M.Hangyo,M.Tani,and T.Nagashima

TERAHERTZ TIME-DOMAIN SPECTROSCOPY OF SOLIDS:

A REVIEW

International Journal of Infrared and Millimeter Waves,Vol. 26, No. 12,December 2005. p.1661

Ashida Lab.Onishi Yohei

ContentsTerahertz regionTerahertz time-domain spectroscopySemiconductorDerivation of complex refractive indexBiomolecule Application to securitySuperconductor(my research)Summary

Terahertz region frequency region between 0.1 and 10THz Includes information on a lot of physical

properties of materials

0.1THz 1THz 10THz 100THz 1PHz

THz region0.1 ～ 10THz

3mm 300m 30m 3m 300nmWavelength

Frequency

FIR MIR NIR

4.1meV

Terahertz time-domain spectroscopy (THz-TDS)

enables us to observe a waveform ”E(t)” directly Information on both amplitude and phase are directly obtained.

Pump beam

Delay stage

Probe beam

THz-TDS

fs pulse laser

rE~ sE

~THz emitter

THz detectorsample

Terahertz time-domain spectroscopy (THz-TDS)

)()(

)(2

1)(

~

eE

dtetEE ti

phaseE

amplitudeEE

　 ･･･

　 ･･･

))(~

arg(

)(~

)(

Figure shows wave form and power spectrum of THz wave.

Fourier transformation

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

Semiconductor(Si)

Peak delay Amplitude decrease

Amplitude increase

100K

Figure shows the temperature dependence of the wave form transmitted through the sample.

Derivation of complex refractive index

)()()(~index refractivecomplex inn ：

d

Sample

inn )(~)(tEreference )(tEsample

Transmission Fresnel constants

)(exp)()(~ rrr EE

)(exp)()(~ sss EE

Complex transmittance

Complex refractive index

Fourier transformed spectrum

phase modulation

change of amplitude(absorption)

Semiconductor(Si)

)1()(,)(

)()()(:tyconductivi

10220

2

ielectricalcomplex 　　

　

：

nn

i

2)(,)(

)()()(~constant dielectriccomplex

222

1

21

Semiconductors(Si)

The real and imaginary parts of the complex conductivity fit to Drude model.

Drude model

THz-TDS is very effective to characterize the electrical properties of semiconductors without electrical contact.

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

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 .

Biomolecules

minimum

maximum THz spectra can determine the compositional ratio of D-and L-alanine in a mixture.

THz-TDS is a new method of the chemical analysis for molecules.

measure the THz spectra for various composition ratios Figure shows the dependence of the absorbance of the

74 and 41.5 bands.1cm

Application to security

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

THz wave

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

Application to security Figure 18 shows the absorption spectra of a C-4 pellet

concealed in an envelope.

The most prominent peak(27.3 )1cm

signature of C-4

Superconductors(my research)

6

7

8

91

2

3

4

5

6

7

8

910

Ampl

itud

e(a.

u.)

3. 53. 02. 52. 01. 51. 00. 50. 0

Frequency(THz)

11K15K20K25K30K40K

0. 001

0. 01

0. 1

1

10

Ampl

itud

e(a.

u.)

14121086420

Frequency(THz)

11K15K20K25K30K40K

Tc ＝ 28K

Fourier transformed spectra of a 50nm thin film MgB2 on the Sapphire at 11-40K

I would like to observe the higher superconducting energy gaps

highest critical temperature (Tc) of 39K among conventional metal-based superconductors.

two superconducting gaps The higher energy gap is not observed by THz-TDS.

MgB2

SummaryTHz-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.