Spectral Position Of Resonances Research Articles

Dual-band polarization-insensitive bound states in the continuum in a permittivity-asymmetric membrane metasurface

It is generally accepted that the symmetry-protected quasi-bound states in the continuum (qBICs) are induced by breaking geometry symmetry in the regular quasi-planer nanostructure. However, it is very challenging to allow precise control of the asymmetry parameter in the process of fabrication and realize a tiny tailoring geometry symmetry. Here, a novel permittivity-asymmetric membrane metasurface, which can also induce polarization-insensitive qBICs by a symmetry breaking in the permittivity of the comprising host and guest medium is proposed and investigated. Simulation results show the line width and spectral position of resonances can be controlled by increasing or decreasing the refractive index of the guest medium. The far-field multiple decompositions and the near-field distributions reveal the physical mechanism of dual qBIC modes, which are dominated by the magnetic dipole and toroidal dipole and display opposite optical properties. Furthermore, it is found that such BICs are robust against the shape of the guest medium. Eventually, the sensing performance of the proposed structure is evaluated. This study provides a simple and promising structure for the excitation of qBICs and conceives a feasible approach to achieving a high Q factor.

Various Types of Light Guides for Use in Lossy Mode Resonance-Based Sensors

A comparative study of figure-of-merit fiber sensors of the mass concentration of NaCl solutions based on single-mode and multi-mode fibers was carried out. Lossy mode resonance is realized on chemically thinned sections of optical fibers to various diameters (from 26 to 100 μm) coated with ZnTe. Thin-film coatings were applied using the method of metalorganic chemical vapor deposition (MOCVD). Samples of single-mode and multi-mode fiber sensors were created in such a way that the depth and spectral position of resonances in aqueous NaCl solutions coincided. Sensors implemented on a single-mode fiber have a higher sensitivity (5930 nm/refractive index unit (RIU)) compared to those on a multi-mode fiber (4860 nm/RIU) and a smaller half-width of the resonance in the transmission spectrum. According to the results of experiments, figure-of-merit sensors are in the range of refractive indices of 1.33–1.35 for: multi-mode fiber—25 RIU−1, single-mode fiber—75 RIU−1. The sensitivity of the resulting sensors depends on the surface roughness of the ZnTe coating. The roughness of films synthesized on a single-mode fiber is four times higher than this parameter for a coating on a multi-mode fiber. For the first time, in the transmission spectrum during the synthesis of a thin-film coating on a multi-mode fiber, the possibility of separating the first nine orders of resonances into electric and magnetic transverse components has been demonstrated. The characteristics of sensors with the operating wavelength range in the visible (500–750 nm) and infrared (1350–1550 nm) regions of the spectrum are compared. The characteristics of multi-mode lossy mode resonance sensors are demonstrated, which make them more promising for use in applied devices than for laboratory research.

Structural color filters based on an all-dielectric metasurface exploiting silicon-rich silicon nitride nanodisks.

An all-dielectric metasurface is deemed to serve a potential platform to demonstrate spectral filters. Silicon-rich silicon nitride (SRN), which contains a relatively large portion of silicon, can exhibit higher refractive indices, when compared to silicon nitride. Meanwhile, the extinction coefficient of SRN is smaller than that of hydrogenated amorphous silicon, leading to reduced absorption loss in the shorter wavelength. SRN is therefore recommended as a scattering element from the perspective of realizing all-dielectric metasurfaces. In this work, we propose and embody a suite of highly efficient structural color filters, capitalizing on a dielectric metasurface that consists of a two-dimensional array of SRN nanodisks that are embedded in a polymeric layer. The SRN nanodisks may support the electric dipole (ED) and magnetic dipole (MD) resonances via Mie scattering, thereby leading to appropriate spectral filtering characteristics. The ED and MD are identified from field profile observation with the assistance of finite-difference time-domain simulations. The manufactured color filters are observed to produce various colors in both transmission and reflection modes throughout the visible band, giving rise to a high transmission of around 90% in the off-resonance region and a reflection ranging up to 60%. A variety of colors can be realized by tuning the resonance by adjusting the structural parameters such as the period, diameter, and height of the SRN nanodisks. The spectral position of resonances might be flexibly tuned by tailoring the polymer surrounding the SRN nanodisks. It is anticipated that the proposed coloring devices will be actively used for color displays, imaging devices, and photorealistic color printing.

Analysis of a log periodic nano-antenna for multi-resonant broadband field enhancement and the Purcell factor

Broadband nano-antennas play a central role in many areas of science and technology. However, a more intuitive understanding for rational design of nano-antennas with broadband response is desirable. A log periodic nano-antenna was studied in the paper. The finite-difference time-domain method was used to explore the spectral characteristics of the log periodic nano-antenna by the excitation mode of reception and emission. The effects of geometry on field enhancement and the Purcell factor were systematically described and investigated. The field enhancement of the nano-antenna can be tuned by geometric parameters such as the outer radius, the tooth angle, and the ratio of the radial sizes of successive teeth, which provide control over both the spectral resonance position and the field enhancement peak amplitude. The Purcell factor mainly depends on the outer radius, the tooth angle, and the bow angle. In addition, multi-resonant field enhancement was analyzed in detail by conformal transformation. Furthermore, a careful comparison of the characteristics of a bowtie nano-antenna demonstrated that the log periodic nano-antenna has considerable potential for multi-resonant field enhancement and improvement of the Purcell factor. The results provide a promising prospect for designing and optimizing the log periodic nano-antenna in a broad range of wavelengths.

