Monocrystalline $AlxGa1−xAs$ heterostructures for high-reflectivity high-$Q$ micromechanical resonators in the megahertz regime

We present high-performance megahertz micromechanical oscillators based on freestanding epitaxial $AlxGa1−xAs$ 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 $4K$⁠. 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.