The physics of localization-thermalization transitions is currently under active investigation. It holds the key to applications like noise-tolerant quantum memories and, more fundamentally, to our understanding of non-equilibrium statistical mechanics. In my project, I derived a Lindbladian master equation for a 1-dimensional tight-binding model interacting with a pure dephasing bath to study localization in open quantum systems. I basically solved the Liouville-von Neumann equation and evaluated disorder averaged populations at each site in the long time limit. Through these back-of-the-envelope style quick numerical calculations, we found that localization could be altered as a function of system-bath interactions.

I briefly explored the lattice random walks to model the transport process in mesoscopic systems. During the summer, I spent time working my way through the textbooks by Abraham Nitzan and Frank Spitzer to understand classical random walks. I did some rudimentary calculations to explore their link to exciton transport models.  

In organic semiconductors, disorder-induced localization of exciton presents a challenge to the development of sustainable organic photovoltaics. In my project, I developed a model of an exciton coupled to rotational degrees of freedom of each sub-unit(chromophore) in a conjugated polymer to study the effects of torsional disorder on exciton migration rates. As a first step, I studied the diffusion of excitons in ordered chains and derived an analytic expression for time-averaged transition probability. Further, I used exact diagonalization methods to numerically calculate disorder averaged quantities for static and dynamic torsional disorder cases. In general, I found that the exciton migration rates in the disordered molecules decreased inversely with the increasing length of the polymer.

[ Thesis | Slides ] 

The optical transmission through thin metal films is known to be poor but, the observation of enhanced optical transmission through sub-wavelength(nano-meter sized) holes due to the coupling with surface modes has attracted a lot of interest. This is popularly known as 'Extra-ordinary Optical Transmission.' The optical properties of such air-hole arrays on a metal-dielectric interface have been studied in many different configurations and arrangements. One such system is the Plasmonic quasi-crystal that has a unique property of tunable broadband optical transmission.

We attempted to understand the underlying mechanism of interaction between light and surface plasmon modes during my internship. My work involved using electron beam lithography, reactive-ion etching, and other cleanroom techniques to design and fabricate quasi-periodic patterns of these holes on gold thin films. I also used different experimental techniques in tandem, like pump-probe spectroscopy, to study the dynamics of surface-plasmon polaritons in these nanophotonic structures. The science we discovered through these experiments could help develop novel photonic devices for applications ranging from solar-energy conversion to optical communications.

[ Report ]