arXiv subjects

Nitish Kumar Gupta

Publications and source records attributed to Nitish Kumar Gupta.

7 recordsLinked to original sources

arXiv2026

Nonlinearity Selective Quasi Bound States in the Continuum via Symmetry Protected Decoupling in \chi(2) Thin Films

Second-harmonic generation in resonant structures is commonly evaluated in terms of intracavity field enhancement at the fundamental and harmonic frequencies. Here, we formulate nonlinear frequency conversion within a symmetry-resolved overlap framework that explicitly separates resonant field buildup from nonlinear mode projection. Using a simple and analytically tractable Fabry--Perot thin-film-on-substrate geometry, we show that, even in the presence of spectrally bright resonances at both $\omega$ and $2\omega$, the emitted second-harmonic signal can be strongly suppressed when the spatial parity of the pump-induced nonlinear polarization is incompatible with that of the radiating $2\omega$ standing-wave mode. This mechanism gives rise to nonlinearity-selective quasi-bound states in the continuum. Beyond providing a compact interpretation of these nonlinear dark states, the framework unifies pump enhancement, harmonic enhancement, and symmetry-controlled modal overlap within a single predictive metric. More broadly, it identifies thickness regimes in which resonant buildup is accompanied by constructive nonlinear coupling, and distinguishes them from regimes in which apparently favorable resonance conditions remain conversion-inactive because the nonlinear source is orthogonal to the radiating harmonic mode.

physics.optics↗

arXiv2025

A High Responsivity Broadband Photodetector Based on a WSe2 NiO Nanowire Heterostructure with Engineered Nanophotonic Enhancement

Engineering nanoscale light matter interaction in mixed dimensional semiconductor heterostructures offers a pathway to mitigate the intrinsic gain bandwidth trade off in photodetectors. Here, we report a broadband, high responsivity 2D and 1D photodetector formed by integrating monolayer p type WSe2 with electrospun p type NiO nanowires. The device photoresponse spans 350 to 780 nm and is governed by a nanophotonic field confinement mechanism rather than bulk optical absorption. The high index NiO nanowire acts as a dielectric Mie type nanoresonator that supports geometry defined optical modes and produces antenna like near field concentration at the nanoscale WSe2 and NiO junction. This localized optical mode increases the local absorption cross section and enhances the photocarrier generation rate within the junction region, identified as the dominant active volume for photocurrent. A coupled optoelectronic model linking full wave electromagnetic simulations to carrier generation, recombination, and extraction accurately captures the measured responsivity spectrum and its power dependence using only two electronic fitting parameters. The device achieves responsivities of 627 A/W in the visible region, 227 A/W in the UV, and 167 A/W in the NIR, demonstrating broadband operation with ultrahigh gain. These results show that geometric resonance in mixed dimensional junctions is a powerful design principle for next generation high gain optoelectronic detectors.

physics.optics↗

arXiv2023

Direct Determination of Photonic Stopband Topological Character: A Framework based on Dispersion Measurements

Ascertainment of photonic stopband absolute topological character requires information regarding the Bloch eigenfunction spatial distribution. Consequently, the experimental investigations predominantly restrict themselves to the bulk-boundary correspondence principle and the ensuing emergence of topological surface state. Although capable of establishing the equivalence or inequivalence of bandgaps, the determination of their absolute topological identity remains out of its purview. The alternate method of reflection phase-based identification also provides only contentious improvements owing to the measurement complexities pertaining to the interferometric setups. To circumvent these limitations, we resort to the Kramers-Kronig amplitude-phase causality considerations and propose an experimentally conducive method for bandgap topological character determination directly from the parametric reflectance measurements. Particularly, it has been demonstrated that in case of one-dimensional photonic crystals, polarization-resolved dispersion measurements suffice in qualitatively determining bandgap absolute topological identities. By invoking the translational invariance of the investigated samples, we also define a parameter Differential Effective Mass that encapsulates bandgap topological identities and engenders an experimentally discernible bandgap classifier.

