arXiv subjects

Juan-Feng Zhu

Publications and source records attributed to Juan-Feng Zhu.

5 recordsLinked to original sources

arXiv2026

Temporal Fourier Optics Reveals Hidden Hybridized Light-Matter States

Spectral measurements provide fundamental insights into wave systems by revealing resonances, mode hybridization, and light-matter interactions. However, intrinsic dissipation and measurement-related spectral broadening often obscure the spectral signatures of the underlying hybridized light-matter states. Here, we establish a temporal Fourier optics framework based on a space-time Fourier correspondence, which interprets spectral broadening as the Fourier counterpart of temporal attenuation. This perspective introduces a temporal point-spread function (TPSF) that enables direct, synthesis-free reconstruction of the underlying spectral response from experimentally measured spectra by compensating for effective temporal decay before transformation back to the frequency domain. We experimentally validate the framework using deterministic single-molecule Au nanosphere dimers and open Au@Ag nanorod- and nanotriangle-based plasmonic nanocavities coupled to J-aggregate excitons. Across these distinct platforms, TPSF consistently resolves hidden upper and lower polaritonic branches, revealing hybridized light-matter states and strong coupling that remain inaccessible in conventional scattering spectra. The reconstructed spectra agree closely with the recently developed complex-frequency formalism while providing a substantially simpler and experimentally accessible implementation. More broadly, temporal Fourier optics establishes a general framework for recovering dissipation-obscured spectral information, opening new opportunities for spectroscopy, imaging, sensing, and inverse wave measurements across photonics and wave physics.

physics.optics↗

arXiv2026

Observation of the Inherent Chiral Smith-Purcell Effect via Symmetry Breaking

The Smith-Purcell effect arises when charged particles move near a periodic structure, emitting radiation. Conventional approaches for generating chiral Smith-Purcell radiation rely on metasurface phase engineering or resonant mode interference, typically producing narrow-band, weakly chiral emission. Here, we introduce a resonance-interference-free mechanism that leverages the properties of the charged particles themselves. Using a non-chiral, non-resonant silicon grating, we demonstrate broadband, tunable Smith-Purcell radiation with high chirality, achieving a record-high degree of polarization of 0.87. This is enabled by converting the transverse spin angular momentum of electron-induced evanescent waves into a longitudinal form, producing opposite chirality at different azimuthal angles. Beam twisting or displacement offers precise control over chirality, paving the way for compact chiral light sources, advanced X-ray imaging, and integrated particle diagnostics platforms.

physics.optics↗

arXiv2025

Spectral localization of single-nanoparticle plasmons through photonic substrate engineering

Surface plasmon resonances (SPRs) are crucial for confining light beyond the diffraction limit, yet heavy metal losses often limit their spectral localization. Here, we propose a practical strategy for enabling the spectral localization of single-nanoparticle SPRs through photonic substrate engineering, which creates distinct optical pathways (OPs) to tailor the electromagnetic environments around plasmonic nanoparticles. By analyzing the multiplication factor spectrum of the projected local density of states, we can trace and control these OPs, enabling strong spatial and spectral confinement of single-nanoparticle SPRs. Simulations reveal that a photonic crystal substrate can reduce the mode volume by fivefold and boost the quality factor by over 80 times compared to a metal nanoparticle on a dielectric substrate. Proof-of-concept experiments using two types of leaking Fabry-Perot photonic substrates demonstrate active manipulation of SPRs in both "open" and "closed" OP states. This multidimensional photonic substrate engineering establishes a customizable platform for single-nanoparticle plasmonics, potentially transforming applications that were previously limited by spectral localization.

physics.optics↗

arXiv2025

Probing dynamics of time-varying media: Beyond abrupt temporal interfaces

This work investigates the effects of time-varying media, where optical properties change over time, on electromagnetic wave propagation, focusing on plane waves and free-electron evanescent waves. We introduce a switching parameter, $τ$, to model ultrafast switching in the femtosecond to nanosecond range. For plane-wave incidence at angular frequency $ω_0$, we derive a generalized expression for the backward-to-forward flux ratio as a function of $ω_0$ and $τ$, aligning with recent experimental data and providing a unified interpretation framework. For free-electron incidence, we observe intensity saturation in temporal transition radiation at $I_{\textrm{max}}$ for $τ\leq τ_{\textrm{0}}$, with both $I_{\textrm{max}}$ and $τ_{\textrm{0}}$ depending on electron speed. These results highlight the importance of precise $τ$ control in experiments to probe time-varying media effectively.

physics.optics↗

arXiv2023

Smith-Purcell radiation from time grating

Smith-Purcell radiation (SPR) occurs when an electron skims above a spatial grating, but the fixed momentum compensation from the static grating imposes limitations on the emission wavelength. It has been discovered that a temporally periodic system can provide energy compensation to generate light emissions in free space. Here, we introduce temporal SPR (t-SPR) emerging from a time grating and propose a generalized t-SPR dispersion equation to predict the relationship between radiation frequency, direction, electron velocity, modulation period, and harmonic orders. Compared to conventional SPR, t-SPR can: 1) Provide a versatile platform for manipulating SPR emission through temporal modulation (e.g., period, amplitude, wave shape). 2) Exhibit strong robustness to the electron-grating separation, alleviating the constraints associated with extreme electron near-field excitation. 3) Introduce additional energy channels through temporal modulation, enhancing and amplifying emission.

physics.optics↗