Ultrafast and Incoherent Raman Studies of Optical Phonons of Different Symmetries in Sb, Bi, Bi2Te3 and Bi2Se3.

Abstract

Illumination with coherent light sources leads not only to a population of photoexcited carriers, but to the generation of coherent electronic states of particular symmetries which can drive vibrations of the same symmetry. In this thesis, I use a combination of temperature dependent ultrafast stimulated Raman scattering (ISRS) and CW spontaneous Raman scattering (RS) to determine the lifetime of electronic coherences of A1g and Eg symmetry in Bi, Sb, Bi2Te3 and Bi2Se3. It is shown that the coherent A1g and Eg modes have different temperature dependent behaviors, and the ratio between the initial amplitudes of the two modes increases with temperature, different from the results observed in RS. An extremely short-lived Eg electronic coherence with lifetime around 10fs is obtained, a timescale much shorter than the laser pulse. The fragile Eg electronic coherence, resulting from an unequal distribution of carriers in equivalent k points in the Brillouin zone, is quickly destroyed by the fast redistribution of carriers. In Bi2Te3 and Bi2Se3, the mechanism for the generation of coherent phonons in opaque materials is further investigated by comparing two modes of A1g symmetry. It is validated that the driving forces for the two A1g modes have similar lifetimes, both much longer than the periods of coherent phonons. Phenomena under high laser fluences have become a popular topic in recent years due to the capability of high fluence radiation to produce high density plasma and photo-induced transient phase transitions. Coherent phonons of A1g and Eg symmetries in Sb are studied at high laser fluences. Both modes demonstrate chirped frequencies, nonlinear growth of amplitudes and different fluence dependent behaviors of decay rates. These experimental data provide useful information for future theoretical works. Bi2Te3 and Bi2Se3 have been receiving increasing interest recently due to their peculiar property as topological insulators. Attempts are made to fabricate and characterize ultrathin films of topological insulator Bi2Te3 with mechanical exfoliation, optical microscopy, AFM and Raman spectroscopy. The thinnest pieces found have thickness around 10nm. An additional peak is observed in the Raman experiments, which may be related to the breaking of symmetry in ultrathin films.