Evolution of mode positions and Q -factors with increasing coating layers. (a) Shift of mode (m = 70, main mode and first sub-mode) with increasing HfO2 coating layers. The dark squares and open circles represent the positions of the main mode and the first sub-mode, respectively. (b) Evolution of the Q-factor of mode (m = 70) with the coating layer. The triangles are the experimental results and the dashed line is the corresponding linear fit.