Coherent optical phonons are the lattice vibrations of the optical branches excited in-phase over a macroscopic spatial region by ultrashort laser pulses. Their generation and relaxation dynamics have been investigated by optical pump-probe techniques in a broad range of solid materials. When probed with a linear optical process such as reflection, the probing depth is from nanometers to micrometers determined by the absorption coefficient and the laser wavelength. By employing a nonlinear optical process such as second harmonic generation, however, it is possible to monitor coherent phonons at surfaces and interfaces exclusively. When the photoexcitation creates electron-hole plasma, coherent phonons can also serve as an ultrafast probe of nonequilibrium electron-phonon coupling. Using an optical pulse train with a designed repetition rate, coherent phonons can be excited to large amplitudes, which is expected to be a promising strategy toward nonthermal phase transitions in solids. New femtosecond beam sources such as X-ray and THz pulses offer complementary techniques to probe the ultrafast phononic and electronic dynamics under extremely nonequilibrium conditions.
10.1 Coherent Phonons in Group V Semimetals
Bismuth (Bi) and antimony (Sb) have been model systems for optical studies on coherent phonons because of their relatively low frequencies and large amplitudes in the optical reflectivity. They both have an A7 crystalline structure and sustain two Raman-active optical phonon modes shown in Figure 10.1e. The coherent phonons of A 1g and E g symmetries have been detected as periodic modulations of the reflectivity (Figure 10.1a) in the order of DR=RΒΌ 10 Γ6 to 10 Γ2 depending on the excitation density. Recently, the coherent A 1g phonons of Bi have gained a renewed interest as the benchmark for femtosecond X-ray diffraction experiments.