Picosecond acoustics in vegetal cells: non-invasive in vitro measurements at a sub-cell scale.

A 100 fs laser pulse passes through a single transparent cell and is absorbed at the surface of a metallic substrate. Picosecond acoustic waves are generated and propagate through the cell in contact with the metal. Interaction of the high frequency acoustic pulse with a probe laser light gives rise to Brillouin oscillations. The measurements are thus made with lasers for both the opto-acoustic generation and the acousto-optic detection, and acoustic frequencies as high as 11 GHz can be detected, as reported in this paper. The technique offers perspectives for single cell imaging. The in-plane resolution is limited by the pump and probe spot sizes, i.e. approximately 1 microm, and the in-depth resolution is provided by the acoustic frequencies, typically in the GHz range. The effect of the technique on cell safety is discussed. Experiments achieved in vegetal cells illustrate the reproducibility and sensitivity of the measurements. The acoustic responses of cell organelles are significantly different. The results support the potentialities of the hypersonic non-invasive technique in the fields of bio-engineering and medicine.