CasebyCase: Halfing the amplitude of light

[renrut13]

Is there a way to reduce the amplitude of a light wave to half its value in one case and do nothing in the other cases?

Three Cases:

Case A: Light amplitude input is 2 - reduce to 1 on output (cut in half)
Case B: Light amplitude input is 1 - output is 1 (do nothing/pass through)
Case C: Light amplitude input is 0 - output is 0

I would like to design a hi-speed device that would allow this to occur. By hi-speed I mean instantaneous or without converting to/from any electronics. I can almost envison a possibility where you have some kind of photon absorbing medium that doesn't kick in until a certain intensity is reached. Does anyone have any insight or could point me into a(ny) direction with this?

 

[cesiumfrog]

Broadly, look into passive nonlinear optics.

 

[Andy Resnick]

Both Kerr lenses and saturable absorbers can be employed to do this. See how femtosecond pulses are generated.