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.

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 (ϭ0.136 ⍀ cm) in the terahertz region agrees with the simple Drude model.

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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.

Articles you may be interested inUltrafast time-domain spectroscopy based on high-speed asynchronous optical sampling Rev. Sci. Instrum. 78, 035107 (2007);Free-carrier Faraday ellipticity and Faraday rotation are measured for a moderately doped n-type silicon wafer with the resistivity of 1.1 ⍀ cm under magnetic fields of ±3 T 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 effect͒. The Faraday ellipticity and Faraday rotation angle are obtained for the directly transmitted pulse ͑first terahertz pulse͒ and the pulse reflected twice at the sample surfaces ͑second terahertz pulse͒ 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.