Abstract

On-chip optical waveguides with low propagation losses and precisely engineered group velocity dispersion are important to nonlinear photonic devices such as soliton microcombs, and likewise can be employed for on-chip gyroscopes, delay lines, or Brillouin lasers. Yet, despite intensive research efforts, nonlinear integrated photonic platforms still feature propagation losses orders of magnitude higher than in standard optical fiber. The tight confinement and high index contrast of integrated waveguides make them highly susceptible to fabrication-induced surface roughness, causing dominant scattering losses. Therefore, microresonators with ultra-high-Q-factors are, to date, attainable only in polished bulk crystalline or chemically etched silica-based devices, which pose, however, challenges for full photonic integration. Here, we demonstrate the fabrication of silicon nitride (Si3N4) waveguides with unprecedentedly smooth sidewalls and tight confinement with record-low propagation losses. This is achieved by combining the photonic Damascene process with a novel reflow process, which reduces etching roughness, while sufficiently preserving dimensional accuracy. This leads to previously unattainable mean scattering Q-factors of 12×106 for tightly confining waveguides with anomalous dispersion. Via systematic process step variation and two independent characterization techniques, we differentiate the scattering and absorption loss contributions and reveal metal-impurity-related absorption to be an important loss origin. Although such impurities are known to limit optical fibers, this is the first time, to the best of our knowledge, they are identified—and play a tangible role—in absorption of integrated microresonators. Taken together, our work provides new insights into the origins of propagation losses in Si3N4 waveguides and provides the technological basis for integrated nonlinear photonics in the ultra-high-Q regime.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. INTRODUCTION

Low propagation losses are a central requirement for planar on-chip optical waveguides in diverse application areas such as narrow linewidth lasers [1], integrated delay lines [2], gyroscopes [3], and quantum photonic circuits [4]. Recently, the ability to achieve low-loss and tight-confinement waveguides with precisely engineered dispersion properties has become central to nonlinear integrated photonics, notably microresonator-based optical frequency combs [5,6] operating in the dissipative Kerr soliton regime [7,8] (soliton microcombs). Such soliton microcombs have enabled synthesis of broadband and coherent optical frequency combs with microwave line spacing in integrated devices, which have enabled counting of optical frequencies [911], dual comb spectroscopy [12], terabit coherent communication [13], ultrafast dual comb ranging [14,15], low-noise microwave generation [16], astrophysical spectrometer calibration [17,18], and an all-photonic integrated frequency synthesizer [19].

Yet, while optical fibers with propagation losses below 0.5 dB/km form the backbone of today’s global communication infrastructure, on-chip waveguides exhibit several orders of magnitude higher attenuation coefficients. The low losses of optical fibers were enabled by high-purity glasses developed in response to the seminal work of Kao, which predicted low-loss optical fibers when reducing impurities [20]. So far, ultra-high-Q microresonators could attain comparable values only when mitigating scattering losses via low confinement geometries or chemical polishing, in platforms such as silica wedges [21] or bulk crystalline resonators [22]. These platforms rely, from a materials perspective, on high-purity glass, as used in optical fibers, or ultra-pure crystalline materials, originally developed for deep UV lithography [23,24]. However, such platforms are not easily compatible with photonic integration: the low refractive index of silica waveguides requires an air cladding, complicating photonic integration, while the fabrication of crystalline resonators is incompatible with common CMOS technology [25]. As a consequence, materials that achieve levels of loss similar to silica, but with a higher index for strong light confinement, could have significant benefit in the technological development of photonic integrated ultra-high-Q microresonator technology.

Figure 1(a) compares the attenuation and nonlinear coefficient, α and γ, of the diverse low-loss waveguide and (ultra-)high-Q microresonator platforms. High index materials allow for tight confinement of light (small effective mode area Aeff) and, following Miller’s rule, a higher nonlinear refractive index n2. Together, these features allow to attain effective nonlinear coefficients γ=(2πn2)/(λAeff) significantly higher than for low-loss crystalline or silica-based platforms.

 figure: Fig. 1.

