Two-photon holographic stimulation

Using pulsed near-infrared lasers together with holographic beam-shaping to excite opsins in three dimensions with near-cellular resolution deep in scattering tissue. Two-photon absorption confines excitation to a small focal volume because it scales with the square of light intensity; holography (CGH) plus temporal focusing then places and shapes many such volumes at once. This is how researchers play defined spatiotemporal spike patterns into specific identified neurons — the write side of all-optical interrogation.

Practical operation requires high-photocurrent, soma-targeted opsins so that a small delivered light dose reliably reaches spike threshold in the targeted cell and nowhere else. Depth is limited to roughly the superficial few hundred micrometers of cortex without three-photon excitation or implanted optics, and total addressable targets are ultimately capped by laser power and heating.