Temperature Dependence of Nonlinear Optical Coefficients in Ferroelectric NaNO2

In ferroelectric NaNO₂, optical rotation as a function of temperature was found to be linearly proportional to the spontaneous polarization as a function of temperature. When a single‐domain crystal was switched by applying a dc electric field, the sense of optical rotation was reversed. Multiple‐domain crystals exhibited no net rotation. The temperature behavior is explained in terms of the long‐range order parameter and crystal symmetry. It is concluded that coefficients for nonlinear effects which are elements of odd‐rank tensors will have a similar temperature dependence. The predicted temperature dependence of each second‐harmonic coefficient X_ijl is given by $X_{\mathrm{ijl}}{\mathrm{(T)=K}}_{\mathrm{ijl}}{P}_z^0\mathrm{(T)}$ , where K_ijl is constant and P⁰ the spontaneous polarization. This prediction is in good agreement with measurements reported by Yanagi et al.