Department of Chemistry Seminar - Chirality Effects: Spin-Switchable Magneto-Optical Properties of Chiral Excitons in Helical Structures

Speaker: Prof. Bin HU

Institution: Frontier Institute of Science and Technology, Xi’an Jiaotong University, China

Hosted by: Prof. Haipeng LU

Abstract

Organic molecules are known to form spin excitons named as S excitons with spin characteristics but lacking orbital characteristics. The S excitons can be shown as singlets and triplets, leading to fluorescence and phosphorescence. Because of lacking orbital characteristics, the S excitons in organic molecules are difficult to generate circularly-polarized luminescence (CPL). On contrast, the excitons in inorganic semiconductors possess both orbital and spin characteristics, leading to the so-called J excitons where J = L + S (L: orbital momentum, S: spin momentum). Importantly, the orbital characteristics can demonstrate right-handed and left-handed circular polarizations to form right-handed and left-handed circularly-polarized excitons. Consequently, the J excitons can commonly demonstrate CPL. Therefore, the S and J excitons can be named as non-circularly polarizable and circularly-polarizable excitons, respectively. Interestingly, when the S excitons are helically ordered in chiral structures, CPL can then be demonstrated by helically-ordered S excitons. This phenomenon presents a breakthrough principle: the helically-ordered S excitons lacking orbital characteristics can be transformed into the J excitons intrinsically having orbital characteristics in chiral structures. Furthermore, by using 2D-superlattice perovskite thin-film systems, our magneto-optical studies found that the circularly-polarized J excitons can develop an extremely long-distance interaction with magnetic spins to demonstrate spin-switchable handiness of chiral excitons. This presents Chirality Effects in Excited States, as contrast to chirality effects in ground states known as CISS (Chirality Induced Spin Selection). Here, Chirality Effects in Excited States enabled by long-distance interaction between chiral excitons and electron spins can generate room-temperature optically-induced ferro-magnetization in 2D-superlattice perovskite thin-film systems. This presents a deeper understanding on chirality effects in excited states. This presentation will discuss the underlying mechanism for spin-switchable magneto-optical properties of chiral excitons.