J. Eur. Opt. Society-Rapid Publ. 21, 9( 2025) 83
[ 26 – 31 ], taking advantage of the nonlinear nature of Brillouin light scattering. ISBS makes full use of the higher SNR to decrease the temporal resolution in contrast to SBS, because the signal is evaluated in the time domain, removing the need for a wavelength scan. The higher temporal resolution of ISBS, facilitated by the interaction of two pump pulses and a probe beam, can significantly reduce measurement duration’ s. This advancement is crucial for both rapid imaging in clinical applications and fundamental research, such as studying morphogenesis at the organism level during gastrulation in Drosophila [ 20 ]. The measurement of elastic properties also plays an important role in organoids. In the optogenetics of transgenic organoids, elasticity measurements are desirable to learn about the transfer functions from light stimulus to mechanical reaction [ 32 ].
The interference fringes formed by two intersecting laser pulses create phonons at the intersection point, where the probe photons are inelastically scattered. The stiffness of the material is linked to the Doppler frequency shift of the inelastically scattered photons. A investigation was performed on the influences of the pulse and probe parameters on the SNR and it was concluded that the choice of high pulse energies offers the greatest leverage [ 33 ]. However, when focusing sharper to increase the spatial resolution, the increasing excitation energy density needs to be taken into account to avoid phototoxic effects. Recently Li et al. [ 29 ] performed a similar study, where they investigated the influence of the probe beam parameters and acquisition time. In this paper, we expand the investigation on optimizing the ISBS excitation process, which requires balancing the influence of excitation parameters, like the repetition rate on the spatial and spectral resolution. The temporal resolution of ISBS is increased by incorporating the exponential window function [ 34 ] into the signal analysis process.
2 Materials and methods
ISBS microscopy relies on creating a transient density grating using an ultra-short pulsed laser and probing it through Bragg diffraction with a continuous wave( cw) laser. The pulsed laser beam is divided into two coherent beams via a diffraction grating. The interference of these coherent laser beams forms an interference fringe system [ 35 ], with fringe spacing d determined by the excitation beam’ s wavelength k pump and the half-crossing angle / pump:
d ¼ k pump 2 sin / pump
: ð1Þ
The impulsive excitation of a standing acoustic wave is induced by either electrostrictive or thermal coupling of the laser pulse to the material within the measurement volume. This density variation also alters the refractive index. Consequently, a portion of the probe beam is diffracted at a temporally varying refractive index grating under the Bragg condition, resulting in intensity modulation. For electrostrictive excitation, the expected modulation frequency is f 2 = 2c S / d, wherec S is the sound velocity in the sample. If the signal is thermally excited, the expected frequency is f 1 = c S / d [ 36 ]. Electrostriction exerts a force on matter along the gradient of the absolute electric field strength [ 37 ], causing displacement of matter toward regions of high light intensity. Thermal excitation heats the matter in regions of higher light intensity, leading to impulsive expansion. The resulting temporal changes in matter are governed by thermodynamic material equations. The single diffracted probe beam can either be detected alone or in a heterodyne fashion [ 38 ]. In the heterodyne scenario, the probe beam is reflected by the standing wave and another coherent part of the probe beam are superimposed on the photodetector. This additional part can be intentionally generated or arise inadvertently in the setup. For purely electrostrictive excitation with heterodyne detection, the expected frequency is f 1. Both frequencies can be utilized to measure sound velocity and, consequently, deduce mechanical properties.
The experimental setup is depicted in Figure 1. It is essentially the same architecture that was used in our previous work [ 33 ], with the only difference being that we now use pump laser with k pump = 1035 nm, while the probe beam is still at k probe = 895 nm( DL100 Toptica Photonics). The pump laser is a Coherent Monaco, which offers a tunable pulse-length, repetition-rate and pulse energy. The probe beam diffraction orders propagate between the diffraction orders of the grating for the pump laser. Both pump and probe beams are focused on the diffraction grating( GT) and the ± 1 diffraction orders are focused with the 4f-system to create the measurement volume. The choice of lenses is essential for the ISBS process. The �1 diffraction order of the probe beam is used to adjust the optical setup, but is later blocked to only detect the Brillouin scattered light. After the measurement volume, a long-pass filter removes the remaining pump light in the optical pathway and the Brillouin scattered probe beam is focused on an avalanche photodiode( APD). The oscilloscope( Tektronix MSO64B) reads out the time signal from the APD and sendsittoaPCforanalysis.
3 Signal strength
The primary objective of this work is to find a balance between multiple experimental parameters that optimize the SNR of ISBS, resulting in faster measurement rates while simultaneously maintaining a high spatial and spectral resolution.
3.1 Excitation parameters
With the configuration depicted in Figure 1, we studied an excitation cross-section diameter of 40 lm and a matched diameter for the probe beam. The fringe spacing in the excitation volume is 3 lm. The measurements were all conducted on a water filled cuvette with 500 lm axial extent and lateral dimensions of 5 mm. We use signal strength, simply defined as the fitted peak height in the Fourier spectrum, as a measure.
At first the signal strength was measured for different pulse length and different pulse energies at a fixed repetition