Sub-diffraction limited focusing with quasi-periodic photonic crystal multilayers

Direct writing lithography plays a crucial role in the fabrication of arbitrary nano-patterns. In order to improve the resolution, a quasi-periodic photonic crystal (QPC) multilayer structure is proposed for manipulating the evanescent waves, which could be used in direct writing lithography by companying with a concentric annular grating. Because of the low loss and spatial frequency selection function of the QPC multilayers, only the desired high- k diffracted waves generated by the grating could be passed through and propagated over a long distance. Eventually, a focused light beam with a sub-diffraction limited full width at half maximum (FWHM) could be formed in the final photoresist (PR) layer. The calculated results show that the FWHM of the focused light beam could be reduced to ∼96 nm (0.23λ), and the sidelobe ratio is less than 13%. In addition, the shape characteristics of the focused light beam in PR could be maintained excellently, and the penetration depth could reach 300 nm. This work presents a new applicative scene in photolithography for photonic crystals. Meanwhile, the sub-diffraction limited feature of the focused beam indicates the strategy is feasible for controlling the evanescent waves.