Device applications of specialty optical waveguides hosting exceptional points
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Date
29-09-2023
Researcher
Dey, Sibnath
Supervisor
Ghosh, Somnath
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Publisher
Indian Institute of Tehcnology, Jodhpur
Abstract
The burgeoning field of non-Hermitian formalism in photonics has recently garnered considerable attention due to its capacity to unveil unconventional phenomena. Exceptional Points (EP s) within nonHermitian systems are particularly interesting, representing critical junctures where system behavior deviates significantly from the norm. EP s occur when coupled eigenvalues and corresponding eigenstates coalesce, leading to the collapse of the eigenspace-dimensionality of the underlying Hamiltonian. This collapse endows EP s with the characteristic of topological defects, and they have found applications in generating unique optical phenomena such as unidirectional lasing, enhanced sensing, asymmetric mode conversions, and optical isolation. This thesis explores EP s of various orders and investigates the resulting unconventional light dynamics in specialty planar optical waveguides with spatial refractive index variations and photonic bandgap waveguides. Customized planar optical waveguides and photonic bandgap waveguides featuring non-conservative elements like optical loss/gain are studied within a non-Hermitian framework to reveal topological avoided-crossing phenomena between coupled states/modes around specific singularities. The investigation delves into the intricacies of second-order, third-order, and fourth-order EP s within optical waveguides, uncovering unique mode excitation phenomena, chiral/non-chiral characteristics, and innovative methods for higher-order mode conversions. The study also extends to one-dimensional photonic bandgap waveguides, where the gain-loss processes enable precise control over TE modes through EP s. Dynamic encirclements around the EP lead to asymmetric mode conversions resilient to high fabrication tolerances. Conjugate EP s within dual-mode planar waveguides featuring T -symmetric optical potentials and complex refractive index profiles are explored. Reverse chiral response achieved through dynamic encirclements of a second-order conjugate EP s (EP2s) yields varying dominant modes dependent on encirclement direction. This concept is extended to a 1D photonic bandgap waveguide system, emphasizing robustness in the face of high parameter tolerances. Moreover, higher order conjugate EP s in two complementary systems have also been investigated, which leads to unconventional light dynamics by exploiting the time-reversal T symmetry. On the other hand, the dynamical encirclement of EP s in the presence of local nonlinearity has also been studied. By incorporating Kerr nonlinearity in the presence of gain-loss, optical waveguides are able to exhibit direction-independent mode conversion. Higher-order mode-collapsing phenomenon is observed during dynamical encirclements around higher-order conjugate EP s in multimodal complementary waveguides, offering the potential for one-way light transmission and intricate optical responses. This comprehensive exploration advances the understanding of EP s in photonics and their application in unconventional optical devices, paving the way for transformative developments in integrated all photonic components and devices for next-generation communication as well as emerging quantum technological applications.
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Citation
Dey, Sibnath (2018). Device applications of specialty optical waveguides hosting exceptional points (Doctor's thesis). Indian Institute of Tehcnology, Jodhpur