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We demonstrate a structure for a vertical-cavity surface-emitting laser (VCSEL) with a mode filter and half-wavelength (λ/2) cavity. The effects of λ/2- and λ-cavities on the threshold current are studied. The thickness and etched diameter of the mode filter are optimised to achieve single-transverse-mode output. The results show that when changing from λ-cavity to λ/2-cavity, the threshold current of the VCSEL decreases by 30.7% (from 0.85 mA to 0.65 mA). In addition, for the mode-filter thickness of 80 nm, and the etched diameter of 2.5 μm, the side mode suppression ratio reaches 90 dB, which meets the requirement of single-transverse-mode output. The VCSELs with the optimised design can overcome many difficulties and have great potential in 3D face recognition.

An increasing number of applications require an electrical source with good insulation and high power. Power over fiber (POF) technology has excellent insulating characteristics and thus is attracting increasing attention. The high-voltage laser power converter (HVLPC) is the most important component in the entire POF system, and the monolithically stacked HVLPC, because of its excellent performance characteristics, is particularly suitable for the requirements of high power. In this paper, we designed and prepared a compact high-power assembly with a 20-junction monolithically stacked HVLPC, and the performance characteristics of the designed assembly were separately tested under laser powers from 2 to 43 W with an 808 nm wavelength. More than 20 W of electric power was extracted under 43 W of laser power, and a maximum photon-energy conversion efficiency of 50.4% was observed with an open-circuit voltage of 22.15 V. Additionally, the temperature characteristics of the designed assembly under different laser powers and different loads were separately discussed, and a three-dimensional thermal simulation model for the designed assembly was established to predict the optimized passive heat sink structure. According to the research conclusions in this paper, additional types of high-power assemblies can be similarly designed in the future.

Fabrication and characteristics of 2004-nm wavelength InP-based ridge-waveguide distributed feedback lasers with InGaAs multi-quantum wells are described. By means of a relatively simple fabrication process, the device can operate stably in a single mode without any mode hopping. A high side-mode suppression ratio of 49 dB is achieved. The emission wavelength can be finely controlled from 2000 to 2008 nm with a current-tuning rate of 0.0047 nm/mA and a temperature-tuning rate of 0.127 nm/K. In addition, the influence of facet coating conditions on the single-mode property of these distributed feedback lasers was investigated.

We report on a short wavelength p-i-n detector using type II InAs/GaAsSb superlattices with AlSb insertion layers. The photoresponse is narrow-band like ( $\delta \lambda /\lambda $ =7.6%) and the wavelength at the response maximum is $1.08~\mu \text{m}$ at 77 K, the shortest wavelength reported until now. Insertion of AlSb thin layers not only helps reach such a short detection wavelength, but also reduces the dark current significantly. When a large positive bias voltage like +3 V is applied to the detector even at room temperature, the photoresponse is still clearly obtained, testifying a very small dark current of the device. The quantum efficiency of the device is 23% at $1.08~\mu \text{m}$ at 77 K, corresponding to the responsivity of 0.21 A/W. The resistance–area product at zero bias, $R_{0}A$ , is $1.7\times 10^{7}~\Omega \cdot $ cm2 at 77 K and is $121~\Omega \,\cdot $ cm2 at 300K. The Johnson noise limited detectivity ${D^{*}}$ of the device is $1.1\times 10^{13}$ Jones for the wavelength of $1.08~\mu \text{m}$ at 77 K.

We report on ultra-short pulse single-section mode-locked lasers emitting at 1.55 μm, based on self-assembled InAs/InGaAsP/InP quantum dot active regions grown by metal-organic vapor phase epitaxy (MOVPE). For a 1.5-mm-long Fabry-Perot laser, mode-locking at a repetition rate of 29.8 GHz with pulse duration of 855 fs is obtained without any external pulse compression techniques. The mode-beating exhibits a narrow RF linewidth less than 30 kHz, and a wide frequency tuning range up to 73 MHz can be achieved by simply changing the injection current. Moreover, a higher repetition rate of 55.6 GHz and the transform limited Gaussian-pulse with the 707 fs pulse duration are achieved from a device with a shorter cavity length of 0.8 mm.

