Diffraction can mimic saturation in multiphoton absorbers

Many traditional investigations of saturation in multiphoton absorbers with the z-scan method use an approximate analytical formula that assumes a steady-state approximation. Using a numerical simulation for Maxwell’s equations for laser propagation including diffraction and coupled electron population dynamics, we show that the commonly used analytical formula for determining saturation in multiphoton absorbers is often incorrect, even when the sample thickness is only one diffraction length. Using published experimental data on an organic chromophore, we show that saturation, in fact, does not occur at the laser intensity values predicted for these two and three photon absorbers. We numerically fit the published experimental z-scan data and obtain new absorption coefficients for multiphoton absorbers that accurately reflect their intrinsic values. The new values are from three to ten times larger than the published values. Because multiphoton absorbers are being used more extensively in many applications such as optical limiter, medical diagnostics and two photon microscopy, it is important to have accurate values for the two and three-photon absorption coefficients. Knowing the real value of the multiphoton absorber coefficients, even for a single diffraction length, is therefore of the utmost importance. In particular, the laser intensity at which the absorber saturates can determine which absorber is useful for a particular application.