Ultra-sharp nanofocusing of graded index photonic crystal-based lenses perforated with optimized single defect

The single defect structure of a two-dimensional graded index photonic crystal (PC) is investigated. By introduction of an air hole located at center of the photonic crystalbased lenses, we can obtain an extremely small focusing spot, sited at full-width and half maximum (FWHM) as fine as λ/75, which is positioned at the subsurface and top surface of the PCs respectively. Computational calculations were performed on the basis of finitedifferent time-domain (FDTD) algorithm for the purpose of verifying the feasibility of our design. To study influence of the defect on nanofocusing property of the PC lenses, we set different length of the air holes at center of the PC lenses. The influence of wavelength and material on the nanofocusing performance of the PC lenses is discussed. New applications in optoelectronic devices, nanometrology, bioimaging, and biosensing from the graded index of PC lenses is possible.