Low Threshold Current Density Research Articles

Indium-flush technique for C-band InAs/InP quantum dots

High-quality InAs/InP quantum dots (QDs) emitting at 1550 nm are indispensable to realize high-performance telecom C-band lasers. In general, a longer emission (>1550 nm) with a broad spectral character has been obtained with InAs/InP QDs. Here, we proposed the use of the indium-flush (IF) method to shorten the emission and improve the optical properties of InAs/InP QDs. By exploiting IF, the full-width at half-maximum of the room-temperature QD photoluminescence spectra is narrowed from 89.2 to 47.9 meV, with a blue shift of 300 nm (from 1824 to 1522 nm). The scanning transmission electron microscopy and electron energy loss spectroscopy results reveal the atomic-level mechanism of the IF method, which uniformly modify the height of InAs/InP QDs in a controlled manner and form distinct Al-rich and In-rich regions. Finally, InAs/InP (001) QD lasers with the IF method have been demonstrated with a low threshold current density per QD layer of 106 A/cm2. We demonstrated both in terms of mechanism model and device performance that the IF method could serve as a robust strategy for the growth of high-performance C-band InAs/InP QD lasers via molecular beam epitaxy.

Comparison between InAs-based and GaSb-based interband cascade lasers with hybrid superlattice plasmon-enhanced claddings

We compare InAs-based and GaSb-based interband cascade lasers (ICLs) with the same 12-stage active region designed to emit at a wavelength of 4.6 µm. They employ a hybrid cladding architecture with the same geometry and inner claddings consisting of InAs/AlSb superlattices but different outer claddings: The InAs-based ICL employs plasmon enhanced n-type doped InAs layers while the GaSb-based ICL employs plasmon-enhanced n-type doped InAs0.915Sb0.085 claddings lattice matched to GaSb. Due to the lower refractive index of n + -InAsSb (n = 2.88) compared to n+ -InAs (n = 3.10) and higher refractive index of separate confinement layers, the GaSb-based ICL shows a 3.8% higher optical mode confinement in the active region compared to the InAs-based ICL. However, the InAs-based ICL has 15.3% lower free carrier absorption losses than the GaSb-based ICL, resulting in approximately equal threshold gains. Experimentally operated in pulsed mode and at room temperature, the GaSb-based ICL shows a 15.3% lower threshold current density, but also 12.8% higher threshold voltage resulting in comparable threshold power densities. Also presented is the influence of geometry and doping variation on confinement factors and calculated free carrier absorption losses in the GaSb-based ICL.

Reducing dislocations for room-temperature continuous-wave InGaAs/AlGaAs multiple quantum well lasers monolithically grown on Si

As dislocation-sensitive light-emitting devices, quantum well lasers monolithically grown on Si substrates operate poorly compared to their counterparts on native ones. In this work, a GaAs/Si virtual substrate with a low threading dislocation density of 8.9 × 106/cm2 was achieved by using a combined dislocation-reducing method. Meanwhile, by using two In-containing strained layer separately inserted into upper and lower AlGaAs cladding layers, the active region was virtually free of gliding dislocations. With the reduced dislocations in the active region, the fabricated Si-based broad-stripe ridged lasers exhibited continuous-wave lasing at room temperature, and a low threshold current density of 1015 A/cm2, high operation temperature of 90 °C as well as a device lifetime of over 87.25 h were achieved. These results demonstrate an experimentally feasible way to optimize a Si-based InGaAs/AlGaAs quantum well laser and further promote the research on monolithic integration.

Design of tensile-strained GeSn/SiGeSn structure for low threshold mid-infrared laser application

The plasticity of GeSn alloy energy band has promoted the development of silicon-based photoelectric integration and optical interconnection. A tensile-strained GeSn/SiGeSn double heterostructure laser wrapped with Si3N4 stress liner is designed and characterized theoretically. The triaxial tensile strain is introduced into the GeSn/SiGeSn heterostructure laser by the Si3N4 linear stressor. The lower threshold current density and higher optical gain of the GeSn/SiGeSn laser can be achieved by tuning the band structure and carrier distribution in the active region with tensile strain and Sn compositions. Compared with the relaxed device, the value of ne ,Γ/ne ,total for the Ge0.90Sn0.10/Si0.315Ge0.499Sn0.186 heterostructure laser wrapped with 300 nm Si3N4 linear stressor is increased to 30.6% at n e,total of 1018 cm−3, the laser λ can be extended to 3.44 μm, and the J th is reduced from 206 to 59 A/cm2.

