arXiv subjects

Jianfa Zhang

Publications and source records attributed to Jianfa Zhang.

17 recordsLinked to original sources

arXiv2025

Polarization-Independent Zero Directional Scattering Without Geometric Symmetries

As the characteristic feature of generalized Kerker effect in Mie theory, directional scattering elimination has been playing a pivotal role in nanophotonics and many other photonic disciplines, such as singular optics and topological photonics. Generally, zero directional scattering can be obtained only for a specific incident polarization, and to make it fully independent of arbitrary polarizations would require scatterers that exhibit geometric (\textit{e.g.} mirror) symmetries. Here we revisit the generalized Kerker effect and directional scattering elimination from the perspective of not the conventional electromagnetic multipoles, but rather quasi-normal modes supported by non-Hermitian systems. We reveal how to obtain zero directional scattering that is independent of arbitrary incident polarizations, even for scattering structures that do not exhibit the required geometric symmetries. Such geometric symmetry-free and polarization-independent responses are made accessible through a synchronous exploitation of electromagnetic reciprocity and geometric phase. Our discovery can stimulate fundamental explorations and practical applications in not only photonics, but also many other wave physics branches where scattering and geometric phase are pervasive.

physics.optics↗

arXiv2025

Polarizations Underdescribe Vectorial Electromagnetic Waves

Electromagnetic waves are described by not only polarization ellipses but also cyclically rotating vectors tracing out them. The corresponding fields are respectively directionless steady line fields and directional instantaneous vector fields. Here we study the seminal topic of electromagnetic scattering from the perspective of instantaneous vector fields and uncover how the global topology of the momentum sphere regulates local distributions of tangent scattered fields. Structurally-stable generic singularities of vector fields move cyclically along lines of linear polarizations and at any instant the index sum of all instantaneous singularities has to be the Euler characteristic $χ=2$. This contrasts sharply with steady line fields, of which generic singularities constrained by the Euler characteristic locate on points of circular polarizations. From such unique perspective of instantaneous singularities, we discovered that for circularly-polarized waves scattered by electromagnetic duality-symmetric particles, since linearly-polarized scatterings are prohibited by helicity conservation, there must exist at least one dark direction along which the scattering is strictly zero. Two such dark directions can be tuned to overlap, along which the scattering would remain zero for arbitrary incident polarizations. We have essentially revealed that \textit{polarizations underdescribe vectorial electromagnetic waves and the instantaneous perspective is indispensable}. The complementarity we discover provides broader and deeper insights into not only electromagnetism, but also other branches of wave physics where singularities are generic and ubiquitous.

physics.optics↗

arXiv2024

Intrinsic polarization conversion and avoided-mode crossing in X-cut lithium niobate microrings

Compared with well-developed free space polarization converters, polarization conversion between TE and TM modes in waveguide is generally considered to be caused by shape birefringence, like curvature, morphology of waveguide cross section and scattering. Here, we reveal a hidden polarization conversion mechanism in X-cut lithium niobate microrings, that is the conversion can be implemented by birefringence of waveguides, which will also introduce an unavoidable avoided-mode crossing. In the experiment, we find that this mode crossing results in severe suppression of one sideband in local nondegenerate four-wave mixing and disrupts the cascaded four-wave mixing on this side. Simultaneously, we proposed, for the first time to our best knowledge, one two-dimensional method to simulate the eigenmodes (TE and TM) in X-cut microrings, which avoids the obstacle from large computational effort in three-dimensional anisotropic microrings simulation, and the mode crossing point. This work will provide an entirely novel approach to the design of polarization converters and simulation for monolithic photonics integrated circuits, and may be helpful to the studies of missed temporal dissipative soliton formation in X-cut lithium niobate rings.

physics.optics↗

arXiv2024

Enhanced Robustness via Loss Engineering in Detuned Non-Hermitian Scattering Systems

