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Somnath Ghosh

Publications and source records attributed to Somnath Ghosh.

At least 37 records · Page 2Linked to original sources
arXiv2020

Nonadiabatic Modal Dynamics Around a Third-order Exceptional Point in a planar waveguide

Dynamical parametric encirclement around an Exceptional Point (EP) and corresponding asymmetric state transfer phenomenon have attracted considerable attention recently. In this context, beyond the reported time-asymmetric state dynamics around a second-order EP (EP2) in a two-level system, the investigation of similar state dynamics around a third-order EP (EP3) in a multi-state system, having comparably complex topology with rich physics, is lacking. Here, we report a fabrication-feasible few-mode planar optical waveguide with a customized gain-loss profile and investigate the effect of dynamical parametric encirclement around an EP3 in the presence of multiple EP2s. The cube-root branch point behavior is established in terms of successive switching between the propagation constants of the coupled modes following an adiabatic encirclement process. Now, while considering the dynamical encirclement process, the breakdown in system adiabaticity around an EP3 leads to a unique light dynamics, where we have shown the breakdown of chirality of the device.

physics.optics↗
arXiv2020

Third-order Exceptional Point and Successive Switching among Three States in a Degenerate Optical Microcavity

One of the most intriguing topological features of open systems is exhibiting exceptional point (EP) singularities. Apart from the widely explored second-order EPs (EP2s), the explorations of higher-order EPs in any system requires more complex topology, which is still a challenge. Here, we encounter a third-order EP (EP3) for the first time in a simple fabrication feasible gain-loss assisted optical microcavity. Using scattering-matrix formalism, we study the simultaneous interactions between three successive coupled states around two EP2s, which yield an EP3. Following an adiabatic parametric variation around the identified EP3, we present a robust successive-state-conversion mechanism among three coupled states. The proposed scheme indeed opens a unique platform to manipulate light in integrated devices.

physics.optics↗
arXiv2020

Ultra-sensitive Light Confinement Driven by Multiple Bound States in the Continuum

We propose a unique framework to study the topological properties of an optical bound state in the continuum(BIC). We employ the interactions between proximity resonances undergoing avoided resonance crossing in a specialty optical microcavity. We utilize a continuous system parameter tuning to induce destructive interference between the resonances with cancelling leakage losses. Similar to the physical insight of Friedrich-Wingten type-BIC, we demonstrate the evolution of an ultra-high quality mode. We report the formation of a special-BIC line in the system parameter space connecting locations of multiple quasi-BICs. Aiming to develop a novel scheme to enhance the performance of optical sensing in microcavity, we study the sensitivity of transmission coefficients and quality factor to sense even ultra-small perturbations in the system configuration.

physics.optics↗
arXiv2019

Higher order topological degeneracies and towards unique successive state-switching in a four-level open system

The physics of topological singularities, namely exceptional points (EPs), has been a key to wide range of intriguing and unique physical effects in non-Hermitian systems. In this context, the mutual interactions among four coupled states around fourth-order EPs (EP4s) are yet to be explored. 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 exhibit 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 coupled 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 new successive state-switching phenomenon among them; which proves to be robust even in the presence of different order of EPs. Implementing the relation of the perturbation parameters with the coupling control parameters, we exclusively report a region to host multiple EP4 in a specific system. The chiral behaviour of successive state-exchange has also been established near EP4. Proposed scheme enriched with physical aspects of EP4s should provide a new light manipulation tool in any anisotropic multi-state integrated system.

physics.optics↗
arXiv2019

Nonlinearity Induced Anomalous Mode Collapsing and Non-chiral Asymmetric Mode Switching around multiple Exceptional Points

