Controllable Coherent Transverse Acoustic Phonon Generation in Jahn-Teller active spinel FeCr₂O₄

Light-generation of coherent acoustic phonons by ultrafast laser has drawn enormous attention due to its wide spectrum of applications. Among most of the reported studies, the dominant generated coherent strain is propagating at longitudinal acoustic (LA) speed rather than at transverse acoustic (TA) speed, limiting potential applications of this technique. Utilizing FeCr₂O_4, a magnetic material with spinel structure and Jahn-Teller effect, we observed coherent strain propagating at both LA and TA speed. Comparing different excitations, we find the use of on-site d-d transition from Jahn-teller active ions (Fe²⁺) is a key to generating the TA strain. Furthermore, we demonstrate a precise control of vibrational phase by changing a relative angle between a linear pump polarization and the Jahn-teller distortion, which cannot be achieved via a coherent LA strain generation. The phase control remains robust even under strong magnetic fields, opening up a new opportunity for developing high precision optoacoustic applications.