The sonic screwdriver: a model system for study of wave angular momentum

When samples of interest are small enough, even the relatively weak forces and torques associated with light can be sufficient for mechanical manipulation, can offer extraordinary position control, and can measure interactions with resolution three orders of magnitude better than atomic force microscopy. However, as the components of interest grow to slightly larger length scales (which may yet be of interest for microfluidic, "lab-on-a-chip" technologies and for research involving biomedical imaging), other approaches gain strength. This paper includes discussion of the angular momentum carried by sonic beams that we have implemented both to levitate and controllably rotate disks as large as four inches across. Discussion of such acoustic beams complements the discussion of the angular momentum carried by light and, by further analogy, how we view stationary states discussed in quantum mechanics. Hence, a primary use of the sonic screwdriver is as a model system, although these beams are useful for a variety of other reasons as well (not least of which is aberration correction for ultrasonic array systems). Methods, including the use of holographically structured fields, are discussed.