Delivering Broadband Light Deep Inside Diffusive Media

TL;DR

This paper introduces a broadband deposition matrix technique that significantly enhances deep light delivery in diffusive media, overcoming coherence limitations and enabling efficient energy transfer to extended targets.

Contribution

The authors develop and experimentally validate a broadband wavefront shaping method that maximizes energy delivery deep inside diffusive media, revealing fundamental limits and practical implications.

Findings

Six-fold energy enhancement for targets with over 1500 speckle grains

Effective energy delivery at depths up to ten transport mean free paths

Enhancement becomes nearly independent of depth and dissipation in broadband limit

Abstract

Wavefront shaping enables targeted delivery of coherent light into random-scattering media, such as biological tissue, by constructive interference of scattered waves. However, broadband waves have short coherence times, weakening the interference effect. Here, we introduce a broadband deposition matrix that identifies a single input wavefront that maximizes the broadband energy delivered to an extended target deep inside a diffusive system. We experimentally demonstrate that long-range spatial and spectral correlations result in a six-fold energy enhancement for targets containing more than 1500 speckle grains and located at a depth of up to ten transport mean free paths, even when the coherence time is an order of magnitude shorter than the diffusion dwell time of light in the scattering sample. In the broadband (fast decoherence) limit, enhancement of energy delivery to extended targets becomes nearly independent of the target depth and dissipation. Our experiments, numerical simulations, and analytic theory establish the fundamental limit for broadband energy delivery deep into a diffusive system, which has important consequences for practical applications.