Cavity optomechanics (OM) is a topical issue of growing interest due to the potential applications ranging from sensing and communications to quantum information technology. However, the number of phonons created in a cavity by an external optical fibre remains relatively low. An alternative way to induce large phonon population is the use of a phonon pump by transforming an RF signal into surface acoustic waves (SAW). In a simulation work, we demonstrated an efficient focusing and conversion of SAW generated by focusing interdigital transducers (IDT) on piezoelectric aluminum nitride film deposited on silicon on insulator (SOI) into the guided modes of a silicon nanobeam. For a straight nanobeam, we achieve an efficiency of -22dB and displacements about 50pm when applying a voltage of 1V to the IDT. When the nanobeam is structured to form a phononic crystal with a cavity, the guided modes excite some localized modes around 2 GHz inside the cavity with a magnitude of the vibrational motions around 1nm. The preliminary experiments confirm the simulations.