Non-Hermitian optics has revealed a series of counterintuitive phenomena with profound implications for sensing, lasing, and light manipulation. While the non-Hermiticity of Hamitonians is well-recognized, recent advancements in non-Hermitian physics have broadened to include scattering matrices, uncovering phenomena such as simultaneous lasing and coherent perfect absorption (CPA), reflectionless scattering modes (RSMs), and coherent chaos control. Despite these developments, the investigation has predominantly focused on static and symmetric configurations, leaving the dynamic properties of non-Hermitian scattering in detuned systems largely unexplored. Bridging this gap, we extend certain stationary non-Hermitian scattering phenomena to detuned systems. We delve into the interplay between bi-directional RSMs and RSM exceptional points (EPs), and elucidate the global existence conditions for RSMs under detuning. Moreover, we introduces a novel category of EPs, characterized by the coalescence of transmission peaks, emerging independent with the presence of Hamiltonian EPs. The transmission EPs (TEPs) exhibit flat-top lineshape and can be extended to a square-shaped spectrum when detuning is involved, accompanied by a distinctive phase transition. Significantly, we demonstrate the applications of the TEPs in a one-dimensional coupled cavity system, engineered to enhance sensing robustness against environmental instabilities such as laser frequency drifts, which can exceed 10 MHz. This capability represents a substantial improvement over traditional sensing methods and an important improvement of fragile EP sensors. Our findings not only contribute to the broader understanding of non-Hermitian scattering phenomena but also paves the way for future advancements in non-Hermitian sensing technologies.

physics.optics↗

arXiv2023

Magneto-chiral backscatterings by rotationally symmetric nonreciprocal structures

It was proved that the joint operation of electromagnetic reciprocity and $n$-fold ($n\geq3$) rotational symmetry would secure arbitrary polarization-independent backscattering efficiency [Phys. Rev. B \textbf{103}, 045422 (2021)]. Here we remove the restriction of reciprocity and study the backscatterings of plane waves by rotationally symmetric magneto-optical structures, with collinear incident wavevector, rotational axis and externally applied magnetic field. It is revealed that though nonreciprocity removes the degeneracy of backscattering efficiencies for circularly-polarized incident waves of opposite handedness, the remaining rotational symmetry is sufficient to guarantee that the efficiency is related to the polarization ellipticity only, having nothing to do with the orientations of the polarization ellipses. Moreover, the backscattering efficiency reaches its extremes (maximum or minimum values) always for circularly-polarized incident waves, and for other polarizations the efficiency is their ellipticity-weighted arithmetic average. The principles we have revealed are dictated by rotational symmetries only, which are irrelevant to specific geometric or optical parameters and are intrinsically robust against any rotational-symmetry preserving perturbations. The correlations we have discovered could be further exploited for fundamental explorations in nonreciprocal photonics and practical applications including polarimetry and ellipsometry.

physics.optics↗

arXiv2023

Spatio-Temporal Nonlinear Theory in Birefringent Microrings

Frequency-dependent nonlinear process in microresonators is widely acknowledged, but there is no theory available to calculate the conversion efficiency for each resonance of the ring, except for the phase-matching one. Similarly for azimuth-dependent nonlinear process in birefringent rings, there is a lack of theory to analysis the conversion efficiency for each azimuth of the ring. Consequently, it leads to old-fashioned or ill-considered coupling position and inefficient energy conversion in birefringent microrings. Here, we introduce spatio-temporal coupled-mode equation to describe mode spatial properties in the cavity, compensating for the deficiency of temporal coupled-mode equation in describing sophisticated responses. By this equation, we find that over a wide frequency range, the extremely strong second-harmonic generation can be achieved at different azimuth under different pumps in an X-cut lithium niobate microring, which is important for realizing an efficient entangled quantum light source, for example. This work will provide new ideas and guidelines for design and applications of monolithic birefringent photonic integrated circuits with high efficiency.

physics.optics↗

arXiv2023

Momentum-space Scattering Extremizations

Studies into scatterings of photonic structures have been so far overwhelmingly focused on their dependencies on the spatial and spectral morphologies of the incident waves. In contrast, the evolution of scattering properties through another parameter space of incident directions (momentum space) has attracted comparably little attention, though of profound importance for various scattering-related applications. Here we investigate, from the perspective of quasi-normal modes (QNMs), the momentum-space scattering extremizations with respect to varying incident directions of plane waves. It is revealed that for effective single-QNM excitations, scatterings are maximized exactly along those directions where the QNM radiation reaches its maximum, with matched incident and radiation polarizations. For an arbitrary direction, when the incident polarization is tuned to be orthogonal to that of the mode radiation, the QNM cannot be excited and thus the scatterer becomes invisible with null scatterings. The principles we have revealed are protected by fundamental laws of reciprocity and energy conservation (optical theorem), which can be further expanded and applied for other branches of wave physics.

