Showing papers on "Quantum dot laser published in 1999"

Quantum dot heterostructures

Abstract: Fabrication Techniques for Quantum Dots. Self-Organization Concepts on Crystal Surfaces. Growth and Structural Characterization of Self-Organized Quantum Dots. Modeling of Ideal and Real Quantum Dots. Electronic and Optical Properties. Electrical Properties. Photonic Devices. References. Index.

A narrow photoluminescence linewidth of 21 meV at 1.35 μm from strain-reduced InAs quantum dots covered by In0.2Ga0.8As grown on GaAs substrates

Abstract: InAs quantum dots with size fluctuations of less than 4% were grown on GaAs using the self-assembling method. By covering the quantum dots with In0.2Ga0.8As or In0.2Al0.8As, strain in InAs dots can be partly reduced due to relaxation of lattice constraint in the growth direction. This results in low-energy emission (about 1.3 μm) from the quantum dots. The photoluminescence linewidth can be reduced to 21 meV at room temperature. This width is completely comparable to the theoretical limit of a band-to-band emission from a quantum well at room temperature. Because the dots can be uniformly covered by the strain reducing layers, factors that degrade size uniformity during coverage, such as compositional mixing or segregation, will be suppressed, allowing for an almost ideal buried quantum dot structure.

Extremely low room-temperature threshold current density diode lasers using InAs dots in In0.15Ga0.85As quantum well

Abstract: The lowest room-temperature threshold current density, 26 A/cm/sup 2/, of any semiconductor diode lasers is reported for a quantum dot device with a single InAs dot layer contained within a strained In/sub 0.15/Ga/sub 0.85/As quantum well. The lasers are epitaxially grown on a GaAs substrate, and the emission wavelength is 1.25 /spl mu/m.

Origin of Luminescence from InGaN Diodes

Abstract: We report the first direct observation of phase decomposition in a luminescent alloy and show that this decomposition, allied to quantum confinement enhancements, accounts for the surprisingly high efficiency of InGaN-based diodes manufactured by Nichia Chemical Industries. Hence nanostructure, rather than composition, is responsible for the success of these devices. A common nanostructure, in the form of nearly pure InN quantum dots, occurs across a large range of average indium content in InGaN and leads to a universal scalability of the optical spectra.

InAs/InGaAs quantum dot structures on GaAs substrates emitting at 1.3 μm

Abstract: InAs self-organized quantum dots inserted in InGaAs quantum well have been grown on GaAs substrates by molecular beam epitaxy. The lateral size of the InAs islands has been found to be approximately 1.5 times larger as compared to the InAs/GaAs case, whereas the island heights and surface densities were close in both cases. The quantum dot emission wavelength can be controllably changed from 1.1 to 1.3 μm by varying the composition of the InGaAs quantum well matrix. Photoluminescence at 1.33 μm from vertical optical microcavities containing the InAs/InGaAs quantum dot array was demonstrated.

Gain and linewidth enhancement factor in InAs quantum-dot laser diodes

Abstract: Amplified spontaneous emission measurements are investigated below threshold in InAs quantum-dot lasers emitting at 1.22 /spl mu/m. The dot layer of the laser was grown in a strained quantum well (QW) on a GaAs substrate. Ground state gain is determined from cavity mode Fabry-Perot modulation. As the injection current increases, the gain rises super-linearly while changes in the index of refraction decrease. Below the onset of gain saturation, the linewidth enhancement factor is as small as 0.1, which is significantly lower than that reported for QW lasers.

Bound-to-continuum intersubband photoconductivity of self-assembled InAs quantum dots in modulation-doped heterostructures

Abstract: We have designed and fabricated a quantum dot infrared photodetector which utilizes the lateral transport of photoexcited carriers in the modulation-doped AlGaAs/GaAs two-dimensional (2D) channels. A broad photocurrent signal has been observed in the photon energy range of 100–300 meV due to the bound-to-continuum intersubband absorption of normal incidence radiation in the self-assembled InAs quantum dots. A peak responsivity was as high as 4.7 A/W. The high responsivity is realized mainly by a high mobility and a long lifetime of photoexcited carriers in the modulation-doped 2D channels. Furthermore, it is found that the observed photosensitivity survives up to 190 K.

