Dear All,
We have a classic ps sync pumped/cavity dumped dye system which now employs a Spectra Physics Vanguard as the pump. The Vanguard poduces mode locked 355 nm ps pulses at 80 MHz. We want to measure lifetimes expected to be only a few ns where ideal excitation would be ~355 nm. While in principle we could try to change over to a near IR dye and frequency double--a lot of effort--the short lifetimes expected suggest that we could simply use the 80 MHz 355 nm pulses for excitation (pulses every 12.5 ns). We set this up (greatly attenuated 355 nm pulse energies) but have seen very poor quality data--instrumental resolution of several ns, and this with an MCP-PMT. The problem must be related to the photodiode used to generate the stop pulses (or start pulses, with proper delay). We normally use one of several very small surface area reverse biased photodiodes. However, they seem to be very insensitive to 355 nm. One "fix" might be to image most of the power of the 355 nm pulses (2.5 W) on the photodiode, but here's there the fear of destroying the photodiode--so we haven't tried. The only photodiode that could produce adequate response has a rather large surface area, meant for ns or longer triggering (Thorlab). The attainable risetime appears to be several ns, which fact probably accounts for the poor results. (Changing discriminator threshold and zero crossing settings seems to do little to help.)
Can anyone suggest some useful steps? Would "blasting" one of the fast photodiodes not risk destroying it? Is there some extremely short-lived luminescence scintillator material that could be used to produce fairly strong output to the red? Are there any other approaches that are promising?
Thanks
Mark Sulkes
Tulane University
New Orleans, LA USA
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