Transmission Circular Dichroism Research Articles

Circular dichroism of relativistically–moving chiral molecules

Understanding the impact of the relativistic motion of a chiral molecule on its optical response is a prime challenge for fundamental science, but it also has a direct practical relevance in our search for extraterrestrial life. To contribute to these significant developments, we describe a multi–scale computational framework that combines quantum chemistry calculations and full–wave optical simulations to predict the chiral optical response from molecules moving at relativistic speeds. Specifically, the effect of a relativistic motion on the transmission circular dichroism (TCD) of three life–essential biomolecules, namely, B–DNA, chlorophyll a, and chlorophyll b, is investigated. Inspired by previous experiments to detect interstellar chiral molecules, we assume that the molecules move between a stationary observer and a light source, and we study the rotationally averaged TCD as a function of the speed of the molecule.We find that the TCD spectrum that contains the signatures of the molecules shifts with increasing speed to shorter wavelengths, with the effects already being visible for moderate velocities.

Nonlinear Chiral Metasurfaces Based on the Optical Kerr Effect

AbstractChiral nanostructures have immense potential for complete polarization control of light. Given the intrinsic 3D structure of circularly polarized light, nanoarchitectures with structural chirality in 3D space are usually required to achieve “full” chiroptical response. On the other hand, due to the lack of available active materials at optical wavelengths, examples of chiral photonic devices that can be tuned in an active manner remain rare. Here, based on the large Kerr nonlinearities of silicon, the nonlinear chiroptical response from a planar Si metasurface supporting high quality (Q)‐factor guided mode resonances (GMRs) at near‐infrared wavelengths, are numerically demonstrated. In the linear regime, the optical anisotropy of the C2‐symmetric structure associated with the observed GMRs leads to a pair of chiral resonance modes, enabling pronounced cross‐polarization transmission circular dichroism and a strong handedness‐dependent field enhancement effect. Owing to the nonlinear Kerr effect, such Si metasurfaces are seen to exhibit prominent chiral‐selective optical nonlinearity. Importantly, a pronounced co‐polarization transmission circular dichroism is observed, suggesting that the circularly polarized waves in the nonlinear regime possess asymmetric Jones matrices. Dielectric chiral metasurfaces with Kerr nonlinearities can be utilized to facilitate polarization‐state modulators with simple planar structures.

Experimental and Theoretical Studies of Magnetic Circular Dichroism for Chiral Lead Halide Perovskites

Chiral lead halide perovskites (LHPs) exhibit pronounced absorption or transmission circular dichroism (CD), which can be efficiently tuned by externally applied magnetic fields, giving rise to magnetic circular dichroism (MCD). To date, a systematic understanding of natural optical activities (NOAs) and magneto-optical activities (MOAs) in LHPs on an equal footing is still lacking. In this work, a joint experimental and theoretical study is carried out systematically in order to establish a unified picture of CD and MCD via a gyration vector for highly crystalline and solution-processable chiral LHPs. Experimentally, MCD strengths that appear near photoabsorption band edges of the chiral LHPs can be enhanced remarkably with increasing fields up to ±1.6 T. The experimental results can be well accounted for by a theory based on excitonic states, material chirality, and field dependence of the gyration vector. Importantly, numerical values of the exciton’s g-factor and chirality-induced spin–orbit coupling (SOC) strength can be extracted. Our work suggests that the magnetic field can be an important tool to elucidate the microscopic origin of NOA in chiral LHPs.

Chiral biosensing using terahertz twisted chiral metamaterial.

Subwavelength chiral metamaterials with tunable geometries and compositions are essential to advance the development of chiral biochemical samples detection. Here, we report a spatial symmetry breaking chiral terahertz (THz) metamaterial structure with stacked layers of L-shape arranged gold disks as the periodic unit cell. The chiroptical response can be adjusted on-demand by manipulating the number of stacking layers and the twisted angle of the periodic unit between adjacent array layers. We reveal that the chiroptical response originates from the optical resonances of the gold disks and the adjacent gold disks array layers via experiments and numerical simulation analysis. Furthermore, we find that this chiral metamaterial can realize label-free detection of proline in biological samples and label-free enantio-discrimination of chiral molecules. The change of the analyte concentration can also regulate the transmission circular dichroism (TCD) intensity of the chiral metamaterials. Our results not only provide new ideas into the design of functional chiral metamaterials, but also bring new strategies to develop chiroptical biosensing devices.

A circular dichroism technique for the study of adsorbed protein structure

A cell that allows the structure of adsorbed protein layers to be examined by transmission circular dichroism is described. Sensitivity of the technique is demonstrated by presentation of spectra obtained from the adsorption of two blood proteins [bovine fibrinogen and human clotting factor XII (Hageman factor)] on a quartz substrate. The former does not undergo a conformational change on adsorption: the latter becomes activated and suffers a major structural change. It is suggested that such methodology is useful in the investigation of the interaction of blood components with prospective polymeric prosthetic implants.