Microvoid channel polymer photonic crystals with large infrared stop gaps and a multitude of higher-order bandgaps fabricated by femtosecond laser drilling in solid resin

摘要

Photosensitive polymer materials are ideally suited for laser-induced micro- and nanostructuring, as structural and compositional changes are achieved already under exposure to moderate intensities of high-repetition rate ultrashort-pulsed light. Photonic crystals with bandgaps in the infrared or the visible spectral region are a particularly interesting application, because highly correlated structural elements at a size of only a few hundred nanometers are required. We fabricated infrared photonic crystals based on microvoid channels inside solid polymer material. Femtosecond-pulsed visible light was focused into UV-cured Norland NOA63 resin by a high numerical aperture objective. In the focal spot microexplosions drive the material out of the center of the focus. Void channels of 0.7-1.3 μm diameter are generated by translating the sample along a preprogrammed pathway. Woodpile structures of void channels at layer spacings of 1.6-2.6 μm and in-plane channel spacings of 1.2-1.3 μm allowed for bandgap-induced suppression of infrared transmission in the stacking direction of as much as 86% by only 20 layers. As these structures are highly correlated and do not contain many imperfections, up to three higher-order stop gaps are observed. Consistent with theory, the number and gapwidth of higher-order gaps strongly increases with the ratio between layer- and in-plane spacing. Due to their low refractive index contrast and the missing interconnectivity of voids our structures do not provide complete photonic bandgaps. However, their manifold of sizable higher-order gaps allows for the engineering of photonic stop gaps down to the near-infrared wavelength region using comparatively large structural dimensions.