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Old 04-06-2009, 09:37 PM #1
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Default Green Laser Diodes - Hot Article!

Just discovered a fantastic article in the April issue of SciAm, talking about work on creating GREEN laser diodes!!!

The article also has some good info on the invention (and inner workings) of cost-effective Blu-Ray diodes, which was apparently created by the same folks who are now developing blue and green! (Good stuff to know, may shed some light on the peculiarities of the PHR-803T diodes?)

pullbangdead, I know you mentioned you have access to some nice meters, but why didn't you tell us you guys were having so much fun out there at UCSB!

Some key highlights -

The same technological breakthroughs at UCSB that allowed the creation of low-cost high-yield Blue-Ray (actually violet) laser diodes, are now being extended to develop blue and green laser diodes!

Indium - use of an ultra-thin (nanometer-scale) active layer of Indium Gallium Nitride between N and P-doped Gallium Nitride layers, to form a "quantum well".

Growing the diodes on a pure crystalline Gallium Nitride substrate (base), rather than traditional Sapphire. Apparently growing these on top of Sapphire produces a lot of submicroscopic crystalline defects / irregularities, which can propagate upwards through successive layers as the diode is grown. These defects act as centers where electrons and holes recombine to produce undesirable heat, rather than light. Interesting quote -

Quote:
The presence of these defects played havoc with production yields when Nichia and Sony first tried to manufacture blue laser diodes.
Reportedly, the lasers used in Blu-Ray players and PS3 are grown on top of sapphire, which as substrates go, is relatively cheap and widely available. I cannot help but wonder if the # of these defects in a particular LD could be the cause of the wide variance in efficiency and output levels being reported here in individual Blu-ray diodes?

Using a pure Gallium Nitride substrate results in far fewer of these mismatch defects, and can thus produce much more laser light, with a lot less heat! 8-)

By adjusting the amount of Indium in that thin active layer, you can also alter the wavelength of the laser light produced! Key concept here -

Quote:
By increasing the indium concentration in the alloy, one can lower this energy, thereby increasing the wavelength of the light and changing its color from violet to blue to green.
Of course, nothing is ever that simple - there are a few difficulties standing in your way!

First off, the crystalline structure of the Gallium Nitride apparently creates strong electrostatic fields which tend to drive the electrons and holes away from each other, fighting against the applied current, and making it more difficult for them to combine to create light. This "quantum-confined Stark effect" also becomes a much greater problem when you try to shift the wavelength from violet to blue to green.

Even worse, the amount of current/energy required to overcome this effect at the longer wavelengths (like green) results in the electrons & holes combining at higher energy levels, shifting the generated light's wavelength back towards violet!

This is reportedly the main reason why we don't have green LDs.

Another major problem is that Indium Gallium Nitride must be grown at much lower temperatures (700C) than the surrounding Gallium Nitride layers (1,000C), as the higher temps cause the Indium to disassociate from the material!

So when you then try to grow the Gallium Nitride layer on top, the extra heat can cause the Indium in the layer below to form "islands" of inhomogeneous indium alloys, which causes the electron-hole recombination energy levels to vary from one place to another, resulting in different wavelengths of light being generated and a broad emission spectrum - OK for making LED's, but not so good when you are trying to produce monochromatic light for a laser!

Because higher amounts of Indium are needed to produce longer-wavelength light, this Indium island problem becomes more severe when trying to create blue and especially green laser diodes.

But the good boys at UCSB are riding to the rescue! [smiley=thumbsup.gif]

On the seemingly insurmountable electrostatic effects which have reportedly prevented the creation of a green LD for over a decade, they realized that this was a result of the crystalline structure of the traditionally-cut "C-plane" (horizontally sliced like a stack of pancakes) Gallium Nitride crystal, and that by cutting the crystal along the "M-plane" (vertical slices from one of the facets on the side of the crystal, see my funky Corel graphic below) instead, they could solve this problem! 8-)

Of course, finding pure Gallium Nitride wafers sliced this way was a bit of a problem. : But in 2006 Mitsubishi started providing small ones (~1cm) of the required quality to UCSB. On Jan. 27, 2007, a UCSB graduate student created the first working nonpolar GaN laser diode using one of these wafers, generating a blue-violet beam at a wavelength of 405nm! (hmm, sound familiar?)

