Study of optical phase transduction on localized surface plasmon resonance for ultrasensitive detection

Detection of the Chilli Leaf Curl Virus Using an Attenuated Total Reflection-Mediated Localized Surface-Plasmon-Resonance-Based Optical Platform

The development of a nanoparticle-based optical platform has been presented as a biosensor for detecting target-specific plant virus DNA. The binding dynamics of gold nanoparticles has been studied on the amine-functionalized surface by the attenuated total reflection (ATR)-based evanescent wave absorption method monitoring the localized surface plasmon resonance (LSPR). The developed surface was established as a refractive index sensor by monitoring the LSPR absorption peak of gold nanoparticles. This nanoparticle-immobilized surface was explored to establish as a biosensing platform with target-specific immunoglobulin (IgG) antibody-antigen interaction. The IgG concentration-dependent variation of absorbance was correlated with the refractive index change. After successfully establishing this ATR configuration as an LSPR-based biosensor, the single-stranded DNA of the chilli leaf curl virus was detected using its complementary DNA sequence as a receptor. The limit of detection of this sensor was determined to be 1.0 μg/mL for this target viral DNA. This ATR absorption technique has enormous potential as an LSPR based nano-biosensor for the detection of other begomoviruses.

A High-Sensitivity SPR Sensor with Bimetal/Silicon/Two-Dimensional Material Structure: A Theoretical Analysis

In this paper, we reported a theoretical study of a novel Surface plasmon resonance (SPR) biosensor composed of BK7 prism, gold (Au)/silver (Ag) bimetallic layer, silicon and two-dimensional (2D) materials. The bimetallic layer combines the advantages of Au and Ag and the high refractive index silicon layer enhances the electric field on the surface of the sensor, so that the sensor has a better overall performance in terms of sensitivity and figure of merit (FOM). Compared with ordinary dielectrics, 2D materials have excellent photoelectric properties, such as larger specific surface area, higher carrier density and stronger adsorption capacity, which improve the detection ability of the sensor. The sensitivity of the optimized sensor achieves 297.2°/RIU, 274°/RIU and 246°/RIU when the silicon layer is covered with graphene, MXene (Ti3T2Cx) and MoS2, respectively. Compared with the traditional SPR sensor, the sensitivity of the structure has been significantly improved, and its excellent performance has broad application prospects in biosensing and other fields.

Experimental demonstration of broadband light trapping by exciting surface modes of an all-dielectric taper

We design an all-dielectric taper and then excite its surface modes by illuminating a plane wave upon the taper to achieve broadband light trapping spanning from 20 to 100 GHz. Via Lewin’s theory, such excitation of surface modes could be analogous to “trapped rainbow”, i.e., activation of negative Goos-Hänchen effect within a negative refractive waveguide. To further reinforce this statement, the corresponding power flow distributions within the all-dielectric taper are recorded in finite-integration time domain simulation and suggest that a chromatic incident pulse is indeed trapped at different critical thicknesses of the taper, a character of the negative refractive waveguide. Finally, the transmittance is measured and compared to the simulated ones. The two followed the similar trend.

Demonstration of an ultrasensitive refractive-index plasmonic sensor by enabling its quadrupole resonance in phase interrogation

We present an ultrasensitive plasmonic sensing system by introducing a nanostructured X-shaped plasmonic sensor (XPS) and measuring its localized optical properties in phase interrogation. Our tailored XPS exhibits two major resonant modes of a low-order dipole and a high-order quadrupole, between which the quadrupole resonance allows an ultrahigh sensitivity, due to its higher quality factor. Furthermore, we design an in-house common-path phase-interrogation system, in contrast to conventional wavelength-interrogation methods, to achieve greater sensing capability. The experimental measurement shows that the sensing resolution of the XPS reaches 1.15×10(-6) RIU, not only two orders of magnitude greater than the result of the controlled extinction measurement (i.e., 9.90×10(-5) RIU), but also superior than current reported plasmonic sensors.

Plasmonic phase transition and phase retardation: essential optical characteristics of localized surface plasmon resonance

Phase transition that occurs around the spectral position of localized surface plasmon resonance (LSPR) has various applications for light manipulation and refractive index sensing. Previous studies focused on phase responses of specific plasmonic structures, whereas the general theoretical analysis remains inadequate. In this study, we analytically modeled the phase spectra and the intensity spectra of silver nanodots with temporal coupled-mode theory. The phase transition occurs at the transmission dip, whereas the phase of reflection varies much more gradually. We further derived the equation for the slope of the phase at the transmission dip, which is a function of the rates of Ohmic dissipation and emission. The theoretical analysis is also applicable for wide varieties of LSPR systems and provides an intuitive physical mechanism for phase properties. Then, based on the fundamental discussion, we further investigated plasmonic phase retardation in anisotropic nanodots for the application of boosting the figure of merit (FOM) of refractive index sensing. The anisotropic nanodots induce plasmonic phase transitions, which spectrally split, for transmission waves polarized along the symmetric axes. Thus, anisotropy induces relative phase retardation in the narrow spectral region between the wavelengths of the LSPRs. We numerically manipulated the full width at half maximum of the ellipsometric spectra by adjusting the aspect ratio of the nanodots and observed an FOM of 24.3. In addition, experiments were performed to demonstrate the feasibility of this arrangement.