{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "# Focusing Properties of a Binary-Phase Zone Plate\n", "\n", "It is also possible to compute a [near-to-far field transformation](https://meep.readthedocs.io/en/latest/Python_User_Interface/#near-to-far-field-spectra) in cylindrical coordinates. This is demonstrated in this example for a binary-phase [zone plate](https://en.wikipedia.org/wiki/Zone_plate) which is a rotationally-symmetric diffractive lens used to focus a normally-incident planewave to a single spot.\n", "\n", "Using [scalar theory](http://zoneplate.lbl.gov/theory), the radius of the $n$th zone can be computed as:\n", "\n", "