Generation and control of phonons in a CMOS-compatible optomechanical platform

by Virginia Greco

A team of researchers led by ICREA Prof. Dr Clivia Sotomayor-Torres, leader of the ICN2 Phononic and Photonic Nanostructures Group, has developed a waveguide platform in which self-sustained coherent phonons in the GHz regime can be generated at room temperature. As this platform is CMOS-compatible, this technique could impulse further development of phononic circuitry for telecommunication applications.

In electronics and telecommunications, the transfer of information currently relies heavily on photonics (light transmission) and electronics (charge carriers’ movement). However, phononics is a very valuable alternative as phonons –i.e., quanta of lattice vibrations— have great potential as vectors for information interchange. In addition, the frequency regime at which phononics operates is complementary to those of electronic and photonic systems. Nonetheless, the field of phononics is still in its relative infancy, having been less explored and having found less applications since generating, transporting, and manipulating phonons entails a number of unsolved challenges.

Existing approaches are not satisfactory as they may provide poor ability to control and manipulate phonons or may require elaborate optomechanical architectures (or external excitation sources) that do not allow for integration in CMOS platforms. Therefore, researchers are putting effort into trying to develop new techniques to generate and modify high-frequency coherent mechanical vibrations in simple 2D CMOS-compatible systems, without the need for additional complex structures.

Towards this goal, ICREA Prof. Dr Clivia Sotomayor-Torres and members of her Phononic and Photonic Nanostructures Group at the ICN2, in collaboration with colleagues in the group of Prof. Dr Søren Stobbe at the Technical University of Denmark, have developed a 2D silicon phononic crystal (PnC) waveguide in which acoustic waves (at a frequency centred around 6.8 GHz) can be generated and sustained at room temperature. This work has been recently published in Physical Review Letters, with Dr Ryan C. Ng and Dr Pedro García as corresponding authors and Dr Guilhem Madiot as the first author (together with the aforementioned Dr Ng).

The authors transformed the phononic structure in an optomechanical platform by using two identical PnC cavities, each made of a silicon membrane on which shamrock-shaped holes were etched with nanofabrication techniques, but one was flipped to be a mirror image of the other. These two cavities were then brought together while leaving an air-slot in the middle. This configuration allows for exploitation of localised photonic modes, which result from inherent and unavoidable imperfections at the silicon-air interfaces, to transduce and amplify mechanical vibrations.

The technique presented in this study enables the generation of self-sustained coherent phonons in the GHz regime within a 2D CMOS-compatible system at room temperature. These outcomes represent an important step towards the design of phononic circuitry that could be integrated with current electronics. Further developments will allow phononic structures and devices to become important components of future telecommunication systems.

Reference article:

Guilhem Madiot, Ryan C. Ng, Guillermo Arregui, Omar Florez, Marcus Albrechtsen, Søren Stobbe, Pedro D. García, and Clivia M. Sotomayor-Torres, Optomechanical Generation of Coherent GHz Vibrations in a Phononic Waveguide. Phys. Rev. Lett. 130, 106903, 2023. DOI: https://doi.org/10.1103/PhysRevLett.130.106903