This meaning is really great and cannot be simply explained. Some things need to be done before they can be done. The simplest example is a laser pointer. A Nd:YAG laser can emit green light by putting a frequency doubling crystal. People can see it, but they can't see it without frequency doubling. For example, the interaction between some lasers and materials is very weak at long wavelength, and will become stronger after frequency doubling to short wavelength. There is also the use of frequency doubling to measure pulse width when measuring ultrashort pulses, such as SHG's spider technology.

If I don't do too much nonlinearity, the research status will be unclear. There are many applications, and the basic principle should be clear, but people are still exploring frequency doubling materials and phase matching technology.