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In this study, the polarization effect in III-nitride based ultraviolet (UV) light-emitting diodes (LEDs) has been investigated theoretically. Some specific designs in active region are proposed to reduce the polarization effect and, hence, improve the device performance. Simulation results show that by utilizing properly designed quaternary AlInGaN material in active region, the hole injection efficiency can be enhanced due to the reduction of polarization mismatch between hetero-layers. On the other hand, the electron leakage is suppressed owing to that the effective potential height for electrons is increased. Therefore, the performance of UV LEDs is significantly improved by the polarization engineering in active region.

In this study, a specific design on the electron blocking layer (EBL) by band engineering is investigated numerically with an aim to improve the output performance and to reduce the efficiency droop in green LEDs. Systematic analyses including the energy band diagrams, carrier distributions in the active region, and electron leakage current are given and the simulation results show that the proposed lattice-compensated superlattice-AlGaN/InGaN EBL can provide better optical and electrical output performances when compared to the conventional rectangular AlGaN EBL. The output power of the green LED can be enhanced by a factor of 52% and the applied voltage can be reduced from 5.08 V to 4.53 V at an injection current of 1500 mA. The internal quantum efficiency is improved and the percentage of the efficiency droop can also be reduced from 58% to 37%, which is mainly attributed to the successful suppression of electron leakage current and improvement in hole injection efficiency.

Tunable passive Q-switching (781 nm to 806 nm at 300 K) of a flash-lamp pumped Cr/sup 3+/:LiCaAlF/sub 6/ (Cr:LiCAF) laser with a Cr/sup 4+/:Y/sub 2/SiO/sub 5/ (Cr/sup 4+/:YSO) broad-band solid-state saturable absorber has been realized. Typical pulse widths of the Q-switched laser output ranged from 40 ns to 80 ns, depending on the lasing wavelength. Spectral narrowing and reduced beam diameter with the use of the saturable absorber were observed. The ground state and the excited state absorption cross sections of the Cr/sup 4+/:YSO absorber were found by bleaching experiments to be (7.0/spl plusmn/1.4)/spl times/10/sup -19/ cm/sup 2/ and (2.3/spl plusmn/0.5)/spl times/10/sup -19/ cm/sup 2/ at 694 nm, respectively. Numerical simulation was utilized to simulate the Cr:LiCAF passive Q-switching with Cr/sup 4+/:YSO solid-state saturable absorber. >

Numerical simulation based on first-principles calculations is applied to study the energy band structural characteristics and the band-gap properties of wurtzite InGaN. The results show that the direct band gap, the band gap bowing parameter, the width of valence band, and the width of top valence band increase with compressive strain and decrease with tensile strain. The biaxial strain effect on the indirect band gap is little. In general, there is a larger band gap bowing parameter and larger strain-induced band gap bowing variation in Ga-rich alloys. In addition, the direct band gap, the indirect band gap, the width of valence band, and the width of top valence band decrease with increase of indium composition. Wurtzite InGaN remains the characteristic of a direct band gap material under biaxial stress.

The impact of the polarization compensation InGaN interlayer between the heterolayers of Ga-face GaN/InGaN p-i-n solar cells is investigated numerically. Because of the enhancement of carrier collection efficiency, the conversion efficiency is improved markedly, which can be ascribed to both the reduction of the polarization-induced electric field in the InGaN absorption layer and the mitigation of potential barriers at heterojunctions. This beneficial effect is more remarkable in situations with higher polarization, such as devices with a lower degree of relaxation or devices with a higher indium composition in the InGaN absorption layer.

The blue shift of viewing angle in the top-emitting organic light-emitting devices is discussed in this study. For the single-mode cavity, the device of anode metal/ m-MTDATA (40 nm)/ α-NPD (10 nm)/ Alq3 (47.6 nm)/ LiF (1 nm)/ Ag (20 nm) with the metal phase difference of 1.30 π has the minimum blue shift of viewing angle. For the double-mode cavity, the recombination area must be away from the cathode for the device with better performance. However, the double-mode cavity with only one recombination area still has worse FWHM and gets more serious the blue shift of viewing angle than the single-mode cavity. Therefore, the double-mode cavity with a recombination area at each antinode is performed, and the results prove that the blue shift of viewing angle and the FWHM are improved. Finally, we replace the emission layer with Alq3:DCM (0.01%) and adjust the main peak wavelength in the double-mode cavity by adjusting the thicknesses of the cavity. The results show that the FWHM and the blue shift of viewing angle obtain further improvement for the double-mode cavity with a recombination area at each antinode.

Specific designs on the band structures near the active region are investigated numerically by using the APSYS simulation program with the purpose to surmount the efficiency droop in the InGaN blue LEDs. Systematic analyses included the energy band diagrams, radiative and SRH recombination rates, distribution of electrons and holes in the active region, and electron overflow. Simulation results show that, with appropriate designs, the efficiency droop may be effectively reduced due to the increase of hole injection efficiency, the enhancement of blocking capability for electrons, or the uniform carrier distribution of carriers in the active region.

