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"It is difficult to find third order nonlinear optical materials with ultrafast response and low loss that satisfy material figures of merit defined for all-optical switching. The key problem is to be able to achieve a 2πn2ILeff/λ > π nonlinear Phase shift where n2 I is the nonlinear index change and Leff is one absorption α-1 > Leffwith the absorption α containing linear α0 and nonlinear β2I + β3I2 + .. contributions where β2 and β3 are the two and three photon absorption coefficients, respectively. To date, these figures of merit have been satis-\r\nFraction of the power output into the bar and cross channels of a one -half beat length nonlinear directional coupler when the input intensity in one channel (bar) is varied.\r\nfied by glasses and a few organic materials. As a result, it has proven very difficult to make efficient all-optical switching and demultiplexing devices in integrated optics formats.\r\nBelow one half the bandgap of a semiconductor, the two photon absorption approaches zero and the linear absorption can also be very small. For example, for Al0.18GaO.82As, which has a bandgap at 750 nm, operation at 1550 ran satisfies\r\nthese conditions very well and we find that total nonlinear phase shifts >5π are achievable with low loss and sub-picosecond response. This wavelength is also very attractive for communications."
<https://www.optica-opn.org/home/articles/volume_3/issue_12/features/nonlinear_polarization_beats_spectroscopy/>