Abstract

Circular dichroism (CD) spectroscopy is one of the most relevant tools for the discrimination of enantiomers and for the determination of their configuration and conformation. However, CD signals are extremely weak, making the analysis of small amounts of chiral analytes very challenging. Recently, novel ‘superchiral’ approaches have been proposed to enhance the CD signal by tailoring the properties of the electromagnetic field through the control of the associated optical chirality. In this framework, plasmonic chiral sensing holds exciting perspectives but several limitations have also been discussed. In this talk we first introduce a universal limit to plasmonic superchirality, demonstrating that in the quasi-static approximation the average optical chirality in the surrounding of a plasmonic nanostructure is analytically bound and the upper limit poses significant challenges in the visible spectral range [1]. These findings also justify recent proposals to move to dielectric materials for superchiral spectroscopies. Along this line, we introduce the concept of ‘superchiral surface waves’ originating from the coherent superposition of the TE and TM surface modes in a one-dimensional photonic crystal with an anisotropic metamaterial surface defect [2,3]. The resulting platform provides superchiral fields over arbitrarily large areas and wide spectral ranges (up to the UV), with CD signal enhancements of more than 2 orders of magnitude. Moreover, the original spectral fingerprint associated with a specific CD resonance is reconstructed with high fidelity. These findings pave a possible way towards on- chip surface-enhanced chiral sensing, spectroscopy, and all-optical manipulation [2-4].