On the Indium "island" problem, a solution was also forthcoming.

In order to overcome the aforementioned electrostatic problem, Blu-Ray LD manufacturers had been creating extremely thin Indium Gallium Nitride layers (under 4nm, or only about 20 atoms thick!), in order to keep the electrons & holes bunched closer together, maximizing the chances they could combine and create light.

But by solving the electrostatic issue, thicker active layers can now be used (up to 20nm). And while Indium islands would still form, they tend to be confined near the layer boundaries, leaving the center area of this thicker active layer free to create monochromatic laser light!

The article goes into much greater detail on this and other things, but by using all of this magic, they have been able to achieve some pretty amazing results!

This has included -

A blue-violet LED with an efficiency > 45%!

A blue-green Laser Diode, running at 481nm!

An optically-pumped (rather than electrically) Laser Diode, producing 514nm (green) laser light!!!

How cool is that!

On a side-note - I guess I must be learning something here at LPF, because I actually caught the esteemed Scientific American magazine making several laser-related technical errors in a side-bar to this article about green laser pointers! -

Quote:
WHAT ABOUT GREEN LASER POINTERS?
The green lasers that have long been available employ a complicated two-step process...Semiconductor lasers inside these devices emit infrared radiation with a wavelength around 1,060 nanometers. This radiation then pumps a crystal that oscillates at half this wavelength - about 540 nanometers...the second crystal can heat up, altering the wavelength...
Hmmm...what could possibly be wrong with that description? :

So I sent them a correction!

The print magazine has cool graphics & lots of extra info and is definitely recommended, but as it's the current issue, the basic article is also available online here -

http://www.sciam.com/article.cfm?id=...e-green-lasers

Enjoy!


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Old 04-06-2009, 10:36 PM #2
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Default Re: Green Laser Diodes - Hot Article!

Awesome article.. I expect pullbangdead might have even more insight into this development...
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Old 04-06-2009, 11:23 PM #3
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Default Re: Green Laser Diodes - Hot Article!



I'll write some later, just got home now, and getting ready for the big game tonight. I'll definitely have to pick up that issue, though.
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Old 04-07-2009, 04:30 AM #4
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Default Re: Green Laser Diodes - Hot Article!

All sorts of neat stuff happening here at UCSB! *

I haven't really been following the advancements in visible lasers here, but in the lab I work at we have been doing super-far-infared stuff (THz, or submilimeter wave), so far IR that they start to act like radio waves and can travel through a lot of materials (clothes and the likes). *So far record power is on the order of a few mw (from about 150mw of laser power going into it), but we have detectors that are sensitive down into the femto watt (ie, 10^-12w, or .0000000001mw) *so for us that is more power than we know what to do with!

But back to stuff useful to hobby lasers, I am not sure how many of you guys realize it but almost all uv,violet,blue,and 'bright' green semiconductors (leds and lasers included) were born at ucsb, by Nakamura's group. *Nakamura is a professor here, who worked with Nichia until political issues caused him to quit and sue for hundreds of millions of dollars, pullbangdead probably knows more than I do.


Also, from what I understand your article is actually a bit outdated, and the current violet diodes are already grown 'nonpolar' (ie, M planar), which is why we have diodes that put out 500mw instead of the 5mw ones that needed to be cooled with liquid nitrogen and cost gazilions of dollars (ok the c-planer ones weren't quite that bad).
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Old 04-07-2009, 07:33 AM #5
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Default Re: Green Laser Diodes - Hot Article!

As I indicated in my first post, the same technological breakthroughs that allowed the creation of low-cost high-yield Blue-Ray laser diodes are now being extended to develop blue and green laser diodes.

While, as I also stated, the very first experimental non-polar violet LD that was the basis for these further advancements was first demonstrated by a grad student about 2 years ago, the fact that the article references what came before, in order to give the reader a technological underpinning for how that work is now being furthered, does not make it "dated". This article appears in the current issue on the newsstands right now, and was co-authored by Nakamura himself, one of the leading experts in the field!