Fig. 1. Nonlinear waveguide platforms and photonic Damascene process with reflow step. (a) Overview of nonlinear coefficients γ and attenuation α for different nonlinear waveguide platforms. Dashed lines indicate similar nonlinear performance based on a constant ratio of γ/α. (b) Schematic process flow of the photonic Damascene process highlighting the newly introduced preform reflow step, which consists of heating the substrate for sufficient time above its glass transition temperature. (c), (d) Final waveguide cross section from photonic Damascene process without and with reflow step. Rounding of the waveguide corners and an increased sidewall angle of 8° are observed after reflow.

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In this context, silicon nitride (Si3N4) waveguides [26] are an interesting compromise between nonlinearity and propagation losses. First studied in the 1980 s [27], it has emerged as a material platform for efficient nonlinear photonics, based on advances in processing that allowed overcoming the challenges of inherent film stress [28]. The material system’s wide transparency range and large bandgap of 5  eV allows applications at visible wavelengths in the bio-medical context [29] and telecom and mid-infrared wavelengths [30]. Together, these properties allow the realization of tightly confining waveguides with high effective nonlinearity and precisely engineered, anomalous group velocity dispersion (GVD). The resulting low-power threshold for parametric processes has enabled the generation of broadband Kerr soliton frequency combs, including dispersive waves [8] and coherent supercontinuum generation from high-repetition-rate pulse trains [31].

Recent studies have shown that the intrinsic material absorption is compatible with propagation losses below of 1 dB/m and Q-factors exceeding 2×107, at wavelengths around 1550 nm [3234]. However, the studies have not brought forward insights into where the residual losses are emanating from. Moreover, they were based on large waveguide geometries, chosen to limit scattering losses but limiting the ability to precisely engineer dispersion properties. Especially, broadband Kerr frequency comb generation at telecom wavelengths requires tightly confining waveguide dimensions of 1.5–2 μm width at heights of 0.75–0.85 μm. To date, it remains an outstanding challenge to fabricate Si3N4 waveguides with reduced scattering losses, rivaling the nonlinear performance of silica wedge and polished crystalline microresonators.

Here, building on recent advances of the photonic Damascene fabrication process [35], we demonstrate a methodology to create unprecedented smooth waveguides that provide for the first time, to the best of our knowledge, a platform in which (tightly confining) integrated photonics in the ultra-high-Q regime can be made a reality. Importantly, our approach does not sacrifice mode confinement as in previous studies [32,33,36], and allows integrating resonators and bus waveguides within one material platform and layer. We perform an in-depth loss study for waveguides based on this novel process and are able to provide evidence that metal impurities are an important origin of propagation losses. Although such impurities have been known to limit optical fibers, this is the first time they are identified—and play a tangible role—in the absorption of integrated photonic waveguides and microresonators. The photonic Damascene reflow process, as shown here, provides already record mean Q-factors (>5×106) for tightly confining (2×0.6  μm) waveguides, which, when combined with materials without impurities, can attain values well in excess of 107.

2. PHOTONIC DAMASCENE PROCESS WITH PREFORM REFLOW

The photonic Damascene process solves several fabrication challenges of high-confinement Si3N4 waveguides by inverting the common processing order, as schematically illustrated in Fig. 1(b). Instead of etching the waveguide pattern into the highly stressed, micrometer thick Si3N4 film, the latter is deposited onto a preform structure with recesses that form the waveguide pattern. A dense filler pattern surrounding the waveguide pattern relaxes the high tensile Si3N4 film stress and prevents crack formation. Planar top surfaces, enabling heterogenous integration via bonding [37] and with 2D materials, are prepared by removing the excess Si3N4 using chemical mechanical polishing. The process enables the reliable fabrication of closely spaced high-confinement waveguides, which have successfully been applied in a growing number of nonlinear photonics experiments [13,15,38,39].