The dynamical encirclement around a second order exceptional point (EP) and corresponding chirality driven nonadiabatic modal dynamics have attracted enormous attention in the topological study of various non-Hermitian systems. However, dynamical encirclement around multiple second-order EPs in a multi-state system is yet to be explored. Here, exploiting an exclusive design of a planar gain-loss assisted three-mode supported optical waveguide with local Kerr-nonlinearity, we encounter multiple second-order EPs. Judiciously, choosing a specific parameter space by varying the unbalanced gain-loss profile, we encircle multiple EPs simultaneously, and explore the beam-dynamics toward corresponding chiral or non-chiral aspects of the device. While propagating through the designed waveguide, three coupled modes are collapsed into a specific dominating mode, owing to corresponding nonadiabatic corrections around multiple EPs. Even in the absence of chirality, here, the same amount of focusing and de-focusing type nonlinearity gives different dominating output, irrespective of the choice of inputs, for the same topological structure of the waveguide. This exclusive topologically robust compact scheme of nonlinearity induced asymmetric and non-chiral light dynamics should provide a promising opportunity to switch or retrieve a selective mode from a multi-mode signal in integrated devices.

physics.optics↗
arXiv2019

Towards Hosting Bound State in Continuum in Specialty Optical Microcavity

We exploit the interaction between supported proximity resonances in an open non-uniformly pumped optical microcavity to host a Bound State in Continuum (BIC). Using the modeling of the S-matrix, we study the coupling between the interacting states forming a BIC. We report the divergence of Quality Factor (Q-factor) of one of the interacting states, numerically demonstrated with the appropriate tuning of the spatial variation of unequal gain-loss within the system whose stability has been discussed in terms of the Petermann factor at the point of BIC. Such high lifetime has been marked as a signature of BIC.

physics.optics↗
arXiv2019

Exceptional Point and Toward Mode Selective Optical Isolation

Dynamical encirclement of an Exceptional Point (EP) and corresponding time-asymmetric mode evolution properties due to breakdown in adiabatic theorem have been a key to range of exotic physical effects in various open atomic, molecular and optical systems. Here, exploiting a gain-loss assisted dual-mode optical waveguide that hosts a dynamical EP-encirclement scheme, we have explored enhanced nonreciprocal effect in the dynamics of light with onset of saturable nonlinearity in the optical medium. We propose a prototype waveguide-based isolation scheme with judicious tuning of nonlinearity level where one can pass only a chosen mode in any of the desired directions as per device requirement. The deliberate presence of EP enormously enhances the nonreciprocal transmission contrast even up to 40 dB over the proposed device length with a scope of further scalability. This exclusive topologically robust mode selective all-optical isolation scheme will certainly offer opportunities in integrated photonic circuits for efficient coupling operation from external sources and improve device performances.

physics.optics↗
arXiv2019

Optical waveguide hosting multiple exceptional points: Toward selective mode conversion

We investigate the astonishing physical aspects of Exceptional Points (EPs) in a 1D planar few-mode optical waveguide. The waveguide hosts four quasi-guided modes. Here interactions between the selected pair of modes are modulated by a spatial distribution of inhomogeneous gain-loss profile. Both the coupled pairs approach two different second-order EPs in parameter plane. Considering a proper parametric loop to encircle the identified EPs simultaneously, we establish a specific topological feature where one round encirclement in parameter space yields the switching between the propagation constants (β) of the corresponding pairs of couple modes in complex β-plane. Choosing two different patterns of the parametric loop, we establish the immutable topology in β- switching phenomena. This robust mode conversion scheme shall provide a platform to realize selective mode switching devices or optical-mode converters.

physics.optics↗
arXiv2018

Non-adiabatic Modal Dynamics around Exceptional Points in an All-Lossy Dual-Mode Optical Waveguide: Towards Chirality Driven Asymmetric Mode-Conversion

We report a 1D planar optical waveguide with transverse distribution of 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 attenuation profile only. We study the adiabatic dynamics of propagation constants of the coupled modes by quasi-static 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 presence of an EP. Here, during conversion the output mode is irrespective of the choice of 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 open up an extensive way-out for implementation of state-transfer applications in chirality driven waveguide-based devices.

physics.optics↗
arXiv2018

Origin of third order exceptional singularities and its signature in successive state conversion

We report an open three-state perturbed system with quasi-statically varying Hamiltonian depending on the topological parameters. The effective system hosts two second order exceptional points (EP2s). Here a third order exceptional point (EP3) is explored with simultaneous encirclement of two EP2s by adiabatic variation of topological parameters. We study the robust successive state-exchange around the EP3. Applying adiabatic theorem, we estimate the evolution of total phase accumulated by each state during encirclement; where interestingly, the state-common to the pairs of coupled state picks up three times phase shift of 2π. Such an exclusively reported scheme can be exploited in potential applications of exceptional points, manipulating fewer topological parameters in various non-Hermitian systems.

