热效应对光折变晶体两波耦合的特性影响的理论分析

用一种模型对两波耦合进行了分析，导出了强度相位的解析式，对影响两波耦合的参数进行了理论分析。分析了对ＢａＴｉＯ３光折变晶体的两波耦合的测量结果。     

光折变两波耦合对光扇开效应的抑制作用

以多个两波耦合（ＴＷＭ）相位栅模型建立了考虑光扇开效应后的动态两波耦合方程，通过数值求解揭示了在大信号两波耦合对光扇开效应的抑制作用和在小信号下光束扇开损耗对两波耦合影响的规律，并进行了相应的实验研究。     

入射光调制对Ce∶KNSBN晶体两波耦合的影响

研究了入射光调制条件下Ce∶KNSBN晶体两波耦合动态过程.结果表明,入射光调制抑制了光扇噪音对Ce∶KNSBN晶体两波耦合动态过程的影响,提高了透射信号光强度.同时研究了入射光光强比Ip/Is及入射光总光强Io对最佳调制频率及增益改善Gm/Gf的影响.结果显示,同一Io下,Ip/Is为100时,Gm/Gf到达峰值1.52,对应的最佳调制频率为150 Hz;同一Ip/Is下,Io为57 mW/cm2时,Gm/Gf最大为1.53,对应的最佳调制频率为175 Hz.     

非故意掺杂GaN层厚度对蓝光LED波长均匀性的影响

通过调整非故意掺杂氮化镓层的厚度,分析氮化镓基LED外延生长过程中应力的演变行为,以控制外延片表面的翘曲程度,从而获得高均匀性与一致性的外延片。由于衬底与外延层之间的热膨胀系数差别较大,在生长温度不断变化的过程中,外延片的翘曲状态也随之改变。在n型氮化镓生长结束时,外延片处于凹面变形状态。在随后的过程中,外延薄膜"凹面"变形程度不断减小,甚至转变为"凸面"变形,所以n型氮化镓生长结束时外延片的变形程度会直接影响多量子阱沉积时外延片的翘曲状态。当非掺杂氮化镓沉积厚度为3.63μm时,外延片在n型氮化镓层生长结束时变形程度最大,在沉积多量子阱时表面最为平整,这与PLmapping测试所得波长分布以及标准差值最小相一致。通过合理控制非故意掺杂氮化镓层的厚度以调节外延层中的应力状态,可获得均匀性与一致性良好的LED外延片。     

Ce∶KNSBN晶体两波耦合中的光扇效应

采用两波耦合非同时读出实验装置 ,测量了掺铈钾钠铌酸锶钡 (Ce∶KNSBN)晶体两波耦合过程中的信号光和抽运光非同时打开条件下两波耦合增益的时间变化规律 ,讨论了光扇的入射光强阈值及光扇效应对两波耦合动态过程的影响。结果表明 :6 32 .8nmHe Ne激光在Ce∶KNSBN晶体中写入体光栅时 ,光扇效应存在明显的写入光强阈值特性 ,当入射光强大于 30mW /cm2 时 ,才存在强烈的光扇效应。利用修正耦合波方程对Ce∶KNSBN晶体中的两波耦合动态过程、增益随着信号光和抽运光打开时间间隔Δt的变化进行了模拟计算 ,理论模拟结果与实验测量结果基本一致     

Optical Image Combination and Logic AND Operation Using Beam Fanning Effect

Using the beam fanning effect in a BaTiO₃ crystal, we propose two simple set-ups to perform the function of an optical image combiner and an optical AND gate respectively. The optical image combiner combines two mutually coherent as well as mutually incoherent patterns transmitted to it separately into a single coherent pattern. This set-up can also be used as an optical OR gate. For the AND gate, the two input signals may also be mutually coherent or incoherent. Experimental results showed the response time of these two devices to range from 0.01 second to 2.5 seconds.     

界面形核时间对GaN薄膜晶体质量的影响

利用金属有机化学气相沉积技术系统研究了界面形核时间对c面蓝宝石衬底上外延生长GaN薄膜晶体质量的影响机理. 用原子力显微镜、扫描电子显微镜、高分辨X射线衍射仪以及光致发光光谱仪表征材料的晶体质量以及光学性质. 随着形核时间的延长, 退火后形成的形核岛密度减小、尺寸增大、均匀性变差, 使得形核岛合并过程中产生的界面数量先减小后增大, 导致GaN外延层的螺位错和刃位错密度先减小后增大, 这与室温光致发光光谱中得到的带边发光峰与黄带发光峰的比值先增大后降低一致. 研究结果表明, 外延生长过程中, 界面形核时间会对GaN薄膜中的位错演变施加巨大影响, 从而导致GaN外延层的晶体质量以及光学性质的差异.     

