Chiral organic photonics: self-assembled micro-resonators for an enhanced circular dichroism effect in the non-linear optical signal

Abstract

Chiro-optical signals from chiral molecules/structures are inherently weak. In particular, enhancing the circular-dichroism (CD) effect (different absorption of left or right circularly polarized light) in the non-linear optical (NLO) signal from chiral molecular materials is challenging and imperative since it is advantageous in various areas, from biomedical sciences to miniaturized photonic devices. Here we suggest and realize for the first time an approach to enhance the CD effect taking advantage of the NLO effects resonantly enhanced in the nonlinear chiral organic micro-resonators. These R- and S-type resonators in the form of micro-spheres are prepared by the self-assembly technique from the corresponding axially chiral enantiomeric molecules, namely R- and S-4-[2,2′-diethoxy-6′-(4-formylphenyl)-[1,1′-binaphthalen]-6-yl]benzaldehyde in CHCl₃/MeOH mixtures. Each enantiomeric micro-sphere, upon optical pumping, acts as a whispering gallery mode (WGM) resonator as the photo-luminescence (PL) is excited in a single- or in a two-photon regime. We show that the NLO-CD effect and the two-photon absorption (TPA) in the case of micro-spheres are at least twice as high as compared to a continuous film of the same effective thickness. This is due to pronounced field localization effects in micro-spheres with high Q-factor, which increases the photon residence time (τ_P) in a chiral medium and thus strengthens the light–matter interaction. Finite-difference time-domain (FDTD) numerical calculations confirm a strong light localization near the boundary of the micro-spheres.