Polariton probing of attometre displacement and nanoscale strain in ultrashort acoustic pulses

Atomic displacement and lattice strain are inextricably linked to most ultrafast processes in solids, such as optically induced phase transitions or demagnetization. Visualizing lattice dynamics, which is typically done using time-resolved X-ray and electron diffraction techniques, yields information about the physical processes involved. However, the detection of atomic motion of an amplitude much less than a picometre has remained challenging. For this purpose, we suggest exploiting the acoustic pulse generated by a spatially localized ultrafast process in the surrounding volume. Its optical detection in a material possessing a narrow polariton resonance provides superior sensitivity. In the validating experiment, we detect the acoustic pulse generated by a 100 attometre thermal expansion of a 100 nanometre metallic film heated with a temperature increase of 0.2 kelvin by a femtosecond optical pulse. Even though the generated acoustic pulse carries dynamical strain with a magnitude of only 10^-9, being injected into the polaritonic layer, it can be confidently detected through transient reflectivity.