Second Harmonic Generation of Unpolarized Light

Spatially co-registered wide-field nonlinear optical imaging of living and complex biosystems in a total internal reflection geometry

Nonlinear optical microscopy that leverages an objective based total internal reflection (TIR) excitation scheme is an attractive means for rapid, wide-field imaging with enhanced surface sensitivity. Through select combinations of distinct modalities, one can, in principle, access complementary chemical and structural information for various chemical species near interfaces. Here, we report a successful implementation of such a wide-field nonlinear optical microscope system, which combines coherent anti-Stokes Raman scattering (CARS), two-photon fluorescence (TPF), second harmonic generation (SHG), and sum frequency generation (SFG) modalities on the same platform. The intense optical fields needed to drive these high order nonlinear optical processes are achieved through the use of femtosecond pulsed light in combination with the intrinsic field confinement induced by TIR over a large field of view. The performance of our multimodal microscope was first assessed through the experimental determination of its chemical fidelity, intensity and polarization dependences, and spatial resolution using a set of well-defined model systems. Subsequently, its unique capabilities were validated through imaging complex biological systems, including Hydrangea quercifolia pollen grains and Pantoea sp. YR343 bacterial cells. Specifically, the spatial distribution of different molecular groups in the former was visualized via vibrational contrast mechanisms of CARS, whereas co-registered TPF imaging allowed the identification of spatially localized intrinsic fluorophores. We further demonstrate the feasibility of our microscope for wide-field CARS imaging on live cells through independent characterization of cell viability using spatially co-registered TPF imaging. This approach to TIR enabled wide-field imaging is expected to provide new insights into bacterial strains and their interactions with other species in the rhizosphere in a time-resolved and chemically selective manner.

TGFβ signaling curbs cell fusion and muscle regeneration

Muscle cell fusion is a multistep process involving cell migration, adhesion, membrane remodeling and actin-nucleation pathways to generate multinucleated myotubes. However, molecular brakes restraining cell-cell fusion events have remained elusive. Here we show that transforming growth factor beta (TGFβ) pathway is active in adult muscle cells throughout fusion. We find TGFβ signaling reduces cell fusion, regardless of the cells' ability to move and establish cell-cell contacts. In contrast, inhibition of TGFβ signaling enhances cell fusion and promotes branching between myotubes in mouse and human. Exogenous addition of TGFβ protein in vivo during muscle regeneration results in a loss of muscle function while inhibition of TGFβR2 induces the formation of giant myofibers. Transcriptome analyses and functional assays reveal that TGFβ controls the expression of actin-related genes to reduce cell spreading. TGFβ signaling is therefore requisite to limit mammalian myoblast fusion, determining myonuclei numbers and myofiber size.

Second Harmonic Generation Susceptibilities from Symmetry Adapted Wannier Functions

Elucidating the orbital level origin of second harmonic generation (SHG) in materials and identifying the local contributions is a long-standing challenge. We report a first principles approach for the SHG where the contributions from individual orbitals or atoms can be evaluated via symmetry adapted Wannier functions without semiempirical parameters. We apply this method to the common SHG materials KBe_{2}BO_{3}F_{2}, KCaCO_{3}F, and β-BaB_{2}O_{4}, and show that the orbitals on noncentrosymmetric sublattices are responsible for SHG effect and the energies of these orbitals control the magnitude.

Ultra-fast Stokes parameter correlations of true unpolarized thermal light: type-I unpolarized light

We measure Stokes parameter correlations in analogy to the intensity correlation measurements in the original Hanbury-Brown & Twiss configuration by realizing an experimental setup by combining a Schaefer-Collett or Berry-Gabrielse-Livingston polarimeter with a Hanbury-Brown & Twiss intensity interferometer. We investigate true unpolarized light emitted from a broadband thermal light source, which we realize by an erbium-doped fiber amplifier, thus being an ideal source of true unpolarized light. We find that all Stokes parameter correlations ⟨S_nS_n⟩, n∈{1,2,3} are equal to 0.5⟨I⟩². The proven invariance of the Stokes parameter correlations against retardation by wave-plates clearly shows for the first time, to the best of our knowledge, that our true unpolarized thermal light represents type I unpolarized light in accordance with a theoretical prediction for a classification of unpolarized light postulated more than 20 years ago.

Mueller Tensor Nonlinear Optical Polarization Analysis in Turbid Media

A mathematical framework to treat partial polarization in second harmonic generation imaging of nonlinear optical susceptibility is described and applied to imaging tissue sections 5, 40, and 70 μm thick, sufficient to introduce significant depolarization of the incident field. Polarization analysis becomes complicated in turbid media, in which scattering can result in degradation of polarization purity. The simplest framework for describing the polarization of purely polarized light is the Jones framework, which has been applied to great effect in the polarization analysis of second harmonic generation. However, the Jones framework lacks the necessary generality to describe a partially polarized electric field, (i.e., ones positioned within the volume of the Poincaré sphere rather than on the surface). Recent work connecting the Jones framework to the Mueller-Stokes framework has enabled interpretation of results with the more intuitive Jones framework while maintaining generality of the Mueller-Stokes method. The magnitude and nature of linear interactions of the tissue with the incident infrared field are discussed. Despite substantial depolarization, the nonlinear optical susceptibility tensor elements of collagen was recoverable at each pixel images of thick tissue utilizing the described framework. For thick and thin tissues, values of the tensor element ratio ρ were recovered in good agreement with previous studies. Both hyperpolarizing and depolarizing effects of SHG were observed, and the mechanism of hyperpolarization was determined to rest upon the interplay of orientation and relative contribution of polarized and depolarized incident light to elicit SHG.

Spatially encoded polarization-dependent nonlinear optics

A single fixed optic is combined with the sample translation capabilities inherent to most microscopes to achieve precise polarization-dependent second harmonic generation microscopy measurements of thin tissue sections. Although polarization measurements have enabled detailed structural analysis of collagen, challenges in integrating rotation stages or fast electro-optic/photoelastic modulation have complicated the retrofitting of existing systems for precise polarization analysis. Placing a static microretarder array in the rear conjugate plane resulted in spatially encoded polarization modulation across the field of view. A complete set of polarization rotation measurements was acquired at each pixel by sample translation, recovering local-frame tensors relating to structure within collagenous tissue.