A novel complementary grating structure is proposed for plasmonic refractive index sensing due to its strong resonance at near-infrared wavelength. The reflection spectra and the electric field distributions are obtained via the finite-difference time-domain method. Numerical simulation results show that multiple surface plasmon resonance modes can be excited in this novel structure. Subsequently, one of the resonance modes shows appreciable potential in refractive index sensing due to its wide range of action with the environment of the analyte. After optimizing the grating geometric variables of the structure, the designed structure shows the stable sensing performance with a high refractive index sensitivity of 1642 nm per refractive index unit (nm/RIU) and the figure of merit of 409 RIU−1. The promising simulation results indicate that such a sensor has a broad application prospect in biochemistry.

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Rapid assessment of foliar chlorophyll content in tobacco is critical for assessment of growth and precise management to improve quality and yield while minimizing adverse environmental impact. Our objective is to develop a precise agricultural practice predicting tobacco-leaf chlorophyll-a content. Reflectance experiments have been conducted on flue-cured tobacco over 3 consecutive years under different light quality. Leaf hyperspectral reflectance and chlorophyll-a content data have been collected at 15-day intervals from 30 days after transplant until harvesting. We identified the central band that is sensitive to tobacco-leaf chlorophyll-a content and the optimum wavelength combinations for establishing new spectral indices (simple ratio index, RVI; normalized difference vegetation index, NDVI; and simple difference vegetation index, DVI). We then established linear and BackPropagation (BP) neural network models to estimate chlorophyll-a content. The central bands for leaf chlorophyll-a content are concentrated in the visible range (410 -680 nm) in combination with the shortwave infrared range (1900 -2400 nm). The optimum spectral range for the spectral band combinations RVI, NDVI, and DVI are 440 and 470 nm, 440 and 470 nm, and 440 and 460 nm, respectively. The linear RVI, NDVI, and DVI models, SMLR model and the BP neural network model have respective R 2 values of 0.76, 0.77, 0.69, 0.78 and 0.86, and root mean square error values of 0.63, 1.60, 1.59, 2.04 and 0.05 mg chlorophyll-a/g (fresh weight), respectively. Our results identified chlorophyll-a sensitive spectral regions and new indices facilitate a rapid, non-destructive field estimation of leaf chlorophyll-a content for tobacco.

We propose a two-dimensional metal grating with rhombus particles on a gold film structure for refractive index sensing due to its perfect absorption at near-infrared wavelength. Via two-dimensional metal grating coupling, the incident light energy is effectively transformed into the surface plasmons which propagate along the upper surface of the gold film and interact with the surrounding environment in a wide range. The plasmonic resonance mechanism of the structure is discussed in detail by theoretical analysis and finite-difference time-domain method. After optimizing the geometrical parameters, the designed structure shows the sensing performance with a refractive index sensitivity of 1006 nm/RIU. More importantly, this plasmonic refractive index sensor achieves an ultra wide refractive index sensing range from 1.0 to 2.4 with a stable sensing performance. The promising simulation results of the structure show that the sensor has a broad application prospect in the field of biology and chemistry.