Thermo‐optical switching in Si based etalons has been demonstrated in two device structures. In one experiment the switching time of a Si etalon has been reduced from milliseconds to microseconds by choosing a probe beam of shorter wavelength in an external switching configuration we reported previously [Appl. Phys. Lett.58, 2073 (1991)]. The switching time has further been improved to the ns range by the use of a 1.06 &mgr;m Nd:YAG laser pump which is presumed to give rise to a thermo‐refractive change in the Si etalon and at the same time the switching threshold energy has been reduced to ∼1 &mgr;J as compared to ∼1 mJ for a CO2laser pump. In comparing Si etalons with thicknesses of 400, 72 and 1.5 &mgr;m, we find that the 72 &mgr;m etalon exhibits the best behavior in terms of low threshold power, high speed and contrast. In addition, effects of the pump beam intensity on the signal pulse shape has been investigated which indicates a multiple interference fringe shift and transverse thermal relaxation dynamics. The second experiment used a Si/Si3N4film structure in which the index of the Si3N4is approximately the square root of the index of the Si, and the optical thickness of the Si3N4is an odd quarter wavelength multiple of the probe beam. This yielded a minimum in surface reflection. Based on an increase of the optical thickness with pump beam heating, a probe beam reflection has been switched out with a high contrast ratio (switched on/off) of 61:1. Different structures using different pump beams are discussed and the restriction of the film optical thickness has been investigated.