Figure 1: Coupled photonic crystal nanobeam cavities.

(a) Scanning electron microscope image showing the complete device with the SU-8 coupling pads, balanced Mach–Zehnder interferometer arms, silicon waveguides and the suspended nanobeam cavity region. Scale bar=10 μm. The inset shows the suspended nanocavity in the region given by the black box (inset scale bar=400 nm). (b) The red filled circles and blue filled triangles show the dispersion of the even and odd cavity modes, respectively, for various spacing between the two cavities. The even mode is highly dispersive while the odd mode is not. The device under test had a gap of 70 nm corresponding to an optomechanical coupling coefficient (g_om) of 96 GHz nm⁻¹ for the even mode and 0.73 GHz nm⁻¹ for the odd mode. The inset shows the profiles of the dominant electric field component of the two modes. The cavity modes are localized near the centre of the nanobeams. (c) Simulated transmission of the device for the even electric field profile. The corresponding optical force is in nN W⁻¹ generated by the even mode for various pump wavelengths. The negative sign indicates the attractive nature of the force. The force for the first three modes has been rescaled (multiplied by a scaling factor of 1/2,000 (red), 1/60 (yellow) and 1/4 (green)) for better comparison. The transmission spectrum and the mutual optical force between the nanobeams for low-Q modes (Q <10⁴) were calculated using finite-element simulations. The two highest-Q modes with quality factors 1.8×10⁶ and 5.1×10⁴ were treated using temporal-coupled mode theory.