Gain in injection lasers based on self-organized quantum dots

The analytical form of the dependence of the gain on pump current density for lasers with an active region based on self-organized quantum dots is derived in a simple theoretical model. The proposed model is shown to faithfully describe experimental data obtained for laser diodes based on InGaAs quantum dots in an AlGaAs/GaAs matrix, as well as InAs quantum dots in an InGaAs/InP matrix. The previously observed gain saturation and switching of the lasing from the ground state to an excited state of the quantum dots are studied. The influence of the density of quantum-dot arrays on the threshold characteristics of lasers based on them is examined on the basis of this model. Fiz. Tekh. Poluprovodn. 33, 215–223 (February 1999)     

Lateral association of vertically coupled quantum dots

The modification produced in the structural and optical properties of vertically coupled In_0.5Ga_0.5As quantum dots in a GaAs matrix by increasing the number of deposited layers of quantum dots has been investigated. It was shown that the deposition of a sequence of In_0.5Ga_0.5As quantum-dot planes separated by narrow (of the order of the height of the quantum dots) GaAs layers gives rise to an interaction between neighboring vertically coupled quantum dots. This interaction shifts the photoluminescence line due to the recombination of nonequilibrium carriers via states of the quantum dots into the region of lower photon energies. Fiz. Tekh. Poluprovodn. 31, 851–854 (July 1997)     

Wide range tunable laterally coupled distributed-feedback lasers based on InGaAs-GaAs quantum dots

The authors have investigated tunable distributed feedback (DFB) lasers based on InGaAs quantum dots grown by molecular beam epitaxy. Two-section tunable DFB lasers were fabricated by patterning laterally gain coupling binary superimposed gratings perpendicular to the ridge waveguide. Side-mode suppression ratios of up to 40 dB have been achieved. The tuning range covers 30 nm.     

Stark effect in vertically coupled quantum dots in InAs-GaAs heterostructures

The results of studies of hole energy states in vertically coupled quantum dots in InAs-GaAs p-n heterostructures by deep-level transient spectroscopy are reported. Spectra were recorded at different reverse-bias voltages. Levels related to bonding and antibonding s and p states of vertically coupled quantum dots were revealed. The energies of these states significantly depend on an external electric field applied to a heterostructure. This dependence was attributed to the quantum-dimensional Stark effect for the hole states of vertically coupled quantum dots. In addition to this, it was found that the energy of thermal activation of carriers from vertically coupled quantum dots depends on the conditions of isochronous annealing that was carried out both with the reverse bias switched-on and switched-off and both in the presence and absence of illumination. These changes, as in the case of isolated quantum dots, are typical of a bistable electrostatic dipole formed by carriers, localized in a coupled quantum dot, and ionized lattice point defects. The built-in electric field of this dipole reduces the energy barrier for the carriers in the coupled quantum dot. The investigated structures with vertically coupled quantum dots were grown using molecular-beam epitaxy taking account of self-assembling effects.     

Engineering laser gain spectrum using electronic vertically coupled InAs-GaAs quantum dots

Continuous large-broad laser gain spectra near 1.3 /spl mu/m are obtained using an active region of electronic vertically coupled (EVC) InAs-GaAs quantum dots (QDs). A wide continuous electroluminescence spectrum, unlike that from conventional uncoupled InAs QD lasers, was obtained around 230 nm (below threshold) with a narrow lasing spectrum. An internal differential quantum efficiency as high as 90%, a maximum measured external differential efficiency of 73% for a stripe-length of L=1 mm, and a threshold current density for zero total optical loss as low as 7 A/cm/sup 2/ per QD layer were achieved.     

Vertical-cavity surface-emitting lasers based on submonolayer InGaAs quantum dots

Molecular beam epitaxy-grown 0.98-/spl mu/m vertical-cavity surface-emitting lasers (VCSELs) with a three-stack submonolayer (SML) InGaAs quantum-dot (QD) active region and fully doped Al/sub x/Ga/sub 1-x/As-GaAs DBRs was studied. Large-aperture VCSELs demonstrated internal optical losses less than 0.1% per one pass. Single-mode operation throughout the whole current range was observed for SML QD VCSELs with the tapered oxide apertures diameter less than 2 /spl mu/m. Devices with 3-/spl mu/m tapered-aperture showed high single-mode output power of 4 mW and external quantum efficiency of 68% at room temperature.     

