Time-domain Brillouin scattering assisted by diffraction gratings

Abstract

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 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 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 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 gratings operating in reflection mode provides access to wide range of acoustic frequencies from minimal to maximal possible values in a single experimental optical configuration for the directions of probe light incidence and scattered light detection. This is achieved by monitoring the backward and forward Brillouin scattering processes in parallel. Potential applications include measurements of the acoustic dispersion, simultaneous determination of sound velocity and optical refractive index, and evaluation of samples with a single direction of possible optical access.

Optical generation and detection of gigahertz shear acoustic waves in solids assisted by a metallic diffraction grating

“…A standard optical pump-probe setup is used for the measurement [24]. A Ti:sapphire mode-locked laser is used as the light source.…”

“…The grating can diffract the probe light before and/or after its scattering by the acoustic field in both transmission and reflection configurations, thus multiplexing the propagation directions of the probe laser pulses inside the sample. These multiple combinations of sound and light propagation directions yield the observation of several different oscillation frequencies in both backward and forward Brillouin scattering simultaneously [24].…”

“…The generation and detection of longitudinal acoustic waves in a transparent isotropic medium using metallic gratings were described in detail elsewhere [24,25]. Absorption of ultrashort light pulses at the metallic grating may generate longitudinal acoustic waves propagating not only normally to the plane of the gratings but also in all directions corresponding to the possible nonzeroth diffraction orders of the acoustic waves induced by this grating, thus multiplexing the propagation directions of the generated acoustic waves in the sample.…”

“…In conjunction with this generic concept, we previously conducted time-domain Brillouin measurements in fused silica samples with metallic grating photoacoustic transducers and showed that the longitudinal acoustic waves propagating along multiple directions are observed simultaneously, giving rise to several distinct Brillouin frequencies [24]. Even though shear acoustic waves are expected to be excited in these grating structures, we did not succeeded in revealing them [24].…”

Detection of Hidden Gratings through Multilayer Nanostructures Using Light and Sound

“…Whether the interference is constructive or destructive depends on the distance that the optical pulse has propagated before diffracting again. This in turn depends on the distance that the acoustic echo in the glass has travelled from the glass-Au interface and is thus a periodic function of time [44][45][46][47][48]. This phenomenon is often referred to as Brillouin scattering.…”