Comparison between two kinds of semiconductor absorbers for mode-locking in Nd : YVO4 laser

We have demonstrated passive mode-locking in a diode-end-pumped Nd:YVO4 laser using two kinds of semiconductor absorbers whose relaxation region comes from In0.25Ga0.75As grown at low temperature (LT) and GaAs/air interface respectively Mode-locking, using absorbers of the GaAs/air interface relaxation region, has the characteristics of less Q-switching tendency and higher average output power than that using absorbers of LT In0.25Ga0.75As relaxation region, but is not as stable as the latter.

Resonant tunnelling diodes and high electron mobility transistors integrated on GaAs substrates

A1GaAs/1nGaAs high electron mobility transistors (HEMTs) and AlAs/GaAs resonant tunnelling diodes (RTDs) are integrated on GaAs substrates. Molecular beam epitaxy is used to grow the RTD on the HEMT structure. The current-voltage characteristics of the RTD and HEMT are obtained on a two-inch wafer. At room temperature, the peak-valley, current ratio and the peak voltage are about 4.8 and 0.44 V, respectivcly The HEMT is characterized by a, gate length of 1 mu m, a, maximum transconductance of 125 mS/mm, and a threshold voltage of -1.0 V. The current-voltage, characteristics of the series-connected RTDs are presented. Tire current-voltage curves of the parallel connection of one RTD and one HEMT are also presented.

Fabrication of Ge nano-dot heterojunction phototransistors for improved light detection at 1.55 mu m

Heterojunction phototransistors (HPTs) with several Ge/Si nano-dot layers as the absorption region are fabricated to obtain improved light detectivity at 1.55 mu m. The HPT detectors are of n-p-n type with ten layers of Ge(8ML)/Si(45nm) incorporated in the base-collector junction and are grown by an ultrahigh-vacuum chemical-vapor deposition system. The detectors are operated with normal incidence. Because of the good quality of the grown material and fabrication process, the dark current is only 0.71pA/mu m(2) under 5 V bias and the break-down voltage is over 20 V. Compared to the positive-intrinsic-negative (PIN) reference detector with the same absorption layer, the responsivity is improved over 17 times for normal incidence at 1.55 mu m.

Effect of small-angle scattering on the integer quantum Hall plateau

A GaAs/AlGaAs two-dimensional electron gas (2 DEG) structure with the high mobility of mu(2K) = 1.78 x 10(6) cm(2)/Vs has been studied by low-temperature Hall and Shubnikov de Hass (SdH) measurements. Quantum lifetimes related to all-angle scattering events reduced from 0.64 ps to 0.52 ps after illuminating by Dingle plots, and transport lifetimes related to large-angle scattering events increasing from 42.3 ps to 67.8 ps. These results show that small-angle scattering events become stronger. It is clear that small-angle scattering events can cause the variation of the widths of the quantum Hall plateaus.

Dependence of intrinsic defects in ZnO films on oxygen fraction studied by positron annihilation

Defects in ZnO films grown by radio-frequency reactive magnetron sputtering under variable ratios between oxygen and argon gas have been investigated by using the monoenergetic positron beam technique. The dominate intrinsic defects in these ZnO samples are O vacancies (V-O) and Zn interstitials (Zn-i) when the oxygen fraction in the O-2/Ar feed gas does not exceed 70% in the processing chamber. On the other hand, zinc vacancies are preponderant in the ZnO Elms fabricated in richer oxygen environment. The concentration of zinc vacancies increases with the increasing (2) fraction. For the oxygen fraction 85%, the number of zinc vacancies that could trap positrons will be smaller. It is speculated that some unknown defects could shield zinc vacancies. The concentration of zinc vacancies in the ZnO films varies with the oxygen fraction in the growth chamber, which is in agreement with the results of photoluminescence spectra.

Growth and magnetic properties of zincblende CrSb epilayers on relaxed and strained (In, Ga)As buffers

Zincblende CrSb (zb-CrSb) layers with room-temperature ferromagnetism have been grown on relaxed and strained (In,Ga)As buffer layers epitaxially prepared on (001) GaAs substrates by molecular-beam epitaxy. The structural characterizations of CrSb layers fabricated under the two cases are studied by using synchrotron grazing incidence x-ray diffraction (GID). The results of GID experiments indicate that no sign of second phase exists in all the zb-CrSb layers. Superconducting quantum interference device measurements demonstrate that the thickness of zb-CrSb layers grown on both relaxed and strained (In,Ga)As buffer layers can be increased to similar to 12 monolayers (similar to 3.6nm), compared to similar to 3 monolayers (similar to 1nm) on GaAs directly.