physics.optics↗

arXiv2018

Large Chiroptical Effects in Planar Chiral Metamaterials

Chiroptical effects, characterized by different optical responses for left- (LCP) and right- handed circularly polarized light (RCP), are powerful and valuable tools in optics with wide applications in polarization resolved imaging and sensing. Previously observed strong chiroptical effects are limited to metamaterials with complex three-dimensional chiral structures at the sub-wavelength scale. Although asymmetrical transmission of LCP and RCP have been investigated in planar chiral metasurfaces, the observed weak chiroptical effects result from anisotropic Ohmic dissipation of the metal constituents. Here, we demonstrate by theory and proof-of-concept experiments that large difference in transmittances of LCP and RCP can be attained in a single-layer planar chiral metamaterial with a sub-wavelength thickness. Without violating the reciprocity and mirror symmetry, the strong chiroptical effect, independent of dielectric loss, arise from a novel mechanism of multimode interference. The described effect may lead to new gateway towards chiral manipulations of light and chiral optical devices.

physics.optics↗

arXiv2016

Controlling light-with-light without nonlinearity

According to Huygens' superposition principle, light beams traveling in a linear medium will pass though one another without mutual disturbance. Indeed, it is widely held that controlling light signals with light requires intense laser fields to facilitate beam interactions in nonlinear media, where the superposition principle can be broken. We demonstrate here that two coherent beams of light of arbitrarily low intensity can interact on a metamaterial layer of nanoscale thickness in such a way that one beam modulates the intensity of the other. We show that the interference of beams can eliminate the plasmonic Joule losses of light energy in the metamaterial or, in contrast, can lead to almost total absorbtion of light. Applications of this phenomenon may lie in ultrafast all-optical pulse-recovery devices, coherence filters and THz-bandwidth light-by-light modulators.

physics.optics↗

arXiv2015

Modulating light with light via giant nano-opto-mechanical nonlinearity of plasmonic metamaterial

From the demonstration of saturable absorption by Vavilow and Levshin in 1926, and with invention of the laser, unavailability of strongly nonlinear materials was a key obstacle for developing optical signal processing, in particular in transparent telecommunication networks. Today, most advanced photonic switching materials exploit gain dynamics and near-band and excitonic effects in semiconductors, nonlinearities in organic media with weakly-localized electrons and nonlinearities enhanced by hybridization with metamaterials. Here we report on a new type of artificial nonlinearity that is nano-opto-mechanical in nature. It was observed in an artificial metamaterial array of plasmonic meta-molecules supported by a flexible nano-membrane. Here nonlinearity is underpinned by the reversible reconfiguration of its structure induced by light. In a film of only 100 nanometres thickness we demonstrated modulation of light with light using milliwatt power telecom diode lasers.

physics.optics↗

arXiv2015

Invisible nanowires with interfering electric and toroidal dipoles

By studying the scattering of normally incident planewaves by a single nanowire, we reveal the indispensable role of toroidal multipole excitation in multipole expansions of radiating sources. It is found that for both p-polarized and s-polarized incident waves, toroidal dipoles can be effectively excited within homogenous dielectric nanowires in the optical spectrum regime. We further demonstrate that the plasmonic core-shell nanowires can be rendered invisible through destructive interference of the electric and toroidal dipoles, which may inspire many nanowire based light-matter interaction studies, and incubate biological and medical applications that require non-invasive detections and measurements.

physics.optics↗

arXiv2015

Graphene plasmonics for light trapping and absorption engineering

Plasmonics can be used to improve absorption in optoelectronic devices and has been intensively studied for solar cells and photodetectors. Graphene has recently emerged as a powerful plasmonic material. It shows significantly less losses compared to traditional plasmonic materials such as gold and silver and its plasmons can be tuned by changing the Fermi energy with chemical or electrical doping. Here we propose the usage of graphene plasmonics for light trapping in optoelectronic devices and show that the excitation of localized plasmons in doped, nanostructured graphene can enhance optical absorption in its surrounding media including both bulky and two-dimensional materials by tens of times, which may lead to a new generation of highly efficient, spectrally selective photodetectors in mid-infrared and THz ranges. The proposed concept could even revolutionize the field of plasmonic solar cells if graphene plasmons in the visible and near-infrared are realized.

