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We investigate the anisotropic optical gain in non-c-plane InGaN quantum wells with 20% indium content including band-gap renormalization and the screening of the quantum confined Stark effect. Waveguide modes and their polarizati...
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We investigate the anisotropic optical gain in non-c-plane InGaN quantum wells with 20% indium content including band-gap renormalization and the screening of the quantum confined Stark effect. Waveguide modes and their polarizations are determined as TE and TM modes or extraordinary and ordinary modes, depending on the birefringence and the orientation of the laser diode's ridge waveguide relative to the c axis. The band structures and optical matrix elements along the polarization directions are calculated using a 6×6 k·p Hamiltonian and a self-consistent Schrodinger-Poisson solver. From these calculations the reduced density of states and the optical gain for the different polarizations are determined in the free-carrier picture with an ad hoc inclusion of the band-gap renormalization and compared to a c-plane quantum well. It is found that for high indium concentrations the gain can be significantly increased by going from the c plane to a semipolar or a nonpolar crystal orientation. However, due to birefringence and composition of the topmost valence-band wave function, the ridge has to be oriented along the [1123] direction for semipolar and along the [0001] direction for nonpolar laser diodes.
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We show the temperature-dependent random lasing characteristics of photoexcited GaAs powders from 30 to 300 K. The lasing properties strongly depend on the temperature, i.e., the lasing peak energy increases and the threshold exci...
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We show the temperature-dependent random lasing characteristics of photoexcited GaAs powders from 30 to 300 K. The lasing properties strongly depend on the temperature, i.e., the lasing peak energy increases and the threshold excitation power decreases as the temperature decreases. A theoretical model, in which the gain spectra of heavily doped n-GaAs are taken into consideration, well describes the temperature dependence of the lasing peak energy. The temperature dependence of the threshold excitation power can be also explained by the model qualitatively.
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InGaAsP/InGaAsP multiple-quantum-well (MQW) double-channel planar-buried heterostructure (DCPBH) lasers emitting at λ ～ 1.57 μm were fabricated by optimizing the epitaxial growth with material characterization. At 25 ℃ (85 ℃)...
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InGaAsP/InGaAsP multiple-quantum-well (MQW) double-channel planar-buried heterostructure (DCPBH) lasers emitting at λ ～ 1.57 μm were fabricated by optimizing the epitaxial growth with material characterization. At 25 ℃ (85 ℃), a 1.8-μm-wide and 300-μm-long antireflectivity/high reflectivity coated laser exhibits a threshold current of 8 mA (23 mA) and a slope efficiency of 0.34 mW/mA (0.24 mW/mA) in continuous-wave mode (cw) as a result of the optimized thickness of the p-InP filling layer in the PBH structure with p-n-p-n current blocking layers. The maximum cw output power was approximately 20 MW at 25 ℃, which was reduced to 17 mW at 85 ℃. The peak wavelength varied from 1572 nm at 25 ℃ to 1602 nm at 100 ℃ for a fixed output power of 5 mW, indicating a temperature coefficient of ～ 0.4 nm/K. The resonance frequency in the small-signal modulation response of devices depends on the etching depth of the U-shaped double channel; it increases from 0.4 GHz without channel etching to 4.3 GHz with 7-μm-thick etching. The full-width at half maximum values in the horizontal and vertical far-field patterns were approximately 24.5° (25.2°) and 29.4° (30.1°), respectively, at 25 ℃ (85 ℃).
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High-temperature, high-power, and continuous-wave (CW) operation of quantum-cascade lasers with 35 active/injector stages at λ ～ 8.85 μm above room temperature is achieved without using a buried heterostructure. At this long wa...
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High-temperature, high-power, and continuous-wave (CW) operation of quantum-cascade lasers with 35 active/injector stages at λ ～ 8.85 μm above room temperature is achieved without using a buried heterostructure. At this long wavelength, the use of a wider ridge waveguide in an epilayer-down bonding scheme leads to a superior performance of the laser. For a high-reflectivity-coated 21 μm × 3 mm laser, the output power of 237 mW and the threshold current density of 1.44 kA/cm~2 at 298 K under CW mode are obtained with a maximum wall-plug efficiency of 1.7%. Further improvements were observed by using a 4-mm-long cavity. The device exhibits 294 mW of output power at 298 K and it operates at a high temperature, even up to 358 K (85℃). The full widths at half-maximum of the laser beam in CW operation for the parallel and the perpendicular far-field patterns are 25°and 63°, respectively.
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We discuss various mechanisms of laser diode degradation based on our own experiments and on the available literature data. In most of the cases, degradation of InGaN laser diodes occurs through the increase of the threshold curre...
