Nano-optomechanical displacement sensor reaches 45fm resolution
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.
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.
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.
The waveguides are based on two InP membranes separated by 220-nm-thick InGaAsP (i-Q1.58) sacrificial layer, which is etched in the sensing region to suspend the top membrane, while it is kept and used as absorbing layer in the photodetector section. The top, 330-nm-thick InP membrane is p-doped, with a 20-nm-thick p-InGaAs contact layer, while the bottom membrane consists of a top 50-nm-thick n-doped InP layer and a 220-nm-thick undoped layer. The corresponding p-i-n junction is used to electrostatically actuate the inter-membrane distance in the sensing section, and forms the photodiode in the detection section. This set up was demonstrated to yield a resolution of 45 femtometers (about 1/1000 the size of the smallest atom) in a measurement time of a fraction of a second.
One of the key advantages of this sensor is that it operates in a large range of wavelengths, which eliminates the need for an expensive laser on the device. In terms of cantilever deflection, the sensor also replicates the resolution of cantilevers in traditional, but bulky AFMs. Using this new device as a foundation and building a tip on top of the moveable waveguide, the researchers plan on developing an entire “nanometrology lab” integrated on a chip that can be used for semiconductor metrology and help in the design of the next generation of microchips and nanoelectronics.
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