Mulin Liu

Enhancing refractive index sensing capability with hybrid plasmonic–photonic absorbers

We experimentally report an enhanced refractive index sensor with a maximum figure of merit (FOM*) value of about 1337 based on a hybrid plasmonic–photonic absorber via utilizing a substantial absorption contrast between a perfect absorber (∼99% at normal incidence) and a non-perfect absorber when there are changes in their surroundings.

Achieving an ultra-narrow multiband light absorption meta-surface via coupling with an optical cavity.

Resonant plasmonic and metamaterial absorbers are of particular interest for applications in a wide variety of nanotechnologies including thermophotovoltaics, photothermal therapy, hot-electron collection and biosensing. However, it is rather challenging to realize ultra-narrow absorbers using plasmonic materials due to large optical losses in metals that inevitably decrease the quality of optical resonators. Here, we theoretically report methods to achieve an ultra-narrow light absorption meta-surface by using photonic modes of the optical cavities, which strongly couple with the plasmon resonances of the metallic nanostructures. Multispectral light absorption with absorption amplitude exceeding 99% and a bandwidth approaching 10 nm is achieved at the optical frequencies. Moreover, by introducing a thick dielectric coupling cavity, the number of absorption bands can be strongly increased and the bandwidth can even be narrowed to less than 5 nm due to the resonant spectrum splitting enabled by strong coupling between the plasmon resonances and the optical cavity modes. Designing such optical cavity-coupled meta-surface structures is a promising route for achieving ultra-narrow multiband absorbers, which can be used in absorption filters, narrow-band multispectral thermal emitters and thermophotovoltaics.

One-process fabrication of metal hierarchical nanostructures with rich nanogaps for highly-sensitive surface-enhanced Raman scattering

One-process fabrication of highly active and reproducible surface-enhanced Raman scattering (SERS) substrates via ion beam deposition is reported. The fabricated metal-dielectric-metal (MDM) hierarchical nanostructure possesses rich nanogaps and a tunable resonant cavity. Raman scattering signals of analytes are dramatically strengthened due to the strong near-field coupling of localized surface plasmon resonances (LSPRs) and the strong interaction of LSPRs of metal NPs with surface plasmon polaritons (SPPs) on the underlying metal film by crossing over the dielectric spacer. The maximum Raman enhancement for the highest Raman peak at 1650 cm(-1) is 13.5 times greater than that of a single metal nanoparticle (NP) array. Moreover, the SERS activity can be efficiently tailored by varying the size and number of voids between adjacent metal NPs and the thickness of the dielectric spacer. These findings may broaden the scope of SERS applications of MDM hierarchical nanostructures in biomedical and analytical chemistry.

Monochromatic filter with multiple manipulation approaches by the layered all-dielectric patch array.

Monochromatic filtering with ultra-narrowband and high spectral contrast is desirable for wide applications in display, image, and other optoelectronics. However, owing to the inherent omhic losses in the metallic materials, a broadband spectrum with a low Q-factor down to 10 inevitably limits the device performance. Herein, we for the first time theoretically propose and demonstrate an ultra-narrowband color-filtering platform based on the layered all-dielectric meta-material (LADM), which consists of a triple-layer high/low/high-index dielectrics cavity structure. Owing to the lossless dielectric materials used, sharp resonances with the bandwidth down to sub-10 nm are observed in the sub-wavelength LADM-based filters. A spectral Q-factor of 361.6 is achieved, which is orders of magnitude larger than that of the plasmonic resonators. Moreover, for the other significant factor for evaluation of filtering performance, the spectral contrast reaches 94.5%. These optical properties are the main results of the excitation of the resonant modes in the LADMs. Furthermore, polarization-manipulated light filtering is realized in this LADM. The classical Malus law is also confirmed in the reflective spectrum by tuning the polarization state. More interestingly and importantly, the filtering phenomenon shows novel features of the wavelength-independent and tunable resonant intensity for the reflective spectrum when the LADM-based filter is illuminated under an oblique state. High scalability of the sharp reflective spectrum is obtained by tuning the structural parameters. A single-wavelength reflective filtering window is also achieved in the visible frequencies. These features hold promise for the LADM-based filter with wide applications in color engineering, displaying, imaging, etc.

