Fig. 1.

Vortex beam trap for microbubbles. (A) The simulated pressure field, $\left|p\right|$, and phase, $\arg \left(p\right)$, of the trapping vortex beam are shown in the transverse plane $\left(x,y\right)$. The phase variation results in a helicoidal wavefront in propagation direction, $z$, where the pressure must vanish. This is confirmed in B, where experimental scans of the pressure field along $x$ for three different axial positions relative to the focal plane $z=0,\hspace{0.17em}1.5$, and $3\hspace{0.17em}$ mm are shown in the absence of the bubble. (C) The total pressure field surrounding the bubble scattering the beam is simulated in the propagation plane $\left(x,z\right)$. (D) The same total field seen in the transverse plane $\left(x,y\right)$ . It gives rise to the net pushing force $F_z$ for a bubble centered on the vortex core ($x=y=0$). (E) Same as D for a bubble shifted by a distance $x=0.5\lambda$. The total field illustrates how the bubble oscillations distort the incident beam and give rise to a strong lateral trapping force attracting the bubble back toward the vortex core.