Nanophotonic approaches using plasmonic and dielectric antennas to enhance both the intensity and chirality of light amplify selectively the rate of absorption of one enantiomer over its mirror image. These strategies have primarily aimed to increase enantioselective absorption by chiral molecules at UV and visible frequencies to achieve greater sensitivity in chiral detection using circular dichroism spectroscopy. Alternatively, chiral-optical enhancement of vibrational excitations using nanophotonic platforms in the mid-infrared has not yet been extensively explored. Vibrational optical activity is a spectroscopic probe of molecular chirality that provides complementary structural information to electronic circular dichroism in the UV regime, and enables unambiguous determination of absolute handedness in combination with density functional theory calculations. To enhance circular dichroism strength for chiral chemical analysis by vibrational spectroscopies, we designed periodic arrays of subwavelength dielectric antennas termed “metasurfaces” capable of producing amplified, circularly polarized electromagnetic near fields upon excitation with linearly polarized light in the mid-infrared region. Thus, metasurfaces would enable the use of spectroscopic methods that are inherently incapable of chiral discrimination for stereochemical analysis of chiral compounds, and to monitor enantiomeric purity in chemical reactions. Using full-field electromagnetic simulations, we demonstrate numerically how these dielectric antenna arrays can support near-field enhancements in electromagnetic chirality density (the figure of merit for sensing and spectroscopy) within narrow bandwidths (ca. 25 cm-1). These resonances can be tuned across the full molecular fingerprint region (1500 cm-1– 500 cm-1) by scaling the critical dimensions of the antennas to overlap with individual fundamental modes of vibration of chiral molecules. By embedding the high-index antenna arrays within an effective chiral medium model, we show how spectral overlap of molecular resonances with enhanced optical chirality density can increase chiral-optical absorption.

All-dielectric, mid-infrared metasurfaces for vibrational circular dichroism enhancement

2019 Fall National Meeting

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