I recommend some measuring ideas as :

https://www.newport.com/n/insights-into-high-speed-detectors-and-high-frequency-techniques

https://physicsopenlab.org/2020/11/21/fluorescence-lifetime/

https://www.labonthecheap.com/upgrade-a-2-photon-microscope-with-high-speed-flim-for-2352/

Article Development of Low-Cost Instrumentation for Single Point Aut...

For an estimation we could use a different LED on/off frequency (you could use datasheets) and a constant Photodetector readout frequency by considering FLT. Randomly measured intensities in frames would be useful.

Or we could study the difference between a long & short lasting FLT by exciting a long lasting material with short one! (something like expanding time/multiplying) Geometry of this hypothesis is on you.

Greetings, Efrim Mengesha, PhD.

1. I understand that your laboratory does not have ... "photodiodes, or a gated CCD camera" ... that is, a photodetector, and they need to be replaced.

2. As a TEMPORARY functional replacement, in my opinion and work experience, a light emitting diode (LED) MAY be used if several conditions are met:

a) connected as a photodiode (PD) and subsequent amplification of the I photocurrent;

b) the design of the LED has been changed to ensure the quality of the hit / focus of the detected radiation on its crystal and p-n junction;

c) normalization of the obtained result Iphotocurrent (or photo-EMF).

3. As an example, you can make an experiment with RGB LED: turn on the blue LED crystal and it will illuminate the green and red LED crystals with its light, which at the same time begin to generate photo-EMF. In this case, the current is very small, and the EMF of such a "photodiode" is obtained about 2-3 V.

If my answer was useful to you, then make a mark / recommendation and visit my profile (articles, patents, abstracts, etc. in the field of micropower optoelectronics, LED-PD optocouplers of the UHF range).

Proskurin N, PhD, docent NUZP, UA

I would consider SiPm [Silicon Phtomultiplier]. E.g. https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10680/106800B/Design-and-development-of-portable-fluorescence-reader-using-silicon-photomultiplier/10.1117/12.2305846.short?SSO=1

They rather easy to use and as sensitive as best PMTs. You can get some eval boards like https://www.digikey.ie/product-detail/en/on-semiconductor/MICROFC-SMA-10020-GEVB/MICROFC-SMA-10020-GEVBOS-ND/9744733 combined with some few GHz 50 ohm SMA amplifier. You can operate them in single photon histogram timing mode.

Dear all, thank you so much for your helpful recommendations !! i will dig throgh the materials recommended.

@ Nick Proskurin your recommendations are very helpful as they are directly related with the problem i have at hand right now. As you have suggested (and from a few youtube tutorials), i have seen an LED used as a photon detector giving out about 0.6 V when a cellphone flash is lighting up on it. In my lab, i have developed a simple TEA Nitrogen laser with just enough power to excite liquid (rhodamine dye) sample (i can detect the emission with my eyes). Since the pulse width of nitrogen laser is on the order of a few nanosecond ca 4ns and the lifetime of the sample is of the same order of magnitude ca 4ns, I was expecting that the voltage response of the LED (connected to a 25MHz oscilloscope) should show a voltage decay profile of a few nanosecond. However. what i saw on my Oscilloscope is a signal rise and fall time about 600 ns which differs significantly from the reference lifetime of Rhodamine. it looks like either the LED or oscilloscope. Have ever faced similar problem?

The intrinsic risetime 10/90% of 25MHz oscilloscope should be order of 14nsec (the proportional rule is 50MHz --> 7nsec).

However, the coax cable capacitance is 100pF/meter, typical LED may have zero-bias capacitance of tens of pF plus few tens ohms of series resistance. Zero-bias is not good way for high speed operation, one needs strong reverse bias.

You may also have a look at vaccum phototubes like (if you want large area at high speed) https://www.ebay.co.uk/itm/124522822494?hash=item1cfe237b5e:g:4UoAAOSwcbhf~CnX

Thanks Nikolay Pavlov !! with the appropriate reverse bias (and terminating resistor) the system should be able able to measure life times on the order of a few tens of nanoseconds. is that right?

I guess the duration (into 50 ohm load ) of response time at high bias (hundreds of Volts) is few nanosecond, likely one can go to several kV and few hundred psec.

https://www.optixs.cz/data/ke-stazeni/fotonky---katalog-d688.pdf

You may have a look at https://www.photek.com/products/photodiodes.html but they are expensive. Sometimes you can buy similar devices cheaply on ebay.

The Fast Output Sensl Sipm also have rather short response, depending on size (1mm2 is < 1nsec wide).

Also, you can find a lot of ancient almost tubes here, they are usually ok, I used that shop a lot. https://tubes-store.com/product_info.php?cPath=103_143&products_id=1062&osCsid=35furihcievcvampdo5kfhbpo7

You replied that ... "I saw on my oscilloscope that the rise and fall times of the signal are about 600 ns, which is significantly different from the reference lifetime of rhodamine; it looks like either an LED* or an oscilloscope**" .... - I note that I am inclined to note the first factor *.

1. It should be borne in mind that the difficulties in measurement are not so much in the method of measuring the t(r-f) value with an HRF oscilloscope (25 ... 100 MHz), but (my opinion) in the fact that you use DISCRETE LEDs as PD. Their p-n semiconductor structures can have a significant value t(r-f); therefore, it is necessary to find integral LEDs of the UHF range with small values ​​of t(r-f) in 1... 10 ns.

2. In general, I agree with the suggestions and comments of N / Pavlov about the true values ​​of the measured rise & fall times t (r-f) - the second factor **.

Or your option. NP.