News and Reviews (July 2013)

Optically Actuated Micro-Machines

Publication of the University of Bristol

(David Phillips) Nano-scale topography can be imaged by raster scanning a sharp tip over a substrate, and measuring its deflection as it glides over surface features. When imaging biological samples in this way, it is crucial to avoid any disruption to the surface by minimising contact forces.
In this work [1] we use the Photonic ­Professional direct laser writing ­system to fabricate probes that can be controlled in three dimensions using optical tweezers, and demonstrate their use in imaging surface topography with nanometre precision whilst applying ultra-low femto-Newton scale forces.
The flexibility of the Photonic Professional systems allowed us to take a rapid prototyping approach to our probe design. A variety of designs were fabricated, tested and iteratively improved. This resulted in probes that are equipped with cylindrical trapping handles to ensure a low force is exerted on the sample during imaging, and large spherical tracking points giving nanometre accuracy position tracking of the position of the probe. By scanning our probes over a test sample, also fabricated with the Photonic Professional system, we demonstrated that this technique has a spatial resolution of approximately 11 nm and applies an average force of only 140 fN normal to the sample, making it ideal for examining soft biological specimens that would otherwise be deformed or damaged.
This project was carried out at the University of Bristol in the nano­physics and soft matter group, headed by Professor Mervyn Miles, in collaboration with Professor John Rarity of electrical and electronic engineering at the University of Bristol, and Miles Padgett at the ­University of Glasgow.

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(left) A rendering of our probe design showing the four cylindrical trapping handles and three tracking spheres.
(right) An optical image of an optically trapped probe.


Measurement of the surface topography of a test sample.
(a) The trajectory of the probe tip (grey line) as it approaches the sample, and then scans laterally over steps of 100, 200 and 500 nm in depth. The red line indicates the measured interface. (b) A scan over shallower steps (40, 50, 60, 70, 80, 90, 100, and 200 nm in depth) to test the height resolution. In (b) the horizontal axis has been compressed to more clearly reveal the steps, the scale bars show the relative scaling along each axis. (d) A scan over a corrugated part of another test sample, similar to that shown in (e). All scale bars on (a), (b) and (c) represent 500 nm. (d), (e), and (g) are scanning electron microscope images of the test sample. (f) ‘Left eye’ and ‘right eye’  stereo-microscope images of the probe held adjacent to the test sample prior to the start of the experiment.

[1] „An optically actuated surface ­scanning probe“, DB Phillips, GM Gibson, R Bowman, MJ Padgett, S Hanna, DM Carberry, MJ Miles & S. Simpson. 2012, Optics Express, 20 (28), 29679.