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Non-Hermitian systems host unconventional physical effects that can be exploited to design
next-generation all optical devices. Exceptional points (EPs) are a special kind of singularity
that appear as topological defects in the parameter-space of the open systems. EPs in non-
Hermitian systems have recently attracted a considerable attention owing to the unique state
flipping and peculiar phase accumulation features. The dynamical encirclement of an EP in
parameter space and corresponding time-asymmetric mode evolution properties have been
the key to a range of exotic physical effects in various open optical, atomic, and other
systems [1]. When light propagates through the open systems in the form of a planar optical
waveguides and a dual core optical fiber with suitable amount of simultaneous gain and loss,
the output mode becomes independent of the choice of the input excitation [2]. This
“asymmetric mode conversion” phenomenon can be explained by the swapping of mode
identities in the vicinity of the exceptional point, together with the failure of adiabatic
evolution in non-Hermitian systems hosing EPs. In addition, the hosting of two conjugate
EPs in two complementary systems connected with time-reversal () symmetry by using the
framework of a gain-loss assisted dual-mode planar optical waveguide has been proposed [3].
The existence of exceptional points of higher orders and the associated intriguing physics to
control the light dynamics has also been revealed [4].
Moreover, exploiting the dynamical EP encirclement scheme, a highly enhanced
nonreciprocal effect in the dynamics of light has been explored with the onset of saturable
local nonlinearity in the optical medium. A prototype waveguide-based isolation scheme with
the judicious tuning of level of nonlinearity where one can pass only a chosen mode in any of
the desired directions as per device requirement is reported. The deliberate presence of EP
enormously enhances the nonreciprocal transmission even up to 40 dB with a scope of further
scalability [5]. This exclusive robust, mode-selective all-optical scheme to achieve isolation
will certainly offer opportunities in integrated photonic circuits for efficient coupling
operation from external sources and hence improve device performances.
1. S. Ghosh and Y. D. Chong, “Exceptional points and asymmetric mode conversion in quasi-guided
dual-mode optical waveguides,” Scientific Reports 6, 19837 (2016).
2. A. Roy, S. Dey, A. Laha, A. Biswas, and S. Ghosh, “Exceptional Point induced asymmetric mode
conversion in a dual-core optical fiber segment”, Optics Letters, 47, 2546-2549 (2022)
3. A. Laha, S. Dey, and S. Ghosh “Reverse-chiral response of two T-symmetric optical systems hosting
conjugate exceptional points” Physical review A, 105, 022203, (2022).
4. A. Laha, D. Beniwal, and S. Ghosh, “Successive switching among four states in a gain-loss-assisted
optical microcavity hosting exceptional points up to order four,” Physical Review A (APS) 103(2),
023526 (2021)
5. A. Laha, S. Dey, H. K. Gandhi, A. Biswas, and S. Ghosh, “Exceptional point and toward mode
selective optical isolation,” ACS Photonics 7, 967—974 (2020). |