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The influence of two-photon absorption (2PA) on GaAs-based high-power lasers receives increasing attention in recent years. 2PA generates an electron-hole pair by absorbing two photons at once, mainly inside the waveguide layers. In absence of self-heating and catastrophic optical damage, 2PA is considered a possible reason for the strong pulse power saturation that hampers various applications (picture). But published simulations of such saturation often contradict each other.
Dogan et al. reproduced the measured pulse power saturation by including 2PA as well as free-carrier absorption (FCA) in a traveling-wave optical model. This FCA is only caused by 2PA-generated carriers and was found to dominate at pulsed laser powers above 30W. However, the model ignores carrier transport effects such as carrier leakage which was previously established as main cause for the pulse power saturation of the same laser, while neglecting 2PA. More recently, Zeghuzi et al. identified a significant 2PA influence but included a very high gain compression factor in order to match the measured power saturation. Vertical carrier leakage from the active layers, which may lead to strong free-carrier absorption in the waveguide layers, was still ignored. In other words, none of these modeling approaches includes all possible saturation processes simultaneously. Read more of this post
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