UHF acoustic microscopic imaging of resonator motion

An important characteristic of acoustic resonator design at ultra high frequencies (UHF) is the efficiency of the device, which is affected by leakage radiation beyond the active transducer area. In order to directly measure leakage radiation, acoustic microscopic imaging of the ultrasonic resonant motion at 880 MHz has been performed with an autocompensating interferometer and heterodyne demodulation. A method of interferometric detection based on two-wave mixing in a photorefractive material was employed to allow for automatic quadrature detection and compensation of low frequency vibrational effects. Heterodyne phase modulation was used to produce a beat frequency of 25 kHz to allow for narrowband detection utilizing lock-in amplifier techniques. A sensitivity of 5×10^-5 nm was obtained with a 10 sec time constant and detection light power of about 0.5 mW. Over 90 dB dynamic drive range was recorded at the UHF operating frequency without vibration isolation of the microscope or optics. The method of calibration and results of scanning measurements on various resonators are presented and discussed in connection with current limitations and future improvements.