Study on strain-induced polarization mode of strained-layerquantum-well 630-nm AlGaInP LD's

Low-threshold operation of 630-nm AlGaInP LD's is examined by introducing compressive and tensile strains into quantum wells (QW's), and the dependence of threshold currents on the effects of strain is investigated. We also try to determine the correlation between the strain-induced polarization mode and threshold currents so as to optimize the strained QW structure for low threshold operation. Experimental results reveal a good correlation when the same device parameters are used. Based on that correlation, applying the larger strain and the QW-structure design that enhances the main polarization mode is effective in obtaining a low threshold. Opposite dependences of the relative intensity of the polarization mode on such device parameters as cavity length and facet reflectivity are observed under the two types of strain. Under tensile strain, the polarization-mode intensity increases as the mirror loss increases and the device temperature rises     

适于InGaAsP光放大器偏振不灵敏的增益介质

研制了适于InGaAsP光放大器偏振不灵敏的增益介质 ,采用有源区内交替的张应变和压应变排列的混合应变量子阱结构 ,器件做成带有倾角的扇形。实验中发现该结构既抑制了激射又改善了器件的偏振灵敏性 ,实现了偏振灵敏度小于 0 5dB ,10 0mA偏置时可达 0 1dB。在较大的电流范围内 ,峰的半高全宽 (FWHM)为 4 0nm。     

Analysis of GaInP/AlGaInP compressive strainedmultiple-quantum-well laser

We have analyzed GaInP/AlGaInP compressive strained MQW lasers, with theoretical calculation and experimental results. Our calculations of TE polarized gain, where the valence subband mixing and the heterobarrier leakage current are taken into account, are in good agreement with the experimental results. When a compressive strain of up to 0.5% is induced in the quantum wells, the density of states near the valence band edge is decreased, due to the reduction of heavy-hole and light-hole subband mixing. At the threshold condition, the compressive strain reduces not only the radiative recombination current, but also the hetero-barrier leakage current. Therefore, the threshold current is reduced, and its temperature dependence is found to be small, In the analysis, we also show that when larger compressive strain of more than 0.5% is induced in the 40-Å-thick quantum wells, the threshold characteristics are degraded     

Uniaxial strain effect on the electronic and optical properties ofwurtzite GaN-AlGaN quantum-well lasers

The valence subband structures of uniaxial-strained wurtzite (WZ) GaN-AlGaN quantum wells (QW's) are calculated using multiband effective-mass theory. The optical gain is investigated using a numerical approach in which we account for the subband structure modification and mixing due to the anisotropic strain in the QW plane. We show that the mixing of the HH and LH bases in the uniaxial-strained (0001) GaN-AlGaN QW decouples |X〉 and |Y〉 at the Γ point, giving two topmost subbands, Y1 and X1, which can be more widely separated than the HH1 and LH1 subbands in the biaxial-strained (0001) GaN-AlGaN QW. We resolve the states of the subband dispersion in terms of the |X〉, |Y〉, and |Z〉 bases, and show the compositional variation as a function of the in-plane wavevector. Under uniaxial strain, it is possible to exploit the existence of the preferred symmetry at the valence band maximum and the reduced band-edge density-of-states due to the anisotropic in-plane energy dispersion to achieve lower transparency carrier and current densities and higher differential gain in comparison with a pseudomorphic biaxial-strained QW. We show that, for a QW laser structure with the optical cavity along the x axis, uniaxial compressive strain in the y direction shows greater improvement than the uniaxial tensile strain in the x direction of the same magnitude. Thus, a suitable uniaxial strain could be used to improve the threshold performance of WZ GaN-based QW lasers     

Polarization of the optical emission of polaron excitons in anisotropic quantum dots

A theory of large polarons in ellipsoidal quantum dots is developed. The optical spectrum of polaron excitons and its dependence on the degree of anisotropy of a quantum dot are analyzed. It is shown that the polaron ground state exhibits specific anisotropic polarization of the medium. The symmetry of the wave function of the ground state depends on the band structure of the material and on the shape of the quantum dot. The conditions under which strong polarization of the zero-phonon emission line produced by interband optical transitions occurs are determined. The possible polarization of this line is determined for various relationships between the polaron energy and the energy of the exchange interaction.     

