Experimental demonstration for precisely tuning the focal length of finite-aperture focused beams and vortices.

High-frequency focused ultrasound is widely used in biomedical applications such as high-resolution imaging, neuromodulation, and particle manipulation. However, dynamic tuning of the focal plane in conventional systems often relies on mechanically adjustable components or array-based control with high cost. In this work, an optically transparent, planar compact piezoelectric ultrasonic transducer was designed and fabricated by encoding phase profile corresponding to an ideal converging spherical wave onto a planar aperture, thereby generating focused fields while maintaining compatibility with microscopic imaging platforms. The acoustic field was experimentally characterized in the focal plane at design frequency and in the propagation plane at several excitation frequencies around the design frequency. The focal shift originates from the mismatch between the fabricated phase pattern and the propagation wavenumber at the excitation frequency. An approximate linear relation between focal length and frequency near the design one is derived theoretically, and the finite-range tuning behavior is interpreted using the stationary-phase condition. Both theory and experiment show the focal length varies approximately linearly with excitation frequency near the design frequency. Water-tank measurements agree well with the theoretical prediction, confirming the proposed model. This work provides a simple and cost-effective approach for focal tuning in compact high-frequency ultrasound devices.