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

1. 研究了用10 fs脉冲激光偶极子型光导（PC）天线对太赫兹电场的超宽带探测。使用两种类型的产生方法演示了从0.1太赫兹到100太赫兹的超宽带频率响应：使用薄GaSe晶体的光学整流和使用具有大孔径的PC天线的偶极子辐射。将PC天线与单色仪和MCT检测器组合校准的频谱进行比较，表明PC天线检测具有检测远高于100thz的频率区域的潜力。利用光导天线对超宽带太赫兹进行快速扫描检测。测量时间缩短到传统方法的十分之一。观测到0.1 ~ 15thz的超宽带太赫兹频谱。该技术对太赫兹时域谱有显著改善。我们测量了沉积在MgO上的SrTiO_3薄膜在0.1 ~ 10THz波段的透射率。然后直接从透射率的幅值和相位计算出薄膜的复介电常数。在室温下，宽软模式（TO1）色散在3太赫兹左右被观察到，TO2模式在5太赫兹被观察到。在室温下，两种模式的谐振频率几乎与厚度无关。另一方面，在低温（15K）下，介电色散强烈依赖于厚度。前人报道的SrTiO_3薄膜在SrRuO_3衬底上的软化与实验结果相似。这一事实表明SrTiO_3薄膜中存在固有的尺寸效应，因为软化与衬底无关。然而，仅在沉积在MgO上的极薄薄膜中观察到展宽，这表明来自衬底的应变强烈影响软模色散的宽度。

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.