Nonlinear crystals are special materials that change their optical properties in response to intense light. Unlike linear crystals, where the response is directly proportional to the light intensity, nonlinear crystals can generate new frequencies of light, allowing for various applications in optics and telecommunications. These crystals are essential in devices like lasers and frequency converters, where they help produce different wavelengths of light. Common examples of nonlinear crystals include beta barium borate (BBO) and potassium titanyl phosphate (KTP), which are widely used in scientific and industrial applications to manipulate light in innovative ways.