Chirality sensing employing parity-time-symmetric and other resonant gain-loss optical systems

Abstract

Molecular chirality detection and enantiomer discrimination are very important issues for many areas of science and technology, prompting intensive investigations via optical methods. However, these methods are hindered by the intrinsically weak nature of chiro-optical signals. Here, we investigate and demonstrate the potential of gain materials and of combined gain-loss media to enhance these signals. Specifically, we show that the proper combination of a thin chiral layer with a gain-loss bilayer can lead to large enhancements of both the circular dichroism (CD) response and the dissymmetry factor $g$ compared with the chiral layer alone. The most pronounced enhancements are obtained in the case of a parity-time ($\mathit{PT}$) symmetric gain-loss bilayer, while deviations from the exact $\mathit{PT}$ symmetry lead to only moderate changes of the CD and $g$ response, demonstrating also the possibility of tuning the system response by tuning the gain layer properties. In the case of $\mathit{PT}$-symmetric gain-loss bilayers, we found that the largest CD enhancement is obtained at the system lasing threshold, while the $g$ enhancements are at the anisotropic transmission resonances of the systems. Our results clearly demonstrate the potential of gain materials in chirality detection. Moreover, our gain-involving approach can be applied in conjunction with most of the nanophotonics/nanostructures-based approaches that have already been proposed for chirality sensing, further enhancing the performance/output of both approaches.