We have just learned that for the first time, a green, 531nm wavelength, direct-emitting semiconductor laser diode has been demonstrated — and at long last. Although Osram and Nichia have both also recently claimed diodes lasing at ~500nm and ~ 515nm respectively, these wavelengths are green-blue and are not quite green enough to be useful in RGB displays for standard gamuts. That’s why the demonstration of a 531nm emitting laser diode is so important. This could be a momentous moment in laser history — and probably for the history of displays as well.
Direct emitting green laser diodes have been anticipated for more than a decade as this region formed the so-called ‘green gap’ where direct bandgap lasers had not been demonstrated. The gap has now been filled with Sumitomo Electric reporting their success in the July 17th online edition of Applied Physics Express, 2, 082101 (2009); Enya et al.
We have solved the green gap problem by developing green lasers built with a frequency-doubling crystal to output ~ 532nm light. Such devices can bulky and electrically very inefficient. But Corning, Osram and others have shown that the combination technology (LD & frequency-doubling crystal) can be miniaturized to a considerable extent — to the size of a piece of chewing gum, or thereabouts.
With Sumitomo’s new result however, we can now contemplate a ‘full house’ solution - all the necessary R, G, & B colors at useful wavelengths for making direct-emission semiconductor laser displays in essentially the same materials system — GaN-InGaN. This is a very significant desire from a materials-growth and device-fab/processing perspective.
Sumitomo’s achievement has important implication for many types of displays in the future. But don’t think this new result will create waves this year, or next - it is early stage research just now - a confirmation of possibility that has been debated and attacked for many years.
To achieve this breakthrough, the direct-green LD was grown on the (2 0 -2 1) semi-polar crystal plane to avoid potentially performance-damaging effects of internal piezo-electric polarization. It exhibited 500ns-pulsed-operation at 0.5% duty cycle, emitting at 531nm. The lasing cavity was 600 microns long and 10 microns wide, gain guided. The threshold current was pretty high, some 15.4 kA/cm2. Power output at 531nm is not stated. But the laser worked - a successful ‘hero’ experiment.
The research paper omits the vitally important cavity-loss numbers that contribute significantly (with poor p-type contact) to the present rather poor overall device performance. The paper also omits specific details of the active region’s quantum-well and barrier system, but it is stated that the quantum wells were 3nm to 4nm wide. There is no data on the barriers, nor an indication of the Indium-concentrations used. From my quick calculations, the quantum-wells will probably contain around 40% Indium incorporation, with the barriers probably containing just a few percent of Indium (extrapolating from a recent publication of the Santa Barbara team).
There is clearly plenty of development-work to be done in the near future to improve Sumitomo’s basic green laser diode performance, and get to continuous-wave output at useful power-levels for displays and many other applications.
I’m reminded strongly of early-1996, when those of us then pursuing frequency-doubled ~ 860nm laser diode light to obtain ~ 430nm blue light (aimed at eventual DVD use, perhaps) - were blind-sided by Nichia’s sudden announcement of the blue laser diode. Nakamura-san and his colleagues created a storm. Much frequency-doubled laser development activity was rapidly halted and a new focus given to repeating and developing the Nichia blue result.
I suspect the same is going to happen again - déjà vu happens. There may now be a frantic move by the world’s leading laser diode labs to repeat and improve-upon the Sumitomo result. Today’s frequency-doubling programs may fade away, for the most part, if history repeats itself.
We may indeed look back at this announcement as the shot in the arm the display industry needed to generate excitement around a laser solution. It’s time to be imaginative, once more, and see what can result.