EEVblog #869 - Counting LED Photons!

How may photons does a LED emit at very low currents?
What is the lowest current where a LED will actually emit photons? Dave does some measurements to answer this quantum physics level stuff!

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[walts]

Negative curvature over right half of curve looks like detector saturation. As detection avalanche events start overlapping, pulse counts are missed. Actual LED efficiency is still increasing in that region.

[PlasmaHH]

Some things that come to my mind that might warp this figure:

- Missed photons due to dead time (50-60ns for counts below 5MC/s it says). Someone might want to calculate this.
- Shifting of the LEDs spectrum with higher current
- Shift in emission direction of the active LED region so that more photons are emitted into another direction than the sensor.

Page 6 of the datasheet has an actual formula to correctly defer the count rate from the measurements.

For measuring higher currents you might want to place some ND filter between the LED and the sensor. For the spectrum part maybe there exist some very narrow optical filters, or you can run things through a prism and a slot.

[KennethScharf]

You almost have an S shaped curve there. The non-linear portion at the top may be saturation of your equipment, but at the low end it must be real. Interesting!

[swfsef]

I work with that detector in my research!


This is unlikely to cause a problem with your LED demonstration, but has potential to reduce sensitivity as the light increases in orders of magnitude. Saturation and statistical distribution vs. deadtime are really only a concern when you get to 30Kcts+.

If you are concerned by the odd negative curvature on the right of your graph it may be worth repeating the experiment with an optical attenuator (camera ND filter would do) to confirm the behavior is the LED and not a detector effect!

Although the datasheet allows very high dark counts, all 6 of my units are at approx 70 dark counts per second, with minimal shielding effort... so the donor may have marked it as an unsuitable device to perform low photon rate experiments with, the SiAPDs in these units are binned by dark count and better units cost more.

[Schmidt]

That's a nice experiment ;)

Well, to make this measurement reliable a few extra steps need to be taken.

For once, the wavelength of the light emitted from a diode is not completely independent from the voltage applied. This means the wavelength shifts (and with that the efficency of the detector changes) during the measurement. If you have a spectrometer you can check this and the applied voltage at the diode should be measured during the experiment aswell.

Further more: as the LED gets warm, its efficiency lowers. Meaning that as you drive more current through it, you get a lower efficency for the light emittance. Coupling the LED to a heat baths at different temperatures could be used to mitigate the problem (or driving it with pulsed currents of equal average power so as to have the diode run at potentially similar average power but different currents).

Then, when you count high rates of photons you need to account for the dead time of the detector. As you have an avalanche detector, after detecting a photon, the detector stays insensitive for further photons for a short period of time. This error is largest for high count rates. You can easily correct the count rates for the dead time when you know the dead time of the detector (or measure it).

At very low currents you also have the problem of noise with the current source. So the current-errorbars need to be measured aswell. I might also have unintentionally underestimated the errorbars for the photon rates slightly. They should be calculated according to gaussian error propagation.

Depending on the precise construction of the LED, the directional distribution of photon emittance also might change as the applied voltage is changed. This is easy to control for as one has to simply repeat the measurements with a differently oriented diode and see of the resulting count rates are proportional to the original ones.

These, and other unaccounted for effects may lead to the drop of in count rates for increased currents you observed.

But generally yes: at very low applied voltages even the metallic contacts to the semiconductor material start to behave non-linearly. From experience it is already not that easy to get simple contacts completely linear on e.g. p-GaAs if less than 10 mV are applied. And of course the n-p-junction is also not completely linear in behaviour. Most electronics are only approximately linear over the range over their typical range of operation.

But well done ;)

[WobblycogsUk]

Interesting experiment. I suspect that the rolling off you see at the highest current is an artefact of the experiment rather then a real effect. We know from the data sheets that the output increases with increasing current up to some point. If you extrapolated the graph you have the output would have to level off and then start increasing again to join up with the data sheet graphs. That would indicate that there was one effect at very low currents and then another at quite low currents and a third at high currents. To me that feels like one effect to many but who knows, we'd have to do the experiment and find out.

[shana_dmr]

This measurement setup discussion is a nice companion for your synchrotron tour and all this "why there are 1 GHz LeCroy scopes standing everywhere and they're treated like basic pocket multimeters" in nearly every experimental physics lab:)

[DJignyte]

That is absolutely amazing, looks like so much fun.

[wormytom]

i found that most linear relationships when scrutinized at the extremes display non linear relationships, they are reported as linear because that is the region in which the thing under scrutiny usually happens at, which coincides with the rising limb of the curve and is mostly linear. So the reporting of linear relationships is usually to simplify the condition irrespective of limiting conditions at the extremes.

[whatthefunction]

Please build a cnc tripod for this collector, then go into the darkest room you can find and try to make a high exposure image.

[Gabbos]

This was very interesting. Now you gotta go out and buy some fancy/expensive LED and give that a go. I need to know how the data changes!

[Pocahonkers]

I just watched EEVblog 713 and i shit you not, your son is smarter than some people in my advanced electronics course.

[LucasHartmann]

I bought a DS1054Z, which still believe is a nice tool for the price. It has several limitations, though. Try saving the full memory data to the flash drive as a CSV file... 2 weeks later you may tell me how it went. :-)

[lyntonr]

a 1 minute tech tip ? Dave to make a 1 minute video ? this i want to see !

[bikingmnviking]

Maybe the reason the diode started up is a combination of factors:
a) it had a few seconds to warm up
b) your shielding of AL foil and electrical tape is not going to stop AM/FM so I'd hypothesize that some of that got through and got detected and rectified just like it does in a Germanium Diode used in an AM radio set.

[Pithead]

I haven't watched the whole vid, but I am wondering - how sensitive is an LED to incoming photons?

[nickmartinblue]

what about temp of the led does it make it move efficient when running at nA

[bryancbeck]

Where can i get a 2016 special edition MSO 3000?

[ivanblogs]

If you ran out of range on the detector, can't you move the LED further away (or put a darkening filter in the way) and re-run the test? Then use the overlapping measurements to come up with a multiplier value that will align the two sets?
Repeating this until the LED blows?
(I'm assuming the response to distance/filters stay linear)

[Skwisgar]

you can reduce the dark counts by cooling the sensor down, the device already does some of that, but some dry ice would probably make it even better.