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How do high-intensity lasers cause biphotonic processes?

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Underwater view of a glowing chain of tunicates with a scuba diver and boat visible near the water's surface in the background. — photochemical reaction Chain of fluorescent tunicates.

High-intensity light sources, such as lasers, enable biphotonic processes by providing the extreme photon density required for a molecule to absorb two photons of light.1

Under conventional conditions with moderate light intensity, the photochemical equivalence law dictates that one molecule reacts for every single quantum of radiation absorbed. However, when light intensity is sufficiently high, the probability of a molecule interacting with multiple photons simultaneously or in rapid succession increases significantly. This allows the molecule to reach higher excited electronic levels or undergo chemical changes that are not possible through the absorption of a single photon.123

These processes are a form of nonlinear interaction, where the response of the atomic oscillators is no longer simply proportional to the intensity of the incoming light. By utilizing these high-intensity pulses, researchers can study complex photochemical reactions and the evolution of excited states with extremely high temporal resolution.432