We demonstrate an ultrashort pulse and high power single-section mode-locked laser using chirped multiple InAs/InP quantum dot (QD) layers as the active region of the laser. The chirped QD active region consists of seven layers of InAs QDs of different heights, which is beneficial in broadening the material gain spectrum. A transform-limited Gaussian pulse with a pulse duration of 322 fs is obtained from a device of 1 mm in length. Moreover, the femtosecond pulse with highest peak power of 6.8 W is achieved for the 45.5-GHz mode-locked laser. These results show the potential of the mode-locked laser for femtosecond pulse generation with high peak power and high repetition rate in the 1.55- $\mu \text{m}$ wavelength band.

We describe the controlled growth of planar InAsSb nanowires (NWs) on differently oriented Si substrates without any foreign catalysts. Interestingly, the planar InAsSb NWs grew along four criss-crossed ⟨110⟩ directions on an [100]-oriented substrate, two ⟨100⟩ directions plus four ⟨111⟩ directions on an [110]-oriented substrate, and six equivalent ⟨112⟩ directions on an [111]-oriented substrate, which correspond to the projections of ⟨111⟩ family crystal directions on the substrate planes. High-resolution transmission electron microscopy (HRTEM) reveals that the NWs experienced a transition from out-of-plane to in-plane growth at the early growth stage but still occurred on the {111} plane, which has the lowest surface energy among all the surfaces. Furthermore, the NWs exhibit a pure zinc-blende crystal structure without any defects. A growth model is presented to explain growth of the NWs. In addition, conductive atomic force microscopy shows that electrically rectifying p-n junctions form naturally between the planar InAsSb NWs and the p-type Si substrates. The results presented here could open up a new route way to fabricate highly integrated III-V nanodevices.

We investigate stacked structures of InAs/AlAsSb/InP quantum dots using temperature- and power-dependent photoluminescence. The band gap of InAs/AlAsSb QDs is 0.73 eV at room temperature, which is close to the ideal case for intermediate band solar cells. As the number of quantum dot layers is increased, the photoluminescence undergoes a blue-shift due to the effects of accumulated compressive strain. This PL red shift can be counteracted using thin layers of AlAs to compensate the strain. We also derive thermal activation energies for this exotic quantum dot system.

We present a study of Au-free InAsSb nanowire (NW) growth on Si (111) substrate under different growth parameters including V/III ratio, group Sb flow rate fraction (Sb-FRF, TMSb/(TMSb+AsH3)), and temperature. It was found that two different kinds of growth mechanisms for the Au-free InAsSb NW growth may be dominant depending on the growth parameters. At low V/III ratio and relatively high Sb-FRF, the NWs grow via vapor–liquid–solid (VLS) mode, while at high V/III ratio and relatively low Sb-FRF, they grow via vapor–solid (VS) mode. The NWs obtained by the two growth modes show clear differences in morphology, growth direction, and crystal quality. Under VS mode, the NWs exhibit unified growth direction and a uniform composition distribution, which are beneficial to integration devices of multiple NWs. On the other hand, under VLS mode, the NWs are first reported with pure crystal phase, which will be useful for the development of single NW devices.

We have determined the band overlap in type II InAs/GaSb superlattice (SL) structure based on the temperature dependent photoluminescence (PL) results of a short wavelength SL sample. The band overlap value is treated as a temperature variable and is simulated by fitting the PL peak position using the 8K·P method. It is found that the band overlap monotonically decreases from 0.325 to 0.225 eV when temperature is increased from 12 to 90 K. The calculated e1–hh1 transition using the obtained band overlap data shows an agreement with the PL results of another SL sample.

We demonstrate electrically pumped continuous-wave (CW) lasing near 1.5 pm at room temperature in microcylinder lasers with InAs quantum dot active regions. The microcylinder radius can reduce to 4μm at room temperature and contentious-wave lasing.

We present a study on the growth of InAs1−xSbx alloy nanowires directly on Si (111) substrates via a self-seeded mechanism for the first time. Through varying group V flow rate ratios, InAs1−xSbx nanowires with x=from 0 to 0.43 are obtained. It is found that Sb content has a significant effect on the morphology and crystal quality of the formed InAs1−xSbx nanowires. Furthermore, the axial and radial growth rates of the nanowires change in opposite trends with increasing group V flow rate ratio. This indicates that the growth rate of InAs1−xSbx nanowires is ultimately determined by Sb compositions of the nanowires. In addition, the scanning electron microscopy and transmission electron microscopy measurements reveal that the dimensional uniformity and crystal quality of InAsSb nanowires with a small amount of Sb compositions are greatly improved compared to the reference InAs nanowires. The effect mechanism of Sb on the growth of InAs1−xSbx nanowires is clarified, which will be a guide for making high-quality InAs1−xSbx nanowires and relevant heterostructure devices in the future.

