Laser Threshold

We investigate many-body and nonequilibrium effects on the dynamical behavior of a quantum-dot laser diode. Simulations, based on the Maxwell-semiconductor-Bloch equations, show strong dependence of the turn-on delay on initial cavity detuning, because of a dynamical shift in the quantum-dot distribution caused by band gap renormalization. Gain switch behavior is found to be insensitive to inhomogeneous broadening, because the balancing between many-body and free-carrier effects inhibits a cavity resonance walk-off. Both the relaxation oscillation damping and frequency are found to increase with decreasing inhomogeneous broadening widths. However, in contrast to bulk and quantum-well lasers, oscillation damping increases less than the frequency.

The relative contributions of the photon and thermal coupling mechanisms to the behavior of self-assembled InAs/ GaAs quantum dot lasers are studied. A theoretical model, which takes into account a photon coupling process between the ground and first excited states of different sized dots, is proposed to fully explain the temperature dependence of the threshold current density ͑J th ͒ of both undoped and p-doped lasers. The simulation results suggest that the carrier distribution between the different energy states in a dot is modulated by the intradot thermal excitation of carriers. This process, when combined with the photon coupling mechanism, can account for the negative characteristic temperature ͑T 0 ͒ appearing in different temperature ranges for undoped and p-doped devices. Thermal coupling, which involves thermal carrier escape and recapture among different dots, has also been studied. Below threshold, thermal coupling is found to be significant but is weakened as threshold is approached because of the decreased carrier lifetime. Near and above threshold, the photon coupling mechanism is important and can be used to model the different temperature behaviors of the lasing spectra observed experimentally for the undoped and p-doped lasers.

The addition of elevated temperature steps (annealing) during the growth of InAs/GaAs quantum dot (QD) structures on Si substrates results in significant improvements in their structural and optical properties and laser device performance. This is shown to result from an increased efficacy of the dislocation filter layers (DFLs); reducing the density of dislocations that arise at the Si/III-V interface which reach the active region. The addition of two annealing steps gives a greater than three reduction in the room temperature threshold current of a 1.3 μm emitting QD laser on Si. The active region of structures grown on Si have a room temperature residual tensile strain of 0.17%, consistent with cool down from the growth temperature and the different Si and GaAs thermal expansion coefficients. This strain limits the amount of III-V material that can be grown before relaxation occurs.

In this paper, a theoretical model is used to study the optical gain characteristics of  quantum dot lasers. The model is based on the density matrix theory of semiconductor lasers with relaxation broadening. The effect of doping with varying the side lengths of the box in the structure is taken into account. A comparative study of the gain spectra of p-doped, undoped and n-doped structures of  cubic quantum-dot (QD) laser respectively, is presented for various side lengths. The variation of peak gain on carrier density is also presented. The effect of side length on the variation in modal gain versus current density is plotted too. The results indicate that the p type doping is efficient to reach a better optical gain value, and to achieve low threshold current densities compared with undoped and  n-doped structures, and the optimum value for quantum dot width to achieve the lower threshold current density for the three cases is L=100A .   

Heparin-binding hemagglutinin (HBHA) is a 199 amino acid virulence factor at the envelope of Mycobacterium tuberculosis that contributes to latent tuberculosis. The binding of HBHA to respiratory epithelial cells, which leads to extrapulmonary dissemination of the pathogen, is mediated by cell-surface heparan sulfate (HS). We report the structural characterization of the HBHA/HS complex by NMR spectroscopy. To develop a model for the molecular recognition, the first chemically synthesized uniformly (13) C- and (15) N-labeled HS octasaccharide and a uniformly (13) C- and (15) N-labeled form of HBHA were prepared. Residues 180-195 at the C-terminal region of HBHA show large chemical shift perturbation upon association with the octasaccharide. Molecular dynamics simulations conforming to the multidimensional NMR data revealed key electrostatic and even hydrophobic interactions between the binding partners that may aid in the development of agents targeting the binding event.

We present a 160 GHz pulsed source, based on an on-chip repetition rate multiplier scheme to double the repetition rate of an 80 GHz multiple pulse colliding mode-locked (mCPM) laser. For the first time to our knowledge, we demonstrate the monolithic integration of the mCPM laser and the pulse rate multiplier structures, using a generic foundry approach. From a 20 GHz fundamental repetition rate we achieve a x4 increase, up to 160 GHz. The FWHM pulsewidth of 160 GHz pulse train is 2.77 ps.