5.0 μm emitting interband cascade lasers with superlattice and bulk AlGaAsSb claddings

We present a comparison between interband cascade lasers (ICLs) with a six-stage active region emitting at 5 μm with AlSb/InAs superlattice claddings and with bulk Al0.85Ga0.15As0.07Sb0.93 claddings. Utilizing bulk AlGaAsSb claddings with their lower refractive index compared to the more commonly used AlSb/InAs superlattice claddings, the mode-confinement in the active region increases by 14.4% resulting in an improvement of the lasing threshold current density. For broad area laser and under pulsed excitation, the ICL with AlGaAsSb claddings shows a lower threshold current density of Jth=396A/cm2 compared to Jth=521A/cm2 of the ICL with superlattice claddings. Additionally, a higher characteristic temperature was obtained for the ICL with bulk claddings. Emission in pulsed operation is observed up to 65 °C.

InAs quantum dots with a narrow photoluminescence linewidth for a lower threshold current density in 1.55 µm lasers

Uniform quantum dots (QDs) with a narrowed linewidth of photoluminescence (PL) are crucial for developing high-performance QD lasers. This study focuses on optimizing the growth conditions of InAs QDs on (001) InP substrates using metal-organic chemical vapor deposition (MOCVD), targeting applications in 1.55 µm QD lasers. By fine-tuning growth parameters such as the V/III ratio, deposition thickness, and growth temperature, we attained a QD density of 4.13 × 1010 cm−2. Further, a narrowed PL full width at half maximum (FWHM) of 40.1 meV was achieved in a five-stack InAs QD layer. This was accomplished using the double-cap technique, which reduced the height dispersion of QDs and shifted the emission wavelength to 1577 nm. Broad-area lasers incorporating a five-stack optimized InAs/InAlGaAs structure demonstrated a low threshold current density of 80 A/cm2 per QD layer, and a saturation power of 163 mW in continuous-wave (CW) mode at room temperature.

Watt-level continuous-wave antimonide laser diodes with high carrier-confined active region above 2.5 µm

Thanks to high performance above room temperature, antimonide laser diodes have shown great potential for broad application in the mid-infrared spectral region. However, the laser`s performance noticeably deteriorates due to the reduction of carrier confinement with increased emission wavelength. In this paper, a novel active region with higher carrier confinements both of electron and hole, by the usage of an indirect bandgap material of Al0.5GaAs0.04Sb as the quantum barrier, was put up to address the poor carrier confinement of GaSb-based type-I multi-quantum-well (MQW) diode lasers emission wavelength above 2.5 µm. The carrier confinement and the differential gain in the designed active region are enhanced as a result of the first proposed usage of an indirect-gap semiconductor as the quantum barrier with larger band offsets in conduction and valence bands, leading to high internal quantum efficiency and low threshold current density of our lasers. More importantly, the watt-level output optical power is obtained at a low injection current compared to the state of the art. Our work demonstrates a direct and cost-effective solution to address the poor carrier confinement of the GaSb-based MQW lasers, thereby achieving high-power mid-infrared lasers.

Design of enlarged-asymmetric-power single-mode silicon evanescent lasers with asymmetric distributed feedback gratings

Great achievements in the bonded III–V lasers have pushed the productization process of silicon photonic chips. In this work, we report the design of silicon evanescent lasers with asymmetric gratings, which can provide high output power and stable single-longitudinal-mode operation. The high coupling efficiency and low threshold current density are achieved by linearly expanding the width of rib waveguide from 0.4 µm to 2 µm through 150 µm taper coupler. By optimizing the λ/4 phase-shift position ratio from conventional 0.50 to 0.64, the ratio of output power at both sides of the bonded lasers (P2/P1) is improved from1.0 to 5.0, and the normalized grating coupling coefficient (κL) remains at 2.78. By optimizing the duty cycle at one side of λ/4 phase-shift from 0.5 to 0.8 and another side remains at 0.5, P2/P1 is 3.3, and κL lowers from 2.78 to 2.21. Besides, keeping the phase-shift position ratio at 0.64 while changing the duty cycle to 0.7, P2/P1 increases significantly to 7.5 with κL of 2.59. This work provides a step forward towards the development of high-power silicon-based light sources.

