EXPERIMENTAL AND THEORETICAL DEVELOPMENTS FOR WAVEFRONT SHAPING

Abstract

Scattering media, such as biological tissues, multi-layer materials, and opaque materials, inherently distort and diffuse light, which pose significant challenges for larger-depth optical microscopy and imaging techniques, especially for deep tissue microscopy in the biological domain, which is currently a highly active research area. Focusing of light, or the formation of complex light patterns inside or through turbid media is a challenging task due to refractive index inhomogeneity, random light scattering, and speckle noise formation. However, amplitude or phase modulation-based iterative wavefront shaping techniques with a suitable optimized cost function show a potential way to focus the light inside scattering media. This thesis work addresses feedback assisted binary phase modulation and wavefront shaping techniques both in the experimental domain and theoretical methodologies to overcome specific challenges in focusing light and structured light formation through scattering media for next-generation optical microscopy and industrial applications. The main contribution of the present work is the development of robust, efficient, and cost-effective experimental setups with binary phase modulation, as well as the development of cost functions for advancing the focusing of light in a controlled manner at the target location, where pixel-to-pixel intensity and spatial correlation have been established. Both theoretical and experimental contributions towards the development of the wavefront shaping system and optimizing the wavefront of light with computational algorithms are discussed. This thesis demonstrates experimental systems development for feedback based wavefront shaping with binary phase modulation using an affordable ferroelectric liquid crystal (FLC) based spatial light modulator (SLM). FLC SLMsprovide several advantages over nematic liquid crystal SLMs (NLC-SLMs) and digital micromirror devices (DMDs). FLC-SLMs do not necessitate phase calibration due to their simplified binary-phase operation and are significantly faster in terms of pixel response times (∼40 µs) as compared to the NLC-SLM (∼10 ms). Moreover, FLC-SLMs provide double the enhancement compared to xi DMDsandoffer a cost-effective solution for wavefront shaping with high-speed binary phase modulation (up to 4.5 kHz). In this thesis work, we have developed a feedback-assisted, cost-effective, binary-phase based wavefront shaping system with FLC-SLM that can construct high-resolution multiple complex hetero structures simultaneously in 3D volume using an optimized single-phase mask. The FLC-SLM based wavefront shaping setup has been further reconfigured to operate in both transmission and diffuse-reflection modes for its implementation in focus-spot formation and complex light structure formation with the scattering media, suchasbiological tissue and standard groundglassdiffuser. Furthermore, we have advanced the experimental setup for controlling the fluorescence light inside the fluorescence-stained biological tissue in diffuse-reflection mode. In the field of wavefront shaping, obtaining tightly focused light spots with controlled intensity, uniformity over the focus spot, and advancing contrast in a controlled manner is highly desired and, it adds advantages across multiple research domains. In feedback-based wavefront shaping, traditional cost func tions such as target intensity (η) and peak-to-background ratio (PBR) overshoot the set-value of intensity at focus-spot, lack uniformity over the focus-spot, and struggle with intensity control over the focus-spot when focus through scattering media. To overcome these limitations, we have derived and proposed an ℓ2 normbasedquadraticcostfunction(QCF)thatestablishespixel-to-pixel intensity, spatial correlation and enables contrast achievement in a controlled manner while maintaining uniformity across the light focus-spot. Both simulations and experiments have been performed extensively using the proposed cost function QCF, and thereafter, its performance has been compared with the commonly used η and PBR based cost functions. The results evidently indicate that the QCF achieves superior performance in terms of intensity control at the focus spot, provides better uniformity, and achieves background noise suppression. In contrast, both the η and PBR cost functions exhibit uncontrolled intensity gain compared to the proposed QCF. The QCF is found to be suitable for applications requiring intensity control at the focus-spot, better uniformity, and effective xii background noise suppression. This method holds significant promise for applications where intensity control is critical, and energy transfer in a controlled manner is necessary, such as laser materials processing, bio-incubation systems, next-generation deep-tissue optical microscopy, photolithography, photothermal treatments and many more to mention. To address the challenges of light-scattering in scattering media and forming complex light structures or specific light patterns through highly scattering media, this thesis demonstrates the coupling of the data regression model in the R-squared metric and uses its advantages as a cost function iteratively in the genetic algorithm to advance the resolution and structural uniformity while maintaining the contrast for complex-structures formation through scattering media. The R-squared metric is analyzed in the genetic algorithm to optimize the binary phase mask alongwith the developed FLC-SLM based binary-phase wavefront shaping system and has been validated with a 120-grit ground glass diffuser and fresh chicken tissue samples of thickness 307 µm and 812 µm. The detailed results show that the proposed data regression model assisted R-squared cost function, combined with the developed FLC-SLM based cost effective wavefront shaping system, can construct high-resolution multiple complex hetero-structures simultaneously in 3D volume using an optimized single phase-mask. Multiple complex light structures and gradient contrast light formation have been reconstructed to validate the algorithm and binary-phase modulation based FLC-SLM system for wavefront shaping. Focusing the light into desired patterns at the specific region of interest is significantly more challenging than focusing on spots. There has always been a trade-off between resolution and the contrast enhancement of the structured light patterns in wavefront shaping. When the desired pattern is constructed using conventional cost functions, such as η or PBR, it has been observed that these methods enhance contrast non-uniformly over a large area and show missing intensity pixels in the structure; Moreover, they fail to resolve the complex structure. To address this, our present work demonstrates the xiii coupling of our proposed ℓ2-norm based cost function with Pearson’s correlation coefficient based cost function and proposes regularized cost function (RCF) for advancing the contrast and maintaining the high resolution of structured light patterns at the same time. Both the simulations and experiments have been performed, and it has been found that the proposed RCF significantly advances the contrast and structural uniformity for focusing light through scattering media as well as for diffuse-reflection mode. The algorithm advancement in this thesis, alongwith system development, holds significant promise in the real-life applications, such as 2D/3D holographic displays, structured illumination mi croscopy, optogenetics, optical coherence tomography, endoscopy, fluorescence imaging, fluorescence microscopy, multi-photon microscopy, photodynamic therapy, optical communication, and optical trapping