Versatile multimodal modality based on Brillouin light scattering and the photoacoustic effect

Abstract

Multimodal optical techniques are useful for the comprehensive characterization of material properties. In this work, we developed a new, to the best of our knowledge, multimodal technology that can simultaneously measure a subset of mechanical, optical, and acoustical properties of the sample and is based on the integration of Brillouin (Br) and photoacoustic (PA) microscopy. The proposed technique can acquire co-registered Br and PA signals from the sample. Importantly, using synergistic measurements of the speed of sound and Brillouin shift, the modality offers a new approach to quantifying the optical refractive index, which is a fundamental property of a material and is not accessible by either technique individually. As a proof of concept, we demonstrated the feasibility of integrating the two modalities and acquired the colocalized Br and time-resolved PA signals in a synthetic phantom made out of kerosene and CuSO₄ aqueous solution. In addition, we measured the refractive index values of saline solutions and validated the result. Comparison with previously reported data showed a relative error of 0.3%. This further allowed us to directly quantify the longitudinal modulus of the sample with the colocalized Brillouin shift. While the scope of the current work is limited to introducing the combined Br-PA setup for the first time, we envision that this multimodal modality could open a new path for the multi-parametric analysis of material properties.

Fig. 1.

Schematic of the multimodal modality based on Brillouin and PA techniques. HWP1-HWP3: half-wave plates; PBS1-PBS2: polarized beam splitter; L1-L5: lenses; OBJ: objective; PH: pin hole; C1-C3: cylindrical lenses; LP: lens pair; EMCCD: electron multiplying charge-coupled devices.

Fig. 2.

Experimental sample and signals. (a) Photo of the stratified sample with the scan direction. (b) Raw Brillouin and (c) PA signal (single measurement) of CuSO4 and kerosene. Arrows indicate the PA signal peak.

Fig. 3.

Result of 1D scanning. Vertical 1-D profiling of the stratified CuSO4-kerosene sample based on Brillouin shift and PA amplitude. Error bars are standard deviation for 100 times measurement.

Fig. 4.

Speed of sound measurement. (a) the schematic for measuring SOS; (b) the regression result from the measurement on 4.76% saline.

Fig. 5.

Measured refractive index and longitudinal modulus of saline solutions. Error bar represents standard error of the mean.