Imaging through plasmonic nanoparticles Mehbuba Tanzida, Ali Sobhania, Christopher J. DeSantisb, Yao Cuib, Nathaniel J. Hoganc, Adam Samaniegoa, Ashok Veeraraghavana, and Naomi J. Halasa,b,c,1 aDepartment of Electrical and Computer Engineering, Rice University, Houston, TX 77005; bDepartment of Chemistry, Rice University, Houston, TX 77005; and cDepartment of Physics and Astronomy, Rice University, Houston, TX 77005 Contributed by Naomi J. Halas, March 31, 2016 (sent for review March 2, 2016; reviewed by Alan C. Bovik and Brendan G. DeLacy) The optical properties of metallic nanoparticles with plasmon image are attenuated by transmission through the nanoparticle- resonances have been studied extensively, typically by measuring based medium. These two approaches provide insight toward the transmission of light, as a function of wavelength, through a understanding how our ability to perceive and resolve an image nanoparticle suspension. One question that has not yet been transmitted through a nanoparticle-based medium is dependent addressed, however, is how an image is transmitted through such upon the specific properties of the nanoparticles themselves in a suspension of absorber-scatterers, in other words, how the addition to the spatial frequencies of the image. various spatial frequencies are attenuated as they pass through The problem under investigation is illustrated schematically in the nanoparticle host medium. Here, we examine how the optical Fig. 1. A medium consisting of suspended plasmonic nanoparticles properties of a suspension of plasmonic nanoparticles affect the is placed between an object and the image plane of an observer, transmitted image. We use two distinct ways to assess transmitted or, alternatively, an imaging system. The extinction spectrum of image quality: the structural similarity index (SSIM), a perceptual the plasmonic medium alone provides valuable but incomplete distortion metric based on the human visual system, and the information concerning the quality of the transmitted image. The modulation transfer function (MTF), which assesses the resolvable spatial frequencies. We show that perceived image quality, as well spectrum does not provide the specific wavelength-dependent as spatial resolution, are both dependent on the scattering and scattering and absorption properties of the nanoparticles, nor does absorption cross-sections of the constituent nanoparticles. Sur- it account for the fact that different spatial frequencies in an image prisingly, we observe a nonlinear dependence of image quality on are differentially transmitted or attenuated while passing through optical density by varying optical path length and nanoparticle the medium. concentration. This work is a first step toward understanding the Qualitatively, in wavelength regions where the extinction is low, requirements for visualizing and resolving objects through media we would expect the transmitted image to be more easily visible consisting of subwavelength absorber-scatterer structures, an than for wavelength regions of higher extinction. This extinction approach that should also prove useful in the assessment of will be vastly different for different spatial frequencies—higher metamaterial or metasurface-based optical imaging systems. spatial frequencies will invariably be attenuated far more than lower spatial frequencies. To quantitatively assess the correlation light scattering | imaging | nanoparticles | plasmonics | metamaterials between image visibility and nanoparticle optical properties, we acquired transmitted images at discrete wavelengths across the ur understanding of light scattering in complex media is visible spectrum. We then analyzed these images using compu- Obuilt almost exclusively on the interaction of light with tational image-processing tools to correlate nanoparticle optical broadband low-loss scatterers, characteristic of many natural properties with human-eye image perception quantitatively. systems such as animal tissue, fog, or sea spray (1–6). In contrast, metallic nanoparticles possess plasmon resonances with strongly Significance frequency-dependent absorption and scattering cross-sections, where the resonant frequency is controlled by nanoparticle size, How is an image transmitted through a material consisting of – shape, and local dielectric environment (7 9). This characteristic subwavelength structures? We use two distinct methods to enables the predictive design and fabrication of nanoparticles obtain quantitative answers to that question. The first, struc- and nanoparticle-based media with specific absorption and tural similarity index, is a method related to human perception, scattering properties at precise wavelengths of choice throughout initially developed