Solid state lasers can produce light in red and blue parts of the visible spectrum, generating laser light in all colors except green. But recent research work suggests that this ‘green gap’ could be plugged. New techniques for growing laser diodes could soon make brilliant full spectrum display a reality. Plugging the green gap in the red green- blue triad needed for full-color laser projection and display would help speed the introduction of laser projectors for televisions and movie theaters, which will display much richer colors than other systems, and tiny handheld projectors as in cellphones.
High-power green diodes might be employed in such applications as DNA sequencing, industrial process control and underwater communications. The familiar green laser pointers used by lecturers employ a complicated two-step process to generate light. Semiconductor lasers inside these devices emit infrared radiation having a wavelength of around 1060 nanometers. This radiation then pumps a crystal that oscillates at half this wavelength—530 nanometers, which corresponds to green part of the spectrum. The process is costly, inefficient and imprecise; the second crystal can heat up, altering the wavelength of the resultant green light. Lasers that generate green light directly would avoid this problem.
What the world need now is a semiconductor laser that’s good, cheap, long lasting, powerful, and truly green. Such a device could revolutionize information display, improve certain ophthalmology 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 ultra-vivid 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 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 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. a new way to grow the crystalline layers of gallium nitride and related alloys that The groups from U.C.S.B. and Rohm are developing make up a laser diode.
The early successes of the approach not only promise greater yields but also buoy hopes of an even bigger payoff: rugged, compact GaN diodes that emit green laser light—a goal that has long eluded scientists and engineers. The technique should also lead to high-efficiency green LEDs that emit much more light than existing devices. These achievements would fill a gaping void
in the visible spectrum where evolution has trained our eyes to be most sensitive, plugging the “green gap” in the red-green-blue triad needed for full-color laser projection and displays. They should help speed the introduction of laser projectors for televisions and movie theaters— which will display much richer colors than other systems and of tiny, handheld “Pico projectors” to be used, for example, in cell phones. And high-power green diodes might even be employed in such diverse applications as DNA sequencing, industrial process control and underwater communications.