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Imaging through plasmonic nanoparticles

Significance How is an image transmitted through a material consisting of subwavelength structures? We use two distinct methods to obtain quantitative answers to that question. The first, structural similarity index, is a method related to human perception, initially developed to quantify transmitted image quality following image compression in digital image transmission systems. The second method treats the medium as an optical component itself, where we determine the spatial frequency content of the image transmitted by the medium. This study opens the door to analyzing images transmitted through particulate media that absorb and/or scatter light, which applies generally to imaging systems whose components are composed of subwavelength structures, such as those composed of random particulates or nanoengineered flat-optics metasurface lenses. The optical properties of metallic nanoparticles with plasmon resonances have been studied extensively, typically by measuring the transmission of light, as a function of wavelength, through a nanoparticle suspension. One question that has not yet been addressed, however, is how an image is transmitted through such a suspension of absorber-scatterers, in other words, how the various spatial frequencies are attenuated as they pass through the nanoparticle host medium. Here, we examine how the optical properties of a suspension of plasmonic nanoparticles affect the transmitted image. We use two distinct ways to assess transmitted image quality: the structural similarity index (SSIM), a perceptual distortion metric based on the human visual system, and the modulation transfer function (MTF), which assesses the resolvable spatial frequencies. We show that perceived image quality, as well as spatial resolution, are both dependent on the scattering and absorption cross-sections of the constituent nanoparticles. Surprisingly, we observe a nonlinear dependence of image quality on optical density by varying optical path length and nanoparticle concentration. This work is a first step toward understanding the requirements for visualizing and resolving objects through media consisting of subwavelength absorber-scatterer structures, an approach that should also prove useful in the assessment of metamaterial or metasurface-based optical imaging systems.