to quantify transmitted image quality fol- the UV, visible, and infrared regions of the spectrum (10–16). lowing image compression in digital image transmission sys- Since their use in antiquity as the vivid colorants in stained glass tems. The second method treats the medium as an optical windows, this frequency dependence has been of primary interest. component itself, where we determine the spatial frequency In the context of modern optics, plasmon-resonant lineshapes are content of the image transmitted by the medium. This study studied extensively through spectroscopic measurements. How- opens the door to analyzing images transmitted through par- ever, the problem of resolving an image through a suspension of ticulate media that absorb and/or scatter light, which applies plasmonic nanoparticles, for example, dispersed in an otherwise generally to imaging systems whose components are composed transparent solid or liquid medium, or patterned onto a trans- of subwavelength structures, such as those composed of random parent substrate, has yet to be investigated. Here, we examine how particulates or nanoengineered flat-optics metasurface lenses. the properties of plasmonic nanoparticles in dilute suspension modify a transmitted image. To do so, we use two very different Author contributions: M.T., A. Sobhani, A.V., and N. J. Halas designed research; M.T. and A. Sobhani performed the experiments; C.J.D. synthesized nanorods for the experiments; strategies. First, we determine the Structural SIMilarity index M.T., Y.C., N. J. Hogan, and A. Samaniego analyzed data; and M.T., A.V., and N. J. Halas (SSIM) of the plasmonic medium-transmitted image relative to wrote the paper. the original image (17). This technique allows us to evaluate how Reviewers: A.C.B., University of Texas; and B.G.D., US Army Edgewood Chemical an image is distorted upon transmission by applying a quantita- Biological Center. tive metric related directly to human visual perception. Second, The authors declare no conflict of interest. we consider the plasmonic nanoparticle medium as if it were Freely available online through the PNAS open access option. an optical component and evaluate the medium’s modulation 1To whom correspondence should be addressed. Email: [email protected]. transfer function (MTF) (18). This approach provides quantita- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. tive information concerning how various spatial frequencies in an 1073/pnas.1603536113/-/DCSupplemental. 5558–5563 | PNAS | May 17, 2016 | vol. 113 | no. 20 www.pnas.org/cgi/doi/10.1073/pnas.1603536113 Downloaded by guest on October 1, 2021 wavelength-selective bandpass filters (FWHM, ±10 nm; FB series; Thorlabs) spanning the wavelength range of 450–900 nm were used. For imaging, a CCD camera (PIXIS 400; Princeton Instruments) sensitive in the 400 to 1,100 nm wavelength range was used. The object was a 1951 US Air Force (USAF) reso- lution target conforming to the military standard (MIL-STD- 150A) (SI Appendix,Fig.2). Nanoparticle suspension in a quartz cuvette was placed directly in front of the camera so that the suspension influenced only the target image and not the illumination source. A suspension of plasmonic nanoparticles was designed and prepared with spectrally distinct transmission maxima and min- ima (Fig. 2). The suspension consisted of a mixture of Au nanorods of two different sizes and aspect ratios corresponding Fig. 1. Imaging through a medium of plasmonic nanoparticles. An image of A the object (Left), under white light illumination, is transmitted through a to two spectrally distinct dipolar plasmon modes (Fig. 2 ). One suspension of nanoparticles with a wavelength-dependent extinction spec- type of Au nanorods were 20 nm × 45 nm in size with a dipolar trum. In this case, the suspension has a transmission maximum at green wavelength (G) and transmission minima at blue (B) and red (R) wavelengths. Here, we examine how the observer perceives the transmitted image in these different wavelength regions and how the absorption and scattering prop- erties of the nanoparticles affect the perceived image. The experiment consisted of a bright field Kӧhler illumination setup for imaging an object through plasmonic nanoparticle sus- pensions (details shown in SI Appendix,Fig.1). A fiber optic white light illumination source (MI-150; Edmund Optics) with a series of ENGINEERING Fig. 2. Mixture of two nanorod solutions for imaging through a complex resonant medium. (A) Measured extinction spectrum of the mixture of Fig. 3. SSIM quality metric for images obtained through nanoparticle 20 nm × 45 nm and 30 nm × 120 nm Au nanorod solutions. (B) Measured solution. (A) Normalized extinction spectrum of nanorod-mixture sus- extinction spectrum