Laser polarization as a critical factor in the SERS-based molecular sensing performance of nano-gapped Au nanowires.

Nowadays, Au dimer-based nanostructures are exhaustively studied due to their outstanding potential as plasmonic nanoantennas for future applications in high-sensitivity molecular sensing by Surface-Enhanced Raman Spectroscopy (SERS). In this work, we analyze nano-gapped Au nanowires (NWs) or Au-NW dimers for designing efficient nanoantennas, reporting an exhaustive study about dimer length and laser polarization orientation effects on their SERS-based molecular sensing performance. Arrays of nanoantennas with gaps of about 22 ± 4 nm, nominal square cross-sections of 60 nm × 60 nm, and different segment lengths from 300 nm up to 1200 nm were fabricated by Au evaporation and subsequent e-beam lithography. The SERS performance was studied by confocal Raman microscopy using a linearly-polarized 633 nm laser. A critical impact of the polarization alignment on the spectral resolution of the studied Raman marker imprint was observed. The results show that the Raman signal is maximized by aligning the polarization orientation with the nanowire long axis, it is reduced by increasing the relative angle, and it is abruptly minimized when both are perpendicular. These observations were consistent with numerical simulations carried out by the FDTD method, which predicts a similar dependence between the orientation of linearly-polarized light and electric-near field amplification in the nano-gap zone. Our results provide an interesting paradigm and relevant insights in determining the role of laser polarization in the Raman signal enhancement in nano-gapped Au nanowires, showing the key role of this measurement condition on the SERS-based molecular sensing efficiency of this kind of nanostructure.

Gap-enhanced optical bistability in plasmonic core-nonlinear shell dimers.

Localized surface plasmon resonance in capacitively-coupled metallic nanoparticle dimers accompanied by a substantial local field enhancement in the interparticle gap area can enable boosting of nonlinear optical effects. In this paper, we analyze optical bistability in a plasmonic spherical dimer wrapped by a mutual nonlinear shell. In the common graphical post-processing technique of optical bistability, it is assumed that the refractive index change is homogeneous throughout the whole shell. However, we resolve this issue by taking into account the inhomogeneous nature of the power density in the dimer and linking the refractive index change to the local intensity inside the shell. The hysteresis branches of the normalized scattering and extinction cross-sections, as well as electric near-field strength, were derived by increasing and decreasing the driving field intensity. The analysis shows that optical bistability in the dimer with a 3 nm gap can be achieved at switching intensities of about 375 kW cm-2 and 225 kW cm-2, where each stable state of the C-Sh dimer corresponds to a certain plasmonic mode. The range of driving field intensities can be further decreased by considering smaller interparticle distances. The influence of the nonlinear shell on the spectral response is also examined. Suggested configurations distributed on a planar dielectric substrate have potential applications as all-optical switches and memory elements.

Dielectric coated conductive rod resonantly coupled with a cut transmission line as a tunable microwave bandstop filter and sensor.

The resonant interaction of a dielectric-coated conductive rod with the X-band microwave field is investigated. The magnetic field distribution of the Goubau standing radial surface waves was experimentally visualized by using a thermo-elastic optical indicator microscope, and the corresponding electric field distribution was determined via numerical simulations. These field distributions are characterized by a certain pattern of antinodes distinctive for standing waves. An analysis of these field distributions allows one to couple a coated rod with a cut Goubau line. A rod placed in the gap region perpendicular to the Goubau line results in a sharp rejection band in the transmission spectrum which is extremely sensitive to the changes in the surrounding media. The shifting rate of the resonance as a function of the dielectric shell thickness is approximately 1.4 GHz/mm. The Q-factor of copper rods depends on their size and dielectric shell thickness. Longer rods with more energy localization areas have higher Q-factors, typically 1.7 times higher (12.7 vs. 7.5). Moreover, incorporating a dielectric shell enhances energy confinement and can elevate the Q-factor by as much as 22 %. When a 25 mm Cu rod is situated inside a cut Goubau line system, the Q-factor values are significantly higher, with a ratio of 275 to 13. With the addition of a dielectric shell, the Q-factor can be elevated by 58 %. The versatility of the proposed controllable system makes it possible to tune the operating spectrum towards higher GHz and THz frequencies.

Tunable ultra-broadband terahertz metamaterial absorber based on vanadium dioxide strips.

A simple design of an ultra-broadband metamaterial absorber (MMA) of terahertz (THz) radiation based on vanadium dioxide (VO2) configurations is proposed. The system is composed of a top pattern representing orderly distributed VO2 strips, a dielectric spacer and an Au reflector. Theoretical analysis based on the electric dipole approximation is performed to characterize the absorption and scattering properties of an individual VO2 strip. The results then are used to design an MMA composed of such configurations. It is shown that the efficient absorption characteristics of the Au-insulator-VO2 metamaterial structure can be ensured in a broad spectrum of 0.66-1.84 THz with an absorption band relative to the center frequency reaching as high as 94.4%. The spectrum of the efficient absorption can be easily tuned via the corresponding choice of strip dimensions. Wide polarization and incidence angle tolerance for both transverse electric (TE) and transverse magnetic (TM) polarizations are ensured by adding an identical parallel layer rotated by 90 degrees in respect to the first one. Interference theory is applied to elucidate the absorption mechanism of the structure. The possibility of modulation of the electromagnetic response of MMA relying on the tunable THz optical properties of VO2 is demonstrated.

3D visualization of microwave electric and magnetic fields by using a metasurface-based indicator.

Visualizations of the microwave electric and magnetic near-field distributions of radio-frequency (RF) filters were performed using the technique of thermoelastic optical indicator microscopy (TEOIM). New optical indicators based on periodic dielectric-metal structures were designed for electric field visualization. Depending on the structure orientation, such metasurface-based indicators allow separately visualization of the Ex and Ey components of the in-plane electric field. Numerical simulations were conducted to examine the working principle of the designed indicator structures, and the results were compared to the experimental, showing good agreement. In addition, the 3D visualization of the microwave near-field distribution was built, to show the field intensity and distribution dependencies on the distance from the RF filter.

Near-perfect broadband infrared metamaterial absorber utilizing nickel.

We propose a thin, compact, broadband, polarization and angle insensitive metamaterial absorber based on a tungsten reflector, a silicon spacer, and a top pattern composed of a double square-like ring resonator utilizing nickel (Ni). In such a structure, a high absorption (above 80%) bandwidth ∼4.8µm from 3.52 up to 8.32 µm corresponding to the relative bandwidth ∼81% can be achieved with deeply subwavelength unit cell dimensions. Here the physical origin of the broadband absorption is associated with low Q-factor dipole modes of the top pattern inner and outer sides functioning as rectangular nanoantennas. Owing to the structural symmetry, the absorber shows a good incidence angle tolerance in the relatively wide range for both transverse electric and transverse magnetic polarizations. The effective parameters of the Ni-based absorber were retrieved using the constitutive effective medium theory, and the absorption characteristics of the effective medium and metamaterial were compared.