Towards development of new generation spin-orbit photonic techniques in observational astronomy

Athira, B. S. (2022) Towards development of new generation spin-orbit photonic techniques in observational astronomy. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Athira B S (16RS013)) 16RS013.pdf - Submitted Version Restricted to Repository staff only Download (5MB)

Abstract

Objects in the universe emit electromagnetic radiation at different wavelength depending upon the composition, density, temperature and environments. Radiation with wavelengths between roughly 400 nm and 700 nm is called visible light because these are the waves that humans can perceive with one of the five senses, vision. The amount of radiation from the sky as a function of time, wavelength, direction and polarization is the fundamental measurement, an astronomer makes. Bartholin Erasmus and Étienne Malus are credited for the early days of polarimetry. But, the first polarimeter was built by Jean-François Dominique Arago in 1811 and was used for measuring the polarization of the Sun light reflected from the Moon. It is to note that his polarimeter already used the basic principle of all good polarimeters in astronomy today. Along with the requirement of developing new polarimetry techniques and analysis schemes for probing the dynamics in the astrophysical domain where high cadence and high precision measurement of spatial polarizations are desirable, there is a need to amplify the polarization signal to overcome the detection limit, especially when the incoming photon count is less. Weak measurement is one tool which can amplify tiny effects up to several orders of magnitude under some special cases. The weak measurement process involves preparation of the system state in a definite initial (pre-) state, which due to weak coupling to the observable results in a superposition of "slightly" shifted eigenstates and subsequent post- selection in a final (post-) state which is nearly orthogonal to the initial state. The outcome of a pre- and post- selected weak measurement is weak value and the scheme is called weak value amplification. It is well known that light (photon) carries angular momentum, spin angular momentum (SAM) is associated with polarization and orbital angular momentum (OAM) is associated with phase of wavefront. Observations suggest the existence of inhomogeneities in the interstellar medium. Martin Harwit in 2003 has described several astrophysical processes that generate OAM, light scattering in the interstellar medium, rotating black holes etc. The spin and the orbital degrees of freedom of light can get coupled under certain circumstances leading to interesting consequences like spin orbit interaction (SOI). Thus, knowledge on SOI are crucial for both fundamental understanding of the interactions and for optimizing experimental parameters for practical applications in diverse systems such as biological systems, complex materials apart from astronomy. In astrophysical domain, solar coronal magnetic field measurement is one such example wherein the time scale of various dynamic processes such as eruptions are of the order of several seconds. Also, the circular polarization signal which determines the strength of coronal magnetic field is weak (10−4) to estimate. Therefore, it is main objective of the present thesis to explore SOI and to develop novel spin orbit photonic techniques in observational solar astronomy unlike traditional differential photometric measurements. Although the method described in the thesis are developed with the motivation of studying high dynamic source such as the Sun, it may be extended further in the domains of understanding of the complex structure of different biological samples, uncovering three-dimensional characteristics of chemical bonds, and for the characterization of complex nano-materials and so forth.

Actions (login required)

View Item