Various optical imaging devices have been significantly developed as commercial products including digital cameras, smartphone displays, and three-dimensional microscopes in the electronic industry until now. Such a rapid development makes many people expect more advanced devices which may be not only multifunctional but also smaller and lighter. However, we cannot achieve it only by scaling down conventional optic systems due to the limits of inherent volume needed in classical optic parts. Nanophotonics can be a potential candidate to overcome the intrinsic problem. In particular, plasmonic and metasurface nanostructures have been briskly studied in recent years because they are able to control input lights within a few hundred nanometers of a thin layer. Here we introduce some representative cases of them for optical imaging. We firstly propose a cavity-aperture, which is comprised of a cavity and a metal nanoaperture, to change the color and intensity of the light transmitted through a single pixel. Because a cavity organizes various lights having different wavelengths and a nanoaperture spatially selects one of them without a serious distortion of a light field distribution, we can extract a light with a specific wavelength and amplitude using the cavity-aperture. Some metasurface nanostructures are also suggested for a broadband polarimeter, circular polarizer, directional switching, and holographic imaging. They are useful in dramatically miniaturizing optical devices due to their thin and compact sizes. We expect these plasmonic and metasurface nanostructures have a potential for advanced portable imaging systems.
We propose a highly sensitive hybrid-plasmonic sensor based on thin-gold nanoslit arrays. The transmission characteristics of gold nanoslit arrays are analyzed as changing the thickness of gold layer. The surface plasmon polariton mode excited on the sensing medium, which is sensitive to refractive index change of the sensing medium, is strengthened by reducing the thickness of the gold layer. A design rule is suggested that steeper dispersion curve of the surface plasmon polariton mode leads to higher sensitivity. For the dispersion engineering, hybrid-plasmonic structure, which consists of thin-gold nanoslit arrays, sensing region and high refractive index dielectric space is introduced. The proposed sensor structure with period of 700 nm shows the improved sensitivity up to 1080 nm/RIU (refractive index unit), and the surface sensitivity is extremely enhanced.
We numerically analyzed the refractive index sensing performance of the two-dimensional array of plasmonic V-shaped grooves. The structural parameters are optimized to show a sharp and large reflectance dip with desired sensing region (n=1.33). Acquiring the sensitivity of the environmental refractive index as 400 nm/RIU in the visible region, FWHM of the dip is ~5 nm. It shows that the proposed structure has extreme value of quality factor and good extinction ratio. The localized mode has the hot spot at the bottom of the grooves so that localized sensing based on magnetic field enhancement is possible. Moreover, the localized mode is dependent on the tapered angle of the grooves, not the opening ratio. The performance of the dual V-shaped grooves is also discussed. The array of the closely located grooves has nearly identical reflectance spectra but a moderate amount of dip shift exists. As well as obtaining refractive index sensing by this configuration, magnetic field hot spot generation by coherent excitation can be applied to highly localized sensing and enhancing nonlinear processes.
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