Flexible Distributed Bragg Reflectors from Nanocolumnar Templates

Recently, another alternative route to the use of purely polymeric structures to attain fl exible Bragg mirrors, and in which enhanced dielectric contrast is achieved, has been successfully demonstrated. [ 22,23 ] Such method takes advantage of multilayers possessing an open porous and interconnected network, which arises as a result of piling up layers of nanoparticles, to infi ltrate a polymer that penetrates the multilayer structure. The polymer holds it together and, after the adequate treatment, permits the hybrid structure to be lifted off from the substrate. In this process, the optical quality of the originally deposited Bragg refl ector is preserved. Just by controlling the thickness and the composition of the layers, this approach has been proven suitable to attain fl exible DBRs of high refl ectance intensity (80% < R < 100%) and spectral width (Δλ/λ MAX = 40%) in an ample range of optical wavelengths (200 nm < λ < 1500 nm). In principle, this method is applicable to any porous optical material not as rigid as to prevent the fi nal hybrid structure to inherit the mechanical properties of the infi ltrated polymer. In this communication, we describe the preparation method and properties of a novel highly refl ecting and fl exible DBR. A porous nanocolumnar structure with photonic crystal properties, attained by oblique angle physical vapor deposition, is infi ltrated by a polymer to achieve a new hybrid nanostructure that preserves a high dielectric constant contrast with improved mechanical properties. This type of multilayer structures has been previously used as optofl uidic sensors to determine the concentration of liquid solutions or the proportion of two components in mixtures of liquids. [ 24,25 ] Structural and optical characterization demons trates the uniform infi ltration of the polymer throughout the whole interstitial network of these porous multilayers, which is crucial to the realization of a robust self-standing structure. Such self-standing fi lm could only be lifted off by depositing an intermediate layer of nanoparticles between the substrate and the porous photonic array, which both allows the growth of nanocolumns onto it and favors the detachment of the whole multilayer from the substrate after polymer infi ltration. Periodically alternated nanocolumnar arrays are typically grown on a variety of substrates by using physical vapor oblique angle deposition (PV OAD), in which the substrate is tilted with respect to the source of precursor species (known as target ) in the chamber. A columnar nanostructure with open porosity results from the shadowing imposed by regions of faster growth over others. [ 24,26,27 ] Porosities as high as 60%– 70% of the total thin-fi lm volume can be achieved by this procedure. [ 28 ] This method has been employed in several occasions before to build porous DBRs. [ 29,30 ] In our case, we use specifi cally the methods and experimental conditions reported Flexible Distributed Bragg Refl ectors from Nanocolumnar Templates