A broadband external cavity tunable InAs/GaAs quantum dot laser by utilizing only the ground state emission

A broadband external cavity tunable laser is realized by using a broad-emitting spectral InAs/GaAs quantum dot (QD) gain device. A tuning range of 69 nm with a central wavelength of 1056 nm, is achieved at a bias of 1.25 kA/cm2 only by utilizing the light emission from the ground state of QDs. This large tunable range only covers the QD ground-state emission and is related to the inhomogeneous size distribution of QDs. No excited state contributes to the tuning bandwidth. The application of the QD gain device to the external cavity tunable laser shows its immense potential in broadening the tuning bandwidth. By the external cavity feedback, the threshold current density can be reduced remarkably compared with the free-running QD gain device.     

Exciton emission dynamics in single InAs/GaAs quantum dots due to the existence of plasmon-field-induced metastable states in the wetting layer

A very long lifetime exciton emission with non-single exponential decay characteristics has been reported for single InA-s/GaAs quantum dot(QD) samples,in which there exists a long-lived metastable state in the wetting layer(WL)through radiative field coupling between the exciton emissions in the WL and the dipole field of metal islands.In this article we have proposed a new three-level model to simulate the exciton emission decay curve.In this model,assuming that the excitons in a metastable state will diffuse and be trapped by QDs,and then emit fluorescence in QDs,a stretchedlike exponential decay formula is derived as I(t)=At~(β-1)e~(-(rt)β),which can describe well the long lifetime decay curve with an analytical expression of average lifetime     

Singlet-Triplet Transition in Quantum Dots Confined by Triangular and Bowl-Like Potentials： the Effect of Electric Fields

We theoretically investigate the energy spectra of two-electron two-dimensional （2e 2D） quantum dots （QDs） confined by triangular potentials and bowl-like potentials in a magnetic field by exact diagonalization in the framework of effective mass theory. An in-plane electric field is found to contribute to the singlet-triplet transition of the ground state of the 2e 2D QDs confined by triangular or bowl-like potentials in a perpendicular magnetic field. The stronger the in-plane electric field, the smaller the magnetic field for the total spin of the ground states in the dot systems to change from S = 0 to S = 1. However, the influence of an in-plane electric field on the singlettriplet transition of the ground state of two electrons in a triangular QD modulated by a perpendicular magnetic field is quite small because the triangular potential just deviates from the harmonic potential well slightly. We find that the strength of the perpendicular magnetic field needed for the spin singlet-triplet transition of the ground state of the QD confined by a bowl-like potential is reduced drastically by applying an in-plane electric field.     

Growth and characterization of InAs quantum dots with low-density and long emission wavelength

The growth parameters affecting the deposition of self-assembled InAs quantum dots(QDs)on GaAs sub- strate by low-pressure metal-organic chemical vapor deposition(MOCVD)are reported,The low-density InAs QDs(～5×10~8 cm~(-2))are achieved using high growth temperature and low InAs coverage.Photolu- minescence(PL)measurements show the good optical quality of low-density QDs.At room temperature, the ground state peak wavelength of PL spectrum and full-width at half-maximum(FWHM)are 1361 nm and 23 meV(35 nm).respectively,which are obtained as the GaAs capping layer grown using triethylgal- lium(TEG)and tertiallybutylarsine(TBA).The PL spectra exhibit three emission peaks at 1361,1280, and 1204 nm,which correspond to the ground state,the first excited state,and the second excited state of the QDs,respectively.     

Optical anisotropy and the direction of polarization of exciton emissions in a semiconductor quantum dot:Effect of heavy-and light-hole mixing

The dependence of the directions of polarization of exciton emissions, fine structure splittings(FSS), and polarization anisotropy on the light-and heavy-hole(LH–HH) mixing in semiconductor quantum dots(QDs) is investigated using a mesoscopic model. In general, all QDs have a four-fold exciton ground state. Two exciton states have directions of polarization in the growth-plane, while the other two are along the growth direction of the QD. The LH–HH mixing does affect the FSS and polarization anisotropy of bright exciton states in the growth-plane in the low symmetry QDs(e.g., C_(2V),C_S, C_1), while it has no effect on the FSS and polarization anisotropy in high symmetry QDs(e.g., C_(3V), D_(2d)). When the hole ground state is pure HH or LH, the bright exciton states in the growth-plane are normal to each other. The LH–HH mixing affects the relative intensities and directions of bright exciton states in the growth-plane of the QD. The polarization anisotropy of exciton emissions in the growth-plane of the QD is independent of the phase angle of LH–HH mixing but strongly depends on the magnitude of LH–HH mixing in low symmetry QDs.     

