Kan Takada

Light-illuminated STM studies on photo-absorption in InAs nanowires

We fabricated surface InAs nanowires along GaAs giant step structures on GaAs (110) vicinal substrates, and their photo-absorption properties were studied by scanning tunneling microscopy (STM) under light illumination. In photo-induced current detection, it was difficult to identify the photo-absorption signals in the InAs nanowires because of the electrical isolation of the surface nanostructures from the conductive substrate. On the other hand, optical response measurements of differential conductance under small AC bias modulation in STM enabled us to obtain the photo-absorption signals from the surface nanostructures with avoiding such an electrical isolation problem. We successfully observed optical response images of differential conductance with a clear contrast between InAs nanowires and surrounding GaAs region by the appropriate tuning of the illuminating photon energy to excite the InAs nanowires selectively.

Temperature-stable 25-Gbps direct-modulation in 1.3-μm InAs/GaAs quantum dot lasers

By clarifying the temperature and mirror-loss dependence of modulation bandwidth of 1.3-μm-wavelength InAs/GaAs quantum-dot lasers, temperature-stable 25-Gbps direct-modulation is achieved from 20 to 70°C with fixed bias and modulation currents.

Long-wavelength quantum dot FP and DFB lasers for high temperature applications

High temperature (>125°C) resistant long-wavelength semiconductor lasers are attractive as light sources in a variety of harsh environments. Here, we report extremely high temperature continuous-wave (CW) operation of QD lasers on GaAs substrate emitted at 1300-nm-range by enhancing gain and increasing the quantized-energy separation of the QD active layers. A suppression of the In out-diffusion during MBE from self-assembled InAs QDs significantly reduced inhomogeneous broadening with high QD sheet density maintained. QD-FP laser exhibited record high CW-lasing temperature for long-wavelength laser of 220°C and QD-DFB laser also exhibited high CW-lasing temperature of 150°C by employing high gain QD active media.

Photoabsorption Characterization on Surface InAs Nanostructures Using Light-Illuminated Scanning Tunneling Microscopy

We characterized surface InAs nanostructures grown on a GaAs vicinal substrate by using scanning tunneling microscopy (STM) under light illumination. In local photoinduced current imaging with a DC bias voltage, the photoabsorption signals on InAs nanowires could not be detected when the illuminated photon energy was adjusted between the band gaps of GaAs and InAs, due to electrical isolation of the surface InAs from the conductive substrate. On the other hand, imaging an optical response of differential conductance with AC bias modulation revealed a clear contrast between InAs nanowires and the surrounding GaAs region without the generation of photocarriers in the underlying GaAs.

Sharp-flat reversible change of top of pyramid in gaas molecular beam epitaxy on (111)B patterned substrate

The sharpening and flattening of the top of a pyramid on GaAs(111)B substrate during molecular beam epitaxy (MBE) of GaAs were monitored in real time under a microprobe reflection high-energy electron diffraction/scanning electron microscope (microprobe-RHEED/SEM) installed in a MBE chamber. After the formation of sharp pyramids with {110} side facets, the top of the pyramids was flattened and sharpened reversibly by increasing and decreasing arsenic pressure, respectively. To understand this behavior, we conducted a theoretical analysis based on intersurface diffusion, and we found a good agreement between the calculated and experimental results. Using this knowledge, we succeeded in controlling the size of the top of pyramids in real time.

Effect of carrier transport on modulation bandwidth of 1.3-µm InAs/GaAs self-assembled quantum-dot lasers

We newly modeled the modulation bandwidth of 1.3-µm quantum-dot lasers and analyzed experimental results. The carrier transport through the active layers was found to affect significantly the modulation bandwidth with increasing stacking-number of quantum-dot layers.

High-output-power 10.3-Gb/s operation of 1.27-μm quantum-dot DFB lasers for 10G-EPON

1.27-μm high-density quantum-dot DFB lasers for 10G-EPON were developed. Fabricated lasers exhibited temperature-stable light-current characteristics and clearly-opened 10.3-Gb/s eye-diagrams with a high averaged output power of +4.5dBm from −10°C to 85°C.

High-speed modulation in 1.3-µm InAs/GaAs high-density quantum dot lasers

High-speed modulation characteristics of 1.3-µm InAs/GaAs high-density quantum-dot lasers are presented. The high-density quantum-dot active layers provided high net modal gain beyond 40 cm−1. Fabricated Fabry-Perot lasers have exhibited an extended modulation bandwidth and 25-Gbps direct modulation, for the first time, in 1.3-µm quantum dot lasers.

Temperature-stable 10.3-Gb/s operation of 1.3-µm quantum-dot DFB lasers with GaInP/GaAs gratings

Temperature-stable operation of 1.3-µm quantum-dot DFB lasers with anti-reflection coated facets was realized by utilizing GaInP/GaAs gratings. A 10.3-Gb/s modulation under fixed driving conditions was achieved between 15°C and 70°C.

1240-nm distributed-feedback lasers with high-density InAs/GaAs quantum dots

1240-nm quantum-dot (QD) distributed-feedback (DFB) lasers are developed for the optical time-domain reflectometer. Fabricated DFB lasers with high-density QDs show excellent temperature-stable lasing characteristics in single-longitudinal-mode operation between 0°and 70°C.