<p>Fresnel zone plates (FZPs) are significant optical objects for extreme ultraviolet and x-ray microscopy, both of which are powerful tools due to their high spatial resolution and large penetration depth. Fabrication of FZPs is challenging as it requires high accuracy to achieve the narrow outmost zone and large aspect ratio. The focused ion beam (FIB) has been practically demonstrated as a potential alternative to electron beam lithography for its precision, rapidness, and mask-free essence. However, the common redeposition effect in the FIB process is a major obstacle to distort actual patterns from designed ones, especially for FZPs with outmost zone width of 100 nm and simultaneously aspect ratio beyond 2. We proposed an efficient method noted as single-pixel line-assisted sputtering (SLAS) to eliminate redeposition <italic>in situ</italic> during FZP fabrication. Through systematically optimizing parameters predominating ion dosages and, particularly, the positions of SLAS, we successfully obtained high-quality FZPs with outmost zone width down to 100 nm, perpendicular side walls, aspect ratio of up to 3 and diameter of 38 μm in 1.5 h. The application of SLAS also provides instructive guidance for other FIB processes to finely modulate the nanostructures.</p>
In this paper the single mode condition of silicon-on-insulator (SOI) rib waveguide with large cross section is investigated based on the effective index method (EIM) by using numerical computation and analytical derivation with the consideration of the polarization effects. A polarized single-mode condition for SOI rib waveguide with large cross section is presented, the results from analytical derivation are highly concordant with that from numerical computation. For the vertical single-mode condition, the deviations between HE and EH modes correlate oppositely with the total rib height of rib waveguide, and the critical rib height ratio gradually approaches but never equals to 0.5 with the increase of the total rib height. There, HE mode and EH mode are commonly known as quasi-transverse-electric (TE) mode and quasi-transverse-magnetic (TM) mode respectively. The deviation of the critical rib width between HE and EH modes for the lateral single-mode condition is relatively small, which is a function of the rib height ratio but irrelevant to the total rib height for the specified index profile. The fact that the total rib height, index profile, and polarization of modes have effects on the single-mode condition of SOI rib waveguide with large cross section was demonstrated in this work, which was not discussed in the previous works. The results in this work can give guidance to design, simulation and fabrication of SOI rib waveguide with large cross section in practical applications.