Chemical reactions that occur below the surface in heterogeneous materials are of interest to many applications. However, the optical opaqueness of these materials prevents the use of spectroscopic techniques such as Raman spectroscopy. ASL has set up a microscope with bidirectional focusing and optical phase correction, and demonstrated that this approach is capable of measuring subsurface Raman and laser-induced fluorescence (LIF) measurements in heterogeneous materials.
Using a spatial light modulator (SLM) and an optimization algorithm, the incoming light is focused at the desired location below the sample surface. The phase of the forward scattered light, collected by a second objective, is measured and its phase conjugate is calculated and generated using another SLM. This light is sent back and focused onto the sample spot. The image shows the Raman difference spectra of a subsurface sample as it photodegrades over time.
References:
1.B.R. Anderson and H. Eilers, “Genetic algorithms for focusing inside opaque media,” J. Opt. 22, 085601 (2020).
