Feature

Nonlinear Polarization Beats Spectroscopy

Alain Villeneuve, G.I. Stegeman, Stewart Aitchison and C.C. Yang

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- Fraction 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. fied 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. Below 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 these 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.