In recent literatures, the quantum efficiency of conventional blue InGaN light-emitting diodes (LEDs) is quite limited under relatively high driving current with conventional GaN barriers due presumably to the poor injection efficiency of hole. In this study, the efficiency enhancement of blue InGaN LEDs with indium composition graded InGaN barriers is proposed. The energy band diagram, carrier concentration in the quantum wells, diagram of hole current, radiative recombination rate, L-I curves, and internal quantum efficiency are investigated numerically. The simulation results show that the InGaN LED with graded InGaN barriers has better performance over its conventional counterpart with GaN barriers due to enhanced efficiency of hole injection. The simulation results also suggest that under relatively high current injection, the internal quantum efficiency and output light power are markedly improved when the traditional GaN barriers are replaced by graded InGaN barriers. According to the improved optical properties, the new-designed LED has promising potential in solid state lighting.

Cr 4+ : GSGG has been utilized for the first time to provide a saturable absorber Q -switch for the ruby laser. Single output pulse operation (100 mJ and 27 ns duration) with efficiencies relative to the free-running ruby laser operation of 25–30% was routinely obtained.

The holmium-doped calcium fluoride (Ho:CaF(2)) crystal was shown to be an effective solid-state saturable-absorber Q-switch for a flash-lamp-pumped Tm,Cr:Y(3)Al(5)O(12) laser at 2.017 µm. With a 1-cm-thick Ho(0.5%),Er(5%):CaP(2) saturable absorber and a 6.3% output coupler, a single Q-switched laser pulse of 51 mJ in energy and 60 ns in duration was obtained at a flash-lamp input energy of 85 J. With a 14.6% output coupler, a typical Q-switched laser pulse of 84 mJ and 82 ns was observed.

Passive Q switching of a flash-lamp-pumped ruby laser has been demonstrated with a Dy(2+):CaF(2) saturable absorber at room temperature. Typically a single Q-switched pulse of 4.5 mJ and 94 ns was observed. At 694 nm, the ground-state absorption cross section and the excited-state absorption cross section of the Dy(2+):CaF(2) absorber were found to be 1.2 × 10(-18) cm(2) and 0.9 × 10(-18) cm(2) respectively, with a four-level slow-relaxing saturable-absorber model.

In this paper, the high-efficiency GaInP/GaAs/InGaAs triple-junction solar cells are investigated numerically by using the APSYS simulation program. The solar cell structure used as a reference was based on a published article by Geisz et al. (Appl. Phys. Lett. 91, 023502, 2007). By optimizing the layer thickness of the top and middle cells, the appropriate solar cell structure which possesses high sunlight-to-energy conversion efficiency is recommended. At AM1.5G and one sun, the conversion efficiency is improved by 2.3%. At AM0 and one sun, the conversion efficiency is improved by 4.2%. At AM1.5D and one sun, the conversion efficiency is improved by 1.3%. Furthermore, based on the optimized structures, this device can achieve efficiencies of more than 40% at high concentrations. For the triple-junction solar cell under AM1.5G solar spectrum, the conversion efficiency reaches 40.2% at 40 suns. For the device under AM0 solar spectrum, the conversion efficiency reaches 36.2% at 30 suns. For the device under AM1.5D solar spectrum, the conversion efficiency reaches 40.2% at 50 suns.

A high energy bandgap electron blocking layer (EBL) just behind the active region is conventionally used in the nitride-based laser diodes (LDs) and light-emitting diodes (LEDs) to improve the confinement capability of electrons within the quantum wells. Nevertheless, the EBL may also act as a potential barrier for the holes and cause non-uniform distribution of holes among quantum wells. A most recent study by Han et al. (Appl. Phys. Lett. 94, 231123, 2009) reported that, because of the blocking effect for holes, the InGaN LED device without an EBL has slighter efficiency droop and higher light output at high level of current injection when compared with the LED device with an EBL. This result seems to contradict with the original intention of using the EBL. Furthermore, findings from our previous studies (IEEE J. Lightwave Technol. 26, 329, 2008; J. Appl. Phys. 103, 103115, 2008; Appl. Phys. Lett. 91, 201118, 2007) indicated that the utilization of EBL is essential for the InGaN laser diodes. Thus, in this work, the optical properties of the InGaN LDs and LEDs are explored numerically with the LASTIP simulation program and APSYS simulation program, respectively. The analyses focus particularly on the light output power, energy band diagrams, recombination rates, distribution of electrons and holes in the active region, and electron overflow. This study will then conclude with a discussion of the effect of EBL on the optical properties of the InGaN LDs and LEDs.

