Two photons can be merged to generate one photon with doubled frequency in nonlinear optical materials. This process is known as second harmonic generation (SHG), which was first discovered in 1960s, immediately after the invention of the laser. Since then, the development of SHG has led to many applications in advanced technologies, such as on-chip light sources, imaging, sensing, and communications. For example, SHG-based imaging devices, which capture the near-infrared (NIR) light and emit light in the visible range, are at the heart of developing novel all-optical NIR imaging technologies, such as night vision.