OH absorption in on-chip high-Q resonators

High-Q integrated resonators have become an essential component in nonlinear photonics. Most often, the guided light in these structures has a significant fractional overlap with silica (e.g., all-silica wedge resonators Lee et al. (2012a); Wu et al. (2020) and silica-clad ultra-low-loss silicon nitride waveguides Jin et al. (2021); Puckett et al. (2021)). It is therefore important to understand the loss limits imposed by the silica used in silicon photonic processing. Besides interface scattering loss, optical absorption from bound hydroxyl ions (Si-OH) can be a significant component of loss Gao et al. (2022), especially since thermal silica is prepared using a process involving water. Bound hydroxyl ions produce a well-known fundamental absorption peak at 2720 nm Humbach et al. (1996) and the overtone at 1380 nm is used here to measure OH absorption loss in ultra-high-Q thermal-silica wedge microresonators. Further comparison to scattering loss is made over a wavelength range from 680 nm to 1550 nm. Also, using cavity-enhanced photo-thermal spectroscopy Gao et al. (2022) near the 1380 nm band, the OH ion content level is estimated to be 2.4 ppm (weight). This value also agrees with Secondary Ion Mass Spectroscopy (SIMS) depth profiling performed on the resonator material. Outside of the 1380 nm band, scattering loss is confirmed by measurement and modeling to be the dominant loss mechanism in the samples tested.

Thermally-grown silica wedge whispering-gallery resonator devices were prepared as measurement samples, and featured 8 $\mu$m thermal oxide thickness with resonator diameter 6.5 mm (10 GHz free spectral range, FSR). The fabrication steps are detailed in reference Wu et al. (2020) and a photo micrograph of a typical resonator is the Fig. 1 inset. The thermal silica was grown from float-zone (low background doping level) silicon wafers using the wet oxidation method. As a final step, the devices were annealed for 18-hours at 1000^∘C in N₂. The samples were stored and measured in a dry N₂ environment to minimize environmental impact on optical loss. Tapered fiber couplers Cai et al. (2000); Spillane et al. (2003) were used to couple probe light to the resonator samples.

Fig. 1 summarizes the optical loss measurements using six of these devices measured at wavelengths from 680 nm to 1550 nm. All resonators feature intrinsic quality factor (Q-factor) over 500 million at 1550 nm. The corresponding Q-factors are also given in the plot (dashed contours). Significantly, the devices have record-high Q values for on-chip devices in the shorter wavelength bands: 300 million at 685 nm, 500 million at 780 nm, and 600 million at 1064 nm. As an aside, the loss per unit length $\alpha$ (dB/km) is determined from the optical Q-factor measurement using the relationship $\alpha=4343\times\frac{2\pi n}{\lambda Q_{0}}\;\textrm{(dB/km)}$, where Q₀ is intrinsic Q-factor in billions, $n$ is the mode effective refractive index and $\lambda$ is the wavelength (nm). The intrinsic Q-factor Q₀ is obtained from the relationship $1/Q_{0}=1/Q_{L}-1/Q_{c}$ by measuring the resonance linewidth with Lorentzian lineshape fitting to obtain the loaded Q-factor Q_L followed by measurement of transmission depth to infer the coupling Q-factor Q_c.

A strong increase in loss near 1380 nm is apparent in all six samples, corresponding to the OH absorption band and reaching over 200 dB/km. This absorption quickly decreases for wavelengths above and below 1380 nm. The loss in other spectral regions is believed to be dominated by Rayleigh scattering. The wavelength dependence of this scattering within the resonator mode volume would scale approximately as $\lambda^{-4}$, and does not fit the data. Modeling of surface scattering is described in the Methods and provides better agreement with the wavelength dependence. This theoretical dependence is given by the purple curve where surface roughness variance (1.9 nm) and correlation length (350 nm) are assumed in the plot Barwicz and Haus (2005). The origin of the increased loss at the shortest wavelengths measured is not known, but possibilities include absorption loss from metallic ion impurities and scattering from material density fluctuations. For example, SIMS Wilson (1989) data has shown that Chromium is a residual contaminant in our processing, and Cr³⁺ ions can contribute 1.6 dB/km/ppbw at 800 nm (peak at 625 nm) to absorption Olshansky (1979).

Cavity-enhanced photothermal spectroscopy (CEPS) Gao et al. (2022) was used to further study the OH absorption loss. This method measures the microresonator resonance thermal triangle formation induced by the thermo-optical effect Carmon et al. (2004) to determine mode volume absorption. Details on this method are provided in reference Gao et al. (2022). Fig. 2 summarizes the wavelength dependence of the measured absorption loss in both the 1380 nm band and at 1550 nm. The measured absorption near 1380 nm follows reasonably well the OH overtone lineshape in silica as determined elsewhere Humbach et al. (1996) (blue curve superimposed in plot). To fit the magnitude of the lineshape function to the data, an OH content level of 2.4 ppm weight is used. Also, the data near 1550 nm indicate that by further reduction in scattering loss, the existing thermal silica can provide absorption limited Q factors as high as 8 billion. This value is 2$\times$ larger than measured for wedge resonators in a previous study Gao et al. (2022) and is attributed to application of an improved resist cleaning step. The absorption loss measurement results at 1385 nm and 1550 nm are summarized in Table 1 and compared with values from the literature.

To further confirm the OH ion impurity level and also study its spatial distribution over the resonator oxide thickness, secondary ion mass spectroscopy (SIMS) depth profiling Wilson (1989) was employed (see Fig. 3). The absolute OH ion concentration in this measurement is obtained from the ratio ¹⁷OH/³⁰Si, as calibrated against reference samples Hauri et al. (2002); Mosenfelder et al. (2011). The OH concentration remains nearly constant along the depth direction with a value in reasonable agreement with the CEPS results in Fig. 2. This uniformity is believed to result from diffusion-driven equilibrium Thomas et al. (2000) during the long oxide growth process and the post high-temperature annealing process Wu et al. (2020). The higher OH concentration near the surfaces is consistent with the hydrophilic nature of silica causing water to be adsorbed on the surfaces Gorodetsky et al. (1996); Rokhsari et al. (2004).

In summary, using thermal silica wedge microresonators and by measuring optical loss from 680 nm to 1550 nm, the OH ion absorption peak in 1380 nm band stands out from base line scattering loss. Absorption-only loss in the 1380 nm band was also measured using cavity enhanced photo-thermal spectroscopy. This gave a maximum absorption loss of 152 dB/km at 1385 nm. Fitting of the absorption spectrum gives an OH content level of 2.4 ppm weight, which is consistent with separate measurements performed on the resonator oxide by Secondary Ion Mass Spectroscopy. Outside the OH overtone absorption band, surface scattering was determined to be the dominant loss mechanism. Record high Q factors (limited by scattering) were measured for visible and near-visible wavelengths. Also, with further reduction scattering loss, Q values approaching 8 billion are feasible at 1550 nm. This value is higher than determined in previous measurements Gao et al. (2022). Overall, this investigation highlights the potential for even lower loss levels and higher Q factors using thermal silica in photonic devices.