Measurement of pure optical nonlinearity in carbon disulfide with a high-repetition-rate femtosecond laser

Abstract

Using the close-aperture Z-scan technique, the pure nonlinear refractive index (n₂) of carbon disulfide is measured with a 76 MHz repetition rate femtosecond laser. Strong interference of thermal effects exists with high-repetition-rate lasers that result in negative values of n₂. We remove the thermal effect completely by continuously increasing the sample flow rate (F) in a sample cell as indicated by the change in sign of n₂ from negative to positive. The positive value of n₂ is due to Kerr-type nonlinearity. At sufficiently high values of F of >25 ml/min, all thermal effects are removed, resulting in an n₂ value that matches low-repetition-rate experiments.