[pnacze199204]

The question might be a little bit strange, but I'm interested if the Two Photon Absorption could occur in the shadow? Let's say we have an object that is in the shade and the rays of the sunlight reflect from the snow. As I know it is not true that TPA occurs only with hight intensity light, although it is more prominent using laser to observe the process.

[Borek]

In low light such processes are just below our detection limits.

Think in terms of probability of two photons of the right frequencies hitting a molecule at the same time. In low light the probability is much lower, but it still exists.

[pnacze199204]

On the Internet I found the information: "The long period of 30 years until 2PA was observed for the first time is reasonable considering the fact that a high intensity is needed for 2PA, which can only be achieved using a laser, which was first developed in 1960" or "2-Photon excitation is a very very rare event! In bright day light a good one- or two-photon absorber absorbs in a 1-photon process: once a second, in a 2-photon process: every 10 million years".

Does that mean that the process is so difficult to observe without using laser?

[Borek]

It is difficult to observe when the light intensity is low. That means you need a high intensity light source to make the process experimentally observable. Does it have to be a laser? No - any high intensity light source will do. At the same time laser is the best high intensity light sources we have so it is definitely a source of choice in this case.

[Corribus]

The key is both high power and high fluence, both of which a focused laser has.

[Borek]

The key is both high power and high fluence, both of which a focused laser has.

I am afraid this wording again reinforces the false idea that the process is not taking place at all when the intensity of the light is low.

[Corribus]

That's not implied at all. The reason TPA is for practical purposes only observed under laser irradiation is because lasers are both high power (# of photons/time) and, under focusing, high fluence (# of photons/area, e.g.). This doesn't mean there is a zero probability of TPA under irradiation with an incandescent light bulb, but the lower temporal and spatial photon density, in addition to the fact that the TPA effect is nonlinear with the field strength, means that probability of TPA occurring in this situation is astronomically low probability.*

The point of my post being: many people erroneously understand "high intensity" to be synonymous with "high power". But in fact, in many practical applications of nonlinear (or even linear) optical effects, the fluence is an equally important, or maybe even more important, consideration. Therefore it's important to define what is meant by "intensity" - this will help to understand why many optical effects are not observed (note: not the same as "do not occur") under normal irradiation conditions. A laser can be a very high power coherent light source that is sufficient to drive many optical processes, but this alone may not be enough to stimulate higher order optical effects, even in molecules that are designed to have strong nonlinear optical response. This effect is taken advantage of in what is called a Z-scan, where a nonlinear optical material is gradually brought into the focal plane of a laser (the z-direction) while keeping the laser power the same (see figure attached). This technique is one of the most common to measure the nonlinear absorptive properties of molecules in solution.

*For pedantic reasons, it's instructive to offer caveats to students about how the probability isn't mathematically zero, but on the flip side of the coin: to amateurs this can be counterproductive, and the opening post is a great example of why. It's hard to convey just how low probability some of these things are, and so it can be equally valuable to truthfully state that it basically just can't be observed in the timescale of human experience. I mean, you can rightfully point out that if you have a hydrogen atom just outside of the moon's orbit, there is a chance that its electron could potentially be found at the other end of the universe. The laws of quantum mechanics say so - the wavefunction never actually decays to zero, after all. But is that a useful thing to point out to someone who doesn't really understand the scales of probability involved?

(The figure I found by searching for z-scan on google. Reference: Applegate et al. Optics Express 2013, 21, 29637-29642 https://doi.org/10.1364/OE.21.029637)