One of the things we’re investigating is pulsed laser systems, because they’re fun, but (especially for funding purposes) also because they are the basis of optical frequency combs (as I’ve mentioned). And things are pulsing along. One of the things that was noticed was that light from the pulsed system, running at 1560 nm, was showing up on a Silicon CCD camera. The Silicon response peaks at 900 nm and drops pretty sharply, petering out at 1100-1200 nm. There’s no way it should respond to a 1560 nm photon.

And it isn’t. It’s responding to pairs of 1560 nm photons. This is a pulsed system, so you have high peak power making it a lot easier to see nonlinear responses like two-photon transitions, because they scale as the square of the intensity. (more photons incident per unit time means a better chance to have two interacting at once, Having n photons means that if you look at any photon, the chance of another photon being around is n(n-1)) Two photons have enough energy for the interaction, since that’s the same as having a 780 nm photon, which is well above the “to be detected you must be this tall” energy cutoff

Here are two images. The square is a beamsplitter cube, and the white blob is the light. The top image is the pulsed laser, and the bottom one is a CW beam, both with around 10 mW average power.

The pulsed laser is saturating the heck out of the CCD, so the spot is really a lot brighter than from the CW beam, though we can’t say for sure based on this quick look. Even though the average power is about the same, though, the pulsed laser is repeating at about 10 MHz, and the pulses are less than a picosecond, so all of the light is being delivered in less than 10^-5 of the time, so the peaks have powers measure in kW.