This contribution is concerned with topology optimization of a coupled optical and mechanical problem in photonic crystals. It is motivated by the potential gain in functionality of optical devices where the mechanical loading influences the optical response by distorting the geometry and through the opto-elastic effect. This is a new conquest for the development of topology optimization methods since little work have been carried out on topology optimization of coupled problems. The practical applications of this research cover novel optical modulators, switches and frequency shifters. Furthermore the research can be seen as method development of multi-physics optimization technology which may find future use in advanced optical devices such as an optical computer.