Remote Sub-Wavelength Focusing of Ultrasonically Activated Lorentz Current

We propose the use of a combination of ultrasonic and magnetic fields in conductive media for the creation of RF electrical current via the Lorentz force, in order to achieve current generation with extreme sub-wavelength resolution at large depth. We demonstrate the modeling, generation, and measurement of Lorentz current in conductive solution, and show that this current can be localized at a distance of 13 $cm$ from the ultrasonic source to a region about three orders of magnitude smaller than the corresponding wavelength of electromagnetic waves at the same operation frequency. Our results exhibit greater depth, tighter localization, and closer agreement with prediction than previous work on the measurement of Lorentz current in a solution of homogeneous conductivity. The proposed method of RF current excitation overcomes the trade-off between focusing and propagation that is fundamental in the use of RF electromagnetic excitation alone, and has the potential to improve localization and depth of operation for RF current-based biomedical applications.
Comment: 5 pages, 7 figures; submitted to Applied Physics Letters