I would like to know more about the thermo-optic effect. I've got the following down: The thermo-optic effect is the thermal modulation of the refractive index of a material. The refractive index of a material can be modulated as a function of its thermo-optic coefficient α. [tex]n(\DELTA T)=n_0 + \alpha \cdot \DELTA T .[/tex] A tunable fabry perot structure can operate by change in its refractive index as function of applied heat. But my question is what makes this thermo-optic effect. Let's say it is a thin film of amorphous Silicon, how does this change the refractive index when it is heated? I do not think it is just linear to the change in size. Does anything change internally? I would appreciate it if anybody would have more info on this.