# Anisotropic nonlinear refractive index measurement of a photorefractive crystal via spatial self-phase modulation

## TL;DR

This paper introduces a simple method to measure and control the anisotropic nonlinear refractive index in a photorefractive crystal using spatial self-phase modulation and far-field diffraction analysis.

## Contribution

The study presents an accessible technique for accurately measuring and controlling the anisotropic nonlinear refractive index in photorefractive crystals through far-field diffraction patterns.

## Key findings

- Measured saturation intensity and maximum refractive index modification.
- Characterized the anisotropic nonlinear response of the crystal.
- Demonstrated control of refractive index modification via external electric field.

## Abstract

We show that the refractive index modification photoinduced in a biased nonlinear photorefractive crystal can be accurately measured and controlled by means of a background incoherent illumination and an external electric field. The proposed easy-to-implement method is based on the far-field measurement of the diffraction patterns of a laser beam propagating through a self-defocusing medium undergoing spatial self-phase modulation. For various experimental conditions, both saturation intensity and maximum refractive index modification have been measured. We also clearly evidence and characterise the anisotropic nonlinear response of the crystal in the stationary regime.