Shane Nichols

Model for Photoinduced Bending of Slender Molecular Crystals

The growing realization that photoinduced bending of slender photoreactive single crystals is surprisingly common has inspired researchers to control crystal motility for actuation. However, new mechanically responsive crystals are reported at a greater rate than their quantitative photophysical characterization; a quantitative identification of measurable parameters and molecular-scale factors that determine the mechanical response has yet to be established. Herein, a simple mathematical description of the quasi-static and time-dependent photoinduced bending of macroscopic single crystals is provided. This kinetic model goes beyond the approximate treatment of a bending crystal as a simple composite bilayer. It includes alternative pathways for excited-state decay and provides a more accurate description of the bending by accounting for the spatial gradient in the product/reactant ratio. A new crystal form (space group P21/n) of the photoresponsive azo-dye Disperse Red 1 (DR1) is analyzed within the constraints of the aforementioned model. The crystal bending kinetics depends on intrinsic factors (crystal size) and external factors (excitation time, direction, and intensity).

Mueller matrices in fluorescence scattering

A reappraisal of the 1929 analysis of luminescence by Soleillet reveals the form of the Mueller matrix for fluorescence scattering whose parameters are directly defined in terms of the now-familiar fluorescence anisotropy parameter. If the scattering analyte is optically active, it is further shown how fluorescence detected circular dichroism and circularly polarized luminescence can be recovered, simultaneously and free of artifacts.

Complete polarimetry on the asymmetric transmission through subwavelength hole arrays

Dissymmetric, periodically nanostructured metal films can show non-reciprocal transmission of polarized light, in apparent violation of the Lorentz reciprocity theorem. The wave vector dependence of the extraordinary optical transmission in gold films with square and oblique subwavelength hole arrays was examined for the full range of polarized light input states. In normal incidence, the oblique lattice, in contrast to square lattice, showed strong asymmetric, non-reciprocal transmission of circularly polarized light. By analyzing the polarization of the input and the output with a complete Mueller matrix polarimeter the mechanisms that permits asymmetric transmission while preserving the requirement of electromagnetic reciprocity is revealed: the coupling of the linear anisotropies induced by misaligned surface plasmons in the film. The square lattice also shows asymmetric transmission at non-normal incidence, whenever the plane of incidence does not coincide with a mirror line.

Circular Birefringence of Banded Spherulites

Crystal optical properties of banded spherulites of 21 different compounds--molecular crystals, polymers, and minerals--with helically twisted fibers were analyzed with Mueller matrix polarimetry. The well-established radial oscillations in linear birefringence of many polycrystalline ensembles is accompanied by oscillations in circular birefringence that cannot be explained by the natural optical activity of corresponding compounds, some of which are centrosymmetric in the crystalline state. The circular birefringence is shown to be a consequence of misoriented, overlapping anisotropic lamellae, a kind of optical activity associated with the mesoscale stereochemistry of the refracting components. Lamellae splay as a consequence of space constraints related to simultaneous twisting of anisometric lamellae. This mechanism is supported by quantitative simulations of circular birefringence arising from crystallite twisting and splaying under confinement.

Measurement of transmission and reflection from a thick anisotropic crystal modeled by a sum of incoherent partial waves.

Formulas for modeling ellipsometric measurements of bianisotropic crystals assume perfectly coherent plane wave illumination. As such, the finite coherence of typical spectroscopic ellipsometers renders such formulas invalid for crystals thicker than a few micrometers. Reflection measurements of thick crystalline slabs show depolarization. Researchers have proposed strategies for the full accounting for multiply reflected incoherent waves in anisotropic, arbitrarily oriented crystals [Appl. Opt.41, 2521 (2002).APOPAI0003-693510.1364/AO.41.002521], but to the best of our knowledge these methods have not been tested by explicit measurements. It is shown that by a summation of multiply reflected incoherent waves, transmission and reflection measurements of thick quartz slabs can be interpreted in terms of the constitutive material parameters.

