Time-domain Brillouin scattering assisted by diffraction gratings

Absorption of ultrashort laser pulses in a metallic grating deposited on a transparent sample launches coherent compression/dilatation acoustic pulses in directions of different orders of acoustic diffraction. Their propagation is detected by the delayed laser pulses, which are also diffracted by the metallic grating, through the measurement of the transient intensity change of the first order diffracted light. The obtained data contain multiple frequency components which are interpreted by considering all possible angles for the Brillouin scattering of light achieved through the multiplexing of the propagation directions of light and coherent sound by the metallic grating. The emitted acoustic field can be equivalently presented as a superposition of the plane inhomogeneous acoustic waves, which constitute an acoustic diffraction grating for the probe light. Thus, the obtained results can also be interpreted as a consequence of probe light diffraction by both metallic and acoustic gratings. The realized scheme of time-domain Brillouin scattering with metallic grating operating in reflection mode provides access to acoustic frequencies from the minimal to the maximal possible in a single experimental configuration for the directions of probe light incidence and scattered light detection. This is achieved by monitoring of the backward and forward Brillouin scattering processes in parallel. Applications include measurements of the acoustic dispersion, simultaneous determination of sound velocity and optical refractive index, and evaluation of the samples with a single direction of possible optical access.Comment: 21 pages, 4 figures, 1 tabl

Additive Laser Excitation of Giant Nonlinear Surface Acoustic Wave Pulses

The laser ultrasonics technique perfectly fits the needs for non-contact, non-invasive, non-destructive mechanical probing of samples of mm to nm sizes. This technique is however limited to the excitation of low-amplitude strains, below the threshold for optical damage of the sample. In the context of strain engineering of materials, alternative optical techniques enabling the excitation of high amplitude strains in a non-destructive optical regime are seeking. We introduce here a non-destructive method for laser-shock wave generation based on additive superposition of multiple laser-excited strain waves. This technique enables strain generation up to mechanical failure of a sample at pump laser fluences below optical ablation or melting thresholds. We demonstrate the ability to generate nonlinear surface acoustic waves (SAWs) in Nb:SrTiO$_3$ substrates, at typically 1 kHz repetition rate, with associated strains in the percent range and pressures close to 100 kbars. This study paves the way for the investigation of a host of high-strength SAW-induced phenomena, including phase transitions in conventional and quantum materials, plasticity and a myriad of material failure modes, chemistry and other effects in bulk samples, thin layers, or two-dimensional materials

Ultrafast acousto-optic mode conversion in optically birefringent ferroelectrics

The ability to generate efficient giga-terahertz coherent acoustic phonons with femtosecond laser makes acousto-optics a promising candidate for ultrafast light processing, which faces electronic device limits intrinsic to complementary metal oxide semiconductor technology. Modern acousto-optic devices, including optical mode conversion process between ordinary and extraordinary light waves (and vice versa), remain limited to the megahertz range. Here, using coherent acoustic waves generated at tens of gigahertz frequency by a femtosecond laser pulse we reveal the mode conversion process and show its efficiency in ferroelectric materials such as BiFeO3 and LiNbO3. Further to the experimental evidence, we provide a complete theoretical support to this all-optical ultrafast mechanism mediated by acousto-optic interaction. By allowing the manipulation of light polarization with gigahertz coherent acoustic phonons, our results provide a novel route for the development of next-generation photonic-based devices and highlight new capabilities in using ferroelectrics in modern photonics

Time-domain Brillouin Scattering as a Local Temperature Probe in Liquids

International audienc

Time-domain Brillouin scattering for the determination of laser-induced temperature gradients in liquids

We present an optical technique based on ultrafast photoacoustics to determine the local temperature distribution profile in liquid samples in contact with a laser heated optical transducer. This ultrafast pump-probe experiment uses time-domain Brillouin scattering (TDBS) to locally determine the light scattering frequency shift. As the te mperature influences the Brillouin scattering frequency, the TDBS signal probes the local laser-induced temperature distribution in the liquid. We demonstrate the relevance and the sensitivity of this technique for the measurement of the absolute laser-induced temperature gradient of a glass forming liquid prototype, glycerol, at different laser pump powers - i.e., different steady state background temperatures. Complementarily, our experiments illustrate how this TDBS technique can be applied to measure thermal diffusion in complex multilayer systems in contact with a surrounding liquid.United States. Department of Energy (Grant No. DE-FG02-00ER15087

Additive laser excitation of multiple surface acoustic waves up to the nonlinear shock regime

We introduce a non-destructive method for laser-shock wave generation based on additive superposition of multiple laser-excited strain waves. This technique enables strain generation up to mechanical failure of the sample at pump laser fluences below material ablation or melting thresholds. We demonstrate the ability to generate nonlinear surface acoustic waves (SAWs) in Nb:SrTiO3 substrates, at typically 1 kHz repetition rate, with associated strains in the percent range. This study paves the way for the investigation of a host of high-strength SAW-induced phenomena, including phase transitions, fatigue, chemistry, and other effects in bulk samples, thin layers, or two-dimensional materials

Additive laser excitation of giant nonlinear surface acoustic wave pulses

The technique of laser ultrasonics perfectly meets the need for noncontact, noninvasive, nondestructive mechanical probing of nanometer- to millimeter-size samples. However, this technique is limited to the excitation of low-amplitude strains, below the threshold for optical damage of the sample. In the context of strain engineering of materials, alternative optical techniques enabling the excitation of high-amplitude strains in a nondestructive optical regime are needed. We introduce here a nondestructive method for laser-shock wave generation based on additive superposition of multiple laser-excited strain waves. This technique enables strain generation up to mechanical failure of a sample at pump laser fluences below optical ablation or melting thresholds. We demonstrate the ability to generate nonlinear surface acoustic waves (SAWs) in Nb-SrTiO3 substrates, with associated strains in the percent range and pressures up to 3 GPa at 1 kHz repetition rate and close to 10 GPa for several hundred shocks. This study paves the way for the investigation of a host of high-strain SAW-induced phenomena, including phase transitions in conventional and quantum materials, plasticity and a myriad of material failure modes, chemistry and other effects in bulk samples, thin layers, and two-dimensional materials

Nonlinear Optical Absorption in Nanoscale Films Revealed through Ultrafast Acoustics

International audienceHerein we describe a novel spinning pump-probe photoacoustic technique developed to study nonlinear absorption in thin films. As a test case, an organic polycrystalline thin film of quinacridone, a well-known pigment, with a thickness in the tens of nanometers range, is excited by a femtosecond laser pulse which generates a time-domain Brillouin scattering signal. This signal is directly related to the strain wave launched from the film into the substrate and can be used to quantitatively extract the nonlinear optical absorption properties of the film itself. Quinacridone exhibits both quadratic and cubic laser fluence dependence regimes which we show to correspond to two- and three-photon absorption processes. This technique can be broadly applied to materials that are difficult or impossible to characterize with conventional transmittance-based measurements including materials at the nanoscale, prone to laser damage, with very weak nonlinear properties, opaque, or highly scattering