Technological innovation in the last century was mainly based on the control of electrons or photons. Now, in the emerging research field of phononics, phonons or vibrations of the crystal lattice attract attention. A research team at Berlin showed a graphene-based phononic crystal whose resonant frequency can be tuned over a broad range and has used a helium-ion microscope to produce such a crystal. This is a real breakthrough in the field of phononics, now published in the journal Nano Letters.
Without electronics and photonics, there would be no computers, smartphones, sensors, or information and communication technologies. In the coming years, the new field of phononics may further expand these options. That field is concerned with understanding and controlling lattice vibrations in solids. In order to realize phononic devices, however, lattice vibrations have to be controlled as precisely as commonly realized in the case of electrons or photons.
The key building block for such a device is a phononic crystal, an artificially fabricated structure in which properties such as stiffness, mass or mechanical stress vary periodically. Phononic devices are used as acoustic waveguides, phonon lenses, and vibration shields and may realize mechanical Qubits in the future. However, until now, these systems operated at fixed vibrational frequencies. It was not possible to change their vibrational modes in a controlled manner.
Now, for the first time, the research team has demonstrated this control. They used graphene, a form of carbon in which the carbon atoms interconnect two-dimensionally to form a flat honeycomb structure. Using a focused beam of helium ions, the team was able to cut a periodic pattern of holes in the graphene. This method is available at CoreLab CCMS (Correlative Microscopy and Spectroscopy). The researchers say they had to optimize the process a lot to cut a regular pattern of holes in the graphene surface without touching neighbouring holes.
The researcher calculated the vibrational properties of this phononic crystal. His simulations show that in a certain frequency range no vibrational modes are allowed. Analogues to the electronic band structure in solids, this region is a mechanical band gap. This band gap can be used to localize individual modes to shield them from the environment. The researchers say that 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.
The researchers hope that their results will push the field of phononics further. They expect to discover some fundamental physics and develop technologies that could lead to application in e.g. ultrasensitive photosensors or even quantum technologies.
News Source: HZB
