Khai Q. Le

Broadband absorbers and selective emitters based on plasmonic Brewster metasurfaces

(Received 20 November 2012; revised manuscript received 17 April 2013; published 10 May 2013)We discuss the possibility of realizing utlrabroadband omnidirectional absorbers and angularly selectivecoherent thermal emitters based on properly patterned plasmonic metastructures. Instead of relying on resonantconcentrationeffectsthatinherentlylimitthebandwidth,webaseourdesignonthecombinationoftwoinherentlynonresonant effects: plasmonic Brewster funneling and adiabatic plasmonic focusing. Using this approach, wepropose compact, broadband absorption and emission spanning terahertz, infrared, and optical frequencies, idealfor various energy and defense applications.DOI: 10.1103/PhysRevB.87.205112 PACS number(s): 78

Broadband Brewster transmission through 2D metallic gratings

Recently, we have introduced a mechanism to achieve ultrabroadband light funnelling and total transmission through 1D narrow metallic gratings at a specific incidence angle, the so-called plasmonic Brewster angle. This phenomenon is based on impedance matching between the guided modes supported by ultranarrow linear slits and transverse-magnetic waves at oblique incidence. In this paper, we demonstrate that such phenomenon, representing the equivalent of Brewster transmission for plasmonic screens, can also occur in 2D metallic gratings of various structural forms and shapes, and that it may be made insensitive to the azimuthal, or polarization, angle φ. This finding may have relevant implications to realize large funneling, absorption and squeezing of light in perforated metallic screens.

Broadband light trapping in thin organic photovoltaic cells using plasmonic resonant antennas

A numerical investigation of plasmonic resonant antennas influence on optical absorption efficiency of organic photovoltaic (PV) cells with thin active layers is reported. Integrating cylindrical Ag antennas on PV cells can enhance absorption efficiency by up to 22%, over a broad bandwidth of operation. This corresponds to an increase in integrated AM1.5G absorption at normal incidence from 48 to 58.5%. Broadband enhancement is observed for transverse-electric and transverse-magnetic polarized incident light, over a wide angle range. It is attributed to plasmonic antennas converting incident sunlight to surface plasmon resonant modes, localized around the antenna/polymer active layer interface. This results in a strong field enhancement in the active layer and enhances absorption efficiency.

Enhanced plasmonic Brewster transmission through metascreens by tapered slits

The recently introduced plasmonic Brewster transmission through free-standing perforated metallic screens (metascreens), which offers ultrabroadband total light transmission has been demonstrated both theoretically and experimentally. This anomalous phenomenon is attributed to impedance matching between the guided modes supported by ultranarrow straight slits and transverse-magnetic impinging waves at a specific oblique incidence. However, this impedance matching mechanism is significantly influenced by the presence of realistic substrates leading to reduce the plasmonic Brewster transmission. To circumvent this substrate influence, the author proposes to carve periodically tapered slits on metascreens to enable the impedance matching at the input and output surface and thus enhance transmission at the Brewster angle. This finding is applied to realize ultrathin perfect absorbers with a broad bandwidth of operation.

Nanostructured Metal–Insulator–Metal Metamaterials for Refractive Index Biosensing Applications: Design, Fabrication, and Characterization

Nanofabrication of nanostructured metal-insulator-metal metamaterial (NMIM2) by the electron beam lithography and the liftoff technique is reported in this paper. The NMIM2 consists of periodic arrays of gold (Au) nanotriangles deposited on a gold layer (acting as a mirror) separated by a thin dielectric insulator. Such an NMIM2 stack was fabricated on a silicon wafer. Reflection of the NMIM2 was measured by the Fourier transform infrared spectroscope. The experimental measurement for reflection coefficients are in good agreement with the full-wave finite-different time-domain simulation results. At resonance, we numerically observed a strong field localized inside the gap between the Au nanostructures and the Au mirror, resulting in a strong confinement of incoming light within the insulator spacer and, thus, a pronounced absorption at telecommunication wavelengths. This resonant spectral response is investigated for sensitive label-free refractive index biosensing applications.

