ABSTRACT
Manipulating the wavefront of coherent light incident on scattering media to enhance the imaging depth, sensitivity, and resolution is a common technique in biomedical applications. Local phase variations cause changes in the interference and can be used to create a focus inside or behind a scattering medium. We use wavefront shaping (WFS) to force constructive interference at an arbitrary location. The amount of light transmitted into a given region strongly depends on the scattering and absorption characteristics. These are described by their respective coefficients µs and µa and the scattering phase function. Controlling the scattering and absorption coefficients, we study the behavior of wavefront shaping and the achievable intensity enhancement behind volume scattering media with well-defined optical properties. The phantoms designed in this publication are made of epoxy resin. Into these epoxy matrices, specific amounts of scattering and absorbing particles, such as titanium dioxide pigments and molecular dyes, are mixed. The mixture obtained is filled into 3D-printed frames of various thicknesses. After a precise fabrication procedure, an integrating sphere-based setup characterizes the phantoms experimentally. It detects the total hemispherical transmission and reflection. Further theoretical characterization is performed with a newly developed hybrid PN method. This method senses the flux of light into a particular angular range at the lower boundary of a slab. The calculations are performed without suffering from ringing and fulfill the exact boundary conditions there. A decoupled two-path detection system allows for fast optimization as well as sensitive detection. The measurements yield results that agree well with the theoretically expected behavior.
ABSTRACT
Manipulating the incident wavefront in biomedical applications to enhance the penetration depth and energy delivery in scattering media such as biological tissue has gained a lot of attention in recent years. However, focusing inside scattering media and examining the electromagnetic field inside the medium still is an elaborate task. This is where electromagnetic field simulations that model the wavefront shaping process can help us understand how the focal near field evolves at different depths. Here we use a two-step beam synthesis method to simulate the scattering of complex incident wavefronts by well-characterized media. The approach uses plane wave electromagnetic near-field solutions in combination with an angular spectrum approach to model different light beams. We apply this approach to various two-dimensional scattering media and investigate the focus intensity over depth while scanning with and without phase optimization. We find that the scanned non-optimized beams have two regions characterized by exponential decays. The absolute progression of the focus intensity over depth for phase-optimized beams using all channels can be described by solutions of the radiative transfer theory. Furthermore, the average enhancement factor over depth of the phase-optimized focus intensity compared to that without optimization is investigated for different numerical apertures and scattering media. Our results show that, albeit the incident beam is diffusively scattered, the theoretical enhancement for a large number of optimization channels cannot be reached due to correlations between the channels. An increase in focus depth and an increase in the numerical aperture reduces the difference between the expected theoretical and simulated enhancement factors.