Anomalous in-plane polarization dependence of optical gain in compressively strained GaInAsP-InP quantum-wire lasers

In-plane polarization anisotropy of optical gain in compressively strained GaInAsP-InP quantum wire (Q-wire) lasers including elastic strain relaxation induced band mixing is studied. The interaction between two-dimensional (2-D) quantum confinement and elastic strain relaxation effects is found to be complex depending qualitatively also on the wire width. Additional valence band mixing due to strain relaxation has a strong influence on the polarization dependence of optical gain. In the absence of elastic strain relaxation, gain is the maximum for tranverse electric (TE) polarization with the electric field parallel to the wire axis (TE/sub /spl par//), in agreement with the existing theory. On the other hand, when strain relaxation is strong, contrary to the existing theory, valence band mixing causes the gain to be the maximum in TE polarization with the electric field normal to the wire axis (TE/sub /spl perp//). Moreover, Q-wire lasers without suppression of strain relaxation are more likely to exhibit ground-state lasing for TE/sub /spl perp// polarization. These results suggest that in the presence of strong strain relaxation, a laser cavity parallel to the wire axis would provide higher gain. Therefore, the appropriate orientation of the laser cavity in strained GaInAsP-InP Q-wire lasers should be decided after carefully studying the polarization dependence of gain. Our calculation also shows that strong strain relaxation causes in-plane polarization anisotropy to show complex, nonmonotonic dependence on the wire width. Consequently, in such structures, in-plane polarization anisotropy may not be regarded as a direct measure of 2-D confinement effects.     

The design of electroabsorption modulators with negative chirp and very low insertion loss

Electroabsorption modulators (EAMs) with negative chirp and very low insertion loss are numerically designed with asymmetric intra-step-barrier coupled double strained quantum wells (AICD-SQWs) based on InGaAlAs material. For this purpose, the electroabsorption coefficient is calculated over a range of wells layer strain from compressive (CS) to tensile (TS). The chirp parameter and insertion loss for TE input light polarization are evaluated from the calculated electroabsorption spectra, and their Kramers-Krönig transformed refractive index changes. The results of the numerical simulation show that the best range of left and right wells strain for EAMs based on AICD-SQWs with negative chirp and very low insertion loss are from 0.032% to 0.05% (TS) and -0.52% to -0.50% (CS), respectively.     

Optical intersubband transitions in strained quantum wells utilizing In1−x GaxAs/InP solid solutions

Infrared absorption in strained p-type In_1−x Ga_xAs/InP quantum wells is investigated for both possible types of strain (tensile and compressive). It is observed that the normalincidence absorption increases considerably under compressive strain (when the ground state is a heavy-hole state) and decreases under tensile strain (when the ground state is a light-hole state). The peak absorption in the compressed quantum well can attain very large values, on the order of 5000 cm⁻¹ at a hole density ∼ 10¹² cm⁻²; this attribute makes “compressed” p-type quantum wells attractive for IR detection applications. Fiz. Tekh. Poluprovodn. 33, 83–90 (January 1999)     

Optimization of long wavelength InGaAsP strained quantum-welllasers

A theoretical study of InGaAsP-InGaAsP multiple quantum-well lasers emitting at 1.55 μm has been carried out to investigate the variation of threshold current density and differential gain with strain, well width and well number. We show that the greatest scope for exploiting this quaternary alloy in laser structures is through the use of compressive wells with unstrained or tensile barriers. We consider structures with a fixed compressive strain of 1% but variable well width, and also with fixed well width but variable strain from 0% to 1.75%. For structures with 1% compressive wells and unstrained barriers we find that the optimum structure for lowest threshold current density with sizable differential gain consists of six 35-Å quantum wells. We find also that there is little benefit to having compressive strains greater than 1.2%. In addition we examine zero-net-strain (ZNS) structures with compressive wells and tensile barriers. We show how the conduction band offset can be significantly increased and valence band offset reduced in such structures. Our gain calculations suggest that the large modification in band offset can decrease the threshold current density compared to similar devices with unstrained barriers     

