Low Threshold Current Density Research Articles

Driving skyrmions with low threshold current density in Pt/CoFeB thin film

Magnetic skyrmions are topologically stable spin swirling particle like entities which are appealing for next generation spintronic devices. The expected low critical current density for the motion of skyrmions makes them potential candidates for future energy efficient electronic devices. Several heavy metal/ferromagnetic (HM/FM) systems have been explored in the past decade to achieve faster skyrmion velocity at low current densities. In this context, we have studied Pt/CoFeB/MgO heterostructures in which skyrmions have been stabilized at room temperature (RT). It has been observed that the shape of the skyrmions are perturbed even by the small stray field arising from low moment magnetic tips while performing the magnetic force microscopy (MFM), indicating presence of low pinning landscape in the samples. This hypothesis is indeed confirmed by the low threshold current density to drive the skyrmions in our sample, at velocities of few ∼10 m s−1.

Indium arsenide single quantum dash morphology and composition for wavelength tuning in quantum dash lasers

InAs quantum dot and dash gain media demonstrate performance benefits, such as lower threshold current densities and reduced temperature sensitivity over quantum wells for lasers operating in the C-band telecommunications window. Quantum dashes are of much interest for their higher gain over quantum dots due to an increased density of states. We combine experimental results and simulations to understand how quantum dash morphology and composition can be used to tune the emission wavelengths of these nanoparticles. Atomic force microscopy (AFM) analysis is performed to determine the effect of growth temperature and sublayer type on InAs/InGaAsP/InP nanoparticle morphology and homogeneity. Uncapped InAs nanoparticles grown by CBE on a GaAs sublayer will have dash-like geometries with heights up to 2.36 nm for growth temperatures of 500–540 °C. GaP sublayers will induce taller quantum dots except for a growth temperature of 530 °C, where quantum dashes form. The dimensions extracted from AFM scans are used in conjunction with photoluminescence data to guide parabolic band simulations of an InAs quantum dash with a GaP or GaAs sublayer and InP cap buried within InGaAsP. The calculated emission energy of a buried 30 × 300 nm quantum dash decreases by ∼100 meV for increasing heights from 1.5 to 2.5 nm, or increases by ∼100 meV by addition of 20% phosphorus in the dash and wetting layers. Modifying the quantum dash height and leveraging the As/P intermixing that occurs between the InAs and InP layers are, thus, most effective for wavelength tuning.

Optical gain and threshold current density of strained wurtzite GaN/AlGaN quantum dot lasers

In this work, the influences of biaxial compressive and tensile strains on optical gain and threshold current density are investigated theoretically as a function of the side lengths of the quantum box in the GaN/Al0.2Ga0.8N structure by using a model based on the density matrix theory of semiconductor lasers with relaxation broadening. For various side lengths of the quantum box, we compare the spectra gain curves of compressive, tensile-strained, and unstrained structures of the GaN/Al0.2Ga0.8N cubic quantum-dot (QD) laser. The dependence of peak optical gain on carrier density and the modal gain on current density is plotted too for all cases. The results reveal that many enhancements can be made to the laser structure by introducing -0.5% compressive strain: a higher value of optical gain of 18421 cm-1 at L=60A, a lower value of transparency of carrier density of Ntr=0.13*10¨¨19 cm-3and transparency current density of Jtr=26.9 A/cm and a lower threshold current density of Jth= 78.87 A/cm2 at L=100 A.

∼8.5 μm-emitting InP-based quantum cascade lasers grown on GaAs by metal-organic chemical vapor deposition

Room-temperature, pulsed-operation lasing of 8.5 μm-emitting InP-based quantum cascade lasers (QCLs), with low threshold-current density and watt-level output power, is demonstrated from structures grown on (001) GaAs substrates by metal-organic chemical vapor deposition. Prior to growing the laser structure, which contains a 35-stage In0.53Ga0.47As/In0.52Al0.48As lattice-matched active-core region, a ∼2 μm-thick nearly fully relaxed InP buffer with strained 1.6 nm-thick InAs quantum-dot-like dislocation-filter layers was grown. A smooth terminal buffer-layer surface, with roughness as low as 0.4 nm on a 10 × 10 μm2 scale, was obtained, while the estimated threading-dislocation density was in the mid-range × 108 cm−2. A series of measurements, on lasers grown on InP metamorphic buffer layers (MBLs) and on native InP substrates, were performed for understanding the impact of the buffer-layer's surface roughness, residual strain, and threading-dislocation density on unipolar devices such as QCLs. As-cleaved devices, grown on InP MBLs, were fabricated as 25 μm × 3 mm deep-etched ridge guides with lateral current injection. The results are pulsed maximum output power of 1.95 W/facet and a low threshold-current density of 1.86 kA/cm2 at 293 K. These values are comparable to those obtained from devices grown on InP: 2.09 W/facet and 2.42 kA/cm2. This demonstrates the relative insensitivity of the device-performance metrics on high residual threading-dislocation density, and high-performance InP-based QCLs emitting near 8 μm can be achieved on lattice-mismatched substrates.