Low Density Self-Assembled InAs/GaAs Quantum Dots Grown by Metal Organic Chemical Vapour Deposition

The self-assembled InAs quantum dots （QDs） on GaAs substrates with low density （5×10^8 cm^-2） are achieved using relatively higher growth temperature and low InAs coverage by low-pressure metal-organic chemical vapour deposition. The macro-PL spectra exhibit three emission peaks at 1361, 1280 and 1204nm, corresponding to the ground level （GS）, the first excited state （ES1） and the second excited state （ES2） of the QDs, respectively, which are obtained when the GaAs capping layer is grown using triethylgallium and tertiallybutylarsine. As a result of micro-PL, only a few peaks from individual dots have been observed. The exciton-biexciton behaviour was clearly observed at low temperature.     

Optical properties of 1.3-μm InAs/GaAs quantum dots grown by metal organic chemical vapor deposition

The optical properties of self-assembled InAs quantum dots （QDs） on GaAs substrate grown by metalorganic chemical vapor deposition （MOCVD） are reported. Photoluminescence （PL） measurements prove the good optical quality of InAs QDs, which axe achieved using lower growth temperature and higher InAs coverage. At room temperature, the ground state peak wavelength of PL spectrum and full-width at half-maximum （FWHM） are 1305 nm and 30 meV, respectively, which are obtained as the QDs are finally capped with 5-nm In0.06Ga0.94As strain-reducing layer （SRL）. The PL spectra exhibit two emission peaks at 1305 and 1198 nm, which correspond to the ground state （GS） and the excited state （ES） of the QDs, respectively.     

Barrier Li Quantum Dots in Magnetic Fields

The methods for the few-body system are introduced to investigate the states of the barrier Li quantum dots (QDs) in an arbitrary strength of magnetic field. The configuration, which consists of a positive ion located on the z-axis at a distaneed from the two-dimensional QD plane (the x-y plane) and three electrons in the dot plane bound by the positive ion, is called a barrier Li center. The system, which consists of three electrons in the dot plane bound by the ion,is called a barrier Li QD. The dependence of energy of the state of the barrier Li QD on an external magnetic field B and the distance d is obtained. The angular momentum L of the ground states is found to jump not only with the variation orB but also with d.     

Effect of Sb composition on the band alignment of InAs/GaAsSb quantum dots

Aiming to achieve InAs quantum dots(QDs) with a long carrier lifetime,the effects of Sb component in cap layers on the band alignment of the InAs/GaAsSb QDs have been studied.InAs QDs with high density and uniformity have been grown by molecular beam epitaxy.With increasing Sb composition,the InAs/GaAsSb QDs exhibit a significant redshift and broadening photoluminescence(PL).With a high Sb component of 22%,the longest wavelength emission of the InAs/GaAs_0.78Sb_0.22 QDs occurs at 1.5 μm at room temperature.The power-dependence PL measurements indicate that with a low Sb component of 14%,the InAs/GaAs_0.86Sb_0.14 QDs have a type-Ⅰ and a type-Ⅱ carrier recombination processes,respectively.With a high Sb component of 22%,the InAs/GaAs_0.78Sb_0.22 QDs have a pure type-Ⅱ band alignment,with three type-Ⅱ carrier recombination processes.Extracted from time-resolved PL decay traces,the carrier lifetime of the InAs/GaAs_0.78Sb_0.22 QDs reaches 16.86 ns,which is much longer than that of the InAs/GaAs_0.86Sb_0.14 QDs(2.07 ns).These results obtained here are meaningful to realize high conversion efficiency intermediate-band QD solar cells and other opto-electronic device.     

