Enhanced normal-direction excitation and emission of dual-emitting quantum dots on a cascaded photonic crystal surface.

Large normal-direction excitation and emission of dual-emitting quantum dots (QDs) are essential for practical application of QD sensors based on the ratiometric fluorescence response. We have numerically demonstrated an all-dielectric four-layer cascaded photonic crystal (CPC) structure (alternating TiO2 and SiO2/SU8 layers with two dimensional nanoscale patterns in each layer) which is capable of providing normal-direction high Q-factor leaky modes at excitation wavelengths of QDs and two low Q-factor leaky modes coinciding with the two emission peaks of a dual-emitting QD. Normal-direction excitation and far-field emission of the dual-emitting QDs are enhanced significantly when QDs are distributed on/in the top TiO2 layer of the CPC structure, especially in the spatial distribution areas of the resonant leaky modes. QDs can be positioned differently depending on the applications. Positioning QDs on the top TiO2 layer will improve the signal-to-noise ratios of QD biomedical/chemical/temperature sensors, while embedding QDs in the top TiO2 layer will increase the light extraction from the QD light emitting device, making our CPC a versatile optical coupling structure. Our CPC-QD structure is experimentally feasible and robust against the parameter perturbation in real fabrication.