In this paper, we present a new approach to obtain large size dots in an MBE grown InAs/GaAs multilayer quantum dot system. This is achieved by adding an InAlGaAs quaternary capping layer in addition to a high growth temperature (590°C) GaAs capping layer with the view to tune the emission wavelength of these QDs towards the 1.3 μm/0.95 eV region important for communication devices. Strain driven migration of In atoms from InAlGaAs alloy to the InAs QDs effectively increases the size of QDs. Microscopic investigations were carried out to study the dot size and morphology in the different layers of the grown samples. Methods to reduce structural defects like threading dislocations in multilayer quantum dot samples are also studied.

In this paper, we present a new approach to obtain large size dots in an MBE grown InAs/GaAs multilayer quantum dot system. This is achieved by adding an InAlGaAs quaternary capping layer in addition to a high growth temperature (590° C) GaAs capping layer with the view to tune the emission wavelength of these QDs towards the 1.3 μm/0.95 eV region important for communication devices. Strain driven migration of In atoms from InAlGaAs alloy to the InAs QDs effectively increases the size of QDs. Microscopic ...

The development simulation model of quantum dot (QD) laser is performed based upon rate equations for the carriers and photons in energy states. The rate equation is solved by using Matlab, Runge-Kutta method. In this paper shown that by increasing carrier injection to the active medium of laser, switching-on and stability time of the system would decrease while output power at peak and stationary will be increased. Indirect (non-instantaneous) carrier injection into QD is an essential component of our model and it describes the actual situation for QD laser.

Vertically aligned InAs/GaAs quantum dot structures were investigated. They were grown by atomic layer molecular beam epitaxy, with 10 layers and wedged spacer thickness d varying between 7.7 and 12.5 nm at steps of 0.2 nm. The effects of electronic coupling on the fundamental transition energy and decay dynamics were studied by time-resolved and continuous-wave photoluminescence (PL). The energy of the PL peak position decreases with increasing d, while the corresponding lifetime increases with d. This is attributed to the interplay of the electronic delocalization along the column and the Coulomb interaction between electrons and holes. Furthermore, a bell-shaped behaviour of the PL lifetime as a function of emission energy was observed inside the inhomogeneously broadened PL band.

Gain saturation increases the radiative component, J rad , of the threshold current density, J th , and its contribution to the thermal sensitivity of J th in short cavity or low QD density devices. However, the main cause of their thermal sensitivity is a strong non-radiative recombination.

We report the device characteristics of stacked InAs/ GaAs quantum dots ͑QDs͒ with GaP strain-compensation ͑SC͒ layers grown by metal organic chemical vapor deposition. By inserting GaP SC layers within the stacked structures, decrease in the density of QDs by stacking QDs can be suppressed due to reduction of overall compressive strain within the stacked QDs. We demonstrate ground-state lasing at 1.265 m of six layers of InAs/ GaAs QDs with GaP SC layers. The threshold current density is as low as 108 A / cm 2. We also assess the internal loss and maximum modal gain of fabricated QD lasers by using a segmented contact method. The internal loss is as low as 5 cm −1 , and the maximum modal gain of the ground state of the stacked QDs is approximately 10 cm −1 .

ABSTRACTTransmission electron microscopy (TEM), and photoluminescence (PL) have been used to evaluate defects and the efficiency of defect-reduction techniques in structures with InAs quantum dots (QDs) for the 1.55 µm range grown at low substrate temperature (LT) using molecular beam epitaxy (MBE). We show that capping of the QDs with thin GaAs layer accompanied by growth interruption at 600oC (flash) allows to eliminate large islands, containing dislocations, while the smaller islands containing local defects (e.g. dislocation dipoles) still remain. If the flash procedure is accompanied with further depositing of thin AlAs cap layer, and followed by high temperature (~700oC) annealing (HTA), an almost complete elimination of defects is observed. The structures emit in the range of 1.55 µm due to lateral agglomerates of LTQDs. Simultaneously bright luminescence due to isolated QDs and GaAs matrix are detected at high excitation densities.

Ridge-waveguide lasers with an InAs=InGaAs quantum dot active region have been subjected to accelerated ageing at 65 and 85 C. No sudden failure was found during the 2070 h test. Activation energy of 0.79 eV was estimated, suggesting the 40 C-lifetime > 10 6 h.

