Kenichi Kawaguchi

Demonstration of transverse-magnetic dominant gain in quantum dot semiconductor optical amplifiers

We demonstrated transverse-magnetic (TM)-mode dominated gain at the 1.5μm wavelength in semiconductor optical amplifiers (SOAs) with columnar quantum dots (QDs). We show that we can control the polarization dependence of optical gain in QD-SOAs by changing the height and tensile-strained barrier of columnar QDs. The TM mode gain is 17.3dB and a gain of over 10dB was attained over a wide wavelength range of 200nm. The saturation output power is 19.5dBm at 1.55μm.

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.

Unit cell parameters of wurtzite InP nanowires determined by x-ray diffraction

High resolution x-ray diffraction is used to study the structural properties of the wurtzite polytype of InP nanowires. Wurtzite InP nanowires are grown by metal-organic vapor phase epitaxy using S-doping. From the evaluation of the Bragg peak position we determine the lattice parameters of the wurtzite InP nanowires. The unit cell dimensions are found to differ from the ones expected from geometric conversion of the cubic bulk InP lattice constant. The atomic distances along the c direction are increased whereas the atomic spacing in the a direction is reduced in comparison to the corresponding distances in the zinc-blende phase. Using core/shell nanowires with a thin core and thick nominally intrinsic shells we are able to determine the lattice parameters of wurtzite InP with a negligible influence of the S-doping due to the much larger volume in the shell. The determined material properties will enable the ab initio calculation of electronic and optical properties of wurtzite InP nanowires.

InAs quantum dots and quantum wells grown on stacking-fault controlled InP nanowires with wurtzite crystal structure

Heteroepitaxial growth of InAs was investigated on sidewalls of InP nanowires (NWs) using metal-organic vapor phase epitaxy. InAs quantum wells (QWs) with smooth surface were formed on the InP NWs having perfect wurtzite phase structure. On the other hand, InAs quantum dots (QDs) were formed on wurtzite InP NWs purposely introduced with stacking-fault segments. Photoluminescence from single NWs attributed to both QWs and QDs was observed

Optical properties of columnar InAs quantum dots on InP for semiconductor optical amplifiers

Optical properties of polarization-controlled columnar quantum dots (QDs) grown by metalorganic vapor-phase epitaxy for semiconductor optical amplifier (SOA) applications are reported. The photoluminescence peak wavelength and polarization sensitivity depended on the time of AsH3 preflow before InAs growth as well as the InAs supply amount, and these changes in the optical properties are considered to be attributed to the change in the strain rather than the change in the height of the columnar QDs. Nearly polarization-insensitive QDs in the 1.5 μm region were obtained by 13-fold columnar QDs and finely controlling polarization of columnar-QD SOAs was demonstrated by changing barrier thickness by 0.05 ML steps.

Polarization-Insensitive Quantum Dot Semiconductor Optical Amplifiers Using Strain-Controlled Columnar Quantum Dots

A polarization-insensitive quantum dot semiconductor optical amplifiers (QD-SOAs) have been studied for use in future optical communication systems. A part of our work shows that the optical polarization property in QDs depends on both their aspect ratio and strain. To control these two parameters, we propose the use of strain-controlled columnar QDs (SC-CQDs), which exhibit a high aspect ratio and have strain-controlled side barriers for polarization-insensitive operation in the 1.5-μ m wavelength band. QD-SOAs with these optimized SC-CQDs demonstrated polarization-insensitive characteristics. They showed a gain of 8.0 dB with polarization dependence of the gain as low as 0.4 dB, -3-dB saturation output power of 18.5 dBm at a wavelength of 1550 nm, and error-free amplification at a bit rate of 40 Gbit/s.

Controlling Polarization of 1.55-µm Columnar InAs Quantum Dots with Highly Tensile-Strained InGaAsP Barriers on InP(001)

The optical polarization properties of columnar InAs quantum dots (QDs) on InP substrate grown by metalorganic vapor-phase epitaxy were investigated. The polarization of photoluminescence was found to strongly depend on the strain in QDs as well as the shape of QDs. We successfully changed the polarization properties from a transverse-electric-dominant to a transverse-magnetic-dominant regime by controlling the height of coupled QDs based on the stacking number and by controlling strain within QDs based on the thickness of 3.7%-tensile-strained barriers. Highly strained side barriers were required to change the polarization of QDs, which is considered to be due to wetting layers acting in maintaining biaxial-compressive strain in QDs. Polarization-insensitive QDs with the 1.55-µm telecom region were obtained, which promises to provide polarization-insensitive semiconductor optical amplifiers.

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.

Compact and low power operation optical switch using silicon-germanium/silicon hetero-structure waveguide

We proposed a silicon-based optical switch with a carrier-plasma-induced phase shifter which employs a silicon-germanium (SiGe) / silicon (Si) hetero-structure in the waveguide core. A type-I hetero-interface formed by SiGe and Si is expected to confine carriers effectively in the SiGe waveguide core. The fabricated Mach-Zehnder optical switch shows a low switching power of only 1.53 mW with a compact phase shifter length of 250 μm. The switching time of the optical switch is less than 4.6 ns for the case of a square waveform driving condition, and 1 ns for the case of a pre-emphasis electric driving condition. These results show that our proposed SiGe/Si waveguide structure holds promise for active devices with compact size and low operation power.

Development of high-density single-mode polymer waveguides with low crosstalk for chip-to-chip optical interconnection

High-density single-mode polymer waveguides were fabricated for chip-to-chip optical interconnection. The waveguides were designed as minimized mode field diameters for the lowest inter-channel crosstalk caused by mode coupling. The optimum relative index difference chosen was 1.2% to ensure compatibility with low crosstalk and wide fabrication tolerances. The 60-mm-length linear waveguides demonstrated a low propagation loss of 0.6 dB/cm and -45 dB crosstalk at 1310 nm. Also, a new crosstalk mechanism for a curved waveguide was revealed.