  Thermo-elastic conversion of electromagnetic wave into ultrasound wave has
enabled diverse biomedical applications such as photoacoustic imaging.
Microwave, which has ~10 cm long wavelength, can penetrate deeper into tissue
than photons, heralding exciting applications such as deep tissue imaging via
thermo-acoustic tomography. However, the thermo-acoustic conversion efficiency
is very low even with an exogenous contrast agent such as carbon nanotube.
Here, we break this low conversion limit through using a split ring resonator
(SRR) to effectively collect and concentrate the microwave energy into a
sub-millimeter hot spot and subsequently convert the energy into ultrasound
wave. Our SRR achieves over three orders of magnitude higher thermo-acoustic
conversion efficiency than commonly used thermo-acoustic contrast agents. We
further harness the SRR as a wireless, battery-free ultrasound emitter placed
under a breast phantom. These results promise exciting potential of SRR for
precise thermo-acoustic localization and modulation of subjects in deep tissue.
