Improvement of radiative recombination rate in deep ultraviolet laser diodes with step-like quantum barrier and aluminum-content graded electron blocking layer

The design of the active region structures, including the modifications of structures of the quantum barrier(QB) and electron blocking layer(EBL), in the deep ultraviolet(DUV) Al Ga N laser diode(LD) is investigated numerically with the Crosslight software. The analyses focus on electron and hole injection efficiency, electron leakage, hole diffusion,and radiative recombination rate. Compared with the reference QB structure, the step-like QB structure provides high radiative recombination and maximum output power. Subsequently, a comparative study is conducted on the performance characteristics with four different EBLs. For the EBL with different Al mole fraction layers, the higher Al-content Al Ga N EBL layer is located closely to the active region, leading the electron current leakage to lower, the carrier injection efficiency to increase, and the radiative recombination rate to improve.     

Theoretical estimates of optimal parameters for quantum cascade lasers

The quantum cascade (QC) laser is a new light source which is based on one type of carrier (electrons) making transitions between energy levels created by quantum confinement. In this paper, focusing on the working conditions which a QC laser should satisfy, we have discussed the subband lifespans in QC laser active regions. The results show that the population inversion condition can be achieved by resonant tunneling associated with an optical phonon, and this population inversion can be facilitated by the short escaping time of electrons from one active region to the neighboring active region. Our calculations also show that the lifespans of levels 3 and 2 are dominated by the phonon scattering time, and the escaping time from one active region to the next active region is determined by the thickness of exit barrier and the proper design of the miniband between the active regions.     

Improvement of performance characteristics of deep violet InGaN DQW lasers using a strip DQW active region

The effect of the laser ridge width on the performance characteristics of deep violet In_0.082Ga_0.918N/GaN double quantum well (DQW) laser diodes (LDs) has been numerically investigated. Simulation results indicated that threshold current of LDs is decreased and slope efficiency and differential quantum efficiency (DQE) are increased by decreasing ridge width, whereas output power is decreased. The results also showed that a decrease of more than 1 μm in the ridge width reduces the threshold current, whereas the slope efficiency, output power, and DQE are decreased. A new DQW LD structure with a strip active region has been proposed to obtain a lower current threshold and higher output power, slope efficiency, and DQE. The results showed the InGaN DQW LD with a strip DQW active region has the highest output power, slope efficiency, and DQE; it also has a lower threshold current compared with that of the original LD. The comparative study conducted for the LDs with output emission wavelengths of 390, 414 and 436 nm has also confirmed the enhancement in LD performance using the strip DQW active region structure.     

High-quality MBE growth of AlχGa1-χ As-based THz quantum cascade lasers

High-quality GaAs-based quantum cascade laser (QCL) structures for the terahertz (THz) emission have been grown by solid source molecular-beam epitaxy. Ex-situ high-resolution x-ray diffraction shows that layer thickness and its control is the most critical growth aspect and that the lasing potential of the structure can be determined by the thickness accuracy of the layers. For our samples, the thickness tolerance for working lasing structures emitting approximately 100 μm was determined to be minimally above 1% for a 15 μm active region which was composed of 54.6 nm cascade cells. Increasing interface roughness adversely affects the lasing threshold and power. Presented at 5-th International Conference Solid State Surfaces and Interfaces, November 19–24, 2006, Smolenice Castle, Slovakia     

Optimization of active layer structures to minimize leakage current for an AlGaInP laser diode

Theoretical analysis for different active layer structures under the same waveguide confinement is conducted to minimize the electron overflow from the active layer to the p-cladding layer for the AlGaInP laser diode. An active layer with five quantum wells and a (Al_xGa_1-x)InP barrier with an x composition of 0.5 has found to be the optimal structure for the AlGaInP laser diode suitable for DVD-ROM and DVD player. Experimental results have confirmed that the characteristic temperature can be as high as 110 K at far field angles of 29°/9° for this optimized AlGaInP laser diode. PACS 42.55.Px; 73.40.-c; 78.20.Bh     