That being said, I am sure that some of the earlier work has now been commercialized in some of the newer Blu-Ray LD's (Rohm, after all, is a partner in the UCSB center!) But the article also indicates that at least some of the Blu-Ray LD's we are messing-with are based on the older sapphire substrate designs. It would be interesting to find-out which LD's are based on which technologies, particularly in learning the reasons for their peculiarities, and how far we can reliably push them!

But my greatest interest is in the development of true green laser diodes! I also did not get into the latest work in semipolar designs at UCSB in my quick summary. But the print version of the article does have a lot of extra info (including about upcoming products!), which is one of the reasons I had recommended it over the online version.

P.S. - I also found it humorous that the side-bar on Green Pointers (which was obviously written by a SciAm editor or staffer - who seems to think KTP is some kind of quartz crystal oscillator???:P - and not by Nakamura himself), was so chock-full of errors, or that some hobbyist from LPF could call them on it! ;D
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Old 04-08-2009, 09:12 AM #6
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Default Re: Green Laser Diodes - Hot Article!

Very interesting article indeed! I don't know enought to grasp the whole idea behind the development, but I am just happy that they are developing these diodes despite the obviously lower demand for blue and green lasers unlike red and blu-ray!
Quite embarrasing that you had to correct them on that side bar description though :b It doesn't take many minutes of research to find out what is wrong there, even to someone completely new to lasers.
I will definately take a look at that magazine
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Old 04-08-2009, 12:19 PM #7
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Default Re: Green Laser Diodes - Hot Article!

Quote:
Originally Posted by ...
I haven't really been following the advancements in visible lasers here, but in the lab I work at we have been doing super-far-infared stuff (THz, or submilimeter wave), so far IR that they start to act like radio waves and can travel through a lot of materials (clothes and the likes).
cool. the company whre i will do practise this summer is working on terraherz-with-lasers too!

awesome research on this article. hope we will one day (but plz as soon as possible!) have green diodes
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Old 04-08-2009, 01:41 PM #8
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Default Re: Green Laser Diodes - Hot Article!

Quote:
Originally Posted by Jaseth
Very interesting article indeed! I don't know enought to grasp the whole idea behind the development, but I am just happy that they are developing these diodes despite the obviously lower demand for blue and green lasers unlike red and blu-ray!
Quite embarrasing that you had to correct them on that side bar description though :b It doesn't take many minutes of research to find out what is wrong there, even to someone completely new to lasers.
I will definately take a look at that magazine
The primary force driving the development of green and true blue (as opposed to violet) laser diode technology is laser televisions and monitors. This technology will not be economically feasible until it can be designed without using DPSS lasers. DPSS technology is awkward and inefficient compared to using diodes..
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Old 04-09-2009, 09:16 AM #9
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Default Re: Green Laser Diodes - Hot Article!

Quote:
The primary force driving the development of green and true blue (as opposed to violet) laser diode technology is laser televisions and monitors.
Quite right, in fact, one of the side-bars in the print edition of this article was about various hand-held RGB laser projectors that were in the works (some of the first ones will actually be based on DPSS, believe it or not!)

One of the most amazing things they were predicting was an RGB laser projector built inside your cell phone, that could project DVD-quality video on a wall or other surface, with no focusing required!

Can holographic 3-D TV be far behind?

(Think R2D2's "Help me Obe One - You're my only hope!", but much higher image quality! )

BTW, thanks for the rep! - I just noticed that!

Quote:
Quite embarrasing that you had to correct them on that side bar description though
I thought so too!

Quote:
Awesome article.. I expect pullbangdead might have even more insight into this development...
So do I! We're still waiting for him to get back from his game!

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Old 04-09-2009, 01:54 PM #10
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Default Re: Green Laser Diodes - Hot Article!

Quote:
Originally Posted by seoguy
Quote:
The primary force driving the development of green and true blue (as opposed to violet) laser diode technology is laser televisions and monitors.
Quite right, in fact, one of the side-bars in the print edition of this article was about various hand-held RGB laser projectors that were in the works (some of the first ones will actually be based on DPSS, believe it or not!)

One of the most amazing things they were predicting was an RGB laser projector built inside your cell phone, that could project DVD-quality video on a wall or other surface, with no focusing required!

Can holographic 3-D TV be far behind?

(Think R2D2's "Help me Obe One - You're my only hope!", but much higher image quality! )

BTW, thanks for the rep! - I just noticed that!

