Selective UV Reflecting Mirrors Based on Nanoparticle Multilayers

A new type of nanostructured selective ultraviolet (UV) reflecting mirror is presented. Periodic porous multilayers with photonic crystal properties are built by spin-coating-assisted layer-by-layer deposition of colloidal suspensions of nanoparticles of ZrO2 and SiO2 (electronic band gap at < 220 nm). These optical filters are designed to block well-defined wavelength ranges of the UVA, UVB, and UVC regions of the electromagnetic spectrum while preserving transparency in the visible. The shielding against those spectral regions arises exclusively from optical interference phenomena and depends only on the number of stacked layers and the refractive index contrast between them. In addition, it is shown that the accessible pore network of the as-deposited multilayer allows preparing thin, flexible, self-standing, transferable, and adaptable selective UV filters by polymer infiltration, without significantly losing reflectance intensity, i.e., preserving the dielectric contrast. These films offer a degree of protection comparable to that of traditional ones, without any foreseeable unwanted secondary effects, such as photodegradation, increase of local temperature or, as is the case for organic absorbers, generation of free radicals, all of which are caused by light absorption.