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ArcticMyst Security by Avery

simple "normal" red laser curiosity

Blade

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hey everyone, of course ive been into the laser hobby for some time now and i feel like a noob asking about this. but i looked and searched the forum quite a bit before posting this to make sure it wasnt already discussed, ive found nothing... anyway, i was messing around with an 8 LED / 5mw 650nm red laser i bought from radioshack when i decided to shine it through a few things (glass, sunglasses etc. ) but i knew blue absorbs red light best, so i shined it through a solution of simple copper acetate i made a while back. at first glance i noticed absolutely NO light was coming through. but i moved around and came across i guess the direct center of the diode and noticed a faint but noticeable light... it was green! i checked this a few times and even turned the lights off to see if it was some sort of stray reflection off of something else. but it was in fact coming from the laser! now, my noob question is: 1) what is this light? if the wavelegnth is red, 2) why is green showing? to my knowledge red and blue make purple lol... so theres some pix too. and i have a video and will post it if needed as well :)

btw, i didnt really know where to post this thread, so i posted it here since i thought it best matched the topic.

hope i can get some answers :D thanx

the photos are of me trying to align the camera and laser (in sequence)
 

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Maybe it's some sort of weak fluorescence? Even though I wouldn't suspect that, because it is rare for higher wavelengths to fluoresce things to emit at lower wavelengths....
 
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Most probably an example of fluorescence. It would make sense as well, since copper's primary emission when excited is in the green part of the spectrum. Red diode lasers are spectrally pure pretty much. If it was a HeNe or DPSS I'd say it's just the other lines coming through with red being blocked. It is indeed rare for materials to fluoresce at a higher wavelength than what they absorbed.
 

Blade

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this is one of those times where i wish my o-like 200mw 650nm hadnt died on me :banghead: (actually, i killed it :shhh:)
 

Blade

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well... i... potmodded it lol. i shouldve know the results were going to be bad. but then to make things worse, i began experimenting with it :eg: i hooked up the driver and diode to a 12v power supply and one of the components on the driver EXPLODED! sparks and smoke everywhere. i actually wasnt really worried about it back then, which is why i attempted potmodding it n stuff. but now i regret it :(

lesson learned --> think ahead before you act :D
 
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Blade

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Good lesson lol!

Diode might still be ok, but you can forget about the driver. Fortunately both are pretty cheap.

thank goodness. although i wouldnt ever do such a thing with any other color laser due to there price.
 
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Flourescence is per definition longer in wavelength, exciting copper is only possible at insane high intensities if you're using red light (as in amplified femtosecond lasers), it would require a 2 photon process to occur. I think the most likely cause are impurities in the semiconductor material, at high current the tiny probability of them being excited is noticeable enough.
 
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Flourescence is per definition longer in wavelength, exciting copper is only possible at insane high intensities if you're using red light (as in amplified femtosecond lasers), it would require a 2 photon process to occur. I think the most likely cause are impurities in the semiconductor material, at high current the tiny probability of them being excited is noticeable enough.

Aye, but this is a 5mW red diode, operating at maybe 20mA tops! I wouldn't think the diode would lase if it was contaminated enough to have spectral impurities at these current levels.

Thanks for the technical correction about fluorescence, it's been a while since I studied chemistry.
 
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Just off the top of my head, couldn't the collective energy of the output red beam be enough to stimulate a higher frequency fluorescent emission of a much lower power level ?
The lower frequency being absorbed entirely and then the multiplying of the power producing a higher frequency but much less brightness. Hence a dim green.
Much like a pumped laser medium.
Isn't that what is going on here ?
 

Blade

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Just off the top of my head, couldn't the collective energy of the output red beam be enough to stimulate a higher frequency fluorescent emission of a much lower power level ?
The lower frequency being absorbed entirely and then the multiplying of the power producing a higher frequency but much less brightness. Hence a dim green.
Much like a pumped laser medium.
Isn't that what is going on here ?

thats what i kind of thought, but i really didnt think a 5mw red could do that. and with copper? i thought it would have to be alot more powerful than that. and i didnt even know copper had such properties. maybe someone else can repeat the scenario to gather more info?
 
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Aye, but this is a 5mW red diode, operating at maybe 20mA tops! I wouldn't think the diode would lase if it was contaminated enough to have spectral impurities at these current levels.

Thanks for the technical correction about fluorescence, it's been a while since I studied chemistry.
It doesn not have to be contaminated that much, the output power is in milliwats but the green power will probably be in the tens of nanowatts. 20mW is still ~10^17 electrons/second.

The other option is recombination of the charge carriers outside of the confined area of the heterostructure, at those points the bandgap is higher and the emission wavelength shorter. Recombination is not very probable there, but it does happen. Now that I think of it this is probably the more likely case.

ShortyInCanada: you explanation doesn't make sense. We're talking about a quantummechanical process here. You can't sum input power and say it's enough, the photoelectric effect didn't work that way and fluorescence also doesn't. Even with total absorbtion it requires more than that to get shorter wavelengths out. Power doesn't get multiplied by the way, it may add up but that's about it. Pumping a laser medium is completely different, the pump there is short in wavelength and the emission long in wavelength.

Two photon fluorescence don't even occur at megawatt levels of a standard Ti:Sapphire oscillator. The only laser I've seen capable of doing this was a 5Khz 30fs Ti:sapphire with 2mJ of energy. That's gigawatts of power (assuming unfocussed beams). No diode available is capable of a non-linear process like this, some can do SHG with a special cavity and nonlinear crystal but two photon absorbtion is a third order nonlinear process instead of 2nd order as with SHG.

I'm not familiar with excited state absorbtion or energy transfer upconversion in semiconductors, but I'm pretty sure these processes are not possible in this case. Anyone here with enough solid state physics knowledge?
 
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I didn't say I was right...or even if I was sorta thinking along the right lines.
Just a quick blast off the top of my head.
Apparently I should wear a hat. :D

Now I tried reading that reply and my brain hurts.
Must be quitting time.
Time to log off and out at work...now where are those pain killers...and a bottle opener.
 

Blade

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I didn't say I was right...or even if I was sorta thinking along the right lines.
Just a quick blast off the top of my head.
Apparently I should wear a hat. :D

Now I tried reading that reply and my brain hurts.
Must be quitting time.
Time to log off and out at work...now where are those pain killers...and a bottle opener.

i feel your pain LOL. that reply crushed my brain :p
 
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Thanks again Bluefan! I really appreciate the depth of technical knowledge in your posts.

If it is charge carrier recombination, then this is something we (the community) should look for in other diodes. Unfortunately I don't think most of us would be equipped to do it, and without a spectrometer we'd be limited to the sensitivity threshold of our eyes and our ability to find/make suitable absorption filters.

Re: non-linear frequency change - I don't even think that it could be SHG from the Cu Acetate because half of 650 = 325 (Green is not the second harmonic of Red), the emission would be in the UV band. The pump frequency would have to be in the IR region to generate green.
 




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