(Left) An electron microscopy image showing the periodic pattern of holes that the helium beam creates in the graphene sample. This results in the superposition of vibrational modes and the emergence of a mechanical band gap. (Right) The frequency of this phononic system can be adjusted between 50MHz and 217MHz by mechanical tension. Image: K. Höflich/HZB.Without electronics and photonics, there would be no computers, smartphones, sensors, or information and communication technologies. Over the coming years, the new field of phononics may usher in a whole new generation of devices. This field is concerned with understanding and controlling lattice vibrations (phonons) in solids. In order to realize phononic devices, however, these lattice vibrations have to be controlled as precisely as electrons or photons are in current devices.
The key building block for phononic devices is a phononic crystal, an artificially fabricated structure in which properties such as stiffness, mass or mechanical stress vary periodically. Phononic devices are already used as acoustic waveguides, phonon lenses and vibration shields, and may form the basis for quantum bits, or qubits, in the future. Up to now, however, these phononic devices have operated at fixed vibrational frequencies, because it has not been possible to change their vibrational modes in a controlled manner.
Now, for the first time, a team of researchers at Helmholtz-Zentrum Berlin (HZB) and Freie Universität (FU) Berlin, both in Germany, has demonstrated this control with graphene, a form of carbon in which the carbon atoms interconnect two-dimensionally to form a flat honeycomb structure. To turn graphene into a phononic crystal, the researchers cut a periodic pattern of holes into it with a focused beam of helium ions.
"We had to optimize the process a lot to cut a regular pattern of holes in the graphene surface without touching neighbouring holes," said Katja Höflich, group leader at Ferdinand-Braun-Institut Berlin and a guest scientist at HZB. The researchers report their work in a paper in Nano Letters.
Jan Kirchhof from FU Berlin, who is first author of the paper, calculated the vibrational properties of this novel phononic crystal. His simulations show that in a certain frequency range no vibrational modes are allowed. Analogous to the electronic band structure in solids, this region is a mechanical band gap, which can be used to localize individual modes to shield them from the environment.
"The simulation shows that we can tune the phononic system quickly and selectively, from 50 megahertz to 217 megahertz, via applied mechanical pressure, induced by a gate voltage." says Kirchhof.
"We hope that our results will push the field of phononics further. We expect to discover some fundamental physics and develop technologies that could lead to applications in ultrasensitive photosensors or even quantum technologies," says Kirill Bolotin, head of the working group at FU Berlin. The first experiments on this new phononic crystal from HZB are already underway in his group.
This story is adapted from material from Helmholtz-Zentrum Berlin, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.