic sources to longer wavelengths. In both cases, the efficiency drops rapidly as the frequency approaches the terahertz region. We recently fabricated GaAs∕AlGaAs quantum cascade lasers, in which a high-confinement metal-metal waveguide was employed and fabricated using In–Au metallic bonding technique. The devices demonstrated lasing operation at a wavelength of around 104.6μm (or about 2.9THz in frequency). In this article, we first present the fabrication and electrical and optical characterizations of the terahertz quantum cascade lasers. We then characterized a set of terahertz quantum cascade lasers with otherwise identical device parameters but the doping concentration. The δ-doping density for each period was varied from 3.2×1010 to 4.8×1010cm−2. We observed that both the lasing threshold and the free carrier absorption caused the waveguide loss increase monotonically. Interestingly, however, the observed maximum lasing temperature displayed an optimum at a doping concentration of 3.6×1010cm−2.