Development of the frequency-dependent three-dimensional LOD-FDTD method and its application to the analysis of plasmonic devices

研究成果の概要 (和文) : 3次元の微小プラズモンデバイスを極めて高速に解析できる、局所的一次元(LOD)法に基づく陰的なFDTD法を開発した。畳み込みの計算が一度で済み高い精度の得られる台形則に基づく手法、さらに、Fundamental法と呼ばれる技法の導入により、計算式の右辺に微分項を含まない極めて簡便な定式化を行った。プラズモニックギャップ導波路をPCで解析したところ、陽的なFDTD法で85分かかった計算がほぼ同等の計算精度を維持しつつ37分に低減された。開発した手法を用いて、種々の3次元プラズモンデバイスを解析した。デバイス特性を2次元構造デバイスと比較しながら議論し、3次元解析の重要性を示した。研究成果の概要 (英文) : An implicit FDTD method based on the locally one-dimensional (LOD) scheme has been developed for the efficient analysis of three-dimensional (3-D) plasmonic devices. The trapezoidal recursive convolution technique has been adopted, in which a single convolution integral is required. In addition, a fundamental scheme has been introduced for efficient formulation of the algorithm, in which no spatial derivative exists in the right-hand side of the equations. The developed LOD-FDTD offers a reduced computation time from 85 min with the conventional explicit FDTD to 37 min for the analysis of a plasmonic gap waveguide. The characteristics of several 3-D plasmonic devices have been investigated using the LOD-FDTD. The 3-D results have been compared with those of 2-D models, showing the necessity of the 3-D analysis for an accurate evaluation of the plasmonic devices

Development of polarization conversion and control devices based on periodic and waveguide structures

研究成果の概要 (和文) : 導波路型および周期構造型の偏波変換・制御デバイスを解析し得る、効率のよい数値解法として、陽解法と陰解法に基づく有限差分時間領域法を金属膜のような分散性媒質を含む際にも取り扱えるように拡張した。構築した解法に基づき、周期構造型偏波変換器の動作メカニズムを明らかにした。また、導波路型偏波変換器を広帯域に動作させる手法を考案した。結果として、波長1.3μmから1.65μmの広帯域に渡って、15dB以上の消光比、0.5dB以下の挿入損を実現した。研究成果の概要 (英文) : To analyze polarization conversion and control devices based on periodic and waveguide structures, we develop explicit and implicit finite-difference time-domain methods for dispersion materials. The mechanism for splitting two polarizations has been explained using the surface plasmon effect. For the waveguide-type converter, the polarization conversion is obtained with an extinction ratio of more than 15dB and an insertion loss of less than 0.5dB over a wide wavelength range of1.3 to 1.65 μm

Plasmonics and its Applications

Plasmonics is a rapidly developing field that combines fundamental research and applications ranging from areas such as physics to engineering, chemistry, biology, medicine, food sciences, and the environmental sciences. Plasmonics appeared in the 1950s with the discovery of surface plasmon polaritons. Plasmonics then went through a novel propulsion in the mid-1970s, when surface-enhanced Raman scattering was discovered. Nevertheless, it is in this last decade that a very significant explosion of plasmonics and its applications has occurred. Thus, this book provides a snapshot of the current advances in these various areas of plasmonics and its applications, such as engineering, sensing, surface-enhanced fluorescence, catalysis, and photovoltaic devices

Plasmonic NanoHole Arrays for Label-free Biosensors

The present work focuses on plasmonic properties of nanostructures to be used as optical biosensors. A biosensor is a device able to detect and recognize a specific biological substance, called analyte. The sensitive component of a biosensor is the receptor, which binds to a specific analyte and the interaction is then translated into a measurable and quantifiable signal by another component called transducer. This latter, in this work, is a NanoHole Array (NHA), which has been designed, synthesized, characterized, functionalized and tested. A NHA is a thin metallic film (of noble metals, here Au) of thickness 50-100 nm, patterned with a periodic array of holes (here the hole diameter is around 320 nm), in this case, an hexagonal array of circular holes. The physical property exploited in NHAs is the Surface Plasmon Resonance (SPR), resulting from the coupling of an electromagnetic (EM) field (UV-VIS-NIR) with surface conduction electrons of the metallic nanostructure. In particular, NHAs take advantage of Surface Plasmon Polaritons (SPPs), which are EM waves travelling at the interface between a metal and a dielectric. Moreover, at the resonance, this device exhibit a peculiar optical property called Extraordinary Optical Transmission, in which the light transmitted by the NHA is more than that of a single hole, whose area corresponds to the sum of the nanoholes area. The Fano-like nature of the EOT phenomenon has been investigated. Therefore, NHA transmittance spectrum consists in a sharp band, whose peak position depends on geometrical parameters of the structures (period, radius, thickness) and on the surrounding dielectric environment. Thus, a change of the dielectric environment and hence of the refractive index leads to a change in the resonance condition and hence in a red-shift of the EOT peak, on which the biosensing transduction mechanism is based. Bulk and local sensitivities of the NHA have been experimentally measured and compared to the simulated results (obtained with FEM simulations), with good agreement: Sbulk = 281 nm/RIU (exp.) vs 290 nm/RIU (exp), Slocal = (2.9 +/- 0.1) RIU-1 vs 3.2 RIU-1. Finally, biosensing tests have been performed following a suitable functionalization protocol. A thiols layer is employed as ligand between the gold surface and the receptor, namely, Biotin. Incubations with different analyte concentrations (Streptavidin, between 10-10 M and 10-6 M ) have been performed, obtaining the sensing curve (which follows a Langmuir isotherm) and the corresponding Limit of Detection (LoD) of 4.7·10-8 M