Quantum Coherence in Electron–Phonon Coupled States in a Semiconductor Crystal

This chapter describes the quantum coherence (electronic and phononic coherences) in a bulk solid investigated via ultrafast quantum-path interferometry for coherent optical phonons. A simple quantum mechanical theory for the generation of coherent phonons is presented using a model consisting of two electronic states and a harmonic oscillator. The evolution of the electron–phonon coupled state is calculated using the quantum Lindblad type master equation. An ultrafast quantum–path interferometry experiment on an n-type gallium arsenide crystal is presented. The coherent phonon oscillation decays exponentially and its decoherence time is simply obtained, e.g., approximately 4.0 ps at 90 K for the longitudinal optical phonon. The interference pattern obtained via relative phase-locked double pulse excitation shows fast and slow fringes corresponding to electronic and phononic interference. The electronic interference pattern shows an apparent collapse and revival feature, and is well represented by theoretical calculations. The electronic decoherence time in the n-type gallium arsenide crystal is determined quantitatively from experimental data obtained via polarized pump pulses with a relative angle of \(\pi /4\) , e.g., 23 fs at 90 K. The electronic decoherence is explained by the scattering of photo–excited electrons with surrounding doped and photo–excited electrons.