Analysis on Nonlinear Emission from Gold Nanoparticles and Advanced Phase Retrieval Techniques for Adaptive Optics Microscopes

Abstract

Multiphoton (MP) imaging with short-wave infrared (SWIR) excitation is one of the powerful techniques to achieve deep tissue imaging. Image quality of the MP microscope is improved with adaptive optics (AO) by compensating extrinsic aberration caused by transmission through a sample. However, the AO-MP microscope has issues, such as the requirement of a proper guide star and improvement of the detection and correction techniques of the extrinsic aberration. In this dissertation, three studies are provided to overcome these issues. The first study focuses on characterization of nonlinear emission from aggregated 50 nm gold nanoparticles (GNPs) excited by a femtosecond laser at 1560 nm, which are potentially attractive guide stars for the sensor-less AO microscopy with SWIR excitation. The study clarifies nonlinear emission characteristics of GNP aggregates, including emission spectra, power dependence, polarization properties, and relationships between brightness and morphology. A critical point in the discussion is SWIR fs excitation, because the previously proposed mechanism does not explain observed nonlinear emission properties with this source. Previously unknown results that are characteristics of nonlinear emission from GNPs are explained by plasmon enhanced polarized hot electrons. The second study presents application of phase retrieval for measurement and correction of extrinsic aberrations with random phase diversity and Gerchberg's reconstruction algorithm. Optimum design parameters of the random phase patterns to improve accuracy and convergence are investigated with Fried's parameter applied to Kolmogorov turbulence. A liquid crystal on silicon (LCoS) spatial light modulator (SLM) is used to experimentally generate random diversity as a demonstration of correcting extrinsic aberration. The results provide the first optimization study using an LCoS and the Gerchberg algorithm in a sensor-less AO application. The third study presents a new single-shot phase retrieval technique called complex diversity. Multiple irradiance data are obtained by a computer-generated hologram (CGH) designed to generate multiple diffraction orders with different diversity values. Complex-number pupil filters containing both amplitude and phase values associated with the individual diffraction orders are determined by numerical propagation from the CGH, and they are used for reconstruction of the extrinsic aberration. The complex diversity technique estimates extrinsic Kolmogorov aberration better than conventional single-shot techniques for a distant point object.