Wavefront shaping microscopy: Controlling scattered light for deep tissue imaging

In microscopy, light scattering sets the limit for the maximum imaging depth inside biological tissue. In this thesis, we explore how this fundamental depth limit can be broken by means of wavefront shaping. Wavefront shaping is a powerful tool that provides control over the light inside scattering media. We aim to extend the imaging depth by studying light scattering in tissue-mimicking samples and by developing new wavefront shaping microscopy techniques. In the first part of the thesis, we will form new theoretical models for light scattering phenomena relevant to microscopy. In Chapter 2, we study how light can be focused through a scattering medium in the absence of a localized form of feedback. We will derive under which conditions the ‘blind’ optimization of a nonlinear feedback signal from multiple sources will lead to a single focus. In Chapter 3, we introduce the generalized optical memory effect, describing all first-order spatial correlations of scattered light. This correlation function describes how much an optimized focus can be shifted and tilted inside a scattering medium. From this unified correlation function, we derive the optimal scanning scheme for imaging inside forward scattering media. In the second part of the thesis, we will discuss how wavefront shaping can be implemented to enhance the performance of a two-photon excitation microscope (TPM). Chapter 4 covers the basics of TPM imaging and how the embedded fluorescent structures can be used as feedback for the wavefront shaping algorithm. Furthermore, we demonstrate how the effective field of view through a scattering layer can be greatly enhanced using the generalized optical memory effect. In Chapter 5, we introduce a new class of wavefront shaping techniques, which we term model-based wavefront shaping. Instead of using the signal from a feedback source inside the scattering structure, this technique utilizes light propagation simulations to obtain the wavefront correction. This wavefront can then be constructed with the spatial light modulator in the TPM setup to form a sharp focus. In a proof-of-principle experiment, we demonstrate high-resolution imaging at depths where a feedback signal can no longer be detected. In the third part of the thesis, we will present a new numerical technique for simulating light propagation in large scattering media. In Chapter 6, we show that our method, based on a modified Born series, is orders of magnitude faster and more accurate than the current state-of-the-art. New boundary conditions for the numerical method are introduced in Chapter 7, further increasing the accuracy and the memory efficiency of the method. Our modified Born series method allows us to accurately study light scattering in two and three-dimensional structures that were previously too large to be simulated