physics.optics↗

arXiv2021

Non-Hermitian Topoelectrical Circuits: Expedient Tools for Topological State Engineering with Gain-Loss Modulation

The congregation of topological quantum and classical systems with the ideas of non-Hermitian physics has generated enormous research interest in the last few years. While the concepts associated to non-trivial topological aspects have provided us an access to the disorder immune states, non-Hermitian physics, which was initially developed within the framework of quantum field theories, has contributed significantly to the study of open quantum systems. Particularly in optics and photonics, the study of non-Hermitian Hamiltonians with balanced loss and gain has resulted in many counter-intuitive phenomena. However, the experimental realization of such systems is challenging, and the need for alternative platforms for testing theoretical propositions and proof of concept demonstrations is widely felt. In this context, active electrical and electronic circuitry has proved to be a prolific alternative and has been receiving increasing attention; mainly, the topoelectric circuits, in many instances, have facilitated the investigation of topological conceptions in conjunction with non-Hermitian physics, beyond the limitations of the condensed matter systems. This article provides a succinct introduction to these non-Hermitian topoelectrical circuits and will also discuss some of the novel physics of topological insulators and semimetals that can be conveniently realized and explored in such configurations.

physics.app-ph↗

arXiv2021

Topological Photonic Systems: Virtuous Platforms to Study Topological Quantum Matter

Topological insulators are a new class of materials that have engendered considerable research interest among the condensed matter community owing primarily to their application prospects in quantum computations and spintronics. Many of the associated phenomena, however, can be well reproduced in classical photonic systems with the additional advantage of relatively less demanding fabrication and engineered system characteristics. Therefore, the photonic analogs of topological materials have gained prominence in the last decade, not only in the field of optics but as an active research front of the topological physics at large. In this article, we succinctly review the fundamental concepts of topological physics and provide a concise description of the photonic topological insulators.

physics.optics↗

arXiv2021

Clocking the Quantum Sojourn Time: Spurious Scatterings and Correction to the Larmor Clock

We revisit the notions of the quantum-mechanical sojourn time in the context of the quantum clocks to enquire whether the sojourn time be clocked without the clock affecting the dynamics of the wave motion. Upon recognizing that the positivity of conditional sojourn time is not ensured even in the case of physically co-evolving clock mechanisms, we trace its origins to the non-trivial inadvertent scattering arising from the disparity, however weak, engendered by the very clock potential. Specifically, our investigations focus on the Larmor spin rotation-based unitary clock where the alleviation of these unphysical contributions has been achieved by correcting the mathematical apparatus of extracting the sojourn times. The corrections have been obtained for both the spin precession-based and spin alignment-based scenarios. The sojourn times so obtained are found to have proper high- and low-energy limits and turn out to be positive definite for an arbitrary potential. The regimen provided here is general and appeals equivalently for unitary as well as non-unitary clocks where the clock-induced perturbations couple to the system Hamiltonian.

quant-ph↗

arXiv2021

Emergence in Science

In the scientific literature, the term emergent phenomena is invoked in the context of a collective behavior observed in a complex adaptive system that exhibits no correspondence with the behavior of the system constituents. Although this description is too generic to be termed a definition, it alludes to the fact that emergent phenomena are closely related to esoteric observations and neoteric developments in science. With these impressions, we aim to investigate a variety of observed behavior from the perspective of an emergentist. Starting with a few familiar portrayals of emergence, we devote a large body of this narrative review to explain instances of emergence in condensed matter systems, namely the phase transition phenomena, spontaneous symmetry breaking, and macroscopic quantum phenomena. At the same time, to present a broader perspective, we cross the domain boundaries to also provide a succinct description of emergent phenomena prevailing in social sciences, economics, computing environments, and in biological systems, as well.

physics.soc-ph↗