Enhanced Circular Dichroism and Chiral Sensing with Bound States in the Continuum

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Accessible Superchiral Near-Fields Driven by Tailored Electric and Magnetic Resonances in All-Dielectric Nanostructures

Hatice Altug, Yasaman Jahani, Ershad Mohammadi, Andreas Tittl, Kosmas Tsakmakidis

Detection and differentiation of enantiomers in small quantities are crucially important in many scientific fields, including biology, chemistry, and pharmacy. Chiral molecules manifest their handedness in their interaction with the chiral state of light (e.g., circularly polarized light), which is commonly leveraged in circular dichroism (CD) spectroscopy. However, compared to the linear refractive index molecular chirality is extremely weak, resulting in low detection efficiencies. Recently, it has been shown that these weak chiroptical signals can be enhanced by increasing the optical chirality of the electromagnetic fields interacting with chiral samples. Here, we show numerically and analytically that dielectric structures can provide an optimum chiral sensing platform by offering uniform superchiral near-fields. To illustrate this, we first study a simple dielectric dimer and show that circularly polarized light can induce parallel and out of phase electric and magnetic fields, which are spectrally and spatially overlapped, and therefore produce superchiral fields at the midpoint of the dimer. This behavior is in contrast to, for example, a plasmonic dimer, where the optical chirality is limited by the electric dipolar field, which is not completely out of phase with the incident magnetic field. With the insights gained from this analysis, next we develop an approach for overlapping electric and magnetic fields in a single particle, based on Kerker effect. In particular, we introduce a Kerker-inspired metasurface consisting of holey dielectric disks, which offers uniform and accessible superchiral near-fields with CD signal enhancements of nearly 24 times.

AMER CHEMICAL SOC2019

Hard X-ray helical dichroism of disordered molecular media

Camila Bacellar Cases Da Silveira, Oliviero Cannelli, Majed Chergui, Christian David, Dominik Kinschel, Jérôme Lacour, Giulia Fulvia Mancini, Malte Oppermann

Chirality is a structural property of molecules lacking mirror symmetry that has strong implications in diverse fields, ranging from life sciences to materials science. Chirality-sensitive spectroscopic methods, such as circular dichroism, exhibit weak signal contributions on an achiral background. Helical dichroism, which is based on the orbital angular momentum (OAM) of light, offers a new approach to probe molecular chirality, but it has never been demonstrated on disordered samples. Furthermore, in the optical domain the challenge lies in the need to transfer the OAM of the photon to an electron that is localized on an angstrom-size orbital. Here we overcome this challenge using hard X-rays with spiral Fresnel zone plates, which can induce an OAM. We present the helical dichroism spectra of a disordered powder sample of enantiopure salts of the molecular complex of Fe(4,4 '-diMebpy)(3) at the iron K edge (7.1 keV) with OAM-carrying beams. The asymmetry ratios for the helical dichroism spectra are within one to five percent for OAM beams with topological charges of one and three. These results open a new window into the studies of molecular chirality and its interaction with the OAM of light. Linearly polarized orbital angular momentum-carrying hard X-ray beams are induced using spiral Fresnel zone plates. By sending the hard X-ray beams to disordered enantiopure molecular complexes, the helicity-dependent and chiral-sensitive signal is obtained.

NATURE PORTFOLIO2022