In electromechanical excitation of mechanical or optoelectromechanical nanostructures, surface acoustic waves (SAW) are typically used [1,2]. The challenges are the relatively low coupling efficiency [3] and the, typically, large surface area of the interdigitated transducer (IDT). Here we demonstrate a new concept of a nanoscale and high efficiency electromechanical transducer for phonon generation. Instead of using IDTs, we use small freestanding bulk acoustic wave (BAW) transducers integrated directly into nanostructures. This configuration provides an efficient and broadband source in the GHz range with a small footprint. FEM simulations show that the efficiency of coupling of mechanical power to a nano beam can easily exceed 50 %. The main loss mechanism is the anchor loss from the freestanding transducer to substrate via the supporting beams. This leakage loss can be eliminated by making the supporting beams 1-dimensional phononic crystals and then the coupling efficiency can reach 90 %. In addition, the simulations show that a transducer optimized for operation at 4 GHz can act as an efficient source from 10 MHz to 10 GHz. The first test devices were fabricated using standard MEMS fabrication processes. The layer thicknesses of the transducer were optimized for operation at 4 GHz. The transducers have shapes from concave to straight triangles to convex horn shapes and the size ranging from a few to fifteen um2. To estimate the coupling efficiency, the transducers are integrated to 220 nm thick and 800-2800 nm wide straight nanocrystalline Si nanobeams and characterised by laser Doppler vibrometry (LDV). The vibrometry measurement provides vertical displacement magnitudes, and the ratio of the magnitudes between different locations can be used to estimate the coupling. The magnitude ratio does not give the true coupling efficiency because of interference effects. Consequently, detailed 3-dimensional FEM simulations based on the LDV data are required to quantify the energy fluxes inside the device and to define the true coupling efficiency. The upper frequency limit of the vibrometer is 2.6 GHz. The amplitude magnitudes on the transducer and the nanobeam were measured from 2.0 GHz to 2.4 GHz, showing normalised magnitude ratios of several tens of percent and indicating relatively high phonon generation efficiency. In this talk we will describe in detail the concept, fabrication and experimental vibrometry results.
[1] D. Navarro-Urrios et al., ACS Photonics 9, 413 (2022).
[2] M. Mirhosseini et al., Nature 588, 599 (2020).
[3] A. V. Korovin et al., J. Phys. D: Appl. Phys. 52, 32LT01 (2019).