Uniform TitleUltrafast spectroscopy of semiconducting and multiferroic materials
NameLou, Shitao (author), Zimmermann, Frank (chair), Bartynski, Robert (co-chair), David Vanderbilt, David (internal member), Murnick, Daniel (internal member), Mitrofanov, Oleg (outside member), Rutgers University, Graduate School - New Brunswick,
SubjectPhysics and Astronomy,
DescriptionIn this thesis, we have used ultrafast spectroscopy to study the optical properties of two semiconductors, GaAs and Ge, and one hexagonal multiferroic material, LuMnO3. Both semiconductor and multiferroic materials are of great importance technologically and economically. By using ultrafast spectroscopy, we obtained time resolved electron and phonon dynamics directly, which is unavailable by conventional optical methods.
Electron-hole pairs, coherent phonon oscillations and an optical coherence response are excited when femtosecond laser pulses interact with either GaAs or Ge crystals. The coherent phonon mode excited in GaAs/Ge is of T2/T2g symmetry as determined by probe beam polarization analysis. The pump polarization dependence of the phonon oscillation is consistent with the transient stimulated Raman scattering (TSRS) mechanism. From the pump polarization dependence of the phonon oscillation, we have identified two excitation mechanisms contributing to the coherent phonon in GaAs,one is consistent with TSRS, and the other is consistent with screening effect of photoexcited electrons.
The femtosecond laser pulse, with 800 nm center wavelength and polarized perpendicular to the c axis of LuMnO3, excites a narrow intra-atomic dxy, x2-y2 to d3z2-r2 transition in Mn. This excitation results in a transient reflectivity change for light of the same wavelength and polarization, by partial saturation of the transition. The relaxation time of this electronic excitation is about 1 ps. Furthermore, the electronic excitation resonantly excites a coherent optical phonon with A1 symmetry (TO: 118 cm-1 and LO: 120 cm-1), involving Lu ions motion along the c-axis, which is identified to be the soft mode driving the ferroelectric transition. A remarkable reversal of the sign of the oscillation amplitude ([pi sign] phase shift) of the reflectivity curve was observed upon comparing longitudinal optical (LO) with transverse optical (TO) mode geometries. The phase reversal is attributed to the macroscopic electric depolarization field accompanying IR active longitudinal phonon modes, but absent in TO modes, or to coupling of LO phonon coordinate to a change in ferroelectric polarization upon to excitation via a macroscopic electric field.
NoteIncludes bibliographical references (p. 103-112).
CollectionGraduate School - New Brunswick Electronic Theses and Dissertations
Organization NameRutgers, The State University of New Jersey
RightsThe author owns the copyright to this work.