White organic light-emitting diodes (OLEDs) have attracted great attention recently. In this study, high-efficiency white organic light-emitting diodes with dotted-line doped layers are numerically investigated with the APSYS (abbreviation of Advanced Physical Model of Simulation Devices) simulation program. The APSYS simulation program, developed by Crosslight Inc., is capable of dealing with the optical, electrical, and thermal characteristics of OLED devices. To approach the real situation, the OLED device fabricated by Park et al. (Current Applied Physics 1, 116, 2001) was first modeled by adjusting the appropriate physical parameters. Based on this OLED structure, a new structure of ITO/α-NPD (40 nm)/Alq3:DCJTB (30 nm)/Alq3 (30 nm)/Mg:Ag emitting quasi-white light was then proposed. Then, the single layer of Alq3:DCJTB was replaced by multi-(Alq3:DCJTB/Alq3)n layers, which are the so-called dotted-line doped layers (see, e.g., paper by Han et al., Solid State Communications 141, 332, 2007), to further improve the optical performance. The optical properties of the white OLEDs with different pairs of (Alq3:DCJTB/Alq3)n dotted-line doped layers are investigated and discussed in detail. Optimization of the proposed quasi-white OLED structures is attempted. The simulation results indicate that the OLED with dotted-line doped layers has higher radiative recombination rate and better emission efficiency than that with a single Alq3:DCJTB layer. The physical origin of the improved optical performance for the OLED with dotted-line doped layers could be due to the increased electrons and holes at the interfaces between the Alq3:DCJTB and Alq3 layers, which thus results in higher radiative recombination rate and better emission efficiency.

Effect of polarization on optical characteristics of blue InGaN LEDs with staggered QW are numerically investigated in this article by using APSYS simulation program. Specifically, band diagram, carrier distribution, and output power have been discussed. According to the simulation results, the structure of staggered QW is proposed to reduce the polarization-related effect; furthermore, the staggered QW structure together with thinner well width is beneficial for improvement of the output power of the blue InGaN SQW LEDs. In this work, the best optical performance is obtained when the quantum-well structure is designed as In0.20Ga0.80N (0.9 nm)-In0.26Ga0.74N (1.1 nm) owing mainly to the enhanced overlap of electron and hole wavefunctions inside the QW.

Passive Q switching of a tunable alexandrite solid-state laser with a Cr:Y2SiO5 solid-state saturable absorber is numerically studied over a major portion of the alexandrite tuning range. The theory of passive Q switching with a slowly relaxing saturable absorber is studied and utilized for evaluating the performance of the Cr:YSO Q-switched alexandrite laser system. With a typical laser configuration, a Q-switched laser pulse of 262 mJ in 23 ns is obtained.

The material properties of zincblende Al x Ga 1-x N alloys from first-principles calculations are investigated. It is found that the bulk moduli of zincblende Al x Ga 1-x N increase with an increase of aluminum composition x, but the pressure derivative of bulk modulus decreases with an increase of aluminum composition x. The bulk modulus of 199.82 ± 0.64 GPa for GaN increases to that of 213.03 ± 1.09 GPa for AlN, and the deviation parameter of bulk modulus for zincblende Al x Ga 1-x N is 11.18 ± 3.61 GPa. The pressure derivative of bulk modulus of 4.190 ± 0.020 for GaN decreases to that of 3.589 ± 0.030 for AlN. The refractive indices in the wavelength range of 300-600 nm are also investigated. The one-Sellmeier-term expression is used for curve-fitting analysis of the refractive index as a function of the wavelength in the transparent region, and the results should provide for the design and growth of vertical cavity structures and distributed Bragg reflectors on GaAs substrates. Finally, the direct bowing parameter of 0.446 ± 0.126 eV and indirect bowing parameter of -0.360 ± 0.242 eV are obtained. There is a direct-indirect crossover near x = 0.654 for which the band gap energy is 4.573 eV.

Transferring existing color filter technology to an organic light-emitting diode (OLED) display can greatly simplify the fabrication of a full-color OLED since only a white light emission device is required. In this work, the optical and electronic properties of bright white multilayer OLEDs, typically with a structure of metal/LiF/Alq3/EML/TPD/ITO constructed by Lim et al., are numerically investigated with the APSYS simulation program. Specifically, the emission/absorption spectra of the Alq3 (Green), Alq3:DCM (Red), and SA (Blue) light-emitting layers (EMLs) as well as the energy band diagrams, electron-hole recombination rates, electroluminescence, current-voltage, and luminance-current characteristics of the simulated OLED devices are investigated and compared to the experimental results. The physical models utilized in this work are similar to those presented by Ruhstaller et al. and Hoffmann et al. The simulated results indicate that the emission spectra of the Alq3, Alq3:DCM, and SA light-emitting layers obtained in this study are in good agreement with those obtained experimentally by Zugang et al. We study the optical and electronic properties of the OLEDs consisting of several dotted-line doped layers (DLDLs) and adjust all emission layers to enhance the luminance efficiency. Finally, we insert m-MTDATA and CuPc buffer layers onto the ITO anode. Using the double-buffer layer structure, the device performance can be greatly improved through the relative alignment of the energy levels of the layers to enhance the holes injection and transportation. Structural optimization for the OLED devices with better optical and electronic performance is also discussed.

