What is the most interesting interaction between light and materials that you have discovered?

Question: What is the most interesting interaction between light and materials that you have discovered?
- Keywords:
  discover,
  interaction,
  light,
  material
Asked by 859dagk22 to Hugh on 13 Nov 2017.
- Hugh J. Byrne answered on 13 Nov 2017:
  
  I don’t think I have discovered any new ones, but while doing my PhD, I was looking at a process whitch was “7th order”, which meant you needed to input 7 photons of light to produce 1 out.

Question: What is the most interesting interaction between light and materials that you have discovered?