Submitted by SurprisedPotato t3_10utol4 in askscience
If one browses an online store for laser pointers, there's an abundance of red, green and blue ones - but not yellow or cyan or other spectral colours.
Is there an actual engineering reason why yellow and cyan are harder to make than green or blue?
Additionally, the “yellow gap” is a thing that exists in semiconductor physics. We just don’t have a good junction diode with a band gap that produces photons around 580nm, which is that “banana yellow.” The way we solve that for LEDs is using phosphors, but that’s useless for producing coherent laser light.
And while green is typically doubled, I thought Nichia came up with the gallium nitride blue laser which is direct for low power lasers.
We have direct blue and direct green diodes now, 450nm and 520nm respectively.
Edit: the direct blues are 5+ watts per diode and the green is at about 1.4 watts per diode so not that low powered
This, but I think there's even more to it: When checking a plot of wavelengths vs color, (eg https://www.gentec-eo.com/media/3583/commercial_laser_lines.png ) one can see that the region of wavelengths perceived as yellow or cyan is rather narrow compared to red, green, or blue. So if you want a yellow laser, you need a laser running pretty exactly at the desired wavelength, so when frequency doubling around 1140-1180 nm. It is doable, but red or green is just much easier as its a wider range (and suitable lasers have been lon established, e.g. Nd:YAG at 1064nm to be frequency doubled to 532nm, which is a great green).
And I think another point comes into play: yellow is just really not convenient as a laser pointer, when most slides have a bright background. Red and especially green can just be seen mich easier also with low power.
So I think in the end it's a combination of harder/more expensive to engineer and just little demand for it.
And why can't you just mix red and green, just like we do on a monitor/TV? The eye does not require a specific wavelength to perceive yellow.
True, but the thing that makes lasers lasers and not just tiny flashlights is that they are a single coherent beam of uniform light. This is what allows them to behave reliably for engineering purposes and stay together over long distances, etc.
If I were to build a yellow laser by having a green laser and a red laser, it would be hard to get them to converge on exactly the same point. Or put another way, getting them to converge at a particular distance would be easy, but as soon as you moved slightly closer or further the dots would probably misalign and you'd end up with a red and green dot near each other. Even if the two beams were mirrored into the same trajectory, it's possible they'd refract while traveling due to their different wavelengths and end up as two dots at the end anyway.
That lack of mixing is actually a feature for fiber optic communications. There are optics that use four lasers with different wavelengths to send four data streams simultaneously down the same fiber line.
One doesn't need lasers for that, as long as the medium is linear the mixing of light is just superposition. It is no different than transmitting multiple RF signals across multiple different channels at once.
I think the staying together property is the important aspect. Light diminishes by 4 times every time you double the distance. Except not really because its just that it disperses at that rate by virtue of spheres. Light itself on an individual scale is as far as I know, infinite. So if you convince them to stay in parallel formation, you can transfer them in a vacuum infinitely.
Lasers don't perfectly align the photons but they do a pretty darn good job. Like take a flashlight and shine it at a wall and step back and the light in the wall keeps getting bigger and bigger. Now I have not verified this next part and I am vaguely remembering what someone else said on reddit so the size probably is a bit different than what I'll say but for the distances its kind of moot. But the lazer on space probes only goes from the base lazer size to about the size of a car going from pluto to earth. So they are shining a little light at earth and it only splits a very small amount. They also mentioned there are designs for perfectly straight lasers but to go from like 99.998% to 99.999% is obnoxiously difficult considering pretty much nothing requires that level and the biggest obnoxious part is it requires a larger and larger lens, eventually reaching infinite size in order to make the perfectly straight lazer. Again, any specifics, take them as a general idea and not a point you want to bring up in casual conversation without adding "I heard it was something along these lines" because this is all "something along these lines" after the point I mentioned that I was parroting off someone from reddit. The points before that I know with much greater confidence.
Oddly I’ve experienced this to some degree. I have a scoped air rifle with an illuminated reticle that is capable of red or green crosshair/dot/circle etc. It also has a red laser mounted slightly below the scope. Both are sighted in for 30 yards and when I select the green reticle and turn on the red laser, they do appear almost yellowish when on target at 30 yards. Any closer or father from 30 yards and they diverge, with the red laser dot being above or below the scope reticle. I used to find this frustrating until I realised the amount of divergence actually became a crude range finder of sorts.
>Even if the two beams were mirrored into the same trajectory, it's possible they'd refract while traveling due to their different wavelengths and end up as two dots at the end anyway.
Do you mean that they would refract differently passing through an interface, or that the two beams would interfere with each other? It seems like the former should be able to be controlled on the device side as long as you are careful with the optics (though shining the laser through an interface would split the beams, but nothing we can do about that). You could probably cheat your way around inter-beam interference by using a pair of pulse-width modulated lasers set out of phase so that the beams don't overlap and relying on persistence of vision for the laser to be perceived as yellow.
You certainly can. But it requires some fine tuning and a beam splitter. I have done this In my photonics lab. Really it just comes down to cost.
