Recently, it was noted that losses in plasmonics can also enable several useful optical functionalities. One class of structures that can maximize absorption are metal insulator metal systems. Here, we study 3-layer systems with a nano-composite metal layer as top layer. These systems can absorb almost 100% of light at visible frequencies, even though they contain only dielectrics and highly reflecting metals. We elucidate the underlying physical phenomenon that leads to this extraordinary high and broadband absorption. A comprehensive study of the particle material and shape, mirror material and dielectric spacer thickness is provided to identify their influence on the overall absorption. Thus, we can provide detailed design guidelines for realizing optical functionalities that require near-perfect absorption over specific wavelength bands. Our results reveal the strong role of lossy Fabry-Perot interference within these systems despite their thickness being well below half a wavelength.
Producing colors with colorless metals such as silver or aluminum is a fascinating application of plasmonic systems. [1][2][3][4][5][6] Such systems can be as simple as a single metal particle, where material, size, shape, and refractive index of the surrounding medium determine the color impression of the particle. [7][8][9][10][11] Indeed, all these parameters influence the absorption amplitude and the wavelength of the localized plasmon resonance within the particle. [12,13] On the other hand, one can also use a system supporting propagating surface plasmon-polaritons or in short surface plasmons. [14][15][16] These systems are often periodic at a length scale below the wavelength of visible light and have been optimized for color generation in transmission [17][18][19] as well as reflection, [20,21] where recently polarization-tunable colors were demonstrated. [22,23] In that case of periodic systems, the color is mainly determined by the material, periodicity, shape, and the refractive index of the surrounding medium. [24] The color generation of plasmonic systems is based on absorption due to the excitation of surface plasmons and not on an interference-based effect, where the periodicity determines the wavelengths which can excite surface plasmons. [14] In this article, we generate vivid colors by strongly enhanced absorption of dye lacquers applied conformally on square crossed metallic grating surfaces with identical periodicity in both dimensions. [25] Unlike previously published works, we focus here not only on the generation of color by the grating itself, [20,21] but explore its possible interactions with a dye lacquer to strongly enhance the dye absorption. Instead of the conventional approach that consists in embedding a plasmonic system in a nonabsorbing dielectric medium, [1,20] we apply a partially absorbing dye lacquer conformally and in direct contact to the grating surface.This idea is inspired by works of others, who measured a significant fluorescence emission enhancement for molecules placed at well-defined distances atop a metallic grating. [26,27] This concept was also used for light extraction to improve the efficiency of a light-emitting diode. [28] Instead of fluorescence emission, we focus here on the enhanced absorption of submicrometer thick (semi-) transparent dye lacquers applied conformally to metallic crossed gratings. Rather than varying the distance of the absorbing material to the metal surface, we vary the thickness and extinction coefficient κ of these lacquers. This detailed study reveals the mechanisms of enhancing absorption within the dye lacquer. The findings are not limited to color generation and may find applications in other fields, such as solar cell absorption improvement, [29] Raman spectroscopy, [30] or highly sensitive sensing applications. [31] 2. Results and Discussion 2.1. Measurement and Simulation of Gratings Self-Color
A laser speckle contrast imaging (LSI) setup has been designed and used to estimate heartbeat rate and microvascular perfusion non-invasively. LSI measurements were performed on the human index finger and thumb during various finger perfusion conditions, such as before and after gently rubbing of a finger or the healing of a small inflammation of the eponychium on the finger. Heartbeat was retrieved with 0.5 to 10ms exposure time using LASCA (Laser Speckle Contrast Analysis) and dLASCA (dynamic Laser Speckle Contrast Analysis) processing methods. Additionally, a noise analysis model for laser speckle contrast imaging was established to evaluate Signal to Noise Ratio (SNR) in speckle imaging and provide a guideline for camera selection and imaging system design.
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