http://confocal-microscopy-list.275.s1.nabble.com/two-photon-absorption-tp7581862p7581880.html
themselves. e.g., deriving the sixth power law or explaining what exactly
gets Œtransferred¹ if not a photon. And if it is not a photon they why
bang? And so on down a hole chasing a rabbit with a pocket watch. Note
we specify how far down we want-slash-need to go. Allowing of course that
Timothy Feinstein, Ph.D. | Confocal Manager
>*****
>To join, leave or search the confocal microscopy listserv, go to:
>
http://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDEN>8ogAJZQ&u=http%3a%2f%2flists%2eumn%2eedu%2fcgi-bin%2fwa%3fA0%3dconfocalmic
>roscopy
>Post images on
>
http://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDBZ>98ApdNQ&u=http%3a%2f%2fwww%2eimgur%2ecom and include the link in your
>posting.
>*****
>
>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://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDEw>s8g8KPA&u=http%3a%2f%2fwww%2efalstad%2ecom%2fqmatomrad%2f
>
>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://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDBV>4oQwLPA&u=http%3a%2f%2fresearchspace%2eauckland%2eac%2enz%2fbitstream%2f22
>92%2f433%2f2%2f02whole%2epdf
>
>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]
>Voice 405-325-4391
>FAX 405-325-7619
>
http://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDBF>59QAMNg&u=http%3a%2f%2fwww%2emicroscopy%2eou%2eedu%2f
>
>
>________________________________________
>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
>
>*****
>To join, leave or search the confocal microscopy listserv, go to:
>
http://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDEN>8ogAJZQ&u=http%3a%2f%2flists%2eumn%2eedu%2fcgi-bin%2fwa%3fA0%3dconfocalmic
>roscopy
>Post images on
>
http://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDBZ>98ApdNQ&u=http%3a%2f%2fwww%2eimgur%2ecom and include the link in your
>posting.
>*****
>
>If you're into quantum mechanical calculations, there is actually a
>fairly good derivation of the 2-photon process here at this link:
>
>
http://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDEE>roVoOPA&u=http%3a%2f%2fchemwiki%2eucdavis%2eedu%2fPhysical%5fChemistry%2fS
>pectroscopy%2fElectronic%5fSpectroscopy%2fTwo-photon%5fabsorption
>
>Cheers,
>
>
>John Oreopoulos
>Staff Scientist
>Spectral Applied Research Inc.
>A Division of Andor Technology
>Richmond Hill, Ontario
>Canada
>
http://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDEx>5pVtaZw&u=http%3a%2f%2fwww%2espectral%2eca
>
>On 2014-04-07, at 8:35 AM, Chen, De (NIH/NCI) [C] wrote:
>
>> *****
>> To join, leave or search the confocal microscopy listserv, go to:
>>
>>
http://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDE>>N8ogAJZQ&u=http%3a%2f%2flists%2eumn%2eedu%2fcgi-bin%2fwa%3fA0%3dconfocalm
>>icroscopy
>> Post images on
>>
http://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDB>>Z98ApdNQ&u=http%3a%2f%2fwww%2eimgur%2ecom and include the link in your
>>posting.
>> *****
>>
>> 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
>>
>> *****
>> To join, leave or search the confocal microscopy listserv, go to:
>>
>>
http://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDE>>N8ogAJZQ&u=http%3a%2f%2flists%2eumn%2eedu%2fcgi-bin%2fwa%3fA0%3dconfocalm
>>icroscopy
>> Post images on
>>
http://scanmail.trustwave.com/?c=129&d=zbTC0zpLoupwLE6t2HWM55ZZjbebwN5SDB>>Z98ApdNQ&u=http%3a%2f%2fwww%2eimgur%2ecom and include the link in your
>>posting.
>> *****
>>
>> 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
>>
>> *****
>> To join, leave or search the confocal microscopy listserv, go to:
>>
>>
http://scanmail.trustwave.com/?c=129&d=zrTC0wJ3Btfng3o6Ki8VL3Twis07aGkKBI>>yvzD1R-A&u=http%3a%2f%2flists%2eumn%2eedu%2fcgi-bin%2fwa%3fA0%3dconfocalm
>>icroscopy
>> Post images on
>>
http://scanmail.trustwave.com/?c=129&d=zrTC0wJ3Btfng3o6Ki8VL3Twis07aGkKBN>>munjcFqA&u=http%3a%2f%2fwww%2eimgur%2ecom and include the link in your
>>posting.
>> *****
>>
>> 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
>>
>> --
>> Martin Wessendorf, Ph.D. office: (612) 626-0145
>> Assoc Prof, Dept Neuroscience lab: (612) 624-2991
>> University of Minnesota Preferred FAX: (612) 624-8118
>> 6-145 Jackson Hall, 321 Church St. SE Dept Fax: (612) 626-5009
>> Minneapolis, MN 55455 e-mail:
[hidden email]