Progress and challenges in electrically pumped GaN-based VCSELs

The Vertical-Cavity Surface-Emitting Laser (VCSEL) is an established optical source in the infrared for short-distance optical communication links, computer mice and industrial heating. Its high power efficiency, 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, which can be achieved by using GaN-based instead of GaAs-based materials. The development of GaN-based VCSELs is challenging, but in recent years several groups have demonstrated electrically pumped GaN-based VCSELs with close to 1 mW of optical output power and threshold current densities between 3-16 kA/cm2. This is close to what is adequate for many applications, but in other respects such as lifetime and beam quality these VCSELs lag behind the more mature GaAs-based VCSELs. The performance is limited by the difficulties in achieving efficient transverse current spreading and transverse optical mode confinement, high-reflectivity mirrors, transverse and vertical carrier confinement, and resonator length control. In this presentation different strategies to solve these issues in electrically pumped GaN-VCSELs will be summarized together with state-of-the-art results. In this context 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 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.