Optical characteristics of 1.24-μm InAs quantum-dot laser diodes

Abstract: The optical characteristics of the first laser diodes fabricated from a single-InAs quantum-dot layer placed inside a strained InGaAs QW are described. The saturated modal gain for this novel laser active region is found to be 9-10 cm/sup -1/ in the ground state. Room temperature threshold current densities as low as 83 A/cm/sup 2/ for uncoated 1.24-/spl mu/m devices are measured, and operating wavelengths over a 190-nm span are demonstrated.

The present status of quantum dot lasers

Abstract: We review the present status of the rapidly developing field of semiconductor laser diodes based on self-organized quantum dots (QDs). Several milestones have been achieved since the first realization of such a device in 1994: above room-temperature cw operation, the lowest threshold current density of any semiconductor laser diode, high temperature stability, an extended wavelength range on GaAs substrate and high power operation. After a brief introduction we discuss the tremendous advances in epitaxial growth, device performance, and theoretical understanding of QD lasers. Three applications of large commercial interest are discussed in detail: 1300 nm QD lasers on GaAs substrate, QD surface emitting lasers, and high power QD lasers. Finally, we give an outlook on future developments.

Influence of Spectral Diffusion on the Line Shapes of Single CdSe Nanocrystallite Quantum Dots

Abstract: We study the emission line shapes of single CdSe nanocrystallite quantum dots. Single dot line shapes are found to result from rapid spectral shifting of the emission spectrum rather than the intrinsic physics of the quantum dot. A strong dependence of single dot line widths on excitation intensity, wavelength, temperature, and integration time is found and is correlated with the number of times that the quantum dot is excited during the acquisition of a single spectrum. The observed results are consistent with thermally assisted spectral diffusion, activated by the release of excess excitation energy.

Ultrafast dynamics of inter- and intraband transitions in semiconductor nanocrystals : implications for quantum-dot lasers

Abstract: Application of femtosecond transient absorption in the visible and near-IR spectral ranges and time-resolved photoluminescence allows us to separate electron and hole relaxation paths and to map the structure of interband and intraband optical transitions in CdSe and CdS nanocrystals (NC's) with a wide range of surface properties. In contrast to electron relaxation, which is controlled by NC surface passivation, depopulation of hole quantized states is extremely fast (sub-ps-to-ps time scales) in all types samples, independent of NC surface treatment (including NC's overcoated with a ZnS layer). Our results suggest that ultrafast hole dynamics are not due to trapping at localized surface defects such as a vacancy, but rather arise from relaxation into intrinsic NC states or intrinsically unpassivated interface states.

Enhanced Polar Exciton-LO-Phonon Interaction in Quantum Dots

Abstract: Phonon-assisted exciton transitions in self-organized InAs/GaAs quantum dots (QDs) are investigated by photoluminescence (PL) and PL excitation spectroscopy. An unexpectedly enhanced polar exciton-LO-phonon interaction for such strained low-symmetry QDs is found. Calculations in the adiabatic approximation indicate that the particular quantum confinement and piezoelectric effect together account for the enhanced coupling. The results provide new insights into the long-standing problem of the exciton-phonon interaction in zero-dimensional systems.

Lasers without internal heat generation

Abstract: A new approach to the design of optically pumped solid-state lasers is proposed which permits lasing without detrimental heating of the laser medium. Very high average power lasers should be possible by balancing the radiated and absorbed power density at each point within the laser medium.

Phonon-assisted capture and intradot Auger relaxation in quantum dots

Abstract: We report on calculations of capture and relaxation of carriers in quantum dots, specifically, InAs/GaAs self-assembled dots. We point out that the phonon-assisted carrier capture presents strong resonances versus the dot size and that the intradot Auger relaxation is extremely fast in these structures. This shows that energy relaxation in InAs/GaAs self-organized quantum dots is dominated by capture effects.

Room-temperature continuous-wave operation of a single-layered 1.3 μm quantum dot laser

Abstract: Room-temperature continuous-wave operation of a 13 μm quantum dot laser is reported The threshold current for a single layer active region with p–up mounting is only 41 mA with a threshold current density of 45 A/cm2 The minimum room temperature threshold current density is 25 A/cm2 for pulsed operation Cryogenic and temperature dependent measurements are performed on broad-area lasers fabricated from the same active material At 4 K the broad-area threshold current density for uncoated facets is 6 A/cm2

Spontaneous Emission of Phonons by Coupled Quantum Dots

Abstract: We find an interference effect for electron-phonon interactions in coupled semiconductor quantum dots that can dominate the nonlinear transport properties even for temperatures close to zero. The intradot electron tunneling process leads to a ``shake up'' of the phonon system and is dominated by a double-slit-like interference effect of spontaneously emitted phonons. The effect is closely related to subradiance of photons (Dicke effect) in a laser-trapped two-ion system and explains the oscillations in the nonlinear current-voltage characteristics of coupled dots observed recently.

