Multi-photon microscopy exploits a phenomenon where fluorescent molecules can simultaneously absorb multiple photons of light to provide the energy required to fluoresce. Typically this is achieved with light at double the wavelength used for 'normal' fluorescence excitation i.e. a fluorophore that excites around 500 nm using standard excitation will exhibit 2-photon excitation at ~1000 nm. The multi-photon effect requires very high photon density, so a high power laser is needed (pulsed lasers are the only practical option) and the effect is only seen at the plane of focus making multi-photon microscopy an inherently optical sectioning method. The longer wavelengths of light used for multi-photon imaging are beneficial for biological applications as infrared light is scattered less by biological tissue than visible light. This means that it's possible to image at depths in excess of 500 microns in live tissue, making multi-photon microscopy the method of choice for intravital imaging.
The Imaging Facility has a Nikon AX-MP mutli-photon microscope equipped with a dual laser system, and is optimally configured for intravital imaging applications.
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