Out of plane radiation losses in two-dimensional (2D) 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 μm), close to that of air. Finite 2D 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.