Second-harmonic generation
（重定向自Frequency doubling）
![Energy level scheme of SHG process.](/Images/godic/202501/14/Energy_level_scheme_of_SHG0849.png")
![An electron (purple) is being pushed side-to-side by a sinusoidally-oscillating force, i.e. the light's electric field. But because the electron is in an anharmonic potential energy environment (black curve), the electron motion is not sinusoidal. The three arrows show the Fourier series of the motion: The blue arrow corresponds to ordinary (linear) susceptibility, the green arrow corresponds to second-harmonic generation, and the red arrow corresponds to optical rectification.](/Images/godic/202501/14/Electron_asymmetric_motion_animation0849.gif")
![Diagram of the second harmonic generation process.](/Images/godic/202501/14/Second_Harmonic_Generation.svg0849.png")
![A depiction of the second harmonic generation set up for measuring the orientation of phenol at the air-water interface.](/Images/godic/202501/14/SHG_phenol_air-water0849.png")
Second harmonic generation (also called frequency doubling or abbreviated SHG) is a nonlinear optical process, in which photons with the same frequency interacting with a nonlinear material are effectively "combined" to generate new photons with twice the energy, and therefore twice the frequency and half the wavelength of the initial photons. Second harmonic generation, as an even-order nonlinear optical effect, is only allowed in media without inversion symmetry. It is a special case of sum frequency generation.