自组织量子点各向异性弹性应变场的数值分析

用数值方法计算了量子点的各向异性弹性应变场,得到了对应不同各向异性度的量子点弹性应变场的分布云图和沿量子点对称轴横向、纵向应变、静水应变和双轴应变变化曲线图,讨论了各向异性弹性应变场对量子点超晶格生长时的垂直相关性和量子点电子结构的影响.     

Complete polarization mode control of long-wavelength tunable vertical-cavity surface-emitting lasers over 65-nm tuning, up to 14-mW output power

We experimentally demonstrate a stable polarization mode operation in long-wavelength tunable vertical-cavity surface-emitting lasers over a 65-nm tuning range and the entire output power range (< 14.6 mW) at room temperature. The polarization mode control was achieved by utilizing anisotropic gain properties of quantum wells due to the difference in bond lengths between the constituent atoms at the interfaces combined with uni-axial external strain induced by a stressor. The experiments were conducted to verify this newly proposed polarization control scheme based on the spin flip model (SFM) developed to incorporate the detailed gain properties, cavity standing wave effect, self-heating effect, and strain effect. The experimental results on the tuning characteristics of polarization switching behavior and output powers were reproduced in highly agreeable manner by simulations. The relative importance of the external strain, interfacial strain at quantum wells, and the wavelength dependence of gain anisotropy are also discussed. It is also shown that the fast spin relaxation times for InP-based vertical-cavity surface-emitting lasers (VCSELs) was responsible for the inhibition of elliptic polarization states often observed for GaAs-based VCSELs. The effectiveness of the polarization control scheme was highlighted by the observed high polarization suppression ratio of 34 dB maintained for the entire wavelength and pump power ranges during the reliability testing over 2000 h. The influence of the elliptic polarization state for the optical pump laser was detected which could be explained as a memory effect of the spin-polarized electrons, supporting the validity of the SFM.     

多量子阱有效折射率偏振相关性研究

分析了多量子阱材料各参数对其TE模和TM模有效折射率的影响。结果表明:阱数增多,多量子阱有效折射率降低,当量子阱数目大于3时,其有效折射率的变化不明显。垒厚增加,有效折射率略有降低。存在合适的张应变量使TE模和TM模有效折射率峰值波长接近的同时,折射率差值整体最小,偏振相关性最小。据此提出多量子阱材料有效折射率低偏振相关设计方法,并设计出C波段内(1530~1565 nm)折射率低偏振相关的InGaAs/InGaAsP多量子阱材料。研究结果有助于设计实用化的有效折射率低偏振相关量子阱材料。     

低偏振灵敏度半导体光放大器

报道了基于混合应变量子阱材料的半导体光放大器 (SOA)。利用张应变量子阱加强了TM模的增益 ,使之接近TE模的增益 ,从而使SOA的偏振灵敏度大为降低。在 150mA的偏置下 ,获得了 2 4dB的小信号增益和 1dB的偏振灵敏度。     

一种大功率低偏振度量子阱超辐射发光二极管

设计了一种张应变与压应变相结合的混合应变量子阱结构超辐射发光二极管,研究了TE模和TM模在器件中的模式增益,分析了影响增益偏振性的因素,在此基础上通过改变有源层量子阱的应变类型、应变量以及层数来达到高增益和偏振不敏感性。最后按设计工艺流程生长了芯片,实验结果表明,所设计的SLD芯片单管输出功率在100mA驱动电流下可达3.5mW,出射光谱FWHM约为40nm,20nm波长范围内偏振度为0.3dB,具有较理想的大功率、宽光谱、低偏振度特性。     

