Coherent phonon dynamics in semiconductors

Ultrafast pump-probe spectroscopy is a powerful experimental technique to study the light-matter interaction and ultrafast dynamics in solids. In many semiconductors, under ultrafast laser irradiation, phonons (quantized lattice vibrations) with both temporal and spatial coherence can be generated conveniently. When a stronger laser pulse excites coherent phonons that induce refractive index change, and thus the reflectivity change of the materials, the time-dependent phonon dynamics can be detected by a delayed probe pulse. The generation and detection of coherent phonons provide an opportunity to understand the fundamental physics between light and matter interaction, as well as a path to manipulate other physical processes, for applications such as sound amplification stimulated emission (SASER), phonon mode manipulation, ultrafast phase switching, superconductivity enhancement and manipulation of magnetism¹⁻⁵. This thesis presents a series of time-resolved studies of coherent phonons in three semiconductor systems, including bulk CdSe, Bi₂Te₃/Sb₂Te₃ superlattice and GaAs/AlAs superlattice. In bulk CdSe, a material extensively studied for quantum dot photoelectronics, coherent phonons serve as the probe for the reversible ultrafast melting. In Bi₂Te₃/Sb₂Te₃ superlattice, a material system used for thermoelectrics, the coherent thermal phonons are excited directly and are found to be selectively filtered in the superlattice structure compared with bulk materials. In GaAs/AlAs superlattice, a quantum well structure for photodetectors and lasers, a strong quantum coherent coupling among different phonon modes is observed. A similar coherent coupling between photons and phonons has been used to induce and enhance superconductivity [superscript 6,7] and mimic the magnetic field⁸. However, direct observation of nonlinear phonon coupling is rare. Moreover, a novel technique based on surface plasmon resonance has been implemented into the pump-probe spectrometer to improve detection efficiency