Ghosh, Somnath

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.

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.

We report an asymmetric behavior of optical pulses during their propagation through a time-varying linear optical medium. The refractive index of the medium is considered to be varying with time and complex, such that a sufficient amount of gain and loss is present to realize their effect on pulse propagation. We have exploited the universal formula for optical fields in time-varying media. Numerically simulated results reveal that pulses undergo opposite temporal shifts around their initial center position during their bi-directional propagation through the medium along with corresponding spectral shifts. Moreover, the peak power and accumulated chirp (time derivative of accumulated phase) of the output pulse in both propagation directions are also opposite in nature, irrespective of their initial state. Numerically simulated behavior of the pulses agrees well with the analytical solutions.

Parabolic optical pulses with their characteristic linear chirp have profound importance in nonlinear optics as they have immense capability to withstand strong nonlinearity which prevents wave-breaking. Moreover, their evolution is self-similar, hence they are typically known as similaritons. Such wave-breaking free parabolic similaritons are widely applicable in all-fiber based devices like, high energy fiber laser sources, bio/chemical sensors, supercontinuum generators, bio-medical imaging and non-invasive surgery tools and so on. To date, theoretical and experimental formation of PPs have been reported for both active and passive optical fibers where PPs are asymptotic solutions of nonlinear Schrodinger equation (NLSE) [1]. It is clearly reported that PPs in active fibers are highly stable and evolve self-similarly over longer propagation distance whereas, passive formation of PPs are a mere intermediate transient state of propagation and with further evolution it becomes unstable [2]. In this context, another two types of similaritons (bright and dark) have been reported which are exact solutions of NLSE and found to propagate self-consistently against their asymptotic counterpart [3]. While encountering the stability issue of PPs in passive medium, it has been seen that high values of dispersion are being detrimental leading to wave-breaking. Hence some dispersion managed schemes have reported that effectively suppresses the excessive dispersive phase accumulation and stabilize the pulse over a few meters [4]. In this paper, we report formation and stable propagation of PPs over few hundreds of meters through a soft glass based tapered Bragg fiber. Instead of employing any dispersion managing technique, we simply have exploited the self-consistent nature of a bright (sechyperbolic) and dark (tanhyperbolic) similaritons and formed parabolic pulses in a decreasing dispersion profile.

The physics of topological singularities, namely, exceptional points (EPs), has been a key to a wide range of intriguing and unique physical effects in non-Hermitian systems. In this context, exploration of the mutual interactions among the states in four-level systems around fourth-order EPs (EP4s) is lacking. Here we report a four-level parameter-dependent perturbed non-Hermitian Hamiltonian, mimicking quantum or wave-based systems, to explore the physical aspects of an EP4 analytically as well as numerically. The proposed Hamiltonian exhibits different orders of interaction schemes with the simultaneous presence of different higher-order EPs. Here an EP4 has been realized by mutual interaction between four states with proper parameter manipulation. We comprehensively investigate the dynamics of corresponding coupled eigenvalues with stroboscopic parametric variation in the vicinity of the embedded EP4 to establish a successive state-switching phenomenon among them, which proves to be robust even in the presence of different orders of EPs. Implementing the relation of the perturbation parameters with the coupling control parameters, we report a region to host multiple EP4s in a specific system. The chiral behavior of successive state exchange has also been established near the EP4. The proposed scheme, which is enriched with physical aspects of EP4s, should provide a unique light manipulation tool in any anisotropic multistate integrated system.

We report an open three-state perturbed system that depends on the topological parameters, where the underlying Hamiltonian is varying quasi-statically. The effective system hosts two second order exceptional points (EP2s). Here a third order exceptional point (EP3) is encountered with simultaneous encirclement of two EP2s by adiabatic variation of topological parameters. We study the robust successive state-exchange around the EP3. Maintaining adiabaticity, we estimate the evolution of total phase accumulated by each of the interacting states during encirclement; where interestingly, the state common to the pairs of coupled state picks up three times phase shift of 2π as a signature of EP3. Such an exclusively reported scheme once implemented in an anisotropic optical waveguide can be exploited in potential applications of mode conversion, switching and lasing by manipulating topological parameters.

We report a planar 1D photonic bandgap waveguide exhibiting four second-order exceptional points (EP2s). The interactions between the selective pairs of supported quasi-guided TE modes are modulated by spatial distribution of transverse in-homogeneous gain-loss profile.

We report a specially configured open optical microcavity, imposing a spatially imbalanced gain–loss profile, to host an exclusively proposed next-nearest-neighbor resonance coupling scheme. Adopting the scattering matrix (S-matrix) formalism, the effect of interplay between such proposed resonance interactions and the incorporated non-Hermiticity in the microcavity is analyzed drawing a special attention to the existence of hidden singularities, namely exceptional points (EPs), where at least two coupled resonances coalesce. We establish adiabatic flip-of-state phenomenon of the coupled resonances in the complex frequency plane (k-plane), which is essentially an outcome of the fact that the respective EP is being encircled in the system parameter plane. Encountering such multiple EPs, the robustness of flip-of-states phenomena has been analyzed via continuous tuning of coupling parameters along a special hidden singular line which connects all the EPs in the cavity. Such a numerically devised cavity, incorporating the exclusive next neighbor coupling scheme, has been designed for the first time to study the unconventional optical phenomena in the vicinity of EPs.