Left, Right, and Ultrafast: Chiral Dynamics in Gold Nanoparticles and Perovskite Lattices by Asst Prof Sankaran Ramesh

"Left, Right, and Ultrafast: Chiral Dynamics in Gold Nanoparticles and Perovskite Lattices"

Chirality describes objects that cannot be superimposed on their mirror images. A light-absorbing chiral material exhibits asymmetry in its absorption of left- and right-circularly polarised light. There are several reasons to study chiral light-matter interactions: such as selectively sensing left- or right-handed molecules in biology and chemistry, steering quantum states with circularly polarised light, and designing materials that guide light in asymmetric ways. 

This will be a two-part talk discussing the ultrafast study of two such systems. In the first part, I will present chiral plexcitons realised by functionalising intrinsically chiral gold helicoid nanoparticles with molecular J-aggregates. Using a non-Hermitian framework alongside space-, time-, and polarisation-resolved measurements, we trace how structural chirality and plasmon-exciton coupling shape the chiroptical response, and show that gap-localised fields enhance polarisation-sensitive contrast while accelerating sub-picosecond relaxation. In the second part, I will turn to two-dimensional hybrid perovskites, where chiral organic spacers are known to induce optical activity but the microscopic coupling between molecular chirality and the inorganic lattice has remained unclear. Using ultrafast spectroscopy, we identify a coherent lattice mode involving coupled motion of the chiral spacer and Pb–I framework that couples strongly to excitons only in the chiral material, and is absent in the racemic analogue. Together, these studies establish chirality as a practical control parameter for steering energy pathways, with coherent lattice motion and plasmon-exciton hybridisation serving as microscopic mediators of chiral response on ultrafast timescales.