Multi-band Sensor Research Articles

High Q-Factor, High Contrast, and Multi-Band Optical Sensor Based on Plasmonic Square Bracket Dimer Metasurface.

A high-performance resonant metasurface is rather promising for diverse application areas such as optical sensing and filtering. Herein, a metal-insulator-metal (MIM) optical sensor with merits of a high quality-factor (Q-factor), multiple operating bands, and high spectrum contrast is proposed using plasmonic square bracket dimer metasurface. Due to the complex square bracket itself, a dimer structure of two oppositely placed square brackets, and metasurface array configuration, multiple kinds of mode coupling can be devised in the inner and outer elements within the metasurface, enabling four sensing channels with the sensitivities higher than 200 nm/RIU for refractive index sensing. Among them, the special sensing channel based on the reflection-type surface lattice resonance (SLR) mechanism has a full width at half maximum (FWHM) of only 2 nm, a high peak-to-dip signal contrast of 0.82, a high Q-factor of 548, and it can also behave as a good sensing channel for the thickness measurement of the deposition layer. The multi-band sensor can work normally in a large refractive index or thickness range, and the number of resonant channels can be further increased by simply breaking the structural symmetry or changing the polarization angle of incident light. Equipped with unique advantages, the suggested plasmonic metasurface has great potential in sensing, monitoring, filtering, and other applications.

Remote Sensing of Snow Parameters: A Sensitivity Study of Retrieval Performance Based on Hyperspectral versus Multispectral Data

Snow parameters have traditionally been retrieved using discontinuous, multi-band sensors; however, continuous hyperspectral sensors are now being developed as an alternative. In this paper, we investigate the performance of various sensor configurations using machine learning neural networks trained on a simulated dataset. Our results show improvements in the accuracy of retrievals of snow grain size and impurity concentration for continuous hyperspectral channel configurations. Retrieval accuracy of snow albedo was found to be similar for all channel configurations.

Multiwavelength absorption switching based on the metal-dielectric-metal multilayer Kerr metamaterial

In this paper, quintuple-band absorption switch has been demonstrated and studied. Finite-difference time-domain simulations indicate the absorption originated from localized surface plasmon polaritons and Fabry–Perot resonance effect. Quintuple-band plasmonic perfect absorption with maximum absorption of 99.8%, 86.2%, 99.9%, 95.8%, and 97.8% at the resonance wavelength of 635 nm, 694 nm, 915 nm, 1125 nm, and 1345 nm, respectively. We calculate the sensitivity of the five modes is about s 1 = 74 n m / R I U , s 2 = 376 n m / R I U , s 3 = 518 n m / R I U , s 4 = 512 n m / R I U , and s 5 = 7 98 n m / R I U (RIU is the refractive index unit). Angle and polarization independent of the quintuple-band absorption are also investigated. Moreover, by adjusting the pump light on and off, absorption ratios of 3.9, 10.3, 3.8, 3.4, and 6.8 have been achieved. Besides, the proposed multiwavelength absorption switching has excellent absorption stability and polarization tolerance. We believe the proposed structure can find potential applications in the fields of multiband sensor and all-optical modulation.

Soil water depletion induces discrepancies between in situ measured vegetation indices and photosynthesis in a temperate heathland

Vegetation indices (VIs) derived from optical sensors have been used as proxies for monitoring gross primary productivity (GPP). In contrast to satellite-based VIs, whose temporal resolution is typically limited, especially in cloudy areas, in situ derived VIs may have higher temporal resolution. This fine temporal frequency implies much larger sample sizes to test the performance of VIs by comparing with eddy covariance-based in situ GPP estimates. Here, we tested the potential of in situ measured VIs to estimate GPP in a temperate heathland ecosystem. We compared half-hourly GPP values derived from an eddy covariance CO2 flux measurements with several greenness-, structure- and chlorophyll-sensitive VIs (e.g., the Terrestrial Chlorophyll Index [TCI] and the Inverted Red-Edge Chlorophyll Index [IRECI], among others) obtained by multiband sensors mounted at the tower. Results showed that tested VIs differed in their ability to capture the temporal variability of GPP during non-drought condition, and that all VIs failed in describing GPP during an extreme drought event. After integrating a drought indicator (e.g., soil water content, precipitation/potential evapotranspiration ratio or actual/potential evapotranspiration ratio) in the regression model, the performances of VIs drastically improved. Among all tested indices, IRECI and TCI were the most promising VIs, capturing best the temporal variation in GPP (R2 = 0.73 and RMSE = 1.85, and R2 = 0.77, RMSE = 1.69, respectively), provided that drought stress was properly accounted for. Our findings have implications for the development and improvement of global ecological models for drought monitoring based on proximal and remote sensing data. Our results have also a strong impact on our ability to upscale CO2 fluxes using satellite sensors (e.g. Sentinel-2 and Sentinel-3) of heathland ecosystem characterized by heather vegetation which is relatively resistant to the structural changes in the canopy and strongly affects the interpretation of the remote sensing signals.

