Monocrystalline AlGaAs heterostructures for high-reflectivity high-Q   micromechanical resonators in the MHz regime
  We present high-performance megahertz micromechanical oscillators based on freestanding epitaxial AlGaAs distributed Bragg reflectors. Compared with dielectric reflectors, the low mechanical loss of the monocrystalline heterostructure gives rise to significant improvements in the achievable mechanical quality factor Q while simultaneously exhibiting near unity reflectivity. Experimental characterization yields an optical reflectivity exceeding 99.98% and mechanical quality factors up to 20 000 at 4 K. This materials system is not only an interesting candidate for optical coatings with ultralow thermal noise, but also provides a promising path toward quantum optical control of massive micromechanical mirrors. 
Garrett D. Cole, Simon Groeblacher, Katharina Gugler, Sylvain Gigan,   Markus Aspelmeyer