Ultrafast non-thermal laser excitation of gigahertz longitudinal and shear acoustic waves in spin-crossover molecular crystals [$Fe(PM-AzA)_{2}$$(NCS)_{2}$]

TL;DR

This study demonstrates ultrafast, non-thermal laser excitation of gigahertz acoustic waves in spin-crossover molecular crystals, revealing a link between spin states and phonon excitation with potential for advanced phononic control.

Contribution

It provides the first experimental evidence of non-thermal laser excitation of GHz acoustic phonons linked to spin state transitions in molecular crystals.

Findings

Observation of GHz longitudinal and shear phonons via femtosecond laser pulses

Identification of a non-thermal pathway for phonon excitation during spin crossover

Insights into optical and mechanical property changes across the spin transition

Abstract

We report GHz longitudinal as well as shear acoustic phonons photoexcitation and photodetection using femtosecond laser pulses in a spin-crossover molecular crystal. From our experimental observation of time domain Brillouin scattering triggered by the photoexcitation of acoustic waves across the low-spin (LS) to high-spin (HS) thermal crossover, we reveal a link between molecular spin state and photoexcitation of coherent GHz acoustic phonons. In particular, we experimentally evidence an unconventional non-thermal pathway for the laser excitation of GHz phonons. We also provide experimental insight on the optical and mechanical parameters evolution across the LS/HS spin crossover temperature T$_{1/2}$.