Lower threshold current density of GaN-based blue laser diodes by suppressing the nonradiative recombination in a multiple quantum well.

The influence of the nonradiative recombination in a multiple quantum well of GaN-based blue laser diodes (LDs) has been are studied experimentally and theoretically by analyzing the optical and electrical properties of LDs with various thickness and indium content of quantum wells (QWs). It is found that when keeping the LD emission wavelength nearly unchanged, the LD device performance with thinner QW and higher indium content of InGaN QWs is much better than the LD with thicker QW and lower indium content, having smaller threshold current density, higher output optical power and larger slope efficiency. Typically, the threshold current density is as low as 0.69 kA/cm2, and the corresponding threshold current is only 250 mA. The lifetime is more than 10,000 hours at a fixed injection current of 1.2 A under a room-temperature continuous-wave operation. Characteristics of photoluminescence (PL) microscopy images, temperature dependent PL spectra, time-resolved PL and electroluminescence spectra demonstrate that a reduction of the nonradiative recombination centers and an improvement of homogeneity in QWs are the main reason for the performance improvement of GaN-based LD using thinner QW layers with a higher indium content in a certain range. Moreover, theoretical calculation results demonstrate that using a thinner quantum well is also helpful for improving the device performance if the change of alloy material quality is considered during the calculation.

High-Power Terahertz Quantum Cascade Lasers Based on High-Al-Composition Four Quantum Wells

High-power terahertz frequency quantum cascade lasers operating in both pulsed and continuous wave (CW) modes are demonstrated. High Al composition four quantum wells active region based on diagonal structure are employed to suppress the escape of electrons into continuum and thermally activated LO-phonon scattering, as evidenced by the activation energy value of ~14.4 meV extracted from the experimental data. The devices operate in a frequency range of 3.8 - 4.05 THz and have a pulsed maximum operating temperature of ~119 K. The total optical output power from an uncoated laser is 1.57 W and the maximum single-facet emission power from an HR coated one is 1.03 W in pulsed mode at 7 K. The maximum output power of ~256 mW is obtained by a 2.5-mm-long device operating in CW mode with a low threshold current density of 188.6 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 15 K.

Degenerate skyrmionic states in synthetic antiferromagnets

Topological magnetic textures, characterized by integer topological charge S, are potential candidates in future magnetic logic and memory devices, due to their smaller size and expected low threshold current density for their motion. An essential requirement to stabilize them is the Dzyaloshinskii–Moriya interaction (DMI) which promotes a particular chirality, leading to a unique value of S in a given material. However, recently coexistence of skyrmions and antiskyrmions, with opposite topological charge, in frustrated ferromagnets has been predicted using J 1–J 2–J 3 classical Heisenberg model, which opens new perspectives, to use the topological charge as an additional degree of freedom. In this work, we propose another approach of using a synthetic antiferromagnetic system, where one of the ferromagnetic (FM) layer has isotropic and the other FM layer has anisotropic DMI to promote the existence of skyrmions and antiskyrmions, respectively. A frustrated interaction arises due to the coupling between the magnetic textures in the FM layers, which enables the stabilization and coexistence of 6 novel elliptical topological textures.

Recent Developments of Quantum Dot Materials for High Speed and Ultrafast Lasers.

Owing to their high integration and functionality, nanometer-scale optoelectronic devices based on III-V semiconductor materials are emerging as an enabling technology for fiber-optic communication applications. Semiconductor quantum dots (QDs) with the three-dimensional carrier confinement offer potential advantages to such optoelectronic devices in terms of high modulation bandwidth, low threshold current density, temperature insensitivity, reduced saturation fluence, and wavelength flexibility. In this paper, we review the development of the molecular beam epitaxial (MBE) growth methods, material properties, and device characteristics of semiconductor QDs. Two kinds of III-V QD-based lasers for optical communication are summarized: one is the active electrical pumped lasers, such as the Fabry–Perot lasers, the distributed feedback lasers, and the vertical cavity surface emitting lasers, and the other is the passive lasers and the instance of the semiconductor saturable absorber mirrors mode-locked lasers. By analyzing the pros and cons of the different QD lasers by their structures, mechanisms, and performance, the challenges that arise when using these devices for the applications of fiber-optic communication have been presented.

