arXiv subjects

Zhanghua Han

Publications and source records attributed to Zhanghua Han.

6 recordsLinked to original sources

arXiv2026

Observation of Janus Chirality for Coherent Thermal Emission from Metasurfaces

Metasurfaces emerged as a powerful tool for controlling thermal radiation, yet achieving coherent emission with opposite circular handednesses remains a highly challenging problem. Here, we demonstrate experimentally the Janus chiral thermal emission from metasurfaces with opposite circular handednesses on either side of a single device. We employ anisotropic metasurfaces supporting high-Q resonances with photonic flatbands enabling near-unity circular dichroism through in-plane symmetry control. Our experiments confirm the Janus coherent emission, and they are validated by the results of the coupled-mode theory. The flatband resonant metasurfaces enabling a control of chiral thermal emission provide an efficient platform for spin-controlled light-matter interaction.

physics.optics↗

arXiv2025

Broadband-operational orbital angular momentum generation in nonlocal metasurfaces with maximum efficiency approaching 80%

Nonlocal metasurfaces provide a compact route to generating momentum-space optical vortices but are limited by steep dispersion typically associated with high-quality (Q) factor resonances, resulting in narrowband and inefficient operation. Here, we introduce a reflection-type nonlocal metasurface that hybrid-couples a bound state in the continuum (BIC) with two degeneracy points (DPs). This engineered interaction enables on-demand control of dispersion, radiative Q-factors, and polarization states of guided resonances, yielding quasi-flat dispersion and enhanced scattering strength. Full-wave simulations predict near-unity on-resonance conversion and overall efficiencies above 90%, representing a three- to fourfold efficiency improvement and more than fifteenfold bandwidth expansion over conventional designs. Experiments confirm broadband operation from 1480 to 1600 nm, achieving peak efficiency approaching 80% and orbital angular momentum (OAM) purity up to 91.7% under flat-top illumination, while suppressing edge effects and mitigating positional sensitivity and numerical-aperture (NA) dependence. As a proof of concept, we demonstrate direct conversion of zero-order Bessel beams into OAM Bessel (perfect vortex) beams with enhanced wavelength tunability, underscoring the versatility of this approach over diverse illumination conditions. This record-high performance establishes a practical and scalable pathway toward broadband, high-efficiency vortex generation, opening new opportunities across high-dimensional optical communications, advanced imaging, and quantum photonics.

physics.optics↗

arXiv2022

One-dimensional quasi bound states in the continuum in the ω~k space for nonlinear optical applications

The phenomenon of bound state in the continuum (BIC) with infinite quality factor and lifetime has emerged in recent years in photonics as a new tool of manipulating light-matter interactions. However, most of the investigated structures only support BIC resonances at very few discrete points in the w~k space. Even when the BIC is switched to a quasi-BIC(QBIC) resonance through perturbation, its frequency will still be located within a narrow spectral band close to that of the original BIC, restricting their applications in many fields where random or multiple input frequencies beyond the narrow band are required. In this work, we demonstrate that a new set of QBIC resonances can be supported by making use of a special binary grating consisting of two alternatingly aligned ridge arrays with the same period and zero-approaching ridge width difference on a slab waveguide. These QBIC resonances are distributed continuously over a broad band along a line in the w~k space and can thus be considered as one-dimensional QBICs. With the Q factors generally affected by the ridge difference, it is now possible to choose arbitrarily any frequencies on the dispersion line to achieve significantly enhanced light-matter interactions, facilitating many applications where multiple input wavelengths are required, e.g. sum or difference frequency generations in nonlinear optics.

physics.optics↗

arXiv2017

Cavity enhanced terahertz fingerprint detection with ultrahigh sensitivity

We report a new scheme of realizing terahertz fingerprint detection with ultrahigh sensitivity. Instead of using the direct absorption of terahertz through a bare sample in the regular transmission scheme, a cavity mode resonating at the characteristic frequency of the sample is used and due to the high dependence of the cavity mode transmission on the material loss, an amplified transmission decaying is observed when the sample is loaded into the cavity. Furthermore, this scheme retains the feature of substance identification. A one-dimensional photonic crystal cavity is used as the example for the detection of α-lactose and an efficient detection of 7nm α-lactose can be achieved, which corresponds to 1/80000 of the free space wavelength at the characteristic frequency of 0.529THz, exhibiting sensitivity 500 times higher than the regular method.

physics.app-ph↗

arXiv2016

On-chip detection of radiation guided by dielectric-loaded plasmonic waveguides

We report a novel approach for on-chip electrical detection of the radiation guided by dielectric-loaded surface plasmon polariton waveguides (DLSPPW) and DLSPPW-based components. The detection is realized by fabricating DLSPPW components on the surface of a gold (Au) pad supported by a silicon (Si) substrate supplied with aluminum pads facilitating electrical connections, with the gold pad being perforated in a specific locations below the DLSPPWs in order to allow a portion of the DLSPPW-guided radiation to leak into the Si-substrate, where it is absorbed and electrically detected. We present two-dimensional photocurrent maps obtained when the laser beam is scanning across the gold pad containing the fabricated DLSPPW components that are excited via grating couplers located at the DLSPPW tapered terminations. By comparing photocurrent signals obtained when scanning over a DLSPPW straight waveguide with those related to a DLSPPW racetrack resonator, we first determine the background signal level and then the corrected DLSPPW resonator spectral response, which is found consistent with that obtained from full wave numerical simulations. The approach developed can be extended to other plasmonic waveguide configurations and advantageously used for rapid characterization of complicated plasmonic circuits.

physics.optics↗

arXiv2010

Ultracompact plasmonic racetrack resonators in metal-insulator-metal waveguides

Among various plasmonic waveguides, the metal-insulator-metal (MIM) type is the most promising for true subwavelength photonic integration. To date, many photonic devices based on MIM waveguides have been investigated, including resonators. However, most of the reported MIM ring resonators suffer from low extinction ratios. In this paper, we present a comprehensive analysis of the intrinsic reasons for the low performance of MIM ring resonators, and give the analytical transmission relation for a universal all-pass ring resonator which has coupling loss. Based on the analysis we propose the plasmonic racetrack resonators in MIM waveguides and show that the performance can be greatly improved.

physics.optics↗