Nonlinear ultrasonic research focuses on studying the interactions between high-frequency sound waves and materials in a nonlinear acoustic regime. In this area of research, the traditional linear relationship between the input signal and the output signal of ultrasonic waves is no longer valid, leading to the generation of harmonics and other nonlinear effects. By analyzing these nonlinear behaviors, researchers can gain insights into the microstructural properties of materials, detect defects or damage in structures, and develop new techniques for nondestructive testing and evaluation. Nonlinear ultrasonic techniques have applications in various fields such as material science, structural health monitoring, medical imaging, and industrial inspection.