arXiv subjects

K. Nireekshan Reddy

Publications and source records attributed to K. Nireekshan Reddy.

11 recordsLinked to original sources

arXiv2020

Sum frequency generation from touching wires: A transformation optics approach

We employ transformation optics to study analytically nonlinear wave mixing from a singular geometry of touching plasmonic wires. We obtain the analytic solution of the near-field and complement it with a solution of the far-field properties. We find, somewhat surprisingly, that optimal efficiency (in both regimes) is obtained for the degenerate case of the Second-harmonic generation. We exploit the analytic solution obtained to trace this behaviour to the spatial overlap of the input fields near the geometric singularity.

physics.optics↗

arXiv2020

Optimization of second-harmonic generation from touching plasmonic wires

We employ transformation optics to optimize the generic nonlinear wave interaction of second-harmonic generation from a pair of touching metallic wires. We demonstrate a 10 orders of magnitude increase in the second-harmonic scattering cross-section by increasing the background permittivity and a 5 orders of magnitude increase in efficiency with respect to a single wire. These results have clear implications for the design of nanostructured metallic frequency-conversion devices. Finally, we exploit our analytic solution of a non-trivial nanophotonic geometry as a platform for performing a critical comparison of the strengths, weaknesses and validity of other prevailing theoretical approaches previously employed for nonlinear wave interactions at the nanoscale.

physics.optics↗

arXiv2019

Surface second-harmonic from metallic nanoparticle configurations - a transformation optics approach

We study surface second-harmonic generation (SHG) from a singular plasmonic structure consisting of touching metallic wires. We use the technique of transformation optics and relate the structure to a rather simpler geometry, a slab waveguide. This allows us to obtain an analytical solution to the problem, revealing rich physical insights. We identify various conditions that govern the SHG efficiency. Importantly, our analysis demonstrates that apart from the mode-matching condition, phase-matching condition is relevant even for this sub-wavelength structure. Furthermore, we identify a geometric factor which was not identified before. We support our analysis with numerical simulations.

physics.optics↗

arXiv2017

Revisiting the boundary conditions for second-harmonic generation at metal-dielectric interfaces

We study second-harmonic generation (SHG) arising from surface nonlinearity at a metal-dielectric interface using a spectral decomposition method. Since our method avoids the need to consider the generalized boundary condition across the metal-dielectric interface in the presence of a perpendicular surface source, we retrieve the known discontinuity of the tangential component of the electric field ($E_{\parallel}^ {2ω}$) for a general geometry, based on a purely mathematical argument. Further, we reaffirm the standard convention of the implementation of this condition, namely, that the surface dipole source radiates as if placed outside the metal surface for arbitrary geometries. We also study and explain the spectral dependence of the discontinuity of the tangential component of the electric field at second harmonic. Finally, we note that the default settings of the commercial numerical package COMSOL Multiphysics fail to account for the $E_{\parallel}^ {2ω}$-discontinuity. We provide a simple recipe that corrects the boundary condition within these existing settings.

physics.optics↗

arXiv2016

Coherent perfect absorption mediated enhancement and optical bistability in phase conjugation

We study phase conjugation in a nonlinear composite slab when the counter propagating pump waves are completely absorbed by means of coherent perfect absorption. Under the undepleted pump approximation the coupling constant and the phase conjugated reflectivity are shown to undergo a substantial increase and multivalued response. The effect can be used for efficient switching of the phase conjugated reflectivity in photonic circuits.

physics.optics↗

arXiv2015

A study of PT-symmetric Non-linear Schroedinger Equation

Systems governed by the Non-linear Schroedinger Equation (NLSE) with various external PT-symmetric potentials are considered. Exact solutions have been obtained for the same through the method of ansatz, some of them being solitonic in nature. It is found that only the unbroken PT-symmetric phase is realized in these systems, characterized by real energies.

quant-ph↗

arXiv2014

Nonlinear Coherent Perfect Absorption

We review some of the recent concepts and their realization exploiting the perfect destructive interference of light in micro and nano structures. One refers to optical structures where the effective absorption can be controlled and maximized to perfect absorption. The reported effects depend crucially on the coherent nature of the exciting radiation. Achieved with a single (two or more) incident plane wave (waves) the effect carries the name of critical coupling (coherent perfect absorption). Thus in a system supporting critical coupling (CC) or coherent perfect absorption (CPA) all the incident radiation can be absorbed leading to null scattering. In particular all the incident light energy can be channeled into a specified mode of a multimodal structure if such modes are supported by the system. We present a brief overview of CC and CPA in linear systems to recount their underlying concepts as time-reversed lasing and some of their futuristic applications. Next we review our work on the nonlinear extensions of CC and CPA where one or more of the layered media could be nonlinear with Kerr-type nonlinearity. The dispersive nonlinearity is shown to offer a practical handle over the process of perfect absorption by incident laser power. Further we show that the nonlinear periodic structure can support gap solitons which absorbs all the incident energy and do not scatter any light outside the hetero-guide

physics.optics↗

arXiv2014

Cavity controlled spectral singularity

We study theoretically a PT-symmetric saturable balanced gain-loss system in a ring cavity configuration. The saturable gain and loss are modeled by two-level medium with or without population inversion. We show that the specifics of the spectral singularity can be fully controlled by the cavity and the atomic detuning parameters. The theory is based on the mean-field approximation as in standard theory of optical bistability. Further, in the linear regime we demonstrate the regularization of the singularity in detuned systems, while larger input power levels are shown to be adequate to limit the infinite growth in absence of detuning

physics.optics↗

arXiv2014

Gap solitons with null-scattering

We study excitation of gap solitons under the conditions of coherent perfect absorption (CPA). Our system consists of a symmetric periodic structure with alternating Kerr nonlinear and linear layers, illuminated from both the ends. We show near-total transfer of incident light energy into the gap solitons resulting in null-scattering. We also report on the nonlinear super-scattering (SS) states. Both the CPA and the SS states are shown to be characterized by typical field distributions. Both the exact and the approximate results (based on nonlinear characteristic matrix method) are presented, which show good agreement.

physics.optics↗

arXiv2013

Light-controlled perfect absorption of light

We study coherent perfect absorption (CPA) of light in a Kerr nonlinear metal-dielectric composite medium, illuminated from the opposite ends. Elementary symmetry considerations reveal that equality of the incident light intensities is a prerequisite to ensure CPA in both linear and nonlinear systems for specific system parameters. We also derive the sufficient conditions for having CPA. We further show that while CPA in a linear system is insensitive to the incident power level, that in a nonlinear system can be achieved only for discrete intensities with interesting hysteretic response. Our unified formulation of CPA and waveguiding identifies them as opposite scattering phenomena. We further investigate light-induced CPA in on- and off- resonant systems.

physics.optics↗

arXiv2013

Nonlinearity Induced Critical Coupling

We study a critically coupled system (Opt. Lett., \textbf{32}, 1483 (2007)) with a Kerr-nonlinear spacer layer. Nonlinearity is shown to inhibit null-scattering in a critically coupled system at low powers. However, a system detuned from critical coupling can exhibit near-complete suppression of scattering by means of nonlinearity-induced changes in refractive index. Our studies reveal clearly an important aspect of critical coupling as a delicate balance in both the amplitude and the phase relations, while a nonlinear resonance in dispersive bistability concerns only the phase.

physics.optics↗