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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
Two long sought-after goals for the semiconductor community have been (i) to develop long-wavelength semiconductor lasers on GaAs substrates, to enable exploitation of vertical-cavity architectures as well as monolithic integration with GaAs-based high-speed microelectronics, and (ii) to realise uncooled operation of semiconductor lasers, whereby the external cooling equipment typically required to maintain operational stability in long-wavelength devices can be removed in order to significantly reduce energy consumption without degrading the device performance.
GaAs can be considered as the prototype compound semiconductor and is used for a wide range of applications including infrared light emission, acoustic sensing, transistor technology and photovoltaics. Nanowires (NWs) made from GaAs commonly exhibit polytypism, i.e., some segments of the NW crystallize in the zincblende (ZB) and others in the metastable wurtzite (WZ) crystal structure, thus turning the NW into a crystal-phase nanostructure. While the electron states of the ZB phase are principally derived from a single conduction band (CB), two energetically close bands exist in the WZ modification. Read more of this post
We were puzzled for a long time, as to why our simulated self-heating of vertical-cavity surface-emitting lasers (VCSELs) is always much smaller than the measured temperature rise. After checking all the details of model and measurement, we ended up with the suspicion that the thermal conductivity of the GaAs/AlAs distributed Bragg reflectors (DBRs) is much lower than expected. This was eventually confirmed by direct measurements (see picture). Even 100nm thick layers restrict the phonon mean free path that determines the thermal conductivity of GaAs and AlAs. In other words, the typical averaging of bulk thermal conductivities may lead to a serious underestimation of the thermal resistance of multi-layer structures. In fact, the thermal conductivity of such structures could be strongly anisotropic. Read more of this post
High-power broad-area laser diodes often exhibit an undesired widening of the lateral far-field with increasing current, which is commonly attributed to self-heating. The non-uniform temperature profile inside the waveguide leads to a lateral refractive index gradient (thermal lens) that enhances the built-in index guiding of laser modes. We studied this complex interaction of electronic, thermal, and optical processes using self-consistent numerical simulation. In good agreement with measurements, our calculations quantitatively link the widening of the far field to a rising order of lateral lasing modes which are supported by the thermal lens. But a relevant part of this widening also occurs without any heating, which is attributed to a non-uniform gain distribution. However, some features of the measured far-field cannot be reproduced in our simulation, e.g., the strong asymmetry. Does anybody know about more advanced models for this far-field widening ?  J. Piprek and Z. M. Li, Appl. Phys. Lett. 102, 221110 (2013).