Light-Matter Strong Coupling:
Resonant interaction of matter with confined photons (electromagnetic field) can lead to weak and strong coupling of the components, depending on the nature of interaction.
Under weak coupling the radiative rates and intensity of the emitter are altered while under strong coupling, light and matter exchange energy at Rabi frequency, faster than all other dissipations, resulting in the formation of two new hybrid states known as polaritonic states (P+ and P-), separated by the Rabi splitting energy (ħΩR) as schematically shown below. Molecules may be placed inside a Fabry-Perot optical cavity or on a plasmonic substrate to achieve strong and weak light-matter coupling.
An interesting aspect of light matter strong coupling is that quantum fluctuation (zero-point energy) of the electromagnetic field and the transition dipole of matter can strongly couple resulting in the vacuum Rabi splitting. In other words, Rabi splitting energy can be finite, even in the absence of real photons. Since a given electromagnetic mode has relatively large volume (~mm3 in the visible region) compared to molecules (~nm3), a large number of molecules can occupy the single mode resulting in enhanced Rabi splitting energy. An interesting consequence is that the wavefunction of the polaritonic states are spread over all the molecules involved in the coupling. This research field is fast emerging and has attracted much attention in the scientific community, especially since the demonstration of the modified chemical reactivity and molecular /material properties.
Implications of Light-Matter Strong Coupling:
Polaritonic chemistry: Light-matter strong coupling, especially the vibrational strong coupling (VSC) has a large impact on the chemical reactivity landscape. Since the first observation of the rate modification under VSC in the year 2016, different chemical reactions have been found to be modified, showing the potential of this physical technique as a means to control chemical reactivity through selective vibrations.
Modified physical and transport properties: The delocalized nature of the polaritonic states have been effectively used to enhance the transport, superconducting and magnetic properties of molecules and materials. Strong coupling facilitated the energy transfer between spatially separated donor and acceptor molecules (over 100 nm) by entangling through the cavity mode.
Chiroptics and Polarization Tomography of Molecules and Their Assemblies using Mueller Polarimetry
Mueller polarimetry is an interesting tool to get artifact free chiroptical characteristics of molecules and their dynamic assemblies. In essence, the Mueller polarimetry is the measure of the transformation of the incident polarization of light by a sample.
The input and output polarization states can be connected through Mueller matrices that rigorously describe the transformation of polarization states of light through a medium, which is experimentally followed by recording the intensity traces of the transmitted light. A unique feature of Mueller polarimetry is that it can be used to extract artifact-free circular dichroism (CD), linear dichroism (LD), circular and linear birefringence of the optically active medium from a single experiment. Moreover, the Mueller polarimetry can be extended to emission to shine light on the excited state chiroptical properties. Our group’s research on Mueller polarimetry will be aimed at the understanding of (i) chiral topology of molecular assemblies, (ii) origin of circularly polarized emission in molecular systems and (iii) to follow the evolution of the polarization in the Poincaré sphere.