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

1.利用10 fs脉冲激光，研究了偶极型光电导（PC）天线对太赫兹电场的超宽带探测。超宽带频率响应从0.1太赫兹到100太赫兹的演示使用两种类型的生成方法：使用薄的GaSe晶体的光整流和使用具有大孔径的PC天线的偶极辐射。用PC天线获得的频谱与用单色器和MCT检测器的组合获得的校准频谱之间的比较表明，PC天线检测具有检测远高于100 THz的频率区域的潜力。我们利用快速扫描的方法，利用光导天线进行超宽带太赫兹波探测。测量时间减少到传统方法的十分之一。观测到了0.1 ~ 15 THz的超宽带太赫兹光谱。该技术在太赫兹时域光谱方面提供了显著的改进.测量了MgO基SrTiO_3薄膜在0.1 ~ 10 THz宽带THz波段的透过率。然后由透射率的振幅和相位直接计算出薄膜的复介电常数。在室温下，在3 THz附近观察到宽的软模（TO 1）色散，在5THz观察到TO 2模色散。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（15 K）下，介电色散强烈依赖于厚度。在SrRuO_3衬底上沉积的SrTiO_3薄膜的软化现象与以前报道的相似。这表明SrTiO_3薄膜中存在本征尺寸效应，因为薄膜的软化与衬底无关。然而，仅在沉积在MgO上的极薄的膜中观察到加宽，这表明来自衬底的应变强烈地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.