Ultralow refractive index substrates-a base for photonic crystal slab waveguides

Out of plane radiation losses in two-dimensional ( 2 D ) photonic crystal ( PC ) waveguides occur due to a lack of total internal reflection at the core-substrate∕superstrate boundaries. In order to minimize these losses, either a high vertical refractive index contrast or deep etching into the substrate is required [ G. Boettger , C. Liguda , M. Schmidt , and M. Eich , Appl. Phys. Lett. 81 , 2517 ( 2002 ) ]. The maximum vertical contrast is achieved in air bridge type PC waveguides, which are inherently fragile. In this article, we introduce a concept which combines the advantages of a high vertical index contrast of an air bridge with those of a solid substrate. This approach consists of mesoporous silica as substrate material with an ultralow refractive index ( n = 1.14 at 1.3 μ m ), close to that of air. Finite 2 D PC line defect resonators consisting of an optical polymer as core and mesoporous silica as substrate were fabricated. Compared to ordinary substrates with higher refractive indices like silica or amorphous Teflon [ C. Liguda , G. Boettger , A. Kuligk , M. Eich , H. Roth , J. Kunert , W. Morgenroth , H. Elsner , and H. G. Meyer , Appl. Phy. Lett. 78 , 2434 ( 2001 ) ], measurements show significantly increased transmission at the resonance wavelength and virtually no losses on the sensitive air band side.