HIGH THROUGHPUT STIMULATED BRILLOUIN SCATTERING SPECTROSCOPY

Abstract

Brillouin light scattering arises from the coupled interaction between light and material acoustic phonons. The measurand of Brillouin scattering is the characteristic frequency difference between incident and scattered light which depends on the local longitudinal modulus of the material. Spontaneous Brillouin scattering has been used in combination with confocal microscopy to provide non-contact, label-free mapping at micron-scale resolution in biological media. To date, spontaneous Brillouin microscopy has reached the speed limit (~20-50ms per spectrum) as determined by the theoretical scattering efficiency. While a great deal of research has been directed to speeding up Brillouin microscopy acquisition times, spontaneous Brillouin scattering is fundamentally an inefficient process thus limiting the ability to study faster biological phenomena and rapid processes. To combat this limitation, its nonlinear counterpart, stimulated Brillouin scattering (SBS) has been proposed for microscopy applications. For decades, stimulated Brillouin scattering has been used in fiber sensing and all-optical pulse control and leverages a nonlinear interaction where two counterpropagating light beams stimulate a more efficient scattering relationship. However, the small interaction volumes and photodamage constraints presented in microscopy have hindered the translation of stimulated Brillouin scattering into the biological realm. Recently, continuous wave stimulated Brillouin microscopy has led to competitive acquisition times (~5ms per spectrum) when compared to the spontaneous alternative but has yet to be widely adopted. Due to a plethora of factors, such as an inefficient power balance between pump and probe beams, lack of proper commercial laser sources, and nonoptimal detection schemes, the complete picture of what SBS spectroscopy has to offer has yet to be revealed. As such, there is a need to customize light sources and detection schemes in order to fully take advantage of the enhanced Brillouin efficiency possible in SBS. Herein we introduce novel methodology to improve the acquisition speed of Brillouin microscopy by designing and developing proper laser sources and detection schemes for efficient SBS spectroscopy. First, we showcase the potential utility of our state-of-the-art continuous wave SBS technology in a flow cytometry application, highly suitable for the counterpropagating geometry of SBS where the laser position is fixed while the sample is being moved at high speeds. Additionally, we will present an optimized receiver design based on polarization detection which enables 100x faster spectral measurements in the low-gain regime relevant to biological materials. Finally, we demonstrate an optimal pulsed laser source specifically designed for SBS Brillouin microscopy.