在非线性光学材料领域，什么是THG效应?还有THG强度？

* Second harmonic generation (SHG), or frequency doubling, generation of light with a doubled frequency (half the wavelength);
* Sum frequency generation (SFG), generation of light with a frequency that is the sum of two other frequencies (SHG is a special case of this);
* Third harmonic generation (THG), generation of light with a tripled frequency (one-third the wavelength) (usually done in two steps: SHG followed by SFG of original and frequency-doubled waves);
* Difference frequency generation (DFG), generation of light with a frequency that is the difference between two other frequencies;
* Parametric amplification, amplification of a signal input in the presence of a higher-frequency pump wave, at the same time generating an idler wave (can be considered as DFG);
* Parametric oscillation, generation of a signal and idler wave using a parametric amplifier in a resonator (with no signal input);
* Parametric generation, like p