# High-\(Q\) TeO\({}_{2}\)-Si Hybrid Microring Resonators
## 1 Introduction  
The impact of silicon photonics on the development of photonic integrated circuits (PICs) is considerable, because of device compatibility with complementary metal-oxide-semiconductor (CMOS) technology and the leveraging of decades of research stimulated by the microelectronics industry [1]. The miniaturization of photonic waveguides has emerged as one of the most prominent technology platforms for PICs in recent decades [2]. The reduction in propagation loss associated with silicon waveguides is an ongoing research area because loss is a key parameter for link budget management in on-chip optical networks and many important micro-photonic devices, as well as passive and active systems including sensors, filters, delay lines, and light sources [3; 4; 5; 6]. One device structure for which propagation loss is particularly critical is the microring resonator (MRR). MRRs provide an efficient cavity which has a compact size, wavelength selectivity, tunability, scalability, and functional versatility [7], making them a prominent candidate for a variety of applications including lasers [8; 9], optical sensors [10], nonlinear optics [11; 12], quantum optics [13], (de-)multiplexing systems [14], optical filters [15], and optical modulators [16]. The loss in MRRs can be quantified by the cavity \(Q\) factor, which is typically limited to values on the order of \(10^{5}\) in standard 220 nm-high single-mode silicon MRRs around 1550 nm. Several approaches have been explored for enhancing the \(Q\) factor in silicon MRRs, including minimizing light scattering at roughened sidewalls via shallow-etched or multimode waveguide designs and alternative fabrication methods based on selective oxidation, e-beam lithography, reactive ion plasma etching, inductively coupled plasma (ICP) etching, or low-pressure chemical vapor deposition (LPCVD). High-\(Q\) factor silicon MRRs fabricated using a selective oxidation process have been shown with intrinsic \(Q\) factors of \(5.1\times 10^{5}\)[17] and \(7.6\times 10^{5}\)[18], although challenges remain in the control of the fabrication process [19]. A silicon microring resonator with a \(Q\) factor of \(1.7\times 10^{6}\) was demonstrated using a large cross-section multi-mode waveguide with a severely limited free spectral range (FSR) of \(17.1\) pm [20]. With a similar approach, a silicon microring resonator with a high \(Q\) of \(1.3\times 10^{6}\) and larger FSR was proposed in [21], with a bend radius of 450 \(\upmu\)m. A silicon MRRwith an internal \(Q\) factor of 1.1 \(\times\) 10\({}^{6}\) and FSR of 0.208 nm and utilizing a multi-mode ridge waveguide was fabricated using a standard CMOS-compatible silicon-on-insulator (SOI) process [22]. A multi-mode ultrahigh quality factor racetrack resonator with a 1.6 \(\upmu\)m width was proposed using a standard single-etching process with a quality factor of 2.3 \(\times\) 10\({}^{6}\) provided by a multi-project wafer foundry [23]. An internal quality factor of 2.2 \(\times\) 10\({}^{7}\), corresponding to a 2.7 dB/m propagation loss, was achieved in a silicon microring resonator with a radius of 2.45 mm cladded with silicon oxide [24] by oxidizing the wafer surface in a steam oxidation process and using a reflowing photoresist strategy. An intrinsic \(Q\) factor of 1.57 \(\times\) 10\({}^{6}\), corresponding to a waveguide loss of 0.35 dB/cm, was realized in a silicon MRR with a radius of 150 \(\upmu\)m and an FSR of 0.845 nm using e-beam lithography with a top cladding of a glass-like compound from hydrogen silsequioxane (HSQ) covered with a silicon oxide layer [25]. Low-loss submicron silicon-on-insulator strip waveguides were reported with a 0.5 dB/cm loss at 1310 nm and a 30 \(\upmu\)m bend radius, cladded with silicon oxide [26] using H\({}_{2}\) plasma post-lithography treatment and H\({}_{2}\) thermal annealing after silicon etching.