Excitation and Time‐Evolution of Coherent Optical Phonons

Abstract

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. Coherent Phonons in Group V SemimetalsBismuth (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=R¼ 10 À6 to 10 À2 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.

Ultrafast coupling of coherent phonons with a nonequilibrium electron-hole plasma in GaAs

“…Using these values for the electron and hole diffusion constants and assuming the electron mobility is 1 to 10 times greater than the hole mobility leads to an estimate of the ambipolar diffusion constant in the range from 15 Transient reflectivity experiments are extremely sensitive to changes in the carrier density near the surface of the sample. The surface carrier density can be determined from Eq.…”

“…They have been employed for characterizing the electron and hole dynamics in pump-probe transmission and reflectivity, time resolved photoluminescence, THz spectroscopy, fourwave mixing, and time-resolved two-photon photoemission experiments [4][5][6][7][8][9][10][11]. Femtosecond laser pulses have also been used to excite and observe the coherent q = 0 optical phonon (coherent phonon) dynamics in a broad range of materials including metals, semimetals, and semiconductors [3,8,[12][13][14][15][16][17][18]. Impurity or photo-doping of semiconductors introduces free carrier plasmas in the valence and conduction bands with frequencies typically in the THz range, where it can couple with the optical phonons of comparable frequency.…”

Coherent photo-induced phonon emission in the charge-density-wave state of K0.3MoO3

“…in the absence of a THz probe pulse) is measured. The generation of coherent phonons (typically with frequencies in the THz range) by sub-picosecond optical excitation is a well-established technique [15][16][17][18][19], especially for Raman-active modes (probed by coherent oscillations in the inter-band permittivity), mostly in conventional semiconductors and semimetals [20], but also for more complex inorganic crystals [21,22]. In comparison, studies of phonon emission from IR-active modes are significantly fewer [23][24][25][26], and only recently have such studies been extended to more complex solids [27][28][29].…”