Enabling Access to the Confined Optical Field to Achieve High-Quality Plasmon Sensing

We predict a unique way to achieve high-quality plasmon sensing via enabling access to the originally confined optical field to efficiently interact with the surrounding medium. Refractive index sensing with high sensitivity (690 nm/RIU) and figure of merit (3391) is achieved in the optical region, which are twice larger than those of the original metal-insulator-metal structure. This sensing scheme is based on the purpose of making utmost use of the confined optical field to spatially overlap with the targets and, therefore, improving the sensing performance, which offers new perspectives for achieving high-sensitive chip-based plasmon sensors and integrated devices.

Improved Multispectral Antireflection and Sensing of Plasmonic Slits by Silver Mirror

We propose and demonstrate an improved multiband antireflection from plasmonic slits array using an opaque metal mirror. The introduced mirror could enhance the optical field coupling and confinement, and therefore produce intensified plasmonic resonances in comparison with those of the open slit built by the dielectric substrate. The sharp triple-band nearunity antireflection is with a high scalability at optical frequencies via tuning the structural parameters. High-performance sensing measurements with a remarkably improved sensitivity (S ~ 1.5 λ nm/refractive index unit) and a high signal-to-noise ratio of the spectral intensity difference (AR > 57%) and with a large slope of 18.6%/nm are achieved based on this subdiffraction-limit (λ/5) structure. This proposed scheme and findings hold potential applications, including high-efficiency subtractive polychromatic filtering and ultracompact biosensing.

Robust Optical Transparency of a Continuous Metal Film Sandwiched by Plasmonic Crystals

Light opacity is a natural phenomenon of a continuous metal film. In various applications, such as transparent electrodes in solar cells or touching screens in ultrathin displayers, low transmittance is a major issue. Here, we present a new concept that enhances the transmission of light through a continuous metal film to reach the transmittance of glass. In order to cancel the potential electron scattering when electrons propagate in the metal film, it is sandwiched between two photonic crystal (PC) layers made of nontouching metallic particles. This geometry shows a robust, near-unity, optical transparency behavior that could be highly tuned by changing the geometrical parameters of the structure. These findings suggest potential applications in ultraintegrated optoelectronics based still benefiting from both electric and mechanical properties metals offer while still achieving optical transparency through plasmon coupling effects.

Near-field plasmon effects in extraordinary optical transmission through periodic triangular hole arrays

Abstract. We present a theoretical investigation of the transmission properties of light through a metallic film perforated with different arrays of compound triangular holes. The extraordinary optical transmission (EOT) in the optical region is obtained by employing the finite-difference time-domain method. The excitation of localized surface plasmon resonances (LSPRs) at the top corners and surface plasmon polaritons (SPPs) on the metal surface, plasmon coupling effects between adjacent apertures, and the waveguide modes for delivering light mainly contribute to the EOT in such structures. The optical characteristics can be effectively tailored by changing the arrangement of triangular holes and the structural parameters. This study may be helpful for plasmonic nanostrucutres based on EOT, and has potential applications in optoelectronic devices.

Subradiant, Superradiant Plasmon Modes and Fano Resonance in a Multilayer Nanocylinder Array Standing on a Thin Metal Film

The optical properties of a novel nanostructure consisting of a hexagonal array of aligned vertically three-layered metal-dielectric-metal nanodisks on a silver film are theoretically studied through the finite-difference time-domain method. The novel nanostructure exhibits three obvious optical transmission bands due to the excitation of subradiant plasmon modes, superradiant plasmon modes, and Fano resonances. Surface plasmon polaritons of the underlying Ag film also play a significant role on these three optical transmission bands via coupling with localized surface plasmons of nanodisk pairs. Moreover, the nanostructure also exhibits a good tunability of optical response by modifying the sizes of cylinders, the thickness of underlying metal film, and the dielectric constant of middle layer. These results demonstrate the nanostructure with great advantages in optical sensors and filters.