Micron scale binary periodic phase plate for increased depth of focusing based on a non local analysis and design

In order to focus light at a desired depth, wavefront shaping of the incident light is required. Light focusing is used in applications such as medical imaging, sensing, power dividers, etc. For this purpose, various phase plates can be exploited, but we propose a periodic phase plate. The reason for considering periodic structures is to profit from wavefront shaping of several periods to focus light at larger depths, which can be a challenging task without a periodic phase plate. Using this method, the focus quality is such that its Full Width at Half Maximum (FWHM) reaches in the order of the diffraction limit, while maximizing power concentration. We also assume that the phase plate elements are of micron-scale, so that it does not require nanofabrication. The modelling and analysis method used in this paper accurately takes the coupling among adjacent phase plate elements into account, i.e., it does not rely on local phase approximation. Since the introduced phase plate has a periodic structure, the focusing is repeated periodically, which finds several applications. Using our computational technique, one is able to rigorously analyze and design phase plates as wide as [Formula: see text]. In a sample design, we show that [Formula: see text] of the illuminated power reaches a depth of [Formula: see text] or 1m at a wavelength of [Formula: see text]. Approximately [Formula: see text] of the incident power is focused within only [Formula: see text] of the unit cell width. Moreover, despite large depth of focusing, its FWHM is in the order of the diffraction limit. In this paper, in addition to achieving light focusing at larger depths, we also demonstrate that the proposed methodology can be utilized for focusing with a FWHM close to the diffraction limit down to [Formula: see text], double focusing, and focusing with a high Numerical Aperture (NA) of 0.97.