Asymmetrically engineered metallic nanodisk clusters for plasmonic Fano resonance generation

In this paper, we numerically introduce a planar metamolecule that generates plasmonic Fano resonance. The engineered molecule consists of closely packed asymmetric gold nanodisks deposited on a glass substrate operating at visible and near-infrared wavelengths. The asymmetric arrangement of nanodisks plays a key role in Fano resonance generation. The induced extinction cross-section spectroscopy has a Fano-like shape owing to interference between bright and dark plasmonic modes sustained by the asymmetric nanodisk clusters. The Fano dips are shown to be highly sensitive to the interdisk gaps as well as to the surrounding environment. As a result, we introduce a potential refractive index nanosensor having a sensitivity of 660 nm/RIU and a figure of merit of 4.75. The proposed metamolecule holds potential for various applications, such as Fano-induced enhancement of solar energy harvesting, molecular fluorescence, and photo upconversion.

Fabrication and Numerical Characterization of Infrared Metamaterial Absorbers for Refractometric Biosensors

We report the successful fabrication of infrared plasmonic metamaterial absorbers by electron beam lithography and the lift-off technique. The absorber consists of periodic arrays of gold (Au) nanostructures deposited on a stack of thin silica spacer and gold film (acting as a mirror) on a silicon wafer. At resonance, we numerically observed a strong field enhancement between the metallic nanostructures and the Au film, resulting in a strong confinement of incident light within the silica spacer and thus a high absorption of up to 80% at infrared wavelengths. Our experimental measurement for reflection coefficients are in excellent agreement with the full-wave simulation results. We show that the resonant absorption spectral response can be used for highly-sensitive, label-free refractive-index biosensors. By tailoring various forms of nanostructures, we investigate their refractive index sensitivities to identity the most sensitive sensor at specific infrared wavelengths.

Broadband luminescence of Cu nanoparticles fabricated in SiO2 by ion implantation.

In this study, we investigate optical properties of metal nanoparticle crystals fabricated by implanting copper (Cu) ions into single silica (SiO2) crystals with 400keV at various ion doses. The Cu implanted SiO2 (SiO2:Cu) crystal produces a broadband luminescence emission, ranging from blue to yellow, and having a blue luminescence peak at 546nm. Such anomalous luminescence emission bands suggest that the ion implantation may give rise to aggregation of Cu nanoparticles in the host matrix. The boundary element method-based modelling of a given Cu nanoparticle aggregation was employed to justify the broadband luminescence emission. Formation of Cu nanoparticles in SiO2 is predicted through their optical absorption data. The experimental results are compared with results of Mie calculations and we observe that the higher ion dose produces the larger particle size.

Broadband THz radiation through tapered semiconductor gratings on high-index substrate

We numerically demonstrate an extraordinary optical transmission (EOT) through semiconductor gratings with plasmonic properties on a high-index substrate over a broad terahertz (THz) bandwidth at the plasmonic Brewster channel. The THz EOT is due to impedance matching, an inherently non-resonant mechanism, at the grating entrance and exit faces, which are periodically carved with tapered slits. The optimal grating geometry provides a transmission of over 85% over a broad 0.2–1.0 THz bandwidth at a Brewster angle of incidence of 75°. In addition, we introduce a perfect THz absorber with low-loss and metal-free properties over a broad operating bandwidth based on the plasmonic Brewster transmission concept.

Fano-Induced Circular Dichroism in Three-Dimensional Plasmonic Chiral Metamolecules

This paper introduces a three-dimensional (3-D) artificial chiral nanostructure, which has variant total optical loss spectroscopies under left- and right-handed circularly polarized (LCP and RCP) incident light. The resulting circular dichroism is induced by Fano resonance generated by the engineered chiral metamolecule, which consists of asymmetrically arranged gold (Au) nanoparticles in three dimensions. The Fano resonance generation is a consequence of modal interference between bright and dark plasmonic modes of asymmetric constituent dimers of the metamolecule.