Design and characterization of a photo-acoustic lens to generate tightly-focused and high frequency ultrasound

Ultrasound microscopy uses high frequency (>40 MHz) ultrasound to produce high resolution images. For high resolution microscopy, broadband ultrasound generation and detection is necessary. Because high frequency ultrasound experiences significant absorption loss that results in weaker signals, it is desirable to focus the energy for microscopy applications, which also results in higher lateral resolution. In this work, thermo-elastically generated ultrasound was brought into a tight focus by shining a ns laser pulse onto a thin metal film-coated concave surface. For ultrasound detection, we use polymer microring resonators which have high frequency and wide band response. We experimentally obtained spatial and temporal characteristics of focused ultrasound by optical generation and detection. The 3-dB spot width of the focused ultrasound is ~50 μm. By frequency filtering over 40~100 MHz, 21 μm width is obtained. The temporal profile is close to the time-derivative of laser pulse waveform. Frequency domain analysis for the signal shows that high frequency loss mechanism of our system is dominated by angular directivity of the microring detector. The issues to improve high frequency response are discussed.