Device characteristics of long-wavelength lasers based on self-organized quantum dots

The current state of the field of semiconductor lasers operating in the spectral range near 1.3 ??m and with an active region represented by an array of self-organized quantum dots is reviewed. The threshold and temperature characteristics of such lasers are considered; the problems of overcoming the gain saturation and of an increase in both the differential efficiency and emitted power are discussed. Data on the response speed under conditions of direct modulation and on the characteristics of lasers operating with mode synchronization are generalized. Nonlinear gain saturation, the factor of spectral line broadening, and the formation of broad gain and lasing spectra are discussed.     

Optically pumped intersublevel MidInfrared lasers based on InAs-GaAs quantum dots

We propose an optically pumped laser based on intersublevel transitions in InAs-GaAs pyramidal self-assembled quantum dots. A theoretical rate equations model of the laser is given in order to predict the dependence of the gain on pumping flux and temperature. The energy levels and wave functions were calculated using the 8-band k/spl middot/p method where the symmetry of the pyramid was exploited to reduce the computational complexity. Carrier dynamics in the laser were modeled by taking both electron-longitudinal optical phonon and electron-longitudinal acoustic phonon interactions into account. The proposed laser emits at 14.6 /spl mu/m with a gain of g/spl ap/ 570 cm/sup -1/ at the pumping flux /spl Phi/=10/sup 24/ cm/sup -2/ s/sup -1/ and a temperature of T=77 K. By varying the size of the investigated dots, laser emission in the spectral range 13-21 /spl mu/m is predicted. In comparison to optically pumped lasers based on quantum wells, an advantage of the proposed type of laser is a lower pumping flux, due to the longer carrier lifetime in quantum dots, and also that both surface and edge emission are possible. The appropriate waveguide and cavity designs are presented, and by comparing the calculated values of the gain with the estimated losses, lasing is predicted even at room temperature for all the quantum dots investigated.     

Injection lasers based on InGaAs quantum dots in an AlGaAs matrix

Arrays of vertically coupled InGaAs quantum dots (QDs) in an AlGaAs matrix have been used in injection lasers. Increase in the band gap of a matrix material by replacement of a GaAs matrix with an AlGaAs one led to dramatic increase in quantum dot localization energy. By using this approach, we reduced the thermal population of the matrix and wetting layer states and thus decreased room temperature threshold current density to 63 A/cm², increased differential efficiency up to 65%, and achieved room temperature continuous wave operation with output power of 1 W. Negative characteristic temperature has been observed in temperature dependence of threshold current density of these lasers in some temperature range. A qualitative explanation assuming a transition from non-equilibrium to Fermi population of QD states is proposed.     

Deep-level transient spectroscopy in InAs/GaAs laser structures with vertically coupled quantum dots

Indium arsenide/gallium arsenide structures with vertically coupled quantum dots imbedded in the active zone of a laser diode are investigated by deep-level transient spectroscopy (DLTS), and the capacitance-voltage characteristics are analyzed. The DLTS spectrum was found to undergo significant changes, depending on the temperature of preliminary isochronous annealing of the sample, T _a <T _ac =245 K or T _a >T _ac , and on the cooling conditions, with a bias voltage V _b =0 or with an applied carrier pulse V _f >0. The changes are attributed to the onset of Coulomb interaction of carriers trapped in a quantum dot with point defects localized in the nearest neighborhoods of the quantum dots and also to the formation of a dipole when T _a <T _ac and cooling takes place with V _f >0, or to the absence of a dipole when T _a >T _ac and V _b =0. It is discovered that the tunneling of carriers from the deeper states of defects to the shallower states of quantum dots takes place in the dipole, and the carriers are subsequently emitted from the dots into bands. Fiz. Tekh. Poluprovodn. 31, 1249–1255 (October 1997)     

Lateral waveguiding in stripe-geometry double-heterostructure lasers below the lasing threshold

Structure in the far-field pattern of current-confined stripe-geometry double-heterostructure lasers in the angle rangepm 20degis observed at current levels of less thanfrac{1}{4}of threshold value. The behavior is explained by leaky-mode guiding due to a gain maximum under the stripe and to a negative refractive index step on the order of -0.01.     

