RESUMEN
As an emerging two-dimensional material, graphene offers an alternative material platform for exploring new metamaterial phenomena and device functionalities. In this work, we examine diffuse scattering properties in graphene metamaterials. We take periodic graphene nanoribbons as a representative example and show that diffuse reflection in graphene metamaterials as dominated by diffraction orders is restricted to wavelengths less than that of first-order Rayleigh anomaly, and is enhanced by plasmonic resonances in graphene nanoribbons, as similar to metamaterials made of noble metals. However, the overall magnitude of diffuse reflection in graphene metamaterial is less than 10-2 due to the large period to nanoribbon size ratio and ultra-thin thickness of the graphene sheet, which suppress the grating effect from the structural periodicity. Our numerical results indicate that, in contrast to the cases of metallic metamaterials, diffuse scattering plays a negligible role in spectral characterization of graphene metamaterials in cases with large resonance wavelength to graphene feature size ratio, which corresponds to typical chemical vapor deposition (CVD)-grown graphene with relatively small Fermi energy. These results shed light on fundamental properties of graphene nanostructures and are helpful in designing graphene metamaterials for applications in infrared sensing, camouflaging, and photodetection, etc.
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Narrowband resonance plays an important role in many optical applications, especially for the development of wavelength-selective properties and enhanced light-matter interaction. In this paper, we demonstrate metal-insulator-metal (MIM) waveguide gratings, which exhibit guided-mode resonance (GMR) with reduced bandwidth in mid-infrared absorption and thermal emission. Our fabricated MIM waveguide grating consists of a copper substrate, a lossless ZnSe film, and a top gold stripe grating. Our measurements reveal strong GMRs with a bandwidth of 1.29% of the central wavelength in both mid-infrared absorption and thermal emission spectra. By varying structural parameters of the MIM waveguide grating, strong absorptions and thermal emissions of GMRs are observed and tuned within the 3-5 µm wavelength range. These results manifest the great potential of engineering infrared properties by using GMR and could be useful for spectral control in a variety of infrared devices.
RESUMEN
Metamaterials of metal-insulator-metal structures represent effective ways in manipulating light absorbance for photodetection, sensing, and energy harvesting etc. Most of the time, specular reflection has been used in characterizing resonances of metamaterials without considering diffuse scattering from their periodic subwavelength units. In this paper, we investigate diffuse reflection in metasurfaces made of periodic metallic disks in the mid-infrared region. Integrating sphere-based spectral measurements indicate that diffuse reflection is dominated by grating diffractions, which cause diffuse scattering in a spectral region with wavelengths less than that of the first order Rayleigh anomaly. The diffuse reflection is greatly enhanced by the metasurface resonance and exhibits a general increase towards shorter wavelengths, which not only causes a significant difference in evaluating the metamaterial resonant absorption efficiency but also a small blue-shift of the resonance frequency. These findings are helpful for designing and analyzing metamaterial resonant properties when diffuse scattering is taken into account.
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The area covered by Chinese-style solar greenhouses (CSGs) has been increasing rapidly. However, only a few pyranometers, which are fundamental for solar radiation sensing, have been installed inside CSGs. The lack of solar radiation sensing will bring negative effects in greenhouse cultivation such as over irrigation or under irrigation, and unnecessary power consumption. We aim to provide accurate and low-cost solar radiation estimation methods that are urgently needed. In this paper, a method of estimation of solar radiation inside CSGs based on a least mean squares (LMS) filter is proposed. The water required for tomato growth was also calculated based on the estimated solar radiation. Then, we compared the accuracy of this method to methods based on knowledge of astronomy and geometry for both solar radiation estimation and tomato water requirement. The results showed that the fitting function of estimation data based on the LMS filter and data collected from sensors inside the greenhouse was y = 0.7634x + 50.58, with the evaluation parameters of R2 = 0.8384, rRMSE = 23.1%, RMSE = 37.6 Wm-2, and MAE = 25.4 Wm-2. The fitting function of the water requirement calculated according to the proposed method and data collected from sensors inside the greenhouse was y = 0.8550x + 99.10 with the evaluation parameters of R2 = 0.9123, rRMSE = 8.8%, RMSE = 40.4 mL plant-1, and MAE = 31.5 mL plant-1. The results also indicate that this method is more effective. Additionally, its accuracy decreases as cloud cover increases. The performance is due to the LMS filter's low pass characteristic that smooth the fluctuations. Furthermore, the LMS filter can be easily implemented on low cost processors. Therefore, the adoption of the proposed method is useful to improve the solar radiation sensing in CSGs with more accuracy and less expense.
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Vibrational absorption spectroscopy presents an effective and direct way for molecular detection and identification. In this paper, we propose and demonstrate a simple strategy and structure to amplify molecular detection sensitivity via the example of a monolayer octadecanethiol (ODT). The underlying amplification mechanism operates on both the enhanced surface field in and the coupled-oscillators' energy transfer between the molecules and the cavity underneath. The structure is designed to be simple and free of lithography or patterning with the potential for large-scale uses. It is made of just a quarter wavelength thick dielectric (ZnSe) layer atop a metallic reflecting base. Both angle and polarization dependent reflection spectra reveal signatures of CH2 and CH3 vibrations in theory and experiment. A vibrational signal intensity of 8.54% reached in s-polarization at a large incident angle is comparable to those reported in plasmonic nanostructures with greater sophistications in structure.
RESUMEN
In this paper, a novel heuristic dynamic programming (HDP) iteration algorithm is proposed to solve the optimal tracking control problem for a class of nonlinear discrete-time systems with time delays. The novel algorithm contains state updating, control policy iteration, and performance index iteration. To get the optimal states, the states are also updated. Furthermore, the "backward iteration" is applied to state updating. Two neural networks are used to approximate the performance index function and compute the optimal control policy for facilitating the implementation of HDP iteration algorithm. At last, we present two examples to demonstrate the effectiveness of the proposed HDP iteration algorithm.
