A silicon single-crystal cryogenic optical resonator
  We report on the demonstration and characterization of a silicon optical resonator for laser frequency stabilization, operating in the deep cryogenic regime at temperatures as low as 1.5 K. Robust operation was achieved, with absolute frequency drift less than 20 Hz over 1 hour. This stability allowed sensitive measurements of the resonator thermal expansion coefficient ($\alpha$). We found $\alpha=4.6\times10^{-13}$ ${\rm K^{-1}}$ at 1.6 K. At 16.8 K $\alpha$ vanishes, with a derivative equal to $-6\times10^{-10}$ ${\rm K}^{-2}$. The temperature of the resonator was stabilized to a level below 10 $\mu$K for averaging times longer than 20 s. The sensitivity of the resonator frequency to a variation of the laser power was also studied. The corresponding sensitivities and the expected Brownian noise indicate that this system should enable frequency stabilization of lasers at the low-$10^{-17}$ level. 
Eugen Wiens, Qun-Feng Chen, Ingo Ernsting, Heiko Luckmann, Ulrich   Rosowski, Alexander Nevsky, and Stephan Schiller
physics.atom-ph physics.optics