We describe measurement results of silicon photonic circuits at cryogenic temperatures. The interplay between optically induced heating and free carrier dynamics in nano-photonic ring resonators is investigated at temperatures down to 1.8K. We find that the life-time of free carriers generated by two-photon absorption in silicon waveguides is reduced from 1.9ns at room temperature to less than 100ps below 10K. At the same time the thermal relaxation time is significantly elongated. Our work provides the first cryogenic measurement of ultra-short free-carrier lifetimes in silicon waveguides. The results further indicate that integrated optical chips can be easily thermo-optically stabilized at low temperatures.