Nonlinear crystals are materials that exhibit a nonlinear response to electric fields or light. This means that their optical properties change in a way that is not directly proportional to the intensity of the light passing through them. These crystals are essential in various applications, such as frequency conversion and laser technology, where they help generate new wavelengths of light. Common examples of nonlinear crystals include beta barium borate (BBO) and potassium titanyl phosphate (KTP). They are widely used in optical devices and telecommunications to enhance signal processing and improve the efficiency of light-based technologies.