Three-photon microscopy

Deep-tissue optical read-out using near-infrared excitation (around 1300 or 1700 nanometers) and three-photon absorption. Its higher-order nonlinearity (excitation scaling with the cube of intensity) suppresses out-of-focus background far more effectively than two-photon, and its longer, less-scattered wavelengths reach further, extending high-resolution single-cell imaging through the full depth of cortex and into the hippocampus in the intact brain. It is a leading route to reading deeper without penetrating optics.

The trade-off is much lower excitation efficiency, which requires high pulse energies at low repetition rates and so risks heating and nonlinear photodamage; frame rates and fields of view are correspondingly constrained. It is a specialized, power-hungry method for reaching depth, not a routine replacement for two-photon imaging.