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

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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.