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Connecting Theory and Practice in Optoelectronics
Thus far, the highest output power measured on GaN-based lasers is about 7W, as shown in the picture.  In comparison, some GaAs-based lasers emit more than 30W in continuous-wave operation at room temperature. A key reason for this difference is the inherently large p-side electrical resistance of GaN-based laser diodes. It leads to strong Joule heating which lowers the gain and boosts various loss mechanisms that eventually cause the typical power roll-off at high currents.  Read more of this post
Auger recombination inside the light-emitting InGaN quantum wells (QWs) was recently identified as major cause of output power limitations in GaN-based blue light-emitting diodes (LEDs) which are the core of many modern light sources. In this electron-hole recombination process, the released energy is transferred to another carrier (electron or hole) without light emission. The Auger recombination rate rises strongly with the QW carrier density and therefore intensifies with stronger current injection into the LED.
In contrast to LEDs, GaN-based blue laser diodes are expected to suffer less from Auger recombination, based on the popular opinion that the QW carrier density does not rise with increasing current injection above lasing threshold. Shuji Nakamura, who received the 2014 Nobel Prize in physics for his pioneering work on GaN-LEDs, stated in his Nobel lecture that “Auger recombination, with the resulting efficiency droop, does not appreciably occur in blue laser diodes”. We dispute this claim based on our numerical analysis of high-power InGaN/GaN laser measurements.
One of the key rules of semiconductor laser physics relates to the carrier density inside the active layer. As long as I can remember, this rule states that the carrier density remains constant when the injection current rises above the lasing threshold. The reason lies in the stimulated emission of photons which consumes all additional carriers injected above threshold. The threshold carrier density delivers the threshold optical gain that compensates for the optical loss, which is usually not dependent on the injection current. Thus, the threshold carrier density should also remain constant. However, my recent analysis of high-power lasers yields different results (see picture). Read more of this post
Ever since Shuji Nakamura mentioned in his Nobel lecture in Stockholm that lasers are the future of lighting, I am puzzled by this claim, especially by the now widely circulated statement that laser diodes are free from efficiency droop. It suggests an advantageous energy efficiency of laser diodes, as shown in the last figure 17 of his lecture (which is actually invalidated by his own reference). If you are familiar with the much debated efficiency droop burdening GaN-LED lighting, you will agree that the underlying carrier loss mechanisms are also present in laser diodes. Even worse, laser diodes require a higher carrier density in the active layers and therefore exhibit stronger Auger recombination and possibly also electron leakage already at lasing threshold. Read more of this post
Quantum efficiency is the ratio of emitted photons to injected electrons. So, how can an electron generate more than one photon ? One possible solution was demonstrated numerically by inserting tunnel junctions into the multi-quantum well (MQW) active region of a GaN-based LED (details). With one tunnel-junction (TJ), each electron gets two chances to generate a photon, because it can tunnel back into the conduction band after the first generation process. Inspired by the numerical demonstration, such GaN-based TJ-MQW LED was recently fabricated for the first time (details). With three tunnel-junctions (picture), each electron gets four chances to generate a photon, enabling quantum efficiencies up to 400%. Read more of this post