On the performance improvement of transmitted Bessel beams emitted from sub-wavelength annular aperture coupled with periodic grating

The lens spatial resolution and depth of focus depends on the numerical aperture and the incident light beam wavelength. Traditionally, smaller focal spot size also means short depth of focus, which may hinder the system integration advancement. We investigate the possibility to break free the limitation associated with small focal size and short depth of focus. It was discovered by T. W. Ebbesen that periodic aperture on metallic film may improve the transmission and confine the divergent angle of the emitting light beams beyond the prediction of the Bethe’s theory. We developed single sub-wavelength annular aperture (SAA) first and then used tapered hollow micro-tube to mimic the function of SAA so as to generate Bessel beam in the far-field region. High aspect ratio microstructures were fabricated using the above-mentioned techniques. In addition, coupling annular aperture and periodic grating to further advance the high aspect ratio fabrication technique was also explored. Firstly, to maximize the transmission in specific wavelength, the pitch of grating was modulated. More specifically, we manipulated the periodic grating to search the maximum transmission efficiency in specific waveband. Secondly, in order to improve the depth of focus, we changed the grating slit width to induce proper phase delay. These studies were all verified by FDTD simulations and some experimental results.