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First nonpolar violet VCSEL could boost laser efficiency

Photonics Spectra
Oct 2012
SANTA BARBARA, Calif. – A violet nonpolar vertical-cavity surface-emitting laser (VCSEL) based on m-plane gallium nitride semiconductors has been reported and could lead to more optically efficient lasers for lighting, displays and sensors.

Because VCSELs exhibit low threshold currents, circular and low divergence output beams, and are easily integrated into two-dimensional arrays, they offer advantages over conventional edge-emitting laser technology for some applications. For example, on-wafer testing of VCSEL arrays during manufacture could save costs compared with edge-emitting lasers that require additional steps before they can be tested.

The electrically injected nonpolar m-plane nitride VCSEL platform, developed by LED pioneer Shuji Nakamura and his research team at the University of California, Santa Barbara, lases at room temperature and provides high optical gain, helping to increase optical efficiency. The device is naturally polarization-locked along the crystallographic a-direction of the wurtzite crystal, in contrast to the majority of VCSELs, which are typically randomly polarized, said Dr. Daniel Feezell, a project scientist in Nakamura’s lab.


Shuji Nakamura and his group at UCSB have demonstrated the first violet nonpolar m-plane VCSEL based on gallium nitride. Pictured, from left, Casey Holder, Daniel Feezell (back), Steven DenBaars, Shuji Nakamura.


“Polarization locking, with all devices being polarized in the same direction, is certainly one of the most important features of nonpolar nitride VCSELs,” doctoral candidate Casey Holder told Photonics Spectra. “In addition to this unique attribute, nonpolar VCSELs are expected to have higher gain than c-plane VCSELs. This should lead to improved performance over c-plane VCSELs, such as lower threshold current and higher optical output power.”

Prior attempts by research groups for the back facet and mirror of the fabrication method, such as c-plane GaN VCSELs using an epitaxial distributed Bragg reflector (DBR) mirror or thinned GaN substrates and dielectric DBR mirrors, have offered little control of cavity length. Now, the UCSB team has fabricated a method that offers more precise cavity length control.

“We have developed a novel process where we use photoelectrochemical (PEC) etching of a sacrificial InGaN layer to remove the substrate, allowing us to precisely control cavity length (via placement of the sacrificial InGaN layer during epitaxial growth), while still allowing us to use a dielectric DBR mirror for the rear facet,” Holder said. “We believe this novel method is a significant contribution to the field.”

VCSELs had not been grown on m-plane substrates because high-quality nonpolar substrates only became available in 2006 through Mitsubishi Chemical Corp., Nakamura said. He added that a new process also was required to make VCSELs using a selective etching, which is how they developed the PEC method.

The team has demonstrated working devices as proof of concept, but Nakamura and Holder agree that displays and sensors will be the most immediate applications.

“Commercial applications are of course hard to predict,” Holder said. “We have a lot of work to do in order to take this technology from proof of concept to a commercial product, but we are very optimistic about all of the opportunities listed as well as other applications we haven’t even thought of yet. Lighting and displays are certainly much larger markets, but I anticipate that nitride VCSELs could make very important contributions in niche markets such as sensing as well.”

“Using the VCSEL, we could reduce the cost of the laser diodes dramatically [in the] near future,” Nakamura told Photonics Spectra.

Next, the researchers plan to continue improving the device’s performance.

“This proof of concept is a great first step, but now we must demonstrate the performance advantages that should be achievable with nonpolar GaN VCSELs beyond polarization-locking, such as lower threshold current and improved optical output powers,” Holder said.

Nakamura hopes to achieve continuous-wave operation of the nonpolar VCSEL, then ramp up the power of continuous-wave operation.

The findings have been submitted for publication.


GLOSSARY
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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