# Selective transmission/absorption of light based on amplitude?

1. Feb 1, 2012

### franco1991

Is there any process or device that will allow a transmission of an EM wave of a certain amplitude (say 2) but will block transmission of a lesser amplitude (say 1), such that an EM wave of a given amplitude of blocked, but a EM wave with 2x the amplitude will pass thru (even if that amplitude is reduced/modulated in the process)?

For example, what if we have a material which absorbs light. Increased amplitude means a larger number of photons. If we make the material a small enough quantity such that it absorbs all the photons of the lesser-amplitude wave (amp of 1), but when a higher amplitude wave (amp of 2), which has more photons, will be transmitted b.c there aren't enough atoms in the sample to absorb all the photons of that larger-amplitude wave.

Is this feasible? Is there anything wrong with my thinking here? Also, are there any other processes/devices which could accomplish this (block waves of a certain amplitude, but allow transmission of a wave with a larger amplitude)?

2. Feb 1, 2012

3. Feb 1, 2012

### Lambduh

As Naty1 linked above saturable absorption works in a similar fashion to what you proposed. When light intensity is high enough, there aren't enough electrons left in the ground state(due to finite upper state lifetimes) to absorb incoming photons. Subsequently, transmission increases until those electrons thermalize/recombine back down to the ground state.

http://www.rp-photonics.com/saturable_absorbers.html

4. Feb 1, 2012

### franco1991

Yes, I read about saturated absorption, but I'm not sure whether that is what I'm suggesting.

Particularly, is saturated absorption what I described (more photons than the atoms can absorb at a given time) or it it some specialized phenomena (the atoms taking longer to switch from energized to ground states, thus they don't absorb/re-emit photons at the sam rate as they normally would) caused by energizing the atoms to a higher-than-normal excited state?

If it is the first, it's the same; if it is the second, it isn't. I might be wrong, but I'm skeptical that it's the same thing, because saturated absorption seems to rely completely on high intensity waves, whereas if it were the same as the first (more photons than atoms, thus more photons than can be absorbed per unit time) then we wouldn't need high intensity waves; we could just decrease the quantity of material (thus decreasing the number of atoms, and thus the number of photons that can be absorbed per unit time).

Can anyone resolve this discrepancy??