Sensing ultrasound optically with photonic crystal slabs: a tale of two mechanisms (Conference Presentation)

Abstract

Silicon Photonics-based sensors can provide low cost, high sensitivity optical detection solutions in ultrasound and photoacoustic (PA) imaging. We demonstrate experimentally the measurement of ultrasound (2.5-8.5 MHz) in water using photonic crystal slab (PCS) nanostructure devices. Each PCS is composed of a periodic array of nanoholes, etched into silicon nitride (t=160 nm), on top of a silicon dioxide layer, and silicon substrate. The PCS devices have guided resonances that peak at ~ 1550 nm, with linewidths that vary from 0.7 to 5.5 nm. One type of PCS device includes a PCS nanostructure located above a thin micro-fabricated silicon membrane (~ 10 micron thick). Membrane deformation by incoming ultrasound waves induce resonance changes in the PCS spectral peak location (i.e., drum effect). We observe these drum-effect PCS devices to have acoustic sensitivities that are very narrowband (with bandwidths ~ 1 MHz), with a 300-micron diameter drum device found to have a peak sensitivity at 5 MHz and a noise equivalent pressure (NEP) of 2.0 kPa (72 Pa/rt Hz). In another mechanism, the sensitivity of the PCS nanostructures to changes in the ambient index of refraction is used. A pressure wave in water that impinges the PCS is accompanied by changes in the water's index of refraction, which causes the resonance peak of the PCS to shift. The acoustic sensitivities of these PCS devices is found to be broadband (> 6 MHz), in contrast to the drum-effect devices, with an NEP of less than 0.5 kPa (6.7 Pa/rt Hz). These devices can potentially allow for optics-based monolithic ultrasound sensor arrays, optimized for PA imaging.