Potential‐inserted quantum well design for quantum cascade terahertz lasers

We report on a new design of terahertz quantum cascade laser based on a single, potential‐inserted quantum well active region. The quantum well properties are engineered through single monolayer InAs inserts. The modeling is based on atomistic, spds* tight‐binding calculations, and performances are compared to that of the classical three‐well design. We obtain a 100% increase of the oscillator strength per unit length, while maintaining a high, nearly temperature‐independent contrast between phonon‐induced relaxation times of the upper and lower lasing states. The improved performances are expected to allow THz lasing at room temperature.     

External cavity quantum dot tunable laser through 1.55 μm

The optical performance of InAs/InGaAsP quantum dot (QD) lasers grown on (1 0 0) InP was studied for three different material structures. The most efficient QD laser structure, having a threshold current of 107 mA and an external differential quantum efficiency of 9.4% at room temperature, was used to form the active region of a grating-coupled external cavity tunable laser. A tuning range of 110 nm was demonstrated, which was mainly limited by the mirror and internal losses of the uncoated laser diode. Rapid state-filling of the QDs was also demonstrated by observing the evolution of the spectra with increasing injected current.     

Room‐temperature terahertz emission from ZnSe‐based quantum cascade structures: A simulation study

We identified conditions for room‐temperature operation of terahertz quantum cascade lasers (THz QCLs) where variable barrier heights are used on ZnSe/Zn_1–xMg_x Se material systems. The THz QCL devices are based on three‐level two‐well design schemes. The THz QCL lasers with alternating quantum barriers with different heights were compared with THz QCL laser structures with fixed quantum barrier heights. It is found that the THz QCL device with novel design employing variable barrier heights achieved the terahertz emission of about 1.45 THz at room‐temperature (300 K), and has improved laser performance due to the suppression of thermally activated carrier leakage via higher‐energy parasitic levels. Thus, THz QCL devices employing the design with variable barrier heights may lead to future improvements of the operating temperature and performance of THz QCL lasers. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

InGaN/GaN laser diode characterization and quantum well number effect

The effect of quantum well number on the quantum efficiency and temperature characteristics of In- GaN/GaN laser diodes （LDs） is determined and investigated. The 3-nm-thick In0.13Ca0.87N wells and two 6-am-thick GaN barriers are selected as an active region for Fabry-Perot （FP） cavity waveguide edge emitting LD. The internal quantum efficiency and internal optical loss coefficient are extracted through the simulation software for single, double, and triple InGaN/GaN quantum wells. The effects of device temperature on the laser threshold current, external differential quantum efficiency （DQE）, and output wavelength are also investigated. The external quantum efficiency and characteristic temperature are improved significantly when the quantum well number is two. It is indicated that the laser structures with many quantum wells will suffer from the inhomogeneity of the carrier density within the quantum well itself which affects the LD performance.     

有源区掺杂的AlGaN基深紫外激光二极管性能优化

为了改善深紫外激光二极管的性能,本文提出了有源区量子势垒n掺杂、p掺杂和n-p掺杂三种结构.利用Crosslight软件,对原始结构和有源区掺杂的三种结构进行仿真研究,比较四种结构的P-I特性曲线、V-I特性曲线、载流子浓度、辐射复合速率和能带图.仿真结果表明,有源区量子势垒n-p掺杂结构的性能更优,其阈值电压和阈值电流分别为4.40V和23.8mA;辐射复合速率达到1.64×10²⁸cm^-3/s;同一注入电流下电光转换效率达到42.1%,比原始结构增加了3.9%;改善了深紫外激光二极管的工作性能.     