1.3-μm CW lasing of InGaAs-GaAs quantum dots at room temperaturewith a threshold current of 8 mA

We demonstrate the first 1.3-μm continuous-wave (CW) lasing at room temperature of self-assembled InGaAs-GaAs quantum dots. High-density 1.3-μm emission dots were successfully formed by the combination of low-rate growth and InGaAs-layer overgrowth methods of molecular beam epitaxy. The 1.3-μm ground-level CW lasing occurred at up to 40°C, and the threshold current of 8 mA at 25°C is less than one thirtieth of values ever reported for 1.3-μm dot pulse lasers. The achievement represents a milestone for creating quantum-dot lasers applicable to fiber-optic communication system     

Injection heterolaser based on an array of vertically aligned InGaAs quantum dots in a AlGaAs matrix

Arrays of vertically aligned InGaAs quantum dots in a AlGaAs matrix have been investigated. It is shown that increasing the band gap of the matrix material makes it possible to increase the localization energy of quantum dots relative to the edge of the matrix band, as well as the states of the wetting layer. The use of an injection laser as the active region makes it possible to decrease the thermal filling of higher-lying states, and thereby decrease the threshold current density to 63 A/cm² at room temperature. A model explaining the negative characteristic temperature section observed at low temperatures is proposed. The model is based on the assumption that a transition occurs from nonequilibrium to equilibrium filling of the states of the quantum dots. Fiz. Tekh. Poluprovodn. 31, 483–487 (April 1997)     

High-temperature lasing in a microring laser with an active region based on InAs/InGaAs quantum dots

Lasing at a wavelength of >1.3 ??m has been achieved at temperatures of up to 380 K in a ring microlaser (diameter of 6 ??m) with an active region based on InAs/InGaAs quantum dots.     

Features of simultaneous ground- and excited-state lasing in quantum dot lasers

The lasing spectra and light-current (L-I) characteristics of an InAs/InGaAs quantum dot laser emitting in the simultaneous lasing mode at the ground- and excited-state optical transitions are studied. Lasing and spontaneous emission spectra are compared. It is shown that ground-state quenching of lasing is observed even in the absence of active region self-heating or an increase in homogeneous broadening with growth in the current density. It is found that the intensities of both lasing and spontaneous emission at the ground-state transition begin to decrease at a pump intensity that significantly exceeds the two-level lasing threshold. It is also found that different groups of quantum dots are involved in ground- and excited-state lasing.     

Thermal stability of vertically coupled InAs-GaAs quantum dot arrays

The effect of high-temperature annealing on the optical properties of vertically coupled InAs quantum dots in a GaAs matrix and on the performance of a quantum-dot laser are studied. A strong blue shift of the photoluminescence peak and lasing line, as well as changes in the photoluminescence intensity and temperature dependence of the threshold current density are observed. The reason for this behavior is probably a reduction in the carrier localization energy due to a partial mixing of the In and Ga atoms as well as an improvement in the quality of the low-temperature grown Ga(Al)As layers achieved by high-temperature annealing. Fiz. Tekh. Poluprovodn. 31, 104–108 (January 1997)     

Analysis of the emission efficiency of powerful semiconductor lasers under closed-mode threshold lasing conditions

A new model describing the decrease in the emission efficiency and optical output power of a semiconductor laser above the lasing threshold of the Fabry-Perot mode is suggested. The mechanism of deterioration of the output-power characteristics is described in the suggested model in terms of the achievement of closed-mode threshold conditions. Rate equations are used to analyze how the closed-mode threshold conditions are satisfied in semiconductor lasers.     

Low-threshold 1.3-µm injection lasers based on single InGaAsN quantum wells

Semiconductors - The design of the active region in InGaAsN quantum well (QW) injection lasers is investigated. Long-wavelength (1.27–1.3 mm), low-threshold (&;lt;400 A/cm2), and...     

Observation of lasing from vertically self-organized InAs three-dimensional island quantum boxes on GaAs (001)

We report the first observation of lasing from vertically self-organized multiple stacks of electronically uncoupled InAs three-dimensional island quantum boxes grown via molecular beam epitaxy on GaAs (001) substrates. A low-threshold current density of 310 A/cm/sup 2/ at 79 K is found for a stack of five sets of islands corresponding to 2 ML InAs depositions separated by 36 monolayer GaAs spacers grown via migration enhanced epitaxy. The distribution of the island volumes (1.5/spl times/10/sup 5/ /spl Aring//sup 3/-4 /spl times/10/sup 5/ /spl Aring//sup 3/) gives, as expected, a multitude of laser lines between 980 mm and 996 nm.