We propose and demonstrate compact suspended silicon microring resonators with ultra-high optical quality. We achieve an intrinsic quality factor of 9.2 × 105 for the resonator with a radius of 9 μm. The high optical quality factor, high optical confinement together with the suspended structure of our device enable great potential for broad applications in biosensing, quantum photonics, nonlinear photonics, cavity optomechanics, and optical signal processing.
© 2014 Optical Society of America
The significant advance of silicon photonics in recent years have found very broad applications ranging from signal processing , nonlinear photonics , biosensing [3, 4], quantum photonics [5–7], to cavity optomechanics . An essential underlying component is silicon wire waveguide (SWW) [9, 10] which provides strong mode confinement for light guidance. However, with a high index contrast, SWWs are very sensitive to waveguide sidewall roughness introduced by fabrication imperfection, resulting in significant scattering losses. In recent years, tremendous efforts have been devoted to improve the quality of silicon photonic waveguides [11–21].
In general, a SWW is fabricated on a silicon-on-insulator (SOI) platform, with the silicon core sitting on top of a buried-oxide layer [11,12,14–17], or embedded inside an oxide cladding for reducing scattering loss [18–21]. However, a SWW with a complete air cladding is more favorable for many applications. For example, a complete air cladding increases the exposure of optical guided modes to the ambient medium, leading to an improved sensitivity for sensing application. On the other hand, it dramatically reduces the clamping loss of the waveguide structure, with a greatly improved mechanical Q for optomechanical applications. Moreover, it eliminates any potential impact of the oxide layer, particularly suitable for mid-infrared photonics where silica exhibits significant absorption [22–24].
In this paper, we propose and demonstrate a unique waveguide structure to realize compact high quality silicon microring resonators suspended in the air. For a bending radius of 9 μm, our suspended microrings can achieve an intrinsic optical Q of Qi = 9.2 × 105 in the telecom band. To the best of our knowledge, it is the highest value for silicon microring resonators with such a small radius reported to date [10,12–14,20,21], even without any post-etching processing for surface passivation .
Our proposed devices were fabricated using two-step electron beam lithography (EBL) and inductively-coupled-plasma (ICP) reactive-ion-etching (RIE) on an SOI wafer, with a top silicon layer thickness of 260 nm and a buried oxide thickness of 2 μm. The detailed processing procedure is shown in Fig. 1. First, the inner circle of the suspended microring was patterned with ZEP-520A positive resist by EBL system. In order to reduce the sidewall roughness, a post exposure bake was employed to reflow the resist. The silicon layer was then partially etched by fluorine-based ICP RIE using C4F8/SF6 chemistry. The etching time was precisely controlled, resulting in a thin slab layer of silicon with a thickness of about 60 nm. The etching parameters were optimized to achieve a smooth waveguide sidewall. Subsequently, the outer circle of the microring was patterned by second EBL with high-precision alignment, of which the overlay accuracy is less than 25 nm. After the resist reflow, the silicon layer was thoroughly etched by the fluorine-based ICP. Finally, the buried oxide layer was isotropically etched by a low-pressure vapor hydrogen fluoride (HF) etcher, resulting in a suspended silicon microring supported by a thin silicon slab on an oxide pedestal.
We designed and fabricated two sets of microring resonators with a waveguide core width w = 750 nm but different radii of 9 and 4.5 μm. Figure 2(b) shows a scanning-electron microscopic (SEM) image of the fabricated device with radius of 4.5 μm, showing a suspended waveguide structure sitting on a silica pedestal. The zoom-in image of the device sidewall in Fig. 2(c) clearly shows a very smooth waveguide sidewall, resulting from the optimization of dry-etching process. To characterize the optical quality of the devices, we launched a continuous-wave tunable laser into the devices by near-field evanescent coupling via a tapered optical fiber (typical diameter is ∼ 1 μm) and recorded the cavity transmission as a function of laser wavelength which is calibrated by a Mach-Zehnder interferometer (Fig. 3(a)).
