SM2K.6.pdf CLEO 2017 © OSA 2017 A chip-based silicon nitride platform for mid-infrared nonlinear photonics Clemens Herkommer 1,3 , Hairun Guo 1 , Adrien Billat 2 , Davide Grassani 2 , Martin Pfeiffer 1 , Michael Zervas 1 , Camille-Sophie Br` es 2 , Tobias J. Kippenberg 1 1 ´ Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), LPQM, CH-1015, Lausanne, Switzerland 2 ´ Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), PHOSL, CH-1015,Lausanne, Switzerland 3 Technische Universit¨ at M¨ unchen, Arcisstraße 21, D-80333 M¨ unchen Germany Author email adress: 1 clemens.herkommer@epfl.ch, 3 clemens.herkommer@tum.de Abstract: We developed a chip-based silicon nitride platform with thick waveguides (> 2 μ m) that overcomes the usual fabrication limitation. We demonstrate both microres- onator frequency comb generation at 2.5 μ m and supercontinuum generation extending beyond 4.0 μ m in this platform. OCIS codes: 190.4390 (Nonlinear optics, integrated optics), 190.7110 (Ultrafast nonlinear optics) Introduction. Chip-based silicon nitride (Si 3 N 4 ) photonic waveguides represent an integrated platform compatible for both nonlinear photonics research and related applications [1], which benefits not only from advanced development of nanofabrication technology but also from the unique optical properties of Si 3 N 4 material. Indeed, the large bandgap of Si 3 N 4 supports a wide transparency from visible to mid-infrared (mid-IR), free of muli-photon absorptions. In addition, Si 3 N 4 has strong nonlinear efficiency (10 times of silica) that provides rich nonlinear phenomena within chip-scale devices. A vivid example are chip-based microresonator frequency combs that feature fully coherent optical spectra with large repetition rate (> 10 GHz) and broad bandwidth [2, 3]. Such comb sources have been used in novel applications such as coherent telecommunication [4], dual-comb spectroscopy [5] and low-noise microwave generation [6,7]. However, despite the ultra-broad transparency, Si 3 N 4 photonic diveces to date are mainly working in the telecommunication band. Limitations for the mid-IR include the technical challenge that larger waveguides can possibly lead to cracks, as well as limited flexibility, e.g. in terms of dispersion engineering. On the other hand, a mid-IR chip-based platform is desired particularly for spectroscopy applications. Here we developed a Si 3 N 4 platform with thick waveguides (> 2 μ m) beyond the limitation of conventional fabri- cation, allowing for mid-IR photonics research. As proof of concept we demonstrate both microresonator frequency comb generation (at 2.5 μ m) and supercontinuum generation (extending to 4.0 μ m) in these devices. 1 . 6 u m 2 . 3 u m S i N f i l m t h i c k n e s s 2 x S i N f i l m t h i c k n e s s 1 μ m 1 μ m a ) b ) c ) Fig. 1. a) Cross section of a fabricated waveguide showing the large core dimensions. b) SEM image of a microresonator without the SiO 2 cladding. c) SEM image of a cross section of waveguides after planarization. Trenches with a width below the threshold of 2 times the deposition thickness (right-hand side) are filled completely with SiN due to the isotropic deposition process. Fabrication. We fabricate the devices using a modified Damascene Process that was recently reported [8] as an approach to reliably fabricate crack-free waveguides. Important advantages are the possibility to employ a “Reflow” step that yields highly smooth sidewalls, as well as 3D-tapered bus waveguide ends that enlarge the mode size and facilitate input coupling. Different from subtractive fabrication processes, here Si 3 N 4 is deposited onto a structured SiO 2 preform, where a dense filler pattern is employed to prevent crack formation originating from the tensile stress of LPCVD-deposited Si 3 N 4 . In order to achieve crack-free Si 3 N 4 waveguides with a large core-size geometry, in the fabrication process we circumvent the crack-prone increase of the Si 3 N 4 film thickness by making use of the isotropic deposition of SiN during LPCVD, as can be seen in figure 1. In this way we can freely choose the waveguide height by merely adjusting the etching time, whereas the width of the waveguide is limited to be less than twice the deposited Si 3 N 4 film thickness.