Controlling and Observing Coherent Phonons in Thin van der Waals Materials

In my talk, I will cover recent experimental observations on coherent acoustic and optical phonons. We observe coherent acoustic phonons in the breathing mode of layered van der Waals materials such as graphite. There, we introduced a control scheme to selectively excite harmonic frequencies of this mode. In addition, I will show coherent breathing mode phonons in a heterostructure composed of graphite and hexagonal boron nitride. For the coherent optical phonons, I will focus on the shear mode of graphite. We have investigated how linearly polarized laser pulses induce an effective polarization in the degenerate phonon mode. With this knowledge, we could create and observe elliptically polarized coherent shear phonons with a double-pulse excitation scheme. This scheme should generally apply to induce elliptically or circularly polarized coherent phonons in various crystals.