The spectral shift between near- and far-field resonances of optical nano-antennas.

Within the past several years a tremendous progress regarding optical nano-antennas could be witnessed. It is one purpose of optical nano-antennas to resonantly enhance light-matter interactions at the nanoscale, e.g. the interaction of an external illumination with molecules. In this specific, but in almost all schemes that take advantage of resonantly enhanced electromagnetic fields in the vicinity of nano-antennas, the precise knowledge of the spectral position of resonances is of paramount importance to fully exploit their beneficial effects. Thus far, however, many nano-antennas were only optimized with respect to their far-field characteristics, i.e. in terms of their scattering or extinction cross sections. Although being an emerging feature in many numerical simulations, it was only recently fully appreciated that there exists a subtle but very important difference in the spectral position of resonances in the near-and the far-field. With the purpose to quantify this shift, Zuloaga et al. suggested a Lorentzian model to estimate the resonance shift. Here, we devise on fully analytical grounds a strategy to predict the resonance in the near-field directly from that in the far-field and disclose that the issue is involved and multifaceted, in general. We outline the limitations of our theory if more sophisticated optical nano-antennas are considered where higher order multipolar contributions and higher order antenna resonances become increasingly important. Both aspects are highlighted by numerically studying relevant nano-antennas.

Enhancement of Raman scattering in Subwave plasmonic nanostructures formed by ion-beam lithography

Subwave gratings with optimized architecture corresponding to the spectral position of resonances at desired wavelengths have been fabricated by ion-beam lithography. The structural and optical properties of the nanostructures have been studied. It is shown that experimental transmission (reflection) spectra are in good agreement with the theoretical spectra calculated within the grating-eigenmode model. An analysis of the Raman scattering of molecules adsorbed on the surface of gratings with optimized architecture demonstrates an increase in the Raman signal intensity from these molecules by four orders of magnitude. Highly sensitive sensors and various elements of optoelectronics can be developed based on these nanostructures.

Low-Cost Infrared Resonant Structures for Surface-Enhanced Infrared Absorption Spectroscopy in the Fingerprint Region from 3 to 13 μm

We use low-cost colloidal lithography with micrometer-sized polystyrene spheres to fabricate arrays of triangular gold microstructures on different infrared-transparent substrates while varying the structures’ lateral size. The refractive index n of the substrate in the infrared spectral range can be varied strongly, e.g., from n = 1.4 (calcium fluoride) to n = 4.0 (germanium) for the used materials. Variation of antenna size and substrate material allows us to tune the spectral resonance position of the fabricated antennas from 3 to 13 μm and therefore to cover the absorption bands of the infrared fingerprint region and functional groups. The easy handling and good tunability is demonstrated with surface enhanced infrared absorption (SEIRA) spectroscopy measurements on poly(methyl methacrylate) (PMMA) covered antennas on two different substrate materials (calcium fluoride and silicon) but equal spectral resonance positions of the antennas to ensure the comparability. Additional near-field measurements show that large antennas on a low-index substrate yield stronger local field enhancement compared to smaller antennas on a higher-index substrate, despite the same far-field response.

Coupling of Plasmon Resonances in Tunable Layered Arrays of Gold Nanoparticles

Using bottom-up and self-assembly processes, large scale layered arrays of strongly coupled gold nanoparticles with controllable dimensions were fabricated. By carefully adjusting the distance between adjacent gold nanoparticle arrays, it is possible to control the coupling of the localized surface plasmon polariton resonance as sustained by individual gold nanoparticles. A greater interaction is observed at smaller separations, leading to a well pronounced shift in the spectral position of resonances that can be adjusted with high precision. Simulations showed good agreement with experimental observations in an in-depth investigation of such structures, suggesting minimal separations of only one nanometer are achieved.

High spectral resolution analysis of tunable narrowband resonant grating waveguide structures

A high spectral resolution analysis of narrowband reflection filters based on resonant grating waveguide structures is presented. A tunable high-performance dye laser with ∼ 0.15 cm-1 line width and a beam analyzing system consisting of three simultaneously controlled CCD cameras were used to investigate grating waveguide resonances at wavelengths in the 694 nm and 633 nm ranges. A reflectivity of ∼ 91% and a line width of ∼ 0.55 nm were measured and theoretically modeled for a resonant reflection filter specifically designed for the ruby laser wavelength 694.2 nm. For a second grating waveguide structure, designed for the helium-neon laser emission wavelength 632.8 nm, we observed a thermal shift of its spectral resonance position of several nanometers, when increasing the sample temperature by some degrees. An inverse thermal shift was observed when the structure was subsequently cooled down to room temperature. Our results suggest implementation of grating waveguide devices combining a narrow line width with a tunability of the resonant response into innovative concepts for reflection filter and sensor applications.

Strong Enhancement of the Radiative Decay Rate of Emitters by Single Plasmonic Nanoantennas

We demonstrate a strong, 5-fold enhancement of the radiative decay rate from highly efficient fluorescent dye molecules around resonant optical nanoantennas. The plasmonic modes of individual gold dimer antennas are tuned by the particle length and the antenna gap, providing control over both the spectral resonance position and the near-field mode profile of the nanoantenna. Resonant enhancement of the radiative and nonradiative decay rates of a fluorescent dye is observed, resulting in an increase of the internal quantum efficiency from 40% up to 53% for single antennas, and up to 59% for antenna clusters. This improvement of the already high quantum efficiency of the dye molecules is in agreement with electrodynamic model calculations that predict a maximum attainable efficiency around 80% due to nonradiative losses in the metal.