An InGaAs Vertical-Cavity Surface-Emitting Laser Emitting at 1130 nm for Silicon Photonics Application

A highly strained InGaAs quantum well (QW) vertical-cavity surface-emitting laser (VCSEL) with low threshold current density, high efficiency and output power emissions around 1130 nm was grown by MOCVD. Its static characteristics at room temperature and high operation temperature were studied in detail. The 7 μm oxide aperture device exhibits a threshold current of 0.68 mA, corresponding to a threshold current density of 1.7 kA/cm2. The slope efficiency is 0.43 W/A and the maximum output power is 3.3 mW. Continuous-wave (CW) operation in the 10–80 °C temperature range is observed. The slope efficiency is almost constant at 10–80 °C. The threshold current becomes lower at high temperatures thanks to the alignment between gain peak and cavity mode. The 3 μm oxide aperture device’s lasing in single mode with the RMS spectral width of 0.163 nm and orthogonal polarization suppression ratio (OPSR) is ~15 dB at 25 °C. The small-signal response analysis indicates that reducing the parasitics of the device and refining the fabrication process will improve the dynamics response characteristics. These results indicate that the 1130 nm GaAs-based VCSEL with highly strained InGaAs QWs is expected to be used as source for silicon photonics.

Monolithically integrated 940 nm VCSELs on bulk Ge substrates.

This research successfully developed an independent Ge-based VCSEL epitaxy and fabrication technology route, which set the stage for integrating AlGaAs-based semiconductor devices on bulk Ge substrates. This is the second successful Ge-based VCSEL technology reported worldwide and the first Ge-based VCSEL technology with key details disclosed, including Ge substrate specification, transition layer structure and composition, and fabrication process. Compared with the GaAs counterparts, after epitaxy optimization, the Ge-based VCSEL wafer has a 40% lower surface root-mean-square roughness and 72% lower average bow-warp. After device fabrication, the Ge-based VCSEL has a 10% lower threshold current density and 19% higher maximum optical differential efficiency than the GaAs-based VCSEL.

Continuous-wave operation of 1550 nm low-threshold triple-lattice photonic-crystal surface-emitting lasers

Benefitting from narrow beam divergence, photonic crystal surface-emitting lasers are expected to play an essential role in the ever-growing fields of optical communication and light detection and ranging. Lasers operating with 1.55 μm wavelengths have attracted particular attention due to their minimum fiber loss and high eye-safe threshold. However, high interband absorption significantly decreases their performance at this 1.55 μm wavelength. Therefore, stronger optical feedback is needed to reduce their threshold and thus improve the output power. Toward this goal, photonic-crystal resonators with deep holes and high dielectric contrast are often used. Nevertheless, the relevant techniques for high-contrast photonic crystals inevitably complicate fabrication and reduce the final yield. In this paper, we demonstrate the first continuous-wave operation of 1.55 μm photonic-crystal surface-emitting lasers by using a ‘triple-lattice photonic-crystal resonator’, which superimposes three lattice point groups to increase the strength of in-plane optical feedback. Using this geometry, the in-plane 180° coupling can be enhanced threefold compared to the normal single-lattice structure. Detailed theoretical and experimental investigations demonstrate the much lower threshold current density of this structure compared to ‘single-lattice’ and ‘double-lattice’ photonic-crystal resonators, verifying our design principles. Our findings provide a new strategy for photonic crystal laser miniaturization, which is crucial for realizing their use in future high-speed applications.

Low-threshold 2 µm InAs/InP quantum dash lasers enabled by punctuated growth.

2 µm photonics and optoelectronics is promising for potential applications such as optical communications, LiDAR, and chemical sensing. While the research on 2 µm detectors is on the rise, the development of InP-based 2 µm gain materials with 0D nanostructures is rather stalled. Here, we demonstrate low-threshold, continuous wave lasing at 2 µm wavelength from InAs quantum dash/InP lasers enabled by punctuated growth of the quantum structure. We demonstrate low threshold current densities from the 7.1 µm width ridge-waveguide lasers, with values of 657, 1183, and 1944 A/cm2 under short pulse wave (SPW), quasi-continuous wave (QCW), and continuous wave operation. The lasers also exhibited good thermal stability, with a characteristic temperature T0 of 43 K under SPW mode. The lasing spectra is centered at 1.97 µm, coinciding with the ground-state emission observed from photoluminescence studies. We believe that the InAs quantum dash/InP lasers emitting near 2 µm will be a key enabling technology for 2 µm communication and sensing.