As an addition to the above-mentioned advantages, we incorporate a novel reflow step before the Si3N4 deposition, aiming at smoothing the roughness of the waveguide preform. For this purpose, the wafer is heated slightly above the preform’s glass transition temperature TG in the case of the wet thermally grown SiO2 preform of about 1475 K [40]. The surface-tension-driven smoothing reduces especially the high spatial frequency components of the dry-etch-induced striations on the recess sidewalls. Here, we use a reflow process that consists of an 18 h long anneal at 1523 K in oxygen atmosphere. As the temperature exceeds TG only slightly, long annealing times are required. This is beneficial to control the reflow process and limit the changes in waveguide cross section. Nevertheless, as shown in Figs. 1(c) and 1(d), an increased sidewall angle of 8° and rounding of the waveguide corners are observed after the reflow.

The ultra-low roughness of the SiO2 preform surface after the reflow step can hardly be perceived in scanning electron micrographs (see, e.g., Fig. 2 in Supplement 1). Atomic force microscopy (AFM) provides a method to measure such roughness to sub-nm levels and has been previously employed to asses the sidewall roughness of optical waveguides [41,42]. To this end, the AFM’s tip is scanned along the waveguide sidewall, or, in the present case, along the waveguide recess’ sidewall, as explained in detail in Supplement 1. Denoting the local deviations from the waveguide dimensions by the random variable f(z), the 1D auto-correlation function R(uz)=f(z)f(z+uz) can be calculated. Common theoretical models estimate the scattering loss induced by the sidewall roughness based on the auto-correlation functions’s root mean square (RMS) deviation σ and correlation length d [43]. By fitting with a model composed of an exponential and a periodic part, we extract values of σ=0.18  nm and d=36  nm for the recess sidewall roughness after the reflow step. While the measured correlation lengths of the recess sidewall roughness are similar to previous works on Si and InP waveguides [41,44,45], the estimated RMS deviation is significantly lower than previously reported values for Si3N4 waveguides [42].

3. SYSTEMATIC PROCESS COMPARISON

Next, we systematically compare the influence of the reflow step and other process variations on the propagation losses of high-confinement Si3N4 waveguides. We infer the propagation losses by analyzing microresonator resonances whose linewidth κ/2π=(κex+κ0)/2π is the sum of the coupling losses from the resonator to the bus waveguide κex/2π and the internal losses κ0/2π. The latter are related to the propagation coefficient α[m1] as α=(neffκ0)/c. The waveguide’s core dimensions have strong influence on the propagation losses and, in contrast to previous works [32,33], the here presented analysis focuses on dimensions allowing for broadband anomalous GVD at telecom wavelengths, an important property for nonlinear photonic applications. For our analysis, we choose 100 GHz free spectral range (FSR) microresonators with a radius of 230  μm, a core width of 1 μm, 1.5 μm, or 2 μm, and a core height of 0.6–0.85 μm.

Frequency-calibrated transmission traces of microresonator devices are recorded for a wavelength range from 1500 nm to 1630 nm and processed using a setup and methods similar to the ones described in [39]. The resonances are automatically identified in the recorded transmission trace, and the intrinsic loss rates κ0/2π are extracted through fitting of their lineshape. For this purpose, either a Lorentzian lineshape model or a resonance doublet model is used. The latter can accurately fit resonance doublets with asymmetric lineshapes, as shown in Fig. 2(a), and is discussed in detail in a later section. Based on their mutual FSR, the resonances are manually grouped into mode families, which are identified through comparison with simulations. The resonator coupling regime is identified by comparing the trend of resonance linewidth to power extinction on resonance for resonators with the same geometry but varying resonator–bus waveguide distance on the same chip. The further analysis is based only on resonators in the under-coupled regime for which the intrinsic loss rate κ0 dominates the overall cavity losses and coupling ideality related excess losses are low [39]. As the intrinsic linewidths can vary significantly across different resonances of the same microresonator, a single resonance measurement is not representative for a given device. In order to faithfully compare the performance of different fabrication processes, we therefore measure the statistical distribution of κ0/2π extracted from up to 150 resonances. As shown in Fig. 2(b), the distribution can be well fitted using a Burr distribution [46], and its maximum is chosen as the performance indicator, representing the most probable propagation loss value for the microresonator device.

 figure: Fig. 2.

