Anomalous behavior of nonlinear refractive indexes of CO2 and Xe in supercritical states

Direct measurement of pressure dependent nonlinear refractive index of CO2 and Xe in subcritical and supercritical states are reported. In the vicinity of the ridge (or the Widom line), corresponding to the maximum density fluctuations, the nonlinear refractive index reaches a maximum value (up to 4.8*10-20m2/W in CO2 and 3.5*10-20m2/W in Xe). Anomalous behavior of the nonlinear refractive index in the vicinity of a ridge is caused by the cluster formation. That corresponds to the results of our theoretical assumption based on the modified Langevin theory.