Temperature-Dependent Photoluminescence in Coupling Structures of CdSe Quantum Dots and a ZnCdSe Quantum Well

A coupling structure of CdSe quantum dots (QDs) and a ZnCdSe quantum well (QW) is fabricated by using the molecular-beam epitaxy technique. The effect o~ temperature on the photoluminescence (PL) of the structure is studied. The results reveal that the activation energy of exciton dissociation in the coupling QDs/QW structure is much higher than that of simple CdSe QDs, which is attributed to the exciton tunnelling from the QW to QDs through a thin ZnSe barrier layer. The results also reveal that the position and width of the emission band of the QDs vary discontinuously at certain temperatures. This phenomenon is explained by the QD ionization and exciton tunnelling from the QW to the QDs. It is demonstrated that the coupling structure significantly improves the PL intensity of CdSe QDs.     

Growth and characterization of InAs quantum dots with low-density and long emission wavelength

The growth parameters affecting the deposition of self-assembled InAs quantum dots （QDs） on GaAs substrate by low-pressure metal-organic chemical vapor deposition （MOCVD） are reported. The low-density InAs QDs （- 5 × 10^8cm^-2） are achieved using high growth temperature and low InAs coverage. Photoluminescence （PL） measurements show the good optical quality of low-density QDs. At room temperature, the ground state peak wavelength of PL spectrum and full-width at half-maximum （FWHM） are 1361 nm and 23 meV （35 nm）, respectively, which are obtained as the GaAs capping layer grown using triethylgallium （TEG） and tertiallybutylarsine （TBA）. The PL spectra exhibit three emission peaks at 1361, 1280, and 1204 nm, which correspond to the ground state, the first excited state, and the second excited state of the ODs, respectively.     

AlGaInP-Si glue bonded high performance light emitting diodes

We propose a new method of using conductive glue to agglutinate GaAs based AlGaInP light emitting diodes (LEDs) onto silicon substrate,and the absorbing GaAs layer is subsequently removed by grinding and selective wet etching.It was found that AlGaInP-Si glue agglutinated LEDs have larger saturation current and luminous intensity than the conventional LEDs working at the same injected current.The luminous intensity of the new device is as much as 1007.4 mcd at a saturation current of 125 mA without being encapsulated,while the conventional LEDs only have 266.2 mcd at a saturation current of 105 mA.The luminescence intensity is also found to increase by about 3.2% after working at 50 mA for 768 h.This means that the new structured LEDs have good reliability performance.     

Enhanced performance of GaN-based light-emitting diodes with InGaN/GaN superlattice barriers

GaN-based multiple quantum well light-emitting diodes （LEDs） with conventional and superlattice barriers have been investigated numerically. Simulation results demonstrate using InGaN/GaN superlattices as barriers can effectively enhance performances of the GaN-Based LEDs, mainly owing to the improvement of hole injection and transport among the MQW active region. Meanwhile, the improved electron capture decreases the electron leakage and alleviates the efficiency droop. The weak polarization field induced by the superlattice structure strengthens the intensity of the emission spectrum and leads to a blue-shift relative to the conventional one.     

Activation of silicon quantum dots and coupling between the active centre and the defect state of the photonic crystal in a nanolaser

A new nanolaser concept using silicon quantum dots (QDs) is proposed. The conduction band opened by the quantum confinement effect gives the pumping levels. Localized states in the gap due to some surface bonds on Si QDs can be formed for the activation of emission. An inversion of population can be generated between the localized states and the valence band in a QD fabricated by using a nanosecond pulse laser. Coupling between the active centres formed by localized states and the defect states of the two-dimensional (2D) photonic crystal can be used to select the model in the nanolaser.     

Enhancing light extraction of GaN-based blue light-emitting diodes by a tuned nanopillar array

Surface patterning of p-GaN to improve the light extraction efficiency of GaN-based blue light-emitting diodes(LEDs) has been investigated. Periodic nanopillar arrays on p-GaN have been fabricated by polystyrene(PS) nanosphere lithography; the diameter of the nanopillars can be tuned to optimize the electrical and optical properties of the LEDs. The electroluminescence intensity of the nanopillar-patterned LEDs is better than that of conventional LEDs; the greatest enhancement increased the intensity by a factor of 1.41 at a 20 mA injection current. The enhancements can be explained by a model of bilayer film on a GaN substrate. This method may serve as a practical approach to improve the efficiency of light extraction from LEDs.     

