Re: two-photon absorption

Posted by Smith, Benjamin E. on
URL: http://confocal-microscopy-list.275.s1.nabble.com/two-photon-absorption-tp7581862p7581872.html

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I find Paul Falstad's simulation helpful in visualizing energy state transitions.  The model shows how quantum state transitions can be thought of as continuous: http://www.falstad.com/qmatomrad/

Since virtual states are unmeasurable, two photon excitation can be thought of as a coupling process between the two photons and the induced dipole:  http://researchspace.auckland.ac.nz/bitstream/2292/433/2/02whole.pdf

The way I imagine it is as two separate yet simultaneous steps.  The first photon induces a dipole oscillation to a virtual state.  Effectively simultaneously, a second photon also induces a further dipole oscillation kicking the dipole all the way up to an observable energy state.  The key is that both photons need to act on the molecule simultaneously, or the "virtual state" dipole disappears before the next photon can induce any further energy.

An analogy would be trying to roll a boulder up onto a log.  Two people of equal strength set about the task of rolling over a boulder up onto a log.  Both people have enough strength to get the boulder to rock back and forth, but neither can get it up onto the log by themselves.  If one person tries to move the boulder, the boulder will rock back and forth, but the moment they stop, the boulder will come to a rest as though nothing had been done to it.  The rocking boulder can be thought of as a virtual state, where the boulder will drop back to its "ground state" the moment someone stops rocking it.  

However, two people decide to work together (this is like the coupling model).  They time their rocking together, and by doubling their effort, they are able to roll the boulder all the way over to the meta-stable state of being on top of the log.  The boulder will then reside atop the log for some time before dropping back down, emitting energy equal to both people's contribution, minus some energy due to friction an noise (just like two-photon emission).

The reason why two photon excitation needs to be thought of as a coupling process, is let's think about a third scenario.  Person A starts to rock the boulder.  Then, person A lets go of the boulder and then person B immediately continues rocking the boulder.  Then end result of this effort would be a rocking boulder, however, since A and B aren't acting on the boulder at the same time, they won't ever be able to transition the boulder up onto the log.

Finally, let's imagine a world where the boulder can only be measured based on it's height of the ground.  Rocking neither raises nor lowers the boulder, so it wouldn't be an observable state.  However, we know that the boulder would have had to have been rocked back and forth to get it up onto the log, so while we can't observe it, we know it must exist.

Hope this helps,
   Ben Smith

Benjamin E. Smith, Ph.D.
Samuel Roberts Noble Microscopy Laboratory
Research Scientist II
University of Oklahoma
Norman, OK 73019
E-mail: [hidden email]
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________________________________________
From: Confocal Microscopy List [[hidden email]] on behalf of John Oreopoulos [[hidden email]]
Sent: Monday, April 07, 2014 8:06 AM
To: [hidden email]
Subject: Re: two-photon absorption

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If you're into quantum mechanical calculations, there is actually a fairly good derivation of the 2-photon process here at this link:

http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Electronic_Spectroscopy/Two-photon_absorption

Cheers,


John Oreopoulos
Staff Scientist
Spectral Applied Research Inc.
A Division of Andor Technology
Richmond Hill, Ontario
Canada
www.spectral.ca

On 2014-04-07, at 8:35 AM, Chen, De (NIH/NCI) [C] wrote:

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> To my knowledge, one photon excitation is linear optical process (dipole excitation, which is proportional to light intensity I), while two photon excitation is nonlinear (quadrupole excitation, proportional to I^2) ; In two photon absorption, an intermittent energy level close to one photon resonance will enhance the two photon excitation depending on the offset from the resonance energy level. Two photon excitation requires high laser power to excite. Short pulsed laser can provide the required high enough peak power to excite with two photon. Forbidden in dipole excitation is allowed in quadrupole excitation.
>
> Reference:
>   "The Principles of Nonlinear Optics" Y. R. Shen, 1984 | ISBN-10: 0471889989 | ISBN-13: 9780471889984
>
> ________________________________________
> From: Marcus Knopp [[hidden email]]
> Sent: Monday, April 07, 2014 5:53 AM
> To: [hidden email]
> Subject: Re: two-photon absorption
>
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> As far as I know, the effect, on the one hand, depends on the timing of the subsequent photons to be absorbed. They have to reach a fluorophore within a short time interval of sub-femtoseconds to a few femtoseconds, i.e. quasi simultaneously. On the other hand, it depends on the symmetry of a fluorophore, which, I think, determines the transition matrix between energy levels. Then it all comes down to whether a transition is allowed or not by the selection rules, which constrain transition for example by the need to conserve an electron's angular momentum. What exactly is going on, I don't know (does anybody?), but one interpretation is indeed, that the first photon induces a transition from the ground state to a virtual excited state at an intermediate energy level (what's wrong with that? It's just a model). This is thought to be close to a real state that can be occupied according to the selection rules. The second photon carries the system from the virtual state to the final state, that originally might have been forbidden.
>
> Best,
> Marcus
>
>
> -----Original Message-----
> From: Confocal Microscopy List [mailto:[hidden email]] On Behalf Of Martin Wessendorf
> Sent: Monday, April 07, 2014 4:24 AM
> To: [hidden email]
> Subject: two-photon absorption
>
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> Dear List--
>
> Is there a physicist out there who can offer an intuitive explanation of how 2-photon absorption occurs?  I expect we all know that it isn't that there isn't any half-excited state that allows one photon to boost an electron half-way to the excited state, and the next photon to finish the job.  My sense is that it has to do with time-energy uncertainty (a la Heisenberg) but my quantum mechanics is elementary-school level.
>
> Thanks!
>
> Martin Wessendorf
>
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