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.
In this paper, we propose a bilayer metasurface which is capable of launching helicity-inverted wave only in the forward direction. In order to obtain directional scattering characteristics of individual cells, we employed two layers of thin metasurfaces that are separated by a dielectric spacer. Multiple scattering analysis is used to derive design conditions for single metasurface reflectances for each polarization and it was shown that such target reflectances are realizable with split-ring aperture. The unit cell structure optimized for forward-only scattering of cross-polarization component is shown to have power extinction ratio as high as 32. The proposed structure can potentially form a supercell with reflective cells so that geometric phases of transmitted light and reflected light can be independently controlled. The proposed scheme is expected to pave a way to new types of metasurfaces with multiplexed optical functions.
In general, color filter is an optical component to permit the transmission of a specific color in cameras, displays, and microscopes. Each filter has its own unchangeable color because it is made by chemical materials such as dyes and pigments. Therefore, in order to express various colorful images in a display, one pixel should have three sub-pixels of red, green, and blue colors. Here, we suggest new plasmonic structure and method to change the color in a single pixel. It is comprised of a cavity and a metal nanoaperture. The optical cavity generally supports standing waves inside it, and various standing waves having different wavelength can be confined together in one cavity. On the other hand, although light cannot transmit sub-wavelength sized aperture, surface plasmons can propagate through the metal nanoaperture with high intensity due to the extraordinary transmission. If we combine the two structures, we can organize the spatial distribution of amplitudes according to wavelength of various standing waves using the cavity, and we can extract a light with specific wavelength and amplitude using the nanoaperture. Therefore, this cavity-aperture structure can simultaneously tune the color and intensity of the transmitted light through the single nanoaperture. We expect that the cavity-apertures have a potential for dynamic color pixels, micro-imaging system, and multiplexed sensors.
The plasmonics is expected to be a potential candidate of future sensing technologies, such as single-molecular detections and surface-enhanced Raman scattering, because of concentrated and enhanced electric fields at a sharp edge of a metallic nanostructure. In particular, breaking symmetry of a plasmonic structure results in anisotropic angular optical responses and allows a broad tunability of surface plasmons. In this study, we suggest a structure of asymmetric double metal caps on a dielectric nanosphere in order to increase the asymmetry of component structures and to improve the tunability of a plasmonic sensor.
Surface plasmon resonance (SPR) has been actively researched for sensor applications. Based on the subwavelength scale enhancement of light field and its sensitivity to refractive index, SPR can be used for surface enhanced Raman spectroscopy and various bio and chemical sensors. This talk will provide comparative overview of the potentials of SPR for optical sensors and its practical limitations in implementation.