Low threshold InAs-based interband cascade lasers grown by MBE

Mid-infrared interband cascade laser (ICL) structures aimed for emission at room temperature in the 4–5 μm wavelength region are grown by molecular beam epitaxy (MBE) on InAs substrates. High crystalline quality of the epitaxial structures has been confirmed by X-ray diffraction with thickness deviations less than 1% from the designs. Also, the average surface defect density is in the low ten to the fourth level. The broad-area (BA) devices made from the MBE-grown ICL wafers can lase in continuous wave (CW) mode in a wavelength range from 3.5 to 4.8 μm at temperatures up to 257 K, which is the highest reported for BA InAs-based ICLs at similar wavelengths. Their threshold current densities are low (e.g. 2.7 A/cm2 at 80 K), indicating excellent material quality with a very low Shockley-Reed-Hall recombination. In pulsed mode, the lowest threshold current density is 252 A/cm2 at 300 K, and the maximum operating temperature has reached 379 K. By comparisons in device performance among multiple MBE-grown ICL wafers, the importance of beam equivalent pressure stability and accurate control of layer thicknesses is demonstrated for the desirable performance.

Recent Progress of Quantum Dot Lasers Monolithically Integrated on Si Platform

With continuously growing global data traffic, silicon (Si)-based photonic integrated circuits have emerged as a promising solution for high-performance Intra-/Inter-chip optical communication. However, a lack of a Si-based light source remains to be solved due to the inefficient light-emitting property of Si. To tackle the absence of a native light source, integrating III-V lasers, which provide superior optical and electrical properties, has been extensively investigated. Remarkably, the use of quantum dots as an active medium in III-V lasers has attracted considerable interest because of various advantages, such as tolerance to crystalline defects, temperature insensitivity, low threshold current density and reduced reflection sensitivity. This paper reviews the recent progress of III-V quantum dot lasers monolithically integrated on the Si platform in terms of the different cavity types and sizes and discusses the future scope and application.

Исследование пространственных характеристик излучения поверхностно-излучающих квантово-каскадных лазеров с кольцевым резонатором

The results of studies of ring quantum-cascade lasers with surface emission due to a second-order grating formed in the top cladding layers are presented. Surface emission near 7.85 μm with a low threshold current density (3.8 kA/cm2), in comparison with ridge quantum-cascade lasers of the same cavity length is demonstrated. The results of measurements of the intensity distribution of the near and far fields at different pumping levels are presented. The estimated value of the angle of beam extraction relative to the surface normal is in the range (5.7–6.7)°.

Study of the spatial characteristics of emission of surface-emitting ring quantum-cascade lasers

The results of studies of ring quantum-cascade lasers with surface emission due to a second-order grating formed in the top cladding layers are presented. Surface emission near 7.85 μm with a low threshold current density (3.8 kA/cm2), in comparison with ridge quantum-cascade lasers of the same cavity length is demonstrated. The results of measurements of the intensity distribution of the near and far fields at different pumping levels are presented. The estimated value of the angle of beam extraction relative to the surface normal is in the range (5.7-6.7)o. Keywords: ring cavity, grating, focused ion beam (FIB) etching, superlattices, quantum-cascade laser, epitaxy, indium phosphide.

Heterostructures of quantum-cascade lasers with nonselective overgrowth by metalorganic vapour phase epitaxy

The possibility of fabrication of 4.6 μm spectral range quantum-cascade laser heterostructures by molecular-beam epitaxy technique with non-selective overgrowth by the metalorganic vapour-phase epitaxy is shown. The active region of the laser was formed on the basis of a heteropair of In0.67Ga0.33As/In0.36Al0.64As solid alloys. The waveguide claddings are formed by indium phosphide. The results of surface defects inspection and X-ray diffraction analysis of quantum-cascade laser heterostructures allow to conclude that the structural quality of the heterostructures is high and the estimated value of the root mean square surface roughness does not exceed 0.7 nm. Lasers with four cleaved facets exhibit lasing at room temperature with a relatively low threshold current density of the order of 1 kA/cm2. Keywords: superlattices, quantum-cascade laser, epitaxy, indium phosphide.

Gallium Arsenide Photonic Integrated Circuit Platform for Tunable Laser Applications

An active-passive integration technique for operation near a wavelength of 1030 nm has been developed on a gallium arsenide (GaAs) photonic integrated circuit platform. The technique leverages quantum wells (QWs) that are slightly offset vertically from the center of the waveguide, and selectively removed prior to upper cladding regrowth to form active and passive regions. The active region consists of indium gallium arsenide (InGaAs) QWs, gallium arsenide phosphide (GaAsP) barriers, GaAs separate confinement heterostructure layers, and aluminum gallium arsenide (AlGaAs) cladding. Fabry Perot lasers with various widths were fabricated and characterized, exhibiting high injection efficiency of 98.8%, internal active loss of 3.44 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , and internal passive loss of 4.05 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for 3 μm wide waveguides. The 3 μm, 4 μm, and 5 μm wide lasers demonstrated greater than 50 mW output power at 100 mA continuous wave (CW) current and threshold current as low as 9 mA. 20 μm wide broad area lasers demonstrated 240 mW output power, 35.2 mA threshold current under CW operation, and low threshold current density of 94 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 2 mm long lasers. Additionally, these devices exhibit transparency current density of 85 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and good thermal characteristics with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 205 K, and T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">η</sub> = 577K.

