We demonstrate that a gold split-ball resonator (SBR) in the form of a spherical nanoparticle with a cut supports both optical magnetic and acoustic modes, which have strong field confinement around the cut. Such localization away from the bottom is expected to lead to an immunity to anchor loss and thus potentially high quality factors of acoustic oscillations when positioned on a substrate. As a result, when a planewave pulse excites the optical resonance, it can then efficiently drive the acoustic vibration through laser heating and/or optical forces. We estimate the overall heat variation by modelling the optical energy dissipation inside the SBR due to the dispersive and absorbing nature of gold at optical wavelengths. The optically induced force is given by the time averaged Lorentz force density. We simulate the mechanical vibrations under the optical excitation through time-dependent simulations using solid mechanics module of COMSOL software. Assuming a moderate quality factor of 10, under a plane wave pulsed laser pump which gives 100K temperature change to the SBR, both the laser heating and optical forces lead to the excitation of the acoustic mode at the same frequency with different magnitudes of 200pm and 10pm, resulting 10% and 0.5% modification of the total optical scattering, respectively. These results show that the SBRs support strong opto-mechanical coupling and are promising in applications such as surface-enhanced Raman spectroscopy and detection of localised strain.
We discuss potential advantages of slow-light waveguides compared to cavity-based structures for enhancing opto-mechanical
interactions. Then, we reveal that slow-light enhanced optical forces between side-coupled photonic-crystal nanowire
waveguides can be flexibly controlled by introducing a relative longitudinal shift. We predict that close to the photonic
band-edge, where the group velocity is reduced, the transverse force can be tuned from repulsive to attractive, and the force
is suppressed for a particular shift value. Additionally the shift leads to symmetry breaking that can facilitate longitudinal
forces acting on the waveguides, in contrast to unshifted structures where such forces vanish.