Dual-wavelength distributed feedback laser for CWDM based on non-identical quantum well

Using non-identical quantum wells as the active material, a new distributed-feed back laser is fabricated with period varied Bragg grating. The full width at half maximum of 115 nm is observed in the amplified spontaneous emission spectrum of this material, which is flatter and wider than that of the identical quantum wells. Two wavelengths of 1.51 mu m and 1.53 mu m are realized under different work conditions. The side-mode suppression ratios of both wavelengths reach 40 dB. This device can be used as the light source of coarse wavelength division multiplexer communication systems.

Fabrication of thermo-optic switch in silicon-on-insulator

Silicon-on-insulator technology has been used to fabricate 2 x 2 thermo-optic switches. The switch shows crosstalk of -23.4 dB and extinction ratio of 18.1 dB in the bar-state. The switching speed is less than 30 mus and the power consumption is about 420 mW The measured excess loss is 1.8 dB. These merits make the switch more attractive for applications in wavelength division multiplexing.

A passively mode-locked diode-end-pumped Nd : YAG laser with a semiconductor saturable absorber mirror grown by metal organic chemical vapour deposition

We report the experimental results of a mode-locked diode-end-pumped Nd:YAG laser with a semiconductor saturable absorber mirror (SESAM) from which we achieved a 10 ps pulse duration at 150 MHz repetition rate. The SESAM was grown by metal organic chemical vapour deposition at low temperature. The recovery time was measured to be 0.5 ps, indicating the potential pulse compression to sub-picoseconds.

Photoluminescence of self-assembled InAs/GaAs quantum dots covered by InAlAs and InGaAs combination strain-reducing layer

Self-assembled InAs/GaAs quantum dots covered by the 1-nm InxAl(1-x)As (x = 0.2,0.3) and 3-nm In0.2Ga0.8As combination strain-reducing layer are fabricated, whose height can take up to 30-46 nm. The luminescence emission at a long-wavelength of 1.33 mum and the energy separation between the ground and the first-excited state of 86 meV are observed at room temperature. Furthermore, comparative study proves that the energy separation can increase to 91 meV by multiple stacking.

Diode-pumped passively Q-switched Yb : YAG microchip laser with a GaAs as saturable absorber

A passively Q-switched Yb: YAG microchip laser has been constructed by using a doped GaAs as the saturable absorber as well as the output coupler. At 13.5 W of pump power the device produces high-quality 3.4 muJ 52 ns pulses at 1030nm with a pulse repetition rate of 7.8kHz in a TEM00-mode.

Defects in GaN films grown on Si(111) substrates by metal-organic chemical vapour deposition

We report the transmission-electron microscopy study of the defects in wurtzitic GaN films grown on Si(111) substrates with AIN buffer layers by the metal-organic chemical vapour deposition method. The In0.1Ga0.9N/GaN multiple quantum well (MQW) reduced the dislocation density by obstructing the mixed and screw dislocations passing through the MQW. No evident reduction of the edge dislocations density by the MQW was observed. It was found that dislocations with screw component can be located at the boundaries of sub-grains slightly in-plane misoriented.

Self-organized InAs quantum wires on GaAs (331)A substrates

Self-organized InAs quantum wires (QWRs) were fabricated on the step edges of the GaAs (331)A surface by molecular beam epitaxy. The lateral size of InAs QWRs was saturated by the terrace width (i.e., 90 nm) while the size along the step lines increased with the increasing thicknesses of the InAs layers, up to 1100 nm. The height of InAs QWRs varied from 7.9 nm to 13 nm. The evolution of the morphology of InAs QWRs was attributed to the diffusion anisotropy of In adatoms.

Realization of low threshold of InGaAs/InAlAs quantum cascade laser

By optimizing the molecule beam epitaxy growth condition, the quality of quantum cascade (QC) material has greatly been improved. The spectrum of double x-ray diffraction indicates that the interface between the constituent layers is very smooth, the lattice mismatch between the epilayer and the substrate is less than 0.1%, and the periodicity fluctuation of the active region is not more than 4.2%. The QC laser with the emission wavelength of about 5.1 mum is operated at the threshold of 0.73 kA/cm(2) at liquid nitrogen temperature with the repetition rate of 10kHz and at a duty cycle of 1%. Meanwhile, the performance of the laser can be improved with suitable post process techniques such as the metallic ohmic contact technology.

Theoretical analysis of characteristics of GaxIn1-xNyAs1-y/GaAs quantum well lasers with different intermediate layers

Based on the band anticrossing model, the effects of the strain-compensated layer and the strain-mediated layer on the band structure, gain and differential gain of GaInNAs/GaAs quantum well lasers have been investigated. The results show that the GaNAs barrier has a disadvantage in increasing the density of states in the conduction band. Meanwhile, the multilayer quantum wells need higher transparency carrier density than the GaInNAs/GaAs single quantum well with the same wavelength. However, they help to suppress the degradation of the differential gain. The calculation also shows that from the viewpoint of band structure, the strain-compensated structure and the strain-mediated structure have similar features.