High-Sensitivity Multiband Detection Based on the Local Enhancement Effect of an Electric Field at Terahertz Frequency

The terahertz detection sensitivity interaction with other substances has received extensive attention and remains challenging due to the loss of terahertz waves. Herein, a terahertz, multiband sensor with high sensitivity based on the local enhancement effect of an electric field at terahertz frequency is proposed. The designed terahertz sensor is based on a common metamaterial sandwich absorber of electromagnetic energy that only employs an asymmetric metallic strip, which produces three resonance peaks with an absorption coefficient of greater than 95% in the 0.2-1.5 THz range and a maximum Q -factor of 46 at 1.397. Based on the principle of impedance matching, the high absorption of electromagnetic wave energy is reasonably analyzed. Furthermore, the influence of the top metal structure on the absorptivity and resonance frequency is analyzed by simulation optimization, and the optimum structural parameters are obtained. In addition, the influence of the refractive index (RI) change of the sample and dielectric layer on the resonance characteristics of the sensor is analyzed. When the thickness of the surface sample of the terahertz sensor metamaterial-based absorber is changed so that the RI changes in 1-2, the RI sensitivity increases up to 208 GHz/RIU, which can be used as an RI sensor. Furthermore, the influence of the sensor sensitivity and reflection coefficient on the thickness parameters of the sample covered by the sensor surface is analyzed, and the optimal sample thickness is 6 μm. The influence of the incident angle of the THz wave on the detection performance in the TM polarization mode is analyzed, and it is determined that the change in the incident angle in the range of 0-40 degrees has minimal influence on the resonance peak and absorption rate of multiband detection. Therefore, we suggest that our proposed sensor has considerable potential in biomedicine and trace detection applications.

Glacier Ice Surface Properties in South‐West Greenland Ice Sheet: First Estimates From PRISMA Imaging Spectroscopy Data

AbstractSnow and ice melt processes on the Greenland Ice Sheet are a key in Earth's energy balance and are acutely sensitive to climate change. Melting dynamics are directly related to a decrease in surface albedo, amongst others caused by the accumulation of light‐absorbing particles (LAPs). Featuring unique spectral patterns, these accumulations can be mapped and quantified by imaging spectroscopy. We present first results for the retrieval of glacier ice properties from the spaceborne PRISMA imaging spectrometer by applying a recently developed simultaneous inversion of atmospheric and surface state using optimal estimation. The image analyzed in this study was acquired over the South‐West margin of the Greenland Ice Sheet in late August 2020. The area is characterized by patterns of both clean and dark ice associated with a high amount of LAPs deposited on the surface. We present retrieval maps and uncertainties for grain size, liquid water, and algae concentration, as well as estimated reflectance spectra for different surface properties. We then show the feasibility of using imaging spectroscopy to interpret multiband sensor data to achieve high accuracy, frequently repeated observations of changing snow and ice conditions. For example, the impurity index calculated from multiband Sentinel‐3 Ocean and Land Colour Instrument measurements is dependent on dust particles, but we show that algae concentration alone can be predicted from this data with less than 20% uncertainty. Our study evidences that present and upcoming orbital imaging spectroscopy missions such as PRISMA, Environmental Mapping and Analysis Program, Copernicus Hyperspectral Imaging Mission, and the Surface Biology and Geology designated observable, can significantly support research of melting ice sheets.

