Integrated optical force sensors using focusing photonic crystal arrays

Abstract

Mechanical oscillators are at the heart of many sensor applications. Recently several groups have developed oscillators that are probed optically, fabricated from high-stress silicon nitride films. They exhibit outstanding force sensitivities of a few aN/Hz 1/2 and can also be made highly reflective, for efficient detection. The optical read-out usually requires complex experimental setups, including positioning stages and bulky cavities, making them impractical for real applications. In this paper we propose a novel way of building fully integrated all-optical force sensors based on low-loss silicon nitride mechanical resonators with a photonic crystal reflector. We can circumvent previous limitations in stability and complexity by simulating a suspended focusing photonic crystal, purely made of silicon nitride. Our design allows for an all integrated sensor, built out of a single block that integrates a full Fabry-Pérot cavity, without the need for assembly or alignment. The presented simulations will allow for a radical simplification of sensors based on high-Q silicon nitride membranes. Our results comprise, to the best of our knowledge, the first simulations of a focusing mirror made from a mechanically suspended flat membrane with subwavelength thickness. Cavity lengths between a few hundred µm and mm should be directly realizable.

Suspended silicon nitride thin films with enhanced and electrically tunable reflectivity

“…Combined with piezoelectric actuation of their mechanical modes [36], such enhanced and electrically tunable reflectivity membranes would be particularly interesting for improving and tailoring the optomechanical response of optomechanical arrays of nanomembranes [25,37] and for investigating collective and strong coupling optomechanics [21,22,38,39]. They would also be interesting for realizing tunable optical filters [35] or waveplates [40], strongly focusing lenses [41][42][43], vertical-cavity surface-emitting lasers [29,44], or optical sensors [27,28] for biophysics [45,46] and biomedical [47,48] applications. The silicon nitride membranes used in this work are commercial [49], high stress (∼GPa), 0.5 mm-square and 200 nm-thick films on a 5 mm-square, 500 µm-thick silicon frame.…”

Centimeter-scale suspended photonic crystal mirrors

“…Photonic crystal (PhC) membranes are suspended dielectric sheets patterned with sub-wavelength, low-index two-dimensional periodic structures [1]. These patterns give rise to resonances that couple out-of-plane radiation to in-plane leaky modes, and can be engineered to transform a flat membrane into a mirror [2], a lens [3], or even a curved mirror [3][4][5]. Here we study a PhC consisting of a periodic lattice of holes in a membrane, whose hole radius and lattice constant can be tuned to reflect light at a wavelength of choice.…”

Cavity Optomechanics with Photonic Bound States in the Continuum

“…Relevant to our discussions is that ultrashort cavities built from Fano mirrors have been shown to suffer less from finite-waist effects [44]. There is also the possibility of designing Fano mirrors with focusing abilities [92,93]. Furthermore, resonance-trapped BICs have been shown to display a large Q-factor over a wide range in k-space [52], and recently the merging of multiple BICs has been used to suppress out-of-plane scattering losses [54].…”