Kazuhiko Hirakawa

Fabrication of graphene nanoribbon by local anodic oxidation lithography using atomic force microscope

We conducted local anodic oxidation (LAO) lithography in single-layer, bilayer, and multilayer graphenes using tapping-mode atomic force microscope. The width of insulating oxidized area depends systematically on the number of graphene layers. An 800-nm-wide bar-shaped device fabricated in single-layer graphene exhibits the half-integer quantum Hall effect. We also fabricated a 55-nm-wide graphene nanoribbon (GNR). The conductance of the GNR at the charge neutrality point was suppressed at low temperature, which suggests the opening of an energy gap due to lateral confinement of charge carriers. These results show that LAO lithography is an effective technique for the fabrication of graphene nanodevices.

Charge-sensitive infrared phototransistors : Characterization by an all-cryogenic spectrometer

Charge-sensitive infrared phototransistors (CSIPs) with a 16×4 μm2 active area, which are fabricated in a GaAs/AlGaAs double-quantum-well structure, are studied with an all-cryogenic spectrometer operated at 4.2 K. Extremely low level of background radiation makes reliable determination of detector characteristics at 4.2 K possible: The detection band is found to be centered at the wavelength λ=14.7 μm with a bandwidth (full width at maximum) Δλ=1 μm. The quantum efficiency (η), the current responsivity (R), the noise equivalent power (NEP), and the specific detectivity (D∗) are derived to be η=(2±0.5)%, R=4×104–4×106 A/W, NEP≅6.8×10−19 W/Hz1/2, and D∗≅1.2×1015 cm Hz1/2/W. The dynamic range of detection is demonstrated to exceed 106 (approximately attowatts to picowatts), but the upper limit of the radiation power is limited by the radiation source intensity. The intrinsic dynamic range of the detector is suggested to reach 1013 (approximately attowatts to microwatts). The detection speed is suggested to ...

Single-photon detector in the microwave range

Single-photon counting at microwave frequencies around 500 GHz is demonstrated by using a single-electron transistor (SET) formed by two capacitively coupled GaAs/AlxGa1−xAs parallel quantum dots (QDs). A point contact separating the double QDs allows the prompt escape of an excited electron from one of the QDs to another. The resulting long-lived photoinduced ionization of the QD is detected as a change in the SET current.

MEMS reconfigurable metamaterial for terahertz switchable filter and modulator

We demonstrate a reconfigurable metamaterial developed by surface micromachining technique on a low loss quartz substrate for a tunable terahertz filter application. The device implements a reconfigurable RF-MEMS (radio frequency - micro electro mechanical systems) capacitor within a split-ring resonator (SRR). Time-domain spectroscopy confirms that the tunability of the SRR resonance and thus the terahertz transmittance are electrostatically controlled by the RF-MEMS capacitor. Due to the high transparency and low loss of quartz used as a substrate, the device exhibits a high contrast switching performance of 16.5 dB at 480 GHz, which is also supported by the terahertz dynamic modulation measurement results. The device shows promise for tunable transmission terahertz optics.

Gate-Tunable Large Negative Tunnel Magnetoresistance in Ni–C60–Ni Single Molecule Transistors

We have fabricated single C(60) molecule transistors with ferromagnetic Ni leads (FM-SMTs) by using an electrical break junction method and investigated their magnetotransport. The FM-SMTs exhibited clear gate-dependent hysteretic tunnel magnetoresistance (TMR) and the TMR values reached as high as -80%. The polarity of the TMR was found to be always negative over the entire bias range studied here. Density functional theory calculations show that hybridization between the Ni substrate states and the C(60) molecular orbitals generates an antiferromagnetic configuration in the local density of states near the Fermi level, which gives a reasonable explanation for the observed negative TMR.

Spin transport through a single self-assembled InAs quantum dot with ferromagnetic leads

The authors have fabricated a lateral double barrier magnetic tunnel junction (MTJ) which consists of a single self-assembled InAs quantum dot (QD) with ferromagnetic Co leads. The MTJ shows clear hysteretic tunnel magnetoresistance (TMR) effect, which is evidence for spin transport through a single semiconductor QD. The TMR ratio and the curve shapes are varied by changing the gate voltage.

Nonequilibrium Green’s function calculation for four-level scheme terahertz quantum cascade lasers

We have calculated the performance of a recently proposed four-level scheme terahertz quantum cascade laser (4L terahertz-QCL) with the nonequilibrium Green’s function method. The calculation result for 40 K showed that the 4L QCL has a larger terahertz gain than the conventional resonant phonon QCL. This is because a large number of electrons accumulate in the upper lasing level and contribute to lasing in the new scheme. When the temperature is increased, the advantage of gain decreases due to thermally activated phonon scattering.

Photon-counting microscopy of terahertz radiation

Photon-counting imaging of terahertz radiation is demonstrated with a spatial resolution of 50μm for a free-space wavelength of 132μm by incorporating a quantum-dot single photon detector into a scanning confocal optical system. Terahertz radiation of 10−19–10−16W or 102–105photons∕s is imaged. Applying the microscope to the study of semiconductor quantum Hall devices, a threshold behavior of the cyclotron emission is found and the mechanism is discussed.

Infrared phototransistor using capacitively coupled two-dimensional electron gas layers

A narrow-band infrared phototransistor (14.8μm) is designed and realized based on a GaAs∕AlGaAs double-layer structure. An isolated island formed from the first quantum well (QW) works as a gate, which is capacitively coupled to the remote two-dimensional electron gas (2DEG) layer working as the source/drain channel. Incident radiation excites the intersubband transition within the isolated QW island. Excited electrons tunnel out of the QW causing it to positively charge up. This affects the conductance of the remote 2DEG channel, yielding detectable photosignals. The present detection mechanism makes it possible to design semiconductor infrared detectors with higher sensitivities along with custom designed tunability. The mechanism also holds potentiality of single-photon detection in the infrared region.

Lateral electron transport through single self-assembled InAs quantum dots

The electron transport through single self-assembled InAs quantum dots (QDs) grown on GaAs surfaces has been investigated by using metallic leads with narrow gaps. Clear Coulomb staircases and Coulomb gaps have been observed at 4.2K. Coulomb blockade oscillation which reflects single electron charging in the QDs was also observed when a backgate voltage was swept. It is found that uncapped as-grown InAs QDs with diameter ⩾50nm contain electrons without applying a gate bias.