Pushing the performance limits of long wavelength interband cascade lasers using innovative quantum well active regions

We report significantly enhanced device performance in long wavelength interband cascade lasers (ICLs) by employing a recently proposed innovative quantum well (QW) active region containing strained InAsP layers. These ICLs were able to operate at wavelengths near 14.4 μm, the longest ever demonstrated for III–V interband lasers, implying great potential of ICLs to cover an even wider wavelength range. Also, by applying the aforesaid QW active region configuration on ICLs at relatively short wavelengths, ICLs were demonstrated at a low threshold current density (e.g., 13 A/cm2 at 80 K) and at temperatures up to 212 K near 12.4 μm, more than 50 K higher than the previously reported ICLs with the standard W-shape QW active region at similar wavelengths. This suggests that the QW active region with InAsP layers can be used to improve device performance at the shorter wavelengths.

Continuous-wave operation of Si-based 1.31 μm InAs/GaAs quantum-dot laser grown by molecular beam epitaxy

Excellent performance III-V quantum-dot (QD) lasers grown on Si substrates by molecular beam epitaxy (MBE) are the most promising candidates for commercially viable Si-based lasers. This makes coveted chip-to-chip and system-to-system optical interconnections feasible. This paper reports the realization of high performance 1.31 μm InAs/GaAs QD lasers on a Si substrate with all-MBE The transition from Si to GaAs was realized using Ge as the intermediary layer, and the InAs/GaAs QD laser structure was grown on the GaAs/Ge buffer. Under continuous wave (CW) operation mode, a low threshold current density of 375 A cm−2, high output power of 63 mW, and high operating temperature of 80 °C, have been achieved using Si-based InAs QD lasers with a narrow ridge structure. It has great potential for application in the development of Si-based photonic integration circuits.

Large damping-like torque and efficient current-induced magnetization reversal in Ti/Tb–Co/Cr structures

Seeking the magnetic heterostructures with large current-induced torque efficiency is currently one of the core hotspots in spintronics. In this work, we report the large and composition-dependent damping-like (DL) torque in the structure consisting of light metal Cr and Ti layers and a ferrimagnetic Tb–Co layer. The DL torque efficiency in the structures reaches a maximum (around −0.55) as the Tb content in the Tb–Co layer is in the range of 0.15–0.18. This composition-dependent behavior is different from that observed in the usual heavy-metal/ferrimagnetic structures. We also demonstrated the efficient current-induced magnetization reversal in these structures with a low threshold current density down to 8×1010 A/m2. In comparison, only very low efficiency values of −0.06 and −0.086 are obtained in the Ti/Co/Cr and Ti/Tb–Co/SiN control samples, respectively, suggesting that the large DL torque in the Ti/Tb–Co/Cr structures may originate from the orbital Hall effect in the Cr metal. By fitting the Cr layer thickness dependence of the torque efficiency with a simple orbital current diffusion model, we obtained an effective orbital Hall angle of −0.57±0.02 for the Ti/Tb0.85Co0.15/Cr samples. This work demonstrated the possibility to enhance the orbital torque effect by using the magnetic layer containing the element with strong spin–orbit coupling.

Surface emitting quantum-cascade lasers with a second-order grating and increased coupling coefficient

The results of studying the characteristics of surface-emitting quantum-cascade ring lasers with a second-order grating are presented. The selection of etching receipt by ion beam lithography made it possible to increase the coupling coefficient to 12 cm–1. Lasing close to 7.6 μm with a low threshold current density (about 0.3 kA/cm2) is demonstrated.

Low-threshold visible InP quantum dot and InGaP quantum well lasers grown by molecular beam epitaxy