Fig. 2. Systematic process comparison and resonance doublet analysis for 100 GHz FSR microresonators. (a) Resonance doublet with asymmetric linewidths fitted to extract the doublet asymmetry rate δκ/2π and the resonance splitting γ/2π, in addition to the intrinsic linewidth κ0/2π. (b) Histogram showing the occurrence of κ0/2π values within 5 MHz bins for the quasi-TM mode of a 2 μm wide microresonator from wafer 3. The distribution is fitted using a Burr distribution (red) to extract the most probable linewidth value (red arrow). (c) Comparison of microresonator loss performance upon systematic variation of process parameters. (d) Comparison of intrinsic loss rates for quasi-TE and -TM modes for microresonators with different widths, fabricated with and without reflow process. Improvements through the reflow process are visible for tightly confining waveguides with widths of 1.5 μm and smaller. (e)–(h) Resonance doublet characteristics for 1.5 μm wide microresonators from different wafers. The mean coupling rate γ/2π and mean doublet asymmetry rate δκ/2π are plotted for quasi-TM (e), (g) and quasi-TE (f), (h) polarized fundamental modes. Triangles indicate mean values based on less than three values. Linear correlations between the mean intrinsic loss rate and splitting or asymmetry rates are shown as dashed lines. The expected mean intrinsic loss rate for vanishing scattering is indicated.

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Figure 2(c) summarizes the loss performance observed for samples from six wafers with different process parameters. Per wafer, the most probable loss values of the fundamental quasi-transverse electric (TE) and quasi-transverse magnetic (TM) mode families for several under-coupled 100 GHz FSR microresonators with 1.5 μm or 2 μm width are plotted. The adjacent table highlights the mutual processing differences among the wafers, and further processing details of each wafer can be found in Supplement 1. The process parameters were varied with the goal to observe trends originating from one of the usually suspected loss origins. Scattering losses due to sidewall roughness depend sensitively on the lithography and etching process used, and should be strongly reduced by the reflow step. Absorption losses are often associated with overtones of hydrogen impurities, and high-temperatue annealing steps of the Si3N4 core as well as the SiO2 cladding films are known to reduce their residual hydrogen content.

The results in Fig. 2(c) show no general trend for the loss relation between TE- and TM-polarized mode families, but the best values for both polarizations are achieved in the larger, 2 μm wide waveguides. The influence of annealing time as well as of the lithography process seems to be non-trivial, hinting to the fact that neither hydrogen-related absorption nor sidewall roughness-induced scattering losses have a clearly dominating role in the loss budget. Surprisingly, the comparison of wafers fabricated with and without reflow reveals little improvements. Similar observations are made when comparing different lithography processes, and the best linewidth reached for most wafers is κ0/2π50  MHz. This is different for cladding-free devices, and reveals the negative influence of the low-temperature oxide (LTO) cladding. The lowest mean linewidths, smaller than 35 MHz, are obtained when performing a short buffered hydrofluoric acid (BHF) dip directly after the reflow step. These values correspond to resonator Q-factors well above 5×106 and propagation coefficients of 5  dB/m, record for waveguides with anomalous GVD.

Although unclear before, beneficial effects of the reflow step are revealed when comparing the loss performance of resonators with different widths. In Fig. 2(d) the values for resonators with 1 μm, 1.5 μm, and 2 μm widths fabricated either with or without reflow as well as different lithography methods are compared. As noted before, for 2 μm wide waveguides, no performance difference is visible, but clear improvements through the reflow process are visible for samples with 1 μm and 1.5 μm widths. Moreover, when applying a preform reflow, the loss performance appears to be independent of the lithography technique for samples with widths of 1.5 μm or larger.

A comparison of process step influence on loss performance is interesting, but unfortunately, the above presented data allow only indirect and ambiguous guesses of the propagation loss origins. In fact, knowledge of the relative strength of scattering and absorption losses is desirable to guide fabrication efforts. In the following, we determine their relative fraction using two independent methods. First, we analyze the relation of linewidth and resonance doublet splitting to derive a limiting loss value in the absence of scattering losses [44]. Second, the measurement of the resonances’ thermal bistability allows us to infer a spectrally resolved absorption loss rate [47].

