N. Yu. Gordeev

Low-threshold injection lasers based on vertically coupled quantum dots

We have fabricated and studied injection lasers based on vertically coupled quantum dots (VECODs). VECODs are self-organized during successive deposition of several sheets of (In,Ga)As quantum dots separated by thin GaAs spacers. VECODs are introduced in the active region of a GaAs-A1GaAs GRIN SCH lasers. Increasing the number of periods (N) in the VECOD leads to a remarkable decrease in threshold current density ( ~ 100 A/cm 2 at 300 K for N = 10). Lasing proceeds via the ground state of the quantum dots (QD) up to room temperature. Placing the QD array into an external AIGaAs--GaAs quantum well allows us to extend the range of thermal stability of threshold current density (To = 350 K) up to room temperature. Using (In,Ga)As-(A1,Ga)As VECODs in combination with high temperature growth of emitter and waveguide layers results in further reduction of threshold current density (60-80 A/cm 2, 300 K) and increase in internal quantum efficiency (70%). Room temperature continuous wave operation (light output 160 mW per mirror) and lasing via the states of QDs up to I = (6-7) Ith have been demonstrated.

High-power single mode (>1W) continuous wave operation of longitudinal photonic band crystal lasers with a narrow vertical beam divergence

We report on 980nm InGaAs∕AlGaAs lasers with a broad waveguide based on a longitudinal photonic band crystal concept. The beam divergence measured as full width at half maximum was as narrow as 15W pulsed operation as limited by the current source. Significantly increased modal spot size enabled stable single lateral mode operation in broad ridge 10μm stripes. Maximum continuous wave power in single mode regime of 1.3W for 10μm wide stripe lasers was obtained, being limited by the catastrophic degradation of the unpassivated laser facets.

Time-resolved photoluminescence measurements of InAs self-assembled quantum dots grown on misorientated substrates

Time-resolved photoluminescence decay measurements have been performed on samples with varying-sized self-assembled InAs/GaAs quantum dot ensembles, formed by substrate misorientation. Ground-state radiative recombination lifetimes from 0.8 to 5.3 ns in the incident power density range of 0.05–3400 W cm−2 at a temperature of 77 K have been obtained. It was found that a reduction of the quantum dot size led to a corresponding reduction of the radiative lifetime. The evident biexponential decay was obtained for the ground state emission of the quantum dot array, with the slower second component attributed to a carrier recapturing process.

A 1.33 µm InAs/GaAs quantum dot laser with a 46 cm−1 modal gain

We report on 1.33 µm quantum dot (QD) lasers grown on GaAs substrates that show a modal gain of 45 cm−1, low threshold current density of 150 A cm−2 and room-temperature continuous wave output power of 2.5 W. The active region is based on ten InAs/InGaAs/GaAs quantum dot layers formed by activated phase separation. High structural quality of the active region is achieved, owing to minimization of the total amount of strained material per QD layer. The optical confinement factor is increased by exploiting high Al composition (80%) in the cladding layers. A modal gain over 20 cm−1 in the 1315–1345 nm wavelength range is revealed by the Hakki–Paoli technique at a low current density of 500 A cm−2.

Effect of p-doping of the active region on the temperature stability of InAs/GaAs QD lasers

A detailed study of the effect of p-doping of the active region on characteristics of long-wavelength InAs/GaAs QD lasers is performed. As the doping level increases, the characteristic temperature rises and the range of temperature stability for the threshold current density is broadened. In a laser doped with 2 × 1012 cm−2 acceptors per QD sheet, the characteristic temperature of 1200 K is obtained in the temperature range 15–75°C and the differential quantum efficiency is stable in the range 15–65°C. A maximum CW output power of 4.4 W is reached in an optimized structure.

Single mode cw operation of 658nm AlGaInP lasers based on longitudinal photonic band gap crystal

GaInP–AlGaInP lasers with broad waveguide based on a longitudinal photonic band gap crystal have been studied. Lasers with 10μm stripe width exhibit single transverse mode operation. The vertical beam divergence is about 8° and is insensitive to the drive current. The aspect ratio is ∼2:1. The quality factor for the lateral beam M2 is less than 2 in single mode regime under pulsed excitation. The total maximum continuous wave output power in the single mode regime at 20°C is more than 115mW (for high reflection/antireflection facet coatings), indicating a dramatic reduction in the catastrophic optical mirror damage problem.

High-Power Low-Beam Divergence Edge-Emitting Semiconductor Lasers with 1- and 2-D Photonic Bandgap Crystal Waveguide

We report on edge-emitting lasers based on the 1- and 2-D longitudinal photonic bandgap crystal concept. The longitudinal photonic bandgap crystal (PBC) design allows a robust and controllable extension of the fundamental mode over a thick multilayer waveguide to obtain a very large vertical mode spot size and a narrow vertical beam divergence.

Metamorphic 1.5 µm-range quantum dot lasers on a GaAs substrate

1.5 µm-range laser diodes based on InAs/InGaAs quantum dots (QDs) grown on metamorphic (In, Ga, Al)As layers, which were previously deposited on GaAs substrates using a defect reduction technique (DRT), are studied. More than 7 W total output power operation in the pulsed mode is shown in broad area lasers. It is shown that the narrow stripe lasers operate in the continuous wave (CW) and the single transverse mode at current densities up to 22 kA cm−2 without significant degradation. CW output power in excess of 220 mW at 10 °C heat sink temperature is demonstrated. 800 mW single-mode output power in the pulsed regime is obtained. It is also shown that the lasers demonstrate the absence of beam filamentation up to the highest current densities studied. First studies on the dynamics of the lasers show a modulation bandwidth of ~3 GHz, limited by device heating. Eye diagrams at 2.5 Gbit s−1 and room temperature (RT) have been performed. Aging tests demonstrate >800 h of CW operation at ~50 mW at 10 °C heat sink temperature and >200 h at 20 °C heat sink temperature without decrease in optical output power. The results indicate the high potential of metamorphic growth using the DRT for practical applications, such as 1500 nm GaAs vertical cavity surface emitting lasers (VCSELs).

Degradation-robust single mode continuous wave operation of 1.46μm metamorphic quantum dot lasers on GaAs substrate

Narrow ridge lasers of 1.5μm range based on InAs∕InGaAs quantum dots grown on metamorphic (In,Ga,Al)As layers deposited on GaAs substrates using defect reduction technique are studied. It is shown that the lasers operate continuous wave (cw) in a single transverse mode. Single-mode 800mW output power in the pulsed regime is obtained for a 6μm ridge width. The dynamic studies of the lasers show a modulation bandwidth of ∼3GHz. Aging tests demonstrate >800h of cw operation at ∼50mW at 10°C (60°C) and >200h at 20°C (70°C) heat sink (junction) temperature without noticeable degradation.

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.