arXiv subjects

Xi-Hua Guan

Publications and source records attributed to Xi-Hua Guan.

3 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↗

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↗

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↗