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The apparent C-V profiles and the deep levels in GaAs InAs GaAs quantum dot nanostructure, have been investigated by space charge spectroscopy techniques (C-V and DLTS). Accumulation peaks and/or depletion offree carriers at the QDs plane are observed under the considered growth parameters. It is shown that, both in the cap and at the QD-layerlcap interface, the deposition ofInAs QDs induces deep levels which exhibit a logarithmic dependence of the DLTS signal amplitude from the pulse width. Transmission Electron Microscopy (TEM) analysis shows extended defects in the cap and near the QD/cap interface, which have been correlated to electrical measurement results.

InAs/GaAs quantum dot (QD) bilayer and trilayer structures have been grown on GaAs(001) substrates by molecular beam epitaxy and the properties of the uncapped QDs investigated using atomic force microscopy (AFM). The emphasis is on understanding the influence a variation of the thickness of the GaAs spacer layer (S) between the QD layers has on the morphological properties of the QDs in the up-most layer, which is grown at a significantly reduced temperature compared to the first QD layer. The size distribution of the QDs in the second layer is shown to be exceptionally narrow for a spacer thickness of 10 nm, with a large average QD size. Larger values of S lead to a much broader size distribution and the appearance of significantly smaller dots. Complete strain relief is only achieved upon deposition of f 50 nm GaAs in bilayer and f 60 nm in trilayer structures, a result that has important implications for multiple stacking of QD layers in device applications.

Microdisk lasers with active region made of type-II GaSb/GaAs quantum dots on the GaAs substrate have been demonstrated. A microdisk cavity with diameter of 3.9 m was fabricated from a 225-nm-thick GaAs layer filled with GaSb quantum dots. Lasing at wavelengths near 1000 nm at 150 K was achieved for this microdisk. A high threshold characteristic temperature of 77 K was also observed. It is found that the lasing wavelength matches closely with the first-order whispering-gallery mode of the cavity as obtained from the finite-element method simulation.

The photoluminescence ͑PL͒, its temperature and power dependences, as well as PL inhomogeneity and x ray diffraction ͑XRD͒ has been studied in the symmetric In 0.15 Ga 1−0.15 As/ GaAs quantum wells with embedded InAs quantum dots ͑QDs͒ ͑dot-in-a-well, DWELL͒ with different QD densities, obtained by the variation in QD growth temperatures. It is shown that four reasons are responsible for the difference in emission intensities, PL peak positions and PL inhomogeneity in studied QD structures: ͑i͒ the high concentration of nonradiative ͑NR͒ recombination centers in the capping In 0.15 Ga 1−0.15 As layer at low QD growth temperatures ͑470°C͒, ͑ii͒ the QD density and size distributions for the structures with QD grown at 490-535°C, ͑iii͒ the high concentration of NR recombination centers in the GaAs barrier at high QD growth temperatures ͑535°C͒, and ͑iv͒ the variation nonmonotonous of elastic strain versus QD density. XRD study confirms that with decreasing density of QDs in DWELLs from 1.1ϫ 10 11 cm −2 down to 1.3ϫ 10 10 cm −2 at the rise of QD growth temperatures the level of compressive strain in DWELLs varies nonmonotonously. The reasons of compressive strain variation and the impact of this variation on emission parameters of DWELLs have been discussed as well.

A model of microcavity semiconductor lasers in which both the cavity field and the gain medium are quantized is presented. The equation of motion for the elements of the reduced density matrix for the field in the photon number representation is developed and numerically solved to find the steady-state photon number distribution and the laser linewidth for a variety of operating conditions. For typical semiconductor microcavity operating conditions, the intensity noise is smaller and the laser linewidth is larger for lasers with a larger fraction of spontaneous emission into the cavity mode. However, for very-low-loss microcavity lasers, if the rate of spontaneous emission into the cavity mode exceeds the loss rate, the laser can appear to turn on at pump rates for which the gain medium is not inverted. In this anamolous regime, the laser intensity noise increases with an increased fraction of spontaneous emission into the cavity mode.

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We derive a closed-form expression for the upper limit for the modulation bandwidth of a semiconductor quantum dot ͑QD͒ laser. The highest possible bandwidth increases directly with overlap integral of the electron and hole wave functions in a QD, number of QD-layers, and surface density of QDs in a layer, and is inversely proportional to the inhomogeneous line broadening caused by the QD-size dispersion. At 10% QD-size fluctuations and 100% overlap, the upper limit for the modulation bandwidth in a single QD-layer laser can be as high as 60 GHz.

