Perforated periodic nanostructures (i.e. nanohole arrays) have become of great alternatives for transmissive structural coloration due to high transmission efficiency and high sensitivity upon incident angles. However, structural colors of conventional periodic nanostructures inevitably exhibit a substantial color-crosstalk due to the multiple resonances. Our previous work (M.-S. Ahn et al., nanoscale) had already reported that the complementary plasmonic structures (CPS) effectively attenuate the high-order resonances, and thus improve color-purity in the range from VIS (red) to NIR.
In this work, we successfully demonstrated transmissive structural coloration with high color-purity in fully visible ranges by using inverted CPS (iCPS) of aluminum (Al) nanoholes and nanodisks. Unlike previous Ag CPS, the Al iCPS features inverted configuration of suspended nanoholes and buried nanodisks by high refractive index (polyurethane acrylate; PUA) substrate, which blue-shifts the resonances of Al nanoholes and redshifts the extinction dip of Al nanodisks. As a result, carefully engineered extinction dip effectively suppresses the first-order resonance of Al nanoholes, and thus iCPS exhibit a pure visible-coloration with a single resonance, depending on the incident angles.
iCPS were nanofabricated by UV nanoimprinting lithography (UV-NIL) and thermal evaporation of aluminum, which enables uniform nanopattering with inch scale. Polyurethane acrylate (PUA) was used for a substrate due to its high refractive index and UV-curable property. After the replication of nanohole patterns into PUA, aluminum was evaporated on PUA nanohole substrate. Then the Al nanoholes are formed on top surface of PUA and Al nanodisks are buried in PUA nanohole. Structural coloration of iCPS provides a new direction for a tunable optical filter that highly requires tunability and selectivity.