What is causing a green light to appear when shining a UV laser through glass?

[BernieM]

Ok, so I have a 405nm laser pointer (bluish purple/uv). Had a few miscellaneous glass objects and was shining it through them when I noted a light green line in the glass that it shone through. One of the objects is a sphere of alexandrite, so I assumed that it was a property of the alexandrite (color change property of alexandrite). To test it I then shone it through a glass prism and again noticed that the uv laser emitted a yellow-green light streak (I estimate it to be in the 530nm range) where it passes through the glass.

The color does not seem to change depending on the type of glass it is shone through. The prism is not alexandrite, just normal glass. Other lasers when shone through the prism and the sphere do NOT show a visible line of any color as it passes through them (the color of the laser or any other color unless it is below the limited range of human vision).

So what's up here? Is this photon down conversion or is it uv luminescence or a laser that emits more than one wavelength? I am assuming that the uv laser is monchromatic (standard diode laser pointer, 5mw).

Can anyone shed any light on this for me? :tongue2:

 

[UltrafastPED]

The GaN laser diode used in Blu-Ray players emits a narrow band centered near 405 nm:
http://www.ledmuseum.candlepower.us/eighth/blu-ray.htm

Note the spectrograph! For other blue/violet lasers see:
http://ledmuseum.candlepower.us/vlspec.htm

It is not clear where the green is coming from; green is centered about 530 nm:
http://www.astronomycafe.net/qadir/q72.html

If your laser is based on frequency doubling the laser diode is emitting infrared at 808 nm to yield 404 nm blue. The usual green laser is a frequency doubled ND:YAG which operates at 1064 nm, frequency doubled to 532 nm.

What is the make/model of the laser pointer? At 5 mW it would seem to be based on the Blu-Ray spec.

 

[BernieM]

It's just your typical cheap laser pointer sold on ebay for example:

http://www.ebay.com/itm/Powerful-Blue-Violet-Laser-Pointer-Pen-Beam-Light-5mw-405nm-Professional-Lazer-/291048141651?pt=US_Laser_Pointers&hash=item43c3d23353#ht_6875wt_916

Photo is attached. Notice the faint green light streak between the two violet dots on either side of the sphere. Although very faint it is actually green and not blue violet when viewed in person. I will see if I can get a better camera to get a picture with. My cell phone cam really doesn't show it very well.

The green light, comparing to a color/wavelength chart visually is as stated, around 530nm.
The streak or line seen in the glass is green in color, where the color of the beam entering AND emergent is blue/violet. The light does not exit the glass as a green beam. The green color is observed from the light that is reflected perpendicularly along the axis of the beam in the glass. This effect is not seen using a red laser.

 

[UltrafastPED]

Try a diffraction grating ... and take a picture of the diffraction spectrum. This eliminates the glass.

 

[BernieM]

Well, I don't have a diffraction grating. Ordered one though. However I did put it through some calcite crystal. Does the same there too, with one additional interesting behavior. I see the same green light band along the axis of the laser beam, no matter at what angle it is shone through the calcite, but I also see bands of a violet color (not quite exactly the same color as the laser) at particular regular intervals in the calcite crystal. The positions seem to stay the same (where the violet bands are), regardless of where the laser enters and exits the crystal in relation to these bands, or at what angle.

Interesting phenomenon to say the least, but then obviously I have little (none) knowledge in regards to optical phenomenon and laser light.

 

[Cthugha]

A GaN laser having an extra emission line in the green would be odd. Usually you need InGaN to get to that spectral region. If you have spectral filters for that region, you can check your laser diode.

Besides that, what kind of glass do you use? Green luminescence under near uv excitation is a good indicator for small surface defects and fractures in fused silica glasses. See e.g. "Green luminescence in silica glass: A possible indicator of subsurface fracture" by J. Fournier et al., Appl. Phys. Lett. 100, 114103 (2012). http://scitation.aip.org/content/aip/journal/apl/100/11/10.1063/1.3693393

Unfortunately, I think there is no free version of that paper available oin the ArXiv.

 

[Andy Resnick]

My thought is fluorescence as well, but the observation that the same green color appears regardless of the material (alexandrite, 'glass'- borosilicate? soda-lime? something else?) is confusing.

 

[BernieM]

A GaN laser having an extra emission line in the green would be odd. Usually you need InGaN to get to that spectral region. If you have spectral filters for that region, you can check your laser diode.

Besides that, what kind of glass do you use? ...