The thickness influence of thermal oxide layers on Si(111) substrates on the formation of self-catalyzed InAs nanowires (NWs) grown by metal-organic chemical vapor deposition (MOCVD) has been investigated. It is found that the thickness of the thermal oxide layer has a strong effect on the morphological characteristics of InAs NWs formed on the Si substrates. In particular, within a suitable thickness range, it is possible to achieve vertical InAs NWs with a uniform distribution of their positions, lengths and diameters. In addition, growth on the thermal oxide layer both improves the morphology of the NWs, and suppresses the frequently observed parasitic islands compared to growth on bare Si. A growth model, which is based on the diffusion length of adatoms, the growth temperature, the thickness of thermal oxide layer, the size and the density of thermal oxide holes, has been developed to explain the formation of the self-catalyzed InAs NWs on the Si substrate with a thermal oxide layer.

The impact of a double-cap procedure using two growth temperatures on the optical characteristics of InAs/InGaAsP/InP quantum dots (QDs) grown by metal-organic chemical vapor deposition has been investigated. With a combination of optimized thickness of the first cap layer and elevated growth temperature for the second cap layer, it is found that the photoluminescence (PL) linewidth of QDs can be significantly reduced from 124 meV to 87 meV at room temperature (RT). This reduction in PL linewidth is likely to be due to the enhanced As/P exchange reaction and indium migration at high growth temperature, which lead to a more uniform QD height distribution. Moreover, the uniformity of the PL peak intensity and peak energy on the wafer surface is evidently improved due to the higher material quality achieved when an elevated temperature is used for the SCL growth.

We demonstrate an ultrashort pulse and high power single-section mode-locked laser using chirped multiple InAs /InP quantum dot (QD) layers as the active region of the laser. The chirped QD active region consists of seven layers of InAs QDs of different heights, which is beneficial in broadening the material gain spectrum. A transform-limited Gaussian pulse with pulse duration of 322 fs is obtained from a device of 1 mm in length. Moreover, the femtosecond pulses with peak power of as high as 6.8 W are achieved for the 45.5 GHz mode-locked laser. These results show the potential of the mode-locked laser for femtosecond pulse generation with high peak power and high repetition rate in the 1.55 µm wavelength band.

We report the growth of InAs/GaSb core-shell heterostructure nanowires with smooth sidewalls on Si substrates using metal-organic chemical vapor deposition with no assistance from foreign catalysts. Sb adatoms were observed to strongly influence the morphology of the GaSb shell. In particular, Ga droplets form on the nanowire tips when a relatively low TMSb flow rate is used, whereas the droplets are missing and the radial growth of the GaSb is enhanced due to a reduction in the diffusion length of the Ga adatoms when the TMSb flow rate is increased. Moreover, transmission electron microscopy measurements revealed that the GaSb shell coherently grew on the InAs core. The results obtained here show that the InAs/GaSb core-shell nanowires grown using the Si platform have strong potential in the fabrication of future nanometer-scale devices and in the study of fundamental quantum physics.

Self-assembled In(Ga)As/GaAs quantum dots (QDs), known as "artificial atoms" for their fully quantized electronic states, show unique physical properties and new prospects for applications. Based on the QDs, an increasing number of leading laboratories on the world engage in developing novel optoelectronic devices with special advantages over existing devices. This paper reviews the current research developments in self-assembled QD devices in China, covering QD lasers and inter sub-level QD photodetectors. QD devices in China are undergoing a rapid advance and have achieved the world's best results in terms of certain characteristics.