4. RESONANCE DOUBLET ANALYSIS

Waveguide sidewall roughness couples the guide to radiation modes, causing scattering loss, and can moreover lead to a coherent buildup of the counter-propagating waveguide mode. A resonance doublet is observed if the coupling rate to the counter-propagating mode is similar or larger than the total resonance decay rate κ. The relation between scattering-induced loss and reflection is non-trivial, but strongly correlated, and the analysis of resonance doublets is a common means to estimate scattering losses [44,48,49]. Most previous works used a simple coupled-mode equation (CME) system with a real coupling coefficient γ for the derivation of the splitted lineshape function [49,50]. However, the resulting expression does not provide accurate fitting for resonance doublets with unequal linewidths, as regularly observed in the context of high-confinement waveguide resonators [44,51,52].

In general, the lineshape resulting from reflective scattering can vary strongly, and even resemble a Lorentzian with enlarged linewidth, depending on the relative position and amplitude of the participating scattering centers [53]. Therefore, Li et al. have proposed an extended CME model that includes also second-order coupling processes via radiation modes [52]. The coherent (direct) and dissipative (indirect, via radiation bath) scattering processes are both loss free but can interfere and thus yield a large variety of lineshapes, including asymmetric resonance doublets. Here, we employ this extended model that practically entails a CME system with complex coupling coefficient κc=κc,R+iκc,I:

damdt=(iΔω+κ2)am+iκc2am+δm,CWκexsin
with am and m,m={CW,CCW} being the modal amplitudes of clockwise and counter-clockwise circulating modes, Δω the cavity detuning, and sin the input laser driving the continuous-wave mode. As shown in Fig. 2(a), the resulting lineshape function accurately fits the asymmetric resonance doublet and allows for the extraction of both the real and imaginary parts of κc. The resulting total coupling strength, summing coherent (direct), and dissipative (indirect) coupling processes, can then be expressed as γ=(κc,R/2)2+(κc,I/2)2. The doublet asymmetry rate, representing the competition between both coupling pathways and thus giving information about their relative strength, is expressed as δκ=(κc,Rκc,I)/2.

We are interested in the effect of different lithography techniques and the reflow process on the scattering processes. To this end, Figs. 2(e)2(h) compare the characteristics of doublet lineshapes for 1.5 μm wide microresonators from the three wafers (1, 3, and 4) already analyzed for Fig. 2(d). For each resonator, considering only their visibly splitted resonances, the mean value of the total coupling rate γ/2π and doublet asymmetry δκ/2π is plotted as a function of the mean intrinsic loss rate κ0/2π for the fundamental quasi-TE and -TM modes. We note that the mean intrinsic loss rate is based only on the κ0/2π values extracted from fitting the resonance doublets; values obtained from resonances with Lorentzian lineshape are omitted. The resulting values are indicated as triangles if less than three splitted resonances were found for the resonator.

Both the total coupling rate as well as the doublet asymmetry of the quasi-TM modes exhibit a clear correlation with the mean intrinsic loss rate. Independently, for both correlations, an intrinsic loss rate of κ0/2π23  MHz for the case of vanishing scattering losses is extrapolated by fitting with a linear model. Moreover, the linewidth reduction through the preform reflow step can now be unambiguously related to a reduced waveguide surface roughness. This becomes evident through the observed simultaneous reduction of resonance splitting and linewidth asymmetry rates with intrinsic loss rate. While the quasi-TM mode displays clear trends, the situation is more complex for the values obtained for the quasi-TE polarization. For samples fabricated using electron beam lithography with or without the reflow step, only a few resonances of each resonator show a visible splitting. This is different for resonators fabricated using stepper lithography for which most resonances of a given resonator exhibit visible splitting, with mean values about twice as large as found for the quasi-TM polarization.