Well Plate Circular Dichroism Reader for the Rapid Determination of Enantiomeric Excess

Circular dichroism (CD) spectropolarimeters typically employ one photoelastic modulator. However, spectropolarimeters employing two or even four modulators are more versatile and can be used to subvert common measurement errors arising from imperfectly isotropic samples or sample holders. Small linear anisotropies that can cause large errors in CD measurement can be associated with multi-well sample holders. Thus, high-throughput CD analyses in multi-well plates have not yet been demonstrated. One such application is the determination of enantiomeric excess of a library of reaction products. Herein, a spectropolarimeter employing four photoelastic modulators and a translation stage was used to determine the enantiomeric excess of a family of chiral amine complexes much more rapidly than could be achieved with a robotic fluid injection system. These experiments are proof of concept for high-throughput CD analysis. In practice, commercially available glass bottomed well plates are sufficiently strain free such that a simple instrument with just one photoelastic modulator and a vertical optical train should be able to deliver the CD without special considerations given herein. On the other hand, polystyrene well plates cannot be used in this way.

Gyration and Permittivity of Ethylenediammonium Sulfate Crystals.

Ethylenediammonium sulfate (EDS) crystals were grown from aqueous solution and cleaved into thin (100-500 micron) plates. The 422 point group of EDS was confirmed by X-ray diffraction. The constitutive relations of EDS crystals were determined through generalized ellipsometry with an instrument that uses four photoelastic modulators (4PEM). The optical rotation at 500 nm, for example, was + 22.9°/mm along the optic axis and - 12.1°/mm perpendicular to the optic axis for the P41 21 2 crystals. Enantiomorphous twins frequently form across the (001) plane. Mirrored halves must be separated by cleavage in advance of optical measurements. Chirality 28:460-465, 2016.

Relation between 2D/3D chirality and the appearance of chiroptical effects in real nanostructures

The optical activity of fabricated metallic nanostructures is investigated by complete polarimetry. While lattices decorated with nanoscale gammadia etched in thin metallic films have been described as two dimensional, planar nanostructures, they are better described as quasi-planar structures with some three dimensional character. We find that the optical activity of these structures arises not only from the dissymmetric backing by a substrate but, more importantly, from the selective rounding of the nanostructure edges. A true chiroptical response in the far-field is only allowed when the gammadia contain these non-planar features. This is demonstrated by polarimetric measurements in conjunction with electrodynamical simulations based on the discrete dipole approximation that consider non-ideal gammadia. It is also shown that subtle planar dissymmetries in gammadia are sufficient to generate asymmetric transmission of circular polarized light.

Imaging with photoelastic modulators

Photoelastic modulators (PEMs) are among the most robust and precise polarization modulation devices, but the high frequency free-running nature of PEMs challenges their incorporation into relatively slow CCD and CMOS imaging systems. Current methods to make PEMs compatible with imaging suffer from low light throughput or use high cost intensified CCDs. They are not ideal for some analyses (microscopy, reflectivity, fluorescence, etc.), and likely cannot be extended to polarimeters with more than two PEMs. We propose to modulate the light source with a square wave derived from particular linear combinations of the elementary PEM frequencies and phases. The real-time synthesis of the square waves can be achieved using a field programmable gate array (FPGA). Here we describe the operating principle.

Revisiting polarimetry near the isotropic point of an optically active, non-enantiomorphous, molecular crystal

Accurate polarimetric measurements of the optical activity of crystals along low symmetry directions are facilitated by isotropic points, frequencies where dispersion curves of eigenrays cross and the linear birefringence disappears. We report here the optical properties and structure of achiral, uniaxial (point group D2d ) potassium trihydrogen di-(cis-4-cyclohexene-1,2-dicarboxylate) dihydrate, whose isotropic point was previously detected (S. A. Kim, C. Grieswatch, H. Küppers, Zeit. Krist. 1993; 208:219-222) and exploited for a singular measurement of optical activity normal to the optic axis. The crystal structure associated with the aforementioned study was never published. We report it here, confirming the space group assignment I 4¯c2, along with the frequency dependence of the fundamental optical properties and the constitutive tensors by fitting optical dispersion relations to measured Mueller matrix spectra. k-Space maps of circular birefringence and of the Mueller matrix near the isotropic wavelength are measured and simulated. The signs of optical rotation are correlated with the absolute crystallographic directions.