A theoretical study of modal gain in p-doped 1.3 mu m InAs/GaAs quantum dot (QD) lasers is presented. The expression of modal gain is derived, which includes an effective ratio that describes how many QDs contribute to the modal gain. The calculated results indicate that the modal gain with the effective ratio is much smaller than that without the effective ratio. The calculated maximum modal gain is is a good agreement with the experimental data. Furthermore, QDs with lower height or smaller aspect ratio are beneficial in achieving a larger maximum modal gain that leads to lower threshold current density and higher differential modal gain. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

InAs/InP quantum dot (QD) microcylinder lasers with an output waveguide are fabricated. Single-mode lasing wavelength of 1441 nm with side-mode suppression ratio around 24 dB is achieved from QD excited states emission.

We have investigated the temperature dependence of threshold in p-doped 1.3 μm InAs/GaAs quantum dot (QD) lasers with ten layers of QDs in the active region. It is found that the dependence of threshold current density on the temperature within the temperature range from 10 to 90 °C can be divided into three regions by its characteristic temperature (T0): negative, infinite, and positive T0 regions. Furthermore, the T0 region width is dependent on the cavity length: the longer cavity length of the QD lasers correspondingly the wider T0 region. Additionally, for the broad area laser, the threshold modal gains of the lasers with different cavity lengths can be fitted by an empirical expression as a function of the threshold current density, when at the temperatures of 30, 50, and 70 °C. We find that the transparency current density (Jtr) remains almost unchanged under different temperatures according to the extracted parameters from these fitted results, which indicates that Jtr plays an important role in balancing the T0 between negative region and positive one. Published version

Large-signal modulation capability, as an important performance indicator, is directly related to the high-speed optical communication technology involved. We experimentally and theoretically investigate the large-signal modulation characteristics of the simultaneous ground-state (GS) and the excited-state (ES) lasing in InAs/GaAs quantum dot laser diodes. The large-signal modulation capability of total light intensity in the transition regime from GS lasing to two-state lasing is unchanged as the bias-current increases. However, GS and ES large-signal eye diagrams show obvious variations during the transition. Relaxation oscillations and large-signal eye diagrams for GS, ES, and total light intensities are numerically simulated and analyzed in detail by using a rate-equation model. The findings show that a complementary relationship between the light intensities for GS and ES lasing exists in both the transition regime and the two-state lasing regime, leading to a much smaller overshooting power and a shorter settling time for the total light intensity. Therefore, the eye diagrams of GS or ES lasing are diffuse whereas those of total light intensity are constant as the bias-current increases in the transition regime.

We have fabricated 1.3-mum InAs-GaAs quantum-dot (QD) lasers with and without p-type modulation doping and their characteristics have been investigated. We find that introducing p-type doping in active regions can improve the temperature stability of 1.3-mum InAs-GaAs QD lasers, but it does not increase the saturation modal gain of the QD lasers. The saturation modal gain obtained from the two types of lasers is identical (17.5 cm-1 ). Moreover, the characteristic temperature increases as cavity length increases for the two types of lasers, and it improves more significantly for the lasers with p-type doping due to their higher gain.

We report the optimization of the double-cap (DC) procedure for InAs/InGaAsP/InP quantum dots (QD) grown by metal-organic chemical vapor deposition. By using a combination of optimized thickness of the first cap layer and elevated growth temperature for the second cap layer, the photoluminescence (PL) linewidth of samples with five QD layers is significantly reduced from 124 meV to 87 meV at room temperature. Furthermore, the uniformity of the PL peak intensity and peak energy on the wafer surface is evidently improved. This distribution improvement is especially beneficial for improving device yield per wafer in device fabrication.

We demonstrate high-brightness 1.3 μm tapered lasers with high temperature stability by using p-doped InAs/GaAs quantum dots (QDs) as the active region. It is found that the beam quality factor M(2) for the devices is almost unchanged as the light power and temperature increase. The almost constant M(2) results from the p-doped QD active region.

We demonstrate the generation of 92 nm (−3 dB bandwidth) flat-topped ultrabroad stimulation emission from a chirped InAs/InP quantum dot (QD) laser. A greatly enhanced bandwidth of the gain spectrum is achieved, which is attributed to the additionally broadened quantum dot energy levels utilizing gradually changed height of QDs in the stacked active layers. The laser exhibits a maximum output power of 0.35 W under pulsed conditions, and the average spectral power density of above 3.8 mW/nm is obtained. The ultrabroad lasing spectrum in the wavelength interval of 1.49–1.61 μm covering S-C-L bands makes such a laser potentially useful as an optical source for various applications being compatible with silica fibers.

