Talk:Two-photon absorption

How does this compare to "Two-photon excitation"? Should that term be linked back to here? Should a redirect of that come here? Should this be renamed to that? Or am I just missing it? -RJFerret 05:15, 12 July 2006 (UTC)

Hi,

as far as i know, two photon excitation also includes effects such as sum frequency generation (SFG). Thus, TPA should be a part of two photon excitation (a page with links should be enough). --lagaffe from wikipedia.fr (144.204.16.1 13:56, 22 November 2006 (UTC))

This needs work.
I'm also saying that this is a really poor article. It tries to reason (incorrectly) why the 2 photon absorption process "should" be called 3 photon absorption. This is totally incorrect and that's just the beginning.

I just made some edits in the article, but I'm pretty unsatisfied. It's a bit of a dog's dinner and could do with a rewrite all over. Yeah, I know, I'm whining when what I should really do is get down and do it myself...

In particular the "Measurements" part, which was much needed, could now do with some expansion (I also personally don't like the use of "β" for cross-section, but that's a whole other discussion).

I'll make an effort to come back and do something when I can find time...

* I hope I may write here something.. very nice, that you inserted the 'Measurements' part.. but where from do you have that formula? What about References?? I'm afraid I shouldn't have Wikipedia in my           References in my Bachelor-Thesis.. but I need this formula and can't find it nowhere else Lg           Alex, 31.08.2010 —Preceding unsigned comment added by 141.84.219.200 (talk) 12:44, 31 August 2010 (UTC) • TheBendster (talk) 7 May 2007, 07:56 (UTC)


 * Its me again... I am not an expert at all in this field just trying to learn. I have spent considerable time trying to find the definition of the cross-section and understanding it. I am still clueless why the dimensions are what they are: 10-50cm4.s.photon-1molecules-1 so if you can help out it would be great. V8rik 20:02, 7 May 2007 (UTC)


 * Well, (almost a year later!) I've done that for you. • TheBendster (talk) 28 April 2008, 07:34 (UTC)

Hi, I believe the reference to the first experimental verification is incorrect. I've recently been studying this topic and came across a review paper of the subject, Cagnac, B., 1994, Laser Physics, Vol. 4, No. 2, that references [http://prola.aps.org/abstract/PR/v79/i2/p314_1 Hughes, V.W. and Grabner, L., 1950,Phys. Rev., 79, 314, 819] as the first experimental observation of the two photon absorption phenomenon. Refer to section V. of the Hughes and Grabner paper. - user Generalphas —Preceding unsigned comment added by Generalphas (talk • contribs) 19:29, 21 May 2010 (UTC)

The Hughes and Grabner paper reports a single RF absorption line at half the frequency of a known line. The line didn't exhibit some of the characteristics expected of a 2-photon absorption, such as nonlinear behaviour with RF pump power. Furthermore none of the other observed absorption lines had a half energy twin line, which could not be explained. In addition, the observation of both the single photon and possible two-photon resonance means that parity could not be invoked to discriminate between the two lines. Therefore this assignment was speculative and could not be considered as a definitive observation 2-photon absorption. Of course this paper reveals that people were looking for such an effect. —Preceding unsigned comment added by 130.102.172.3 (talk) 00:17, 3 June 2010 (UTC)

Expert tag
I wouldn't say that this article is great, but I don't think it needs the Expert tag at this point. Any dissenters to the idea of removing it? • TheBendster (talk) 28 April 2008, 07:35 (UTC)

Expert tag
Howdy Bendster:

I do research with TPA and TPEF so this is right up my alley, but I have not done editing in wikipedia before. first - the TPA cross section is usual a lower case delta or with a subscript 2. The units are correct... the cross section takes into account the idea that two separate events must occur in a given region for the extinction of photon in the process (this happens twice though, which is accounted for later in the constitutive equation) - it is sometime best to think of it as "the likelihood of the photon in questions consumption over a give region x the likelihood that another will be there and go with it x the inverse of photons provided/second" so we get cm^2 x cm^2 x (hv/s)^-1 = cm^4 sec / photon. Then the whole mess is per molecule... sometimes per mol or (when MW is high) per repeat unit.

