Effect of experimental parameters on wavefront-shaping-assisted bidirectional focusing in opaque media

Recently, we developed a technique to perform wavefront-shaping-assisted bidirectional focusing in opaque media, which has applications in performing subsurface spectroscopy of heterogeneous materials. In this study, we report in detail the effects of different experimental parameters on the function of this technique. Namely, we consider the effects of target depth, sample positioning, objective spacing, and initial intensity [e.g., signal-to-noise ratio (SNR)]. From these measurements, we find that all four parameters affect the performance of the bidirectional focusing microscope, but the most significant ones are the target depth and the sample/objective spacings. The SNR is found to only be significant for small SNRs as our system functions optimally for initial SNRs > 6.7 (intensities 1.042 × larger than the background). The observed importance of the sample/objective spacings demonstrates that the enhancement depends strongly on the incident beams’ spot sizes. We also estimate that the bidirectional focusing microscope can improve focusing on subsurface particles up to a depth of ≈ 18 mean free paths (which is 1170 μm for our samples).