Terahertz Applications by THz Time Domain Spectroscopy Nature Materials 1, 26 - 33 (2002) Bradley Ferguson and Xi-Cheng Zhang Ashida lab. Iwasa Takuya

Contents About THz THz Spectroscopy System FTS THz-TDS Data of THz wave detection by EO-sampling THz Applications Summary

About THz THz radiation is from 1011 to 1013Hz approximately. THz wave goes through materials such as plastic, paper, ceramic and semiconductor and has low energy. It has been difficult to detect frequencies in this region.

THz Spectroscopy System Fourier Transform Spectroscopy (FTS) In FTS a sample is illuminated with a incoherent broadband thermal source. (ex. an arc lamp) A direct detector such as a helium-cooled bolometer is used to detect the interference signals.

THz Spectroscopy System Time Domain Spectroscopy（THz-TDS） THz-TDS uses short pulses of broadband THz radiation. The transmitted THz electric field is measured coherently, which provides both high sensitivity and time-resolved phase information. t THz wave Probe laser E Time delay THz-TDS pump probe system Principle of THz-TDS

THz Emitter Photoconductive antenna Difference frequency generation Nonlinear crystal Bias voltage fs laser THz wave fs laser Fourier transform THz wave Intensity Intensity J(t): current ω2 ω1 ω３ frequency frequency

THz Detector Photoconductive antenna EO-sampling A Balance detector Bias voltage A Wallaston prism fs laser THz wave λ/4 plate EO crystal Probe laser μ: mobility of electron N: the number of carrier THz wave

Electro-optic sampling (EO-sampling) EO crystal becomes birefringent under the irradiation of THz wave. λ/4 plate change probe laser’s polarization linear into circular. Wallaston prism divides vertical and horizontal polarization. Balance detector Wallaston prism λ/4 plate EO crystal Probe laser THz wave

Data detected by EO-sampling (2002) Frequency spectrum of the THz field. The absorption resonance at 5.3THz is due to phonon modes in the ZnTe crystals. Temporal waveform of a THz pulse. Emitter: 27μm ZnTe Detector: 30μm ZnTe

Data detected by EO-sampling (2004) This data is from “Ultrabroadband detection of multi-terahertz field transients with GaSe electro-optic sensors: Approaching the near infrared” American Institute of Physics 2004 C.Kubler Emitted with 20-μm-thick GaSe and detected with 30-μm-thick GaSe. The amplitude spectrum peaks at 33.8THz and extends from 7 to beyond 120THz. Appl. Phys. Lett. 85, 3360 (2004)

THz Applications Semiconductor and high-temperature superconductor characterization Tomographic imaging Cellular level imaging and chemical and biological sensing

THz Applications An optical image of the test structure. Tomographic imaging An optical image of the test structure. The target was imaged using the tomography system, and the refractive index of each cross-sectional slice was reconstructed. The central slice is shown. The cross-sectional slices are combined to form 3D image.

THz Applications Cellular level imaging and chemical and biological sensing Detected by electro-optic sampling in a 30-μm-thick ZnTe crystal. This spatial resolution is less than 50μm. THz image of an onion cell membrane.

Summary THz-TDS (especially used by EO-sampling) has been developed and observed broader region. It made a lot of applications including tomographic imaging and biological sensing.