Anisotropic molecular alignment occurs ubiquitously and often heterogeneously at different size scales. However, conventional optical imaging approaches can only provide incomplete molecular orientation maps on the 2D polarization plane. Here, we present two new approaches based on polarization-controlled IR microscopy and broadband CARS microscopy to determine the 3D angles of molecular orientations independently at each image pixel. These new approaches are based on concurrent polarization analysis of multiple vibrational modes to map the 3D orientation angles and the order parameter of the local orientational distribution of polymer chains. We demonstrate the new technique using a non-banded poly(e-caprolactone) film and a ring-banded polyethylene film.
Infrared (IR) absorption microscopy is a sensitive label-free chemical imaging tool for biomolecules in cells and tissues. However, quantitative measurements in hydrated biological samples remain a challenge due to the strong water absorption in the fingerprint region, in particular near 1650 cm-1, which critically overlaps with the protein amide I peak. Here, we present a new approach to overcome the challenge by using a Solvent Absorption Compensation (SAC) unit, which dynamically adjusts the intensity of incident light to compensate for the strong water absorption. We demonstrate the capability to perform quantitative absorption imaging of proteins (amide I) in live fibroblast cells in a 25 µm thick layer of water. This work enables future label-free, quantitative chemical imaging of biomolecules in the living cell.
A non-tomographic analysis method is proposed to determine the 3D angles and the order parameter of molecular orientation using polarization-dependent infrared (IR) spectroscopy. Conventional polarization-based imaging approaches provide only 2D-projected orientational information of single chromophores or vibrational modes. The newly proposed method concurrently analyses polarization-dependent absorption profiles of two non-parallel transition dipole moments. The relative phase angle and the maximum-to-minimum ratios of the two polarization-dependent absorption profiles are used to calculate the 3D angles and the order parameter of molecular orientation. The relativity of those intermediate observables makes the analysis output values unaffected by variations in concentration, thickness, absorption peak, and absorption cross-section, which can occur in typical imaging conditions. This analysis is based on a single-step, non-iterative calculation that does not require any analytical model function of an orientational distribution function. This concurrent polarization analysis method is demonstrated using two simulation data examples and the error propagation analysis is discussed as well. Application of this robust spectral analysis method to polarization IR microscopy will provide a full molecular orientation image without tilting that tomographies require. In this talk, I describe this new approach that non-iteratively determines the 3D angles and the orientational order parameter without assuming a model function for an ODF. Then, I will demonstrate an application of this analysis using experimental image data acquired from a semicrystalline polymer film with polarization IR microscopy. The results clearly show how the 3D angles and the order parameter are determined for every pixel using straightforward formulas without iterative calculation.
We demonstrate that pulse shaping of a narrowband pulse can suppress the nonresonant background (NRB) contribution and retrieve resonant Raman signals efficiently in a broadband coherent anti-Stokes Raman scattering (CARS) spectrum. A pulse shaper prepares a probe pulse with two spectral components of differing phase. When the CARS fields
generated by these two out-of-phase components are optically mixed, the NRB signal is greatly reduced while a resonant CARS signal survives with minimal attenuation. We discuss three model schemes for the interfering pulse components: (1) two pulses with different bandwidths and the same center frequency (ps-fs scheme); (2) two pulses with the same bandwidth and shifted center frequencies
(ps-ps scheme); and (3) a pulse with different phases across the center frequency (fs(+/-) scheme). In all schemes, only the resonant signal from the "3-color" CARS mechanism survives. The resonant signal from "2-color" CARS mechanism vanishes along with the NRB. We discuss optimization conditions for signal intensity and shape of resonant CARS peaks.
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