Exploding single electrons in liquid helium using ultrasound focused by a Fresnel Zone Plate

Electrons in liquid helium prefer to stay trapped in nanometer-sized vacuum bubbles. These bubbles can be exploded with high amplitude ultrasound and optically imaged. This serves as a means of tracking single electrons. However, material limitations make it very difficult to generate uniform pressure fluctuations of the required strength. Previous studies have attempted to resolve this issue by using hemispherical piezoelectric transducers and focusing weak sound waves into small regions. While this method allows for the imaging of electrons, observations can only be made at a single focus which is fixed by the transducer dimensions. Also, it is difficult to introduce uniform electric fields in the system due to the curvature involved. We introduce diffraction zone plates to focus plane waves in liquid helium, which allows for multiple focal spots with flexible control on positioning and amplification. We have developed a method to design zone transparency profiles from an intended focal point distribution, and fabricated a few plates using femtosecond laser micromachining technology. Remarkably, our setup has not involved curvature, enabling detailed further studies of individual electron mobilities in liquid helium. Also, our designs directly extend to studies of the long-standing problem of "exotic ions" in liquid helium.