Ghosh, Somnath

We propose a multi-channel specialty topological fiber based on 1D periodic geometry which supports three coexisting and non-interacting topologically-protected robust light states in the mid-infrared wavelength range. Each channel comprises two topologically-distinct 1D periodic lattices that are characterized by the topological invariant, Zak phase. The interface between the trivial and the nontrivial lattices is made unconventional to provide a strong transverse confinement of the light state. Moreover, the observed multiple interface states exhibit robust simultaneous transmission of light through all the channels keeping in contact with each other with an inter-channel cross-talk of ∼−27 dB. Such highly stable, scatter-free, and robust interface states have the potential to increase the transmission capacity of the existing optical communication channels, and can eventually be utilized as an alternative platform via the space-division multiplexing schemes.

Exploration of exceptional points (EPs) and associated unique features in gain-loss assisted optical systems to develop future all optical devices have been a great interest in recent years. However, incorporation and adjustment of gain distribution in a system is quite challenging. Here, we design a fabrication feasible dual-core optical fiber where only the customized transverse loss profile controls the interaction between two coupled modes and results in hosting an EP. Parametric encirclement of the identified EP and corresponding chirality-driven asymmetric mode conversion phenomenon between the supported modes have been reported. The proposed structure features ease of fabrication using state-of-the-art techniques with possible applications in all-optical components for communication and all fiber photonic devices.

Recently, there has been a growing interest in time-varying photonic media owing to their significant potential in the field of wave manipulation. Here, we explore the exotic characteristics of wave amplification in a photonic time crystal (PTC) made of a spatially homogeneous medium where the refractive index varies periodically in time. Based on qualitative and quantitative analysis of the amplification, we show that the amplification not only depends on the choice of wave vector of a propagating light but also attains different values in different bandgaps. Our approach further extends towards achieving the minimum amount of variation of permittivity required to open momentum gaps to facilitate the phase-dependent amplification of PTCs. Further, we investigate the impact of permittivity variation and choice of number of temporal unit cells to truncate a PTC to mimic the properties of infinite PTC and offer new opportunities to manipulate and control the amplification of light for applications including highly tunable PTC lasers and devices.

The occurrence of exceptional points (EPs) is a fascinating non-Hermitian feature of open systems. A level-repulsion phenomenon between two complex states of an open system can be realized by positioning an EP and its time-reversal (T) conjugate pair in the underlying parameter space. Here, we report interesting nonreciprocal responses of such two conjugate EPs by using a dual-mode planar waveguide system having two T-symmetric active variants concerning the transverse gain-loss profiles. We specifically reveal an all-optical scheme to achieve correlative nonreciprocal light dynamics by using the reverse chirality of two dynamically encircled conjugate EPs in the presence of local nonlinearity. A specific nonreciprocal correlation between two designed T-symmetric waveguide variants is established in terms of their unidirectional transfer of light with a precise selection of modes. Here, the unconventional reverse chiral properties of two conjugate EPs allow the nonreciprocal transmission of two selective modes in the opposite directions of the underlying waveguide variants. An explicit dependence of the nonlinearity level on a significant enhancement of the nonreciprocity in terms of an isolation ratio is explored by investigating the effects of both local Kerr-type and saturable nonlinearities (considered separately). The physical insights and implications of harnessing the features of conjugate EPs in nonlinear optical systems can enable the growth and development of a versatile platform for building nonreciprocal components and devices.

Branched flow refers to the bifurcation of waves as they propagate through customized disordered media, leading to the creation of multiple channels or branches. This dynamic wave phenomenon has been observed in various disordered systems under specific conditions. In our study, we demonstrate the branched flow of light within a disordered photonic lattice and establish that the intensity distribution of these branches follows Lévy statistics. This behavior is more pronounced when the wavefront of the input light beam is adjusted, resulting in the propagation of more stable branches through the structure. We also investigated the formation of light branches with varying input beam widths and the impact of two simultaneous input beams on the disordered photonic lattice. Our findings indicate that the distribution of branches in the disordered system can be controlled by shaping the wavefront of the input beam, which has potential applications in imaging, nano-lasing, and integrated photonic components. This research opens new avenues for fundamental studies on the manipulation of light through complex photonic systems.

We present the pulse shape dependent amplification in photonic time crystals. We study two types of pulses Gaussian pulse and hyperbolic secant (sech) pulse propagating through such medium.

The engineering of exceptional points (EP) in gain-loss-assisted optical systems has recently garnered considerable attention for the development of unconventional photonic components. Moreover, the substantial noise associated with EPs poses a significant challenge to realizing their full potential in applications. There is a gap in the exploration of asymmetric mode conversion phenomena in the close proximity of the dynamical encirclement around an EP. In response, we propose the design of a fabrication-feasible dual-mode optical planar waveguide. Non-Hermiticity, in terms of a customized gain-loss profile, is introduced to modulate the interaction between the coupled modes. Contrary to previous assumptions, we establish that the dynamical encirclement of an EP is not a prerequisite for achieving asymmetric mode conversion. Furthermore, we calculate the Petermann factor (K) near the EP and demonstrate a significant two orders of magnitude reduction in the Petermann noise factor when we do not dynamically encircle the EP. Hence, we propose a new scheme to reduce the inherent noise.

We report the hosting of a pair of conjugate exceptional points (EPs) while considering the interactions of coupled lasing states and absorbing states in a Fabry-Pérot-type optical microcavity. Under the scattering matrix formalism, we investigate the characteristics of lasing and absorbing states in terms of poles and zeros of the associated scattering matrix. While encircling two conjugate second-order EPs (EP2s) in the gain-loss parameter space, we exclusively reveal a correlative optical response in adiabatic switching schemes among the two corresponding coupled lasing states (poles) and two coupled absorbing states (zeros). © 2024 IEEE.

We demonstrate self-similar stable propagation of parabolic optical pulses through a highly nonlinear specialty Bragg fiber at 2.8 μm by a numerical approach. To obtain such propagation characteristics over a longer length of a Bragg fiber, we propose and verify a fiber design scheme that underpins passive introduction of a rapidly varying group-velocity dispersion around its zero dispersion wavelength and modulated nonlinear profile through suitable variation in its diameter. To implement the proposed scheme, we design a segmented and tapered chalcogenide Bragg fiber in which a Gaussian pulse is fed. Transformation of such a launched pulse to a self-similar parabolic pulse with full-width-at-half-maxima of 4.12 ps and energy of ∼39 pJ is obtained at the output. Furthermore, a linear chirp spanning across the entire pulse duration and 3 dB spectral broadening of about 38 nm at the output are reported. In principle, the proposed scheme could be implemented in any chosen set of materials.

We report a one-dimensional planar optical waveguide with a transverse distribution of the inhomogeneous loss profile, which exhibits an exceptional point (EP). The waveguide hosts two leaky resonant modes, where the interaction between them in the vicinity of the EP is controlled by proper adjustment of the inhomogeneity in the attenuation profile only. We study the adiabatic dynamics of propagation constants of the coupled modes by quasistatic encirclement of control parameters around the EP. Realizing such an encirclement with the inhomogeneous loss distribution along the direction of light propagation, we report the breakdown of adiabatic evolution of two coupled modes through the waveguide in the presence of an EP. During conversion the output mode is irrespective of the choice of the input excited mode but depends on the direction of light transportation. This topologically controlled, robust scheme of asymmetric mode conversion in the platform of the proposed all-lossy waveguide structure may provide a means for implementation of state-transfer applications in chirality-driven waveguide-based devices.