AG Wolf - Terahertz Spectroscopy - Einleitung

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THz Spectroscopy
 Introduction to THz Spectroscopy

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Prof. Dr. Martin Wolf
Abt. Physikalische Chemie
Fritz-Haber Institut der MPG
Faradayweg 4-6
14195 Berlin, Germany
phone:+49 30 8413-5111
Fax: +49 30 8413-5106

hitherto (2010):
Department of Physics
Free University Berlin
Arnimallee 14
D - 14195 Berlin


Far-infrared radiation with wavelengths of several 10 µm can excite very soft vibrational modes of molecules adsorbed on a solid surface. The respective vibrational frquencies of these so-called external modes (in contrast to internal intramolecular vibrations) fall into the terahertz range.
1 THz = 10 12 Hz correponds to a quantum energy of 4.1 meV.

In THz spectroscopy with ultrashort laser pulses, these long wavelengths are generated by optical rectification: The various frequency components of a 100-nm broad femtosecond pulse are mixed in a nonlinear crystal, which by difference frequency generation results in pulsed electromagnetic radiation of only few THz up to several 10’s of THz. These pulses are detected by electro-optical sampling, in which an additional pulse, a gate or sampling pulse, is overlaid with the THz pulse. The induced birefringence due to the THz electric field causes a change of the gate pulse’s polarization. By scanning the variable time delay between gate and THz pulse, the THz electric field can be retrieved (Fig.1), thus full amplitude and phase information is available. Fourier transformation then yields the respective THz spectrum (Fig. 1). Since measurements are performed in the time domain, this technique is often called THz time domain spectrocopy (TDS).

Fig. 1: Electric field of the THz pulse as measured by the EO sampling technique (pulse duration ~100 fs) with the correponding THz spectrum ranging from 8 - 30 THz
                    E-Field THz              THz Spec Fig1                  

In addition, time-resolved experiments are possible by exciting the sample with a VIS pump pulse and subsequently probing the evolving dynamics of low-energy excitations by the THz probe. An overview of a typical experimental setup for such time-resolved THz spectroscopy is given below in Fig. 2.
Fig. 2: Nonlinear crystal for THz generation: GaSe, detection crystal for electro-optical (EO) sampling: ZnTe, frequency range: 8 - 30 THz = 38 - 10 µm = 33 - 125 meV.
THz Generation

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