Can SimphoSOFT model a reversible saturable absorber?
Where can I define a saturation intensity parameter I0?
SimphoSOFT is capable of simulating reversible saturation absorption. However, it does not require a user to specify a value for saturation intensity I0 as the program takes care of saturable absorption automatically by explicitly modeling ground state depletion.
Introducing the saturation intensity I0 is a way to avoid solving rate equations to the full extent. It shows the intensity level at which the ground state will be depleted which will drastically reduce absorbing power of the active material. Up to some extent, one can just solve a propagation equation by using the intensity-dependent absorption rate A.
[CONVENTIONAL MODEL : with saturation intensity I0]
∂I / ∂z = − A(I,z) I^2
A(I,z) = α/(1+I/I0)
SimphoSOFT goes much further. It models depletion of the ground state explicitly by calculating the dynamics of population densities of all the energy levels. For example, if absorption from the ground state to the first excited state is strong and the relaxation is slow, the ground state will be depleted and the attenuation term in the propagation equation will be decreased substantially, which will stop absorption.
∂I(z,t) / ∂z = − α N0(z,t) I(z,t)^2
∂N0(z,t) / ∂t = − α N0(z,t) I(z,t)^2 /2 h ω0 – k10 (N-N0)
What is important is that a user provides a “intrinsic” value for the two-photon absorption cross-section.
To run this analysis in SimphoSOFT, one needs to define data snapshots in Run Configuration / Output tab. One should choose “Dyn popln dty” to output dynamics of population density at a desired sample depth and the distance from the beam center.