Until now, the idea of an acoustically modulated photonic crystal merely existed in theory, but a new technique is proving that sound waves can control photonic crystals within a nanocavity.
Nanosystems Initiative Munich.
Scientists working for the cluster of excellence Nanosystems Initiative Munich (NIM), the Center for Nanoscience (CeNS), the Augsburg Center for Innovative Technologies (ACIT) and the California NanoSystems Institute (CNSI) at Santa Barbara have fabricated a freestanding nanomembrane of semiconducting material. After drilling a large periodic array of tiny holes using cleanroom nanofabrication into the membrane, they trapped light of a well-defined wavelength or color inside the photonic crystal structure in a region where they skipped three holes. They placed quantum dots inside the nanocavity as light emitters.
The key challenge was overlapping the wavelength of the light trapped in the nanocavity and the light emitted by the quantum dot. When the two wavelengths are in resonance, the quantum mechanical Purcell effect leads to a dramatic increase of the light extraction efficiency.
To solve this problem, the NIM-CNSI scientists used a nanoquake, or surface acoustic waves, which periodically stretch and compress the thin membrane and its precisely ordered array of holes. The nanoquakes deform the photonic crystal at radio frequency, and the wavelength of the light inside the nanocavity oscillates back and forth in less than a third of a nanosecond. This is more than 10 times faster than any other approach worldwide, the scientists say.
“The idea of an acoustically modulated photonic crystal existed in our lab for quite a long time,” said NIM graduate student Daniel Fuhrmann. “After all the hard work, it made me really proud to actually see the wavelength of the nanocavity oscillating with the shaking of the nanoquake.”
The Augsburg group is renowned for its pioneering work and application of surface acoustic waves. The researchers apply these to nanosystems ranging from biological and biophysical systems over microfluidics to fundamental physical effects such as the quantum Hall effect. All of these experiments have attracted large attention worldwide and built the outstanding reputation of their research using their nanoquakes on a chip.
Based on these groundbreaking experiments, researchers expect that a highly efficient, acoustically triggered “single photon source” will be realized. Such a device is crucial for inherently secure quantum cryptography and the optical quantum computer.