Температурные характеристики кольцевых лазеров с активной областью на основе InAs/InGaAs/GaAs-квантовых точек оптического диапазона 1.3 &amp;mu;m

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.

Heterostructures of quantum-cascade lasers with nonselective overgrowth by metalorganic vapour phase epitaxy

The possibility of fabrication of 4.6 μm spectral range quantum-cascade laser heterostructures by molecular-beam epitaxy technique with non-selective overgrowth by the metalorganic vapour-phase epitaxy is shown. The active region of the laser was formed on the basis of a heteropair of In0.67Ga0.33As/In0.36Al0.64As solid alloys. The waveguide claddings are formed by indium phosphide. The results of surface defects inspection and X-ray diffraction analysis of quantum-cascade laser heterostructures allow to conclude that the structural quality of the heterostructures is high and the estimated value of the root mean square surface roughness does not exceed 0.7 nm. Lasers with four cleaved facets exhibit lasing at room temperature with a relatively low threshold current density of the order of 1 kA/cm2. Keywords: superlattices, quantum-cascade laser, epitaxy, indium phosphide.

Low-threshold InP quantum dot and InGaP quantum well visible lasers on silicon (001)

Monolithically combining silicon nitride ( S i N x ) photonics technology with III-V active devices could open a broad range of on-chip applications spanning a wide wavelength range of ∼ 400 − 4000 n m . With the development of nitride, arsenide, and antimonide lasers based on quantum well (QW) and quantum dot (QD) active regions, the wavelength palette of integrated III-V lasers on Si currently spans 400 nm to 11 µm, with a crucial gap in the red-wavelength regime of 630–750 nm. Here, we demonstrate red I n 0.6 G a 0.4 P QW and far-red InP QD lasers monolithically grown on CMOS-compatible Si (001) substrates with continuous-wave operation at room temperature. A low-threshold current density of 550 A / c m 2 and 690 A / c m 2 with emission at 680–730 nm was achieved for QW and QD lasers on Si, respectively. This work represents a step toward the integration of visible red lasers on Si, allowing the utilization of integrated photonics for applications including biophotonic sensing, quantum computing, and near-eye displays.

E‐Band InAs/GaAs Trilayer Quantum Dot Lasers

The introduction of a closely stacked quantum dot (QD) structure is considered as one of the most effective approaches to extend the emission wavelength of InAs/GaAs QDs beyond 1.3 μm (1300 nm). Herein, a trilayer QD structure (three closely stacked QD layers) is proposed to further extend the emission wavelength of QDs. Room‐temperature (RT) emission at 1418 nm from InAs/GaAs trilayer QDs is demonstrated. Moreover, based on these results, an E‐band InAs/GaAs trilayer QD laser is fabricated on a GaAs substrate, achieving RT oscillation at 1370 nm with a low threshold current density of 96 A cm−2 under continuous‐wave mode operation. The results indicate that the trilayer QD structure is promising for realizing high‐performance QD lasers on GaAs substrate over 1300 nm without incorporating metamorphic buffer layers.

Performance characteristics of low threshold current 1.25 μm type-II GaInAs/GaAsSb ‘W’-lasers for optical communications

Type-II ‘W’-lasers have made an important contribution to the development of mid-infrared laser diodes. In this paper, we show that a similar approach can yield high performance lasers in the optical communications wavelength range. (GaIn)As/Ga(AsSb) type-II ‘W’ structures emitting at 1255 nm have been realised on a GaAs substrate and exhibit low room temperature threshold current densities of 200–300 A cm−2, pulsed output powers exceeding 1 W for 100 µm wide stripes, and a characteristic temperature T 0 ≈ 90 K around room temperature. Optical gain studies indicate a high modal gain around 15–23 cm−1 at 200–300 A cm−2 and low optical losses of 8 ± 3 cm−1. Analysis of the spontaneous emission indicates that at room temperature, up to 24% of the threshold current is due to radiative recombination, with the remaining current due to other thermally activated non-radiative processes. The observed decrease in differential quantum efficiency with increasing temperature suggests that this is primarily due to a carrier leakage process. The impact of these processes is discussed in terms of the potential for further device optimisation. Our results present strong figures of merit for near-infrared type-II laser diodes and indicate significant potential for their applications in optical communications.