Ultrafast Spectroscopic Studies of Ferroelectric BaTiO3 Thin-Films on SrTiO3

In this masters thesis ultrafast spectroscopic studies have been performed on single-crystal 111 strontium titanate, SrTiO3, and pulsed-laser deposited (PLD) thin-films of ferroelectric barium titante, BaTiO3. This has been achieved by means of optical Kerr effect spectroscopy (OKE) and terahertz time-domain spectroscopy (THz-TDS). Measurements of SrTiO3 in transmission geometry by THz-TDS, using THz-pulses consisting of a typical frequency band of 0.2-6 THz, resulted in very weak detectable signals, which resulted in the need for a systematic study to determine the various contributing factors to signal loss. It has been found that the low transmission of the terahertz pulse in the 500um thick SrTiO3 probably is due to high absorption or reflection by the sample, possibly due to the reported phonon modes lying in this spectral region. This showed that using SrTiO3 as a substrate material did not enable any reliable measurements for the BaTiO3 thin-films, implying a need for another use of THz-TDS to enable observations of absorption bands in STO and BTO. A series of measurements using the optical Kerr effect spectroscopy have been performed on various samples giving promising results. The measured OKE signals of STO and BTO provided interesting information regarding the relaxational dynamics of the system and unexpected observations in the time-domain signals motivates further study of both $ SrTiO3 and BaTiO3 using this spectroscopic method. To enable comparisons with other studies acetone has been analysed. From the measured OKE transient, the Raman spectra have been retrieved. For the OKE measurements of 111 SrTiO3 samples a clear peak can be seen in a region which suggests this to be an observation of the transverse-optic TO1 phonon mode. Measurements of BaTiO3 thin-films in OKE shows promising peaks which could be caused by a combination mode of the known E and A1 phonon modes which can be found in BaTiO3. As the post-processing of the measured OKE signal which is needed enable extraction of the Raman spectrum was found to be greatly affected by various parameters, further studies would need to be performed in order to determine exact positions, and confirm the physical nature of these measurement