Low-loss Al-free waveguides for unipolar semiconductor lasers

Abstract: A promising waveguide design for midinfrared (λ=5–20 μm) unipolar semiconductor lasers is proposed and demonstrated in (Al)GaAs quantum cascade structures. In the latter, the active region is embedded between two GaAs layers, with an appropriate doping profile which allows optical confinement, with low waveguide losses and optimal heat dissipation. Low internal cavity losses of 20 cm−1 have been measured using different techniques for lasers with emission wavelength at ∼9 μm. At 77 K, these devices have peak output power in excess of 550 mW and threshold current of 4.7 kA/cm2.

Quantum computation with quantum dots and terahertz cavity quantum electrodynamics

Abstract: A quantum computer is proposed in which information is stored in the two lowest electronic states of doped quantum dots. Multiple quantum dots are located in a microcavity, and a pair of gates controls the energy levels in each quantum dot. A controlled NOT (CNOT) operations involving any pair of quantum dots can be effected by a sequence of gate voltage pulses which tune the quantum dot energy levels into resonance with frequencies of the cavity or a laser. The duration of a CNOT operation is estimated to be much shorter than the time for an electron to decohere by emitting an acoustic phonon.

1.3 [micro sign]m GaAs-based laser using quantum dots obtained by activated spinodal decomposition

Abstract: Low threshold current density (Jth = 65 A/cm2) operation near 1.3 µm at room temperature (RT) is realised for lasers using InAs/InGaAs/GaAs quantum dots (QDs). The lasing occurs via the QD ground state for cavity length L > 1 mm. The differential efficiency is 40% and internal losses are 1.5 cm–1. The characteristic temperature near RT is 160 K.

Ultralow threshold laser using a single quantum dot and a microsphere cavity

Abstract: We propose a novel semiconductor microlaser, made by capturing the light emitted from a single InAs/GaAs quantum dot in the whispering-gallery mode of a glass microsphere. We demonstrate that such an arrangement allows the laser threshold condition to be satisfied. The corresponding threshold current should be several orders of magnitude lower than is currently possible in semiconductor lasers.

Temperature dependence of lasing characteristics for long-wavelength (1.3-μm) GaAs-based quantum-dot lasers

Abstract: Data are presented on the temperature dependence of 1.3-/spl mu/m wavelength quantum-dot (QD) lasers. A low-threshold current density of 90 A/cm/sup 2/ is achieved at room temperature using high reflectivity coatings. Despite the low-threshold current density, lasing at the higher temperatures is limited by nonradiative recombination with a rapid increase in threshold current occurring above /spl sim/225 K. Our results suggest that very low threshold current density (/spl les/20 A/cm/sup 2/) can be achieved at room temperature from 1.3-/spl mu/m QD lasers, once nonradiative recombination is eliminated.

Dependence of the device performance on the number of stages in quantum-cascade lasers

Abstract: The cascading scheme is a characteristic feature of quantum cascade (QC) lasers. It implies that electrons above threshold generate one photon per active region they successively traverse. This paper presents a study of the cascading behavior as a function of the number N of stacked active regions. Experimental results are presented for devices with N=1, 3, 6, 12, 20, 30, 45, 60, and 75 active stages. The highest optical power and lowest threshold current density are obtained for laser devices with N as high as possible. However, the lowest threshold voltage and the lowest dissipated power at laser threshold are achieved for N=3 and N=22, respectively. We further present the highest power QC lasers so far, which, using N=75 stages, show in pulsed mode peak powers of 1.4, 1.1, and 0.54 W at 50 K, 200 K, and room temperature, respectively. Finally, we also demonstrate the first few-stage (N<10) QC lasers. These QC lasers show strongly reduced operating voltages. A threshold voltage around 1.5 V is achieved for N=3. This makes the lasers very well compliant with conventional laser diode drivers, which in turn will simplify their immediate use in systems and applications.

