AFM Tip Used to Precisely Map Light's Wave Pattern

Using the tip of an atomic force microscope (AFM), researchers have discovered that it is possible to map the wave pattern of light, trapped in a so-called optical resonator, with unprecedented precision. The discovery could lead to the design of complex optical structures, such as an optomechanical, reconfigurable photonic crystal add/drop multiplexer, the scientists said.

Schematic layout of the measurement setup, for illustrative purposes. In reality, the photonic crystal and the AFM are much smaller.

Wico Hopman, a researcher within the Integrated Optical MicroSystems group of the MESA+ Institute for Nanotechnology, published the results in September in the online journal Optics Express. MESA+ is the largest research institute of the University of Twente in Enschede. It trains graduate and PhD students and conducts research in the fields of nanotechnology, microsystems, materials science and microelectronics.

Contributing to the research were Mesa+ electrical engineering faculty members Anton Hollink and René de Ridder, Mesa+ science and technology faculty members Kees van der Werf and Vinod Subramaniam and Wim Bogaerts of the Department of Information Technology at Ghent University in Belgium.

The shape of the resonator. The pattern of darker circles is formed by the holes; the actual resonator is situated in the middle, where no holes are present.

They also discovered that the AFM is also capable of playing with the light to optimize the performance of the resonator. If the optical crystal doesn’t work at the correct color of light, for example, this mechanical correction works out well. It is even possible to build a optomechanical switch in this way, the researchers said.

While scanning the cavity with the AFM tip, on every position of the needle the transmitted light is detected. Outside the resonator, there is no interaction (the light-colored section), on locations with high concentrations of light, there’s a lot of interaction (darker color).

With the 10-nm AFM tip, Hopman was able to manipulate light that is locked in an optical crystal -- a sort of cage in which light is trapped. An optical crystal has a pattern of holes at which all light reflects and comes together in a cavity where no holes are present. In this cavity the light resonates at a specific color. This makes optical crystals highly suitable to act as selective filters for certain colors of light. Whenever Hopman scans the cavity with the AFM tip, the light "feels" the presence of the needle, the color is influenced lightly and the filter does its work for the new color. In this very precise way, Hopman can demonstrate the way the light is divided in the cavity.

By combining both of the figures above, the hot spots within the resonator immediately become clear.

Thanks to this extremely high precision, Hopman can locate the "hot spots" at which he can manipulate the light best. Within these hot spots, the color can be modified in the best way and also the selectivity of the filter is manipulated: how well will it distinguish one color from another? If the crystal has small defects, it may not perform optimally, and the AFM method is capable of compensating for these defects. Building a fast optomechanical on/off switch is possible in this way too. The distance the needle has to travel is just a few nanometers, which can be done in nanoseconds, Hopman said.

The researchers now need to work on how to integrate the tip and control it from within the crystal.

Hopman closely collaborated with the Biophysical Engineering Group of the University of Twente, which investigates the properties of fluorescent proteins. Manipulating the light properties of these proteins within an optical crystal could be an interesting option, which will be investigated further, he said.

The research was supported by NanoNed, a national nanotechnology program coordinated by the Dutch ministry of Economic Affairs, and the European Network of Excellence (ePIXnet). For more information, visit: www.utwente.nl/en