Quantum-dot-based membrane external-cavity surface-emitting laser (MECSEL) for the 650 to 760 nm range

尽管半导体光盘激光器在波长柔韧性和输出功率方面取得了巨大的成功，但半导体材料的全部潜力仍未完全转移到器件结构中。例如，基于磷化物的半导体，AlGaInP，可以覆盖500 nm到900 nm之间的波长范围。到目前为止，半导体激光器主要工作在630nm到680nm的红色光谱范围内。其中一个原因是这些激光器选择的衬底是砷化镓。然后在该衬底上以一定的应变生长由AlGaInP量子阱等组成的有源区域，以达到目标波长。为了有效发射，活动区域应该没有缺陷或裂纹，因此合并应变的量应该很低。然而，这种与GaAs的晶格匹配限制了AlGaInP的组成，从而限制了可用的波长。对于VECSELs来说，情况变得更加戏剧性，因为半导体结构在活跃区域旁边也包含一个DBR反射镜。这种反射镜也必须是晶格匹配的，但根据目标波长的不同，所用材料的折射率差异可能会变得太低，因此需要包含更多层的反射镜堆叠，而不是没有缺陷的沉积。MECSEL概念在前一个项目期间的成功演示克服了上述限制。我们的最终目标是实现波长在650 ~ 760 nm之间的高功率短脉冲光泵浦半导体膜激光器。该波长范围对于生物光子和医学应用具有吸引力，例如受益于组织中的大穿透深度，例如荧光成像和光动力治疗。这种红色到近红外的光学窗口在水和血红蛋白中都表现出较低的光吸收，为精确靶向的光医学铺平了道路，在这种光医学中，肿瘤可以在专门设计的光敏剂的帮助下消除。有了这个，我们也可以关闭仍然存在的波长差距，在国家的最先进的激光器在红色波长范围和钛蓝宝石激光器与一个紧凑和廉价的设备。

Despite the tremendous success of semiconductor disk laser concerning the wavelength flexibility and output power, still not the full potential of the semiconductor material could be transferred into device structures. For example, the phosphide-based semiconductors, AlGaInP, may cover the wavelength range between 500 nm and 900 nm. Up to now, semiconductor lasers operate mainly in the red spectral range between 630 nm and 680 nm. One reason is that the substrate of choice for these lasers is GaAs. The active region consisting of, e.g., AlGaInP quantum wells are then grown on this substrate with a certain strain to achieve the targeted wavelength. For an efficient emission, the active region should be free of defects or cracks and therefore the amount of incorporated strain should be low. However, this lattice-matching to GaAs limits the AlGaInP composition and thus the available wavelength. For the VECSELs, the situation gets even more dramatic as the semiconductor structure contains next to the active region also a DBR mirror. This mirror must be grown lattice-matched as well, but depending on the targeted wavelength, the difference of the refractive indices of the materials used might be getting too low, thus requiring mirror stacks that contain more layers than can be deposited without defects.The successful demonstration of the MECSEL concept in the previous project period allows overcoming the mentioned limitation. Our final goal is to realize a high-power short-pulse optically-pumped semiconductor membrane laser in the wavelength range between 650 nm and 760 nm. This wavelength range is attractive for bio-photonic and medical applications that benefit for example from a large penetration depth in tissue, such as fluorescence imaging and photodynamic therapy. This red to near infrared optical window exhibit low absorption of light in both water and hemoglobin and paves the way to precisely targeted photo-medicine in which tumors can be eliminated with the help of a specifically designed photosensitize. With this, we can as well close the still existing wavelength gap between state-of-the art lasers in the red wavelength range and the Ti-Sapphire lasers with a compact and inexpensive device.