A spatial light modulator (SLM) is disclosed, wherein the SLM has a substrate comprising an array of MEMS-based mirror elements that is periodic with small pitch ( 80%) in a first dimension. Each mirror element includes a micromirror whose height above the substrate is controlled via a vertical comb-drive actuator, which is located completely beneath the micromirror in the first dimension. As a result, mirror element can control the phase of light reflected from its micromirror. In some embodiments, the SLM includes an array of mirror elements that is periodic with small pitch and high fill factor in two dimensions. In such embodiments, the actuator, as well as tethers for supporting the micromirror above the substrate, are located completely beneath the micromirror, thereby enabling high fill factor in both dimensions.