Setup (refer attachment):diode laser (440nm, polarization-horizontal), sample, detection of fluporescence at 90 degrees from excitation, fluorescence collecting planoconvex lens (1 inch diameter), converging lens, polychromator, detector, TCSPC hardware board, software control for detector and laser.
I am conducting these experiments to find out the lifetimes of quantum dots (size- 10-100 nm). While going through the literature, I found out that these kind of experiments are done at magic angle conditions. We are currently looking for polarizers (Glan Taylor polarizers for high purity of polarization). Now, the problem I am facing is, my fluorescence collimated beam is 25 mm in diameter, so I need a polarizer having a diameter 25 mm, which is hard to find online. I am trying to get it custom made for us.
How important is magic angle detection in this case? Also, does my laser have to be vertically aligned since laser is horizontally polarized.
Please refer the attachment. Thanks!
Saurabh,
Magic angle conditions are used to remove polarization effects resulting from photo-selection. Molecular species will have a greater probability of absorbing light when the absorption transition dipole moment is aligned with the electric field vector of the laser beam. The electric field vector for emission will be aligned with the emission transition dipole moment of the molecule. Emission will have some degree of polarization until random rotation of molecules in solution results in random orientation of those molecules that were photo-selected, resulting in a random orientation of emission transition dipole moments.
Polarization effects will distort the shape of the fluorescence decay, Magic angle conditions remove the polarization effects that distort the shape of the fluorescence decay.
Yes, for magic angle conditions to work the light exciting the sample must be vertically polarized. This is not a difficult problem to overcome. You need to place a quarter wave plate in the path of the excitation beam. This will assure that the horizontally polarized beam will be transformed into a vertically polarized beam.
Reference text: "Polarized Light in Optics and Spectroscopy" by Kliger, Lewis and Randall.
I hope this is of some help.
John
Saurabh,
Magic angle conditions are used to remove polarization effects resulting from photo-selection. Molecular species will have a greater probability of absorbing light when the absorption transition dipole moment is aligned with the electric field vector of the laser beam. The electric field vector for emission will be aligned with the emission transition dipole moment of the molecule. Emission will have some degree of polarization until random rotation of molecules in solution results in random orientation of those molecules that were photo-selected, resulting in a random orientation of emission transition dipole moments.
Polarization effects will distort the shape of the fluorescence decay, Magic angle conditions remove the polarization effects that distort the shape of the fluorescence decay.
Yes, for magic angle conditions to work the light exciting the sample must be vertically polarized. This is not a difficult problem to overcome. You need to place a quarter wave plate in the path of the excitation beam. This will assure that the horizontally polarized beam will be transformed into a vertically polarized beam.
Reference text: "Polarized Light in Optics and Spectroscopy" by Kliger, Lewis and Randall.
I hope this is of some help.
John
Hi Saurabh, you do not need an expensive Glan Taylor polarizer for that. you can use much cheaper thin film polarizers or wire-grid polarizers (e.g. Edmund optics) with large apertures. Those polarizers can be placed in diverging beam as well!
I guess you are using a diode laser, just turn it 90 degrees and everything will be like in the book.
Thanks Ivan! I also looked for thin film polarizers but the extinction ratio for these polarizer is 500:1 whereas for Glan Taylor, it is 100,000:1. Since I want high polarization purity for my experiment, I was planning to go for Glan Taylor polarizers,
Saurabh, you are correct, Glan Taylor is better, but (!) it needs a collimated beam and it is 20 times more expensive. Moreover, in practice you have other sources of errors which gives more problems than 0.5% of light coming from the other polarization if film polarizers are used. For example, re-absorption, reflections in the cell and so on. Also, if you want to reach super accuracy, you need to be sure that your laser is linearly polarized, it is, bit not with 1: 100000. Anyway, if you do not study polarization, and just want to avoid polarization artefacts film polarizers is what you need. By the way, what will be the accuracy you set the magic angle? 0.5 degree? think about it...and 1:100000.
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