Terahertz Applications by THz Time Domain Spectroscopy

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

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

3. 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.

4. THz Spectroscopy SystemFourier 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.

5. E THz wave t Probe laser t Time delay THz Spectroscopy SystemTime 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. THz-TDS pump probe system Principle of THz-TDS

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

7. THz Detector Photoconductive antenna EO-sampling 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

8. Electro-optic sampling (EO-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

9. 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

10. 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. 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 Appl. Phys. Lett. 85, 3360 (2004)

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

12. THz Applications 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.

13. 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.

14. 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.