Dye Laser Part 4: Getting Anwsers

Background

Thanks to help of my friends I managed to contact a very helpful PhD working at Warsaw University, Faculty of Physics that researches ultrafast laser pulses and their use. She got interested in my project and we spend over 6h measuring chlorophyll in various ways.

Measurements

Because 2 high school students also came to learn about spectroscopy we started with redoing excitation ans emission measurements. As expected we found 2 peaks at the excitation spectrum each came with corresponding 2 peaks in the emission spectrum. 

Then we measured lifetimes of each of the transitions. 420nm excitation resulted in 5.36ns lifetime for both 672 and 729nm emission. 468nm excitation resulted in 2.94ns lifetime for 657nm emission and a double exponential decay for 716nm. This double decay was around 90% 2.94ns and around 10% 5.8ns. 

As a sidenote: Generating sub 1ns, tunable laser pulses is "simple". It was just a matter of using  Q-switched Nd:YAG going through nonlinear crystal to generate second harmonic to pump OPO and again to nonlinear crystal achieving 2nd harmonic generation to finally achieve excitation wavelengths with short pulses (thanks to Q-switching).

Analysis

The last measurement is very interesting. It's almost the same as for 657nm emission with just a bit of slower decay mixed in. As the secondary decay has much smaller effect on the overall shape we asked the software how big uncertainty is assuming 2.94ns as a time constant of one decay and we got 5.17-5.89ns. Lifetime of 420nm excitation fits into this range which begs the question if they may be related.

It's hard to describe the entire though process without a lot of graphs and drawings  so I'll just present the conclusions here:

  • 410-430nm excitation excites chlorophyll a which has the fluorescence lifetime of 5.36ns+-0.2ns
  • 468nm excitation excites chlorophyll b which has the fluorescence lifetime of 2.94ns
  • 468nm also hits the very tale of chlorophyll a that's why there's a double decay for 716nm emission and why it's shifted to shorter wavelengths. When we measured a lifetime after the emission peak at 729ns we got pure 2.94ns
  • Same lifetimes for both vis and NIR emissions suggest that the both origin from same level transition just from different vibrant modes.

 Conclusion

As NIR and vis emissions most probably come from same level transition and NIR emissions are fainter than visible it should be simpler to get lasing in thw visible spectrum with lower lasing threshold. This is very good info as it's considerably simpler to align laser if you can see its emission. Some people suggested that 729nm transition is a triplet which seems very unlikely considering just how short its lifetime is.

Additional thoughts

University lend me 2 dielectric mirrors fine tuned to my frequencies of interest. One plane and one convex. I'm very grateful for them as those things are unreasonably expensive and very required to build high Q laser cavity.

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