Ms. Qiqige Wulan Profile

Qiqige Wulan

at Univ of Electronic Science and Technology of China

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Proceedings Article | 22 December 2023 Paper

Tunable coupling between molecular vibration and sub-radiant magnetic quadrupole in a plasmonic metasurface

Qiqige Wulan, Jiachen Yu, Li Xing, Zhijun Liu

Proceedings Volume 12773, 1277309 (2023) https://doi.org/10.1117/12.2686759

KEYWORDS: Vibration, Magnetism, Plasmonics, Molecules, Plasmons, Plane waves, Nanostructures, Nanoparticles, Molecular interactions, Mirrors

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Strong coupling between plasmonic resonances in metallic nanostructures and molecule vibrations has been reported in structures such as metallic dimers and nanoparticles on mirror, forming new hybrid states called ‘plexcitons’. Strong coupling occurs when the energy exchange rate exceeds the damping rates of both optical mode and the molecular vibration. Sub-radiant plasmonic modes with high quality factor have often been considered as promising candidates for achieving strong coupling with molecules. However, the sub-radiant plasmon mode cannot be directly excited by farfield normal incident plane wave due to destructive interaction among the moments with equal magnitudes but opposite directions, completely cancelling each other out. In this work, we demonstrate a plasmonic metasurface, which exhibits a sub-radiant magnetic quadrupole (MQ) resonance and tunable coupling strength with molecular vibrations. Our fabricated metasurface consists of an asymmetric metallic micro-cross array on top of a dielectric spacer backed with a metallic film. A magnetic quadrupole resonance is excited at 37.8THz at far-field normal incidence condition. Its quality factor reaches 34, which is more than four times of regular radiant magnetic dipole excited in the same structure. Coupling of the MQ resonance with vibration band (Si-CH₃ at 37.8THz) of polydimethylsiloxane (PDMS) leads to Rabi splitting phenomenon in the dispersion curve. As thickness of the spin-coated PDMS film increases from 14 nm to 310 nm, the Rabi splitting gap increases from 0.6THz to 1.13THz, exhibiting a tunable coupling strength. Our demonstrated magnetic quadrupole metasurface presents a versatile scheme for manipulating the light-matter interaction in sensing application.

Proceedings Article | 28 November 2023 Paper

Cavity-coupled narrow bandwidth Fano resonance in dielectric metasurface

Jiachen Yu, Qiqige Wulan, Li Xing, Zhijun Liu

Proceedings Volume 12773, 1277304 (2023) https://doi.org/10.1117/12.2685341

KEYWORDS: Dielectrics, Silicon, Silica, Photons, Light absorption, Transparency, Semiconductors, Refractive index, Optical semiconductors, Optical filters

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Dielectric metasurfaces show great potential in light manipulation for applications such as optical filters and semiconductor metasurfaces for generating entangled photons. These applications critically rely on resonant mode properties in micro/nano-structured metasurfaces. Several modes including electric dipole (ED), magnetic dipole (MD), and quasi-bound state in the continuum resonance have been widely explored with the merits of high quality factor and narrow bandwidth. However, these reported metasurface resonances generally do not absorb light due to the transparency of dielectric materials, which limits their usage in some applications such as thermal emission and photon detection etc. In this work, we demonstrate a new class of dielectric metasurfaces, which exhibit the features of both strong light absorption and narrower bandwidth. Our fabricated metasurface consists of a Si cuboid array on top of a SiO₂ spacer layer backed with a metallic Cu film. The cavity mode within the SiO₂ spacer couples with electrical dipole in the Si cuboid, leading to an asymmetric narrow bandwidth Fano resonance within our metasurface structure. Our spectral measurements show that the Fano resonance occurs at the wavelength of 4.19 μm. It has a strong absorption efficiency of 65.8% and a narrow bandwidth of 32.5nm, corresponding to a quality factor as high as 112. As an example to reveal the potentials of our metasurface, the Fano resonance is applied in refractive index sensing with a sensitivity of 518.75nm/RIU and a high figure-of-merit (FoM) of 14.82 RIU^-1. These results indicate that cavity-coupled dielectric metasurface presents an effective way for obtaining both strong light absorption and narrow bandwidth, opening new space for metasurface applications.

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