Picosecond Acoustics

Interferometric picosecond acoustics is a powerful time-domain approach for the study of sound attenuation in amorphous materials. PA is a pump-probe technique in which an ultrashort optical pump pulse is partially absorbed by a thin metallic transducing layer, deposited onto the surface of the sample, and launches by “instantaneous” thermal expansion a longitudinal acoustic wave-packet with a characteristic spectrum extending in the 40–400 GHz region.

The acoustic pulse then travels inside the sample and its motion is monitored by a delayed optical probe pulse with wavelength λpr. The probe pulse is reflected by the strain wave through a (stimulated) Brillouin scattering process, i.e., interacting with a single phonon frequency ν=c*q/(2π)=2n*c*cos(θ)/(λpr), where c is the sound velocity, q the acoustic wave vector, n(λpr) the refractive index of the medium and θ the scattering angle (see figure). The Brillouin scattered intensity interferes with the portion of probe beam reflected by the fixed metallic layer, leading to time-dependent oscillations with a periodicity T set by the Bragg condition: T=λpr/(2n*c*cos(θ)), i.e., corresponding to the period of the phase-matched acoustic phonon. The periodicity and damping of such oscillations then gives direct access to acoustic parameters such as sound velocity and attenuation.

Picosecond acoustics experimental set up. It is based on a regeneratively amplified Ti:sapphire laser producing 150 fs, 500−μJ pulses at 800 nm wavelength and 1 kHz repetition rate. A portion of this beam (the pump, with energy up to 5μJ) is loosely focused on the sample. Another fraction of the pulse is focused in a 2-mm-thick sapphire plate to generate a broadband single-filament white light continuum which, after passing through a delay line, acts as a probe. The visible portion of the continuum, extending from 430 to 760 nm, is also focused on the sample and the reflected light is dispersed on a spectrometer equipped with fast electronics, allowing single-shot recording of the probe spectrum at the full 1-kHz repetition rate. Our setup achieves for each channel sensitivity down to ΔR/R≈10-5, allowing continuous monitoring of the sound parameters (speed and attenuation) over a broad range of frequencies in a single measurement.

We have developped a novel broadband version of interferometric PA, which allows accessing in a single measurement a nearly octave-spanning range of the acoustic frequency wave-packet with unprecedented sampling efficiency. Our system is based on a 1-kHz amplified Ti:sapphire laser and uses a white-light continuum  probe in combination with an optical multichannel analyzer.

We have used our setup to study two samples:

  • A set of thermally grown pure silica (v-SiO2) samples of different thicknesses in the range 70–800 nm, grown on the polished surface of a [111] silicon wafer. A 10-nm-thick Ti transducer layer was then deposited on all samples. The metal/SiO2/Si structure allows extending upwards the accessible acoustic frequency range since, when entering the Si substrate, the phase-matched acoustic frequency becomes approximately five times larger, due to the larger value of sound speed and refractive index compared to silica. Vitreous silica, the prototypical covalently bonded glass former, is a benchmark to study sound damping in disordered materials. We were able to extract the frequency dependence of the sound attenuation in silica, which displays a dynamic crossover around 170 GHz, separating a “low frequency” region  where the sound attenuation follows an ν2 law (in agreement with the predictions of the Akhiezer mechanism) to a “high-frequency” Rayleigh scattering regime when acoustic waves are diffused by uncorrelated pointlike defects. Read more: Pontecorvo, E., Ortolani, M., Polli, D., Ferretti, M., Ruocco, G., Cerullo, G., Scopigno, T. “Visualizing coherent phonon propagation in the 100 GHz range: A broadband picosecond acoustics approach” (2011) Applied Physics Letters, 98 (1), art. no. 011901.
  • SrTiO3(STO) [001] single crystals. STO is a prototype material belonging to the class of transition metal oxides with perovskite structure. They present appealing functional properties including ferromagnetism, ferroelectricity, piezoelectricity, interfacial superconductivity. STO displays a simple cubic perovskite structure at room temperature with a phase transition occurring below 105 K. It is a band gap insulator with indirect band gap of 3.26 eV and large dielectric constant.It is very important to understand the lattice dynamics of STO and the related acoustic properties. Previous measurements of the sound speed in the [001] direction in the range of few tens of megahertz (by ultrasonic measurements) up to 42 GHz (using Brillouin scattering technique) found almost the same value (7900 m/ s), indicating a linear dispersion relation for acoustic phonons.However, a detailed investigation of the same properties at higher frequencies (above a few tens of gigahertz) is still missing. We found that the longitudinal sound velocity is not constant but varies by ≈5% (from 7900 to 7520 m/s) in the investigated phonon wave vector range (60–74 rad/μm), thus indicating a deviation from the linear behavior expected for the phonon dispersion relationship close to the center of the Brillouin zone (extending up to 104 rad/μm). Read more: Brivio, S., Polli, D., Crespi, A., Osellame, R., Cerullo, G., Bertacco, R. “Observation of anomalous acoustic phonon dispersion in SrTiO3 by broadband stimulated Brillouin scattering” (2011) Applied Physics Letters, 98 (21), art. no. 211907.