Large signal modulation characteristics of the simultaneous ground-state (GS) and excited-state (ES) lasing quantum dot lasers are theoretically investigated. Relaxation oscillations of ‘0 → 1’ and ‘1 → 0’ in the GS lasing region (Region I), the transition region from GS lasing to two-state lasing (Region II) and the two-state lasing region (Region III) are compared and analyzed. It is found that the overshooting power and settling time in both Regions I and III decrease as the bias current increases. However, there exist abnormal behaviors of the overshooting power and settling time in Region II owing to the occurrence of ES lasing, which lead to fuzzy eye diagrams of the GS and ES lasing. Moreover, the ES lasing in Region III possesses much better eye diagrams because of its shorter settling time and smaller overshooting power over the GS lasing in Region I.

High brightness InAs/GaAs quantum dot tapered lasers at 1.3μm were first developed with high temperature stability. The devices consisted of a straight index-guided section formed by a ridge waveguide (RW) and a gain-guided tapered section. To enhance the mode filtering, a pair of cavity spoiling grooves had been included. For two different laser lengths L =1.5mm and 3mm with a constant full taper angle φ TR = 4°, the length of the ridge waveguide section L 2 was varied for 1.5 mm long devices, whose L 2 was 0.5mm and 1 mm, respectively. In case of L=3 mm, the L 2 was fixed at 1mm. For above-mentioned geometries, the light power (P) - current (I) characteristics and the beam quality factor (M 2 ) were presented. The lasers with a smaller L 2 have higher threshold current due to the larger area of the tapered section at fixed total length. The beam quality can be improved with longer L and L 2 at fixed total length L. The M 2 was lower than the value of 3 measured under different light power, moreover, the M 2 increased with light power increasing. We also measured temperature dependence of threshold current for two kinds of lasers with L=1.5 and 3 mm. The threshold current was almost constant over a temperature range from 30 to 70°C. It is of important significance for the practical application because that the temperature range overlapped with the operating temperature range. In addition, temperature dependence of the M 2 for the lasers with L=3 mm was also studied.

Large-signal performance of five-stack 1.3 μm InAs/GaAs quantum-dot (QD) lasers has been investigated. Open eye pattern up to 10 Gb/s under direct modulation is demonstrated at 15 °C.

We demonstrate high performance 2150 nm InAs/InGaAs/InP quantum well (QW) lasers grown by metalorganic vapor phase epitaxy. The laser structure consists of two InAs/InGaAs QWs, with a 30 μm-wide ridge waveguide and two cleaved cavity facets. The continuous wave operation at room temperature (RT) is achieved, with an output power of larger than 160 mW per facet and with a low threshold current density of 90.4 A/cm(2) per QW derived for the infinite cavity length. Under pulse injection mode, the maximal peak power per facet is as high as 1.35 W. By varying the cavity length, the lasing wavelength can be tuned in a range from 2142 nm to 2154 nm. Moreover, the highest operating temperature reaches up to 100 °C, and characteristic temperatures are 50 K (T(0)) and 132 K (T(1)) in the temperature range of 20-70 °C, respectively.

We investigate the photoluminescence (PL) properties of a hybrid type-I InAs/GaAs and type-II GaSb/GaAs quantum dot (QD) structure grown in a GaAs matrix by molecular beam epitaxy. This hybrid QD structure exhibits more intense PL with a broader spectral range, compared with control samples that contain only InAs or GaSb QDs. This enhanced PL performance is attributed to additional electron and hole injection from the type-I InAs QDs into the adjacent type-II GaSb QDs. We confirm this mechanism using time-resolved and power-dependent PL. These hybrid QD structures show potential for high efficiency QD solar cell applications.

The influence of facet coating on the performance of InAs/InP quantum dot (QD) tunable external-cavity (EC) lasers has been studied. It is shown that antireflection coating of the EC lasers expands the tuning bandwidth from 39 to 50 nm, but accompanied with a great increase in threshold current density. By adding antireflection/high-reflection facet coating, the performance of the EC lasers can be further enhanced. Compared to the laser without any facet coating, the output optical power of the facet coated QD laser increases from 20 to 34 mW under the identical injection current while the slope efficiency increases from 36.4 to 70 mW/A. Moreover, the tunability of the EC laser is extended to 104 nm covering the wavelengths from 1457 to 1561 nm with a relatively low injection current level.

