Fig. 1. (a) Physical implementation of the coupled-microtoroid cavity system. Here, only microtoroid B is coupled to the fiber taper. (b) Scanning electron microscope (SEM) image of microtoroid B with a large pillar diameter of about 20 μm. (c) SEM image of microtoroid A with a small pillar diameter of about 0.8 μm.

Fig. 2. (a) and (b) Normalized optical transmission spectra of microtoroids A and B with their corresponding intrinsic optical quality factors of 9.7×107 and 2.5×107 at the wavelength of 1535 nm, respectively. (c) Set of representative optical spectra taken over a range of spatial separation distance, which exhibit controllable splitting of the two supermodes in coupled microtoroids.

Fig. 3. (a) Schematic diagram of the phonon laser experimental setup with coupled silica microtoroid cavities. VOA, variable optical attenuator; PC, polarization controller. (b) Measured power spectrum of microtoroid A, which gives a mechanical quality factor of 18,000 in vacuum for the fundamental radial-breathing mode of 59.2 MHz. (c) Numerical FEM simulation of the fundamental radial-breathing mode.

Fig. 4. (a) By carefully tuning the parameters, the system is operated to produce two supermodes with frequency splitting equal to the mechanical frequency of microtoroid A. (b) Typical optical transmitted optical power for input pump powers above the threshold. (c) Mechanical oscillation amplitude as a function of input optical pump power for the compound microtoroid resonators in vacuum, where the phonon lasing threshold is as low as 1.2 μW.

Fig. 5. (a) Typical RF spectra with optical pump power below (blue) and above (red) the threshold. (b) Mechanical energy as a function of input optical pump power.