DEPARTMENT OF COMPUTATIONAL AND DATA SCIENCES
Ph.D. Thesis Defense (Online)
Speaker : Ms. Kritika Jain
S.R. Number : 06-18-01-10-12-15-1-12709
Title : “Understanding spontaneous emission in the strong-coupling regime of an emitter and absorbing matter”
Date & Time : 14th July 2021 (Wednesday), 03:30 PM
Venue : Online
This thesis explains the unexpected large increase in the spontaneous emission of photons from emitters such as molecules and quantum dots, when placed near fully absorbing metal nanoparticles less than 10 nm in dimensions; an anomaly that was highlighted by earlier work at the Institute. These very small metal nanoparticles do not scatter light, are dissipative, and were understood to not increase the local density of optical states (LDOS) available for spontaneous emission. On the other hand, strongly scattering larger plasmonic metal structures (> 50 nm in dimensions) significantly enhance the density of optical states, and this is well-known as the Purcell effect.
This work also unravels the origins of large gains observed in surface-enhanced-Raman-spectroscopy (SERS). In SERS, a rough metal surface (or metal nanopore) effects a near-field enhancement of the incident radiation exciting the proximal molecule, by factors up to 10^5. But the photons emitted from the proximal excited molecule are predicted to be largely dissipated by the metal, making the observed large gains due to the surfaces anomalous in conventional theory. This remarkable divergence of SERS from theoretical predictions has been widening for four decades, during which the reported SERS enhancements have grown from 10^4 to 10^14.
The first part of this work established the divergence of multiple independent experimental observations from the theory, using quantitative evaluations of an emitter coupled to very small metal nanoparticles. It further involved a study of collective spontaneous emission from multiple (strongly coupled) emitters that are also coupled to metal nanoparticles; the study was possible due to a computational method developed earlier for solving such problems. This study established that the collective modes of emission of a photon are not the source of this divergence of theory from the observations. It was also understood from the works of others that a quantization of the emitter, the nanoparticle, or the fields do not resolve the incorrect theoretical predictions. Also note that the emitter and the excited nanoparticle are not placed in a cavity and are only weakly coupled to vacuum.
We then revisited the moderate to strongly coupled nanostructures, where the photon may be re-absorbed by the emitter at ground-state, from the excited dissipating nanostructure. A modification to the conventional (Markovian) partition of optical states into the radiative and non-radiative (dissipative) parts was proposed. We invoked the quantum interference of additional paths involved in this regime to derive a one-loop (first order) correction increasing the radiative decay rates significantly at the cost of the non-radiative decay rates. This phenomenological theory of partition of optical states was shown to predict the experimental observations, and it provides a simple formula to correct the conventional evaluations of decay rates. The new partition of optical states was also incorporated into the model of collective emission from multiple emitters, and it allowed us to elucidate emission in bulk materials dispersed with extremely small metal nanoparticles.
Later, a first-principles microscopic model of the non-Markovian interactions of an emitter and dissipating matter was developed, which validates the above proposed first-order corrections. Though this recent work is not part of the thesis, a few of its results will be presented to conclude.