As I stated above, the 3 things tested, all showing the same effect were an alexandrite sphere (violet in color), a glass prism and a piece of optical calcite. Here is a picture of the objects tested:

 

[BernieM]

Here is a video I did of the prism with blue/uv laser and red laser shone through the prism and the alexandrite sphere.

http://www.youtube.com/watch?v=OLcKTjyUhko&feature=youtu.be

I will do one with the calcite crystal next, however the camera doesn't want to show the green or the violet color in the calcite crystal very well.

 

[BernieM]

Ok, here is a picture of the laser shone through a diffraction grating. It's got the full rainbow of colors! Now I am a bit confused at this point, as I was under a mistaken belief that a laser was usually monochromatic. Anyone have an idea what's up here?

Although the quality of the image is somewhat poor (taken with cheap phone camera), the full range of colors of the rainbow are clearly there. In the upper right corner of the image you can see the yellows and reds.

But I guess it still does not answer the question why a green streak is seen in the different glasses and crystals. Why not the yellow or red instead?

 

[nasu]

Are these spots images on a screen? I mean, you took a picture of the image on a screen or you send the light through the grating and then directly to your camera?

I have a violet laser pointer that shows a similar, interesting behavior.
If I look through a grating at the spot on different types of surfaces I can get either pure violet diffraction spots or spots with continuous spectrum. It depends on the surface.
I thought It may be some fluorescence induced by the laser.
I will post pictures when I have a chance.

 

[BernieM]

There is white paper behind the diffraction grating. The laser is in front of the diffraction grating. What you are seeing is the spectrum created by the laser on the white background. Red laser under same condition shows nothing but red light.

Yeah I was leaning towards fluorescence, something common to the glasses and the alexandrite that fluoresces green, like an additive to the glass or some element in the glass. Really old purple glass was caused by magnesium in the glass when we got most of our glass from Germany pre-1918 or so. Today most of it I think comes from China, and maybe there is an element common in their glass sands there, or something they are adding to their glass batches. ?

But now with the rainbow spectrum from the laser, I wonder. Perhaps it is some kind of fluorescent property of a glass, maybe even the glass of the lens/filter on the uv laser instead of the glass samples themselves??

 

[Drakkith]

I'd guess it's fluorescence. Notice the graphs about 2/3 to 3/4 of the way down on this page: http://www.ledmuseum.candlepower.us/eighth/blu-ray.htm

They show the spectrum of the luminescence emitted by the various objects.

Edit: Also found the following page: http://laserclassroom.com/fluorescence-beauty-brains/?doing_wp_cron=1394516597.7835791110992431640625

Now, for something completely different – try it with the 405nm violet laser. What? Why is this monochromatic laser producing a spectrum?? Because even though the 405nm violet laser is in the visible spectrum, it has a very short wavelength, a high enough frequency and sufficient energy to cause LOTS of things to fluoresce, including a plain piece of white paper. Some other things to try are detergents, olive oil and petroleum jelly – as well as any tonic water containing quinine.

What you see with the diffraction glasses is the emission spectrum caused by fluorescence. When the electrons in the paper absorb the 405nm violet light, they become excited and are “knocked” up to a higher energy level. They can not stay up there for long, (much, much less than a second) and quickly fall back to their original energy state. As they do this, they release the extra energy in the form of light of various wavelengths. So, you see a spectrum.

In the picture the author of the article took, the 405 nm light forms a spectrum instead of a simple diffraction pattern:

Whereas the green laser does not form a spectrum:

 

[Drakkith]

Also, while looking up info on this, I came across the following page: http://www.laserpointersafety.com/illegalpointers/illegalpointers.html

Apparently a large number of 405 nm lasers are being sold on ebay and amazon that are marketed as having less than 5 mW of power output when they have something along the lines of 60-80 mW or more. Apparently these lasers require that much power output just to be visible since the eye responds poorly to 405 nm light. Be careful.

 

[BernieM]

That's an important heads-up on the laser power issue. I know for a fact that these 5mw pointers are far in excess of 5mw. Personally, I use polycarbonate plastic lensed glasses which suck up uv very well.

 

[BernieM]

Ok, so after re-checking the spectrum on the uv laser without a white background, there is only uv. Back to the green color then in the different glass samples. Checked pyrex too. Same green. So what's the odds 4 different glasses all have the same fluorescent impurity? Including a natural crystal of calcite and alexandrite. ?