A Method for the Reduction of Out-of-Band Measurement Errors in Multi-band Instruments using Synthetic Source Distributions

A method to reduce multi-band sensor measurement biases due to finite out-of-band response is described. The method takes advantage of the fact that out-of-band measurement errors cancel if the calibration source and the measured source have the same spectral distributions—independent of their spectral distributions or the magnitude of a sensor band’s out-of-band response. Using a known spectral responsivity, a synthetic, arbitrary source spectral distribution can replace a realized spectral distribution in the measurement equation and the signal can be calculated rather than measured. Given the freedom to select any arbitrary distribution for the synthetic source, the efficacy of the approach depends on the fidelity of the replication of the measured spectrum by the synthetic source spectrum. To illustrate the method, an example application is given of top-of-the-atmosphere measurements of water-leaving radiance by multi-band filter radiometers on celestial Earth-viewing sensors.

Spectral characteristics of sea snot reflectance observed from satellites: Implications for remote sensing of marine debris

In 2021, coastal communities in the Sea of Marmara were impacted by large sea snot events. Based on data collected by several satellite sensors, we analyze and present the spectral reflectance characteristics of sea snots in the visible and near-infrared wavelengths. Sea snots often form elongated image slicks in Red-Green-Blue (RGB) or false-colour RGB (FRGB) images. Most reflectance spectra of sea snots show largely featureless spectral shapes, with continuous reflectance increases from the blue to the red, which then become flat toward the near infrared wavelengths. Some reflectance spectra show a local trough around 670 nm, indicating presence of chlorophyll a pigment, and thereby live algae. A 20-year time series of satellite data also reveals large-scale sea snot events in the Sea of Marmara before 2021. These results suggest that it might be possible to develop algorithms to search and map sea snots at a global scale, as sea snot events also occur in other regions. The results also indicate that remote differentiation of sea snots and marine debris using multi-band sensors may be difficult because of the spectral similarity between them.

Single-Chip Two Antennas for MM-Wave Self-Powering and Implantable Biomedical Devices

Implantable biomedical applications arise the need for multi-band sensors with a wideband frequency channel for RF energy harvesting operation. Using a separate antenna for energy harvesting can simplify device circuit complexity and reduces operation frequency bands interference. This paper demonstrates the design of single chip with two separate integrated antennas for implantable biomedical applications. The two antennas have different structures with orthogonal polarization to achieve low mutual coupling and negligible interaction between them. The first antenna is a multi-band meander line (MBML) designed for multiple channels data communication, with quad operating bands in the MM-wave range from 22-64 GHz with area 1150 × 200μm2. The second antenna is a wideband dipole antenna (WBDA) for RF energy harvesting, operates in the frequency range extend from 28 GHz to 36 GHz with area 1300×250μm2. The proposed antennas are designed by using high frequency structure simulator (HFSS) and fabricated by using UMC180nm CMOS technology with total area 0.55 mm2. The MBML frequency bands operating bandwidths can reach 2 GHz at impedance bandwidth ≤ -10 dB. While, the WBDA antenna has gain -2 dB over the operating band extend from 28 GHz up to 36 GHz. The antenna performance is simulated separately and using the human-body phantom model that describes layers of fats inside body, and shows their compatibility for in body operation. Die measurements is performed using on wafer-probing RF PICOBROBES and shows the matching between simulation and measurement values.

Analyzing the Performance of WBAN Links during Physical Activity Using Real Multi-Band Sensor Nodes

Wireless body area networks (WBANs) present unique challenges due to their specific characteristics of mobility and over-the-body radio propagation. A huge amount of factors—both internal and external to the network—affect WBAN channel conditions, so a reliable and comprehensive theoretical model of these communications is unfeasible and impractical in real scenarios. Thus, an empirical performance analysis of several WBAN channels is presented in this work, based on the receiver signal strength indicator (RSSI) and packet reception rate (PRR) metrics. Four different static and dynamic activities have been characterized: standing, sitting, cycling and walking. This analysis confirms the theoretical notions of path attenuation due to body parts obstructing the signal path, while serving as a benchmark for the design of future algorithms. The experiments have been carried out with real hardware nodes with wireless interfaces in three ISM bands: 433 MHz, 868 MHz and 2.4 GHz, evaluating the effect of the transmit power and node placement for different subjects. In all scenarios, the PRR metric reaches its maximum of 100% for both sub-GHz bands. Finally, our study concludes that the RSSI metric is sufficient to exploit the periodicity of dynamic activities, without the need for any extra hardware resources.

Bio-inspired multimodal 3D endoscope for image-guided and robotic surgery.

