arXiv subjects

Shailendra K. Varshney

Publications and source records attributed to Shailendra K. Varshney.

9 recordsLinked to original sources

arXiv2025

Cauchy-Schwarz and Bell Inequality Violations in Coupled Optomechanical Systems

Destructive interference-based photon-phonon antibunching can lead to violations of classical inequalities in optomechanical cavity systems. In this paper, we explore the violation of the classical Cauchy-Schwarz inequality by examining second-order auto-correlation and cross-correlation functions, as well as Bell's nonlocality, to analyze the quantum correlations of single photon-phonon excitations when the system is driven by two weak probe fields. We propose that the violation of the Cauchy-Schwarz inequality can serve as an indicator for the stronger nonclassical tests associated with Bell's theorem. Our system reveals strong quantum correlations of photon-phonon pairs with distinctive antidiagonal patterns of photon filtering. For numerical analysis, we consider a weak effective optomechanical coupling strength and various optical-to-mechanical field amplitude ratios that enable unconventional photon (phonon) blockades at resonance. The findings are significant for producing sub-Poissonian signals under optimal conditions and have potential applications in hybrid systems for generating on-demand single photon-phonon pairs.

quant-ph↗

arXiv2023

Exceptional point induced quantum phase synchronization and entanglement dynamics in mechanically coupled gain-loss oscillators

The optomechanical cavity (OMC) system has been a paradigm in the manifestation of continuous variable quantum information over the past decade. This paper investigates how quantum phase synchronization relates to bipartite Gaussian entanglement in coupled gain-loss mechanical oscillators, where the gain and loss rates are engineered by driving the cavity with blue and red detuned lasers, respectively. We examine the role of exceptional point in a deterministic way of producing self-sustained oscillations that induce robust quantum correlations among quadrature fluctuations of the oscillators. Particularly, steady phase synchronization dynamics along with the entanglement phenomena are observed in the effective weak coupling regime above a critical driving power. These phenomena are further verified by observing the mechanical squeezing and phase space rotations of the Wigner distributions. Additionally, we discuss how the oscillators frequency mismatches and decoherence due to thermal phonons impact the system dynamics. These findings hold promise for applications in phonon-based quantum communication and information processing.

quant-ph↗

arXiv2021

Intra-cavity field dynamics near avoided mode crossing in concentric silicon nitride ring resonator

Understanding the intra-cavity field dynamics in passive microresonator systems has already been intriguing. It becomes fascinating when the system is complex, such as a concentric dual microring resonator that exhibit avoided mode crossing (AMC). In this work, we present a systematic study of intra-cavity oscillatory field dynamics near AMC in a concentric silicon nitride microring resonator with the help of the coupled Lugiato-Lefever equation (LLE). We identify two regions viz. weakly coupled region (WCR) and strongly coupled region (SCR) based on eigenfrequency separation of the two-hybrid modes, which originate from mode coupling near AMC. In WCR, the mode coupling effect is dominant, leading to intra-cavity power oscillation between these two modes in a periodic manner and non-identical variation of their phases. In SCR, the mode coupling effect reduces gradually with nearly identical characteristics of both the modes. We further verify our numerical findings with the semi-analytical variational method, leading to an in-depth understanding of the mode coupling induced dynamics. We finally analyze the polarization evolving state, and the polarization locked state with the help of Stokes parameters and Jones vectors in WCR and SCR, respectively.

physics.optics↗

arXiv2020

Realistic non-local refrigeration engine based on Coulomb coupled systems

Employing Coulomb-coupled systems, we demonstrate a cryogenic non-local refrigeration engine, that circumvents the need for a change in the energy resolved system-to-reservoir coupling, demanded by the recently proposed non-local refrigerators. We demonstrate that an intentionally introduced energy difference between the ground states of adjacent tunnel coupled quantum dots, associated with Coulomb coupling, is sufficient to extract heat from a remote target reservoir. Investigating the performance and operating regime using quantum-master-equation (QME) approach, we point out to some crucial aspects of the proposed refrigeration engine. In particular, we demonstrate that the maximum cooling power for the proposed set-up is limited to about $70\%$ of the optimal design. Proceeding further, we point out that to achieve a target reservoir temperature, lower compared to the average temperature of the current path, the applied voltage must be greater than a given threshold voltage $V_{TH}$, that increases with decrease in the target reservoir temperature. In addition, we demonstrate that the maximum cooling power, as well as the coefficient of performance deteriorates as one approaches a lower target reservoir temperature. The novelty of the proposed refrigeration engine is the integration of fabrication simplicity along with descent cooling power. The idea proposed in this paper may pave the way towards the realization of efficient non-local cryogenic refrigeration systems.

physics.app-ph↗

arXiv2020

Competition Between Intermodal Modulation Instability and Kerr Beam Self-cleaning in Graded-index Multimode Fiber