Optical characteristics of deep ultraviolet light-emitting diodes with the consideration of spontaneous and piezoelectric polarizations are studied in this article with the APSYS (Advanced Physical Model of Semiconductor Devices) simulation program. The amounts of surface charges caused by different polarizations are calculated and compared. Moreover, the band diagram, carrier distribution, radiative recombination current, and light-current performance curves of the InAlGaN UV LED structures with different polarizations are also discussed and investigated. According to the simulated results, we find that the influence of the spontaneous polarization is more apparent than the piezoelectric polarization on band properties, carrier distribution, radiative recombination and output power in deep UV spectral region. In other words, for nitride materials in deep UV region, the polarization resulted from lattice-mismatch is smaller than that caused by asymmetry of the structure along the c-axis. This conclusion is quite different from the situation of blue InGaN light-emitting diodes. For blue LEDs, the piezoelectric polarization is the dominant polarization mechanism because the lattice mismatch between compound layers is a severe problem for these long-wavelength LED devices.

The effect of spontaneous and piezoelectric polarizations on optical characteristics of blue InGaN light-emitting diodes (LEDs) is investigated in this work with the APSYS simulation program. Specifically, the band diagram, carrier distribution, radiative recombination current, and light-current performance curves are discussed for various polarization situations. According to the simulation results, the amount of polarization charges is dominated by piezoelectric polarization. Therefore, the band diagram and overlap between electrons and holes in quantum wells are improved effectively and the light-current performance is raised apparently as the piezoelectric polarization is removed and only the spontaneous polarization is taken into account. The possible reason is that the influence of piezoelectric polarization due to lattice constant mismatch on optical properties is severe than the spontaneous polarization due to asymmetry of the wurtzite structure along the c-axis. Moreover, the simulation results suggest that the blue InGaN LED structures with spontaneous polarization may provide higher output power but are more sensitive to temperature, in a range from 300 K to 350 K, when compared to the LED structures with piezoelectric polarization.

Visible InGaP/InGaAlP resonant-cavity light-emitting diodes with low temperature sensitivity output characteristics were demonstrated. By means of extending the resonant cavity to a thickness of three wavelength (3 lambda), the degree of power variation between 25 and 95degC for the devices biased at 20 mA was apparently reduced from -2.1 dB for the standard structure design (1-lambda cavity) to -0.6 dB. Numerical simulation showed that the insensitive temperature dependence of output characteristics was attributed to the reduced electron leakage current.

Referred to the laser structure and its experimental results obtained by Selmic et al. and Liu et al., optimized active structure for the 1.55-μm quantum well lasers based on AlGaInAs material system is investigated. A structure with 1.2% compressive-strained wells and a p-type AlInAs electron stopper layer of 20 nm thickness and 5×10 23 m -3 doping concentration is suggested. Using this structure the threshold current is reduced to 17.8 mA, and the electron overflow percentage is decreased to 1.74% at 330 K. Furthermore, the characteristic temperatures of threshold currents are enhanced to 55.6 K, 67.0 K, and 43.3 K in operating temperature ranges of 300 K~350 K, 300 K~330 K, and 330 K~350 K, respectively.

In reference to an AlInGaN UV LED fabricated in laboratory, the optical properties of the 370-nm UV LEDs are investigated with a self-consistent APSYS simulation program. The optical performance of the UV LEDs with different aluminum compositions in AlGaN electron blocking layer and different numbers of quantum wells are investigated in an attempt to optimize the UV LED structure. The simulated results show that the electron leakage current can be effectively reduced with the use of an AlGaN electron blocking layer with an aluminum composition of greater than 0.19, and optimum performance may be obtained when the number of quantum wells is three. Since the built-in polarization is one of the most important factors for the deterioration of III-nitride LED performance, the feasibility of using a latticematched quaternary Al0.18In0.039Ga0.781N electron blocking layer in the UV LED to improve the LED performance is also numerically studied. The simulated results suggest that with the use of a lattice-matched Al0.18In0.039Ga0.781N electron blocking layer, the polarization charge density in each heterostructure interface is reduced, the electrostatic field in quantum wells is reduced, and the maximum output power is sufficiently improved. The simulated results also indicate that better LED performance may be obtained when the Al0.18In0.039Ga0.781N electron blocking layer has a higher pdoping concentration due to reduced electron leakage and increased hole concentration in active region.