physics.optics↗
arXiv2018

Propagation and asymmetric behavior of optical pulses through time-dynamic loss-gain assisted media

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. Possibilities have been explored in context of pulse shaping and unconventional optical devices.

physics.optics↗
arXiv2018

Realization of Third Order Exceptional Singularities in a Three level non-Hermitian System: Towards Cascaded State Conversion

The appearance of topological singularities, namely exceptional points (EPs) is an intriguing feature of parameter-dependent open quantum or wave systems. EPs are the special type of nonHermitian degeneracies where two (or more) eigenstates of the underlying system coalesce. In this paper, we present a three level non-Hermitian Hamiltonian which hosts three interacting eigenstates. The matrix elements are optimized in such a way that the intermediate eigenstate interacts with both the other states and the underlying system hosts at least two different second order EPs. The impact of quasi-static parameter variation along a cyclic contour around the embedded EPs on the dynamics of interacting eigenvalues is well investigated in the context of cascaded state conversion. Such dynamics of the eigenvalues shows a clear signature of the third order EP with a combined effect of both the second order EPs. Moreover, we examine the accumulation of phases around the identified EPs and study the hallmark of phase exchange during cascaded state conversions accompanied by the parametric encirclement of the third order EP.

quant-ph↗
arXiv2018

Net-charge fluctuation in Au+Au collisions at energies available at the Facility for Antiproton and Ion Research using the UrQMD model

We have studied the dynamical fluctuation of net-charge of hadrons produced in Au+Au collisions at energies that in near future will be available at the Facility for Antiproton and Ion Research (FAIR). Data simulated by a microscopic transport model based on ultra-relativistic quantum molecular dynamics are analyzed for the purpose. The centrality and pseudorapidity dependence of the net-charge fluctuation of hadrons are examined. Our simulated results are compared with the results available for nucleus-nucleus collision experiments held at similar energies. The gross features of our simulated results on net-charge fluctuations are found to be consistent with the experiment. At incident beam energy $E_{\rm lab}=10A$ GeV the magnitude of net-charge fluctuation is very large, and in comparison with the rest its centrality dependence appears to be a little unusual. The effect of global charge conservation is expected to be very crucial at FAIR energies. The charge fluctuations measured with varying pesudorapidity window size depend on the collision centrality. The dependence is however exactly opposite in nature to that observed in the Pb+Pb collision at $\sqrt{s_{_{NN}}}=2.76$ TeV.

nucl-th↗
arXiv2018

Radial flow in a multi phase transport model at FAIR energies

Azimuthal distributions of radial velocities of charged hadrons produced in nucleus-nucleus $(AB)$ collisions are compared with the corresponding azimuthal distribution of charged hadron multiplicity in the framework of a multiphase transport (AMPT) model at two different collision energies. The mean radial velocity seems to be a good probe for studying radial expansion. While the anisotropic part of the distributions indicates a kind of collective nature of radial expansion, the isotropic part characterizes a thermal motion. The present investigation is carried out keeping the upcoming Compressed Baryonic Matter (CBM) experiment to be held at the Facility for Anti-proton Ion Research (FAIR) in mind. As far as high-energy heavy-ion interactions are concerned, CBM will supplement the Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) experiments. In this context our simulation results at high baryochemical potential would be interesting, when scrutinized from the perspective of almost a baryon free environment achieved at RHIC and LHC.

nucl-th↗
arXiv2018

Signature of phase singularities in diffusive regime in one dimensional disordered lattices: Interplay and qualitative analysis

Co-existence and interplay between mesoscopic light dynamics with singular optics in spatially random but temporally coherent disordered waveguide lattices is reported. Two CW light beams of 1.55 micron operating wavelength are launched as inputs to 1D waveguide lattices with controllable weak disorder in refractive index profile. Direct observation of phase singularities in the speckle pattern along the length is numerically demonstrated. Quantitative analysis of onset of such singular behavior and diffusive wave propagation is analyzed for the first time.

physics.optics↗