Harsh K. Gandhi

Bio: Harsh K. Gandhi is an academic researcher from Indian Institute of Technology, Jodhpur. The author has contributed to research in topics: Optical medium & Second-harmonic generation. The author has an hindex of 4, co-authored 15 publications receiving 60 citations. Previous affiliations of Harsh K. Gandhi include University of Brescia.

Exceptional Point and toward Mode-Selective Optical Isolation

Abstract: Exceptional points (EPs) in non-Hermitian systems have recently attracted considerable attention owing to unique state-flipping and peculiar phase accumulation features. The dynamical encirclement ...

Gain-loss engineering of bound states in the continuum for enhanced nonlinear response in dielectric nanocavities.

Abstract: We reveal the potential of bound states in the continuum (BIC) to enhance the nonlinear response in specialty optical resonators in the presence of gain and loss. We demonstrate this phenomenon in a square core–shell AlGaAs nanowire having a proper engineered spatial variation of gain and loss to sustain quasi–BICs. The presence of these high-quality modes at both fundamental and second-harmonic wavelengths leads to an extremely high enhancement in second harmonic generation, thus preluding a framework to fabricate composite media with high effective nonlinearity.

Exceptional Point and Toward Mode Selective Optical Isolation

Abstract: 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.

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

Abstract: 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.

Chirality breakdown in the presence of multiple exceptional points and specific mode excitation.

Abstract: The dynamical parametric encirclement around a second-order exceptional point (EP) enables the time-asymmetric nonadiabatic evolution of light, which follows the chirality of the underlying system. Such light dynamics in the presence of multiple EPs and the corresponding chiral aspect is yet to be explored. In this Letter, we report a gain–loss assisted four-mode-supported optical waveguide that hosts a parameter space to dynamically encircle multiple EPs. In the presence of multiple EPs, we establish a unique nonadiabatic behavior of light, where beyond the chiral aspect of the system, light is switched to a particular mode, irrespective of the choice of the input mode. Proposed scheme certainly opens a step-forward approach in light manipulation to facilitate next-generation integrated photonic systems.

Enhanced light–matter interactions in dielectric nanostructures via machine-learning approach

Abstract: A key concept underlying the specific functionalities of metasurfaces is the use of constituent components to shape the wavefront of the light on demand. Metasurfaces are versatile, novel platforms for manipulating the scattering, color, phase, or intensity of light. Currently, one of the typical approaches for designing a metasurface is to optimize one or two variables among a vast number of fixed parameters, such as various materials’ properties and coupling effects, as well as the geometrical parameters. Ideally, this would require multidimensional space optimization through direct numerical simulations. Recently, an alternative, popular approach allows for reducing the computational cost significantly based on a deep-learning-assisted method. We utilize a deep-learning approach for obtaining high-quality factor (high-Q) resonances with desired characteristics, such as linewidth, amplitude, and spectral position. We exploit such high-Q resonances for enhanced light–matter interaction in nonlinear optical metasurfaces and optomechanical vibrations, simultaneously. We demonstrate that optimized metasurfaces achieve up to 400-fold enhancement of the third-harmonic generation; at the same time, they also contribute to 100-fold enhancement of the amplitude of optomechanical vibrations. This approach can be further used to realize structures with unconventional scattering responses.

Bound states in the continuum in quantum-dot pairs (4 pages)

Abstract: Received 20 April 2005; published 17 February 2006It is shown that for two open quantum dots connected by a wire, “bound states in the continuum” of a singleelectron are formed at nearly periodic distances between the dots. This is due to Fabry-Perot interferencebetween quasibound states in each dot. The bound states are nonlocal, describing the electron trapped in bothdots at the same time. Theoretical and numerical results show that trapped states exist even if the wireconnecting the dots is relatively long.DOI: 10.1103/PhysRevA.73.022113 PACS number s : 03.65. w, 73.23. b, 73.63. b

Enhanced Light-Matter Interactions in Dielectric Nanostructures via Machine Learning Approach

Abstract: A key concept underlying the specific functionalities of metasurfaces, i.e. arrays of subwavelength nanoparticles, is the use of constituent components to shape the wavefront of the light, on-demand. Metasurfaces are versatile and novel platforms to manipulate the scattering, colour, phase or the intensity of the light. Currently, one of the typical approaches for designing a metasurface is to optimize one or two variables, among a vast number of fixed parameters, such as various materials' properties and coupling effects, as well as the geometrical parameters. Ideally, it would require a multi-dimensional space optimization through direct numerical simulations. Recently, an alternative approach became quite popular allowing to reduce the computational cost significantly based on a deep-learning-assisted method. In this paper, we utilize a deep-learning approach for obtaining high-quality factor (high-Q) resonances with desired characteristics, such as linewidth, amplitude and spectral position. We exploit such high-Q resonances for the enhanced light-matter interaction in nonlinear optical metasurfaces and optomechanical vibrations, simultaneously. We demonstrate that optimized metasurfaces lead up to 400+ folds enhancement of the third harmonic generation (THG); at the same time, they also contribute to 100+ folds enhancement in optomechanical vibrations. This approach can be further used to realize structures with unconventional scattering responses.