New Compact, Optical Sensor for On-Chip Sensing Platforms

At the Eindhoven University of Technology, a research team has designed a new, integrated optical sensor that offers an enhanced resolution in measurements. The sensor opens the door for completely integrated, small optical sensors such as detectors and lasers for on-chip sensing platforms.

These sensors could play a crucial role in the precise displacement and force measurements at the nanoscale, which is essential for nanodevice and microchip design and evaluation. This study has been reported in the Nature Communications journal.

In the era of nanoelectronics, accuracy is of utmost significance. For instance, nanostructures can be tracked with nano-optical instrumentation—small, light-based systems that quantify the smallest of forces, movements, and surface variations.

As speed and resolution are crucial, optical read-out sensors that rely on optomechanical systems are often utilized in sensing applications, for example, in atomic force microscopes (AFMs). Such devices produce sub-nanometer resolution images by quantifying the laser light that has been reflected by the deflection of a cantilever above a surface of interest.

But conventional laser-based methods like those in AFMs can be heavy, which together with the demand for higher resolution and lower cost, induced the need for a different approach. As a result of the progress in nano-optomechanical systems (NOMS), compact optical sensors for the quantification of force, mass, and motion at the nanoscale are now viable. However, the demand for a tunable laser with a narrow linewidth is a limiting factor, and this can be challenging to integrate into a device.

To avoid this problem, Tianran Liu, Andrea Fiore, together with collaborators from the Institute for Photonic Integration at TU/e have developed a new optomechanical device with a resolution of 45 fm (where a femtometer is around 1/1000^th the size of the smallest atom) in a measurement time of a fraction of a second. Most importantly, the device has an ultra-wide optical bandwidth of 80 nm, thereby eliminating the need for a tunable laser.

Waveguides and Large Wavelength Range

The sensor has been developed based on an indium phosphide (InP) membrane-on-silicon (IMOS) platform, which is perfect for including passive components like detectors or lasers. The sensor itself includes four waveguides—structures that limit light signals to a specific path and direction—with two waveguides suspended over two output waveguides.

The corresponding amount of signal carried by the output waveguides differs upon forcing a suspended waveguide toward the output waveguides on the InP membrane. Fabrication occurs through a sequence of lithography steps to characterize the cantilever and waveguides, and the final sensor includes the photodiodes, actuator, and transducers.

One of the main benefits of this sensor is that it functions in a wide range of wavelengths, which avoids the need to add a costly laser on the device. With regards to the deflection of the cantilever, the sensor also mimics the resolution of cantilevers in conventional, but huge, AFMs.

With the latest device as a basis, the scientists intend to design an entire “nanometrology lab” integrated on a chip that can be utilized for semiconductor metrology and for designing next-generation nanoelectronics and microchips.

Journal Reference:

Liu, T., et al. (2020) Integrated nano-optomechanical displacement sensor with ultrawide optical bandwidth. Nature Communications. doi.org/10.1038/s41467-020-16269-7.