Minimization of the linewidth enhancement factor in compressivelystrained semiconductor lasers

A comprehensive model for the optical response of a semiconductor quantum well, including valence subband mixing and many-body effects, is used to theoretically investigate means of minimizing the linewidth enhancement factor. The effects of well width and compressive strain are analyzed, and the contribution of many-body effects evaluated. Compressive strain in narrow quantum wells generally leads to reduction of the linewidth enhancement factor at gain peak. In addition, many-body effects, particularly bandgap renormalization, admit the possibility that by small detuning to below the gain peak position, a zero value is possible     

Optical Gain of V—groove Zn1—xCdxSe／ZnSe Quantum Wires

The subband structures, distributions of electron and hole wave functions, state density, optical gain spectra, and transparency carrier density of the V-groove Zn 1-x Cd x Se/ZnSe quantum wires are investigated theoretically using four band effective-mass Hamiltonian, which takes into account the effects of the valence band anisotropy and the band mixing. The biaxial strain effect for quantum wires is included in the calculation. The compressive strain in the Zn 1-x Cd x Se wire region increases the energy separation between the uppermost subbands. The optical gain with xy -polarized light is enhanced, while optical gain with z -polarized light is strongly decreased. The xy -polarized optical gain spectrum has a peak at around 2.541 eV, with the transparency carrier density of 0.75×10 18 cm -3 . The calculated results also show that the strain tends to increase the quantum confinement and enhance the anisotropy of the optical transitions.     

High-speed modulation of strain-compensated InGaAs-GaAsP-InGaP multiple-quantum-well lasers

Small signal direct modulation characteristics of InGaAs-GaAsP-InGaP multiple quantum well ridge waveguide lasers (4.5/spl times/220 /spl mu/m/sup 2/) are described. The compressive strain of four InGaAs quantum wells is compensated by the tensile strain of GaAsP barriers. The lasers have a threshold current of 8 mA and an internal differential quantum efficiency of 80%. A 3-dB bandwidth of 25 GHz is obtained at 54 mA. It is found that the strain-compensated lasers have a K factor as low as 0.15 ns, implying a maximum 3-dB bandwidth of 59 GHz.     

Polarization-insensitive electroabsorptive modulation usinginterdiffused InGaAs(P)-InP quantum wells

This is a theoretical study to demonstrate the use of interdiffusion in the realization of polarization insensitivity at the band-edge. Two InGaAs-InP quantum well as-grown structures have been investigated: one with lattice-matched condition and the other with small as-grown tensile strain (0.15%). The interdiffusion is considered to take place on the Group V (As and P) sublattice only. As a result, a tensile strain is produced which merges the heavy- and light-hole states in order to achieve polarization insensitivity. Criteria to develop polarization-insensitive quantum wells (QW's) using interdiffusion are presented here. When the two-phase interdiffusion mechanism is modeled, the results show that the well barrier interfaces of the QW maintain an abrupt profile while the well width remains constant after interdiffusion. The two interdiffused QW structures considered here can produce polarization insensitive electroabsorption at operation wavelengths around 1.55 μm. The one with lattice-matched condition Is particularly attractive since it only requires an easy (high-yield) fabrication process with a simple postprocessing thermal annealing to achieve polarization insensitivity     

Polarization dependence of photo-detection in strained multiplequantum-well semiconductor lasers

We investigated polarization dependence of photo-detection in strained layer multiple quantum-well (SL-MQW) lasers for time compressive multiplex (TCM) application. The polarization dependence of SL-MQW lasers is larger than that of bulk lasers because of the quantum effect. To improve the polarization dependence of photo-detection, we theoretically and experimentally analyzed the effect of strain quantity on the polarization dependence, and found that large tensile strained quantum-well structure is suitable for the small polarization independence of 0.8 dB and low threshold current characteristics. Moreover, we achieved smaller polarization dependence less than 0.5 dB by applying reverse bias voltage in the photo-detection mode