0
$\begingroup$

Optical trapping literature, including the wikipedia article on optical trapping seems to indicate that the transition from the $\lvert^5P_{3/2},F'=4\rangle$ level to the $\lvert^5D_{5/2},F'=5\rangle$ level is forbidden. Why is this?

From https://en.wikipedia.org/wiki/Laser_cooling

Other sources seem to show that it is possible but is not considered.

Improve this question
$\endgroup$
3
  • 2
    $\begingroup$ Your question text and your image are inconsistent. There's no $D_{5/2}$ level in the latter. $\endgroup$
    – Emilio Pisanty
    Commented May 21, 2018 at 20:51
  • $\begingroup$ @EmilioPisanty The image is from the wikipedia article and the "other sources" link shows the other level $\endgroup$
    – Cameron
    Commented May 21, 2018 at 20:59
  • 1
    $\begingroup$ That's irrelevant. Your question is and should be read as a self-contained document, which currently contradicts itself. $\endgroup$
    – Emilio Pisanty
    Commented May 21, 2018 at 21:08

1 Answer 1

Reset to default
2
$\begingroup$

Edit for clarity: optical trapping is done with lasers that are far off-resonance from any atomic transitions. The dominant effect of these lasers is therefore a shift in energy of the bare-atom states, such as the ground state, which is what forms an 'optical potential'. Actual excitation into the excited atomic states occurs only weakly, which is what makes optical trapping so useful.

Rubidium atoms spend nearly all of their time in the ground state manifold ($5S_{1/2}$ states) since the low lying excited states have a very short lifetime (10s of nanoseconds). Therefore the relevant transitions that determine trapping are transitions from the ground state, ie., the transitions from $5S_{1/2}$ to $5P_{1/2}$ and $5P_{3/2}$.

The $5P_{3/2}$ state will indeed be shifted by a light field that off-resonantly couples it to $5D_{5/2}$, but the energy of the ground state should not be affected by this higher transition.

Improve this answer
$\endgroup$
3
  • $\begingroup$ What would happen if an on-resonance laser was applied to the $5S_{1/2}$ would it only oscillate between that state and the $5P_{3/2}$ state or could it reach the higher energy level which has the same $\omega_0$? $\endgroup$
    – Cameron
    Commented May 24, 2018 at 11:04
  • 1
    $\begingroup$ If you apply a laser that is on resonance with the transition from $5S_{1/2}$ to $5P_{3/2}$, then the atom will get excited to $5P_{3/2}$. However, there are no transitions at the same frequency from $5P_{3/2}$. There are higher transitions at different frequencies. $\endgroup$
    – Harry Levine
    Commented May 24, 2018 at 13:47
  • 1
    $\begingroup$ If you have two lasers shining on the atoms, one laser resonant with $5S_{1/2} \to 5P_{3/2}$ and another laser resonant with $5P_{3/2} \to 5D_{5/2}$, then the atom can get excited to the higher state as well. (This would correspond to a laser at 780 nm and a laser at 776 nm). $\endgroup$
    – Harry Levine
    Commented May 24, 2018 at 13:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.