A flat laser-generated focused ultrasound transmitter with extended focal depth

Laser generated focused ultrasound (LGFU) is potential in the field of biomedical science like ultrasound imaging with high resolution and ultrasound treatment with high spatial precision. However, the most commonly used method of realizing LGFU is to coat a photoacoustic (PA) film on to an uneven surface, which brings problem to the real application since a uniform PA film with well controlled thickness must be required to guarantee the LGFU performance. In this paper, a new strategy is employed by directly patterning the excitation region on a flat PA film. A chromium mask with a ring-shape clear window is put on a planar PA transmitter, thus forming a ring-shape PA source. The effects of ring size on the focal characteristics are explored numerically and experimentally for the first time. The focal length is proportional to the central radius of the ring due to the constant beam divergence of the acoustic wave when the ring width is fixed. And the larger the ring width, the smaller the beam divergence. Moreover, the ring-shape PA source is able to focus ultrasound with a small lateral beamwidth of several hundred micrometers within the focal depth of a few centimeters. Such a focal field would be useful in ultrasound imaging when a large imaging depth is needed.