Self-Assembled Colloidal Nanopatterns toward Unnatural Optical Meta-Materials

2D nanoscale patterns have been extensively investigated for various applications including information storage devices, non-volatile memory devices, chemical or biological sensors. Recently, such nanostructured patterns have been gaining lots of attention because of their potential uses as meta-materials or meta-surfaces with unnatural optical properties. In particular, self-assembled colloidal monolayers of gold nanoparticles (NPs) have been demonstrated as optical meta-materials for unnaturally high refractive index (n_eff > 4) at optical frequency. Raspberry-like particles or spherical assembly of NPs are also investigated for negative or near-zero index of refraction. Thanks to the recent advance in colloidal chemistry for shaping NPs with controlling the interaction, non-close-packed, or even non-periodic colloidal nanostructures can be achieved, which may be potentially useful for optical meta-surfaces. In this article, a brief introduction of optical meta-materials is provided and recent progress for high-index and negative-index meta-materials are reviewed. In the second part, the meta-surfaces will be briefly described including fundamental principle and some of the key potential applications. Finally, the recent experimental progress of high-precision self-assembly for the meta-surface fabrication will be discussed.