Bonding, antibonding and tunable optical forces in asymmetric membranes

Abstract

We demonstrate that tunable attractive (bonding) and repulsive (anti-bonding) forces can arise in highly asymmetric structures coupled to external radiation, a consequence of the bonding/anti-bonding level repulsion of guided-wave resonances that was first predicted in symmetric systems. Our focus is a geometry consisting of a photonic-crystal (holey) membrane suspended above an unpatterned layered substrate, supporting planar waveguide modes that can couple via the periodic modulation of the holey membrane. Asymmetric geometries have a clear advantage in ease of fabrication and experimental characterization compared to symmetric double-membrane structures. We show that the asymmetry can also lead to unusual behavior in the force magnitudes of a bonding/antibonding pair as the membrane separation changes, including nonmonotonic dependences on the separation. We propose a computational method that obtains the entire force spectrum via a single time-domain simulation, by Fourier-transforming the response to a short pulse and thereby obtaining the frequency-dependent stress tensor. We point out that by operating with two, instead of a single frequency, these evanescent forces can be exploited to tune the spring constant of the membrane without changing its equilibrium separation

Control of buckling in large micromembranes using engineered support structures

“…The small thickness allows for greater flexibility of the membrane (to enable strong response of the structure to the weak forces), while the large area is needed to assure good confinement of guided resonances supported by the membrane. Furthermore, the membrane and the substrate are separated by as few as hundreds of nm [12], [14]- [16], in order to enhance the optomechanical coupling. For all of these reasons, it is essential to reduce buckling of the membrane due to built-in internal stress and avoid failure of the structure.…”

Nonlinear optomechanical paddle nanocavities

“…In systems with dominantly dispersive optomechanical coupling, this dependence is expressed to second-order in mechanical resonance amplitude x as ω o (x) = ω 0 +g (1) x+ 1 2 g (2) x 2 , where ω o is the cavity resonance frequency, and g (1) = δω o /δx, g (2) = δ 2 ω o /δx 2 are the first and second order optomechanical coupling coefficients. In nanophotonic devices, x parameterizes a spatially varying modification to the local dielectric constant, ∆ (r; x), whose distribution depends on the mechanical resonance shape and is responsible for modifying the frequencies of the nanocavity optical resonances.Insight into nonlinear optomechanical coupling in nanocavities is revealed by the dependence of δω (2) on the overlap between ∆ and the optical modes of the nanocavity [26,27]:where the first term is a "self-term" and gω ,ω represents cross-couplings between the fundamental mode of inter-arXiv:1412.4431v2 [quant-ph]…”

Optomechanical and photothermal interactions in suspended photonic crystal membranes

“…1(b), suspended a few hundred nanometers above a Silicon-on-Insulator (SOI) substrate and is capable of generating strong attractive and repulsive forces. 26 To fabricate our devices, we oxidize the top 35 nm of two SOI wafers (device layer thickness = 220 nm, buried oxide layer thickness = 2 μm) and bond the two oxidized surfaces together. After removal of one of the handle wafers along with its buried oxide layer, we are left with a double-device layer SOI with two thin silicon layers of thickness h = 185 nm separated by a thin oxide layer of thickness s0 = 260 nm.…”

“…In previous studies, which featured wavelength-scale mode-volumes (~ ሺߣ /݊ሻ ଷ ) and poor thermal transport properties, 30,31 the thermo-optic effect obscured the underlying optomechanics, 32 due in large part to the strong two-photon absorption in silicon at near-IR frequencies. By employing a system containing extended photonic crystal modes 26,33 with large optical mode volumes (~1500 ሺߣ /݊ሻ ଷ ) and thermal diffusion rates γt (~ 450 kHz) orders of magnitude larger than those of microcavity geometries, we avoid the problems commonly associated with silicon optomechanical systems while still achieving optical forces comparable to those achieved with micocavities.…”