Visible-light metamaterials with a controllable refractive index based on sub-skin depth nanostructures

Abstract

For the last decade, metamaterials and metasurfaces have attracted much attention because of their extraordinary properties. They can achieve distinguishable performances beyond nature materials. In order to fabricate metasurface working at the visible wavelengths, metal nanostructures smaller than the skin depth scales (tens of nanometers) are needed. First, the size effect on metal nanostructures is analyzed. When the size of the material becomes a nanometer scale, the material properties are changed from bulk’s properties due to the high surface to volume ratio. So the influence of size effect on the final optical performance of the fabricated visible metasurface is analyzed. Second, new design principle for high refractive index metasurface in visible wavelengths is proposed based on effective medium theory and the large difference between Thomas-Fermi screening length of longitudinal electric field and penetration depth (skin depth) of the transverse electromagnetic field. Using Finite-Difference Time-Domain simulations, the proposed design principle is numerically verified and experimentally demonstrated with two different experiments: close-packed gold nanoparticle array and strip nanostructures with aluminum thin films. The refractive index (n) is the single most important optical property, as it governs how materials emit, diffract, refract, and absorb light. The ultimate resolutions in optical microscopy and photolithography are also directly determined by this index. Here, a self-assembled metal nanoparticle array can possess a record-high refractive index over broad wavelength ranges in the visible and infrared regions, with a peak value of 5.0 and a broadband value which exceeds 4.2. This optical material is realized by monodispersed nanoparticle synthesis and evaporation-induced close-packed array formation, with a potential application to large, curved surfaces and which do not require any costly top-down patterning steps. Moreover, the refractive index can be tuned in real time by stretching the material or by substituting the encapsulation material. On the other hand, strip nanostructures with aluminum thin film and hafnium oxide were designed and optimized for CMOS process compatible structures. The optimized structure can achieve a refractive index of 4.0 overall visible wavelengths regime.