Microwave, which has ~10 cm long wavelength, can penetrate deeper into tissue than photons, heralding exciting deep tissue applications such as modulation or imaging via thermo-acoustic (TA) effect. However, the TA conversion efficiency is very low even with an exogenous contrast agent. Here, we break this low conversion limit through using a split ring resonator (SRR) to effectively collect and confine the microwave into a sub-millimeter hot spot for ultrasound emission, and achieve over two thousand times higher conversion efficiency than reported TA contrast agents. Importantly, the frequency of emitted ultrasound can be precisely tuned and multiplexed by modulation of the microwave pulses. This is inaccessible by a piezoelectric-based transducer or a photoacoustic emitter and opens new opportunities to study the frequency response of cells in ultrasound bio-modulations. For applications in deep tissue localization, we further harness the SRR as a wireless, battery-free ultrasound becaon placed under a breast phantom.
Lan Lu, Li Yueming, Yang-Tran Tiffany, Jiang Ying, Cao Yingchun, Cheng Ji-Xin. Ultra-efficient Thermo-Acoustic Conversion through a Split Ring Resonator[J]. Advanced Photonics, , (): .
