Shigeki Nashima

Spectroscopy by pulsed terahertz radiation

Spectroscopies using terahertz (THz) radiation excited by ultrashort laser pulses have been rapidly developing recently. In this paper, the principles of various types of THz time domain spectroscopies (THz TDSs), i.e. transmission-, reflection-and ellipsometry-type THz TDSs, and their applications to the characterization of semiconductors are described. In addition to the THz TDS using a femtosecond laser, a sub-THz TDS system using a cheap and compact continuous multimode laser diode is also described.

Measurement of optical properties of highly doped silicon by terahertz time domain reflection spectroscopy

Optical properties of doped silicon wafers have been measured by means of terahertz time domain reflection spectroscopy. A method is proposed to obtain the relative phase by reflection accurately. By using this method, the relative phase is obtained within an error of less than 10 mrad at 1 THz. The experimentally obtained complex conductivity of relatively high-doped silicon (ρ=0.136 Ω cm) in the terahertz region agrees with the simple Drude model.

Temperature dependence of optical and electronic properties of moderately doped silicon at terahertz frequencies

Complex conductivity of moderately P-doped silicon wafers (1.1±0.2 Ω cm at room temperature) has been measured by using a terahertz (THz) time-domain spectroscopy for the temperature and frequency ranges of 20–300 K and 0.2–1.1 THz, respectively. The strong frequency dependence of the complex conductivity due to the free carriers in the THz region is observed and it changes rapidly with temperature, which is interpreted in terms of the increase in mobility and freezing of the free carrier as analyzed by using the simple Drude model. The experimental data deviate slightly from the simple Drude model at low temperatures and becomes apparent with decreasing temperature.

Optical actuation of micromirrors fabricated by the micro-origami technique

Micromirrors were fabricated by the micro-origami technique. This technique allows the fabrication of simple and robust hinges for movable parts, and it can be applied to any pair of lattice mismatched epitaxial layers, in semiconductors or metals. A multilayer structure, including AlGaAs/GaAs component layers and an InGaAs strained layer, was grown by molecular beam epitaxy on a GaAs substrate. After definition of the hinge and mirror’s shape by photolithography, the micromirrors were released from the substrate by selective etching. They moved to their final position powered by the strain release in the InGaAs layer. Optical actuation was achieved by irradiation with the 488 nm line of an argon laser, and the mirror’s position was measured by sensing the reflection of a He–Ne laser. Continuous wave irradiation with a power density of 450 mW/mm2 produced an angular deflection of the mirror of around 0.5°. The frequency response of the mirrors shows a resonance at 25 kHz.

Quantum-well microtube constructed from a freestanding thin quantum-well layer

We fabricated and experimentally investigated a nanostructure known as a quantum-well (QW) microtube, which is a fine tube with a micron- or nanometer-order diameter fabricated by rolling a semiconductor GaAs QW. Although the wall thickness is only 40 nm, the system retains the quantum properties of a QW, and photoluminescence from the QW subband can be clearly observed. Even though the QW width is sufficiently small to make the QW subband type-II band-aligned, a type-II to type-I transition caused by uniaxial strain in the microtube allows for optical emission.

Faraday ellipticity and Faraday rotation of a doped-silicon wafer studied by terahertz time-domain spectroscopy

Free-carrier Faraday ellipticity and Faraday rotation are measured for a moderately doped n-type silicon wafer with the resistivity of 1.1Ωcm under magnetic fields of ±3T using the terahertz time-domain spectroscopy. From the experimental data, we obtain the time evolution of the electric-field vector of the terahertz radiation pulses. When the magnetic field is applied to the sample, the transmitted radiation has an elliptic polarization with its major axis rotated from the polarization direction of the incident radiation (Faraday effect). The Faraday ellipticity and Faraday rotation angle are obtained for the directly transmitted pulse (first terahertz pulse) and the pulse reflected twice at the sample surfaces (second terahertz pulse) separately. They are compared with the calculations using the Drude model. A slight deviation is observed between the experimental and calculated Faraday ellipticities and Faraday rotation angles probably due to the energy dependence of the carrier scattering time.Free-carrier Faraday ellipticity and Faraday rotation are measured for a moderately doped n-type silicon wafer with the resistivity of 1.1Ωcm under magnetic fields of ±3T using the terahertz time-domain spectroscopy. From the experimental data, we obtain the time evolution of the electric-field vector of the terahertz radiation pulses. When the magnetic field is applied to the sample, the transmitted radiation has an elliptic polarization with its major axis rotated from the polarization direction of the incident radiation (Faraday effect). The Faraday ellipticity and Faraday rotation angle are obtained for the directly transmitted pulse (first terahertz pulse) and the pulse reflected twice at the sample surfaces (second terahertz pulse) separately. They are compared with the calculations using the Drude model. A slight deviation is observed between the experimental and calculated Faraday ellipticities and Faraday rotation angles probably due to the energy dependence of the carrier scattering time.

