# Non-invasive light focusing in scattering media using speckle variance optimization

## TL;DR

This paper introduces a non-invasive method for focusing light deep inside scattering media by optimizing speckle variance, enabling improved bio-imaging without invasive procedures.

## Contribution

The authors propose a novel speckle variance optimization technique for non-invasive light focusing using linear signals in scattering media, expanding the capabilities of deep bio-imaging.

## Key findings

- Successfully focused light on a single fluorescent source deep inside scattering media.
- Demonstrated robustness and efficiency of the speckle variance optimization method.
- Shown equivalence to two-photon fluorescence-based optimization strategies.

## Abstract

Optical imaging deep inside scattering media remains a fundamental problem in bio-imaging. While wavefront shaping has been shown to allow focusing of coherent light at depth, achieving it non-invasively remains a challenge. Various feedback mechanisms, in particular acoustic or non-linear fluorescence-based, have been put forward for this purpose. Non-invasive focusing at depth on fluorescent objects with linear excitation is, however, still unresolved. Here we report a simple method for focusing inside a scattering medium in an epi-detection geometry with a linear signal: optimizing the spatial variance of low contrast speckle patterns emitted by a set of fluorescent sources. Experimentally, we demonstrate robust and efficient focusing of scattered light on a single source, and show that this variance optimization method is formally equivalent to previous optimization strategies based on two-photon fluorescence. Our technique should generalize to a large variety of incoherent contrast mechanisms and holds interesting prospects for deep bio-imaging.