We present a free-running 80-MHz polarization-multiplexed solid-state dual-comb laser which delivers 1.8 Watts of average power with 110-fs pulse duration per comb. We apply this free-running dual-comb laser to picosecond ultrasonic measurements via a high-sensitivity pump-probe setup. We demonstrate ultrasonic measurements on thin-film samples, and compare our measurements to ones obtained with a pair of locked femtosecond lasers and x-ray diffraction measurements. Our data show that a free-running dual-comb laser is well-suited for picosecond ultrasonic measurements and thus it offers significant reduction in complexity and cost for this widely adopted non-destructive testing technique.
We use femtosecond laser pulses absorbed in a metallic transducer, namely the picosecond ultrasonics technique, for the
remote optical generation and detection of GHz acoustic frequencies in single cells by pump-probe sampling. Samples
are MC3T3 cells adhering on a TiAl4V alloy substrate. Both pump and probe beams are focused at the cell/transducer
interface. The pump absorption yields a temperature rise in the absorbing substrate and a picosecond acoustic pulse is
generated through the thermoelastic effect. The probe beam is partially reflected from the metallic interface and partially
scattered by the acoustic wavefront propagating in the transparent cell. The change of reflectivity of the cell is measured
as a function of the pump-probe time delay. Interferences arise from the two probe contributions causing the so-called
Brillouin oscillations. Optical phase variations due to acoustic-induced changes in cell thickness are simultaneously
measured. The result of a semi-analytical calculation is fitted to the experimental data. Acoustic frequencies are detected
at 30 GHz in the nucleus of single osteoblast cells.
Transient reflectivity pump-probe experiments are performed on a single gold particle to analyze the vibration
of the particle as well as the propagation of the resulting GHz acoustic wave in the embedding medium. In a
first part, the vibration of a single 430 nm diameter gold particle embedded in a silica matrix is investigated.
A semi-analytical model is presented and demonstrates that the detection mechanism relies on an intrinsic
common-path interferometer which directly images the particle interface displacement. The coherent phonon
propagation inside the embedding medium is then studied in the case of a gold particle embedded in an
agarose gel. A comparison between experimental results and calculations suggests a detection of the Brillouin
scattering in agarose, so long as the Brillouin frequency at the considered probe wavelength matches the
fundamental breathing mode frequency (or one of its harmonics) of the particle.
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