Two-photon Laser

Two-photon absorption requires a fluorescent molecule to encounter two photons essentially simultaneously, making the event unlikely except where photon density is very high. Focusing the beam concentrates this intensity into a small region, so fluorescence is generated predominantly at the focal point rather than throughout the light path. This spatial restriction enables localized image formation in biological samples.

The laser delivers brief, high-intensity femtosecond pulses that create the photon density needed for simultaneous absorption. Its near-infrared light uses longer-wavelength photons than the emitted fluorescence, while still providing enough combined energy to excite the molecule. This arrangement supports imaging in biological specimens where longer-wavelength illumination is especially valuable, including relatively deep and scattering samples.

Because two-photon absorption occurs mainly at the beam focus, fluorescent molecules outside that focal region receive little excitation. The microscope therefore forms an image from a confined optical section instead of exciting the entire illuminated volume. Limited out-of-focus excitation reduces unwanted fluorescence and helps preserve spatial information when imaging cells, tissues, or neuronal activity.

A pulsed near-infrared beam is focused into a biological specimen containing fluorescent molecules. At the focal point, molecules absorb two photons and then emit a shorter-wavelength photon. Detecting this emitted fluorescence while the focus is positioned within the sample produces an optically sectioned image. The sequence can be applied to living cells, tissues, and neuronal activity.

This approach is useful when investigators need high-resolution fluorescence images from living cells or tissues, particularly in samples that scatter light or extend relatively deep. It also supports studies of neuronal activity, where localized excitation and optical sectioning can help distinguish signals from different focal regions. Reduced out-of-focus excitation further favors observations intended to limit photodamage.

The method can reveal fluorescence with high spatial localization in living cells, tissues, and neural samples. Since excitation is concentrated near the focal plane, images can separate structures or activity occurring at different depths while limiting signals from outside that plane. The resulting optical sectioning is especially relevant for examining biological organization and neuronal activity in scattering specimens.