An apparatus and method for switching optical signal is disclosed. An optical signal switch array includes a plurality of optical switches, each optical switch having a first trench adapted to hold index-matching fluid, and a second trench adapted to hold the index-matching fluid, but initially containing gas, the second trench connected to said first trench. A first expansion chamber is connected to the first trench, the first expansion chamber adapted to hold gas. A second expansion chamber is connected to the second trench, the second expansion chamber adapted to hold gas. To effect switching, the first expansion chamber is heated expanding gas in the first expansion chamber, the expanding gas shifting the index-matching fluid in the first trench to the second trench. Bistability is achieved by the use of capillary action, which passively holds the droplet of working fluid in either of two stable states until actuation.