The cross section , for a transition from the ground state to an excited state that is resonant with an EM field of = 808 nm, for a neodymium (Nd) atom doped into a YAG (yttrium aluminum garnet) crystal is ~3 1020 cm2. Assume that the dopant density of Nd in the YAG crystal is 10^20 atoms/cm3 and that the YAG crystal itself is transparent to 808-nm light. A diode laser with = 808 nm is used to pump an Nd:YAG laser rod. (a) Estimate the small-signal absorption coefficient for the Nd:YAG crystal. (b) Estimate the depth of penetration into to the Nd:YAG crystal at which the intensity of the diode laser beam attenuates to 1/e of its initial value. (c) One can define saturation intensity , for an absorptive medium as the intensity for which the loss coefficient is reduced by a factor of 2 from its small-signal value. Find the expression for ,.

Solve it last time you just gave me theotical answer that is not correct.