Quote:
Quite embarrasing that you had to correct them on that side bar description though
I thought so too!

Quote:
Awesome article.. I expect pullbangdead might have even more insight into this development...
So do I! *We're still waiting for him to get back from his game!
Well, they've already built prototypes for devices that use very high-powered "eye-safe" lasers to create plasma points in mid air. By intersecting two beams to create the point, it becomes possible to place the plasma point at any position within the field of the device. The ultimate goal is a holographic projection system.
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Old 04-09-2009, 09:08 PM #11
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Default Re: Green Laser Diodes - Hot Article!

hmm.. eye-safe and plasma in one sentence.. i am curious about how they will (try to) do that.. :-)
and no matter how eye-safe this will turn out, i will get it, eventually! hehe

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Old 04-10-2009, 03:40 PM #12
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Default Re: Green Laser Diodes - Hot Article!

Quote:
Originally Posted by ElektroFreak
The primary force driving the development of green and true blue (as opposed to violet) laser diode technology is laser televisions and monitors. This technology will not be economically feasible until it can be designed without using DPSS lasers. DPSS technology is awkward and inefficient compared to using diodes..
Thanks, I figured that out for myself after a bit.
In future we will be breaking into cinemas to harvest diodes ;D
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Old 04-19-2009, 12:13 AM #13
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Default Re: Green Laser Diodes - Hot Article!

Wow, totally forgot about this thread, sorry guys.

And you have a pretty good synopsis of what's going on there, seoguy. *Let me add some more details. *I'll just fill in within your writing in red, if you don't mind, it's easier to link thoughts that way.

Quote:
Originally Posted by seoguy
Just discovered a fantastic article in the April issue of SciAm, talking about work on creating GREEN laser diodes!!!

The article also has some good info on the invention (and inner workings) of cost-effective Blu-Ray diodes, which was apparently created by the same folks who are now developing blue and green! *(Good stuff to know, may shed some light on the peculiarities of the PHR-803T diodes?)

pullbangdead, I know you mentioned you have access to some nice meters, but why didn't you tell us you guys were having so much fun out there at UCSB!

Everyone should be so lucky as to be where I'm at and doing what I'm doing. *

Some key highlights -

The same technological breakthroughs at UCSB that allowed the creation of low-cost high-yield Blue-Ray (actually violet) laser diodes, are now being extended to develop blue and green laser diodes!

Indium - use of an ultra-thin (nanometer-scale) active layer of Indium Gallium Nitride between N and P-doped Gallium Nitride layers, to form a "quantum well".

Quantum wells are one of the major keys for all the LEDs and laser diodes out there today, they all use quantum well active layers. *The exact reasons for this are not clear without a quantum mechanics explanation, so I'll leave at: they're essential. *Basically all the LEDs and laser diodes out there today use a double heterostructure design of some kind (google double heterostructure). *Basically, when the inner layer of the double heterstructure is made thin enough, it becomes a quantum well, because of what that "thinness" does to the quantum mechanics of the situation. *Having quantum wells and the way they work makes lasers and LEDs much more effective and efficient. *Also, it's generally not just 1 quantum well, it's usually more, often 3 or 5 quantum wells separated by equally-thin barrier layers. *I can maybe make a picture later of what the structure of a laser diode or LED looks like.

Growing the diodes on a pure crystalline Gallium Nitride substrate (base), rather than traditional Sapphire. *Apparently growing these on top of Sapphire produces a lot of submicroscopic crystalline defects / irregularities, which can propagate upwards through successive layers as the diode is grown. *These defects act as centers where electrons and holes recombine to produce undesirable heat, rather than light. *Interesting quote -

Quote:
The presence of these defects played havoc with production yields when Nichia and Sony first tried to manufacture blue laser diodes.
Reportedly, the lasers used in Blu-Ray players and PS3 are grown on top of sapphire, which as substrates go, is relatively cheap and widely available. *I cannot help but wonder if the # of these defects in a particular LD could be the cause of the wide variance in efficiency and output levels being reported here in individual Blu-ray diodes?