We report the suppression of intermodal modulation instability peaks as a consequence of Kerr induced self beam-cleaning in a 90m long graded-index multimode optical fiber under various specific launching conditions. The output spectrum and the modal beam profiles for three modes (LP01, LP11, and LP21) have been recorded for several values of pump pulse energy. Experimental findings establish that the nonlinear coupling among the guided modes leads to reshaping the output speckle pattern into a bell shape or higher-order spatially clean beam profile as the pump peak power increases, whereas in spectral-domain intermodal modulation instability peaks are observed whose amplitude increases gradually to its maximum value and beyond certain threshold power, the intermodal modulation instability peaks diminish. This suggests the possibility to generate intermodal modulation instability free broadband spectra at high pump peak powers.

physics.optics↗

arXiv2019

Variational approach to study soliton dynamics in a passive fiber loop resonator with coherently driven phase-modulated external field

We report a detailed semi-analytical treatment to investigate the dynamics of a single cavity soliton (CS) and two co-propagating CSs separately in a Kerr mediated passive optical fiber resonator which is driven by a phase-modulated pump. The perturbation is dealt with by introducing a Rayleigh's dissipation function in the framework of variational principle that results in a set of coupled ordinary differential equations describing the evolution of individual soliton parameters. We further derive closed-form expressions for quick estimation of the temporal trajectory, drift velocity and the phase shift accumulated by the CS due to the externally modulated pump. We also extend the variational approach to solve two solitons interaction problem in the absence as well as in the presence of the externally modulated field. In absence of phase modulated field, the two copropagating solitons can attract, repulse or can propagate independently depending on their initial delay. The final state of interaction can be predicted through a second-order differential equation which is derived by the variational method. While in presence of the phase modulated field, the two solitons interaction can result in annihilation, merging, breathing or two soliton state depending on the detuning frequency and the pump power. Variational treatment analytically predicts these states and portrays the related dynamics that agrees with full numerical simulation carried out by solving the normalized Lugiato-Lefever equation. The results obtained through this variational approach will enrich the understanding of complex pulse dynamics under phase modulated driving field in passive dissipative systems.

physics.optics↗

arXiv2019

Phase Synchronization of Stimulated Raman Process in Optical Fiber For Long Pulse Regime

We investigate the evolution of coherence property of noise-seeded Stokes wave in short (< 1 ps) and long pulse (> 1 ps) regimes. Nonlinear equations expressing the evolution of pump and Stokes wave are solved numerically for both the regions. The simulations include quantum noise by incorporating noise seed in the pump field where one photon per mode with random phase. The spectral phase fluctuations of the Stokes wave for both the regions, are characterized by performing multiple simulations and finally, the degrees of first-order mutual coherence are calculated as a function of wavelength for different conditions. Our statistical analysis proclaim that noise-seeded stimulated Raman process, which plays the role in degradation of coherence in short pulse region, exhibits strong phase synchronization in long pulse regime. The manifestation of phase synchronization occurs by the transition of the Stokes wave from incoherent to coherent spectra in long pulse regime.

physics.optics↗

arXiv2019

Modal Group Velocity Mismatch Induced Intermodal Modulation Instability in Step-index Fiber

We present detailed experimental study on noise-seeded intermodal modulation instability (IM-MI) in normal dispersion region of a conventional step-index fiber. The sharp refractive index contrast between core and cladding leads to large group velocity mismatch between the spatial modes, coaxing to efficient IM-MI and generation of multiple spectral peaks along with Raman peaks. Evolution of the spectrum with pump powers and fiber lengths are observed. Experimental findings are well supported with the theoretical framework based on bimodal-MI model considering the distinct dispersion parameters of the participating modes.

physics.optics↗

arXiv2018

Generalized Mathematical Formalism Governing Free-carrier Driven Kerr Frequency Comb in Optical Micro-cavities

Continuous-wave pumped optical microresonators have been vastly exploited to generate frequency comb (FC) utilizing the Kerr nonlinearity. Most of the nonlinear materials used to build photonic platforms exhibit nonlinear losses such as multi-photon absorption, free-carrier absorption (FCA), and free-carrier dispersion (FCD) which can strongly affect the nonlinear characteristics of the devices made out of these materials. In this work, we model the Kerr FC based on modified Lugiato-Lefever Equation (LLE) along with the rate equation and develop analytical formulations to make quick estimations of the steady-state, modulation instability (MI) gain, bandwidth and the dynamics of Kerr Frequency-Comb (FC) in presence of nonlinear losses. Our analytical model is valid over a broad wavelength range of interest as it includes the effects of all nonlinear losses. Higher order (>3) characteristic polynomial of intra-cavity power describing the steady-state homogeneous solution of the modified LLE are discussed in detail. We derive the generalized analytical expressions for the threshold of normalized pump detuning to initiate the optical bistability which is a necessary condition for the FC generation. Free-carrier dispersion-led nonlinear cavity detuning is observed through the reverse Kerr-tilt of the resonant-peaks. We further deduce the expressions for the threshold pump intensity and the range of possible cavity detuning for the initiation of the MI when all the nonlinear losses are present. To corroborate our analytical findings, LLE along with the rate equations are solved numerically through split-step Fourier method. Our theoretical study can explain several experimental results which are previously reported and thereby is able to provide a better understanding of the comb dynamics.

physics.optics↗