5566
Letter
Vol. 40, No. 23 / December 1 2015 / Optics Letters
Analysis of silicon-on-insulator slot waveguide ring resonators targeting high Q-factors WEIWEI ZHANG, SAMUEL SERNA, XAVIER LE ROUX, CARLOS ALONSO-RAMOS, LAURENT VIVIEN, AND ERIC CASSAN* Institut d’Électronique Fondamentale (IEF), CNRS, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France *Corresponding author: eric.cassan@u‑psud.fr Received 7 October 2015; accepted 29 October 2015; posted 3 November 2015 (Doc. ID 251541); published 24 November 2015
Vertical slot waveguide micro-ring resonators in silicon photonics have already been demonstrated in previous works and applied to several schemes, including sensing and hybrid nonlinear optics. Their performances, first quantified by the reachable Q-factors, are still perceived to be restrained by larger intrinsic propagation losses than those suffered by simple Si wire waveguides. In this Letter, the optical loss mechanisms of slot waveguide micro-ring resonators are thoroughly investigated with a special focus on the coupler loss contribution that turns out to be the key obstacle to achieving high Q-factors. By engineering the coupler design, slotted ring resonators with a 50 μm radius are experienced with a loaded Q-factor up to 10 times improvement from Q 3;000 to Q 30;600. The intrinsic losses due to the light propagation in the bent slot ring itself are proved to be as low as 1.32 0.87 dB∕cm at λ 1;550 nm. These investigations of slot ring resonators open high performance potentials for on-chip nonlinear optical processing or sensing in hybrid silicon photonics. © 2015 Optical Society of America OCIS codes: (230.5750) Resonators; (230.7370) Waveguides; (130.3120) Integrated optics devices. http://dx.doi.org/10.1364/OL.40.005566
Silicon photonics is now accepted as the most potential platform for future low-cost and high-speed integrated optics due to its CMOS compatibility and versatility for nonlinear active photonic devices. Benefiting from high index contrast between silicon and its cladding oxide, nonlinear optical processing in silicon photonic nanowaveguides consumes much less power than the one needed in optical fibers [1]. Remaining challenges are related to the silicon centro-symmetric lattice and its indirect energy bandgap, which prevent electro-optic χ 2 -based nonlinear effects and stimulated light emission, respectively. Last, χ 3 nonlinear optical properties in silicon basically suffer in this material from the detrimental effect of the two-photon-absorption (TPA) process at telecommunication wavelengths. One possible solution to hinder TPA in silicon while still benefiting from high index contrast is to use slot 0146-9592/15/235566-04$15/0$15.00 © 2015 Optical Society of America
waveguides composed of two silicon nanorails separated by a void gap [2,3]. Different from traditional silicon nanowires, the gap in slotted waveguides can be infiltrated with low-index highly nonlinear materials that experience enhanced lightmatter interaction from electric field n2si ∕n2slot reinforcement and have been successfully applied on all optical high-speed processing [4], electro-optic modulations [5], and bio detection/ sensing [6]. A further step to reinforcing light–matter interactions consists of using a ring resonator configuration, which now plays an important role in silicon photonic devices for a broad set of applications, including optical modulation [7], frequency comb generation [8], and sensing [9,10]. Transposing wire waveguide ring resonators into their slot waveguide counterparts is straightforward and has been done for years [11–13]. Yet this approach faces the obstacle that the slot scheme leads to higher propagation losses in straight and bent waveguides, leading to ring resonators with a moderate Q-factor of typically a few thousands in standard situations. In this context, a better understanding of the loss limitations of slot waveguide ring resonators and the way to circumvent them to push Q-factors to higher values is welcome. The aim of the Letter is to contribute to this goal. Intrinsically, slot waveguides are known to suffer from larger roughness-induced scattering extrinsic losses than Si wires due to a strong electric field interaction with the inner rails’ walls defined by lithographic and etching technological processes. Hence, different ratios of slot width to total slotted waveguide width and waveguide aspect ratio lead to different linear loss and Q-factors. Typically, the loss levels of straight symmetric slot waveguides are reported around 7 dB∕cm [14] and after sidewall roughness smoothing of around 4 dB∕cm [15], which is still larger than nanowire waveguides (
