Seminar by Dr Kishan Menghrajani
Title: Polaritons in confined optical fields: near-field probing, single-particle spectroscopy, and plasmonic function
Speaker: Dr Kishan Menghrajani, Research Fellow at Monash University
Abstract: Molecular and material resonances placed within confined optical fields can hybridise with light to form polariton states, altering the optical response and, potentially, the underlying physical and chemical behaviour. This talk examines how such coupling depends on the detailed structure of the optical mode, and what is missed when the response is characterised only in the far field. The first part concerns vibrational strong coupling on metallic gratings, where surface-enhanced Raman scattering is used as a near-field probe of polariton states. The lower polariton is found to be spatially localised at the grating slots, where the local coupling strength substantially exceeds the value obtained from far-field infrared transmission, placing the near-field response in the ultrastrong coupling regime while the far field remains conventionally strongly coupled. A complementary study of planar microcavities establishes that the usual Rabi splitting criterion is insufficient to guarantee polaritonic emission, and that a condition on cavity finesse must also be met, since spectrally overlapping photonic modes dilute the excitonic content of the lower polariton. The second part turns to plasmonic resonances in individual nanostructures, introducing a correlative approach in which dark-field scattering spectroscopy is combined with electron microscopy and elemental mapping on the same particles, separating the contributions of geometry and composition to the optical response, with wavelength-resolved measurements showing that photocatalytic activity tracks a specific plasmon mode. The talk closes on current work extending these near-field and single-structure methods to phonon-polariton resonators.
Venue: Seminar Room (202), Physics Department IIT Bombay, Powai, Mumbai