DCMPMS Seminars

Nano optics with fast electrons: Optical excitations in electron microscope

by Dr. Pabitra Das (Laboratoire de Physique des Solides, Université Paris-Sud, France)

Monday, April 2, 2018 from to (Asia/Kolkata)
at AG69
Description
Nano-optics aims at the understanding of optical phenomena at the nanometer length scale. A rapid dvancement of nanoscience and nanotechnology in last decades makes it essential to look for adequate tools and strategies for fabrication, manipulation and characterization at the nanometer scale as well as sophisticated analytical and computational methodology for better understanding of the underlying physics.1 Nano-optics covers a large span ranging from fundamental physics and material science to biology and medicine. The increasing trend towards nanoscience and nanotechnology makes it unavoidable to study optical phenomena on the nanometer scale. A central goal of research in nano optics is to develop and use optical technique at a length scale beyond the diffraction limit of light. In recent times several new approaches have been put forward to “shrink” the diffraction limit (e.g. confocal microscopy) or even to overcome it (near field microscopy). Another parallel technique that completely bypass the diffraction limit of light by using energetic electron beam as excitation source instead of light has also been developed in the domain of electron microscopy and in a very advanced stage now.2,3 In my talk I will mostly focus on my personal experience of last few years in doing optics and more specifically plasmonics within electron microscope. Plasmonics deals with light confinement at the nanoscale. This is achieved by binding light to coherent electron oscillations at the surfaces of metallic nanoparticles, the so-called surface plasmons (SPs) or surface plasmon polaritons. These SPs come together with large field enhancements and evanescent fields in the vicinity of metallic nanoparticles, which allow light confinement to sub-diffraction volumes. Besides being of fundamental interest, this topic holds promise for a variety of photonic applications, such as optical communication and storage, surface enhanced spectroscopies and quantum optics. Localized surface plasmons (LSPs) are non-propagating excitations of the conduction electrons of metallic nanostructures coupled to the electromagnetic field. In my talk, I will describe the Physics of localized surface plasmons. I will start with instrumentation for this purpose.4 Next I would discuss my research in this direction during last few years. I will discuss about the evolution of plasmon modes of single metallic with shape, size and dielectric property of the local environment.56 I will discuss about the numerical procedure to simulate the experimental results that we developed. Substrate effects and plasmon hybridization will be highlighted and discussed.78 Next I will shift to modal engineering with a cross like geometry and finally I will show how we can exploit the geometry of cross to probe non-Hermiticity and obtain counter intuitive phenomena like plasmon self-hybridization.9 
1 	Novotny, L., & Hecht, B. (2012). Principles of nano-optics. Cambridge university press. 
2 	De Abajo, F. G. (2010). Optical excitations in electron microscopy. Reviews of modern physics, 82(1), 209. 
3 	Kociak, M., & Stéphan, O. (2014). Mapping plasmons at the nanometer scale in an electron microscope. Chemical Society Reviews, 43(11), 3865-3883. 
4 	Das, P., & Chini, T. K. (2011). An advanced cathodoluminescence facility in a high-resolution scanning electron microscope for nanostructure characterization. Current Science, 101(7), 849-854. 
5 	Das, P., Chini, T. K., & Pond, J. (2012). Probing higher order surface plasmon modes on individual truncated tetrahedral gold nanoparticle using cathodoluminescence imaging and spectroscopy combined with FDTD simulations. The Journal of Physical Chemistry C, 116(29), 15610-15619. 
6 	Das, P., Kedia, A., Kumar, P. S., Large, N., & Chini, T. K. (2013). Local electron beam excitation and substrate effect on the plasmonic response of single gold nanostars. Nanotechnology, 24(40), 405704. 
7	Das, P., & Chini, T. K. (2014). Substrate induced symmetry breaking in penta-twinned gold nanorod probed by free electron impact. The Journal of Physical Chemistry C, 118(45), 26284-26291. 
8 	Das, P., Lourenço-Martins, H., Tizei, L. H. G., Weil, R., & Kociak, M. (2017). Nanocross: a highly tunable plasmonic system. The Journal of Physical Chemistry C, 121(30), 16521-16527. 
9 	Lourenço-Martins, H., Das, P., Tizei, L. H., Weil, R., & Kociak, M. (2018). Self-hybridization within non-Hermitian localized plasmonic systems. Nature Physics, 1.