Cascaded optical field enhancement (CFE) can be realized in some specially designed multiscale plasmonic nanostructures, in which the generation of extremely strong fields at nanoscale volume is crucial for many applications, for example, surface-enhanced Raman spectroscopy (SERS). In this paper, we propose a strategy for realizing a high-quality plasmonic nanoparticle-in-cavity (PIC) nanoantenna array, in which strong coupling between a nanoparticle (NP) dark mode with a high-order nanocavity bright mode can produce strong Fano resonance at the target wavelength. The Fano resonance can effectively boost the CFE in a PIC. A cost-effective and reliable nanofabrication method is developed using room temperature nanoimprinting lithography to manufacture high-quality PIC arrays. This technique guarantees the generation of only one gold NP at the bottom of each nanocavity, which is crucial for the generation of the expected CFE. To demonstrate the performance and application of the PIC array, the PIC array is employed as an active SERS substrate for detecting 4-aminothiophenol molecules. An experimental SERS enhancement factor of 2 3 10 7 is obtained, which verifies the field enhancement and the potential of this device. 1,2 SPs can be considered to be the resonant photon-induced collective oscillations of free electrons confined on the surfaces of metallic nanostructures, which can be driven by an incident light field. The importance of these photonelectron interactions is their ability to concentrate light energy in nanoscale volumes in the metallic nanostructures and subsequently boost the intensity of the optical near field by several orders of magnitude.2-5 Due to these properties, localized SPs can facilitate many applications based on the enhanced light-matter interaction, such as the detection of trace element chemical varieties in the vicinity of metallic nanostructures by surface-enhanced Raman spectroscopy (SERS). The detection limit can be reduced to a single molecular level.

The efficient steering of surface plasmon polariton (SPP) fields is a vital issue in various plasmonic applications, such as plasmonic circuitry. We present a straightforward and efficient method for generating unidirectionally propagating SPP beams with arbitrary amplitude and phase profiles by manipulating Δ-shaped nanoantennas. As an example, a second-order Hermite-Gauss SPP beam is generated with this method. The near-field distribution of the generated SPP beam is experimentally characterized to validate the effectiveness of the method.

Plasmonic nanostructures separated by nanogaps enable strong electromagnetic-field confinement on the nanoscale for enhancing light-matter interactions, which are in great demand in many applications such as surface-enhanced Raman scattering (SERS). A simple M-shaped nanograting with narrow V-shaped grooves is proposed. Both theoretical and experimental studies reveal that the electromagnetic field on the surface of the M grating can be pronouncedly enhanced over that of a grating without such grooves, due to field localization in the nanogaps formed by the narrow V grooves. A technique based on room-temperature nanoimprinting lithography and anisotropic reactive-ion etching is developed to fabricate this device, which is cost-effective, reliable, and suitable for fabricating large-area nanostructures. As a demonstration of the potential application of this device, the M grating is used as a SERS substrate for probing Rhodamine 6G molecules. Experimentally, an average SERS enhancement factor as high as 5×10⁸ has been achieved, which verifies the greatly enhanced light-matter interaction on the surface of the M grating over that of traditional SERS surfaces.

Surface plasmon polariton (SPP) sources and launchers are highly demanded in various applications of nanophotonics. Here, we propose a general approach that can realize complete control of the complex extinction ratio (including amplitude and phase) of any two linearly independent SPP modes excited by any elementary SPP excitation architecture just by manipulating the incident polarization state. In an optical system, it suffices to simply tune the orientation angles of a linear polarizer and a quarter wave plate, which may greatly simplify the design and application of SPP launchers and diversify their functionalities. As an example to show the broad application prospect of this method, we design and realize a metaline consisting of Δ-shaped plasmonic nanoantennas, which can effectively realize dual functionalities, i.e., the tunable directional SPP excitation at an arbitrarily chosen wavelength and the complete unidirectional SPP excitation over a broad bandwidth. This general approach can also be extended to the control of the complex extinction ratio of any two linearly independent excited modes in many other linear optical systems, such as two modes in a waveguide or two diffraction orders in a grating, over a broad bandwidth.

We present an experimental study on a unidirectional surface plasmon polariton (SPP) launcher based on a compact binary area-coded nanohole array, where the symmetry breaking is realized via effective-index modulation in the binary pattern of the gold film, thus avoiding the challenge of modulating nanostructure in its depth. It is shown that SPPs can be unidirectionally and effectively excited at normal incidence. The SPP intensity and asymmetric excitation ratio, which are two key figure-of-merits of SPP launchers, can be improved by increasing the number of array rows. The proposed device is compatible with most mature top-town nanofabrication techniques and thus is perspective for low-cost mass production.