A high-frequency, 2-D array element using thermoelastic expansion in PDMS

Optical generation of ultrasound is a promising alternative to piezoelectricity for high-frequency arrays. An array element is defined by the size and location of a laser beam focused on a suitable surface. Optical generation using the thermoelastic effect has traditionally suffered from low conversion efficiency. We previously demonstrated an increase in conversion efficiency of nearly 20 dB with an optical absorbing layer consisting of a mixture of polydimethylsiloxane (PDMS) and carbon black spin coated onto a glass microscope slide. Radiation pattern measurements with an 85 MHz spherically focused transducer indicated an array element size of 20 microm. These measurements lacked the spatial resolution required to reveal fine details in the radiated acoustic field. Here we report radiation pattern measurements with a 5-microm spatial sampling, showing that the radiated acoustic field is degraded by leaky Rayleigh waves launched from the PDMS/glass interface. We demonstrate that replacing the glass with a clear PDMS substrate eliminates the leaky Rayleigh waves, producing a broad and smooth radiation pattern suitable for a two-dimensional (2-D) phased array operating at frequencies greater than 50 MHz.