Abstract
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, medical applications, 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 is challenging, but during recent years several groups have managed to demonstrate electrically pumped GaN-based VCSELs with close to 1 mW of optical output power and threshold current densities between 3-16 kA/cm2. The performance is limited by challenges such as achieving high-reflectivity mirrors, vertical and lateral carrier confinement, efficient lateral current spreading, accurate cavity length control and lateral optical mode confinement. This paper summarizes 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 TiO2/air high contrast gratings (HCGs). Fabricated HCGs of this type show a high reflectivity (>95%) over a 25 nm wavelength span.
Highlights
The Vertical-Cavity Surface-Emitting Laser (VCSEL) was first proposed in 1977 by Professor Kenichi Iga from Tokyo Institute of Technology, who two years later achieved lasing for the first time under pulsed operation at 77 K.1,2 In earlier research by Ivars Melngailis in 1965, lasing parallel to the current injection was achieved for the first time and some of the advantages of this type of laser structure were highlighted such as easy array formation and low divergent output beam due to coherent emission from a large area.[3]
The first electrically injected GaN-based VCSEL was demonstrated by the National Chiao Tung University (NCTU) in Taiwan in April 2008.30 It had one bottom epitaxial AlGaN/GaN Distributed Bragg Reflectors (DBRs), one top dielectric DBR, and a 240 nm indiumtin-oxide (ITO) layer as a current spreading layer
In long-wavelength InP-based VCSELs, which share many of the same challenges as GaN-VCSELs such as low p-type conductivity, high metal contact resistance to p-doped material, and poorly-conductive DBRs, tunnel junctions are a standard technology to achieve efficient lateral current spreading[125]
Summary
Applications for today’s VCSELs include short-distance optical links, computer-mouse applications, laser printers, sensors, infrared illumination and tailored infrared power heating systems They have become a popular light source because of their advantages over their edge-emitting laser counterpart such as high modulation speed at low drive currents, circular symmetric low-divergent output beam, low threshold currents, ease to fabricate into two-dimensional arrays, and low-cost manufacturing due to on wafer-testing.[6] If VCSELs were available with emission wavelengths in the ultraviolet to visible regime, many more applications could benefit from such a light source. It is encouraging to see many companies investing time and effort into this field, and as a result Nichia[27], Panasonic[28] and Sony[29] all have demonstrated electrically injected GaN-VCSELs
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