Mitsuru Sugawara

Recent progress in self-assembled quantum-dot optical devices for optical telecommunication: temperature-insensitive 10?Gb?s?1 directly modulated lasers and 40?Gb?s?1 signal-regenerative amplifiers

This paper reviews the recent progress of self-assembled quantum-dot optical devices, highlighting temperature-insensitive 10 Gb/s directly modulated lasers at 1.3 mum and 40 Gb/s signal-regenerative amplifiers in the 1.5 mu band

Temperature-Insensitive Eye-Opening under 10-Gb/s Modulation of 1.3-µm P-Doped Quantum-Dot Lasers without Current Adjustments

We demonstrate temperature-insensitive eye-opening under 10-Gb/s direct modulation of 1.3-µm p-doped quantum-dot lasers without using any current adjustments. The lasers show a 6.5-dB extinction ratio between 20°C and 70°C. An active region consisting of ten quantum-dot layers with p-type doping enabled this highly temperature-stable dynamic performance, which was much superior to conventional 1.3-µm quantum-well lasers. These results make it possible to use uncooled 1.3-µm quantum-dot lasers without any current adjustments.

Modeling room-temperature lasing spectra of 1.3-μm self-assembled InAs∕GaAs quantum-dot lasers: Homogeneous broadening of optical gain under current injection

We studied the injection current dependence of room-temperature lasing spectra of a 1.3-μm self-assembled InAs∕GaAs quantum-dot laser both experimentally and theoretically. Starting from the ground-state lasing with a few longitudinal modes, the spectra showed splitting, broadening, excited-state lasing, and quenching of the ground-state lasing as the current increased. We could explain this unique current dependence by numerical simulation based on our quantum-dot laser theory, taking into account the inhomogeneous and homogeneous broadening of the optical gain as well as the carrier relaxation processes in the spatially isolated quantum dots. Through the simulation, we found that the homogeneous broadening of the ground state is kept between 5 and 10 meV under the ground-state lasing, while it increases up to 20 meV under the excited-state lasing.

Photon lifetime dependence of modulation efficiency and K factor in 1.3μm self-assembled InAs∕GaAs quantum-dot lasers: Impact of capture time and maximum modal gain on modulation bandwidth

We studied small-signal modulation characteristics of 1.3μm InAs∕GaAs self-assembled quantum-dot lasers in terms of the modulation efficiency and the K factor as a function of the photon lifetime. We could explain the measured photon-lifetime dependence based on the rate equation model considering explicitly the carrier-capture process and Pauli blocking in quantum dots. Our model shows how the modulation bandwidth of quantum-dot lasers is limited by the carrier-capture time and by the maximum modal gain via the K factor. We present prerequisite designs of quantum-dot active regions for over 10GHz modulation.

1.28μm lasing from stacked InAs∕GaAs quantum dots with low-temperature-grown AlGaAs cladding layer by metalorganic chemical vapor deposition

We report the device characteristics of stacked InAs∕GaAs quantum-dot lasers cladded by Al0.4Ga0.6As layer grown at a low temperature by metalorganic chemical vapor deposition. A blueshift in emission energy by the effect of postgrowth annealing can be suppressed when the annealing temperature is below 570°C. We achieved the 1.28μm continuous-wave lasing at room temperature of five layer stacked InAs∕GaAs quantum dots embedded in In0.13Ga0.87As strain-reducing layer whose p-cladding layer is grown at 560°C. From the experiments and calculations of the gain spectra of fabricated quantum-dot lasers, the observed lasing originates from the first excited state of stacked InAs quantum dots.

InAs∕GaAs self-assembled quantum-dot lasers grown by metalorganic chemical vapor deposition—Effects of postgrowth annealing on stacked InAs quantum dots

We investigated the effects of postgrowth annealing on stacked InAs∕GaAs quantum dots. The blueshift in emission energy by postgrowth annealing depends on the temperature of postgrowth annealing and the growth conditions of stacked InAs quantum dots, such as a spacer thickness or a stacking number. We can control the peak wavelength of stacked InAs quantum dots by changing the temperature of postgrowth annealing and the growth conditions of stacked InAs quantum dots. We achieved continuous-wave lasing with a threshold current of 16.4mA at the wavelength of 1.245μm from five layer vertically aligned InAs quantum dots whose upper cladding layer was grown at 600°C.

Control of optical polarization anisotropy in edge emitting luminescence of InAs/GaAs self-assembled quantum dots

We have studied the polarization properties of cleaved-edge photoluminescence (PL) from InAs/GaAs self-assembled quantum dots. Transverse-electric (TE) and transverse-magnetic (TM) mode PL intensities have been analyzed for the dots having 8 nm InxGa1−xAs capping layer with indium (In) composition of x=0 and 0.13. Polarization results show a dramatic change with the capping layer In compositions; TE-mode dominant PL is observed for dots with x=0, on the other hand, TM-mode dominant PL for dots with x=0.13. This polarization change has been attributed to the dot shape change using transmission electron microscopy images. These results suggest that the optical polarization anisotropy of the quantum dots can be controlled by manipulating the capping layer In composition.

Fabrication of InAs quantum dots on InP(100) by metalorganic vapor-phase epitaxy for 1.55 μm optical device applications

A change in the density and wavelength of InAs quantum dots (QDs) on InGaAsP/InP(100) substrate grown by metalorganic vapor-phase epitaxy (MOVPE) in accordance with variation in the growth conditions was studied, aiming at optical device applications in the 1.55 μm region. In the moderate V/III ratio region, the size of QDs was found to decrease while the density increased as the group-V source was reduced, but on the other hand, both of them increased monotonously with increasing InAs supply. The combination of changing the V/III ratio and InAs supply allowed us to control the density and wavelength of QDs independently so that QDs with a density as high as 5.6×1010 and a 1.6 μm emission were obtained. The letter reports the MOVPE growth technique of QDs on InGaAsP/InP(100), which connects QDs with mature 1.55 μm device technology.

Extended wavelength emission to 1.3μm in nitrided InAs∕GaAs self-assembled quantum dots

We have studied long-wavelength emission in 1.3-μm optical communication range from nitrided InAs∕GaAs quantum dots (QDs). Atomic-layer nitridation just after the growth of InAs QDs realizes a remarkable redshift of the photoluminescence peak by more than 150nm. Growth temperature plays a key role in achieving room-temperature emission from the QDs. The emission wavelength can be adjusted by controlling the growth conditions of the initial InAs QDs. As compared with conventional QDs grown without nitridizing the QD surface, it is found that the nitridation results in larger dots with a large aspect ratio.

Internal Strain of Self-Assembled InxGa1-xAs Quantum Dots Calculated to Realize Transverse-Magnetic-Mode-Sensitive Interband Optical Transition at Wavelengths of 1.5 µm bands

We calculated the interband optical transition energies in self-assembled InxGa1-xAs quantum dots on InP as functions of the strain, composition, and height of dots on the basis of the kp perturbation theory. We found that a transverse-magnetic (TM)-mode-sensitive optical transition at wavelengths in the range from 1.5 to 1.6 µm is achieved due to the light-hole valence band when the crystal lattice is expanded in the same direction of growth as the in-plane lattice matched to the substrate at x≤0.4, or when the lattice is compressed in the growth direction with the in-plane lattice relaxed at x≥0.6. We propose barrier structures covering the dots in order to realize these conditions, leading to TM-mode-sensitive quantum dots for polarization-independent optical amplifiers.