sentences:  Also, MultiLevel Recording, where data is written in "grayscale" rather than as "on" and "off" signals, is technically feasible.Although there are many nonlinear optical phenomena, only multiphoton absorption is capable of injecting into the media the significant energy required to electronically excite molecular species and cause chemical reactions.Two-photon absorption is the strongest multiphoton absorbance by far, but still it is a very weak phenomenon, leading to low media sensitivity.