Because neither red nor green, in terms of frequency/wavelength, are yellow. The colour wheel is about visual perception.
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I don't understand your point. The brain can't tell the difference between yellow wavelength and the mix of red+green that give the same excitation of red/green photoreceptors.
I get that it's not a coherent light/laser anymore, but it should still be possible to create a yellow pointer, if you can join the two beams and align them precisely.
I was typing a wrong response but I can tell you why it doesn't work: lasers run parallel and it's kind of an important aspect of them. In order to mix something, they need to converge. So you need to adjust the focus point of the two lasers to account for distance. I was thinking maybe fiber optics but that wouldn't change the fact you can't change the position of light in that way or it will diffuse or not converge to mix the colors.
Lasers are not great light sources for displays. You have to do something to get rid of the coherence, otherwise you get a lot of speckles that either distract you or make you sick.
Good question, I don't know. Sounds like it could work. A quick search only gave me some single wavelength yellow ones though. My guess is that they are just more expensive. You'll need two lasers, and also optics compensated for the chromatic shift and suitably coated. That's straightforward, but probably just makes it too expensive for normal laser pointers. But yeah, i don't see a fundamental reason not to make it like this
Lasers have a lot of use outside slideshows though. It’s very possible that yellow could be useful outside of that narrow isecase
Yellow also just doesn't travel as far. Which is why from far away, tress on mountains appear blue, not green.
Or at least that's what I've always heard.
Consumer lasers aren't going to be powerful or well calibrated enough for this to be an issue. The main phenomenon is Rayleigh scattering, the same thing that causes blue sky/red sunset, so red would travel farthest, and a hypothetical yellow would travel farther than green. There are some other less common factors like air pollution that could change how this plays out.
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No, it’s because the atmosphere is scattering blue light between you and the mountain. Yellow light is unscattered.
Which is adding blue light, not removing yellow light (though the scattering also removes some blue light coming from the mountains: the scattered blue light from the much brighter sun more than makes up for it)
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I read the word 'diodes' wrong the first time I read it. I was utterly confused
Cyan and yellow solid state lasers exist. They are not common or cheap though.
Red and blue lasers technology was driven by the CD and DVD industries. A lot was invested to make them fast and cheap. Green was pushed later to make RGB displays and because humans are very sensitive to that color. Green pointers are perceived as very bright. There hasn’t been much invested in yellow or cyan.
Indeed, the diodes for cyan and yellow are very expensive and have only very recently been publicly available
Is this a recent breakthrough (similar to blue diodes 10 years ago), or is it something that has been known for a while, but just far too expensive for the benefit that it provided?
Could we see the cost of yellow diodes drop dramatically over the next decade?
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Laser diodes are made from a semiconducting material. We have different materials available, and the choice of material determines the colour. We have good working and widely used materials available for red and blue, but not for the space inbetween (green, yellow, cyan, orange). As someone else described, it is relatively easy to produce green from infrared. For the other colours, we do not have materials available for home consumer use. There are some, but they are not in high demand which is why they are not as cheap and widely available.
We absolutely do have laser diodes that can output green and cyan (520nm green diodes have been around for years, and 488/492nm diodes are newer but also avaliable), but yellow is still in a gap that we don't have any known way of making a diode for at the moment.
Most likely because development cost of red and green/blue semiconductor diode lasers have been paid for by other applications, and those devices are available cheap. Selling laser pointers probably does not generate enough profit to develop their own laser technology.
Optical data storage used to be pretty big, and paid for development of semiconductor lasers. CDROM used AlGaAs/GaAs/AlGaAs thin film 'stack' which emits in red spectrum. BlueRay uses GaN/InGaN/GaN stack, which emits green or blue depending on In content in the middle layer. So these lasers were/are produced in volume.
You can get yellow/orange LED (light emitting diodes), but not lasers. Old LEDs used to be doped GaP, which are pretty dim. More recent ones are AlInGaP layer wafer-bonded to GaP substrate. These are frequently used in traffic lights, and very bright.
The reason data storage lasers skipped yellow/orange is because timing of invention of green/blue lasers. Shorter the wavelength (red > yellow > green > blue), higher the data density on the storage disk. Green/blue lasers were invented before red-laser CDROM had gone obsolete. So when the time came for data storage industry to move to a shorter wavelength, they decided to put development money into green/blue.
There's some history to laser availability, but right now, it's because you can make all those readily available colors from a light emitting diode. LED's are the same way where the only available yellows use phosphorescence to convert blue to yellow, which you can't do for a laser, and doesn't work all that well comparatively. If someone invents a yellow diode, they will get very, very rich because it is something the biomedical field needs as well as for displays and lighting.
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ellipsis31 t1_j7f4giw wrote
Infrared and red laser pointers are direct laser diodes (put current in and light squirts out). Green and blue laser pointers are made by using an infrared laser diode to pump a nonlinear optical crystal which doubles the frequency (halves the wavelength... however you want to say it) to put out photons with a new color. That doubling leaves a gap of wavelengths that are more difficult to access.