Measurement of third-order nonlinear refraction using a nonlinear-imaging technique with positive-negative phase object

A unique phase object (PO) with both positive and negative phase retardation is used in the nonlinear-imaging technique with a phase object (NIT-PO) to measure the nonlinear refractive index of materials. Using positive-negative PO, one can increase the sensitivity of the NIT-PO compared with the conventional uniform PO. The improvement of sensitivity is especially significant at negative nonlinear shift regions, and can reach an order of magnitude near the nonlinear phase shift Δℓ_NL=-π. The sensitivity increase of the NIT-PO with positive-negative PO is from 1.7 to 3.0 times greater when compared with the Z-scan technique within Δℓ_NLℓ|π|. An empirical expression is obtained which allows for direct calculation of the Kerr coefficient from measured the nonlinear image. CS₂ is used to demonstrate the measurement using a positive-negative PO.