Paper in proceedings, 2015
The Vertical-Cavity Surface-Emitting Laser (VCSEL) is an established optical source in short-distance optical communication links, computer mice and tailored infrared power heating systems. Its low power consumption, easy integration into two-dimensional arrays, and low-cost manufacturing also make this type of semiconductor laser suitable for application in areas such as high-resolution printing, bio-medical and general lighting. However, these applications require emission wavelengths in the blue-UV instead of the established infrared regime, which can be achieved by using GaN-based instead of GaAs-based materials. The development of GaN-based VCSELs have shown to be challenging, and so far only a handful research groups have demonstrated lasing from such electrically pumped devices [1-6]. The presented performance is typically orders of magnitudes lower compared to that from electrically driven GaAs-based VCSELs. Some of the challenges are to achieve efficient transverse current spreading, transverse optical mode confinement, high-reflectivity mirrors and resonator length control. This talk will summarize the different strategies to solve these issues in electrically pumped GaN-VCSELs together with state-of-the-art results. We will highlight our work on combined transverse current and optical mode confinement, where we show that many structures used for current confinement result in unintentionally optically anti-guided resonators. Such resonators can have a very high optical loss, which easily doubles the threshold gain for lasing . We will also present an alternative to the use of distributed Bragg reflectors as high-reflectivity mirrors, namely a TiO2/air high contrast gratings (HCGs). Fabricated HCGs of this type show a high reflectivity (>95%) over a 25 nm wavelength span, which is in excellent agreement to the reflectivity spectrum predicted by numerical simulations assuming an ideal HCG geometry .
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