Yutaka Kadoya

Directional control of light by a nano-optical Yagi–Uda antenna

A Yagi–Uda directional antenna — the work horse of radiofrequency communications for more than 60 years — has now been demonstrated at visible wavelengths. An array of appropriately tuned nanoparticles replicate the reflecting and directing elements of the original design. Directional control of radiation from the nano-optical Yagi–Uda antenna was experimentally shown.

Extraordinary carrier multiplication gated by a picosecond electric field pulse

The study of carrier multiplication has become an essential part of many-body physics and materials science as this multiplication directly affects nonlinear transport phenomena, and has a key role in designing efficient solar cells and electroluminescent emitters and highly sensitive photon detectors. Here we show that a 1-MVcm−1 electric field of a terahertz pulse, unlike a DC bias, can generate a substantial number of electron–hole pairs, forming excitons that emit near-infrared luminescence. The bright luminescence associated with carrier multiplication suggests that carriers coherently driven by a strong electric field can efficiently gain enough kinetic energy to induce a series of impact ionizations that can increase the number of carriers by about three orders of magnitude on the picosecond time scale.

Design parameters for a nano-optical Yagi–Uda antenna

We investigate the possibility of directing optical emissions using a Yagi–Uda antenna composed of a finite linear array of nanoparticles. The relevant parameters characterizing the plasma resonances of the nanoparticles are identified and the interaction between the array elements is formulated accordingly. It is shown that the directionality of the optical emission can be observed even in the presence of non-negligible absorption losses in the nanoparticles. We conclude that a finite array of gold nanorods may be sufficient for the realization of a working nano-optical Yagi–Uda antenna.

Transport properties of two-dimensional electron gas in AlGaAs/GaAs selectively doped heterojunctions with embedded InAs quantum dots

Transport properties of two‐dimensional electron gas (2DEG) are studied in selectively doped GaAs/n‐AlGaAs heterojunctions, in which nanometer‐scale InAs dots are embedded in the vicinity of the GaAs channel. When the distance Wd between the InAs dot layer and the channel is reduced from 80 to 15 nm, the mobility μ of electrons at 77 K decreases drastically from 1.1×105 to 1.1 ×103 cm2/V s, while the carrier concentration increases from 1.1×1011 to 5.3×1011 cm−2. Such a reduction of mobility is found only when the average thickness of InAs layer is above the onset level (∼1.5 monolayer) for the dot formation. Origins of these changes in μ and Ns are discussed in connection with dot‐induced modulations of the electronic potential V(r) in the channel.

Terahertz wave emission and detection using photoconductive antennas made on low-temperature-grown InGaAs with 1.56μm pulse excitation

Photoconductive antennas made on low-temperature-grown Be doped InxGa1−xAs (0.45⩽x⩽0.53) have been investigated focusing on the terahertz emission properties. In the antenna of x=0.45, the resistance as high as 3MΩ enabled us to increase the bias field up to 60kV∕cm, and the terahertz waves emitted from the antenna were significantly enhanced. In addition, terahertz waves with the spectral range over 2.5THz and the peak to noise ratio of 45dB were generated and detected using only 1.56μm pulses.

Real-time terahertz near-field microscope

We report a terahertz near-field microscope with a high dynamic range that can capture images of a 370 x 740 μm2 area at 35 frames per second. We achieve high spatial resolution (14 μm corresponding to λ/30 for a center frequency at 0.7 THz) on a large area by combining two novel techniques: terahertz generation by tilted-pulse-front excitation and electro-optic balanced imaging detection using a thin crystal. To demonstrate the microscope capability, we reveal the field enhancement at the gap position of a dipole antenna after the irradiation of a terahertz pulse.

Detection of terahertz waves using low-temperature-grown InGaAs with 1.56 μm pulse excitation

The authors have investigated the dc and terahertz-detection characteristics of the photoconductive antennas made on low-temperature-grown (LTG) InxGa1−xAs (0.4<x<0.53). It was found that the resistivity of the LTG In0.4Ga0.6As can be as high as 700Ωcm, with which the resistance of the antenna becomes higher than 3MΩ. Terahertz waves were detected by the antennas with the pulse excitation at 1.56μm, with a spectral range exceeding 3THz, and a dynamic range of about 55dB. The results also indicate that the photocarrier dynamics depend on the In content.

Micro-strip-line-based sensing chips for characterization of polar liquids in terahertz regime

We have developed embedded thin-film micro-strip-line-based sensor chips working in terahertz regime. In the chip, terahertz waves are generated and detected with femtosecond optical pulses accessed from the back side of the chips. Moreover, the spectroscopic sensitivity can be freely adjusted by changing the thickness of a polyimide cover layer. They make it easy to measure polar liquids. The measurement of water is demonstrated with spectral range from 30GHzto1–1.5THz, where the upper limit depends on the thickness of the cover layer.

THz spectroscopic characterization of biomolecule/water systems by compact sensor chips

We demonstrate the terahertz (THz) spectroscopic performance of highly integrated sensor chips based on microstrip lines by measuring biomolecule/water systems. The concentration resolution of the present chips reaches down to 0.05g∕ml. We have confirmed that the number of bound water molecules per biomolecule can be obtained with precision using solid state transmission lines. The chips are highly suitable for the inspection of small amounts of specimen and for the application to a wide range of water rich materials. Our method may therefore be a good candidate for a simple liquid sensor working in the THz regime.

Electrical properties and dopant incorporation mechanisms of Si doped GaAs and (AlGa)As grown on (111)A GaAs surfaces by MBE

Abstract The relation of Si incorporation site and the growth parameters in MBE growth of GaAs and (AlGa)As on (111)A surface is systematically investigated. Both n- and p-type GaAs and (AlGa)As layers with reasonably low compensation are achieved, and the dependencies of Si incorporation on the parameters can be qualitatively understood by the change of the population of As atoms at the surface. Two-dimensional electron and hole gas (2DEG, 2DHG) structures are successfully grown on (111)A substrates.