Pingping Pan

Automatically acquired broadband plasmonic-metamaterial black absorber during the metallic film-formation.

Broadband electromagnetic wave absorbers are highly desirable in numerous applications such as solar-energy harvesting, thermo-photovoltaics, and photon detection. The aim to efficiently achieve ultrathin broadband absorbers with high-yield and low-cost fabrication process has long been pursued. Here, we theoretically propose and experimentally demonstrate a unique broadband plasmonic-metamaterial absorber by utilizing a sub-10 nm meta-surface film structure to replace the precisely designed metamaterial crystal in the common metal-dielectric-metal absorbers. The unique ultrathin meta-surface can be automatically obtained during the metal film formation process. Spectral bandwidth with absorbance above 80% is up to 396 nm, where the full absorption width at half-maximum is about 92%. The average value of absorbance across the whole spectral range of 370-880 nm reaches 83%. These super absorption properties can be attributed to the particle plasmon resonances and plasmon near-field coupling by the automatically formed metallic nanoparticles as well as the plasmon polaritons of the metal film with the induced plasmonic magnetic resonances occurring between the top meta-surface and the bottom metal mirror. This method is quite simple, cost-effective for large-area fabrication, and compatible with current industrial methods for microelectro-mechanical systems, which makes it an outstanding candidate for advanced high-efficiency absorber materials.

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.

Improving Plasmon Sensing Performance by Exploiting the Spatially Confined Field

A universal method for improving sensing performance has been computationally studied based on a common triple-layer metal-dielectric-metal (MDM) plasmonic perfect absorber (PA). Calculation results show that the originally idled resonant optical field in the middle dielectric spacer can be exploited to enhance the sensing capability via etching the dielectric spacer to open a channel for analyte. By comparing with the sensitivity (S) of the common PA-based sensor, an enhancement factor up to 5.0 can be achieved for an etched PA-based sensor. Moreover, in order to maintain the mechanical stability of the structure, a modified PA-based sensor platform with a solid support for the suspended plasmonic disks array was further employed to study the sensing properties. In comparison with the referenced sensor without suspension technique, all the three sensing factors of the S, the figure of merit (FOM), and the spectral intensity difference related figure of merit (FOM*) are noticeably improved with the enhancement factors up to 2.5, 3, and 57, respectively. In addition, since there is no special dealing with the structure except the need of hollowing the dielectric spacer to open the sensing channel for the analysis, the predicted method profiles itself as a simple and universal strategy to improve the sensing performance of a wide variety of nanoplasmonic sensors.

III–V semiconductor resonators: A new strategy for broadband light perfect absorbers

Broadband light perfect absorbers (BPAs) are desirable for applications in numerous optoelectronics devices. In this work, a semiconductor-based broadband light perfect absorber (S-BPA) has been numerically demonstrated by utilizing plasmonlike resonances of high-index semiconductor resonators. A maximal absorption of 99.7% is observed in the near-infrared region. By taking the absorption above 80% into account, the spectral bandwidth reaches 340 nm. The absorption properties mainly originate from the optical cavity modes induced by the cylinder resonators and ultrathin semiconductor film. These optical properties and simple structural features can maintain the absorber platform with wide applications in semiconductor optoelectronics.

High-index dielectric meta-materials for near-perfect broadband reflectors

All-dielectric meta-materials offer a potential alternative to plasmonic meta-materials at optical frequencies. Herein, we take advantage of loss-less as well as simple unit cell geometry to demonstrate near-perfect broadband reflectors made from all-dielectric materials. These near-perfect reflectors, consisting of high-index cross-shaped resonators (n = 3.5, Si), operating in the telecommunications bands, exhibit novel optical properties including polarization-independent, wide-angle near-unity reflection. The average reflectance is exceeding 98% at the wavelength range from 1.261 μm to 1.533 μm. At 1.310 μm, the average reflectance (R) reaches 99.7%, surpassing the reflectance of metallic mirrors. A near-perfect super-broadband reflection spectrum with bandwidth of 0.330 μm (R > 98%) is achieved for a system with a higher index dielectric resonator array (n = 4.0, Ge). Moreover, the optical properties of the reflector provide high scalability across the wavelength range via tuning of dielectric resonators. The whole structure, with common triple-layer features, can be mass-produced using standard lithography methods and deposition techniques. These optical and structural features make the proposed near-perfect broadband reflectors feasible avenues for manipulating light in important applications in spectroscopy, photovoltaics and light emission.

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.

All-dielectric resonant cavity-enabled metals with broadband optical transparency

Metal films with broadband optical transparency are desirable in many optoelectronic devices, such as displays, smart windows, light-emitting diodes and infrared detectors. As bare metal is opaque to light, this issue of transparency attracts great scientific interest. In this work, we proposed and demonstrated a feasible and universal approach for achieving broadband optical transparent (BOT) metals by utilizing all-dielectric resonant cavities. Resonant dielectrics provide optical cavity modes and couple strongly with the surface plasmons of the metal film, and therefore produce a broadband near-unity optical transparent window. The relative enhancement factor (EF) of light transmission exceeds 3400% in comparison with that of pure metal film. Moreover, the transparent metal motif can be realized by other common metals including gold (Au), silver (Ag) and copper (Cu). These optical features together with the fully retained electric and mechanical properties of a natural metal suggest that it will have wide applications in optoelectronic devices.

Intensity distribution and spectral degree of coherence for array beams in non-Kolmogorov atmospheric turbulence

The propagation expressions of the intensity distribution and the spectral degree of coherence for radial array Gaussian Schell-model beams in non-Kolmogorov atmospheric turbulence have been derived. There exists a maximum of the on-axis intensity for radial array beams on propagation. The optimal propagation distance where the maximum appears has been analyzed in detail. The influence of the generalized structure constant and the initial spatial coherence parameter for the correlated superposition beam is different from the uncorrelated one. In particular, the on-axis intensity maximum exhibits a minimum when the generalized exponent α = 3.1. The spectral degree of coherence for the correlated superposition beam is larger than the uncorrelated one, and the difference between the two superposition styles becomes larger when the spatial coherence parameter becomes larger and the turbulence becomes weaker. Moreover, when α = 3.1 the spectral degree of coherence exhibits a minimum.