Study on carrier dynamics of transition metal oxides using broadband infrared pump-probe spectroscopy

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1. We investigated ultrabroadband detection of terahertz electric field with a dipole-type photoconductive (PC) antenna with 10 fs pulse laser. Ultrabroadband frequency response from 0.1 THz up to 100 THz was demonstrated using two types of generation methods: optical rectification using a thin GaSe crystal and dipole radiation using a PC antenna with a large aperture. Comparison between the spectrum obtained with the PC antenna and the calibrated one with a combination of a monochromator and an MCT detector suggests that PC antenna detection has potential for detecting a frequency region much higher than 100THz.2. We utilized rapid scanning method to ultrabroadband terahertz detection using photoconductive antennas. Measurement time was reduced to one tenth of the conventional method. Ultrabroadband terahertz spectrum from 0.1 to 15 THz was observed. This technique provides significant improvement in THz timedomain spectroscopy.3. We measured transmittance of SrTiO_3 thin films deposited on MgO in broadband THz region from 0.1 THz to 10THz. Then we calculated the complex dielectric constant of the thin films directly from amplitude and phase of the transmittance. At room temperature, broad soft-mode (TO1) dispersion is observed around 3 THz and TO2 mode at 5 THz. The resonant frequencies of both modes are almost independent of the thickness at room temperature. On the other hand, at cryogenic temperature (15K), dielectric dispersion is strongly dependent on the thickness. The previously reported softening of the SrTiO_3 thin films deposited on SrRuO_3 substrate is similar to the obtained one. This fact indicates the existence of the intrinsic size effect in SrTiO_3 thin films, since the softening is independent of the substrate. However, the broadening is only observed in extremely thin films deposited on MgO, suggesting that the strain from the substrate strongly affects the width of the soft mode dispersion.