PHOTOLUMINESCENCE STUDY OF InAs/GaAs SELF-ORGANIZED QUANTUM DOTS WITH BIMODAL SIZE DISTRIBUTION

We have investigated the temperature dependence of the photoluminescence (PL) spectrum of self-organized InAs/GaAs quantum dots. A distinctive double-peak feature of the PL spectra from quantum dots has been observed, and a bimodal distribution of dot sizes has also been confirmed by scanning tunneling microscopy image for uncapped sample. The power-dependent PL study demonstrates that the distinctive PL emission peaks are associated with the ground-state emission of islands in different size branches. The temperature-dependent PL study shows that the PL quenching temperature for different dot families is different. Due to lacking of the couple between quantum dots, an unusual temperature dependence of the linewidth and peak energy of the dot ensemble photoluminescence has not been observed. In addition, we have tuned the emission wavelength of InAs QDs to 1.3 μm at room temperature.     

Resonance Transmission in Graphene-Nanoribbon-Based Quantum Dot and Superlattice

By using a decomposition elimination method for Green＇s function, the transport properties of Graphenenanoribbon-based quantum dot （QD） and/or QD superlattice are studied. It is shown that relatively small changes of both QD size and magnetic field intensity can induce strong variations in the electron transmission across the structure. For a QD device, electrons can be either totally reflected or totally transmitted through the QD region at some energies, and the quasibound peaks have been observed to have a small shift due to quasibound state energy varying. In the case of QD superlattice, the electrons within the miniband energy region can transmit through a device, similar to a QD device. Therefore, the transmission spectrum can be tailored to match with requirement by modulating the size of quantum dot and the number P of superlattice.     

Fluorescence spectrum characteristic of ethanol--water excimer and mechanism of resonance energy transfer

The steady-state fluorescence spectrum characteristic of ethanol--water excimer has been studied in this paper. By analysing the features of the sharp emission spectrum with fine structures in a shortwave band and the characteristics of the broad and featureless fluorescence peaks in the longwave band, one can conclude that the excimers are formed between the new ethanol--water cluster molecules in the excited state and the ground state through the interaction among different chromophores. The excitation spectra in the two fluorescence bands have been studied, and their emission mechanisms have been ascertained based on the energy transfer theory. Furthermore, the critical distance of the resonance energy transfer has been calculated.     

Effect of GaAs/GaSb Combination Strain-Reducing Layer on Self-Assembled InAs Quantum Dots

Self-assembled quantum dots capping with a GaAs/Gasb combined strain-reduced layer （CSRL） are grown by MBE. Their structural and optical properties are investigated by AFM and photoluminescence （PL）. PL measurements have shown that stronger emission about 1.3μm can be obtained by Sb irradiation and capping QDs with 3 ML GaAs/2 ML GaSh CSRL at room temperature. The full width at half maximum （FWHM） of the PL spectrum is about 20.2 meV （19.9 meV） at room temperature （2OK）, indicating that the QDs have high uniform, The result of FWHM is much better than the recently reported result, which is due to the fact that lower QD growth rate and growth interruption after the QDs deposition are adopted in our experiments.     

Effect of Al Doping in the InGaN/GaN Multiple Quantum Well Light Emitting Diodes Grown by Metalorganic Chemical Vapour Deposition

The effect of Al doping in the GaN layer of InGaN/GaN multiple quantum-well light emitting diodes （LEDs） grown by metalorganic chemical vapour deposition is investigated by using photoluminescence （PL） and highresolution x-ray diffraction. The full width at half maximum of PL of A1 doped LEDs is measured to be about 12nm. The band edge photoluminescence emission intensity is enhanced significantly. In addition, the in-plane compressive strain in the Al-doped LEDs is improved significantly and measured by reciprocal space map. The output power of Al-doped LEDs is 130mW in the case of the induced current of 200mA.