Computer-generated holography (CGH)

A technique that shapes a laser wavefront with a spatial light modulator (SLM) to project an arbitrary three-dimensional pattern of light — dozens to hundreds of foci at once — instead of scanning a single beam point by point. Combined with two-photon excitation and temporal focusing (which confines each spot axially), CGH lets an experimenter write to many precisely chosen neurons simultaneously with millisecond timing. It is the enabling optics of modern all-optical control.

The number of simultaneously addressable targets is bounded by available laser power (split among the foci), tissue heating, and scattering; deeper or larger ensembles demand more power and better aberration correction. Phase-only SLMs also refresh slowly (tens to a few hundred hertz), limiting how fast the pattern can change, though multiplexed temporal focusing and acousto-optic methods push these limits.