Thermoelastic generation of ultrasound for high frequency phased arrays

High frequency two-dimensional phased arrays for ultrasound microscopy are difficult to construct using piezoelectric technology. A promising alternative involves optical generation of ultrasound, where an array element is defined by the size and location of a laser beam. Optical generation of ultrasound is possible through a variety of mechanisms, the most common being the thermoelastic effect. Optical absorption of a laser pulse rapidly heats a localized volume, where thermal expansion launches an acoustic wave containing frequencies corresponding to the envelope of the absorbed optical pulse. Acoustic pulse shaping is therefore possible by suitable modulation of the laser pulse. An erbium doped fiber amplifier (EDFA) was built to amplify pulses from a diode laser to peak powers of several watts, sufficient to generate ultrasound detectable with conventional transducers. Preliminary experiments used a chromium film deposited on a glass substrate as the light absorbing structure. This structure is found to generate ultrasound with a bandwidth comparable to the excitation laser. This is very promising, but the poor SNR of the measured signals call for an improvement in the optical to acoustical conversion efficiency. Efficiency can be improved by choosing materials with high thermal expansion coefficients. An increase in conversion efficiency of nearly 20 dB was obtained using an optical absorbing layer consisting of a mixture of polydimethylsiloxane (PDMS) and carbon black spin coated onto a glass microscope slide. Radiation pattern measurements using high frequency piezoelectric transducers show that the thermoelastic element is small enough for a 75 MHz array. However, measurements with an optoacoustic detection array show that the radiated acoustic field is corrupted by leaky Rayleigh waves excited along the PDMS/glass interface. Replacing the glass with a clear PDMS substrate eliminates this problem to produce a smooth radiation pattern. These results show that a 75 MHz 2-D transmitting array can be produced using thermoelastic generation of ultrasound in an “all PDMS” structure.