What the world needs now is a semiconductor laser that's good, cheap, long-lasting, powerful, and truly green. Such a device could revolutionize information display, improve certain ophthalmological therapies, and give us affordable televisions with bigger, more dazzling pictures than the best available today.
For display, it would make possible a full-color projector small enough to fit inside a mobile telephone. It may even one day be possible to project ultravivid images directly onto your retina.
It could also cut the cost of a major eye treatment, because green light is ideally suited to burn thousands of spots into the retina, stopping the proliferation of new blood vessels and ameliorating diabetic retinopathy, one of the main causes of blindness in Europe and North America. Green light propagates to greater distances through water than any other color, so it would improve underwater communications. You may also find it in laser light shows, industrial process control, and one day, in DNA sequencing machines.
But the biggest market of all would arguably be for use in television sets. Laser TVs are already available—Mitsubishi began selling a 65-inch model in the United States in 2008, for US $6999. The eye-watering price, which has since dropped hardly at all, reflects the high costs of the green and blue lasers (there's a red one, too, of course, but it's a relatively inexpensive descendant of the semiconductor laser chip in DVD players). The blue is a longer-wavelength sibling of the ultraviolet laser used in Blu-ray players. The real headache, though, is the green laser. Because there is no commercial semiconductor laser chip in this color range, the device's manufacturer had to cobble together a cumbersome contraption known as a frequency doubler. It starts with a laser that emits light at the wavelength of 808 nanometers, in the part of the infrared range just beyond visible red light.
That radiation pumps a crystal that emits infrared light at 1064 nm. This light bounces in a cavity between two crystals, doubling the frequency to 532-nm emission. Voilà: green light.
For every watt of light that comes out of the original, infrared laser, you get about 0.4 watt of green light. What's even worse—for a TV manufacturer, at any rate—is that the power and space needs of that green-laser kludge add appreciably to the complexity and cost of the control circuits for the TV.
Since the 1960s, academic and industrial research teams around the globe have been running a race to build the first reliable, manufacturable, green-emitting semiconductor laser. After a flurry of research in the late 1960s and early 1970s ended in failure, practically no one in the field saw that the key to victory was an obscure material called gallium nitride.
The story of the green laser actually starts with a different color and a different device: LEDs—blue ones.
To understand how and why, you'll need to sit through Lasers 101 (skip this and the next four paragraphs if you've already taken this class). Lasing happens in a semiconducting chip when a free electron skittering through the semiconductor's crystalline lattice drops from a higher to a lower energy band, emitting a photon of a very specific wavelength. As the photon bounces back and forth between the reflective ends of the chip, it stimulates the emission of still more photons of the same wavelength and phase. The resulting cascade of photons wrings energy from the system in the form of coherent, monochromatic light. But such a cascade is possible only if a great fraction of the electrons are in the higher energy band to begin with—an oversupply of 'excited' electrons that's called a population inversion. Such an inversion can be obtained in gallium nitride.
Gallium nitride is a III-V material—that is, its first element, gallium, is in Group III of the periodic table, and its second, nitrogen, is from Group V. The resulting crystal is a semiconductor. If you add just a trace of silicon to the mix, the sprinkling of additional 'dopant' atoms will sit on only those sites in the lattice that would normally be occupied by gallium. However, because silicon is from Group IV, each atom contributes an additional, chemically unbound electron to the structure. Such free electrons can move throughout the structure, where together they occupy what's known as the conduction band. Meanwhile, those electrons that are chemically bound end up in the valence band, which has a lower energy. The 'energy gap' between the two bands determines how energetic the laser photons will be, and their energy, in turn, determines their wavelength. A high-energy gap means a short wavelength—green, blue, or violet, for example.