We theoretically investigated the current-voltage characteristic of InAs/GaAs quantum dot (QD) intermediate band solar cells by changing the QD layer’s position in i-region. The open circuit voltage, short current density, fill factor, and conversion efficiency all vary with the position of QD layer. If the light generation coefficients through intermediate band (IB) are small, the IB mainly plays the role of a recombination energy level. If the light generation coefficients are improved, in order to ensure the highest QD layer performance, QD layer should be placed in an appropriate range.

The carrier induced refractive index change and linewidth enhancement factor α due to ground-state (GS) and excited-state (ES) transitions have been compared by measuring the optical gain spectra from an InAs/GaAs quantum dot (QD) laser structure. It is shown that the ES transition exhibits a reduced α-factor compared to the value due to the GS transition. This result can be explained by the α-factor due to the ES transition having a smaller increase from the non-resonant carriers in the combined state of the wetting layer and InGaAs strain reducing layer than the α-factor increase due to the GS transition, since the relaxation time for carriers from the combined state of the wetting layer and InGaAs strain reducing layer to the ES is shorter than to the GS. The result reported here shows another advantage of using ES QD lasers for optical communication, in addition to their higher modulation speed.

We investigate the band structure of a compressively strained In(Ga)As/In0.53Ga0.47As quantum well (QW) on an InP substrate using the eight-band k ? p theory. Aiming at the emission wavelength around 2.33 ?m, we discuss the influences of temperature, strain and well width on the band structure and on the emission wavelength of the QW. The wavelength increases with the increase of temperature, strain and well width. Furthermore, we design an InAs/In0.53Ga0.47As QW with a well width of 4.1 nm emitting at 2.33 ?m by optimizing the strain and the well width.

In this paper we discuss the fabrication of high-density InAs quantum dots (QDs) on GaAs (1 0 0) substrates by molecular beam epitaxy, via antimony surfactant-mediated growth. The structural and optical properties of the QDs were studied as a function of the amount of antimony irradiation. It was found that with appropriate antimony irradiation, the dot density significantly increases while simultaneously restraining giant QDs. However, when the irradiation is beyond a certain amount, the dot density inversely decreases significantly. Moreover, temperature-dependent photoluminescence (T-PL) measurements reveal that antimony irradiation can cause a larger redshift of the PL peak wavelength at all temperatures, a faster decrease of integrated PL intensity below 100 K and a slower decrease of integrated PL intensity within 100–300 K as compared to measurements taken in the absence of antimony irradiation. It was also found that by increasing the amount of antimony irradiation, the samples' behavior is closer to that of conventional QDs.

We theoretically studied the role of a InGaAs and InAlAs strain-reducing layer (SRL) with different thicknesses and indium compositions covered on InAs/GaAs self-assembled quantum dots (QDs). The ground-state transition wavelength increases as the thickness and indium composition of the SRL increase. The energy separation between ground state and excited state can achieve the maximum by a proper design. The redshift is due to (1) the strain reducing in QD, (2) the potential barrier, and (3) effective mass reducing in SRL, but the latter two tend to cancel each other and the energy level separation is mainly determined by (2) and (3).

We report the fabrication of intermediate-band solar cells (IBSCs) based on quantum dots (QDs), which consists of a standard P-I-N structure with multilayer stacks of InAs/GaAs QDs in the I-layer. Compared with conventional GaAs single-junction solar cells, the IBSCs based on InAs/GaAs QDs show a broader photo-response spectrum (> 1330 nm), a higher short-circle current (about 53% increase) and a stronger radiation hardness. The results have important applications for realizing high efficiency solar cells with stronger radiation hardness.

Experimental and theoretical study of the self-heating effect on the two-state lasing behaviors in 1.3-µm self-assembled InAs–GaAs quantum dot (QD) lasers is presented. Lasing spectra under different injected currents, light–current (L–I) curves measured in continuous and pulsed regimes as well as a rate-equation model considering the current heating have been employed to analyze the ground-state (GS) and excited-state (ES) lasing processes. We show that the self-heating causes the quenching of the GS lasing and the ES lasing by the increased carrier escape rate and the reduced maximum modal gain of GS and ES.