Effect of overdrive voltage on PBTI trapping behavior in GaN MIS-HEMT with LPCVD SiN_x gate dielectric

The effect of high overdrive voltage on the positive bias temperature instability(PBTI)trapping behavior is investigated for GaN metal–insulator–semiconductor high electron mobility transistor(MIS-HEMT)with LPCVD-SiNx gate dielectric.A higher overdrive voltage is more effective to accelerate the electrons trapping process,resulting in a unique trapping behavior,i.e.,a larger threshold voltage shift with a weaker time dependence and a weaker temperature dependence.Combining the degradation of electrical parameters with the frequency–conductance measurements,the unique trapping behavior is ascribed to the defect energy profile inside the gate dielectric changing with stress time,new interface/border traps with a broad distribution above the channel Fermi level are introduced by high overdrive voltage.     

静电放电对GaN基功率型LED老化特性的影响

对GaN基蓝光功率型LED在老化前和老化期间施加反向人体模式静电放电(ESD),并对静电打击前后及老化前后的LED光学电学参数进行分析。实验结果及理论分析表明,ESD使LED芯片有源层及限制层中产生缺陷,最终导致电学特性及光学特性的变化。ESD给LED带来的损伤可在老化前期过程中被局部恢复,但随着老化时间推移,电参数漂移程度及光衰幅度不断增大,而老化过程中LED对ESD的敏感度增加,使LED抗ESD能力减弱。     

The Observation of Photorefractive Effect of Prism Coupling in Ion-implanted Fe-doped LiNbO_3 Plannar Waveguide

1.IntroductionTheFe--dopedLithiumNiobate(LiNbO3)crystalhavebeenintensitystudiedbecauseoftheirimprovedphotorefractivepropertiesandutilizationinawiderangeofapplicationsinopticalstorage,opticalphaseconjugationandtwo-andfour--wavemixingtechniques.ThewaveguidefabricatedwithsuchcrystalisdesirablebecauseitprovidescomP8tibilitywithotherminiaturizedintegrateddevicescommonlyusedtoday,particularwithlaserdiodesandfibergeometries.Comparingtootherwaveguidefabricationtechniques,suchasmetaldiffusionorionex…     

Effect of heavy ion irradiation on the interface traps of AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) were irradiated with heavy ions at various fluences. After irradiation by 2.1 GeV¹⁸¹ Ta³²⁺ ions, the electrical characteristics of the devices significantly decreased. The threshold voltage shifted positively by approximately 25% and the saturation currents decreased by approximately 14%. Defects were induced in the band gap and the interface between the gate and barrier acted as tunneling sites, which increased the gate current tunneling probability. According to the pulsed output characteristics, the amount of current collapse significantly increased and more surface state traps were introduced after heavy ion irradiation. The time constants of the induced surface traps were mainly less than 10 μs.     

Effects of ion implantation on yellow luminescence in unintentional doped n-type GaN

The effects of Si, O, C and N ion implantation with different implantation doses on yellow luminescence (YL) of GaN have been investigated. The as-grown GaN samples used in the work were of unintentional doped n-type, and the photoluminescence (PL) spectra of samples had strong YL. The experimental results showed that YL of ion implanted samples exhibited marked reductions compared to samples with no implantation, while the near band edge (NBE) emissions were reduced to a lesser extent. The deep-level centers associated with YL may be produced in GaN films by O and C ion implantation, and identities of these deep-level centers were analyzed. It was also found that the dose dependence of YL was analogous with the one of the intensity ratios of YL to the near band edge (NBE) emission (I _YL /I _NBE ) for ion implanted samples. The possible reason for this comparability has been proposed.      

Crystal orientation dependence of the SNR for signal beam amplification in photorefractive BaTiO3

In signal beam amplification by two-beam coupling in BaTiO₃ photorefractive crystals, beam fanning in the direction of the amplified signal reduces the signal-to-noise ratio (SNR). The dependence of the SNR and the signal beam gain on the crystal orientation are analysed using a HeNe laser. It is found that orientating the crystal for maximum gain gives poor signal-to-noise ratio. A compromise has to be made between the SNR and high gain for optimum signal amplification.     