The zincblende In x Ga 1-x N, Al x Ga 1-x N, and Al x In 1-x N alloys are studied by numerical analysis based on first-principles calculations. The results show that the lattice constant of the three alloys obeys the Vegard's law. For In x Ga 1-x N the direct band gap bowing parameter obtained with the equilibrium lattice constant is 1.890 ± 0.097 eV. For Al x Ga 1-x N the direct and indirect bowing parameters of 0.574 ± 0.034 eV and 0.055 ± 0.038 eV are obtained, and there is a direct-indirect crossover near x = 0.56. For Al x In 1-x N the direct and indirect bowing parameters of 3.5694 ± 0.028 eV and 0.1953 ± 0.054 eV are obtained, and there is a direct-indirect crossover near x = 0.807.

Vertical-cavity surface-emitting lasers with variant compressively strained InGaAlAs quantum wells have been investigated. The valence band structures, optical gain spectra, and threshold properties of InGaAlAs/AlGaAs quantum wells are compared and analyzed. The simulation results indicate that the characteristics of InGaAlAs quantum wells can be improved by increasing the amount of compressive strain in quantum well. Furthermore, the properties of VCSELs with these compressively strained InGaAlAs quantum wells are studied numerically. The results of numerical calculations show that the threshold current and maximum output power can be enhanced by using higher compressively strained InGaAlAs quantum well. However, when the compressive strain is larger than about 1.5%, further improvement of the laser performance becomes minimal. The effects of the position and aperture size of the oxide-confinement layers on the laser performance are also investigated. Variation of the oxide layer design is shown to affect the current distribution which makes the temperature in the active region different. It is the main reason for the power roll-off in the VCSEL devices.

Organic light-emitting diodes (OLEDs) have been extensively developed in the past few years. The OLED displays have advantages over other displays, such as CRT, LCD, and PDP in thickness, weight, brightness, response time, viewing angle, contrast, driving power, flexibility, and capability of self-emission. In this work, the optical and electronic properties of multilayer OLED devices are numerically studied with an APSYS (Advanced Physical Model of Semiconductor Devices) simulation program. Specifically, the emission and absorption spectra of the Alq 3 , DCM, PBD, and SA light-emitting layers, and energy band diagrams, electron-hole recombination rates, and current-voltage characteristics of the simulated OLED devices, typically with a multilayer structure of metal/Alq 3 /EML/TPD/ITO constructed by Lim et al., are investigated and compared to the experimental results. The physical models utilized in this work are similar to those presented by Ruhstaller et al. and Hoffmann et al. The simulated results indicate that the emission spectra of the Alq 3 , DCM, PBD, and SA light-emitting layers obtained in this study are in good agreement with those obtained experimentally by Zugang et al. Optimization of the optical and electronic performance of the multilayer OLED devices are attempted. In order to further promote the research results, the whole numerical simulation process for optimizing the design of OLED devices has been applied to a project-based course of OLED device design to enhance the students' skills in photonics device design at the Graduate Institute of Photonics of National Changhua University of Education in Taiwan. In the meantime, the effectiveness of the course has been proved by various assessments. The application of the results is a useful point of reference for the research on photonics device design and engineering education. Therefore, it proffers a synthetic effect between innovation and practical application.

A 370-nm LED with an AlGaN electron-block layer is fabricated. Simulation results suggest that optimal performance is obtained when the LED has more than 3 wells and the AlGaN has Al composition of 19-21%.

In this work, we numerically investigate the passive Q-switching performance of the tunable Cr:YSO Q-switched Cr:LiSAF laser over its entire tuning range. Specifically, the optical performance of the Cr:YSO Q-switched Cr:LiSAF laser as functions of the initial population in the ground state of the Cr:YSO saturable absorber, the pumping rate, the reflectivity of the output coupler, and the dissipative loss inside the laser cavity are studied. Simulation results show that the Cr:YSO is an effective saturable absorber Q switch for the Cr:LiSAF laser over its entire tuning range. Unlike the Cr:YSO Q-switched alexandrite laser and the Cr:YSO Q-switched Cr:LiCAF laser, the Cr:YSO Q-switched Cr:LiSAF laser has similar passive Q-switching performance when the laser polarization is along each of the three principal axes of the Cr:YSO. The results obtained numerically in this work are in good agreement with those obtained experimentally by other researchers. Our simulation results indicate that, a Q-switched laser pulse with an output energy of 10 mJ and a pulse width of 17 ns may be obtained at 850 nm, the peak of its tuning spectrum.

