Monitoring sub-surface chemical reactions in heterogeneous materials using wavefront-shaping-assisted bidirectional focusing

Abstract

We have developed a bidirectional focusing microscope that utilizes feedback-assisted wavefront shaping to focus light inside a heterogenous material in order to monitor sub-surface chemical reactions. The bidirectional geometry is found to provide superior intensity enhancement relative to single-sided focusing, owing to increased mode control and long-range mesoscopic correlations. Also, we demonstrate the microscope’s capability to measure sub-surface chemical reactions by optically monitoring the photodegradation of a Eu-doped organic molecular crystal embedded in a heterogeneous material using both fluorescence and Raman spectroscopy as probe techniques.

Absorption-invariant focusing efficiency for wavefront-shaping controlled reflection from absorbing disordered media

“…Control of the optical properties of disordered media using active wavefront shaping was first predicted by Freund in 1990 [1] and then experimentally demonstrated by Vellekoop and Mosk in 2007, when they used a liquid crystal on silicon spatial light modulator (LCOS-SLM) to focus coherent light through an opaque material [2]. Since then, the technique of wavefront shaping to control the optical response of a disordered system has been applied to a wide range of systems and applications, including: the compression of ultrashort pulses [3], spectral control of broadband light sources [4], control of light polarization [5], spectral and spatio-temporal control of random lasers [6,7], enhanced fluorescence microscopy [8], imaging through scattering media [9], focusing through dynamic scattering media (which has applications in biological and astronomical imaging) [10], subsurface spectroscopy in heterogeneous materials [11,12], enhanced nonlinear optical effects [13], attaining perfect focusing [14,15], control of reflected light [16], tunable beamsplitters [17], and the implementation of optical physically unclonable functions [18,19].…”

Subsurface Spectroscopy in Heterogeneous Materials Using Self-Healing Laser Beams

“…To address this challenge, we recently developed a sub-surface spectroscopy technique [1-4] that utilizes feedback-assisted wavefront shaping (FAWs) [5] to focus probe light inside of a heterogeneous material and onto a guidestar particle; while we have successfully used this technique to probe slow subsurface reactions such as photodegradation [1] and thermal degradation [3,4], this technique is far too slow to probe faster reactions, such as those that occur during detonation [6]. Additionally, this technique requires a guidestar particle to be embedded inside of the heterogeneous material, which can alter the chemistry we are trying to measure.…”