Ever wonder why projector systems and televisions doesn't use laser illumination? It isn't for safety reasons, and it isn't for efficiency reasons—laser diodes have efficiencies ranging from 30 to 50 percent. No, the problem is green light. We have red laser diodes, and blue laser diodes turned up nearly 15 years ago. But green—where the heck is the green laser diode?

A group of Japanese researchers have answered that question: in our lab. Yes, they have the first "true green" laser diode. It doesn't work that well yet but, based on past history, expect rapid progress from here and commercial laser diodes before the end of next year.

I guess the big question is "what took them so long?" And the answer to that question is a little bit complicated. First, lets take a look at "normal" lasers. The color of light generated by a normal laser, such as the red of a helium-neon laser or the blue-green of an Argon ion laser, is not under our control. Lasers require a rather special set of conditions to be met before they will work, and, nature only satisfies these with certain material/color combinations. This was, and continues to be, an immense source of frustration to scientists.

Laser diodes, which are made using the junction between two or more semiconductor materials, are slightly different. In this case, the color of the light is determined by the energy difference between the conducting electrons and the lowest available states in the nonconducting valence band. This gap can be adjusted by combining different materials and making sandwich structures, called quantum wells.

So, you end up with complicated materials, like alloys of aluminum, indium, gallium, and arsenide, combined with careful layering. One layer might have gallium, arsenide, and aluminum, while the next will have gallium, indium, and aluminum, and so on. To adjust the gap, one changes the ratios between the different components.

In principle, one can get any color, from blue right through to the mid-infrared, by combining the appropriate semiconductor materials. But the world had to wait about 20 years for the blue laser diode and another 15 years for green, so what went wrong?

Pooling indium and other difficulties

The general problem is that certain combinations of materials don't alloy very well. For instance, blue laser diodes use a gallium nitride system, and figuring out how to get nitride to mix through the gallium evenly turned out to be quite difficult. Green, it turns out, requires a high level of indium in certain layers of the quantum well structures. Unfortunately, the indium diffuses and pools together, and that messes up the whole structure.

The other problem is that gallium nitride has a natural electric field associated with it. Good gallium nitride substrates are typically grown in such a way that this field acts on electrons passing through the quantum well structures, preventing them from losing energy and emitting light. In other words, the electric field quenches emission in the blue-green and green part of the spectrum.

The new researchers, from Semiconductor Technologies R&D Laboratories at Sumitomo Electric Industries, got around the electric field problem by changing the orientation of the gallium nitride substrate. That is a story in itself, but, briefly, they found an appropriate template that encouraged the gallium nitride to grow so that it presents a different surface to the world, one that is electrically neutral. On to this, they grew the various layers required for a green laser diode. However, what they don't tell us is how they prevented the indium from diffusing. I would guess that the orientation of the substrate somehow slows the diffusion down.

In any case, the structure worked. They report laser emission for colors between 520 and 531nm, which is pretty much dead center on the green color required for display technology.

Of course, the laser wasn't very good: it was operated in pulsed mode (all lasers operate in pulsed mode when they are first switched on). The efficiency was also shockingly bad at 0.1 percent, mostly due to the pulsed nature—if we ignore the off time, the efficiency goes up to about 20 percent. The authors observe that the electrical contacts weren't optimized, leading to high resistive losses; the laser operated at about 20V, while a normal laser diode operates between 1 and 3V.

You can guarantee that those problems will be solved pretty quickly now that the researchers know they are on the right track. Once volume production starts, prices will fall into line with red and blue laser diodes, and then the cost of projection display systems should fall dramatically. In the end, what many researchers are aiming for are hand-held projection display systems.

For those of you wondering why green laser pointers exist, here is the short answer: take an infrared diode laser, use it to power another laser that is deep into the infrared. Use an optical nonlinear crystal to double the frequency and half the wavelength of that laser. You get 530nm light and profit from a complicated little device. The overall efficiency of this process, however, is something like 6 percent.

Applied Physics Express, 2009, DOI: 10.1143/APEX.2.082101