arXiv subjects

Xiao-Jing Du

Publications and source records attributed to Xiao-Jing Du.

7 recordsLinked to original sources

arXiv2026

Merging the characteristics of an exceptional point and a quasi-bound state in the continuum in nanophotonic cavities

In conventional eigenvalue analyses of non Hermitian two mode systems, mode coupling cannot produce the simultaneous occurrence of an exceptional point (EP) and a quasi bound state in the continuum (QBIC) at the same spectral position. This work shows that this limitation originates from the eigenvalue framework itself. By introducing an excitation phase degree of freedom, the interference between radiation channels can be reshaped without modifying the intrinsic eigenmodes of the system, thereby overcoming this constraint. Based on coupled mode theory, we demonstrate that the excitation phase enables the merging of an EP and a QBIC (and even a BIC) in nanophotonic cavities, and we validate this mechanism through full wave simulations of practical stacked structures. In the EP-QBIC regime, the mode quality (Q) factor is enhanced by more than one order of magnitude. We further systematically analyze the formation conditions of EP-QBIC states and conventional QBICs. Moreover, in a purely plasmonic structure, introducing an excitation phase leads to a more than 15 fold increase in the Q factor due to QBIC formation-surpassing the theoretical limit imposed by intrinsic material loss.

physics.optics↗

arXiv2026

Near-Perfect Chirality and Giant Spin-Orbit Conversion in a Single Plasmonic Cavity

To overcome the difficulty of single nanostructures in approaching the theoretical limit of chiroptical performance, we design a single plasmonic twisted dimer cavity whose magnetic gap plasmon mode enables magnetic polarization near-field engineering for high chirality. The structure exhibits strong extinction under circularly polarized excitation with one handedness, while its response to the orthogonally circularly polarized light is almost perfectly suppressed, yielding a chiral g-factor as high as 1.94. Meanwhile, the structure demonstrates strong chiral-selective spin-orbit angular momentum conversion: the conversion efficiency is ~95% under circularly polarized excitation with one handedness and only ~1% under the other. By tuning geometric parameters, the g-factor can be continuously adjusted from 0 to 1.94. Without relying on periodic coupling or collective effects, this work achieves near-perfect chirality and highly efficient angular momentum manipulation solely through intrinsic near-field matching, providing a new design strategy and theoretical basis for highly selective, ultra-compact integrated chiral photonic devices.

physics.optics↗

arXiv2025

Quantum versus Classical Descriptions of Spontaneous Emission in Nanophotonic Cavities

Here, we demonstrate that quantum and classical descriptions generally yield different results for the spontaneous emission in nanophotonic cavities. Starting from the quantized single-mode field in a general context of dispersive and lossy cavities, we derive the expression for emission rate enhancement as well as key relevant parameters such as mode volume and quality factor. For general nanophotonic cavities, this ratio of the quantum to the classical description is typically below unity and varies with the material dispersion properties, scattering-to-absorption ratio and morphology of the cavity. Notably, the two descriptions converge for lossless, non-dispersive dielectric cavities and for noble-metal plasmonic cavities with sufficiently low scattering losses.

physics.optics↗

arXiv2025

Subwavelength plasmonic antennas based on asymmetric split-ring-resonators for high near-field enhancements

As for plasmonic antenna structures that generate localized near-field enhancement, the most effective current implementations are based on electric dipole resonance modes, but this approach also imposes limitations on their further optimization. Here we introduce an ASRR structure whose ASR mode enables differential charge distribution across both sides of the split. Through asymmetric regulation, charges at one end can become highly localized, thereby achieving efficient near-field enhancement. The formation of this structure was initially driven by a hybrid computational framework integrating evolutionary optimization with residual neural networks, and subsequently simplified into an ASRR prototype using the Occam's Razor principle. The ASRR dimer structure can achieve an electric field intensity enhancement over 6.5 times larger than a traditional nanorod dimer, while maintaining a compact size (<1/3 the working wavelength). The ASRR configuration also demonstrates superior Purcell factor and fluorescence enhancement. These results can find applications in surface-enhanced spectroscopy, nonlinear optics, and quantum light-matter interactions.

physics.optics↗

arXiv2024

Relations between near-field enhancements and Purcell factors in hybrid nanostructures of plasmonic antennas and dielectric cavities

Strong near-field enhancements (NFEs) of nanophotonic structures are believed to be closely related to high Purcell factors (FP). Here, we theoretically show that the correlation is partially correct; the extinction cross section (σ) response is also critical in determining FP. The divergence between NFE and FP is especially pronounced in plasmonic-dielectric hybrid systems, where the plasmonic antenna supports dipolar plasmon modes and the dielectric cavity hosts Mie-like resonances. The cavity's enhanced-field environment can boost the antenna's NFEs, but the FP is not increased concurrently due to the larger effective σ that is intrinsic to the FP calculations. Interestingly, the peak FP for the coupled system can be predicted by using the NFE and σ responses. Furthermore, the limits for FP of coupled systems are considered; they are determined by the sum of the FP of a redshifted (or modified, if applicable) antenna and an individual cavity. This contrasts starkly with the behavior of NFE which is closely associated with the multiplicative effects of the NFEs provided by the antenna and the dielectric cavity. The differing behaviors of NFE and FP in hybrid cavities have varied impacts on relevant nanophotonic applications such as fluorescence, Raman scattering and enhanced light-matter interactions.

physics.optics↗

arXiv2023

Magnetic light amplification by stimulated emission of radiation in subwavelength systems of a dielectric cavity and magnetic quantum emitters

We propose a magnetic laser in a subwavelength system consisting of a high-refractive-index dielectric cavity and an active medium formed by magnetic quantum emitters. Stimulated emissions of magnetic quantum emitters induced by their coherent interactions with quantized magnetic fields of a cavity are theoretically considered. The condition to archive such a magnetic laser is obtained. Numerical results show that magnetic lasers are feasible in some realistic systems, for example, a silicon disk of high-quality whispering gallery modes with embedded emitters. Furthermore, the competitions between the electric interaction and magnetic one in terms of their Purcell factors are also considered in some magnetic laser achievable systems. In a wavelength-scale silicon block of a high-order magnetic mode, the ratio of magnetic Purcell factor to the electric one can reach more than ~10^3 large. Our results open up ways to enhanced magnetic light-matter interactions.

physics.optics↗

arXiv2022

Strong Superradiance of Coherently Coupled Magnetic Dipole Emitters Mediated by Whispering Gallery Modes of a Subwavelength All-Dielectric Cavity

The interaction of magnetic dipole (MD) emitters and common photonic cavities is usually weak, which is partially due to the low magnetic near field enhancements of the cavities. Here, we show that whispering gallery modes (WGMs) of a subwavelength dielectric cavity can not only greatly boost the emission rate of a MD emitter but also bring efficient couplings between coherent MD emitters. In a WGM cavity, the maximal emission rate (γmax) of a single emitter occurs at an antinode of the field pattern. The emission of the MD emitter can also be greatly affected by another coherent one depending on the magnetic field response of the WGM. The maximal contribution can also reach γmax. Notably, the cooperative emission rate of the coherent MD emitters does not decay with distance in the considered range due to the high-quality feature of a WGM. In contrast to the emission, the absorption of an emitter is hardly affected by the coherent couplings between emitters mediated by a WGM. The difference between the performances of emission and absorption is highly related to the excitation behaviors of WGMs. Our results are important for enhanced magnetic light-matter interactions.

physics.optics↗