An optimized 650-nm AlGaInP multiple-quantum-well (MQW) laser, which has a compressively strained graded-index separate confinement heterostructure (GRIN-SCH), with improved characteristic temperature, is described. We theoretically show that the parabolic GRIN-SCH has a better carrier injection and smaller overflow than the conventional step-SCH for the AlGaInP LD under identical optical confinement. We have also calculated the electron distribution in the quantum wells for both GRIN-SCH-4QW and SCH-4QW at high temperature. The results indicate that the electron leakage to the p-cladding layer is greatly reduced if the GRIN-SCH-4QW structure is used. We have also compared the performance of LDs with different GRIN-SCH profiles and found that the parabolic GRIN-SCH is better than linear GRIN-SCH in terms of carrier confinement. We have further demonstrated the performance of AlGaInP LDs with four different structures (4-QW step-SCH, 5-QW step-SCH, 4-QW parabolic-GRIN-SCH and 5-QW parabolic-GRIN-SCH). Both theoretical and experimental results indicate that the laser diode with GRIN-SCH-4QW shows the best laser performance among the three structures. A characteristic temperature of 110 K has been demonstrated.

The optical properties of the violet-blue InGaN quantum-well lasers with an emission wavelength of 400-480 nm are studied with a LASTIP simulation program. Assuming that the In x Ga 1-x N/In y Ga 1-y N heterostructure has a band-offset ratio of 7/3, our simulation results indicate that the use of an AlGaN blocking layer can help reduce the electronic current overflow, and the non-uniform carrier distribution in the quantum wells plays an important role in the laser performance. If the piezoelectric effect is taken into account, the lowest threshold current of the violet-blue InGaN quantum-well lasers is obtained when the number of InGaN well layers is two if the emission wavelength is shorter than 412 nm, and one if the emission wavelength is longer than 412 nm. At a laser wavelength of 478 nm, the slope efficiency of the InGaN single quantum-well laser is decreased by ~2.4% and that of the double quantum-well laser is decreased by ~13.9% when the thermal effect is taken into account.

The wurtzite AlxGa1-xN, InxGa1-xN, and AlxIn1-xN alloys are studied by numerical simulation based on first-principles calculations. For AlxGa1-xN the Vegard's law deviation parameter is 0.018 ± 0.001 a for the a lattice constant and -0.036 ± 0.005 a for the c lattice constant. For InxGa1-xN that is 0.047 ± 0.011 a for the a lattice constant and -0.117 ± 0.026 A for the c lattice constant. For AlxIn1-xN that is 0.063 ± 0.014 a for the a lattice constant and -0.160 ± 0.015 a for the c lattice constant The results indicate that the band gap bowing parameters obtained with the equilibrium lattice constant and with the lattice constants derived from the Vegard's law are 0.341 ± 0.035 eV and 0.351 ± 0.043 eV respectively for AlxGa1-xN, 1.782 ± 0.076 eV and 1.916 ± 0.068 eV respectively for InxGa1-xN, and 3.668 ± 0.147 eV and 3.457 ± 0.152 eV respectively for AlxIn1-xN.

Optimization of a 1300-nm AlGaInAs/InP strained multiple quantum-well structure with an electron stop layer, which is located between the active region and the p-type GRIN-SCH layer, is studied numerically with a LASTIP simulation program. Specifically, the effect of the electron stop layer on the characteristic temperature and the temperature dependence of the slope efficiency are investigated. Various physical parameters at different operating temperatures are adjusted so that the threshold currents of the simulated laser structure can be matched to the results measured experimentally by Selmic et al. Our simulated results suggest that the AlInAs is a better material for the electron stop layer than the GaAsP. With the use of a p-type Al 0.5 In 0.5 As electron stop layer and an active region consisting of Al 0.175 Ga 0.095 In 0.73 As(6 nm)/Al 0.27 Ga 0.21 In 0.52 As(10 nm), a characteristic temperature of as high as 94.7 K is achieved for the 250-µm-long AlGaInAs/InP strained quantum-well laser under study.

A novel folded-cavity design for the Ti:sapphire laser provided improved performance compared to a linear cavity when pumped by a Q-switched Nd:YAG laser.

Passive Q switching of a flashlamp‐pumped alexandrite laser with a Cr4+:Y2SiO5 (Cr4+:YSO) solid‐state saturable absorber has been demonstrated at room temperature. An output of a single Q‐switched laser pulse of 20 mJ in energy and 70 ns in duration was obtained. The output wavelength of the Q‐switched laser pulse and the free‐running laser was 753 nm.

In a previous work [Appl. Phys. Lett. 65, 3060 (1994)] we experimentally demonstrated that passive Q switching of a 2,017-nm, flashlamp-pumped Tm,Cr:YAG laser with a Ho:YLF saturable absorber could be obtained with an internal focusing lens. We numerically investigate the optical performance of the Ho:YLF Q-switched Tm:YAG laser system by solving the coupled rate equations. The simulation results indicate that the results obtained numerically are in good agreement with those obtained experimentally. With typical laser configuration, a Q-switched laser pulse of 35 mJ in 30 ns is obtained.

