Abstract
The generation of sub-diffraction-limit focal spots is essential for nanoscale manufacturing, and tight focusing of radially polarized beams provides a promising route to achieve this goal through the longitudinal field. This work proposes an annular-modulation-enhanced focusing method that combines numerical aperture (NA) optimization, annular ring width control, and Bessel–Gaussian beam parameter tuning to enhance the longitudinal field and compress the focal spot. The method selectively transmits high-angle components that contribute dominantly to the longitudinal field, while filtering out low-angle components that broaden the focal spot. The physical boundary of the effective regulation window was identified through parameter space mapping. By optimizing the parameters, the simulated longitudinal field content exceeded 85%, and the focal spot size was reduced below a 0.2 wavelength (λ). Moreover, under far-field conditions, etching was performed on single-crystal silicon to achieve a straight line with a width of approximately 254 nm, corresponding to approximately λ/4. These results indicate that Bessel–Gaussian beam annular modulation can not only achieve sub-wavelength spots but also has excellent potential in the field of nanomanufacturing.