Optimization of cubic GaN/AlGaN quantum cascade structures for negative refraction in the THz spectral range

In this work we theoretically investigate a possibility to use cubic nitride based multi-layer periodic nanostructure as a semiconductor metamaterial. The structure design is based on an active region of a quantum cascade laser optimized to achieve optical gain in the Terahertz (THz) spectral range. In particular, we test the GaN/AlGaN quantum well configurations, which should exhibit important advantages compared to GaAs-based structures, namely room temperature operation without the assistance of magnetic field and lower doping densities. Our numerical rate-equations model is solved self-consistently and it takes into account electron-longitudinal optical phonon scattering between all the relevant states among the adjacent periods of the structure. A global optimization routine, specifically genetic algorithm is then used to generate new gain-optimized structures. This work confirms the advantages of cubic GaN designs over GaAs ones, namely feasibility of negative refraction at room temperature without the assistance of magnetic field while keeping the doping densities of the same order of magnitude.     

Lateral Current Injection (LCI) Multiple Quantum-Well 1.55 μm Laser with Improved Gain Uniformity Across the Active Region

A simulation study of lateral current injection 1.55 m laser with strain-compensated multiple quantum-well (MQW) active region (InGaAsP well, InGaAlAs barrier) is presented using self-consistent 2D numerical simulations. The effects of different mesa width and p-doping in the QWs on the carrier and gain uniformity across the active region are explored. A high p-doping in the quantum wells is found to increases the carrier and gain non-uniformity across the active region. The QW region close to the n-contact side does not provide much gain at high optical powers. An asymmetric optical waveguide design is proposed to help reduce the gain non-uniformity across the active region. By shifting the optical modal peak toward the p-side, the modal overlap between the gain region and the optical mode is improved and a more even carrier and gain distribution is obtained. However, due to reduced bandgap of the quaternary InGaAsP p-cladding, an enhanced electron leakage out of the QWs into the p-cladding degrades the laser efficiency and increases the threshold current. Transient time–domain simulations are also performed to determine the small-signal modulation response of the laser promising a simulated high modulation bandwidth suitable for direct-modulation applications.     

The effects of quantum wells number and the built-in polarization on the performance of quaternary AlInGaN UV laser diode

The performance of quaternary Al_0.08In_0.08Ga_0.84N multi-quantum well (MQW) laser diodes (LDs) using the simulation program of Integrated System Engineering Technical Computer Aided design (ISE TCAD) was studied. The simulation results show that the low threshold current, high output power and slope efficiency can be obtained when the quantum wells number is 4. Although, the fourth quantum well which placed in the right side (n-side) of the active region has a negative value of optical gain this means that the optical gain does not occur in this quantum well of laser structure. However, high external differential quantum efficiency (DQE) inside the active region was also observed. Optical gain and intensity were increased when the numbers of quantum wells increase reached 4. The built-in electric field effect inside the quantum well leads to the reduction of the overlap integral between the electrons and holes by separating their wave function was included. As well as, Al_0.25In_0.08Ga_0.67N electron blocking layer (EBL) employed to enhance the performance of Al_0.08In_0.08Ga_0.84N MQW LDs by increasing the optical confinement factor (OCF) inside the quantum wells.     

Infrared semiconductor lasers for sensing and diagnostics

There is an increasing demand for spectrally agile compact solid-state lasers for spectroscopic sensing and diagnostics. This demand can be met by III–V semiconductor-based lasers employing various design concepts for the active region. These concepts include, apart from the classical type-I interband diode laser, the type-II W-laser and the unipolar quantum cascade laser. Representative device data will be presented for these three types of lasers in conjunction with a discussion of the relative merits of the different active-region concepts. PACS 42.55.Px; 42.62.Fi; 81.05.Ea; 81.07.St     

Amplification of kW peak power femtosecond pulses in single quantum well InGaAs tapered amplifiers

The amplification of ps and fs pulses with peak powers of up to 4.5 kW has been investigated in a single quantum well InGaAs tapered amplifier. The pulses with durations of 100 fs or 2 ps were generated by a modelocked titanium-sapphire laser. The amplified pulses indicate strong gain saturation and carrier generation due to photon absorption in the laser active region which causes a temporal broadening of the amplified pulses as well as modifications of the optical spectrum. The gain recovery time was measured by a pump-probe experiment. The experimental results are analyzed with respect to the sub-ps gain dynamics which is described by a relaxation time approximation.     