The details of dynamic phenomena of broadband lasing in quantum-dot lasers in association with the role of both inhomogeneous and homogeneous optical gain broadening is theoretically investigated. The theoretical results of room temperature broadband laser signature are in well agreement with the experimental observation confirming the broadband stimulated emission from the laser is a result of the occurrence of multiple-state lasing in highly inhomogeneity quantum-dot systems.

Comparing simulation results with experimental findings, it is found that considering nonlinear optical gain is quite essential to accurately obtain dynamic and static characteristics of self-assembled quantum-dot lasers (SAQDLs). In fact, the nonlinear optical gain prevents extreme decline or growth of photon population as the time increases and of output power as the injected current enhances. It also results in multi-mode lasing and increasing the number of lasing modes with elevation of the injected current. In addition, the best performance of SAQDLs, at a certain injected current, depends on homogeneous and inhomogeneous broadening. Thermal carrier excitation results in degradation of light-current characteristics. It also leads to a red shift in dominant lasing modes at low injected currents, the dominant lasing modes move toward higher energies as the current enhances until the most dominant mode becomes the central one.

Annealing at higher temperature (700 °C) of structures with two-dimensional and three-dimensional arrays in InAs–GaAs quantum dots (QDs) results in an increase in the size and in a corresponding decrease in the indium composition of the QDs. The change in the In composition is monitored by the contrast pattern in the plan-view transmission electron microscopy (TEM) images viewed under the

Transmission electron microscopy studies of low indium composition In x Ga 1Ϫx As insertions ͑xϽ0.4͒ in a GaAs matrix deposited by molecular beam epitaxy or by metal-organic chemical vapor deposition reveal nanoscale quasiperiodic compositional and morphological modulations. The luminescence peak wavelength is found to be a strong function of deposition parameters and can be tuned from 1.1 to 1.3 m for the same x and average thickness of the deposit. Strained ͑In, Ga, Al͒As lasers with such a laterally modulated In x Ga 1Ϫx As active region demonstrate significant depolarization of the electroluminescence. For short cavity length, lasing occurs at energies corresponding to transitions between In x Ga 1Ϫx As conduction band and light holelike valence band states. Devices lase at low threshold current densities and demonstrate continuous wave operation up to 3 W at room temperature.

Annealing at higher temperature ͑700°C͒ of structures with two-dimensional and three-dimensional arrays in InAs-GaAs quantum dots ͑QDs͒ results in an increase in the size and in a corresponding decrease in the indium composition of the QDs. The change in the In composition is monitored by the contrast pattern in the plan-view transmission electron microscopy ͑TEM͒ images viewed under the strong beam imaging conditions. Increase in the size of the QDs is manifested by the plan-view TEM images taken under ͓001͔ zone axis illumination as well as by the cross-section TEM images. We show that the dots maintain their geometrical shape upon annealing. Luminescence spectra demonstrate a shift of the QD luminescence peak toward higher energies with an increase in the annealing time ͑10-60 min͒ in agreement with the decrease in indium composition revealed in TEM studies. The corresponding decrease in the QD localization energy results in an effective evaporation of carriers from QDs at room temperature, and the intensity of the QD luminescence decreases, and the intensity of the wetting layer and the GaAs matrix luminescence increase with the increase in the annealing time.

We report on quantum dot ͑QD͒ lasers made of stacked InAs dots grown by metalorganic chemical vapor deposition. Successful growth of defect-free binary InAs/GaAs QDs with high lateral density (d l у4ϫ10 10 cm Ϫ2 ) was achieved in a narrow growth parameter window. The room-temperature photoluminescence ͑PL͒ intensity is enhanced up to a factor of 3 and the PL peak width is reduced by more than 30% when a thin layer of In 0.3 Ga 0.7 As is deposited onto the InAs QDs. A QD laser with a single sheet of such InAs/InGaAs/GaAs QDs exhibits threshold current densities as low as 12.7 and 181 A/cm 2 at 100 and 300 K, respectively. Lasers with threefold stacked QDs show ground-state lasing and allow for cw operation at room temperature.

