Meanwhile, in Japan...


PHOTO: Tony Mastres/UC Santa Barbara

In the same journal issue, researchers at Rohm Co., in Kyoto—another partner in the UCSB Center—published a paper stating that they had fashioned a similar blue-violet laser in early February using a similar approach. According to the paper, their laser operated in the preferred, continuous-wave mode, at current densities as low as 4.0 kA/cm². (In fact, Hiroaki Ohta of the Rohm group said his team had achieved lasing even earlier than that, but this claim could not be confirmed at press time.) 


These two rapid-fire successes make it clear that the availability of high-quality GaN substrates was crucial to the breakthrough. So commercialization of the new lasers will depend on getting enough good material. Mitsubishi indicates that right now it has only enough capacity to satisfy the research needs of its partners in the center. But the company will probably try to ramp up its production of m-plane substrates—as will others, such as Kyma Technologies in Raleigh, N.C., which specializes in GaN substrates.


ANIMATION: MIKE SPECTOR

The cost of the substrates, however, remains a major concern. Kyma’s chief technical officer, Drew Hanser, citing his company’s earlier experiences with c-plane GaN substrates, estimates that the first 2-inch wafers of m-plane GaN substrates will cost more than US $10 000 apiece. Compare that with the $20 to $30 that laser and LED makers now pay for comparable-size wafers of c-plane sapphire, according to a report from Strategies Unlimited, a Mountain View, Calif., market research firm. So even though laser-diode production yields may be 10 times better than on sapphire, the cost of GaN substrates will still have to come down by an order of magnitude for this approach to be competitive. 


UCSB’s DenBaars estimates that it will take manufacturers two to three years to resolve all the production problems and bring the new GaN lasers to market. When that happens, the units could begin to replace the blue-violet lasers currently used in Sony’s Blu-ray and competing high-definition DVD players, which boast 27 gigabytes of storage capacity—enough to hold five feature-length movies. According to DenBaars, the GaN lasers in these units now account for 20 to 30 percent of the product’s retail price of about $600.


Nonpolar lasers have another advantage in that their light is naturally polarized. This makes them ideal for use in compact liquid-crystal displays, which require polarized light. 


Target: green light


The real payoff should come in producing green laser light—something no semiconductor laser has done in the 45 years since these devices were invented—according to Noble Johnson, an IEEE Fellow and a laser scientist at Xerox’s Palo Alto Research Center, in California. Electron-hole segregation and related effects in polar c-plane diodes cause quantum efficiencies to plummet rapidly toward zero at green wavelengths of 550 nanometers, says Nakamura. But nonpolar GaN diodes lose only a few percent in efficiency in this range, because many more electrons and holes are able to recombine. The new approach may allow for much brighter green LEDs, Johnson says, but it will prove more difficult getting similar nonpolar diodes to lase at these wavelengths. Rohm is taking dead aim at this target, however, and others will certainly follow suit. In early February the company claimed its researchers were working on a green GaN laser that should operate at 532 nm by year-end, a timetable that Johnson thinks is optimistic.


Once someone makes a green solid-state laser diode at an acceptable cost, it will be possible to combine it with existing red and blue laser diodes in several exciting new applications for consumer electronics. Rohm predicts that they will be used in large-screen, HDTV screens with a color range more than 50 percent better than current analog television standards. Even miniature, laser-driven pocket projectors may become possible. And who knows, they may also end up in cellphones, the way everything else electronic has.