蓝紫光氮化镓光子晶体面射型激光器

设计、制作了蓝紫光氮化镓光子晶体面射型激光器结构,并测量其光学性质,探讨了光子晶体的晶格常数、边界形状及晶格种类对激光器特性的影响。激光器结构采用有机金属化学气相沉积法配合电子束光刻及感应耦合等离子体干蚀刻等技术制作。由角度解析光致发光系统测得绕射图案、激光发射光谱及发散角等光学性质。同时,使用平面波展开法及多重散射法计算光子晶体的能带结构与阈值增益。由实验结果得出,可由改变光子晶体的晶格常数达到调变激光器操作模态的目的。此外,光子晶体的边界形状对激光器波长及半高宽并无显著的影响,但圆形边界的阈值激发能量密度比六角形边界低0.3 mJ/cm²。另一方面,将六角晶格、四角晶格与蜂巢晶格的晶格种类进行比较,蜂巢晶格具有较小的激发能量密度（1.6 mJ/cm²）及发散角（1.3°）,而四角晶格的激发能量密度（3.8 mJ/cm²）及发散角（2.2°）为三者之中最大。多重散射法求得的阈值增益与实验结果相吻合,可视为快速有效设计光子晶体激光器结构的工具。本文研究成果对今后发展高功率蓝紫光氮化镓光子晶体面射型激光器具有指导意义。     

Effect of the optical excitation signal intensity on the impulse response of a metal-semiconductor-metal photodiode

The effect of the optical excitation signal intensity on the impulse response of a photodetector based on a set of metal-semiconductor-metal (MSM) rectifier contacts is studied. The response of the detector is better at a low optical excitation signal. When the energy of an optical excitation pulse is high, the response can be improved by increasing the bias voltage. The advantages of a GaN-based MSM diode in detecting high- energy radiation pulses are established. It is shown that the speed of the GaN-based MSM detector may reach 25 ps for a 60-pJ optical excitation pulse at a wavelength of 290 nm.     

Multiwavelength GaN-Based Surface-Emitting Lasers and Their Design Principles

Dual-wavelength lasing operations are demonstrated in GaN-based vertical-cavity surface-emitting lasers (VCSELs) comprising ingeniously designed asymmetric InGaN quantum wells (AS-QWs). The dual laser modes show exact positive-correlated polarization dependences with a high degree of polarization of up to 98%. By simply tuning the pump energy, the components and intensity of the laser outputs can be continuously changed, making wavelength selection and switching available for the GaN-based VCSELs. Detailed theoretical analysis and experimental measurements show that the intensity of optical gain and the coupling between the active layer and optical field, namely the electron–photon interaction, as well as carrier tunneling and photon reabsorption play a crucial role in the multiwavelength lasing processes. Moreover, the design principles of the proposed AS-QWs and multistacked size-varied quantum dot (MS-QD) active regions are elaborated to provide guidelines for controllable multiwavelength emissions in GaN-based surface-emitting lasers. These results not only provide better understanding of lasing in nitride-based microcavity systems but also shed insight into the more fundamental issues of electron–photon coupling in such systems. Importantly, such controllable multiwavelength laser operations may extend nitride-based VCSELs to previously inaccessible areas, for example, flip-flop, ultrafast switches, and other functional devices such as Raman lasers and sensors.     

Internal power loss in GaN-based lasers: mechanisms and remedies

GaN-based laser diodes transform only a relatively small fraction of the electrical input power into laser light. The inherently large series resistance of these devices causes significant self-heating that leads to the typical power roll-off at high current. We analyze recently reported measurements using advanced numerical laser simulation and investigate the physical mechanisms that limit the lasing power in continuous-wave operation. Contrary to common expectations, our analysis reveals a strong influence of Auger recombination since the self-heating leads to a rising quantum well carrier density above the lasing threshold. As possible remedy, we investigate the effect of a tunnel-junction contact and predict a significant enhancement of lasing power and efficiency.     

