A swinging molecule of light

A team of NIM researchers at the Institute of Physics at Augsburg University and the Walter Schottky Institute at TU Munich (TUM) successfully used nanomechanical sound waves to control a ‘molecule of light’ formed by two neighboring nanophotonic resonators. In their recent publication in Nature Communications, the team led by NIM-Professor Hubert Krenner showed that the vibrating sound wave switches on and off the bond of their photonic molecule at unprecedented speeds.

For their experiments NIM-graduate student Stephan Kapfinger and his supervisor Prof. Hubert Krenner at the chair of Experimental Physics I (Prof. Achim Wixforth) at Augsburg University used nanometer-thin membranes of semiconducting material into which they drilled a large periodic array of tiny holes using cleanroom nanofabrication. In such a structure, a ‘photonic crystal’, light of well-defined energy (or color) can be trapped inside a region where three holes are skipped, thus forming a tiny ‘defect’ and acting as a nanocavity. Together with the group of Dr. Michael Kaniber and Prof. Jonathan Finley at TUM, they designed and fabricated a structure of two adjacent nanocavities, in which photons, single quanta of light, can oscillate back and forth. “In our photonic molecule, photons behave just like electrons, which create a chemical bond in a hydrogen molecule. While the two hydrogen atoms forming an H2 molecule are totally identical by nature, the two artificial, man-made nanophotonic cavities usually are not,” Stephan Kapfinger explains. Until now, these tiny nanoscale imperfections have hampered the realization of coupled photonic elements or even larger scale photonic circuits. The Augsburg research team used a smart trick to solve this pressing problem: they designed and employed a tiny sound wave, a nanometer earthquake on a chip, such that it compresses one of the nanocavities and simultaneously stretches the other. This way, they are able to overcome the tiny fabrication related imperfections and make the two nanocavities identical for a very short snatch. Moreover, this happens also at a precisely defined time during the cycle of the sound wave, thus providing total control over the coupling.

Stephan Kapfinger is really enthusiastic on the success of his experiment: “It was fascinating to see that the two nanocavities don’t emit at the same color as one would naively expect. In fact they “repel” each other and the difference is simply the bond strength of the photonic molecule! Many scientists have tried hard to measure this effect but with little success.” Hubert Krenner notes: “Our hallmark experiment not only demonstrates scaling and control at unprecedented speed. It also shows that nanomechanical waves can be efficiently converted to optical signals. This is quantum-mechanical control in the truest sense of the word.” The pioneering work and longstanding expertise of the Augsburg group on the application of surface acoustic waves (SAW) led to numerous important results and application. These cover the entire spectrum of the very active field of nanoscience and attracted large attraction worldwide. “We are very happy, that surface acoustic waves, our special tool for which we are famous here in Augsburg, led to another outstanding result in the field of nanophotonics,” Achim Wixforth proudly adds.

The studied photonic crystal devices are highly attractive because they can be scaled to large integrated circuits for light even in the quantum regime. Based on their groundbreaking experiments, the NIM research team expects that this system can now finally be extended to an optical quantum computer. Shaking and rattling them using well defined nanoquakes will be the clock of the quantum processor.

This work is supported by the Deutsche Forschungsgemeinschaft (DFG) via the Emmy Noether Programme (KR3790/2-1) and Sonderforschungsbereich SFB 631, and by the Excellence Initiative via the Cluster of Excellence Nanosystems Initiative Munich (NIM).

Stephan Kapfinger, Thorsten Reichert, Stefan Lichtmannecker, Kai Müller, Jonathan J. Finley, Achim Wixforth, Michael Kaniber and Hubert J. Krenner
Dynamic acousto-optic control of a strongly coupled photonic molecule
Nature Communications 6, 8540 (2015), doi:10.1038/ncomms9540


message of 06.10.2015