Polarization gratings allow for real-time and artifact-free circular dichroism measurements P. Pagliusi* a,b , E. Lepera a , C. Provenzano a , A. Mazzulla b , G. Cipparrone* a,b a Dept. of Physics, University of Calabria, Ponte P. Bucci, Cubo 33B, Rende (CS), 87036, ITALY b CNR-IPCF, UOS Cosenza, and Excellence Centre CEMIF.CAL, Ponte P. Bucci, Rende (CS), 87036, ITALY ABSTRACT We propose a simple and innovative diffractive method for circular dichroism (CD) spectroscopy. It enables real-time measurements and suppress the artifacts introduced by anisotropic specimens and nonideal optical elements in conventional CD spectrometers. The method is based on a single cycloidal optical axis grating and takes advantage of its peculiar optical diffraction behavior. We prove that the true CD spectrum of a general anisotropic medium could be measured in the spectral range of interest, exploiting unpolarized white light and the intrinsic spectral selectivity of the grating. Two experimental approaches have been pursued to create the cycloidal optical axis grating, both based on polarization holography and liquid crystal photoaligning technology. The gratings are replicas of the polarization holograms in thin-films of azodyes, either in low molar mass liquid crystal cells or in reactive mesogen layers. Keywords: Circular dichroism, Spectrometers and spectroscopic instrumentation, Polarization-selective devices, Polarization holography, Cycloidal optical axis grating, Liquid crystals, Reactive mesogens. 1. INTRODUCTION Chiral objects, just like left and right hands, are not superimposable upon their mirror images. 1 Chirality is ubiquitous in nature and is a major feature of the living world. 2 Building blocks of biomolecules (i.e., nucleic acids, carbohydrates, lipids, steroid and proteins) are chiral and their interactions with chiral chemicals (both natural compounds and synthetic drugs) are expected to be stereoselective. 3-5 Indeed, one enantiomer of a molecule is often physiologically active, while the other enantiomer may be either inactive or toxic. For these reasons, chirality is of foremost importance for proteomics, agrochemistry, biochemistry and pharmaceutics. 6 Chirality in artificial materials, like synthetic polymers, liquid crystals and metamaterials, has also gained scientific and technological interest because of their promise to allow for manipulation of light polarization and propagation for photonic and optoelectronic applications. 7,8 The most prominent optical properties of natural and artificial chiral materials are polarization rotation and circular dichroism (CD). 9,10 CD spectroscopy, which measures the differential absorption between the left-handed (LCP) and the right-handed (RCP) circularly polarized light, is able to provide unique information on chiral molecular, supramolecular and nanofabricated structures and, hence, has a broad range of applications in different fields of research and industry. 10 In particular, CD spectroscopy is widely implemented in biochemical and biophysical research, to investigate protein secondary and tertiary structure, protein folding, biomolecular interactions and kinetics, as well as in pharmaceutical industry, for looking at protein stability, formulation optimization and pharmacokinetics of small drug molecules. 9 CD spectroscopy is commonly carried out in the ultra-violet (UV) and visible part of the electromagnetic spectrum, where it provides unique information on the secondary (190-250nm) and tertiary (>250nm) structure of proteins and on metal- protein interactions (visible range). Measurements are generally restricted to the solution phase, because of the inherent weakness of the conventional CD instruments in dealing with solid state samples. 11-15 This is often a major limitation, in that many proteins are embedded in membranes in their native state and their solid state structure and supramolecular properties are not obtainable from solution studies. * P.P.: email pasquale.pagliusi@fis.unical.it; phone +39 0984 496121; fax +39 0984 494401; * G.C.: email gabriella.ciapprrone@fis.unical.it; phone +39 0984 496148; fax +39 0984 494401; Integrated Photonics: Materials, Devices, and Applications, edited by Ali Serpengüzel, Giancarlo C. Righini, Alfred Leipertz, Proc. of SPIE Vol. 8069, 806910 · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.886969 Proc. of SPIE Vol. 8069 806910-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/19/2012 Terms of Use: http://spiedl.org/terms