Modulated polarization profiles overcome diffusion to cheaply focus light through tissue
Modulated polarization profiles overcome diffusion to cheaply focus light through tissue lead image
The obstacle to imaging biological tissue is often the tissue itself, especially when trying to focus light through any appreciable thickness. Overcoming scattering to focus light deeper than about 1 millimeter requires shaping of the incident wavefront, typically with specific input about the local scattering profile.
Phase and amplitude modulation have demonstrated such success, and now a report in Applied Physics Letters demonstrates a polarization modulation scheme to shape wavefronts for imaging through tissue. Moreover, the polarization based mechanism uses inexpensive, high pixel count spatial light modulators (SLMs) from common electronic displays.
The authors focused light through a 3-millimeter sample of chicken breast tissue. They mapped scattered light from the sample and determined the necessary linear polarization profile for focusing by using an optimization algorithm. Similar to phase modulation approaches, the algorithm found optimal electric field correlation coefficients.
Twisted, nematic liquid crystal based SLMs, widely used in commercial 4K displays, shape the wavefront polarization profile for experimental application. The focusing quality this achieves through tissue is similar to what phase modulation achieves, and at a fraction of the cost. The phase-only SLMs required by phase modulated focusing methods are not commercially available and can cost tens of thousands of dollars, while the polarizing SLMs cost only a few hundred dollars.
Because the SLMs are so cost-effective, this polarization approach affords the potential for scaling and further improvement. A mechanism to cascade a large number of SLMs for significantly more imaging controls is now experimentally realizable.
Source: “Focusing light through scattering media by polarization modulation based generalized digital optical phase conjugation,” by Jiamiao Yang, Yuecheng Shen, Yan Liu, Ashton S. Hemphill, and Lihong V. Wang, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.5005831 .
