Plasmonics offer an exciting way to mediate the interaction between light and matter, allowing strong field enhancement and confinement, large absorption and scattering at resonance. However, simultaneous realization of ultra-narrow band perfect absorption and electromagnetic field enhancement is challenging due to the intrinsic high optical losses and radiative damping in metals. Here, we propose an all-metal plasmonic absorber with an absorption bandwidth less than 8 nm and polarization insensitive absorptivity exceeding 99%. Unlike traditional Metal-Dielectric-Metal configurations, we demonstrate that the narrowband perfect absorption and field enhancement are ascribed to the vertical gap plasmonic mode in the deep subwavelength scale, which has a high quality factor of 120 and mode volume of about 10−4 × (λres/n)3. Based on the coupled mode theory, we verify that the diluted field enhancement is proportional to the absorption, and thus perfect absorption is critical to maximum field enhancement. In addition, the proposed perfect absorber can be operated as a refractive index sensor with a sensitivity of 885 nm/RIU and figure of merit as high as 110. It provides a new design strategy for narrow band perfect absorption and local field enhancement, and has potential applications in biosensors, filters and nonlinear optics.

The concept of parity-time symmetry (PT symmetry) originates from the canonical quantum mechanics and has become a hot topic recently. As a versatile platform to investigate the intriguing concept, both theoretical and experimental works in optics have been implemented. In this paper, the PT symmetry breaking phenomenon is investigated in a coupled nanobeam cavity system. An exceptional point is observed during the tuning of the gain/loss level and the coupling strength of the closely placed nanobeam pair. Unidirectional light propagation is investigated, as well as enhanced sensitivity of single particle detection in the vicinity of the exceptional point. The proposed system is easy to be integrated with photonic integrated circuits and can be strongly coupled to optical waveguides.

A fast two-dimensional fluorescence correlation spectroscopy technique based on light emitting diodes is developed, which uses light intensity and excitation wavelength as quickly changeable and easily controllable external perturbations. A compact and automatic system is set up to detect tea quality. A partial least square regression method is used to create predictive models for tea grades. Compared to the traditional fluorescence spectroscopy method, this convenient two-dimensional correlation spectroscopy technique is more accurate according to our experimental results and is promising for practical applications.

Abstract-We propose a novel design of broadband plasmonic nanoantenna that is suitable for fluorescence and Raman enhancement. The structure consists of a gold nanoring and bowties at the center. We numerically investigate the near field and far field performance by employing the finitedifference time-domain method. High Purcell enhancement and large SERS are demonstrated in a record wide spectral bandwidth of 700 nm based on a single emitter-antenna configuration. Moreover, unlike a traditional antenna design, the proposed nanoantenna has low heat generation and high field enhancement at the gap simultaneously when operating off resonance.

We propose a novel design of a broadband plasmonic nanoantenna, investigate it numerically using finite-difference time-domain methods, and explain its performance using the analysis of charge distribution in addition to a multipole expansion.