Fumito Nakajima

120-GHz-band millimeter-wave photonic wireless link for 10-Gb/s data transmission

A 120-GHz-band wireless link that uses millimeter-wave (MMW) photonic techniques was developed. The output power and noise characteristics of 120-GHz-band MMWs generated by converting a 125-GHz optical subcarrier signal were evaluated. It was then shown that the noise characteristics of the 125-GHz signal generated with these photonic technologies is sufficient for 10-Gb/s data transmission. We constructed a compact 120-GHz-band wireless link system, and evaluated its data transmission characteristics. This system achieved error-free transmission of OC-192 and 10-GbE signals over a distance of more than 200 m with a received power of below -30 dBm.

Continuous THz-wave generation using antenna-integrated uni-travelling-carrier photodiodes

Photonic generation of continuous millimetre- and sub-millimetre waves up to the THz range using antenna-integrated uni-travelling-carrier photodiodes is described. A device integrating a wideband log-periodic antenna exhibits a maximum output power of 2.6 ?W at 1.04 THz with good linearity. A module with a quasi-optical output port fabricated for practical use generates almost the same output power as the chip at around 1 THz and operates at frequencies of up to 1.5 THz. The output power level and the operation frequency are records for wideband photodiodes operating at 1.55 ?m. Devices integrating resonant narrowband dipole antennae have also been fabricated and the output power increases at resonant peak frequencies confirmed. The device having a peak at 1.04 THz exhibits a maximum (detected) output power of 10.9??W at 1.04 THz with good linearity. This output power is the highest value ever directly generated from a photodiode in the THz range, and several times higher than the maximum value reported by the low-temperature-grown GaAs photoconductive switch at around 1 THz.

Two-stage Kondo effect in a quantum dot at a high magnetic field.

We report a strong Kondo effect (Kondo temperature approximately 4 K) at high magnetic field in a selective area growth semiconductor quantum dot. The Kondo effect is ascribed to a singlet-triplet transition in the ground state of the dot. At the transition, the low-temperature conductance approaches the unitary limit. Away from the transition, for low bias voltages and temperatures, the conductance is sharply reduced. The observed behavior is compared to predictions for a two-stage Kondo effect in quantum dots coupled to single-channel leads.

InP-Based Planar-Antenna-Integrated Schottky-Barrier Diode for Millimeter- and Sub-Millimeter-Wave Detection

An InP-based Schottky-barrier diode (SBD) is monolithically integrated with a wideband log-periodic toothed antenna for detecting millimeter- and sub-millimeter-waves at frequencies of up to the terahertz (THz) range. A module with a quasi-optical collimation lens fabricated for practical use exhibits sensitivities of 1000 V/W at 300 GHz and 125 V/W at 1.2 THz with good linearity. Near-distance wireless transmission of 5 Gbit/s data with a 240 GHz carrier is also examined using the fabricated SBD module.

Single-electron AND/NAND logic circuits based on a self-organized dot network

We experimentally demonstrated single-electron operations of an AND/NAND logic circuit based on a self-organized GaAs quantum-dot (QD) network fabricated by applying a selective-area metalorganic vapor-phase epitaxy technique. Single-electron logic operations using four cooperating single-electron tunneling (SET) transistors has been tested. This logic circuit has an architecture based on a binary decision diagram (BDD) using a Coulomb blockade (CB) in GaAs QDs, which is a representation of digital logic functions using directed graphs. BDD node devices consisting of two SET transistors achieved a two-way path switching operation in single-electron mode due to the CB effects which appeared complementarily in the two SET transistors at 1.9 K. We also demonstrated an AND/NAND operation in a logic circuit by integrating two BDD nodes.

Fabrication of semiconductor Kagome lattice structure by selective area metalorganic vapor phase epitaxy

Artificial two-dimensional semiconductor Kagome lattice structures formed by quantum wires can show ferromagnetism when the flatband is half filled, even though it does not have any magnetic elements. Experimental realization of such a Kagome lattice structure is reported. The structure, with different pattern periods, was formed with GaAs quantum wires by selective area metalorganic vapor phase epitaxy on GaAs (111)B substrates. To overcome the lateral overgrowth and to improve the shape of smaller period pattern, flow rate modulation epitaxy was employed and a GaAs Kagome lattice structure with 1 μm period was effectively grown.

GaAs dot-wire coupled structures grown by selective area metalorganic vapor phase epitaxy and their application to single electron devices

We describe a method for fabricating GaAs dot arrays and dot-wire coupled structures having periodic nanofacets which uses selective area metalorganic vapor phase epitaxy. First, a thin GaAs buffer layer and an AlGaAs layer are grown on a masked substrate having wirelike openings with periodic width modulation. The width of AlGaAs wirelike structure is naturally squeezed by the periodic combination of nanofacets, and its top (001) surface is partially isolated by a self-limited region. Next, an AlGaAs/GaAs quantum well structure is fabricated on the substrate to form dots on the narrower top terraces, wires on the wider terraces, and ridge wires in the self-limited region. Cathodoluminescence images clearly showed dot arrays and dot-wire coupled structures were formed using this method. A single electron transistor with the same structure was also fabricated, and clear Coulomb blockade oscillation was observed. We also describe single electron tunneling devices with these dot arrays and dot-wire coupled s...

GaAs Single Electron Transistors Fabricated by Selective Area Metalorganic Vapor Phase Epitaxy and Their Application to Single Electron Logic Circuits

GaAs single electron transistors (SETs) are successfully fabricated using selectively grown GaAs/AlGaAs modulation doped structures by metalorganic vapor phase epitaxy (MOVPE) on (001) GaAs masked substrates. SET shows clear Coulomb oscillations and Coulomb gaps modulated by gate voltage. GaAs single electron tunneling inverter circuits having a SET and a variable load resistance are also formed. The operation of a resistance-load inverter circuit is confirmed at 1.9 K from the transport properties of this SET and input-output characteristics.

A 1 bit binary-decision-diagram adder circuit using single-electron transistors made by selective-area metalorganic vapor-phase epitaxy

We demonstrate single-electron operation of a 1 bit adder circuit using GaAs single-electron tunneling transistors (SETs). GaAs dot and wire coupled structures for the fabrication of SETs were grown by a selective-area metalorganic vapor-phase epitaxy technique. The logic circuit was realized based on a binary decision diagram architecture using Coulomb blockade (CB) in GaAs dots and switching operations were achieved in a single-electron mode because of the CB effects. Through this architecture, a 1 bit adder circuit was realized with three SETs, two of which were for AND logic and one with two input gates for exclusive OR (XOR). Both AND and XOR operations were demonstrated at 1.9 K, which indicated successful fabrication of the 1 bit adder.

Formation and characteristics of 100-nm scale GaAs quantum wires by selective area MOVPE

We fabricated quantum wires (QWRs) with sub-micron wire width using GaAs/AlGaAs selectively doped structures grown by selective area metalorganic vapor phase epitaxy (SA-MOVPE) on (0 0 1) masked GaAs substrates partially covered by SiON. From the measurement of a two-terminal conductance as a function of geometrical wire width, QWRs with effective channel width <100 nm are formed without application of any gate bias. The magnetoresistance measurement at 1.7 K also suggests the formation of narrow QWRs, although it also indicates a presence of potential fluctuation along the QWRs. The effective channel width of present QWRs are much narrower than the previously reported values (∼300 nm) of those formed by SA-MOVPE.