physics.optics↗

arXiv2015

Tunable asymmetric transmission of THz wave through a graphene planar chiral structure

In this letter, we show that asymmetric transmission of circularly polarized waves through a nanostructured planar chiral graphene film can be observed in terahertz range. The asymmetric transmission effect of monatomic layer graphene closely resembles that of metallic planar chiral nanostructures which has previously been demonstrated. And the relative enantiomeric difference in the total transmission varies with the change of graphene's Fermi level. The plasmonic excitation in the graphene nanostructure is the enantiometically sensitive which is asymmetric for opposite propagating directions. This phenomenon will deepen our understanding of light-matter interactions in planar chiral structures and may find applications in polarization-sensitive devices, sensors, detectors and other areas.

physics.optics↗

arXiv2015

Toroidal dipole induced transparency for core-shell nanoparticles

We investigate the scattering properties of spherical nanoparticles by employing a Cartesian multipole expansion method which has incorporated radiating toroidal multipoles. It is shown that toroidal dipoles, which are negligible under long-wavelength approximations, can be excited within high-permittivity dielectric nanoparticles and significantly influence the scattering profile in the optical regime. We further reveal that the scattering transparencies of core-shell plasmonic nanoparticles can be classified into two categories: i) the trivial transparency with no effective multipole excitations within the particle, and ii) the non-trivial one induced by the destructive interferences of induced electric and toroidal multipoles. The incorporation of toroidal moments offers new insights into the study into nanoparticle scattering in both the near- and far-fields, which may shed new light to many related applications, such as biosensing, nanoantennas, photovoltaic devices and so on.

physics.optics↗

arXiv2014

Circular Polarizer Realized by a Single Layer of Planar Chiral Metallic Nanostructure

As a basic optical element, circular polarizer plays significant roles in signal transmission, measurements and life science microscopy. Three-dimensional (3D) chiral structures have been thought to be necessary to realize circular polarizers. Here we demonstrate theoretically and experimentally for the first time that a high-efficiency circular polarizer could be realized by a single layer of planar 2D chiral structure. Our proposed circular polarizer is based on unidirectional polarization conversion instead of circular polarization stop bands. Since two-dimensional planar structures present obvious advantage for fabrication and integration on chip, the proposed circular polarizer is of great interest in integrated optics and microscopy. It provides a novel scheme to manipulate polarizations of light wave, as well as Terahertz wave and microwave.

physics.optics↗

arXiv2014

Ultra-directional forward scattering by individual core-shell nanoparticles

We study the angular scattering properties of individual core-shell nanoparticles that support simultaneously both electric and optically-induced magnetic resonances of different orders. In contrast to the approach to suppress the backward scattering and enhance the forward scattering relying on overlapping electric and magnetic dipoles, we reveal that the directionality of the forward scattering can be further improved through the interferences of higher order electric and magnetic modes. Since the major contributing electric and magnetic responses can be tuned to close magnitudes, ultra-directional forward scattering can be achieved by single nanoparticles without compromising the feature of backward scattering suppression, which may offer new opportunities for nanoantennas, photovoltaic devices, bio-sensing and many other interdisciplinary researches.

physics.optics↗

arXiv2010

Controlling the Colour of Metals: Intaglio and Bas-Relief Metamaterials

The fabrication of indented ('intaglio') or raised ('bas-relief') sub-wavelength metamaterial patterns on a metal surface provides a mechanism for changing and controlling the colour of the metal without employing any form of chemical surface modification, thin-film coating or diffraction effects. We show that a broad range of colours can be achieved by varying the structural parameters of metamaterial designs to tune absorption resonances. This novel approach to the 'structural colouring' of pure metals offers great versatility and scalability for both aesthetic (e.g. jewellery design) and functional (e.g. sensors, optical modulators) applications. We focus here on visible colour but the concept can equally be applied to the engineering of metallic spectral response in other electromagnetic domains.

physics.optics↗