Using a pure Gallium Nitride substrate results in far fewer of these mismatch defects, and can thus produce much more laser light, with a lot less heat! 8-)

This is a KEY area of research, and could huge in the future. *This is also combined later with the talk about internal electric fields/quantum-confined stark effect, and so on, so I'll talk about it all at once down there, and the 2 biggest reasons for wanting to grow on GaN substrates instead of sapphire (or SiC) as is traditionally done (Cree is the only company that grows on SiC I think, and they really only do LEDs, so sapphire is the big one).


By adjusting the amount of Indium in that thin active layer, you can also alter the wavelength of the laser light produced! *Key concept here -

Quote:
By increasing the indium concentration in the alloy, one can lower this energy, thereby increasing the wavelength of the light and changing its color from violet to blue to green.
Increasing indium content is key, you have to get the wavelength up, so you have to get the bandgap down, and the only way to do that is adding indium. *Ideally, you want pure, homogeneous indium gallium nitride, all one even phase, all interdispersed. *The problem comes in that if you have too much indium or aren't careful, the material can sometimes prefer to become an uneven mixture of indium nitride and gallium nitride, and that's a Bad Thing. *you mention this later, that the InGaN for the wells is grown at a lower temperature, but then you have to grow p-doped gallium nitride on top of the wells, and to grow good p-doped GaN, you have to raise the temperature. *Raising the temperature like that can hurt the wells, and actually can be a factor that can cause the InGaN to become InN + GaN, which is, as I said, a Bad Thing. *Of course it's not that simple, but that's the idea.

Of course, nothing is ever that simple - there are a few difficulties standing in your way!

First off, the crystalline structure of the Gallium Nitride apparently creates strong electrostatic fields which tend to drive the electrons and holes away from each other, fighting against the applied current, and making it more difficult for them to combine to create light. *This "quantum-confined Stark effect" also becomes a much greater problem when you try to shift the wavelength from violet to blue to green.

Even worse, the amount of current/energy required to overcome this effect at the longer wavelengths (like green) results in the electrons & holes combining at higher energy levels, shifting the generated light's wavelength back towards violet!

This is reportedly the main reason why we don't have green LDs.

This is why we want to grow the diodes on GaN substrates instead os sapphire, along with reducing the number of defects cause by lattice mismatch. *As you mention below, when you can grow on GaN, you can choose which plane of the crystal you want to grow on. *You can grow on m- plane, which has no built-in electric field, or a plane, which has a lessened effect, or a whole host of other planes that will reduce that effect. *Here are some photos to accompany yours below. *That is a good representation of what m vs c plane, now I'll just stick the atoms in there to show why it's better.

This is the unit cell of GaN/InGaN, it's called wurtzite after the prototypical mineral of this structure. *this is a hexagonal unit cell: the 1st photo given is of 1/3 of that hexagon, and 2nd photo shows how the 1/3 fits in the hexagon. *This is the same hexagon that seoguy posted.




Now, when you grow GaN on sapphire, is automatically aligns itself so that the c-plane is on the top, as seoguy shows. *So as you're growing your diode from top to bottom, this hexgon is how the diode will end up: you'll have this hexagonal prism sitting in the middle of your active layer, with electron coming up through the bottom and holes down from the top. *But looks at the atoms: when coming from the top or coming from the bottom of the hexagon, the atoms are layered by type! *There's a plane of Ga, then a plane of N, then a plane of Ga, and so on, and those atoms all have a charge associated with them! *As the electron is coming in, it has to travel through a capacitor with every plane of atoms it passes through, because there's an electric field between each layer of atoms!

How do we fix this? *Well look at the m-plane, the "side" of the hexagonal prism: the Ga and N atoms are on the same plane! *If you're an electron or a hole traveling straight into the side of that hexagon, there's no "net" electric field, because the Ga and N are on the same plane that you're traveling through!

