Air-suspended TiO 2 -based HCG reflectors for visible spectral range Ehsan Hashemi 1 , J¨orgen Bengtsson 1 , Johan Gustavsson 1 , Stefan Carlsson 1 , Georg Rossbach 2 , and ˚ Asa Haglund 1 1 Photonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, Sweden; 2 Laboratory of Advanced Semiconductors for Photonics and Electronics, Institute of Condensed Matter Physics, ´ Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), Switzerland; ABSTRACT For GaN-based microcavity light emitters, such as vertical-cavity surface-emitting lasers (VCSELs) and resonant- cavity light emitting diodes (RCLEDs) in the blue-green wavelength regime, achieving a high reflectivity wide bandwidth feedback mirror is truly challenging. The material properties of the III-nitride alloys are hardly compatible with the conventional distributed Bragg reflectors (DBRs) and the newly proposed high-contrast gratings (HCGs). Alternatively, at least for the top outcoupling mirror, dielectric materials offer more suitable material combinations not only for the DBRs but also for the HCGs. HCGs may offer advantages such as transverse mode and polarization control, a broader reflectivity spectrum than epitaxially grown DBRs, and the possibility to set the resonance wavelength after epitaxial growth by the grating parameters. In this work we have realized an air-suspended TiO 2 grating with the help of a SiO 2 sacrificial layer. The deposition processes for the dielectric layers were fine-tuned to minimize the residual stress. To achieve an accurate control of the grating duty cycle, a newly developed lift-off process, using hydrogen silesquioxan (HSQ) and sacrificial poly- methyl-methacrylate (PMMA) resists, was applied to deposit the hard mask, providing sub-10 nm resolution. The finally obtained TiO 2 /air HCGs were characterized in a micro-reflectance measurement setup. A peak power reflectivity in excess of 95% was achieved for TM polarization at the center wavelength of 435 nm, with a reflectivity stopband width of about 80 nm (FWHM). The measured HCG reflectance spectra were compared to corresponding simulations obtained from rigorous coupled-wave analysis and very good agreement was found. Keywords: vertical-cavity surface-emitting laser, gallium nitride, high-contrast grating, dielectric materials, TiO2 1. INTRODUCTION A high index-contrast sub-wavelength grating (HCG) is a thin structure with extraordinary features, that is expected to have various implementations in optoelectronic devices. 1 The most important application of HCGs has been to serve as the top reflector in vertical-cavity surface-emitting lasers (VCSELs), in which they are believed to have several advantages over the already well-established distributed-Bragg reflectors (DBRs). These advantages include interesting properties such as broadband high reflectivity, wavelength setting capability, transverse mode control, and polarization selectivity. Up to now, HCGs have been explored in different material systems and a few electrically pumped VCSELs incorporating an HCG structure have been realized, mostly in GaAs and InP-based material systems emitting at infrared wavelengths. In these experiments, the HCG structures have either been applied as a part of the top reflector, in combination with a couple of DBR pairs, 2, 3, 4 or as the one and only part of it, fully substituting the top DBR. 5, 6 One should also mention an optically pumped VCSEL using two HCGs as the top and the bottom reflectors, which proves the concept of DBR-free VCSELs. 7 In the GaN-based material system, the HCG reflectors are of particular interest since they may potentially solve some of the key challenges regarding VCSELs emitting in the visible. The large lattice-mismatch and/or (Send correspondence to Ehsan Hashemi) E-mail: ehsan.hashemi@chalmers.se, Telephone: +46 31 772 1888 High Contrast Metastructures IV, edited by Connie J. Chang-Hasnain, David Fattal, Fumio Koyama, Weimin Zhou, Proc. of SPIE Vol. 9372, 93720D · © 2015 SPIE CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2078951 Proc. of SPIE Vol. 9372 93720D-1