Coherent phonon spectroscopy for materials science

Coherent phonon spectroscopy for materials science

Dr. Kunie Ishioka
National Institute for Materials Science, Tsukuba, Japan

Coherent phonon spectroscopy is an experimental scheme to investigate lattice dynamics of solids, where an ultrashort light pulse excites spatially in-phase optical phonons and another pulse monitors their temporal evolution as a periodic modulation of an observable quantity on femto- to nanosecond time scales. In this talk I will present my recent research results on the coherent optical phonons and their interaction with photoexcited carriers by pump-probe reflectivity measurements.

The first part targets high-temperature superconductor YBa₂Cu₃O_7-δ, whose Raman-active phonons include five A_g-symmetry modes. By performing transient reflectivity and spontaneous measurements on the same sample at room temperature we found that the relative amplitudes of the coherent Cu(2) and in-phase O(2,3) modes were apparently enhanced compared to their relative Raman intensities. The results reveal that these two modes were coupled particularly strongly with the intraband transition within the CuO₂ induced by the near infrared light pulse [1].

The second part of the talk presents on the behavior of the shear (E_g-symmetry) phonons of elementary crystal bismuth under intense photoexcitation. This phonon mode has been much less explored compared to the internal displacement (A_1g) phonons of bismuth, which have been extensively studied as a benchmark of state-of-the-art experimental techniques. With increasing excitation density we found that the Eg phonon amplitude reached a maximum and turned to a decrease at a lower density than the A_1g phonon amplitude reached saturation. This peculiar behavior can be understood as a result of strong dynamic coupling of the E_g mode with the large-amplitude A_1g phonon [2]. These two examples showcase the usefulness of the coherent phonon spectroscopy in the understanding of ultrafast dynamics of electron-phonon and phonon-phonon couplings in a variety of solid materials.

[1] K. Ishioka et al., Phys. Rev. B 107, 184302 (2023).External link
[2] K. Ishioka and O.V. Misochko, arXiv: 2403. 10046 [cond-mat.mrtl-sci].External link