I'm finishing up my phd in molecular optics, and I've only published twice in the area... not sure if this qualifies me as an expert. What is the criteria..? Anyway, hope that helped. J.e.raymond (talk) 17:47, 8 May 2008 (UTC)


 * Hi there. If you think that you can make a better rationalization for the units of cross-section than I did, then please go right ahead. As to the symbol for moleucular cross-section, although delta is very common, sigma is also seen a lot, particularly among chemists. As far as I know there's no accepted "correct" symbol, but correct me (and the article) if I'm wrong. Beta is generally used for the bulk cross-section (i.e. the 2-photon equivalent to OD). In general, any additions you can make to the article would be great. You're certainly expert enough, and anyway the community is here to correct any errors. If you wat to go further, you could start a new article on TPEF... WP could certainly do with one, and I'll pledge to help with it if you can put up the bones. • TheBendster (talk) 9 May 2008, 06:09 (UTC)

Selection Rules
I think the discussion of selection rules is misleading as written:

"The selection rules for TPA are therefore different than for one-photon absorption (OPA), which is dependent on the first-order susceptibility. For example, in a centrosymmetric molecule, one- and two-photon allowed transitions are mutually exclusive. In quantum mechanical terms, this difference results from the need to conserve spin. Since photons have spin of ±1, one-photon absorption requires excitation to involve an electron changing its molecular orbital to one with a spin different by ±1. Two-photon absorption requires a change of +2, 0, or −2."

Only circularly polarized light carries angular momentum. Linearly polarized light does not, but there are still different selection rules for one- and two-photon absorption. These selection rules have to do with the symmetry of the states under inversion (or parity). The spin of the states is the same for both processes.

Perhaps the article could be amended so that the example refers explicitly to circularly polarized light, or generalized to include other examples of one- and two-photon processes that probe states with different symmetries.

Jmkinder1 (talk) 20:34, 3 June 2008 (UTC)


 * All photons carry one unit of spin, regardless of polarization. -- Fuzzyeric (talk) 15:17, 3 November 2009 (UTC)

The selection rules for two-photon absorption are primarily distinguished from those of single-photon absorption by parity: Using a quantum mechanical treatment, time dependent perturbation theory expands the initial quantum state into an infinite series of powers in the interaction Hamiltonian. The first order term can be used to study one photon absorption, while the second term, which is quadratic in the interaction can be used to derive the two-photon absorption term. For a light matter interaction, the appropriate interaction term is the electric dipole operator. As the electric dipole operator has odd parity, the parity of the initial and final states following a single photon absorption event must be opposite for the transition matrix element to be non-zero. This provides the well known selection rules for atomic gases for transitions between different angular momentum states. In the case of two-photon absorption the interaction is proportional to the square of the dipole operator, which then has even parity. Therefore the initial and final states must have the same parity following a two-photon absorption event, regardless of any possible polarisation selection rules. This property has been used to test the parity of quantized nanocrystal states. For example, it was found that the band edge exciton in CdSe nanocrystals had a finite two-photon absorption cross-section, indicating that the state has a mixed parity, contrary to theoretical expectation.

In general any possible polarisation selection rules will be additional to the stronger parity selection rule, and are much more dependent on the details of the initial and final state magnetic fine structure. Often the energy degeneracy of different levels mean that polarisation selection rules modify the total TPA cross-section. In atomic gases, when explicitly considering transitions between individual fine structure states, then polarisation selection rules become important. —Preceding unsigned comment added by 130.102.172.3 (talk) 06:14, 1 June 2010 (UTC)

TPA Coefficients
Hi, I am a little confused with the formula for the two-photon absorption coefficient \beta, because when I insert the magnitudes N and \sigma in units with cm how can then \beta have the unit of m/W? A formula for \beta is published in Kato et. al. Chem. Eur. J. 2006, 12, 2303-2317, unfortunately the units of \beta are not mentioned. However it seems as if in that paper \beta is cm/W because all length are given in cm in that paper. The rest of the formula is equal to that given here on Wikipedia. Could some expert comment on that! — Preceding unsigned comment added by 140.78.174.177 (talk) 08:43, 10 September 2012 (UTC)

I want to make a comment on the I(x) for 2-photon absorption. Is x the the cross section or beta? — Preceding unsigned comment added by 218.248.45.66 (talk) 12:47, 14 January 2014 (UTC)