Tatsuya Usuki

50-Gb/s ring-resonator-based silicon modulator.

We achieved 50-Gb/s operation of a ring-resonator-based silicon modulator for the first time. The pin-diode phase shifter, which consists of a side-wall-grating waveguide, was loaded into the ring resonator. The forward-biased operation mode was applied, which exhibited a V(π)L as small as 0.28 V · cm at 25 GHz. The driving voltage and optical insertion loss at 50-Gb/s were 1.96 V(pp) and 5.2 dB, respectively.

First demonstration of high density optical interconnects integrated with lasers, optical modulators, and photodetectors on single silicon substrate

Optical interconnects integrated with lasers, silicon optical modulators and germanium photodetectors on a single silicon substrate were demonstrated for the first time. A 5 Gbps line bit rate and 3.5 Tbps/cm2 transmission density were achieved.

Single-Photon Generation in the 1.55-µm Optical-Fiber Band from an InAs/InP Quantum Dot

We first succeeded in generating single-photon pulses in the C-band (1.55-µm band: the highest transmittance in optical telecommunication bands) from a single InAs/InP quantum dot. The quantum dot with 1546.1-nm exciton emission was prepared by controlling the growth conditions. A well-designed mesa structure presented efficient injection of the emitted photons into a single-mode optical fiber. A Hanbury-Brown and Twiss measurement has proved that the photons through the fiber were single photons. We also performed to transmit single-photon pulses through 30-km optical fiber. This preliminary trial is a milestone toward quantum telecommunication using ideal single photons.

Microstructure and electrical properties of Sn nanocrystals in thin, thermally grown SiO2 layers formed via low energy ion implantation

We have developed a simple technique for fabricating Sn nanocrystals in thin thermally grown SiO2 layers using low energy ion implantation followed by thermal annealing. The formed Sn nanocrystals have excellent size and depth uniformity. Their average diameter is 4.2 nm with a standard deviation of 1.0 nm. Our experimental results clearly reveal that a stable depth of Sn exists in the SiO2 layer at about 2 nm from the SiO2/Si interface. Most of the Sn nanocrystals reside near this stable depth. The I–V characteristics of the diode structure show a clear Coulomb blockade region of 0.12 V and a Coulomb staircase at 4.2 K. A Coulomb blockade region around 0 V was observed until reaching a temperature of 77 K. The features of these nanocrystals will open up new possibilities for the creation of novel devices.

Site-controlled photoluminescence at telecommunication wavelength from InAs∕InP quantum dots

We fabricated ordered InAs∕InP quantum-dot (QD) arrays using atomic-force-microscopic oxidation, wet etching, and regrowth by metalorganic chemical vapor deposition. The QDs exhibit single-dot photoluminescence peaking at wavelengths ranging from 1.22 to 1.45μm, mostly matching the telecommunication band of optical fibers. The site dependence of single peaks indicates the site controllability of single-dot light emitters, which might be useful in quantum information processing.

12.5-Gb/s operation with 0.29-V·cm V π L using silicon Mach-Zehnder modulator based-on forward-biased pin diode

We present high-speed operation of pin-diode-based silicon Mach-Zehnder modulators that have side-wall gratings on both sides of the waveguide core. The use of pre-emphasis signals generated with a finite impulse response digital filter was examined in the frequency domain to show how the filter works for different filter parameter sets. In large signal modulation experiments, V(π)L as low as 0.29 V·cm was obtained at 12.5 Gb/s using a fabricated modulator and the pre-emphasis technique. Operation of up to 25-Gb/s is possible using basically the same driving configurations.

V/sub th/ fluctuation induced by statistical variation of pocket dopant profile

This paper studies effect of pocket (halo) profile on V/sub th/ fluctuation due to statistical dopant variation by measurement and simulation. A pocket profile significantly enhances V/sub th/ fluctuation by a factor of >15% at worst even if the implantation process variations would be negligible. This is because pocket dopants shrink the area which controls V/sub th/.

Transmission Experiment of Quantum Keys over 50 km Using High-Performance Quantum-Dot Single-Photon Source at 1.5 µm Wavelength

We have developed a high-performance single-photon source (SPS) operating at 1.5 µm wavelength. The source is an InAs/InP quantum dot with a horn-shaped nanostructure. A resonant excitation to the p-shell state helps achieve a single-photon efficiency of 5.8% after coupling into a single-mode fiber with a second-order correlation value of g(2)(0)~0.055. The performance of the source has been assessed by integrating it into a conventional quantum key distribution system. We have successfully transmitted secure keys over a 50 km commercial fiber, exceeding the previously reported range for an SPS operating below 1.3 µm.

Compact PIN-Diode-Based Silicon Modulator Using Side-Wall-Grating Waveguide

We developed PIN-diode-based silicon Mach-Zehnder modulators, which have side-wall-gratings in the phase-shifter sections. Such passive waveguides with gratings were fabricated using ArF immersion lithography, which showed a small scattering loss of 0.4 dB/mm. We extensively investigated the forward-biased operation of the modulators by using equivalent circuit analysis and the measurement of the fabricated devices. We argue carrier recombination time only plays a minor role for the overall performance of the modulator. Dependences of the modulation efficiency on other various critical parameters are discussed. In particular, if we use relatively short phase shifter, the forward-biased operation provides smaller VπL than reversed one even at high frequency of 20 GHz, at the expense of the narrow bandwidth. Our approach enables high-speed operation up to 50 Gb/s, by using phase shifter as short as 250 μm and preemphasis signals. For 12.5-Gb/s operation, the modulator cell size was only 300 μm × 50 μm, which was suitable for the applications of high-density optical interconnects.

Laterally coupled self-assembled InAs quantum dots embedded in resonant tunnel diode with multigate electrodes

We study the electronic properties of submicron vertical resonant tunneling structures containing several self-assembled InAs quantum dots (SADs) surrounded by four gate electrodes. The four gates are designed not only to squeeze the conductive channel containing a few SADs but also to differently modulate the electrochemical potential of each SAD. We measure the stability diagram and distinguish the features of lateral interdot coupling, such as the type of coupling (quantum mechanical or capacitive), the number of coupled dots, and the relative coupled dot position. This technique will be useful in characterizing the electronic properties of coupled SAD systems.