Terahertz is the name of frequency (10¹² Hz), just between infrared and microwave in the electro-magnetic wave, which has been called "Far Infrared" in past.  From the viewpoint of molecular spectroscopy, a lot of information about molecular structure and molecular motion can be obtained from the spectra in this region, such as rotational spectra of light molecules, lower vibrational spectra carbon chaines, the internal rotational motion of molecules and intermolecular vibrations.

2.THz-TDS spectrometer
In our lab. we are focusing on the molecular spectroscopy by a recently developed Terahertz Time Domain Spectroscopy (THz-TDS).  Since THz-TDS is more sensitive and more stable than conventional FT-IR spectrometer, we expect to make more accurate measurement by using our system.
In THz-TDS system, transient change of the electric field made by a femto second laser (pulse width = ~100 fs) is used as a THz wave source, and the change of the pulse shape after passing through the sample is detected.  In our system, the production and extinction of carriers on the biased photo-conductive antenna make transient change of the electric field, and the time profile of the electric field is detected on by a probe pulse with changing the detecting timing which was scanned by a optical delay stage.  
The obtained waveform of the electric field is then Fourier transformed to the frequency domain power spectrum.  The spectra below show the example of the THz time domain waveform with and without sample (H₂O 500 Pa).  Due to the coherent rotational motion of the sample, the time domain wave form shows structure after main pulse, called "free induction decay (FID)".  After Fourier Transform, these FID are transformed to the absorption lines in the frequency domain spectra.
The table below shows comparison of conventional spectroscopic apparatus with THz-TDS.  Compared to the continuous wave light sources such as Tu-FIR and BWOs, THz-TDS has no advantage for high resolution spectroscopy.  However, for the spectroscopy in lower resolution, it can make more accurate measurement of line shape and line intensity in better S/N ratio than FT-IR.

	CW　laser, BWO	FT-IR	THz-TDS
周波数分解能	~MHz	~100 MHz	1GHz
測定範囲	~10 THz	~100 THz以上	~4.0 THｚ
光源のパワー	mW ~ nW	mW	mW
検出器	Si Bolometer / InSb Detector　(4K)		EO/ 光伝導アンテナ (300K)
NEP@3THz	10-9 ~10-10 W･Hz1/2		~10-16 W･Hz1/2
ダイナミックレンジ		~103	~108
S/N		○	◎
光源の安定性		○	◎
変調した検出	◎	△	△
実験の容易さ	×	○	◎

3. Pressure broadening parameters of rotational lines of water vapor

As one of the application of THz-TDS for molecular spectroscopy, we measured precise pressure broadening parameters of water vapor.