Time-domain Brillouin scattering is the workhorse of picosecond ultrasonics. Typically the reflectivity change of a sample with a variable delay tau after the optical pump pulse is detected at a single probe wavelength. The probe wavelength and the reflection angle define the probed acoustic phonon wavevector. Due to the presence of propagating acoustic phonons inside the sample, the transient reflectivity is modulated at the oscillation frequency of the phonon corresponding to the probed wavevector.
We apply an advanced version of this technique using broadband probing. Here, an ultrashort white-light continuum pulse is reflected by the laser-excited sample and a spectrometer simulaneously detects the reflectivity changes over a broad spectral range spanning the entire visible and the beginning near-infrared regions. This allows measuring the amplitude spectrum and frequencies of phonons simultaneously for a continuous set of phonon wave vectors.
The example shows broadband time-domain Brillouin scattering in SrTiO3 after photoexciting broadband acoustic phonon wavepacket using a metallic SrRuO3 transducer with (a) low and (b) high excitation fluence (left panel). Each probe wavelength is sensitive to a specific phonon wavevector which is indicated on the right-side vertical axis. In the high-fluence regime, the complex time-dependence of the oscillation amplitudes encode variations of the phonon wavepacket spectrum as it propagates. These changes are due to the nonlinear propagation of the high-amplitude acoustic phonon wavepacket. The experimental broadband Brillouin scattering data allows the determination of the time-dependen shape changes of the phonon wavepacket as it moves through the SrTiO3 crystal (right panel).
Bojahr et al., Phys. Rev. B 86, 144306 (2012).