Passive Q‐switching of the flashlamp pumped Tm,Cr:Y3Al5O12 2‐μm laser with a Ho:YLiF4 saturable absorber has been demonstrated at room temperature. A single Q‐switched laser pulse of 11 mJ in energy and ∼45 ns in duration at 80 J flashlamp input energy was obtained. The wavelength of the Q‐switched laser output pulse and the Tm,Cr:Y3Al5O12 free‐running output was 2.017 μm.

A Cr4+:Gd3Sc2Ga3O12 (Cr4+:GSGG) broad‐band saturable absorber has been demonstrated to be an excellent passive Q‐switch for the flashlamp‐pumped tunable Cr3+:LiCaAlF6 (Cr:LiCAF) laser at room temperature. A single Q‐switched laser output pulse of 11 mJ in energy and 37 ns in duration at 778 nm was obtained in a nonoptimized laser.

In this paper we report the tunable passive Q-switching of the Cr/sup 3+/:LiCaAlF/sub 6/ laser with a broad-band Cr/sup 4+/:Y/sub 2/SiO/sub 5/ solid-state saturable absorber.

Although several solid-state saturable absorbers have been developed in the past few years, the research on passive Q-switching with solid-state saturable absorbers in 2 /spl mu/m region is still at its early stage. In this report we introduce the use of a 4% Ho:YVO/sub 4/ and a 10% Ho:YLiF/sub 4/ (Ho:YLF) solid-state crystals as saturable absorber Q-switches for the Tm,Cr:YAG laser operating at 2.017 /spl mu/m. The optical characteristics of the two saturable absorbers are also investigated.

In this work, we study the optical properties of the 570-nm, yellow-green, AlGaInP semiconductor materials experimentally with a photoluminescence (PL) measurement system and numerically with a commercial Lastip simulation program. Specifically, the effects of the aluminum composition in wells of the AlGaInP MQW structures, the barrier height in quantum wells, the tensile strain barrier cladding (TSBC) next to the MQW region, the compensated strain in MQW, and the distributed Bragg reflector (DBR) on the optical properties of the AlGaInP devices are investigated. The optical properties of the blue and ultraviolet InGaN quantum well devices have also been investigated. The effects of the indium composition in quantum well, the well width, and the bowing parameter on the optical properties of the InGaN quantum well structures are studied numerically with the Lastip simulation program.

The vertical-cavity surface-emitting lasers (VCSEL) operating in the spectral range near 850 nm usually utilize GaAs/AlGaAs as the active layer materials. In this work, in addition to the traditional unstrained GaAs/AlGaAs semiconductor laser, the characteristics of the strained InGaAs/AlGaAs vertical-cavity surface-emitting laser and the distributed Bragg reflectors (DBR) used in this semiconductor laser are investigated with a PICS3D (abbreviation of Photonic Integrated Circuit Simulator in 3D) simulation program. The simulation results show that the strained InGaAs/AlGaAs VCSEL has a better optical performance than that of the traditional unstrained GaAs/AlGaAs VCSEL. That is, when compared with the unstrained GaAs/AlGaAs quantum well structures, the strained InGaAs/AlGaAs VCSEL has a higher stimulated recombination rate, a lower threshold current, a higher main-side mode suppression ratio, and a higher characteristic temperature, which might be owing to its narrower well width and smaller carrier effective masses.

The III-nitride semiconductor materials attract much attention in the past few years owing to their important application in light-emitting diodes and semiconductor lasers. Since the III-nitride semiconductor devices are usually grown on the sapphire substrate, they all have wurtzite crystal structures. The energy bandgaps of the wurtzite III-nitrides are usually obtained experimentally. Several researchers have investigated the energy bandgaps and the bandgap bowing parameters of the wurtzite InGaN, AlGaN, and AlInN alloys; however, the results are quite diverging. In this work we investigate the band structures of the wurtzite InGaN, AlGaN, and AlInN alloys with a CASTEP simulation program. The simulation results suggest that the wurtzite InGaN, AlGaN, and AlInN have a bandgap bowing parameter of 1.21 eV, 0.35 eV, and 3.33 eV respectively. Our simulation results also indicate that the widths of the top valance bands of the wurtzite InGaN and AlGaN alloys decrease when the indium and aluminum compositions increase while the width of the AlInN top valence band has a maximum value of about 6.57 eV when the aluminum composition is near 0.53. In this paper, the investigation of the band structures and bandgap bowing parameters for the zincblende InGaN, AlGaN, and AlInN alloys is also reported.

