Nanocavities inscribed in a gold thin film are optimized and designed to form a metasurface. We demonstrate both numerically and experimentally the creation of surface plasmon (SP) vortex carrying orbital angular momentum in the metasurface under linearly polarized optical excitation that carries no optical angular momentum. Moreover, depending on the orientation of the exciting linearly polarized light, we show that the metasurface is capable of providing dynamic switching between SP vortex formation or SP subwavelength focusing. The resulting SP intensities are experimentally measured using a near-field scanning optical microscope and are found in excellent quantitative agreements as compared to the numerical results.

An optical fiber sensor based on the change of optical confinement in a subwavelength tip is presented. The optical spot is substantially increased when the environmental refractive index (RI) increases from 1.3 to 1.4. By measuring the intensity of low angular spectral components, an intensity sensitivity up to 8000% per RI unit is achieved. The fiber tip sensors take advantage of the small detection volume and real-time responses. We demonstrate the application of the nanofiber sensors for measuring concentrations of acids and evaporation rates of aqueous mixtures.

Plasmonic hot-carriers, which are induced by plasmons at metal surfaces, can be used to convert photon energy into excited carriers over a subwavelength region and provide a new means to realize photodetection within the subband-gap region of semiconductor materials. However, the barrier height of the metal−semiconductor junction affects the behavior of the plasmon-induced hot-carriers and limits the electrical response of photodetection. High electrical responsivity, achieved by manipulating the barrier height using plasmon-induced hot electrons, is desired to broaden the possible applications. Here we report a plasmonic channelcoupled nanogap structure, where the barrier height of the metal−semiconductor junction is altered upon the excitation of plasmon-induced hot-carriers. The structure consists of semiconductor channels and metal slabs forming nanogaps, which sustain coupled plasmons and confine light to the semiconductor−metal interfaces. In contrast to conventional Schottky barriers and ohmic contacts, in which plasmon-induced hot-carriers and the generation of electron−hole pairs by photoabsorption cause an increase in the photocurrent, the generation of plasmon-induced hot-carriers at the resonant wavelength results in an increase in the junction barrier height and a decrease in the photocurrent induced by photoabsorption. By modifying the barrier height, the plasmon resonance can be monitored from the electrical response with a high spectral resolution and a large modulation.

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