Fig. 1: Optomechanical device.

a Device assembly process. We start with three chips, on which the different structures are fabricated from thin film silicon nitride. They are patterned with (i) a photonic crystal, (ii) spacers, and (iii) center part of a mechanical structure, respectively. A fiber is used to first pick up the spacers and place them on the mechanical chip. Then the photonic crystal is stacked on top of the spacers. The sketch in the box at the bottom-right shows the cross-section of the fully assembled structure. The thickness of the spacers determines the gap distance between the photonic crystal and the mechanical resonator. b False-colored SEM image of the assembled device. c Simulation of the fundamental out-of-plane mechanical mode of the fractal structure. The mechanical motion is gradually damped from the center to the clamping points. The green dashed box highlights the part of the mechanical structure shown in (a) and (b). d Electric field distribution (top) on the upper surface of the mechanical layer and (bottom) on the center vertical plane. The green dashed line on the upper plot shows the projection of the photonic crystal. The mechanical layer in the simulation has a width of 1 µm, matching the width of the photonic crystal. e Measured power spectral density (PSD) of an assembled device, showing how clean the spectrum is around the fundamental mode, with negligible excess noise from other modes. The PSD is measured using a balanced homodyne detection scheme. The green arrow indicates the fundamental high-Q_M mechanical mode, the orange dashed lines the higher mechanical modes, while the mechanical mode of the photonic crystal is marked in purple. All other peaks are either electronic noise or mixing between modes in the read-out³¹