A polished blue

Abstract

Researchers at the University of California, Santa Barbara, have achieved the highest ever power output and efficiency for a blue laser diode on a semipolar GaN substrate. Semipolar substrates are cut at a different angle from the standard c-plane of the crystal, and are predicted to result in more effective laser operation. Despite the advantages, such as higher gain and efficiency, several technological challenges have limited the efficacy of these semipolar substrates in most applications. Blue laser diodes can excite other phosphors to emit different wavelengths, resulting in white light The Santa Barbara team, however, have overcome these challenges through a polishing process, and the resulting laser demonstrates stable, continuous wave operation with high optical power densities. This could lead to greatly improved solid-state systems in all lighting, imaging, and display systems. Solid-state lighting systems are lighting systems based on semiconductor devices, typically diodes, and use considerably less power than traditional light sources, as explained by Robert Farrell, one of the authors and a researcher at UC Santa Barbara: “they are used primarily because they are more efficient than traditional white light sources such as incandescent or fluorescent lighting.” Quantifying this difference in efficiency makes the advantages of solid-state systems very clear, said Farrell, telling us that “incandescent bulbs and linear florescent bulbs use ∼15 lm/W and ∼100 lm/W, respectively, while state-of-the-art solid-state lighting systems operate at ∼150 lm/W and this is expected to surpass 200 lm/W as further developments in the technology are made.” The current generation of commercially available systems use LEDs for excitation. For lighting purposes, “the semiconductor device emits photons in the violet or blue range of the spectrum,” said Farrell, “and these excite one or more phosphors that emit in other regions of the visible spectrum. The combined emission of light from the semiconductor devices and the phosphors is used to produce high quality white light.” While LED lighting technology is relatively mature and developed, laser diode approaches show even more promise in terms of power and efficiency. Efficiency is not the only advantage, however, as the laser diodes have both the potential to decrease the cost of solid-state lighting systems, as well as to improve the directional control of light emission as lasers are a natural point source. Laser diodes naturally require a substrate cavity for operation. This cavity can be oriented at various angles to the substrate's crystal structure. Standard (polar or c-plane substrate) lasers use mirror facets formed by cleaving the semiconductor wafer along a crystallographic plane. This is not, however, the best solution. Non-polar substrates show greater promise than their polar counterparts, but for laser cavity orientations on semipolar crystal planes “there is not an orthogonal crystallographic plane that can be used for cleaving mirror facets,” explained Farrell. The consequences of this are fairly drastic as other techniques, such as dry etching or polishing, are therefore necessary. Unfortunately, it has not been clear if these techniques are stable under high optical power densities. The achievement of the UC Santa Barbara team has been, according to Farrell, “to show that polishing can be used to create mirror facets for semipolar blue laser diodes that are smooth, perpendicular to the laser cavity, and stable under CW operation with high optical power densities (greater than 30 MW/cm2).” Showing that blue semipolar laser diodes with polished mirror facets are stable during CW operation with high optical power densities firmly establishes them as a viable alternative to c-plane laser diodes. Is the story then complete? The team's results certainly represent an important step in the development of semipolar laser diodes for laser-based solid-state lighting. However, laser diode lighting is still at an early stage - shortcomings in the technology are still apparent. “In particular,” said Farrell, “the wall plug efficiency (currently around 40%) needs to be improved, and new phosphor geometries need to be developed for integration with laser diodes.” The mechanically polished facet Once these issues have been addressed, Farrell told us that the team expect, as the efficiency of laser diodes improves over time, “it will be possible for laser-based solid-state lighting systems to find applications in all areas of general lighting.”