Figure 3(b) and 3(c) show the recorded cavity transmission spectra of a fundamental quasi-transverse electric (quasi-TE) mode for the devices with radii of 9 and 4.5 μm, respectively. By fitting the transmission spectrum (Fig. 3(b)), we obtain an intrinsic quality factor of Qi = 9.2 × 105 for the microring with a radius of 9 μm, which is the highest optical Q for such compact silicon microring resonators, to the best of our knowledge [10, 12–14, 20, 21]. The corresponding propagation loss  is 0.76 dB/cm. The microring with a radius of 4.5 μm (Fig. 3(c)) also exhibits a high intrinsic optical Q of Qi = 3.5 × 105, which is very attractive for such a small footprint. Its lower Q compared with the 9-μm device is primarily due to its higher sensitivity to the device outer sidewall, as inferred by its doublet feature which originates from the coupling between the degenerate clockwise and counter-clockwise cavity modes introduced by light scattering from outer sidewall roughness.
The demonstrated high-Q suspended silicon microring resonators exhibit great potential for a variety of applications. One example is to produce high-purity photon pairs through cavity-enhanced four-wave mixing [5–7]. The high optical Q introduces significant Purcell effect on all cavity modes to drastically enhance the photon generation efficiency and the quantum-state purity . The complete air cladding of the microring eliminates any potential noise from the cladding material , e.g., the Raman noise photons from the oxide substrate. In particular, the suspended microring allows very flexible engineering of waveguide dispersion which is crucial for the underlying four-wave mixing process. The group-velocity dispersion (GVD) can be controlled through both the thickness of the thin supporting slab (t, see Fig. 2(a)) and the size of the waveguide core. As shown in Fig. 4(a), for a suspended silicon waveguide with a core area of 800 nm × 260 nm, the zero-dispersion wavelength (ZDWL) of the fundamental quasi-TE mode can be tailored by 70 nm within the telecom band as t varies from 50 nm to 90 nm. Figure 4(b) shows that ZDWL can be tailored with a tuning range of larger than 200 nm by adjusting the waveguide width and/or slab thickness.
As a second example, the demonstrated suspended waveguide structure is ideal for highly sensitive biological and chemical sensing based upon refractive index variations in the waveguide cladding. The fundamental quasi-transverse magnetic (quasi-TM) mode of a suspended silicon waveguide exhibits strong evanescent field on both top and bottom surfaces of the waveguide (Fig. 5(c), inset). Consequently, the optical guided mode would be very sensitive to any perturbation to the cladding medium, either on the core-cladding interface or over the bulk cladding layer. As an example to illustrate the power of the proposed device, we consider the case of bulk index sensing. A figure of merit characterizing the sensing capability in this case is the bulk waveguide sensitivity (BWS) defined as 
Figure 5 compares the simulated BWS of a suspended silicon waveguide with a conventional SWW, with the same dimension, sitting on top of an oxide layer (Fig. 5(a) and 5(b)). For both cases, we assume that the devices are immersed in an aqueous solution with nc = 1.333, with a probing laser wavelength at 1550 nm. Figure 5 shows clearly that the quasi-TM mode of the suspended waveguide exhibits a BWS much higher than a conventional SOI wire waveguide. In particular, when the waveguide width is smaller than 350 nm, the field-analyte interaction becomes so strong that the BWS of the suspended waveguide is even greater than unity. Among all other dielectric waveguides, this magnitude of BWS can only be achieved with optimized SOI slot waveguides . However, our proposed suspended structure exhibits a significant structure simplicity and is much easier to fabricate in practice, compared with a slot waveguide which requires extremely critical control of the subwavelength-size air slot and generally suffers from serious scattering losses due to surface roughness on multiple interfaces. Therefore, our proposed device can offer a much higher optical quality, thus implying a better detection limit .
In summary, we have demonstrated high-quality compact suspended silicon microring resonators with an intrinsic Q factor of 9.2 × 105 for a bending radius of 9 μm, the highest Q reported to date for such compact silicon ring resonators. It corresponds to a propagation loss as low as 0.76 dB/cm. Our simulations show that the proposed waveguide structure has great capability of dispersion engineering for nonlinear photonic and quantum photonic applications. In particular, it is ideal for biological and chemical sensing, with excellent combination of high sensitivity and low detection limit. Although we demonstrate the suspended microring structure on silicon, the same idea can readily be applied for other material platforms such as silica, silicon nitride, etc. We expect that the proposed suspended microring resonators would become an ideal device platform for broad applications in biosensing, quantum photonics, nonlinear photonics, cavity optomechanics, and optical signal processing.
This work was supported by the National Science Foundation (NSF) under grant ECCS-1351697, and was performed in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by NSF (Grant ECS-0335765).
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