Metal mesh device sensor immobilized with a trimethoxysilane-containing glycopolymer for label-free detection of proteins and bacteria

Biosensors for the detection of proteins and bacteria have been developed using glycopolymer-immobilized metal mesh devices. The trimethoxysilane-containing glycopolymer was immobilized onto a metal mesh device using the silane coupling reaction. The surface shape and transmittance properties of the original metal mesh device were maintained following the immobilization of the glycopolymer. The mannose-binding protein (concanavalin A) could be detected at concentrations in the range of 10(-9) to 10(-6) mol L(-1) using the glycopolymer-immobilized metal mesh device sensor, whereas another protein (bovine serum albumin) was not detected. A detection limit of 1 ng mm(-2) was achieved for the amount of adsorbed concanavalin A. The glycopolymer-immobilized metal mesh device sensor could also detect bacteria as well as protein. The mannose-binding strain of Escherichia coli was specifically detected by the glycopolymer-immobilized metal mesh device sensor. The glycopolymer-immobilized metal mesh device could therefore be used as a label-free biosensor showing high levels of selectivity and sensitivity toward proteins and bacteria.

Measurement of Electrical Properties of GaN Thin Films Using Terahertz-Time Domain Spectroscopy

We demonstrate the noncontact and nondestructive evaluation of electrical properties of n-type GaN thin films on sapphire substrates using time domain spectroscopy in the THz frequency region (THz-TDS). DC resistivities of the GaN films with various free carrier densities are deduced by fitting the transmission spectra of the sample to the Drude model. The DC resistivities obtained by the THz-TDS show good agreement with those obtained by the conventional contact measurements. Mobilities of the free carriers in lightly doped GaN films are also determined by the Drude fit. It is found that the temperature dependence of the mobilities for the lightly doped films shows a peak at ~150 K. The temperature dependences of the free carrier densities for the lightly doped films obtained by the THz measurements are compared with that predicted by a model with two kinds of donors reported previously.

Tunable Picosecond Terahertz-Wave Parametric Oscillators Based on Noncollinear Pump-Enhanced Signal-Resonant Cavity

We have succeeded in developing tunable picosecond terahertz (THz)-wave parametric oscillators (ps-TPOs) by employing a noncollinear pump-enhanced signal-resonant cavity. As a parametric gain medium, we use two different shapes of unpoled, 5 mol% MgO-doped lithium niobate (MgO:LiNbO3) crystal: 1) a rectangle for Si-prism output-coupler technique and 2) a trapezoid for surface-emitted configuration. Unlike conventional nanosecond TPOs (ns-TPOs), these ps-TPOs are synchronously pumped by a mode-locked 1.5-ps Ti:sapphire laser operating at 780 nm. To overcome the high pump threshold due to the strong absorption by MgO:LiNbO3 in the THz region, we employ a pump-enhanced cavity which is carefully designed for the noncollinear dual resonance of both pump and signal waves. By slightly translating the position of one of the ps-TPO cavity mirrors, we experimentally find that the THz-wave peak frequency is continuously tunable from 0.9 to 3.3 THz, approximately, with the average output power of dozens of nanowatts. In all the above tuning range, especially above 2 THz, the THz-wave output of the surface-emitting ps-TPO using the trapezoidal MgO:LiNbO3 crystal is enhanced several times more than that of the Si-prism-coupled ps-TPO using the rectangular MgO:LiNbO3 crystal due to the suppression of the absorption loss in MgO:LiNbO3.

Comparison of the electromagnetic pulse emission from YBa2Cu3O7−δ and Y0.7Pro0.3Ba2Cu3O7 excited by femtosecond laser pulses

Abstract We observe terahertz radiation from Y 0.7 Pr 0.3 Ba 2 Cu 3 O 7 (YPBCO) thin films excited with femtosecond optical pulses, and compare its waveform and efficiency with those from YBa 2 Cu 3 O 7−gd (YBCO). It is found that YPBCO can produce much stronger radiation than YBCO. We discuss the origin of the enhancement as well as the temperature dependence of the emission properties.