In(Ga)As/GaAs self-organized quantum dot lasers: DC and small-signal modulation properties

Abstract: Self-organized growth of InGaAs/GaAs strained epitaxial layers gives rise to an ordered array of islands via the Stranski-Krastanow growth mode, for misfits >1.8%. These islands are pyramidal in shape with a base diagonal of /spl sim/20 nm and height of /spl sim/6-7 nm, depending of growth parameters. They therefore exhibit electronic properties of zero-dimensional systems, or quantum dots. One or more layers of such quantum dots can be stacked and vertically coupled to form the gain region of lasers. We have investigated the properties of such single-layer quantum dot (SLQD) and multilayer quantum dot (MLQD) lasers with a variety of measurements, including some at cryogenic temperatures. The experiments have been complemented with theoretical calculations of the electronic properties and carrier scattering phenomena in the dots. Our objective has been to elucidate the intrinsic behavior of these devices. The lasers exhibit temperature independent threshold currents up to 85 K, with T/sub 0//spl les/670 K. Typical threshold currents of 200-/spl mu/m long room temperature lasers vary from 6 to 20 mA. The small-signal modulation bandwidths of ridge waveguide lasers are 5-7.5 GHz at 300 K and increased to >20 GHz at 80 K. These bandwidths agree well with electron capture times of /spl sim/30 ps determined from high-frequency laser impedance measurements at 300 K and relaxation times of /spl sim/8 ps measured at 18 K by differential transmission pump-probe experiments. From the calculated results we believe that electron-hole scattering intrinsically limits the high-speed performance of these devices, in spite of differential gains as high as /spl sim/7/spl times/10/sup -14/ cm/sup 2/ at room temperature.

Light emission spectra of columnar-shaped self-assembled InGaAs/GaAs quantum-dot lasers: Effect of homogeneous broadening of the optical gain on lasing characteristics

Abstract: We examined the current–output power characteristics and light emission spectra for columnar-shaped self-assembled InGaAs quantum-dot lasers with a room temperature lasing threshold of 6 mA. Lasing threshold currents became obscure as temperature decreased below 180 K. While lasing occurred with one line including a series of longitudinal modes at room temperature, spectra at 80 K showed broad lasing emission over a range of 50–60 meV. We conclude that dots with different energies start lasing independently at low temperatures due to their spatial localization, while at room temperature the dots contribute to one-line lasing collectively via homogeneous broadening of optical gain.

Lead-Orientation-Dependent Wave Function Scarring in Open Quantum Dots

Abstract: Experimental studies of the conductance of open quantum dots show a series of highly regular oscillations at low temperatures as the voltage applied to their defining gates is varied. Simulations of quantum transport through these dots reveal the oscillations to be correlated to the recurrence of specific groups of wave function scars. We furthermore find that nominally identical dots, differing only in the orientation of their input and output contacts, may be used to excite different families of scars, giving rise in turn to measurable transport results.

Determination of photoluminescence mechanism in InGaN quantum wells

Abstract: We report on the unambiguous experimental determination of the photoluminescence mechanism in a set of In0.25Ga0.75N quantum wells. Instead of studying the photoluminescence for different In contents, we have investigated it as a function of the quantum well width in combination with a similar study performed on GaN quantum wells. In this way, we show that the photoluminescence is not coming from quantum dots or very localized states in the quantum well, but from the quantum well itself under the influence of a piezoelectric field induced by strain. The previously reported abnormal photoluminescence shifts and temperature dependencies can thus be explained.

Long wavelength superlattice quantum cascade lasers at λ≃17 μm

Abstract: We report the realization of a semiconductor injection laser based on intraband transitions with emission wavelengths extending beyond the atmospheric windows. The structure uses the quantum cascade scheme with “chirped” superlattices as active material. Laser action in pulsed operation is achieved at λ≃17 μm up to 150 K, with peak output powers of ∼12 mW at cryogenic temperatures.

Colloidal CdTe/HgTe quantum dots with high photoluminescence quantum efficiency at room temperature

Abstract: We have used an aqueous colloidal growth technique to form hybrid CdTe/HgTe quantum dots with a broad, strong fluorescence in the infrared (800–1200 nm). The quantum efficiency is high, around 44%, when pumped in the visible (488 nm), and the excited state lifetime is around 130 ns, making the material interesting as an optical amplifier medium. Using a pump-probe experiment, we have demonstrated weak optical amplification in a dilute aqueous suspension of CdTe/HgTe dots in the short wavelength wing of the emission spectrum at 808 nm.