Novel mid-IR laser based on hybrid AlGaAsSb/InAs/CdMgSe heterostructure

We report on the development of a novel design of a mid-IR laser combining III–V and II–VI compounds in a “hybrid” double heterostructure. It possesses large (1.5 eV) potential barriers both for injected electrons and holes, suppressing their leakage from the active region, and provides strong optical confinement. An AlGaAsSb/InAs/CdMgSe laser diode with a III–V/II–VI heterovalent interface at the 0.6 μm-InAs active region has been grown by molecular beam epitaxy on an InAs substrate. Despite a far from optimal defect density at the CdMgSe/InAs interface and high losses inherent for bulk active region of the laser, the structure demonstrates lasing at 2.8 μm (up to 100 K) in the pulsed regime with a threshold current density of 3–4 kA/cm². Type II InSb monolayer insertions into an InAs layer show bright photoluminescence at 3.8 μm (77 K), confirming the great potential of the InAs-based nanostructure active region for longer wavelength applications.     

A review on quantum well structures in photonic devices for enhanced speed and span of the transmission network

Quantum well devices feature heterostructures of very thin epitaxial layers of group III-V and II-VI semiconductor materials. Quantum well devices are integrated monolithically with various optoelectronics devices to provide photonic integrated circuits. The representative structure could be realized with GaAs wells with GaAlAs barriers for wavelengths around 0.9 μm and InGaAsP are used for longer wavelengths. Together with quantum well, superlattice structure is another popular design for InGaAs Avalanche Photo Diode (APD). Quantum well structures find their applications in improved lasers, superlattice for photodiodes, modulators and switches. Consequences of quantum well theory are available today in terms of quantum wires and quantum dots. Upon the application of the normal electric field to quantum well structures, exciton pairs becomes more and more confined and the sharp exciton absorption peaks are observed. The effect is termed as “Quantum Confined Stark Effect”. The electro-absorption effect is approximately 50 times larger in multiple quantum well structures than it is in bulk semiconductors. Another electro-absorption effect known as “Franz Keldysh Effect” has been employed in monolithic waveguide detector. These effects lead to electro-absorption lasers (EAL) as well as electro-absorption laser modulators (EML).     

量子级联激光器有源核中界面声子的特性研究

研究了量子级联激光器有源核中界面声子的色散关系和静电势分布. 根据有源核内部的平移不变性导出了界面声子的色散关系. 计算显示有源核中的界面声子可以分为体声子和表面声子模式. 体声子的色散曲线构成一系列准连续的声子子带，其静电势分布于整个有源核并呈现出Bloch波的特征. 表面声子的色散曲线位于各体声子子带的带隙内，其静电势局域在有源核一侧. 这些结果将有助于量子级联激光器和子带跃迁激光器的优化设计.     

980nm OPS-VECSEL关键参数的理论分析

光抽运半导体垂直外腔面发射激光器(Optically Pumped Semiconductor Vertical External Cavity SurfaceEmission Laser)的输出特性受到诸多参数的影响,理论分析和模拟显得尤为重要.设计了一种以808 nm二极管激光耦合模块为光抽运光源,In_0.159Ga_0.841As/GaAs_0.94P_0.06为增益介质的980 nm光抽运垂直外腔面发射激光器,并借助于PICS3D软件计算了器件各种特性参数.分析了芯片半径、量子阱的周期数以及外腔镜反射率对器件性能的影响,特别是对阈值和光-光转换效率的影响.模拟结果表明,器件的半径影响光-光转换效率.量子阱个数和外腔镜反射率对器件的输出功率和光-光转换效率都有影响,所以要根据实际需要,设计、生长结构和进行实验.