GaN-based VCSELs

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 [7]. 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 [8]. References [1] T.-C. Lu, et al., Applied Physics Letters, 92, 14, (2008). [2] Y. Higuchi, et al., Applied physics express, 1, 12, 121102, (2008). [3] G. Cosendey, et al., Applied Physics Letters, 101, 15, (2012). [4] C. Holder, et al., Applied Physics Express, 5, 092104, (2012). [5] T. Onishi, et al., IEEE J. of Quantum Electronics, 48, 9,1107–1112, (2012). [6] W.-J. Liu, et al., Applied Physics Letters, 104, 251116 (2014). [7] E. Hashemi, et al., Optics Express, vol. 22 1, p. 411-426, (2014). [8] E. Hashemi, et al., Proceedings of SPIE, (0277-786X), vol. 9372 (2015)