Kohki Mukai

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

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

EMISSION FROM DISCRETE LEVELS IN SELF-FORMED INGAAS/GAAS QUANTUM DOTS BY ELECTRIC CARRIER INJECTION : INFLUENCE OF PHONON BOTTLENECK

We studied the excitation‐power dependence of photoluminescence (PL) spectra and the current dependence of electroluminescence (EL) spectra of self‐formed InGaAs/GaAs quantum dots. We observed five peaks in the PL and EL spectra. From the size dependence of the peak interval and the carrier‐density dependence of peak energy and peak intensity, we assigned the peaks to the discrete energy levels in the quantum dots. We simulated the EL spectra with rate equations, taking into account the carrier relaxation between the discrete levels, and found that the relaxation lifetime was about 10–100 ps.

Lasing at three-dimensionally quantum-confined sublevel of self-organized In/sub 0.5/Ga/sub 0.5/As quantum dots by current injection

A laser oscillation from self-organized In/sub 0.5/Ga/sub 0.5/As quantum dots is achieved at 80 K by current injection. Lasing at a three-dimensionally quantum confined sublevel of the In/sub 0.5/Ga/sub 0.5/As quantum dots is clearly demonstrated for the first time by electroluminescence and diamagnetic energy shift measurement. The results predict the possibility of ultra-low threshold current operation of quantum dot lasers.

EFFECT OF PHONON BOTTLENECK ON QUANTUM-DOT LASER PERFORMANCE

The effect of phonon bottleneck on quantum-dot laser performance is examined by solving the carrier-photon rate equations including the carrier relaxation process into the quantum-dot ground state. We show that the retarded carrier relaxation due to phonon bottleneck degrades the threshold current and the external quantum efficiency. We also show that quantum-dot lasers are quite sensitive to the crystal quality outside as well as inside quantum dots. Our results clarified that the relaxation lifetime should be less than about 10 ps to fully utilize the laser potential originating from the quantum-dot discrete energy states.

Nonlinear gain dynamics in quantum-dot optical amplifiers and its application to optical communication devices

Ultrafast gain dynamics in quantum-dot (QD) optical amplifiers has been studied. It was found that there are at least three nonlinear processes, which are attributed to carrier relaxation to the ground states, phonon scattering, and carrier capture from the wetting layers into the QDs. The relevant time constants were evaluated to be /spl sim/90 fs, /spl sim/260 fs, and /spl sim/3 ps, respectively, under a 50-mA bias condition. The dephasing time was evaluated to be /spl sim/85 fs. The third-order optical susceptibility (/spl chi//sup (3)/) has been evaluated by means of both nonlinear transmission and four-wave mixing experiments. The results show that the nonlinearity expressed by /spl chi//sup (3)//g/sub 0/ is quite similar to that of bulk and quantum wells, which can be explained by similar relaxation times. Applications to optical communication devices are also discussed.

1.3-/spl mu/m CW lasing characteristics of self-assembled InGaAs-GaAs quantum dots

This paper presents the lasing properties and their temperature dependence for 1.3-/spl mu/m semiconductor lasers involving self-assembled InGaAs-GaAs quantum dots as the active region. High-density 1.3-/spl mu/m emission dots were successfully grown by the combination of low-rate growth and InGaAs-layer overgrowth using molecular beam epitaxy. 1.3-/spl mu/m ground-level CW lasing occurring at a low threshold current of 5.4 mA at 25/spl deg/C with a realistic cavity length of 300 /spl mu/m and high-reflectivity coatings on both facets. The internal loss of the lasers was evaluated to be about 1.2 cm/sup -1/ from the inclination of the plots between the external quantum efficiency and the cavity length. The ground-level modal gain per dot layer was evaluated to be 1.0 cm/sup -1/, which closely agreed with the calculation taking into account the dot density, inhomogeneous broadening, and homogeneous broadening. The characteristic temperature of threshold currents T/sub 0/ was found to depend on cavity length and the number of dot layers in the active region of the lasers. A T/sub 0/ of 82 K was obtained near room temperature, and spontaneous emission intensity as a function of injection current indicated that the nonradiative channel degraded the temperature characteristics. A low-temperature study suggested that an infinite T/sub 0/ with a low threshold current (/spl sim/1 mA) is available if the nonradiative recombination process is eliminated. The investigation in this paper asserted that the improvement in surface density and radiative efficiency of quantum dots is a key to the evolution of 1.3-/spl mu/m quantum-dot lasers.

Nonlinear processes responsible for nondegenerate four-wave mixing in quantum-dot optical amplifiers

Wavelength conversion using nondegenerate four-wave mixing in quantum-dot optical amplifiers is investigated. From the detuning frequency dependence of χ(3), derived from the conversion efficiency, we consider that, within the range of detuning in the experiment, spectral-hole burning and carrier heating are responsible, and that their time constants, i.e., carrier relaxation time to the ground state and the phonon scattering time, are in the range of 60–140 and 200–400 fs, respectively. This indicates that the carrier supply to the ground level via relaxation from the higher levels is very fast and that a broad conversion bandwidth comparable to that of quantum-well devices is ensured.

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

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.

Temperature dependent lasing characteristics of multi-stacked quantum dot lasers

Temperature dependence of self-formed quantum dot lasers with a multi-stacked dot layer has been investigated in detail. Lasers oscillating at different subbands exhibit different behaviors against temperature change both in the spectral characteristics and the threshold current. A discontinuous shift of lasing wavelength from the second subband to the ground state is observed with lowering temperature, which is strongly related to emission efficiency of quantum dots and thermal excitation of carriers to higher-order subbands. High characteristic temperature over 300 K has been achieved in a laser with high-reflection coating on both facets in the temperature range 60–200 K.

High photoluminescence efficiency of InGaAs/GaAs quantum dots self-formed by atomic layer epitaxy technique

We investigated the photoluminescence (PL) intensity of InGaAs/GaAs quantum dots self-formed by atomic layer epitaxy (ALE) technique as a function of temperature. We report that the reduction of PL intensity following a temperature increase was two-orders smaller in the ALE-grown dots than in the dots grown by molecular beam epitaxy via Stranski–Krastanov mode. Measuring the radiative lifetimes and evaluating the temperature dependence of PL intensity, we conclude that the nonradiative channel outside the dots caused the PL reduction.