The InGaN semiconductor materials have important application in visible light-emitting diodes (LED) and short-wavelength laser diodes. In this work we investigate the electronic current overflow and the inhomogeneous hole distribution of the blue InGaN quantum well structures with a LASTIP (abbreviation of LASer Technology Integrated Program) simulation program. The simulation results show that the InGaN quantum well structure has an appreciable electronic current overflow at room temperature. The electronic current overflow problem becomes even more severe at elevated temperatures, which not only affects the emission efficiency of the quantum well structure, but also deteriorates the operation lifetime of the InGaN optical devices. The simulation results indicate that it is possible to improve the electronic current overflow by increasing the doping level of the p-type epi-layers and adding an AlGaN blocking layer in the p-type region. On the other hand, our numerical simulation also shows that, in addition to the electronic current overflow issue, the distribution of the holes in the InGaN active region is very inhomogeneous. It turns out that the laser performance of a single quantum well InGaN laser is better than that of the multiple quantum well lasers. According to our studies, if the barriers between the quantum wells are properly doped, the inhomogeneous hole distribution in the active region may be improved and hence the laser performance of the multiple quantum well InGaN lasers may be enhanced.

The absorption cross-section of the Ho:YLF crystal is close to the emission cross-section of the Tm:YAG laser. According to the passive Q-switching theory, a giant laser pulse cannot be generated from the Ho:YLF Q-switched Tm:YAG laser system unless an internal focusing lens is utilized. In a previous work we experimentally demonstrated that passive Q-switching of the 2017-nm, flashlamp pumped Tm,Cr:YAG laser with a Ho:YLF saturable absorber could be obtained with an internal focusing lens. In this paper, we theoretically investigate the optical performance of the Ho:YLF Q-switched Tm:YAG laser system by solving the coupled rate equations. The simulation results indicate that the results obtained numerically are in good agreement with that obtained experimentally. Moreover, we study the passive Q-switching performance of the Ho:YLF Q-switched Tm:YAG laser system as functions of the reflectivity of the output coupler, the initial population of the saturable absorber ground state, the laser pumping rate, and the loss inside the laser resonator. On the other hand, assuming that a polarizer is utilized inside the laser cavity, we explore the passive Q-switching performance of the Ho:YLF Q-switched Tm:YAG laser system when the polarization of the laser light is along different direction between the two saturable absorber principal axes. Effect of the relative position between the saturable absorber and the output coupler is also investigated.

Both Cr:LiCAF and Cr:LiSAF solid state lasers were developed by Payne et al. As transition-metal vibronic lasers the Cr:LiCAF and Cr:LiSAF exhibit broad emission spectra, long lifetime of the upper laser levels, low nonlinear refractive indices, low thermal lensing, and low excited state absorption that make both of them unique sources for tunable or short pulse lasers. In previous work we had experimentally demonstrated that the Cr:YSO could work as a saturable absorber Q switch for the Cr:LiCAF laser near 780 nm, i.e., the peak of its tuning range. In this work, we theoretically investigate the optical performance of the Cr:YSO Q-switched Cr:LiCAF laser system over its entire tuning range by solving the coupled rate equations. The simulation results indicate that the results obtained numerically are in good agreement with that obtained experimentally. The theoretical simulation also shows that the Cr:YSO may be used as an effective saturable absorber Q switch for the tunable Cr:LiCAF laser over a major portion of its entire tuning range from 725 nm to 840 nm. On the other hand, the Cr:YSO had also been experimentally demonstrated to be an effective saturable absorber Q switch for the tunable Cr:LiSAF laser by Munin et al. The optical performance of the Cr:YSO Q-switched Cr:LiCAF and Cr:LiSAF lasers is numerically studied in this paper.

In this study, the design and fabrication schemes of back-side illuminated InGaN/GaN solar cells with periodic via-holes etching and Bragg mirror processes are presented. Compared to typical front-side illuminated solar cells, the improvements of open-circuit voltage (V(oc)) from 1.88 to 1.94 V and short-circuit current density (J(sc)) from 0.84 to 1.02 mA/cm(2) are observed. Most significantly, the back-side illuminated InGaN/GaN solar cells exhibit an extremely high fill factor up to 85.5%, leading to a conversion efficiency of 1.69% from 0.66% of typical front-side illuminated solar cells under air mass 1.5 global illuminations. Moreover, the effects of bottom Bragg mirrors on the photovoltaic characteristics of back-side illuminated solar cells are studied by an advanced simulation program. The results show that the J(sc) could further be improved with a factor of 10% from the original back-side illuminated solar cell by the structure optimization of bottom Bragg mirrors.

AlGaInP LEDs with emission wavelengths near 570 nm are important in liquid crystal display backlight application. However, high brightness in this spectral region is difficult to achieve due to the reduction of the radiation efficiency in the high-aluminum-containing active region and the smaller band offset between the active and the cladding region. In order to improve the performance of the 570-nm AlGaInP LEDs, we have grown several wafers with different structure designs and studied the optical properties as functions of the device temperature and the excitation power experimentally with a photoluminescence measurement system and numerically with a commercial Latsip simulation program. Specifically, important factors such as the barrier height in quantum wells, the tensile strain barrier cladding next to the MQW region, the compensated strain in MQW, and the disturbed Bragg reflector are investigated. Good agreement between the experimental and numerical results is observed.