# Direct three-dimensional measurement of refractive index via dual photon-phonon scattering

## TL;DR

This paper introduces a novel microscopy technique that directly measures the local refractive index in three dimensions using dual photon-phonon scattering, eliminating the need for phase delay sampling and enabling high-resolution biological tissue imaging.

## Contribution

The authors developed and experimentally validated a new method for 3D refractive index measurement based on co-localized Brillouin scattering interactions within a confocal microscope.

## Key findings

- Refractive index can be mapped at micron-scale resolution.
- Method does not require assumptions about sample geometry.
- Potential applications in biological tissue characterization.

## Abstract

We developed a microscopy technique that can measure the local refractive index without sampling the optical phase delay of the electromagnetic radiation. To do this, we designed and experimentally demonstrated a setup with two co-localized Brillouin scattering interactions that couple to a common acoustic phonon axis; in this scenario, the ratio of Brillouin frequency shifts depends on the refractive index, but not on any other mechanical and/or optical properties of the sample. Integrating the spectral measurement within a confocal microscope, the refractive index is mapped at micron-scale three-dimensional resolution. As the refractive index is probed in epi-detection and without assumptions on the geometrical dimensions of the sample, this method may prove useful to characterize biological cells and tissues.

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Source: https://tomesphere.com/paper/1901.10918