A narrowband terahertz (THz) filter based on metal–dielectric–metal structure with a circular loop array patterned on metallic films was proposed. The transmission properties in the THz range of the structure were experimentally and theoretically analyzed. The structure’s transmission spectrum has two narrow bands of which band 1 in the lower frequency range was dominated by the transmission mode due to localized plasmon polaritons and band 2 in the higher frequency range was dominated by propagated plasmon polaritons mode and guided mode. Both band 1 and band 2 have high Q-factors. By designing an asymmetry periodic constant in orthogonal directions, band 2 exhibits potentials as a narrowband polarizer. Furthermore, the transmittance of band 2 can be promoted to 100% by laterally shifting the metallic arrays.
Chiral plasmonic structures provide an effective way to couple optical spin with orbital angular momentum, which is suitable to perform manipulation of light beam. This work presents simulation results for a type of spiral-ring plasmonic structures that provide circular polarization selectively in the visible spectrum. The structure consists of discrete spiral gratings that are truncated by rings. With circular polarized lights transmitting the subwavelength structure, surface plasmon polaritons and localized surface plasmons are excited together. Thus, multi-rings spiral plasmonic structures are introduced to obtain high extinction ratio circular dichroism. Moreover, the wavefront phase singularity is also observed in the simulation.
Visible and near infrared (Vis-NIR) reflectance spectroscopy technology, which is rapid, cost-effective, in-situ and non-destructive, is getting more and more widely used in improving the prediction and digital mapping for soil properties. Soil available nitrogen (AN) is closely related to soil fertility and quality, assessing its content and mapping the spatial variability greatly satisfies precision agriculture. In this study, the Vis-NIR diffuse reflectance spectra collected by ASD FieldSpec Pro FR spectrometer with a performance of spectral range from 350 nm to 2500 nm, 1 nm resampling intervals, was used to model and characterize the spatial variability of available nitrogen. Firstly, the raw soil spectra was pre-processing by reducing to 400 nm - 2450 nm with transformation into apparent absorbance spectral using Log(1/R) and Savitzky-Golay smoothing. Secondly, spectral indices (normalized spectral index-NDI; difference index-DI; ratio index-RI) were convinced for seeking further relationship between AN. Afterwards, Partial Least Squares Regression (PLSR) method was employed to predict AN. The results indicated good predictions with RPD more than 1.4. Finally, the spatial variability of AN was mapped by (co)kriging method, digital mapping of the measured and predicted AN showed similar patterns and value ranges, though there are some minor differences. The resultant prediction and mapping demonstrated a promotion of assessing and mapping of soil properties by a rapid and reliable approach from lab to field in-situ.
In this paper, square lattices of air holes were fabricated on a three-layer structure of metal-dielectric-metal using
micromachining technology. The metal-dielectric-metal structure is based on RT/duroid 5870 produced by Rogers
corporation. The square period is 400 μm and the radius of circular hole is 100 μm. The thickness of the structure is
about 863 μm with metal thickness of 39.2 μm and dielectric layer thickness of 785 μm. The loss and dispersion of the
dielectric layer with the dielectric constant of 2.33 are low at microwave frequencies. Terahertz transmission spectra
through the sample were measured by the state-of-the-art THz-TDS system. Experimental results show that there is a
transmittance centered at 1 THz with a wide pass-band exceeding 100 GHz. Transmission spectra calculated by FDTD
method were given for comparisons and showed good agreements with the experimental results. Through analysis, the
extraordinary transmission phenomena are caused by both the Febry-Parot effect and surface plasmon polaritons existing
on the metal arrays.
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