Second-order optical nonlinearities from χ(2) gratings induced by holographic all-optical poling

The paper presents the second-order optical nonlinearities from χ(2) gratings induced by holographic all-optical poling for azobenzene polymer. Second harmonic (SH) signal along the directions with two different vectors was measured. One is strong SH signal diffracted in the same direction as 2ω writing beam with wave vector k2ω and the other is weak SH signal diffracted in the direction of wave vector of 4kω - k2ω + Δk where kω is wave vector of ω beam and Δk is the wave vector mismatch whose vector is parallel to kω. The latter signal was used as a tool to monitor the formation of holographic χ(2) gratings in real-time because it has off-axis wave vector different from both kω and k2ω. The increase of 2ω intensity on poling process led to the large increase of second-order optical nonlinearity. The real-time monitoring showed that it also gave the large relaxation of second-order optical nonlinearity on poling process. The increase of 2ω (532 nm) energy enhanced the increase of local heating, which led to easier alignment of azobenzene chromophore and also larger relaxation of aligned chromophore.