TERAHERTZ TIME-DOMAIN SPECTROSCOPY OF SOLIDS: A REVIEW

1. TERAHERTZ TIME-DOMAIN SPECTROSCOPY OF SOLIDS:A REVIEW M.Hangyo,M.Tani,and T.Nagashima International Journal of Infrared and Millimeter Waves,Vol. 26, No. 12,December 2005. p.1661 Ashida Lab. Onishi Yohei

2. Contents • Terahertz region • Terahertz time-domain spectroscopy • Semiconductor • Derivation of complex refractive index • Biomolecule • Application to security • Superconductor(my research) • Summary

3. Wavelength 3mm 300mm 30mm 3mm 300nm FIR MIR NIR THz region 0.1～10THz 0.1THz 1THz 10THz 100THz 1PHz Frequency Terahertz region • frequency region between 0.1 and 10THz • Includes information on a lot of physical properties of materials 4.1meV

4. Terahertz time-domain spectroscopy (THz-TDS) • enables us to observe a waveform ”E(t)” directly • Information on both amplitude and phase are directly obtained. Probe beam fs pulse laser Pump beam sample THz detector Delaystage THz emitter THz-TDS

5. Terahertz time-domain spectroscopy (THz-TDS) Fourier transformation Figure shows wave form and power spectrum of THz wave.

6. Application of THz-TDS to solids • various kinds of low energy excitations in various materials

7. Semiconductor(Si) • Figure shows the temperature dependence of the wave form transmitted through the sample. Peak delay Amplitude decrease 100K Amplitude increase

8. Derivation of complex refractive index Fourier transformed spectrum Transmission Fresnel constants Complex transmittance Sample d Complex refractive index phase modulation change of amplitude (absorption)

9. Semiconductor(Si)

10. Semiconductors(Si) Drude model • The real and imaginary parts of the complex conductivity fit to Drude model. N:density of carriers :scattering time of carriers :effective mass of carriers • The temperature dependence of the carrier density and mobility is obtained. carrier mobility • THz-TDS is very effective to characterize the electrical properties of semiconductors without electrical contact.

11. Biomolecules • Biomolecules have relatively sharp spectral lines in THz region. • Alanine has optical isomers, D and L. • Figure shows the spectra of D-,L- ,and 1:1 mixture of D and L alanine. • The enantiomer D(L) and racemic compound show quit different spectra with each other .

12. Biomolecules • measure the THz spectra for various composition ratios • Figure shows the dependence of the absorbance of the 74 and 41.5 bands. • THz spectra can determine the compositional ratio of D-and L-alanine in a mixture. maximum minimum THz-TDS is a new method of the chemical analysis for molecules.

13. Application to security THz wave • penetrates papers, plastics, ceramics, woods, and so on • useful to probe unknown materials concealed in containers made of these materials • It is possible detect plastic bombs in mails by the THz-TDS

14. Application to security • Figure shows the refractive index and absorption spectra of a plastic bomb called C-4. The most prominent peak signature of C-4

15. Application to security • Figure 18 shows the absorption spectra of a C-4 pellet concealed in an envelope. The most prominent peak(27.3 ) signature of C-4

16. Superconductors(my research) MgB2 • highest critical temperature (Tc) of 39K among conventional metal-based superconductors. • two superconducting gaps • The higher energy gap is not observed by THz-TDS. I would like to observe the higher superconducting energy gaps Fourier transformed spectra of a 50nm thin film MgB2 on the Sapphire at 11-40K Tc＝28K

17. Summary • THz-TDS is very effective to study properties of semiconductors, superconductors, and biomolecule. • THz spectroscopy become one of the most exciting fields in the basic science and technology, and also expected to be useful in industries.