Letter
Vol. 40, No. 23 / December 1 2015 / Optics Letters
Analysis of silicon-on-insulator slot waveguide ring resonators targeting high Q-factors WEIWEI ZHANG, SAMUEL SERNA, XAVIER LE ROUX, CARLOS ALONSO-RAMOS, LAURENT VIVIEN, AND ERIC CASSAN* Institut d’Électronique Fondamentale (IEF), CNRS, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France *Corresponding author: eric.cassan@u‑psud.fr Received 7 October 2015; accepted 29 October 2015; posted 3 November 2015 (Doc. ID 251541); published 24 November 2015
Vertical slot waveguide micro-ring resonators in silicon photonics have already been demonstrated in previous works and applied to several schemes, including sensing and hybrid nonlinear optics. Their performances, first quantified by the reachable Q-factors, are still perceived to be restrained by larger intrinsic propagation losses than those suffered by simple Si wire waveguides. In this Letter, the optical loss mechanisms of slot waveguide micro-ring resonators are thoroughly investigated with a special focus on the coupler loss contribution that turns out to be the key obstacle to achieving high Q-factors. By engineering the coupler design, slotted ring resonators with a 50 μm radius are experienced with a loaded Q-factor up to 10 times improvement from Q 3;000 to Q 30;600. The intrinsic losses due to the light propagation in the bent slot ring itself are proved to be as low as 1.32 0.87 dB∕cm at λ 1;550 nm. These investigations of slot ring resonators open high performance potentials for on-chip nonlinear optical processing or sensing in hybrid silicon photonics. © 2015 Optical Society of America OCIS codes: (230.5750) Resonators; (230.7370) Waveguides; (130.3120) Integrated optics devices. http://dx.doi.org/10.1364/OL.40.005566
Silicon photonics is now accepted as the most potential platform for future low-cost and high-speed integrated optics due to its CMOS compatibility and versatility for nonlinear active photonic devices. Benefiting from high index contrast between silicon and its cladding oxide, nonlinear optical processing in silicon photonic nanowaveguides consumes much less power than the one needed in optical fibers [1]. Remaining challenges are related to the silicon centro-symmetric lattice and its indirect energy bandgap, which prevent electro-optic χ 2 -based nonlinear effects and stimulated light emission, respectively. Last, χ 3 nonlinear optical properties in silicon basically suffer in this material from the detrimental effect of the two-photon-absorption (TPA) process at telecommunication wavelengths. One possible solution to hinder TPA in silicon while still benefiting from high index contrast is to use slot 0146-9592/15/235566-04$15/0$15.00 © 2015 Optical Society of America
waveguides composed of two silicon nanorails separated by a void gap [2,3]. Different from traditional silicon nanowires, the gap in slotted waveguides can be infiltrated with low-index highly nonlinear materials that experience enhanced lightmatter interaction from electric field n2si ∕n2slot reinforcement and have been successfully applied on all optical high-speed processing [4], electro-optic modulations [5], and bio detection/ sensing [6]. A further step to reinforcing light–matter interactions consists of using a ring resonator configuration, which now plays an important role in silicon photonic devices for a broad set of applications, including optical modulation [7], frequency comb generation [8], and sensing [9,10]. Transposing wire waveguide ring resonators into their slot waveguide counterparts is straightforward and has been done for years [11–13]. Yet this approach faces the obstacle that the slot scheme leads to higher propagation losses in straight and bent waveguides, leading to ring resonators with a moderate Q-factor of typically a few thousands in standard situations. In this context, a better understanding of the loss limitations of slot waveguide ring resonators and the way to circumvent them to push Q-factors to higher values is welcome. The aim of the Letter is to contribute to this goal. Intrinsically, slot waveguides are known to suffer from larger roughness-induced scattering extrinsic losses than Si wires due to a strong electric field interaction with the inner rails’ walls defined by lithographic and etching technological processes. Hence, different ratios of slot width to total slotted waveguide width and waveguide aspect ratio lead to different linear loss and Q-factors. Typically, the loss levels of straight symmetric slot waveguides are reported around 7 dB∕cm [14] and after sidewall roughness smoothing of around 4 dB∕cm [15], which is still larger than nanowire waveguides (