We have solved the rate equations for InGaAs/GaAs self-assembled quantum-dot laser considering homogeneous and inhomogeneous broadening of optical gain numerically using fourth-order Runge-Kutta method. Dynamic-characteristics are analyzed; relaxation oscillation frequency, modulation bandwidth, and turn-on delay improve as injection current increases. With enhancing of the full width at half maximum (FWHM) of homogeneous broadening, threshold current and turn-on delay increase, because of elevating of central group density of states. Simulation results of static-characteristics show that slope-efficiency increases as the FWHM of homogeneous broadening heightens due to enhancing of central group carriers. Exceeding of the FWHM of homogeneous broadening from FWHM of inhomogeneous broadening results in degradation of slope-efficiency and static-characteristics because empty DOS increas. Nonlinearity appears in light-current characteristics at a special interval from ratio of FWHM of homogeneous broadening to FWHM of inhomogeneous broadening. Differential gain decreases as initial relaxation time and recombination times heighten. Consequently, relaxation oscillation frequency and modulation bandwidth decline.

InAs quantum dash ͑QDH͒ and quantum dot ͑QD͒ lasers grown by molecular beam epitaxy on InP substrate are studied. The laser active zones with multiple stacked layers exhibit lasing wavelength at 1.55 m. On these devices, the experimental threshold current density reaches its minimum value for a double stacked QDH/QD structure. Other basic laser properties such as gain and quantum efficiency are compared. QD lasers exhibit better threshold current densities but equivalent modal gain per layer than QDH. Finally, the analysis of the modal gain on QD lasers shows a promising potential for improvement.

The Bose-Einstein condensation (BEC) temperature $T_{c}$ of Cooper pairs (CPs) created from a very general interfermion interaction is determined for a {\it linear}, as well as the usual quadratic, energy {\it vs}% center-of-mass momentum dispersion relation. This $T_{c}$ is then compared to that of Wen & Kan (1988) in $d=2+\epsilon $ dimensions, for small $\epsilon $, in a geometry of

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We studied nonradiative recombination centers in MBE-grown InAs/GaAs quantum dot (QD) structures with photoluminescence (PL) peak energies between 1.12 and 1.29 eV by the scheme of two-wavelength excitation. Temporally chopped below-gap excitation (BGE) light of 0.75 eV was superposed on a CW above-gap excitation light of 1.59 eV on the sample surface, and the resultant PL intensity change due to the BGE (BGE effect) was measured. Observed 25-35% decrease in PL intensity at 80-90 K implies a first discrimination of a pair of nonradiative centers activated at 0.75 eV inside the band gap of the QD region. Due to trap-filling, the BGE effect showed saturation with increasing BGE power. Its temperature dependence below 50 K was different from that of quantum wells, reflecting carrier dynamics peculiar to the QD. r

Transmission electron microscopy (TEM), and photoluminescence (PL) have been used to evaluate defects and the efficiency of defect-reduction techniques in structures with InAs quantum dots (QDs) for the 1.55 µm range grown at low substrate temperature (LT) using molecular beam epitaxy (MBE). We show that capping of the QDs with thin GaAs layer accompanied by growth interruption at 600 o C (flash) allows to eliminate large islands, containing dislocations, while the smaller islands containing local defects (e.g. dislocation dipoles) still remain. If the flash procedure is accompanied with further depositing of thin AlAs cap layer, and followed by high temperature (~700 o C) annealing (HTA), an almost complete elimination of defects is observed. The structures emit in the range of 1.55 µm due to lateral agglomerates of LTQDs. Simultaneously bright luminescence due to isolated QDs and GaAs matrix are detected at high excitation densities.

A detailed study of the carrier transfer and photoluminescence ͑PL͒ quenching in stacked InAs/ GaAs quantum dots ͑QDs͒ is presented. Vertically aligned QD structures, grown by atomic layer molecular beam epitaxy, with different numbers N of dot planes and different spacer thicknesses (d) were prepared and studied. The dependencies of carrier transfer from the GaAs barriers to the InAs QDs and of the PL quenching channels on the design parameters N and d have been identified by performing continuous wave and time-resolved PL measurements. We have found that both the radiative recombination and capture efficiency into the QDs are reduced by increasing N and by decreasing d, as a consequence of the deterioration of the interdot GaAs spacers induced by stacking.

InAs quantum dot lasers grown on (311)B InP substrates with AlGaInAs barriers have been fabricated and studied. A large decrease of the threshold current with temperature was observed from 110 to 140 K. In the same temperature range, electroluminescence spectra showed a shape change, an energy shift with temperature, which can not be fitted with a Varshni law, and a large decrease of the laser linewidth. These results can be related to a delayed thermalization of carriers within quantum dot ensemble.