Refractive index sensing with Fano resonances in silicon oligomers

Dark-field microscopy studies of single silicon nanoparticles fabricated by e-beam evaporation technique: effect of thermal annealing, polarization of light and deposition parameters

Herein, we report the dark-field microscopy studies on single silicon nanoparticles (SiNPs) fabricated using different deposition parameters in the electron beam evaporation technique. The morphology of the fabricated SiNPs is studied using theAtomic Force Microscope. Later, for the first time, the effect of thermal annealing and deposition parameters (i.e. beam current and deposition time) on the far-field scattering images and spectra of single SiNPs is studied using a transmission-mode dark-field optical microscope to estimate the wavelength locations and full-width at half maxima of the optical resonances of single SiNPs. Finally, the role of polarization of incident light on the optical resonances of single SiNPs is also studied by recording their scattering images and spectra.

Design of highly perceptible dual-resonance all-dielectric metasurface colorimetric sensor via deep neural networks

Colorimetric sensing, which provides effective detection of bio-molecular signals with one's naked eye, is an exceptionally promising sensing technique in that it enables convenient detection and simplification of entire sensing system. Though colorimetric sensors based on all-dielectric nanostructures have potential to exhibit distinct color variations enabling manageable detection due to their trivial intrinsic loss, there is crucial limitation that the sensitivity to environmental changes lags behind their plasmonic counterparts because of relatively small region of near field-analyte interaction of the dielectric Mie-type resonator. To overcome this challenge, we proposed all-dielectric metasurface colorimetric sensor which exhibits dual-resonance in the visible region. Thereafter, we confirmed with simulation that, in the elaborately designed dual-Lorentzian-type spectra, highly perceptible variations of structural color were manifested even in minute change of peripheral refractive index. In addition to verifying physical effectiveness of the superior colorimetric sensing performance appearing in the dual-resonance type sensor, by combining advanced optimization technique utilizing deep neural networks, we attempted to maximize sensing performance while obtaining dramatic improvement of design efficiency. Through well-trained deep neural network that accurately simulates the input target spectrum, we numerically verified that designed colorimetric sensor shows a remarkable sensing resolution distinguishable up to change of refractive index of 0.0086.

Advances and applications of nanophotonic biosensors

Nanophotonic devices, which control light in subwavelength volumes and enhance light-matter interactions, have opened up exciting prospects for biosensing. Numerous nanophotonic biosensors have emerged to address the limitations of the current bioanalytical methods in terms of sensitivity, throughput, ease-of-use and miniaturization. In this Review, we provide an overview of the recent developments of label-free nanophotonic biosensors using evanescent-field-based sensing with plasmon resonances in metals and Mie resonances in dielectrics. We highlight the prospects of achieving an improved sensor performance and added functionalities by leveraging nanostructures and on-chip and optoelectronic integration, as well as microfluidics, biochemistry and data science toolkits. We also discuss open challenges in nanophotonic biosensing, such as reducing the overall cost and handling of complex biological samples, and provide an outlook for future opportunities to improve these technologies and thereby increase their impact in terms of improving health and safety.

On the performance of a tunable grating-based high sensitivity unidirectional plasmonic sensor

Optical biosensing is currently an intensively active research area, with an increasing demand of highly selective, sensitivity-enhanced and low-cost devices where different plasmonic approaches have been developed. In this work we propose a tunable optimized grating-based gold metasurface that can act both as a high sensitivity sensor device (up to 1500 nm/RIU) and as an unidirectional plasmon source. The theory behind surface plasmon polariton generation is recalled to thoroughly understand the influence that every parameter of the grating source has on the performance of the proposed device. The results and conclusions discussed here offer a key step toward the design of biosensors based on excitation of surface plasmons polaritons by grating-based structures or in the process of creating new nanophotonic circuit devices.

Dual polarized engineering the extinction cross-section of a dielectric wire using graphene-based oligomers

In this paper, graphene-coated spherical nanoparticles are arranged around an infinite length dielectric cylinder to enhance its extinction cross-section. Initially, a single longitudinal one-dimensional periodic array is considered in different loci concerning the transverse electric (TE) incident plane wave. It is observed that regardless of the position of the particles, the extinction cross-section of the dielectric cylinder is considerably enhanced with respect to the bare one. Later, by increasing the number of longitudinal plasmonic arrays around the cylinder, each residing in a different azimuthal direction, the extinction cross-section is further manipulated to observe double pronounced Fano resonances. The origin of the Fano resonances is described by considering their planar counterparts constructed by the periodic assembly of plasmonic oligomers. Finally, the hexamer configuration is considered as the prototype, and the effect of various optical, geometrical, and material parameters on the optical response is investigated in detail. Interestingly, due to the spherical symmetry of the cells, the extinction cross-section is also enhanced for the transverse magnetic (TM) incident wave, which is unattainable using a continuous plasmonic cover made of metal or graphene. The potential application of our proposed structure is in the design of reconfigurable conformal optical absorbers and sensors.

