Snapshot circular dichroism measurements

Abstract

Two coherent waves carrying orthogonal polarizations do not interfere when they superpose, but an interference pattern is generated when the two waves share a common polarization. This well-known principle of coherence and polarization is exploited for the experimental demonstration of a novel method for performing circular dichroism measurements whereby the visibility of the interference fringes is proportional to the circular dichroism of the sample. Our proof-of-concept experiment is based upon an analog of Young's double-slit experiment that continuously modulates the polarization of the probing beam in space, unlike the time modulation used in common circular dichroism measurement techniques. The method demonstrates an accurate and sensitive circular dichroism measurement from a single camera snapshot, making it compatible with real-time spectroscopy. IntroductionCircular dichroism (CD) is the differential absorption of circularly polarized light. Despite this differential absorption being typically very small in optically active enantiomers (10 −5 -10 −4 ), CD spectroscopy has become a well-established technique, especially as a method for identifying structural motifs in chiral proteins [1]. Due to its intrinsic weakness, the detection of CD requires relatively complex and expensive instruments that often need to integrate the low signal over significant measurement time.Modern approaches to CD measurement are based on a polarization-modulation scheme in which linearly polarized probing light passes through a quartz crystal that has been subjected to periodic time-varying mechanical stress inducing birefringence due to the photoelastic effect. The modulation device, commonly called photoelastic modulator, produces circular polarizations with alternating handedness in time [2]. In the early realizations of spectroscopic CD measurements from 1960's [3], the polarization modulation was also accomplished with electrically driven Pockels cells, but nowadays virtually all commercial CD instruments use photoelastic modulators. Regardless whether Pockels cells or photolastic modulators are used as polarization-modulation elements, the measurement approach based on the analysis of the temporal variation of the polarization of the modulated probing light is the same.In this work we present a novel method for spectroscopic CD characterization that is based on spatial rather than on temporal polarization modulation. The application of multi-domain spatial, spectral, angular, etc modulation approaches has been previously explored by Tyo and coworkers in the more general context of polarimetry [4,5]. In our method, different points of the sample are illuminated with different, well-controlled states of polarization with alternating handedness. The presence of CD in the sample results in the formation of a periodic interference pattern at the detector. As chiroptical data are obtained from the analysis of a single snapshot of the interference pattern, the novel approach is not subject to measurement time constraints typi...

Interference-contrast optical activity: a new technique for probing the chirality of anisotropic samples and more

“…Interestingly, the initial optical field here (which sports one-dimensional helicity fringes) is essentially the same as the optical field proposed by one of the authors for exerting discriminatory optical forces on chiral molecules [35]; a phenomenon recently demonstrated in the laboratory for chiral liquid crystal microspheres [36]. It is also essentially the same as the optical field employed in the new technique of snapshot circular dichroism, which might enable faster measurements of circular dichroism than is possible using traditional circular dichroism spectrometers [37].…”

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Circular Intensity Differential Scattering for Label-Free Chromatin Characterization: A Review for Optical Microscopy

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