Figure 11.7 — HOT configurations Figure 11.7 — HOT configurations. Holographic optical tweezers (HOTs) can be configured in different ways. (a) The spatial light modulator (SLM) can be positioned exactly in the Fourier plane and the 1st-order diffracted beam can be deflected away from the 0th-order beam and spatially selected using an iris, or (b) close to the Fourier plane to allow the 0th-order beam to be removed with a spatial filter (beam stop). (c) A counter-propagating optical trapping configuration can be generated by placing a mirror behind the sample plane, allowing for the use of objectives with lower numerical apertures and longer working distances. Furthermore, the SLM can also be placed (d) in a Fresnel plane or (e) in a plane conjugated to the image plane, e.g., to use the phase contrast approach discussed in Subsection 11.6.2.