Green laser makes LEDs glow!

Boy, was I surprised when I tried this! I was bored, so I took my green laser and happed to have some LEDs laying around. I put on my goggles and shined the beam
into a red LED in order to kill it (I was bored, remember that ;D). But instead it made a bright glow! It works with orange, yellow and yellow-green LEDs too. I've done
some experiments in the past with my Blu-Ray and it does the same thing, but with green and blue LEDs.
Here's a video.
http://www.youtube.com/watch?v=PxKKH-vpNrE

Nice fluorescence.. its it visible at all without the goggles, or completely overwhelmed by the green?

Yes, it's overwhelmed by the green without goggles, and it hurts my eyes too. You really think it's fluorescence, or maybe it's someting else..?
The LED make some good voltage (I'm gonna measure that tomorrow), it stings when the leads touches my tongue, this is no news though.
Btw, I tried with a IR LED too, but no luck.

Yes, the photons coming from the laser are exciting electrons up through the bandgap, and when they fall back down to the valence band they give off of the color of light characteristic of the bandgap (the same color as the LED emits when voltage is applied). You're just putting energy in in a different way, but the light is leaving the LED for the same reason as when it's in normal operation. This is the same idea as a solar cell, which is why it can give you that voltage output: you have a diode, a photon hits it and excites an electron creating an electron-hole pair. In a solar cell, you want that electron and hole to get separated from one-another and not recombine, that gives you the power output. However, LEDs want the opposite, they are designed to MAXIMIZE electron-hole recombination, so they re-emit light instead of sending all that energy into electrical energy.

Typically, applying a voltage lowers the energy barrier present in the pn junction (aka diode), allowing current to flow. During the course of current flow, electrons and holes recombine (electron in the conduction band and hole in the valence band, the electron falls through the bandgap into the hole), and the electron gives off its energy (the bandgap energy) in the form of a photon. In your case, the photons hitting the junction excite the electrons up into the conduction band, making an electron hole pair which very quickly recombines and emits the photon that is characteristic of the bandgap of the material.

At least that's the general theory, there are some more details that I kindof skipped over. If it makes sense, awesome. If not and if you're interested, I can provide some helpful diagrams.

Pretty cool, eh?

This is amazing, I'm learning new things today! You know your stuff, no question about that. Feel free to show the diagrams.
Hehe, your avatar is a LED, right?

So this means LEDs create voltage when light is shone upon them? So they can be used as inefficient solar cells? :-/ Hmmm.... Does the wavelength hitting the LED have to be shorter than the wavelength characteristic to the bandgap or it doesn't matter? I mean can a red laser do this? Or sunlight for example? I mean, I'm sure sunlight can do it, but will all the wavelengths it's composed of contribute to this or only the short ones?
This is new stuff to me.....you should try to power a LED with another LED, that'd be awesome I bet it can be done ;D

This is new stuff to me.....you should try to power a LED with another LED, that'd be awesome I bet it can be done ;D[/quote]
There is a circuit diagram floating around on the web to do just that power a L.E.D with another L.E.D. If i can find it again i'll post it.

Yes LED's generate power when you shine light into them. using the same colour LED as your light source will produce more power though. Yes can power a LED with a LED, but the one being powered is dull, and the on generating the power requires alot of light

Hey, look here.


If I use a green LED or laser to illuminate an orange LED, the orange led will be able to power a red LED .
It seems like green light and orange LED is the best combo.
When I built the BR I also built kindof a powermeter, the purpose was to document if the BR would get better or worse in time. It was basically made from a yelllow LED with a white paper diffuser and my DMM.
Every time I check my BR I get a reading of 23mv, so it has the same power today from the time I built it ;D.

Edit: the link doen't work.

strange I posted a reply to this topic and it never showed up anyway here is a schematic for a blinky LED powered by the same LED.

^^ about above post
I'm not sure what the hell the point of that circuit is, really. But hey, neat proof of concept. Perhaps it could be altered to make a throwie that recharges itself via the LED in daytime? XD if only.

Switch said:

So this means LEDs create voltage when light is shone upon them? So they can be used as inefficient solar cells? :-/ Hmmm.... Does the wavelength hitting the LED have to be shorter than the wavelength characteristic to the bandgap or it doesn't matter? I mean can a red laser do this?

Click to expand...

The wavelength of the irradiating laser or led must be shorter to get any usuable result. Most leds will contain some impurities allowing them to capture photons of lower energy to some degree, but this is hardly noticeable. The same effect is visible the other way around: driving a blue led at a very low current often results in red, orange or yellow light, but only a tiny amount (hardly visible if at all).

The blinky led circuit is a cool idea, though i'm not sure it actually works. If it does, the blink rate should be somewhat proportional to the light going into it, giving a qualitative power indication that could be used to compare lasers... sounds like its worth trying just for the fun of it.

I think the circuit would be much better if it was designed with 2 L.E.D's instead of one. One to capture the light and one to emit it.

FireMyLaser said:

Hey, look here.


If I use a green LED or laser to illuminate an orange LED, the orange led will be able to power a red LED .
It seems like green light and orange LED is the best combo.
When I built the BR I also built kindof a powermeter, the purpose was to document if the BR would get better or worse in time. It was basically made from a yelllow LED with a white paper diffuser and my DMM.
Every time I check my BR I get a reading of 23mv, so it has the same power today from the time I built it ;D.

Edit: the link doen't work.

Click to expand...

Google "LED museum", pictures and click on the one with a green and red laser dot.

FireMyLaser said:

Yes, it's overwhelmed by the green without goggles, and it hurts my eyes too. You really think it's fluorescence, or maybe it's someting else..?
The LED make some good voltage (I'm gonna measure that tomorrow), it stings when the leads touches my tongue, this is no news though.
Btw, I tried with a IR LED too, but no luck.

Click to expand...

Sure you're not poking your tongue with the leads? I'm pretty sure LEDs can only output a volt or two at most. 1 volt stings your tongue? Holy placebo effect batman!

"Sting" is probably not the right word. I get that sour tase, you know, what's the word... Oh well.

I've done some measurements with different LED colours, here are the results:

880nm IR = 1v
Red = 1.7v
Orange = 1.9v
Yellow, yellow-green = 1.8v
Green, blue ~150mv
Violet ~10mv
385nm UV ~ 5mv