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I'm in the process of building a setup for Single photon counting in 300 - 400 nm. Basically I'm trying to detect SHG (Second harmonic generation) reflected off of a surface. (SHG is a non-linear optics physics surface spectroscopy method widely used for measuring electronic structures and band gaps, magnetic ordering, electron-phonon coupling, and many other properties of surfaces, interfaces, and monolayers.)

We have a 7 fs ultrafast laser with a 80 MHz repetition rate. I've already tried a SPC (Single Photon Counter) with an APD (Avalanche PhotoDiode) but it wasn't successful as expected due to the high dark count. Now I'm shopping for PMTs (PhotoMultiplier Tubes) to work with time correlated SPC and I should pick a TTS (Transit Time Spread) value for this. For this configuration, what would be the typical TTS values that I should seek for?

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    $\begingroup$ You have to read the data sheets and pick the right product for your application. If your sensitive parameter is TTS, then you are probably better off with a micro-channel plate: hamamatsu.com/content/dam/hamamatsu-photonics/sites/documents/…. $\endgroup$ Commented Oct 23, 2022 at 7:35
  • $\begingroup$ If you want to try to get the question reopened, it might help to at least briefly describe the physics you are trying to do. Also, what are the expected single photon rates, and are there any other time scales involved besides the 7 fs pulse width and 80 MHz rep rate? $\endgroup$ Commented Oct 24, 2022 at 1:09
  • $\begingroup$ Thank you @FlatterMann I have already contacted Hamamatsu about this, but they need a TTS value. What I'm finding it difficult to understand is the optimal TTS value/range for our specific laser. $\endgroup$
    – GayanW
    Commented Oct 24, 2022 at 19:11
  • $\begingroup$ Thank you @DavidBailey. I've edited the question. There are no other time scales involved other than the pulse width and the rep rate. $\endgroup$
    – GayanW
    Commented Oct 24, 2022 at 19:12
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    $\begingroup$ @GayanW Generally speaking the manufacturer can not understand your experiment for you. That is your job. They can at most tell you the performance of their products and then you have to decide if it is sufficient. At 80MHz a tts of 1ns or less should be more than sufficient to cleanly separate individual pulses. If you are hoping to resolve the timing down to the fs range, then you are going to be disappointed, though. A streak camera could get you below 1ps, however. I have never done anything faster than that. $\endgroup$ Commented Oct 25, 2022 at 0:19

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@FlatterMann has well covered the essential points in their comments, but I will note that the TTS is not the width of the output pulse. The rise time of the PMT output pulse is normally longer than the TTS, and the fall time is (even with well designed and matched electronics) 1.5-2 times longer still . That said, there are PMTs with nanosecond rise times (or even less) which should work for you, but be sure your whole PMT module - including the amplifier - can operate at 80 MHz.

I suggest checking out a reference such as the Hamamatsu Photomultiplier Tubes Basics and Applications. (For example, see Sections 4.3 and 5.3.5 in that guide.) FlatterMann is certainly correct that you are responsible for the final choice of your PMT, but I have found a good manufacturer's sales rep can be very helpful in avoiding mistakes when purchasing instruments. They sometimes identified issues I hadn't thought of.

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  • $\begingroup$ Thanks a lot, @DavidBailey This is really helpful. This resource explains the whole concept really well. $\endgroup$
    – GayanW
    Commented Oct 26, 2022 at 12:53

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