Protection of isolated and active regions in AlGaN/GaN HEMTs using selective laser annealing

Al Ga N/Ga N high-electron-mobility transistors with Au-free ohmic contacts are fabricated by selective laser annealing and conventional rapid thermal annealing. The current transport mechanism of ohmic contacts is investigated. Hightemperature annealing can be avoided in the isolated region and the active region by selective laser annealing. The implanted isolation leakage current is maintained 10-6 m A/mm even at 1000 V after selective laser annealing. On the contrary, hightemperature annealing will cause obvious degradation of the isolation. The morphology of Al Ga N surface is measured by atomic force microscope. No noticeable change of the Al Ga N surface morphology after selective laser annealing, while the root-mean-square roughness value markedly increases after rapid thermal annealing. The smaller frequency dispersion of capacitance–voltage characteristics confirms the lower density of surface states after selective laser annealing. Thus,dynamic on-resistance is effectively suppressed.     

Light-induced scattering in SBN：Cr crystal under external electric fields and its suppression

In this paper, the enhancement of light-induced scattering in congruent SBN:Cr (Sr_{0.61}Ba_{0.39}Nb_2O_6:Cr) crystals in the presence of an externally applied electric field and its suppression are studied. If a coherent image is focalized in SBN:Cr crystal without applying external electric field, the output image will remain clear, because of the weak photorefractive effect in the crystal. When a field is applied properly along the crystal axis, markedly enhanced scattering from the signal beam and the output image dispersion can be observed due to the increase of the photorefractive two-beam coupling gain and the light-induced index change in SBN:Cr crystals. By introducing a coherent or incoherent beam with higher intensity the light-induced scattering can be suppressed through the erasure of scattering gratings. The difference between coherent and incoherent beam is that the former can also amplify the signal beam as the scattered light is removed, whereas the latter can only make the signal beam revert to its initial state. The results obtained under different experimental conditions are consistent with theoretical analysis.     

Self-Bending of a Laser Beam in Photorefractive Crystals with Variable Nonlinearity Coefficient

The self-bending of a laser beam in photorefractive crystal is considered in the case in which the coefficient of photorefractive diffusion nonlinearity changes along deflection path. The bent trajectory is calculated analytically and the obtained result is compared with those reported before.     

光折变晶体Ce:BaTiO_3中的各向同性扇形散射

用一束ｅ偏振的激光入射到一块掺ＣｅＢａＴｉＯ３晶体上，在出射方向上观察到了相同偏振的散射光扇。测量了散射光强度、建立时间与入射光波长、入射角度、入射功率的依赖关系，以及散射光强的角分布。对实验现象进行了解释。使用一组经过简化的基于二波耦合理论的方程进行了计算。在所得结果中，散射光强的角分布与实验值定量地符合；透射光强与入射角度、波长的关系能够反映出其变化趋势。     

金属层亚波长狭缝中光波耦合

亚波长波导能够控制光在亚波长的尺寸中以很小损耗传输,在集成光学中有广泛的应用。利用二维时域有限差分(FDTD)法,研究了光波在金属层中亚波长两狭缝之间的耦合过程。分别在较厚的金属层前表面和后表面刻上两个狭缝,纵向错开一定距离(间距),横向重叠一定长度(耦合长度),两个狭缝能够将光波从金属层的前表面耦合到后表面。改变两个狭缝长度、间距和耦合长度等参数,耦合波长和效率发生明显变化。结合振幅分布,认为光波在两狭缝形成波导共振,前表面狭缝的共振将入射波能量耦合进入狭缝中,后表面狭缝的共振将能量耦合出去,两个狭缝之间通过隧穿效应耦合。     

Photorefractive effect in Fe-doped GaN

The photorefractive effect was observed in Fe-doped semi-insulating GaN. We measured the two-beam coupling constant and the grating formation time as a function of pump intensity at a wavelength of 458 nm in reflection geometry. The photorefractive gain coefficient was 0.39 cm⁻¹, and the grating formation time was 7 ms at a pump intensity of 1.0 W/cm². Besides the refractive index grating, the contribution of an absorption grating was also observed in a two-wave mixing experiment. The coupling constant of the absorption grating was negative and increased with the pump intensity. The origin of the absorption grating was attributed to light-intensity-dependent photochromism.