The World's First CW Non-polar GaN VCSEL
Polarization-locked arrays of violet, blue and green lasers are now on the horizon, thanks to the first demonstration of non-polar GaN VCSELs delivering continuous-wave operation by Charles Forman, Steven Denbaars and Shuji Nakamura from the solid state lighting and Energy Electronics Center at University of California, Santa Barbara
The VCSEL has many desirable attributes. Sporting a far smaller size than its edge-emitting cousins (see Figure 1), it is capable of higher modulation speeds for data communication, and lower power consumption, which is ideal for battery-powered devices. Its only significant drawback is its relatively low output power, but this can be overcome by arranging these devices in densely-packed, two-dimensional arrays.
VCSEL production is now well established. Leading the way are infrared VCSELs based on the GaAs family, which have been capable of CW operation since the late 1980s and were commercialised in the 1990s. These devices have replaced edge-emitters in computer mice and laser printers, and are now being deployed in mobile phones. In the latter application, Finisar is enjoying tremendous success, having just been awarded $390 million to increase its R&D and production of VCSELs, which are key components in the iPhone X TrueDepth camera and AirPod proximity sensor.
Finisar's success could be just the tip of the iceberg for VCSEL manufacturers. Today, this market is limited to red and infrared emission, using GaAs and InP-based VCSELs. But if the spectral domain could expand to encompass blue, green and shortwavelength sources, VCSELs could start to penetrate a whole world of untapped applications in display, illumination, and sensing technology.
Combining red VCSELs with those emitting in the green and blue could create full-colour light engines for displays and projectors, while the low power of all these sources makes them ideal for portable electronics, such as wearable displays. In addition, VCSELs could be deployed in laser-based lighting, and in Li-Fi, where they could provide even faster modulation speeds than edge-emitters, which are already hundreds of times faster than LEDs, enabling a hike in data transfer rates.
At the Nakamura Lab at the University of California, Santa Barbara, we are taking important strides in this direction: we have demonstrated the world's first non-polar GaN-based VCSEL that is capable of lasing under continuous-wave (CW) operation. This is a significant breakthrough because non-polar VCSELs offer many advantages over their polar cousins, including: an absence of polarization-related electric fields in the active region that can improve radiative efficiency; and anisotropic gain, which enables the fabrication of arrays that provide a fully polarized emission source. The latter attribute increases the opportunities for the GaN VCSEL, by opening up applications ranging from optical sensing to increased data transmission rates via polarization division multiplexing.
GaN VCSEL progress
Progress in GaN VCSELs has not been easy. 12 years elapsed between Nakamura's report of the first blue laser in 1996 and the unveiling of the first electrically pumped GaN VCSEL, by Tien-Chang Lu and colleagues from National Chiao Tung University (NCTU). Even a decade on from that first great success, just eight research groups have successfully demonstrated an electrically-injected, GaN-based VCSEL.
Figure 1. Violet GaN VCSEL (left) and blue edge-emitting laser (right) under electrical injection. In comparison to edge-emitters, VCSELs have many merits: orders of magnitude smaller active volumes, lower thresholds for lasing with lower power consumption, high-speed modulation, 2D arraying capability, onwafer testing, circular output beam with low divergence, higher spectral purity, and ability for single-longitudinal mode operation.
Understand the difficulties associated with making a GaN VCSEL, and it is easy to appreciate this apparent lack of progress. Due to an extremely short gain path length of typically 10-30 nm, this class of laser requires a pair of mirrors with a reflectivity in excess of 99 percent. It's a requirement that is relatively easy to fulfil for GaAs VCSELs, wherein mirrors can be formed from alternating, lattice-matched layers of doped, quarterwavelength-thick GaAs and AlGaAs "“ a pairing that creates electrically conductive epitaxial distributed Bragg reflectors (DBRs). However, with a GaN-based VCSEL, fully epitaxial DBRs have not been an option. There have been several growth challenges, along with difficulty activating p-GaN that is buried below epitaxial layers.
Figure 2. Focused ion-beam cross-sectional scanning electron microscopy revealed problems with earlier GaN VCSELs. The thermally insulating bottom DBR forces heat to flow through a thin gold path toward the flip-chip substrate The thermal performance is further impaired by cracks in this thin metal that form during the Au-Au flip-chip bond.
Instead, teams have produced VCSELs with either two sets of dielectric DBRs or a hybrid design with a bottom epitaxial DBR; however, even the growth of just one epitaxial DBR is challenging, due to a lattice mismatch between AlGaN and GaN that leads to cracking.
One team that has had success with the latter approach is that of Tetsuya Takeuchi and co-workers at Meijo University. Using lattice-matched, n-type conducting layers of AlInN and GaN, they have made a mirror with the required level of reflectivity. However, this requires 46 pairs of AlInN and GaN, due to the low index contrast. Consequently, it's a lengthy growth process, demanding precise thickness control to match the peak reflectance with the lasing wavelength.