Image-guided and robotic surgery based on endoscopic imaging technologies can enhance cancer treatment by ideally removing all cancerous tissue and avoiding iatrogenic damage to healthy tissue. Surgeons evaluate the tumor margins at the cost of impeding surgical workflow or working with dimmed surgical illumination, since current endoscopic imaging systems cannot simultaneous and real-time color and near-infrared (NIR) fluorescence imaging under normal surgical illumination. To overcome this problem, a bio-inspired multimodal 3D endoscope combining the excellent characteristics of human eyes and compound eyes of mantis shrimp is proposed. This 3D endoscope, which achieves simultaneous and real-time imaging of three-dimensional stereoscopic, color, and NIR fluorescence, consists of three parts: a broad-band binocular optical system like as human eye, an optical relay system, and a multiband sensor inspired by the mantis shrimp's compound eye. By introducing an optical relay system, the two sub-images after the broad-band binocular optical system can be projected onto one and the same multiband sensor. A series of experiments demonstrate that this bio-inspired multimodal 3D endoscope not only provides surgeons with real-time feedback on the location of tumor tissue and lymph nodes but also creates an immersive experience for surgeons without impeding surgical workflow. Its excellent characteristics and good scalability can promote the further development and application of image-guided and robotic surgery.

Polarization independent septuple-band plasmonic perfect absorber based on hexagonal nanorods structure

In this paper, polarization-independent multi-band infrared perfect absorption has been demonstrated by using hexagonal nanorods coupled with the gold substrate. Numerical simulations of the finite-difference time-domain (FDTD) method indicate that perfect absorption can be achieved due to the Fabry-Perot gap plasmonic resonances, all the incident electromagnetic energy is confined in the Al2O3gap layer. Septuple-band perfect absorption with maximum absorbance up to 99.4% and the lowest above 82.5% is achieved in the range of 700−2000 nm. Angle and polarization-dependent performance of the septuple-band absorption is also investigated. Moreover, the spectral position of the absorption peak can be tuned by adjusting the geometrical parameters of the proposed absorber. Besides, the proposed septuple-band absorber has excellent absorption stability and polarization-independent. The proposed absorber may find potential applications in the fields of multiband sensor and plasmonic detection.

Diffusion-based gradient index optics for infrared imaging

New moldable, infrared (IR) transmitting glasses and diffusion-based gradient index (GRIN) optical glasses enable simultaneous imaging across multiple wavebands including short-wave infrared, midwave infrared, and long-wave infrared, and offer potential for both weight savings and increased performance in optical sensors. Lens designs show potential for significant reduction in size and weight and improved performance using these materials in homogeneous and GRIN lens elements in multiband sensors. An IR-GRIN lens with Δn = 0.2 is demonstrated.

Multiband plasmon-induced transparency based on nanometals-graphene hybrid model

<sec> In this paper, we have proposed a multiband plasmon-induced transparency (PIT) hybrid model based on silver nanorods, silver nanodisk and graphene. The electromagnetic properties are numerically and theoretically studied in this paper. The research results show that using the bright-bright mode coupling between silver nanorods and silver nanodisk, based on the weak hybridization effect induced by the detuning of each bright mode unit, the single-band, dual-band and triple-band PIT effects can be achieved. By changing the chemical potential of graphene, the tunability of the resonant frequencies and transmission amplitude can be achieved simultaneously in each PIT model. </sec><sec> When the chemical potential of graphene is 0 in each of the three PIT models, that is, without graphene, the resonant frequencies of its transparent window is the smallest. As the chemical potential of graphene increases from 0 to 0.5 eV, the resonant notches of the transparent peak in all three PIT models are both enhanced and blue shifted. Especially, when the chemical potential is 0.5 eV, the absolute increment of resonance notch generated by the sing-band PIT transparent window is <inline-formula><tex-math id="M1">\begin{document}$\Delta f = 1.01$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200200_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200200_M1.png"/></alternatives></inline-formula> THz and the relative increment is 2.91% while the largest absolute increment of resonance notch generated by the dual-band PIT transparent window is <inline-formula><tex-math id="M2">\begin{document}$\Delta f = 1.77$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200200_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200200_M2.png"/></alternatives></inline-formula> THz and the largest relative increment is 5.97%. In the next place, when the chemical potential is 0.3 eV, the absolute increment of resonance notch generated by the triple-band PIT transparent window is <inline-formula><tex-math id="M3">\begin{document}$\Delta f = 1.26$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200200_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20200200_M3.png"/></alternatives></inline-formula> THz and the relative increment of the window is 4.02%.</sec><sec> On the other hand, when graphene is existent in none of the three models, the resonance between silver nanodisk and silver nanorods, and the resonance between silver nanorods and silver nanorods are the weakest and the transmission amplitude of transparent window is the strongest in each of the three PIT models. Thereafter, with the increase of chemical potential, the number of surface charges on the silver nanodisk and silver nanorods increases and the intensity of electric field is enhanced. At the same time, the coupling strength between silver nanodisk and silver nanorods, and the coupling strength between silver nanorods and silver nanorods are also gradually enhanced. As a result, the transmission amplitude of each PIT model will gradually decrease. Especially, when the chemical potential is 0.5 eV, the amplitude modulation depth of the single-band PIT transparent peak is 20.2% and the amplitude modulation depth of the two transparent windows in dual-band PIT model are 31.2% and 24.2% respectively. In addition, when the chemical potential is 0.3 eV, the amplitude modulation depths of the three transparent windows in triple-band PIT model are 29.8%, 33.8%, and 20.5%. Finally, the sensing properties of the single-band PIT model are further investigated. The results show that the sensitivities of the model with refractive index of different background materials reach 3906.6 nm/RIU all, which provides a theoretical reference for the design of multiband filtering and ultrasensitive sensors. </sec>

