J. Eur. Opt. Society-Rapid Publ. 21, 9( 2025) 87
Figure 5.( left) A low spatial resolution image( 4 6 pixels) of a hydrogel cube embedded in water using the Brillouin frequency as contrast.( right) Example spectra of water and hydrogel from two pixels of the left image.
Figure
6.( left) Time signals for a symmetric gaussian and line excitation( offset added for clarity).( right) Images of the two excitation profiles.
40 lm( Fig. 6). Additionally, the line excitation also allows for the usage of higher pulse energies, as the line excitation spreads the deposited energy over a large area. However, the increase in spectral resolution is achieved at the expense of lower spatial resolution in the x-direction.
This trade-off between spectral and spatial resolution becomes particularly interesting when considering material interfaces, where the unique properties of phonon propagation can be observed. If the excitation cross section overlaps at an interface of two different materials, it will create phonons in both materials with the same wavelength, because the same interference grating is imprinted in both materials. However, due to different sound velocities in the materials the phonons will have different frequencies. When phonons transmit through an interface they change their wavelength, but their frequency will stay constant. This frequency will be detected when the phonon passes through the probe area( Fig. 7a). The detection of the frequency depends on the transmission efficiency of phonons through the interface and the attenuation of the phonons, rendering it therefore sample dependent. If the probe beam is in close proximity to the interface, the intersection between the probe and the interference grating under an angle also leads to the detection of both frequencies( Fig. 7b). The probe beam gets reflected from the grating periods in both materials. The strength of this volumetric effect depends on the intersection angle and the axial extent of the interference fringes.
To investigate phonon transmission across interfaces, we conducted an experiment using a line excitation profile on a wet hydrogel cube immersed in water. We selected a wet hydrogel cube for its relatively smooth surface compared to its dried counterpart, minimizing potential scattering effects at the interface. To analyze the signal behavior at varying distances from the interface, we performed a lateral scan of the sample, systematically altering the position at which the probe beam entered the sample. Figure 8 presents the spectra obtained from this horizontal scan, revealing the evolution of the Brillouin signal as a function of distance from the hydrogel-water interface. Starting far from the interface, the ISBS signal shows only the frequency of hydrogel f = 500 MHz. The Brillouin frequency of this cubes differs from the one used in Figure 4, because the cube was made out of a softer hydrogel. Moving closer to the interface, the excited phonons in water transmit through the water / hydrogel interface and propagate through the probe area, leading to the detection of the water frequency( 493 MHz). The position, where the two peaks are of equal height, corresponds to the interface position, because the line excitation excites an equal amount of phonons in both media. In our case this would be at the position x = 90 lm. In the vicinity of the interface, both the volume effect and the crosstalk effect superimpose, whereas in the outer regions only the crosstalk effect exists. The data suggest that the crosstalk in that sample is limited to 90 lm around the interface, because at x = 190 lm only the water