III-V lasers based on self-assembled quantum dots (QDs) have attracted widespread interest due to their unique characteristics, including low threshold current density (Jth), low sensitivity to backreflections, and resistance to threading dislocations. While most work to date has focused on 1.3 μm InAs/GaAs QDs, InP QDs have also aroused interest in lasers emitting at visible wavelengths. Molecular beam epitaxy (MBE) enables the growth of high-density InP/AlGaInP QDs on exact (001)-oriented GaAs substrates but requires a relatively low substrate temperature of <500 °C. The low substrate temperature used for phosphide growth in MBE leads to degraded optical properties and makes post-growth annealing a crucial step to improve the optical quality. Here, we report the exceptional thermal stability of InP/AlGaInP QDs grown using MBE, with up to 50× improvement in room temperature photoluminescence intensity with the optimization of annealing temperature and time. We also demonstrate the room temperature pulsed operation of InP multiple quantum dot (MQD) lasers on GaAs (001) emitting close to 735 nm with Jth values of 499 A/cm2 after annealing, a factor of 6 lower than their as-grown counterparts and comparable to such devices grown by MOCVD. In0.6Ga0.4P single quantum well (SQW) lasers on GaAs (001) also exhibit a substantial reduction in Jth from 340 A/cm2 as-grown to 200 A/cm2 after annealing, emitting at 680 nm under pulsed operation conditions. This work shows that post-growth annealing is essential for realizing record-performance phosphide lasers on GaAs grown by MBE for applications in visible photonics.

Effect of dislocations on the performance of GaSb-based diode lasers grown on silicon

Silicon photonics is a promising technology for the fabrication of dense photonic chips, thanks to the very mature silicon industry. The direct epitaxial growth of III–V lasers on silicon is one of the main challenges for the realization of compact and robust mid-infrared sensors based on photonic integrated circuits. The crystal defects arising from this heteroepitaxial growth affect the laser performance and, therefore, need to be mitigated but also studied to better understand their impact on the laser operation. Here, we studied the effect of threading dislocations on laser performance by comparing the series of GaSb-based diode lasers grown on native GaSb and Si substrates with different numbers of quantum wells (nQW) in their active zones. As expected, the laser threshold currents are higher in the case of diode lasers on Si, and they rapidly vary with nQW. Still, the lowest threshold current densities are achieved with nQW = 1 for both substrates. With the help of a theoretical gain model, we attribute these results to the fact that dislocations create non-radiative recombination but do not introduce additional optical losses. This work allows a better understanding of the origin of performance degradation and the decision to be made regarding the heterostructure design.

Room temperature spin-orbit torque efficiency and magnetization switching in SrRuO3 -based heterostructures

Spin-orbit torques (SOTs) from transition metal oxides (TMOs) in conjunction with magnetic materials have recently attracted tremendous attention for realizing high-efficient spintronic devices. ${\mathrm{SrRuO}}_{3}$ is a promising candidate among TMOs due to its large and tunable SOT efficiency as well as high conductivity and chemical stability. However, a further study for benchmarking the SOT efficiency and realizing SOT-driven magnetization switching in ${\mathrm{SrRuO}}_{3}$ is still highly desired so far. Here, we systematically study the SOT properties of high-quality ${\mathrm{SrRuO}}_{3}$ thin film heterostructuring with different magnetic alloys of both IMA and PMA configuration by the harmonic Hall voltage technique. Our results indicate that ${\mathrm{SrRuO}}_{3}$ possesses pronounced SOT efficiency of about 0.2 at room temperature regardless of the magnetic alloys, which is comparable to typical heavy metals (HMs). Furthermore, we achieve SOT-driven magnetization switching with a low threshold current density of $3.8\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.28em}{0ex}}\mathrm{A}/{\mathrm{m}}^{2}$, demonstrating the promising potential of ${\mathrm{SrRuO}}_{3}$ for practical devices. By making a comprehensive comparison with HMs, our work unambiguously benchmarks the SOT properties and concludes the advantages of ${\mathrm{SrRuO}}_{3}$, which may bring more diverse choices for SOT applications by utilizing hybrid-oxide/metal and all-oxide systems.

Near-Full Current Dynamic Range THz Quantum Cascade Laser Frequency Comb.

The present study proposes a terahertz quantum cascade laser frequency comb (THz QCL FC) with a semi-insulated surface plasma waveguide characterized by a low threshold current density, high power and a wide current dynamic range. The gain dispersion value and the nonlinear susceptibility were optimized based on the combination of a hybrid bound-to-continuum active region with a semi-insulated surface plasmon waveguide. Without any extra dispersion compensator, stable frequency comb operation within a current dynamic range of more than 97% of the total was revealed by the intermode beat note map. Additionally, a total comb spectral emission of about 300 GHz centered around 4.6 THz was achieved for a 3 mm long and 150 µm wide device. At 10 K, a maximum output power of 22 mW was obtained with an ultra-low threshold current density of 64.4 A·cm-2.