  Suspended monolayer transition metal-dichalcogenides (TMD) are membranes that
combine ultra-low mass and exceptional optical properties, making them
intriguing materials for opto-mechanical applications. However, the low
measured quality factor of TMD resonators has been a roadblock so far. Here, we
report an ultrasensitive optical readout of monolayer TMD resonators that
allows us to reveal their mechanical properties at cryogenic temperatures. We
find that the quality factor of monolayer WSe2 resonators greatly increases
below room temperature, reaching values as high as 1.6E^4 at liquid nitrogen
temperature and 4.7E^4 at liquid helium temperature. This surpasses the quality
factor of monolayer graphene resonators with similar surface areas. Upon
cooling the resonator, the resonant frequency increases significantly due to
the thermal contraction of the WSe2 lattice. These measurements allow us to
experimentally study the thermal expansion coefficient of WSe2 monolayers for
the first time. High Q-factors are also found in resonators based on MoS2 and
MoSe2 monolayers. The high quality-factor found in this work opens new
possibilities for coupling mechanical vibrational states to two-dimensional
excitons, valley pseudospins, and single quantum emitters, and for quantum
opto-mechanical experiments based on the Casimir interaction.