RASPBERRY PI 3 MULTISPECTRAL LOW-COST SENSOR FOR UAV BASED REMOTE SENSING. CASE STUDY IN SOUTH-WEST NIGER

Abstract. The technology of UAV (Unmanned Aerial Vehicles) is rapidly improving and UAV-integrated sensors have kept up with it, providing more efficient and effective solutions. One of the most sought-after characteristics of on-board sensors is the low costing associated to good quality of the collected data. This paper proposes a very low-cost multiband sensor developed on a Raspberry device and two Raspberry Pi 3 cameras that can be used in photogrammetry from drone applications. The UAV-integrated radiometric sensor and its performance were tested in in two villages of South-west Niger for the detection of temporary surface water bodies (or Ephemeral water bodies): zones of seasonal stagnant water within villages threatening the viability and people’s health. The Raspberry Pi 3 cameras employed were a regular RGB Pi camera 2 (Red, Green, Blue) and a NoIR Pi 3 camera v2 (regular RGB without IR filter) with 8MPX resolution. The cameras were geometrically calibrated and radiometrically tested before the survey in the field. The results of the photogrammetry elaborations were 4 orthophotos (a RGB and NoIRGB orthophoto for each village). The Normalized Difference Water Index (NDWI) was calculated. The index allowed the localization and the contouring of the temporary surface water bodies present in the villages. The data were checked against the data collected with a Sony (ILCE-5100). Very high correspondence between the different data was detected. Raspberry-based sensors demonstrated to be a valid tool for the data collection in critical areas.

Development of a multi-band sensor for crop temperature measurement

A system combining a miniature long wavelength infrared (LWIR) camera with a visible, or RGB, camera was developed to capture a field of view and derive a plant-specific temperature measurement. The electronic and software development of the instrument, including calibration, field operation, and post-processing of data, are described. Calibration of the LWIR camera was accurate to 0.65 °C when relating pixel output to a thermal measurement, allowing it to act as a thermal camera. A processing algorithm was developed to identify plants within a visible camera image using a binary mask to identify crop/non-crop components within the field of view. The mask was then used to obtain a crop-only region of interest from the thermal image, over which data were integrated to create a temperature measurement. The instrument was tested against an infrared thermometer on soybean plots during September and October 2016. It was capable of removing shaded areas and soil from thermal images to produce temperature measurements more representative of the crop canopy.