When using sapphire, we don't have a choice, we ALWAYS have to travel through the base plane, where there is a built in electric field. *But, if we have a chunk of GaN for a substrate, we can turn that puppy sideways and grow the GaN in any direction we want! *We can use the m-plane and do completely away with that built-in electric field, if the electrons and holes are traveling perpendicular to the field, it won't affect us at all! *

And, when growing GaN no GaN, there is no mismatch like when growing GaN on sapphire, so the crystal doesn't have all the problems that come from that mismatch. *Really, it's a miracle that GaN works at all with as many defects as it does. *That's why nobody got it to work sooner, no one thought it could possibly work with how many defects there were in it. *But low and behold, it does work, even with enough defects to cripple other materials (red diodes would NEVER work with this many defects, NEVER). *No one really knows why the defects don't bother GaN as much. *And no one really knows either just how well GaN will work when we can get rid of those defects. *For comparison, GaN now works with defects on the order of 10[sup]9[/sup] defects per cm[sup]2[/sup]; with today's technology, we can make silicon with LESS THAN ONE defect per cm[sup]2[/sup]. *Just imagine how good these things will work if we could ever make GaN even close to as well as we make silicon?



Another major problem is that Indium Gallium Nitride must be grown at much lower temperatures (700C) than the surrounding Gallium Nitride layers (1,000C), as the higher temps cause the Indium to disassociate from the material!

So when you then try to grow the Gallium Nitride layer on top, the extra heat can cause the Indium in the layer below to form "islands" of inhomogeneous indium alloys, which causes the electron-hole recombination energy levels to vary from one place to another, resulting in different wavelengths of light being generated and a broad emission spectrum - OK for making LED's, but not so good when you are trying to produce monochromatic light for a laser!

This is what I was saying above about getting more indium into the wells, it's tough. *

Because higher amounts of Indium are needed to produce longer-wavelength light, this Indium island problem becomes more severe when trying to create blue and especially green laser diodes.

But the good boys at UCSB are riding to the rescue! *[smiley=thumbsup.gif]

On the seemingly insurmountable electrostatic effects which have reportedly prevented the creation of a green LD for over a decade, they realized that this was a result of the crystalline structure of the traditionally-cut "C-plane" (horizontally sliced like a stack of pancakes) Gallium Nitride crystal, and that by cutting the crystal along the "M-plane" (vertical slices from one of the facets on the side of the crystal, see my funky Corel graphic below) instead, they could solve this problem! 8-)

Of course, finding pure Gallium Nitride wafers sliced this way was a bit of a problem. : *But in 2006 Mitsubishi started providing small ones (~1cm) of the required quality to UCSB. *On Jan. 27, 2007, a UCSB graduate student created the first working nonpolar GaN laser diode using one of these wafers, generating a blue-violet beam at a wavelength of 405nm! (hmm, sound familiar?)

On the Indium "island" problem, a solution was also forthcoming.

In order to overcome the aforementioned electrostatic problem, Blu-Ray LD manufacturers had been creating extremely thin Indium Gallium Nitride layers (under 4nm, or only about 20 atoms thick!), in order to keep the electrons & holes bunched closer together, maximizing the chances they could combine and create light.

But by solving the electrostatic issue, thicker active layers can now be used (up to 20nm). *And while Indium islands would still form, they tend to be confined near the layer boundaries, leaving the center area of this thicker active layer free to create monochromatic laser light!

The article goes into much greater detail on this and other things, but by using all of this magic, they have been able to achieve some pretty amazing results!

This has included -

A blue-violet LED with an efficiency > 45%!

A blue-green Laser Diode, running at 481nm!

An optically-pumped (rather than electrically) Laser Diode, producing 514nm (green) laser light!!!

How cool is that! *

The published record now for electrical-pumped stimulated emission is 500nm by Rohm and another group (can't remember from where).

On a side-note - I guess I must be learning something here at LPF, because I actually caught the esteemed Scientific American magazine making several laser-related technical errors in a side-bar to this article about green laser pointers! -

Quote:
WHAT ABOUT GREEN LASER POINTERS?
The green lasers that have long been available employ a complicated two-step process...Semiconductor lasers inside these devices emit infrared radiation with a wavelength around 1,060 nanometers. This radiation then pumps a crystal that oscillates at half this wavelength - about 540 nanometers...the second crystal can heat up, altering the wavelength...
Hmmm...what could possibly be wrong with that description? :

So I sent them a correction!

The print magazine has cool graphics & lots of extra info and is definitely recommended, but as it's the current issue, the basic article is also available online here -

http://www.sciam.com/article.cfm?id=...e-green-lasers

Enjoy!


my funky Corel graphic -

Hope that helps a little, let me know if you have any questions or want further explanations of "how stuff works"
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