Two-Photon Absorption Calculator

Two-photon absorption (TPA) occurs when a molecule simultaneously absorbs two photons of identical or different frequencies to transition from a lower energy state to a higher excited state. The computation relies on first determining the photon flux from the laser parameters, and then calculating the excitation per molecule. The photon flux (F) is derived using laser power (P), wavelength (λ), full-width at half-maximum pulse duration (FWHM), Planck's constant, and the speed of light: photons_flux = 0.0001 * ((2 * 1.17741^2) / (π * h * c)) * power * wavelength / (fwhm^2). Once the flux is established, the excitation per molecule (E) is calculated using the TPA cross-section (σ in Goppert-Mayer units) and exposure time (t): excitation_per_molecule = (photons_flux^2) * cross_section * exposure_time * 10^(-50) / 2.

Consider an experiment using a pulsed femtosecond laser with a power (P) of 0.5 Watts, a wavelength (λ) of 800 nanometers, and a pulse duration FWHM of 100 femtoseconds. Assume the fluorescent dye has a two-photon absorption cross-section (σ) of 50 Goppert-Mayer (GM), and the exposure time is 1 second. First, compute the photon flux using the optical parameters, yielding a high-density photon stream essential for nonlinear optical events. Next, substitute the squared flux, cross-section, and exposure time into the excitation equation. Factoring in the scaling constants, the calculator determines the average excitation events per molecule, providing vital insight into fluorophore photobleaching and signal brightness.

Ensure your input units match standard SI conventions before running calculations, particularly converting nanometers and femtoseconds appropriately. Remember that two-photon absorption depends quadratically on laser intensity, meaning minor fluctuations in laser power or pulse width will dramatically alter your excitation rates. Always account for collection efficiency and fluorophore saturation when translating theoretical calculations into practical laboratory setups.