The measurement of picosecond ultrasonic pulses reflected from contacting interfaces using the optical pump and probe technique has recently been demonstrated to be a promising new line of investigation for the characterization of nanoscale mechanical contacts between solids. [1] We here demonstrate the ability of this technique to image non-specific contacts of single biological cells with metallic substrates. We develop new signal analysis tools to perform such studies at various frequencies.
Monocytes are key players in the development of atherosclerosis. We culture these cells on top of a thin biocompatible Ti metal film, supported by a transparent sapphire substrate. Low-energy femtosecond pump laser pulses are focused at the bottom of the Ti film to a micron spot. This light is modulated for lock-in detection. The subsequent ultrafast thermal expansion launches a longitudinal acoustic pulse in Ti, with a broad spectrum extending up to 200 GHz. The absorbed heat simultaneously diffuses on a larger timescale, and partially sinks into the substrate.
The Ti film thickness and the modulation frequency of the pump are chosen so that the thickness remains larger than the thermal diffusion length at the modulation frequency. This ensures that no thermal confinement occurs that could induce a thermal stress to the cell. We monitor the temperature variations at the pump modulation frequency across the contact to validate this assumption.
We also measure the acoustic echoes reflected from the film/cell interface through the transient optical reflectance changes. The time-frequency analysis of the reflected acoustic pulses with a wavelet transform gives access to a map of the stiffness of the film-cell interface at frequencies ranging from 20 to 60 GHz. The existence of a frequency-dependent behavior at some particular points in the cell and its relation to biological processes is discussed.