Microwave Frequency Comb Interrogation of High-Overtone Bulk Acoustic Resonators for High-Speed, Multi-Modal Mass Sensing

The application of microwave frequency combs to the interrogation of a high-overtone bulk acoustic resonator (HBAR) is presented for high-speed measurement of resonances with sub-millisecond time resolution. Unlike conventional approaches such as a phase locked loop (PLL), this method tracks the complete frequency response, providing resonance frequencies, damping coefficients, and line shapes for multiple modes simultaneously and at a much higher speed than traditional approaches, such as vector network analysis (VNA). When applied to mass sensing using an HBAR, it is shown that both frequency shifts and increased damping induced by the application of a drop of isopropyl alcohol (IPA) can be tracked with 0.2 ms time resolution. These are the first published results using microwave frequency combs to excite and measure micromechanical resonators, and this work will enable high-speed and data-rich mass sensing with HBARs for biological, chemical, and process control applications.