Abstract
Optical read-out of motion is widely used in sensing applications. Recent developments in micro- and nano-optomechanical systems have given rise to on-chip mechanical sensing platforms, potentially leading to compact and integrated optical motion sensors. However, these systems typically exploit narrow spectral resonances and therefore require tuneable lasers with narrow linewidth and low spectral noise, which makes the integration of the read-out extremely challenging. Here, we report a step towards the practical application of nanomechanical sensors, by presenting a sensor with ultrawide (∼80 nm) optical bandwidth. It is based on a nanomechanical, three-dimensional directional coupler with integrated dual-channel waveguide photodiodes, and displays small displacement imprecision of only 45 fm/Hz1/2 as well as large dynamic range (>30 nm). The broad optical bandwidth releases the need for a tuneable laser and the on-chip photocurrent read-out replaces the external detector, opening the way to fully-integrated nanomechanical sensors.
Highlights
Optical read-out of motion is widely used in sensing applications
Nanophotonic cavities have shown the potential for highresolution displacement sensing on an integrated chip3–12. sensitivity is realized by micro-rings[3,8], micro-disks[4,5,6,7] and photonic crystal cavities[9,10,11,12] with optical quality factors up to hundreds of thousands
In order to appreciate the corresponding limitations, we note that a commercial atomicforce microscopy (AFM) system combines a displacement imprecision in the tens of fm/Hz1/2 range, together with a maximum displacement amplitude of a few hundred nanometers using a simple red read-out laser[13,14]
Summary
Recent developments in micro- and nano-optomechanical systems have given rise to on-chip mechanical sensing platforms, potentially leading to compact and integrated optical motion sensors These systems typically exploit narrow spectral resonances and require tuneable lasers with narrow linewidth and low spectral noise, which makes the integration of the readout extremely challenging. Resonant-cavity-based nanooptomechanical systems can have ultrahigh sensitivity below fm/Hz1/2 but at cost of a limited dynamic range of a few nanometers, and require a high-performance tuneable laser with picometer linewidth and low-frequency noise. Only a few nano-optomechanical systems without cavities have been reported, e.g., based on side- or top-coupled Mach–Zehnder interferometers[16,17], and suspended waveguide with a free end[18] Their displacement imprecision has remained limited to ≫ 1 pm/Hz1/2, with no demonstration of Brownian motion sensing. The use of an InP platform further opens the way to the integration of lasers for the read-out, potentially resulting in fully-integrated optical sensors
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