The Coupling Effects of Surface Plasmon Polaritons and Magnetic Dipole Resonances in Metamaterials

Beyond Conventional Sensing: Hybrid Plasmonic Metasurfaces and Bound States in the Continuum

Fano resonances result from the strong coupling and interference between a broad background state and a narrow, almost discrete state, leading to the emergence of asymmetric scattering spectral profiles. Under certain conditions, Fano resonances can experience a collapse of their width due to the destructive interference of strongly coupled modes, resulting in the formation of bound states in the continuum (BIC). In such cases, the modes are simultaneously localized in the nanostructure and coexist with radiating waves, leading to an increase in the quality factor, which is virtually unlimited. In this work, we report on the design of a layered hybrid plasmonic-dielectric metasurface that facilitates strong mode coupling and the formation of BIC, resulting in resonances with a high quality factor. We demonstrate the possibility of controlling Fano resonances and tuning Rabi splitting using the nanoantenna dimensions. We also experimentally demonstrate the generalized Kerker effect in a binary arrangement of silicon nanodisks, which allows for the tuning of the collective modes and creates new photonic functionalities and improved sensing capabilities. Our findings have promising implications for developing plasmonic sensors that leverage strong light-matter interactions in hybrid metasurfaces.

Composite Structure of Ag Colloidal Particles and Au Sinusoidal Nanograting with Large-Scale Ultra-High Field Enhancement for SERS Detection

In this study, a novel composite Surface-Enhanced Raman Scattering (SERS) substrate is proposed for ultrasensitive detection. Consisting of gold sinusoidal nanograting and silver colloidal nanoparticles (AgNPs-AuSG), this type of SERS substrate is easy for fabrication by maskless laser interference lithography, and capable of providing large-scale ultra-high field enhancement, attributed to localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs). The enhancement factor (EF) of this composite substrate is as high as up to 10 orders of magnitude in the simulation experiment. Experimental results show that this large-area, productive SERS substrate of AgNPs-AuSG has realized sensitive TNT and RDX detection with the limit of detection (LOD) of 10−10 M, which may be a potential candidate for trace explosives detection.

Narrowband Light Reflection Resonances from Waveguide Modes for High-Quality Sensors

Designing various nanostructures to achieve narrowband light reflection resonances is desirable for optical sensing applications. In this work, we theoretically demonstrate two narrowband light reflection resonances resulting from the excitations of the zero-order transverse magnetic (TM) and transverse electric (TE) waveguide modes, in a waveguide structure consisting of an Au sphere array on an indium tin oxide (ITO) spacer on a silica (SiO₂) substrate. The positions of the light reflection resonances can be tuned easily, by varying the array periods of gold (Au) spheres or by changing the thickness of the ITO film. More importantly, the light reflection resonances have a very narrow bandwidth, the full width at half maximum (FWHM) of which can be reduced to only several nanometers for the zero-order TM and TE waveguide modes. The conventionally defined performance parameters of sensors, sensitivity (S) and figure of merit (FOM), have quite high values of about 80 nm/RIU and 32, respectively, in the visible wavelength range.

Plasmonic Colour Printing by Light Trapping in Two-Metal Nanostructures

Structural colour generation by nanoscale plasmonic structures is of major interest for non-bleaching colour printing, anti-counterfeit measures and decoration applications. We explore the physics of a two-metal plasmonic nanostructure consisting of metallic nanodiscs separated from a metallic back-reflector by a uniform thin polymer film and investigate the potential for vibrant structural colour in reflection. We demonstrate that light trapping within the nanostructures is the primary mechanism for colour generation. The use of planar back-reflector and polymer layers allows for less complex fabrication requirements and robust structures, but most significantly allows for the easy incorporation of two different metals for the back-reflector and the nanodiscs. The simplicity of the structure is also suitable for scalability. Combinations of gold, silver, aluminium and copper are considered, with wide colour gamuts observed as a function of the polymer layer thickness. The structural colours are also shown to be insensitive to the viewing angle. Structures of copper nanodiscs with an aluminium back-reflector produce the widest colour gamut.

Angular dependent strong coupling between localized waveguide resonance and surface plasmon resonance in complementary metamaterials

We give direct evidence of both surface plasmon resonance (SPR) and localized waveguide resonance (LWR) contribution to the extraordinary optical transmission in complementary metamaterials. Strong coupling between SPR and LWR are also observed with clear evidence of Rabi splitting and anti-crossing phenomena. The splitting introduces sharp phase shift, which in turn enhances group velocity delay by the incident angle without geometric parameter change. The results not only clarify SPR and LWR effects in the extraordinary optical transmission, but also provide a novel route to control light-metamaterial interaction by angular modulation for on-chip slow light devices.

Broadband light absorption of an Al semishell-MIM nanostrucure in the UV to near-infrared regions

A plasmonic broadband light absorber, whose absorption is insensitive to incident angles and polarizations, in the UV to near-infrared regions is demonstrated. In experimental observations, the maximum average absorption of 83% over a wavelength range from 300 to 1000 nm was confirmed. Our proposed plasmonic absorber is based on a three-layer stack of metal-insulator-metal, and the top metal layer is nanostructured by colloidal lithography. This structure is composed of Al, which is an excellent and cost-effective plasmonic material. This fabrication simplicity and economical material allows us to produce a large-scale device of solar absorbers.

Multiband and Broadband Absorption Enhancement of Monolayer Graphene at Optical Frequencies from Multiple Magnetic Dipole Resonances in Metamaterials

It is well known that a suspended monolayer graphene has a weak light absorption efficiency of about 2.3% at normal incidence, which is disadvantageous to some applications in optoelectronic devices. In this work, we will numerically study multiband and broadband absorption enhancement of monolayer graphene over the whole visible spectrum, due to multiple magnetic dipole resonances in metamaterials. The unit cell of the metamaterials is composed of a graphene monolayer sandwiched between four Ag nanodisks with different diameters and a SiO₂ spacer on an Ag substrate. The near-field plasmon hybridizations between individual Ag nanodisks and the Ag substrate form four independent magnetic dipole modes, which result into multiband absorption enhancement of monolayer graphene at optical frequencies. When the resonance wavelengths of the magnetic dipole modes are tuned to approach one another by changing the diameters of the Ag nanodisks, a broadband absorption enhancement can be achieved. The position of the absorption band in monolayer graphene can be also controlled by varying the thickness of the SiO₂ spacer or the distance between the Ag nanodisks. Our designed graphene light absorber may find some potential applications in optoelectronic devices, such as photodetectors.