Optomechanical Generation of Coherent GHz Vibrations in a Phononic Waveguide

13 pages, 4 main text figures, 6 appendix figuresNanophononics has the potential for information transfer, in an analogous manner to its photonic and electronic counterparts. The adoption of phononic systems has been limited, due to difficulties associated with the generation, manipulation, and detection of phonons, especially at GHz frequencies. Existing techniques often require piezoelectric materials with an external radiofrequency excitation that are not readily integrated into existing CMOS infrastructures, while nonpiezoelectric demonstrations have been inefficient. In this Letter, we explore the optomechanical generation of coherent phonons in a suspended 2D silicon phononic crystal cavity with a guided mode around 6.8 GHz. By incorporating an air-slot into this cavity, we turn the phononic waveguide into an optomechanical platform that exploits localized photonic modes resulting from inherent fabrication imperfections for the transduction of mechanics. Such a platform exhibits very fine control of phonons using light, and is capable of coherent self-sustained phonon generation around 6.8 GHz, operating at room temperature. The ability to generate high frequency coherent mechanical vibrations within such a simple 2D CMOS-compatible system could be a first step towards the development of sources in phononic circuitry and the coherent manipulation of other solid-state properties.We acknowledge the support from the project LEIT funded by the European Research Council, H2020 Grant Agreement No. 885689. ICN2 is supported by the Severo Ochoa program from the Spanish Research Agency (AEI, Grant No. SEV-2017-0706) and by the CERCA Programme/Generalitat de Catalunya. R. C. N. acknowledges funding from the EU-H2020 research and innovation programme under the Marie Sklodowska Curie Individual Fellowship (Grant No. 897148). O. F. is supported by a BIST PhD fellowship under the Marie Sklodowska Curie Grant Agreement (No. 754558). M. A. and S. S. gratefully acknowledge funding from the Villum Foundation Young Investigator Program (Grant No. 13170), the Danish National Research Foundation (Grant No. DNRF147—NanoPhoton), Innovation Fund Denmark (Grant No. 0175-00022—NEXUS), and Independent Research Fund Denmark (Grant No. 0135-00315—VAFL).Peer reviewe