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 2×10^(7) is obtained,which verifies the field enhancement and the potential of this device.
Zhendong ZhuBenfeng BaiOubo YouQunqing LiShoushan Fan
The longitudinal optical field is a peculiar physical phenomenon that is always involved with the domain of near-field optics. Due to its extraordinary properties, it has recently attracted increasing attention in research and application. In this work, the longitudinal fields generated by the evanescent illumination of tightly focused, different polarized hollow beams are investigated. The focused light fields are numerically simulated according to vector diffraction theory, and their vector analysis is also carried out. The longitudinal fields on the focal plane are demonstrated experimentally using tip-enhanced scanning near-field microscopy. The simulation and experimental results show that the tightly focused radially polarized beam is suited to generating a stronger and purer longitudinal optical field at the focus.
Phase is one of the most important parameters of electromagnetic waves. It is the phase distribution that determines the propagation, reflection, refraction, focusing, divergence, and coupling features of light, and further affects the intensity distribution. In recent years, the designs of surface plasmon polariton(SPP) devices have mostly been based on the phase modulation and manipulation. Here we demonstrate a phase sensitive multi-parameter heterodyne scanning near-field optical microscope(SNOM) with an aperture probe in the visible range, with which the near field optical phase and amplitude distributions can be simultaneously obtained. A novel architecture combining a spatial optical path and a fiber optical path is employed for stability and flexibility. Two kinds of typical nano-photonic devices are tested with the system. With the phase-sensitive SNOM, the phase and amplitude distributions of any nano-optical field and localized field generated with any SPP nano-structures and irregular phase modulation surfaces can be investigated. The phase distribution and the interference pattern will help us to gain a better understanding of how light interacts with SPP structures and how SPP waves generate, localize, convert, and propagate on an SPP surface. This will be a significant guidance on SPP nano-structure design and optimization.
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.
Surface plasmon polaritons(SPPs)have been widely exploited in various scientific communities,ranging from physics,chemistry to biology,due to the strong confinement of light to the metal surface.For many applications,it is important that the free space photon can be coupled to SPPs in a controllable manner.In this Letter,we apply the concept of interfacial phase discontinuity for circularly polarizations on a metasurface to the design of a novel type of polarization-dependent SPP unidirectional excitation at normal incidence.Selective unidirectional excitation of SPPs along opposite directions is experimentally demonstrated at optical frequencies by simply switching the helicity of the incident light.This approach,in conjunction with dynamic polarization modulation techniques,opens gateway towards integrated plasmonic circuits with electrically reconfigurable functionalities.
