  We provide a compact 200 line MATLAB code demonstrating how topology
optimization (TopOpt) as an inverse design tool may be used in photonics,
targeting the design of two-dimensional dielectric metalenses and a metallic
reflector as examples. The physics model is solved using the finite element
method, and the code utilizes MATLAB's fmincon algorithm to solve the
optimization problem. In addition to presenting the code itself, we briefly
discuss a number of extensions and provide the code required to implement some
of these. Finally, we demonstrate the superiority of using a gradient-based
method compared to a genetic-algorithm-based method (using MATLAB's ga
algorithm) for solving inverse design problems in photonics. The MATLAB
software is freely available in the paper and may be downloaded from
https://www.topopt.mek.dtu.dk.
