Metallo-dielectric photonic crystals (MDPCs) are used as ultrasensitive molecular detectors for concentrations down to picomolar level based on surface-enhanced Raman spectroscopy (SERS). Calculations show that the amorphous silicon photonic crystals (a-Si PCs) embedded in multiple metallo-dielectric (MD) units can significantly increase the electromagnetic fields at the air-dielectric interface, leading to remarkable Raman enhancement. Corresponding experiments show the multiple MDPC structures can serve as an ultrasensitive SERS substrate with excellent reproducibility and stability, capable of quantitative analysis down to 10 pM level. The MDPC structure can be generalized to other applications, such as plasmonic devices, ultrasensitive sensors, and nanophotonic systems.

We fabricated ordered hexagonal-packed vertical silicon nanowire ͑SiNW͒ arrays with varying diameters of 450-900 nm and varying lengths of 0.54-7.3 m, and studied their Raman enhancement properties. We found the Raman enhancement per unit volume ͑REV͒ increased with decreasing wire diameters and oscillated with wire length, and the REV of seven 450-nm-diameter, 3-m-long SiNWs was about twice that of a single SiNW having the same size. The differences were attributed to the vertical finite-length cylinder structures of the SiNW array, as supported by finite-difference-time-domain simulation results based on the helical resonant surface wave model.