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We discuss various mechanisms of laser diode degradation based on our own experiments and on the available literature data. In most of the cases, degradation of InGaN laser diodes occurs through the increase of the threshold current with almost constant slope efficiency. The threshold current change follows frequently the square root on time dependence. Though this type of behavior has usually been attributed to magnesium acceptor diffusion, no firm proof of such a hypothesis has so far been presented. In contrast, there is an increasing number of reported experiments showing that the most important factor contributing to fast (hours), and medium time (hundreds of hours) degradation is the process of carbon deposition. This process involves photochemical reactions leading to the decomposition of hydrocarbons existing in the laser diode environment. This process resembles very closely the mechanism responsible for 980-nm laser diode degradation and known as Package Induced Failure.
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We successfully developed high-power and long-lived pure blue laser diodes (LDs) having an emission wavelength of 440-450 nm. The pure-blue LDs were grown by metalorganic chemical vapor deposition (MOCVD) on GaN substrates. The di...
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We successfully developed high-power and long-lived pure blue laser diodes (LDs) having an emission wavelength of 440-450 nm. The pure-blue LDs were grown by metalorganic chemical vapor deposition (MOCVD) on GaN substrates. The dislocation density was successfully reduced to ～ 10~6 cm~(-2) by optimizing the MOCVD growth conditions and the active layer structure. The vertical layer structure was designed to have an absorption loss of 4.9 cm~(-1) and an internal quantum efficiency of 91%. We also reduced the operating current density to 6 kA/cm~2 under 750 mW continuous-wave operation at 35℃ by optimizing the stripe width to 12 μm and the cavity length to 2000 μm. The half lifetimes in constant current mode are estimated to be longer than 10000 h.
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Nitride based violet laser diodes grown on GaN substrates have been investigated. A grooved substrate, having a striped pattern with a pitch of 400 μm, has been prepared and epitaxial growth has been performed. No cracks have bee...
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Nitride based violet laser diodes grown on GaN substrates have been investigated. A grooved substrate, having a striped pattern with a pitch of 400 μm, has been prepared and epitaxial growth has been performed. No cracks have been observed in the epi-layers. The violet laser device has been fabricated and characterised. At a pulsed output power of 210 mW, the operating current and the voltage have been 164mA and 6.6V, respectively. The mean-time-to-failure has been estimated to be over 3000 h under pulsed condition with an output power of 210 mW at 80℃.
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Photomodulated reflectance (PR) studies are conducted on the wafer of a vertical-cavity surface-emitting laser (VCSEL) designed for oxygen sensing up to high temperatures. By varying the angle of incidence, the VCSEL cavity mode (...
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Photomodulated reflectance (PR) studies are conducted on the wafer of a vertical-cavity surface-emitting laser (VCSEL) designed for oxygen sensing up to high temperatures. By varying the angle of incidence, the VCSEL cavity mode (CM) is tuned through the positions of two static PR features, revealing the ground-state electron to heavy-hole and light-hole quantum well (QW) transition energies. The PR is also measured over a large temperature range, revealing that the CM becomes tuned to the heavy-hole QW transition at a temperature of 115 ± 1℃, where the energies coincide at ～1.631 eV (760 nm). It is observed that when tuned, the width of the CM feature in the reflectance spectrum is maximised. Reflectance simulations confirm that this broadening occurs when the CM is tuned with an excitonic transition, where the cavity absorption is maximised. Therefore, monitoring this broadening as the energies of the CM and QW are varied relative to each other could provide an additional, non-destructive technique for determining the VCSEL ground-state QW transition energy and the conditions under which it is tuned to the CM.
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The paper presents an overview of recent activity in fabrication by molecular beam epitaxy and study of AlGaSbAs/InAs/(ZnTe)/Cd(Mg)Se hybrid pseudomorphic heterostructures with an InAs/Ⅱ-Ⅵ heterovalent interface either in the ac...
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The paper presents an overview of recent activity in fabrication by molecular beam epitaxy and study of AlGaSbAs/InAs/(ZnTe)/Cd(Mg)Se hybrid pseudomorphic heterostructures with an InAs/Ⅱ-Ⅵ heterovalent interface either in the active region (in case of mid-IR laser diodes) or in the InAs quantum well (QW). Different approaches to fabrication of the defect-free InAs/Ⅱ-Ⅵ interface are discussed, as well as their effect on crystalline properties and an electronic band structure at the interface. The hybrid mid-IR laser diodes are characterized by a dramatically improved carrier (hole) and optical confinement in the InAs-based active region. Clear confining effects are observed in the photoluminescence from a ～7-nm-thick InAs QW with CdMgSe and AlAsSb barriers. An existence of 2D electron gas in such QWs of ～16 nm in a thickness is proved by observation of Shubnikov-de Haas oscillations. The obtained results are prospective both for mid-IR optoelectronics and spintronics.
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