Plasmonic nanoresonators for spectral color filters and structural colored pigments

Color perception is an important part of our daily life and is normally achieved by incorporating colored pigments or dyes into objects or display devices. These pigments elements produce colors because they absorb a certain band of visible light. In the context of display devices, chemical pigments are widely used as color filters in liquid crystal displays (LCDs). Current color filter technology has room to improve mainly in two aspects: simplifying the manufacturing process and improving brightness [1-3]. In nature, colors can also be produced by light interacting with physical structures, for 362example, the blue color of the Morpho butterfly is due to the photonic crystal-like volume-diffractive nanostructures on its wings. Inspired by these natural phenomena, structural colors can be produced by exploiting light interaction with photonic crystals [4, 5] and plasmonic nanostructures [2, 3, 6-9]. In the past years many new phenomena have been reported in plasmonic nanostructures and nanocavities and have gathered considerable interest [10-16]. By exploiting plasmonic nanostructures, such as nanohole or nanoslit arrays, efficient conversion between incident and outgoing plane waves and plasmons can be managed at the subwavelength scale and produce color-filtering functions [17]. For example, the resonance effect in a plasmonic nanohole array for filtering color has been reported by sweeping the resonant transmission peaks at the visible spectrum [9]. However, the transmission band of such filters is too broad to meet the bandwidth requirement for display and imaging applications. Other approaches for spectral filtering such as nanoslits combined with periodic grooves [8] or in a metal-insulator-metal (MIM) waveguide [18] also demonstrated color filtering. However, in these structures, two neighboring output slits have to be separated by additional structures or by specific coupling distances; therefore, the device dimension and efficiency are restricted. An even greater limitation is the low efficiency of the plasmonics-based spectrum filters reported previously (typically less than 10%), which cannot satisfy the requirement for practical display applications. Among the above efforts, filters generated by MIM waveguide resonators are of particular interest. MIM waveguide geometries have the ability to support surface plasmon (SP) modes at visible wavelengths and have been widely investigated for various applications, such as guiding waves at the subwavelength scale [19-22], concentrating light to enhance the absorption for photovoltaic applications [23, 24], achieving a near-field plate for super-resolution at optical frequency [25-28], or composing metamaterials for magnetic resonance and negative refraction [25-28]. In addition to enabling efficient subwavelength optical confinement, the top and bottom metal layers of MIM structures can be potentially used as electrodes in an electro-optic system for a compact device size. In this chapter, we introduce a number of structural color designs, and many of them can be traced 363back to the basic MIM structure. Several important aspects such as color purity, efficiency, and angle dependence will be discussed and correlated to the physical parameters of the structure.