Effective medium theory to the description of plasmonic resonances: Role of Au and Ti nanoparticles embedded in MoO3 thin films

The growing interest in functional transition metal oxides for efficient energy consumption or in the bio-sensing process; indicates that is necessary to develop a new theoretical method that describes experiments. This article presents a new theoretical methodology to characterize molybdenum trioxide (MoO₃) thin films doped with resonant gold – nanoparticles (Au – NPs) and non-resonant titanium – nanoparticles (Ti – NPs). The modulation of surface plasmon resonance (SPR) and the implications in the MoO₃ transmittance spectrum is described by applying an effective medium theory. The transmittance modulation was modified by variating three parameters, the radius of the NPs, the concentration of the NPs as well as the variation of the MoO₃ thin films thickness. It was found that the nanoparticles concentration is the most important parameter in the transmittance modulation. Additionally, the orthorhombic and monoclinic structure of MoO₃ was studied, from which it was obtained that the monoclinic structure of the MoO₃ doped with Au – NPs favors the reduction in the transmittance values in the visible region which is associated with the increase of the SPR signal. Similar analyses are performed for non-resonant nanoparticles such as Ti, where it was found that optical modulation is not as marked as the case of gold nanoparticles.

Reversible Image Contrast Manipulation with Thermally Tunable Dielectric Metasurfaces

Increasing demand for higher resolution of miniaturized displays requires techniques achieving high contrast tunability of the images. Employing metasurfaces for image contrast manipulation is a new and rapidly growing field of research aiming to address this need. Here, a new technique to achieve image tuning in a reversible fashion is demonstrated by dielectric metasurfaces composed of subwavelength resonators. It is demonstrated that by controlling the temperature of a metasurface the encoded transmission pattern can be tuned. To this end, two sets of nanoresonators composed of nonconcentric silicon disks with a hole that exhibit spectrally sharp Fano resonances and forming a Yin-Yang pattern are designed and fabricated. Through exploitation of the thermo-optical properties of silicon, full control of the contrast of the Yin-Yang image is demonstrated by altering the metasurface temperature by ΔT ≈ 100 °C. This is the first demonstrated technique to control an image contrast by temperature. Importantly, the turning technique does not require manipulating the external stimulus, such as polarization or angle of the illumination and/or the refractive index of this environment. These results open many opportunities for transparent displays, optical switches, and tunable illumination systems.

Optical Coherent Reflection from a Confined Colloidal Film: Modeling and Experiment

We study the optical reflectivity of confined colloidal films as a function of the angle of incidence in an internal reflection configuration. Two effective medium models and an extended coherent-scattering model for thin colloidal films are compared against experimental measurements with gold, latex, and titanium dioxide colloids. A derivation of the coherent scattering model for confined colloidal films used in this work is presented in a comprehensive way. The model lies within the framework of the multiple-scattering theory and is valid for any angle of incidence and for colloids of small or large particles compared to the wavelength of light, however, only for small and moderately small particles' volume fraction. Reflectivity versus angle of incidence curves for an opaque colloidal film in an internal reflection configuration show the effects of two critical angles. Within the two critical angles, there is a high sensitivity to the presence of colloidal particles, while the volume of colloidal samples needed is in the microliter range. Upon comparing theory with experiment, no model fitting was done in any case. The experimental setup and its calibration procedure are discussed. The results provide physical insight into applications involving optical properties of colloidal systems.

Tunable compact nanosensor based on Fano resonance in a plasmonic waveguide system

An ultracompact plasmonic refractive index sensor based on Fano resonance is proposed. The sensor comprises a metal-insulator-metal waveguide with a stub and a side-coupled split-ring resonator. The effect of structural parameters on Fano resonance and the refractive index sensitivity of the system are analyzed in detail by investigating the transmission spectrum. Simulation results show that Fano resonance has different dependences on the parameters of the sensor structure. The reason is further discussed based on the field pattern. The peak wavelength and lineshape can be easily tuned by changing the key parameters. Furthermore, dual Fano resonance effects with different frequency intervals are obtained, which are mainly induced by the symmetry breaking of the structure. The proposed sensor yields sensitivity higher than 1.4×10³  nm/RIU and a figure of merit of 1.2×10⁵. The sensitivity and figure of merit can be further improved by optimizing the geometry parameters.