Micromirrors with electrically tunable curvature

Micromirrors with current-controlled curvature have been fabricated and tested. The principle behind the devices is that resistive heating changes the temperature of the micromirrors, causing a change in optical coating stress. For highly symmetric mirrors, the variable stress induces a uniform and reversible change in curvature. The mirrors used in this study were surface-micromachined from polysilicon and had gold coatings. The radius of curvature of a typical mirror could be tuned from 2.5 mm to 8.2 mm over a current-induced temperature range from 22°C to 145°C (estimated). The corresponding change in focus over this range, as measured by the Seidel parameters of the wavefront, was 0.38λ while the largest aberration (astigmatism) varied from its zero current value by less than 0.02λ. These results should be readily extendable to tuning or correcting flat mirrors by changing the design stress of the reflective coating.