Low threshold laser cavities of two-dimensional photonic crystal line defect mode

Two-dimensional (2D) rod-type photonic crystal (PC) line defect waveguide (LDW) laser cavities based on three types of line defect modes with zero group velocity are studied by using finite-difference time-domain (FDTD) method. These laser cavities have high quality (Q) factor, better localization of light, non-uniform gain distribution and small overlap between gain medium and light field. Therefore, they have the advantages over conventional and air-bridge PC cavities with uniform gain, such as low threshold, single mode lasing and effectively avoiding thermal effect. From their comparison, one can find the mode at middle Brillouin zones (BZ) is the best one to be used as lasing mode. Its dynamic lasing process and lasing features are demonstrated by the numerical experiment where the FDTD method coupling Maxwell's equations with the rate equations of electronic population is used.     

The single-longitudinal-mode operation of a ridge waveguide laser based on two-dimensional photonic crystals

An electrically driven, single-longitudinal-mode GaAs based photonic crystal (PC) ridge waveguide (RWG) laser emitting at around 850 nm is demonstrated. The single-longitudinal-mode lasing characteristic is achieved by introducing the PC to the RWG laser. The triangle PC is etched on both sides of the ridge by photolithography and inductive coupled plasma (ICP) etching. The lasing spectra of the RWG lasers with and without the PC are studied, and the result shows that the PC purifies the longitudinal mode. The power per facet versus current and current-voltage characteristics have also been studied and compared.     

Generation of dual-wavelength domain-wall rectangular-shape pulses in HNLF-based fiber ring laser

The generation of dual-wavelength domain-wall rectangular-shape pulses in a highly nonlinear fiber (HNLF)-based fiber ring laser is experimentally demonstrated. The dual-wavelength lasing operation is realized by employing the intracavity birefringence-induced spectral filtering effect. An 85 m long HNLF is introduced into the fiber ring laser to enhance the nonlinear effect, which is favorable for the cross coupling between the two lasing beams. Experimentally, it was found that the interval of two domain walls in the time domain could be adjusted by simply tuning the linear cavity phase delay, which results in the achievement of different output pulse shapes. By properly rotating the polarization controller (PC), the dual-wavelength rectangular-shape pulses could be efficiently obtained. The proposed fiber laser provides a simple and efficient way to generate rectangular-shape pulse.     

Solar-blind MSM-photodetectors based on Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>N heterostructures

Solar-blind MSM photodetectors based on the AlGaN heterostructures have been fabricated and investigated. The influence of material properties on device parameters is discussed. Effect of different buffer layers on the detector performances has been examined. Detectors exhibit low dark currents and high sensitivity within the range of 250–290 nm. Effect of optical excitation energy on GaN-based MSM-detector performance is analyzed and discussed. At high excitation level the detector speed of response is limited by the field screening caused by the space-charge of the holes. The impulse response of GaN-based MSM-detector is compared favorably with GaAs MSM-device.     

Study on GaN-based light emitting diode with InGaN/GaN/InGaN multi-layer barrier

The current study investigates GaN-based light-emitting diodes (LEDs) with InGaN/GaN/InGaN multi-layer barrier (MLB). Simulation results show that GaN-based LEDs with MLB have better performance than conventional GaN-based LEDs with only one GaN barrier because of the enhancement in hole injection into the quantum well and decrease in electron leakage current.     

Light extraction of GaN LEDs with 2-D photonic crystal structure

Ultraviolet photo-lithography is employed to introduce two-dimensional （2D） photonic crystal （PC） structure on the top surface of GaN-based light emitting diode （LED）. PC patterns are transferred to 460-rimthick transparent indium tin oxide （ITO） electrode by inductively coupled plasma （ICP） etching. Light intensity of PC-LED can be enhanced by 38% comparing with the one without PC structure. Rigorous coupled wave analysis method is performed to calculate the light transmission spectrum of PC slab. Simulation results indicate that total internal reflect angle which modulated by PC structure has been increased by 7°, which means that the light extraction efficiency is enhanced outstandingly.     

Active region design of a terahertz GaN/ Al0.15Ga0.85N quantum cascade laser

We propose the idea of developing THz quantum cascade lasers (QCLs) with GaN-based quantum well (QW) structures with significant advantages over the currently demonstrated THz lasers in the GaAs-based material system. While the ultrafast longitudinal optical (LO) phonon scattering in AlGaN/GaN QWs can be used for the rapid depopulation of the lower laser state, the large LO-phonon energy (∼90 meV) can effectively reduce the thermal population of the lasing states at higher temperatures. Our analysis of one particular structure has shown that a relatively low threshold current density of 832 A/cm² can provide a threshold optical gain of 50/cm at room temperature. We have also found that the characteristic temperature in this structure is as high as 136 K.     

