Two Photon Absorption Calculator

Two Photon Absorption Calculator

The Two Photon Absorption Calculator estimates how many two-photon excitations occur per molecule during laser irradiation. Two-photon absorption (TPA) is a nonlinear optical process where a molecule simultaneously absorbs two photons to reach an excited state.

The excitation rate per molecule is:

$$N = \frac{1}{2} \cdot \delta \cdot \phi^2 \cdot \tau$$

where $\delta$ is the TPA cross-section in GM (1 GM = $10^{-50}$ cm$^4 \cdot$ s $\cdot$ ph$^{-1}$), $\phi$ is photon flux in ph/(cm$^2 \cdot$ s), and $\tau$ is exposure time in seconds.

Photon flux at the beam center is derived from laser power $P$, wavelength $\lambda$, and focus FWHM:

$$\phi = \frac{I \lambda}{hc}, \quad I = \frac{2P}{\pi w^2}, \quad w = \frac{\mathrm{FWHM}}{\sqrt{2 \ln 2}}$$

For a 10 W, 840 nm laser with 20 μm FWHM, 210 GM cross-section, and 1 s exposure, the calculator gives about $9.33 \times 10^{24}$ ph/(cm$^2 \cdot$ s) and roughly 91 excitations per molecule.

Related tools: Beer Lambert Law Calculator and Wavelength Calculator.