微流控-超材料集成多带太赫兹传感器

微流控-超材料集成多带太赫兹传感器

Improved Color Mapping Methods for Multiband Nighttime Image Fusion

Previously, we presented two color mapping methods for the application of daytime colors to fused nighttime (e.g., intensified and longwave infrared or thermal (LWIR)) imagery. These mappings not only impart a natural daylight color appearance to multiband nighttime images but also enhance their contrast and the visibility of otherwise obscured details. As a result, it has been shown that these colorizing methods lead to an increased ease of interpretation, better discrimination and identification of materials, faster reaction times and ultimately improved situational awareness. A crucial step in the proposed coloring process is the choice of a suitable color mapping scheme. When both daytime color images and multiband sensor images of the same scene are available, the color mapping can be derived from matching image samples (i.e., by relating color values to sensor output signal intensities in a sample-based approach). When no exact matching reference images are available, the color transformation can be derived from the first-order statistical properties of the reference image and the multiband sensor image. In the current study, we investigated new color fusion schemes that combine the advantages of both methods (i.e., the efficiency and color constancy of the sample-based method with the ability of the statistical method to use the image of a different but somewhat similar scene as a reference image), using the correspondence between multiband sensor values and daytime colors (sample-based method) in a smooth transformation (statistical method). We designed and evaluated three new fusion schemes that focus on (i) a closer match with the daytime luminances; (ii) an improved saliency of hot targets; and (iii) an improved discriminability of materials. We performed both qualitative and quantitative analyses to assess the weak and strong points of all methods.

A Dark Target Algorithm for the GOSAT TANSO-CAI Sensor in Aerosol Optical Depth Retrieval over Land

Cloud and Aerosol Imager (CAI) onboard the Greenhouse Gases Observing Satellite (GOSAT) is a multi-band sensor designed to observe and acquire information on clouds and aerosols. In order to retrieve aerosol optical depth (AOD) over land from the CAI sensor, a Dark Target (DT) algorithm for GOSAT CAI was developed based on the strategy of the Moderate Resolution Imaging Spectroradiometer (MODIS) DT algorithm. When retrieving AOD from satellite platforms, determining surface contributions is a major challenge. In the MODIS DT algorithm, surface signals in the visible wavelengths are estimated based on the relationships between visible channels and shortwave infrared (SWIR) near the 2.1 µm channel. However, the CAI only has a 1.6 µm band to cover the SWIR wavelengths. To resolve the difficulties in determining surface reflectance caused by the lack of 2.1 μm band data, we attempted to analyze the relationship between reflectance at 1.6 µm and at 2.1 µm. We did this using the MODIS surface reflectance product and then connecting the reflectances at 1.6 µm and the visible bands based on the empirical relationship between reflectances at 2.1 µm and the visible bands. We found that the reflectance relationship between 1.6 µm and 2.1 µm is typically dependent on the vegetation conditions, and that reflectances at 2.1 µm can be parameterized as a function of 1.6 µm reflectance and the Vegetation Index (VI). Based on our experimental results, an Aerosol Free Vegetation Index (AFRI2.1)-based regression function connecting the 1.6 µm and 2.1 µm bands was summarized. Under light aerosol loading (AOD at 0.55 µm < 0.1), the 2.1 µm reflectance derived by our method has an extremely high correlation with the true 2.1 µm reflectance (r-value = 0.928). Similar to the MODIS DT algorithms (Collection 5 and Collection 6), a CAI-applicable approach that uses AFRI2.1 and the scattering angle to account for the visible surface signals was proposed. It was then applied to the CAI sensor for AOD retrieval; the retrievals were validated by comparisons with ground-level measurements from Aerosol Robotic Network (AERONET) sites. Validations show that retrievals from the CAI have high agreement with the AERONET measurements, with an r-value of 0.922, and 69.2% of the AOD retrieved data falling within the expected error envelope of ± (0.1 + 15% AODAERONET).

Dark Mode Driven Extra-narrow and Multiband Absorber

Recently, metamaterial absorbers have received tremendous amount of interest because of their remarkable ability to manipulate the amplitude, phase, and polarization of light. However, most absorbers rely on the direct coupling of electric or magnetic field with external excitation, which lead to inevitable energy leakage to the surrounding environment and depress the quality factor of the structure. In this work, we investigate the multiband absorption property by exciting dark plasmonic modes in reflective symmetric and asymmetric metamaterials. Theoretically, the existence of dark plasmonic modes in asymmetric metamaterials is unambiguously illustrated by the improved eigen-mode theory. With the introduction of asymmetry, dark modes in metamaterials can be easily excited by normal incident plane wave. Moreover, we also directly excite the dark modes in symmetric absorber with oblique incidence. The dark modes splitting mechanism is also clarified with the excitation of designer surface plasmon. Dominated by magnetic dipole or higher-order multipole, these dark modes possess high quality factors (Q). Numerical results indicate that the metamaterial absorber maintains high absorbance within a wide-angle incidence (0~50°). The high Q asymmetric metamaterial absorber can be an excellent candidate for multiband plasmonic sensor.