Nano-optomechanical displacement sensor reaches 45fm resolution

May 26, 2020 //By Julien Happich
displacement sensor
Optical read-out sensors based on optomechanical systems are frequently used in sensing applications such as in atomic force microscopes (AFMs), able to measure the smallest of surface variations, forces and movements.

These devices generate sub-nanometer resolution images by measuring the laser light reflected by the deflection of a cantilever over a surface of interest. Now, researchers from Eindhoven University of Technology have drastically increased the resolution of such systems, leveraging a newly designed nano-optomechanical system (NOMS) with unprecedented measurement resolution. The transducer described in a Nature Communications paper titled "Integrated nano-optomechanical displacement sensor with ultrawide optical bandwidth" consists of four evanescently-coupled waveguides, with two waveguides suspended above two output waveguides.

Simulated electric field distribution (|E|) before and
after displacement (55 nm) of an ideal device.

The structure is designed in such a way that, before displacement, light coming from one input waveguide excites a superposition of symmetric and anti-symmetric supermodes which, after traveling for a beating length in the directional coupler, interferes constructively at the “cross” output port. A displacement of one suspended waveguide changes the propagation constants of the supermodes and makes the interference destructive, resulting in increased transmission from the other waveguide. These change in the relative transmission from the two output waveguides result from a combination of vertical and horizontal evanescent coupling, the authors explain.

Scanning electron microscope (SEM) image of the
nanomechanical directional coupler used as a transducer (a).
b Schematic illustration of light passing through the direction
coupler before (up) and after (down) actuation.
Credit: Eindhoven University of Technology.

The sensor is built out of an indium phosphide (InP) membrane-on-silicon (IMOS) platform, fabricated via a series of lithography steps to define the waveguides and cantilever, while the final sensor integrates the transducers, actuator, and photodiodes. Compared to silicon photonics, this platform allows integration of passive components, lasers and detectors in a micron-thick and high-confinement InP membrane.

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