Improved light extraction of GaN-based light-emitting diodes with surface-textured indium tin oxide electrodes by nickel nanoparticle mask dry-etching

GaN-based light-emitting diodes (LEDs) with surface-textured indium tin oxide (ITO) as a transparent current spreading layer were fabricated.The ITO surface was textured by inductively coupled plasma (ICP) etching technology using a monolayer of nickel (Ni) nanoparticles as the etching mask.The luminance intensity of ITO surface-textured GaN-based LEDs was enhanced by about 34% compared to that of conventional LED without textured ITO layer.In addition,the fabricated ITO surface-textured GaN-based LEDs would present a quite good performance in electrical characteristics.The results indicate that the scattering of photons emitted in the active layer was greatly enhanced via the textured ITO surface,and the ITO surface-textured technique could have a potential application in improving photoelectric characteristics for manufacturing GaN-based LEDs of higher brightness.     

Stable zirconia-erbium doped multiwavelength fiber laser by precise control of polarization states

A compact zirconium-erbium doped fiber (Zr-EDF) based multiwavelength fiber laser (MWFL) with stable output comb is proposed and demonstrated. The MWFL utilizes a 3 m long Zr-EDF with an erbium concentration of 3000 ppm and pumped by a 1480 nm laser diode (LD) as the active gain medium. A fiber based multimode polarization controller (PC) is used to precisely control the polarization states of the oscillating modes in the MWFL, distributing the total energy of the system among the lasing wavelengths in order to overcome the mode-suppression and mode-competition that arises from homogenous line broadening. The MWFL is capable of generating up to four lasing wavelengths with average peak powers of 0.08 dBm in the extended L-band of 1600 nm. The lasing wavelengths have a 3-dB linewidth of 0.1 and an extinction ratio of 40 dB as well as highly stable, with minimal fluctuations of less than 0.6 dB observed in the peak powers of the lasing wavelengths over a period of 1 h. The proposed system allows for the realization of a compact, cost-effective and stable erbium based MWFL capable of operating at room temperature.     

Distributed feedback laser with optoelectronic tunability in dye-doped cholesteric liquid crystal with coated photoconductive layer

This work investigates, for the first time, an optoelectronically tunable distributed feedback laser that is based on a planar DDCLC cell with a coated photoconductive (PC) layer. Experimental results show that the CLC reflection band and the lasing wavelength of the DDCLC can both be tuned optoelectronically by varying the intensity of one irradiating CW green beam or the magnitude of the applied dc voltage. The tunability of the DDCLC laser depends on the controllability of the optoelectronic properties of the PC layer and, therefore, on the voltage dropping on the CLC layer. Therefore, the CLC pitch can be controlled by exploiting the optoelectronically induced electrohydrodynamic effect which causes the spatially periodic deformation of the CLC structure. In addition, the dependences for other critical lasing parameters, e.g., energy threshold, lasing efficiency, and lasing linewidth, on external controlling signals are also measured and discussed in the current study.     

Progress and prospects of GaN-based LEDs using nanostructures

Progress with GaN-based light emitting diodes(LEDs) that incorporate nanostructures is reviewed,especially the recent achievements in our research group.Nano-patterned sapphire substrates have been used to grow an Al N template layer for deep-ultraviolet(DUV) LEDs.One efficient surface nano-texturing technology,hemisphere-cones-hybrid nanostructures,was employed to enhance the extraction efficiency of In GaN flip-chip LEDs.Hexagonal nanopyramid GaN-based LEDs have been fabricated and show electrically driven color modification and phosphor-free white light emission because of the linearly increased quantum well width and indium incorporation from the shell to the core.Based on the nanostructures,we have also fabricated surface plasmon-enhanced nanoporous GaN-based green LEDs using AAO membrane as a mask.Benefitting from the strong lateral SP coupling as well as good electrical protection by a passivation layer,the EL intensity of an SP-enhanced nanoporous LED was significantly enhanced by 380%.Furthermore,nanostructures have been used for the growth of GaN LEDs on amorphous substrates,the fabrication of stretchable LEDs,and for increasing the3-d B modulation bandwidth for visible light communication.     

