Material Conductivity: Material with Variable Ohmic Resistance

[sondreL]

Is there any such material which can be made to conduct from a non conducting state by external means like shining a light or any other EM wavelength on it or other means , that could alter the material ohmic resistance to electric current

 

[BOAS]

Do you mean something like an LDR?

http://en.wikipedia.org/wiki/Photoresistor