Dual-wavelength switchable erbium-doped fiber ring laser based on merged sagnac and intermodal interferences in sagnac loop mirror

A simple but novel concept of switchable dual-wavelength erbium-doped fiber ring laser is proposed and experimentally demonstrated. It is based on a Sagnac loop mirror incorporating a high birefringence few-mode fiber and polarization controller (PC). Since its transmission properties depend on the PC state, the proposed fiber ring laser can operate in the random combination of two wavelengths by adjusting only the PC settings. The potential lasing lines are determined by simultaneous occurrence of the Sagnac and intermodal interferences in the Sagnac loop mirror. All lasing emissions have the optical signal to noise extinction ratio higher than 45 dB and their power fluctuations lower than 0.5 dB.     

Low-frequency excess noise characterizations of laser-assisted de-bonded GaN films

Systematic characterization of flicker noise was conducted on GaN-based metal-semiconductor-metal (MSM) interdigitated devices. The devices were fabricated on both the regular GaN-on-sapphire (type A) and laser de-bonded films followed by layer transfer of hydride vapor phase epitaxy-grown GaN films to Si substrates (type B). Experimental results indicated no significant degradation in the I-V characteristics for Schottky MSM devices fabricated on type B films compared to those fabricated on type A films. However, substantial increase in the flicker noise level, particularly in the low-temperature regime, is observed among the ohmic MSM devices fabricated on type B films. The experimental data suggest that material degradation occurs at the vicinity of the GaN-sapphire interface, while in regions close to the GaN film surface there is practically no change in the film quality. This is supported by finite element simulation of the temperature of the film during laser irradiation. The results indicate that the temperature dropped from 1400 K at the GaN-sapphire interface to about 1000 K within 0.5 μm away from the interface stipulating that material degradation occurs only within 0.5 μm from the GaN-sapphire interface.     

Multiwavelength ultralow-threshold lasing in quantum dot photonic crystal microcavities

We demonstrate multiwavelength lasing of resonant modes in linear (L3) microcavities in a triangular-lattice 2D photonic crystal (PC) slab. The broad spontaneous emission spectrum from coupled quantum dots, modified by the PC microcavity, is studied as a function of the intensity of incident optical excitation. We observe lasing with an ultralow-threshold power of approximately 600 nW and an output efficiency of approximately 3% at threshold. Two other resonant modes exhibit weaker turnon characteristics and thresholds of approximately 2.5 and 200 microW, respectively.     

激光剥离技术实现垂直结构GaN基LED

为改善GaN基发光二极管(Light-emitting diode,LED)的电学特性和提高其输出光功率,采用激光剥离技术,在KrF准分子激光器脉冲激光能量密度为400mJ/cm2的条件下,将GaN基LED从蓝宝石衬底剥离,结合金属熔融键合技术,在300℃中将GaN基LED转移至高电导率和高热导率的硅衬底,制备出了具有垂直结构的GaN基LED,并对其电学和光学特性进行了测试。结果表明:在110mA注入电流下,垂直结构器件的开启电压由普通结构的3.68V降低到了3.27V;在560mA注入电流下,器件输出光功率没有出现饱和现象;采用高电导率和高热导率的硅衬底能有效地改善GaN基LED的电学和光学特性。     

Surface Light Extraction Mapping from Photonic Quasicrystal on Current Two-Dimensional Array of 12-Fold Injected GaN-Based LEDs

A two-dimensional array of dodecagonal photonic quasicrystal （12PQC） is fabricated on the surface of current injected GaN-based LEDs to out-couple guided modes. The spatially-resolved surface light extraction mapping of 12PQC is observed and compared with that of triangular lattice photonic crystal （3PC） by microscopic electrical luminescence and scanning near-field microscopy. The higher enhancement factor of 12PQC is obtained to be larger than that of 3PC. It is shown that 12PQC is more favourable and efficient for light extraction of guided lights.