The latter is expected, as the scattering for the quasi-TE polarization is dominated by the waveguide sidewall roughness, which is higher than the waveguide’s top and bottom surface roughness. In contrast, the little visibility of resonance doublets for the electron beam lithography samples is puzzling. The loss performance of 2 μm wide samples, as presented in Figs. 2(c) and 2(d), suggests that the losses other than scattering are similar for all three wafers. Moreover, for the non-reflowed, 1.5 μm wide samples, scattering rates higher than for reflowed samples are expected and thus larger than the values of γ/2π80  MHz obtained for wafer 4 (green) in Fig. 2(h). In a simple scattering model, the microresonators from the non-reflowed wafer 1 (blue) should thus support many visibly split resonance doublets, as their expected intrinsic linewidth κ0/2π100  MHz [see Figs. 2(c) and 2(d)] is on the same order. A similar reasoning can be made for wafer 3 (red), and overall, we explain these observations by the non-trivial correlation between scattering losses and coherent reflection, which depends on the statistical properties of the roughness, which are different for stepper or e-beam lithography. To extract a value for the intrinsic linewidth of the quasi-TE polarization in the absence of scattering losses, we base the correlation thus only on the values obtained for stepper lithography samples. However, a reasonable value of κ0/2π=32  MHz is obtained only via the values of the resonance asymmetry δκ.

In conclusion, we estimate a scattering loss rate of 45  MHz for 1.5 μm wide waveguides when applying a preform reflow step, accounting for about two-thirds of the total propagation losses. For larger waveguides, lower values are expected, and further experiments are needed to clarify the origin of the remaining losses.

5. THERMAL BISTABILITY SPECTROSCOPY

Next, we perform systematic measurements of the resonance’s thermal bistability to estimate the absorption loss rate [47,54]. To this end, the resonance frequency shift δω as function of dropped power Pd is measured, a quantity called thermal susceptibility χth in the following. δω and Pd are related via the local temperature increase ΔT in the mode volume, which originates from the absorbed fraction ζ of the dropped power: Pabs=ζPd. The thermo-refractive effect and the thermal expansion of the mode volume, both described by the frequency-dependent coefficient β(ω), cause the resonance frequency shift upon heating as δω=β(ω)ΔT. In practice, β(ω) can be easily measured by observing the resonance shift upon global heating of the sample, as described in detail in Supplement 1. The structure’s thermal resistance Rth determines the local temperature increase upon heating with the absorbed power as ΔT=RthPabs and can be estimated via finite element simulations. Thus, measuring χth allows to calculate the absorption fraction ζ=χth(Rthβ(ω))1, which is alternatively expressed as absorption loss rate κabs=ζκ0.

The thermal susceptibility is measured by recording the skewed, triangular lineshapes of resonance upon red detuning a sufficiently intense laser across them, as shown in Fig. 3(a). Such skewing originates from the thermal self-lock between driving laser and the resonance [55] and can be fitted with a steady-state model describing the bistability via a cubic equation to extract the associated resonance frequency shift δω. In order to precisely measure the dropped power on resonance, the frequency shift is measured for both on-chip coupling directions. As shown in Fig. 3(b), a linear correlation of the measured resonance frequency shifts versus dropped powers allows then to determine the resonance’s thermal susceptibility χth. Limiting values of the thermal resistance are simulated by considering a constant heating power in the core area and a fixed ambient temperature or perfect thermal isolation on the cladding top surface. This allows to calculate boundaries for χth, shown in Fig. 3, for the case of complete absorption of the power. Based on the measured and simulated values for the thermal susceptibility, as well as the measured intrinsic loss rates κ0/2π, an absorption rate κabs/2π is then calculated. Details of the measurement setup, data processing, and simulations can be found in Supplement 1.

 figure: Fig. 3.

Fig. 3. Determination of the absorption loss rate κabs via thermal bistability spectroscopy of 100-GHz FSR microresonators with 1.5 μm wide waveguides. (a) Measurement and fit of a skewed resonance lineshape due to the transient heating-induced bistability. (b) Linear correlation of extracted resonance drags for different dropped powers revealing the resonance’s thermal susceptibility. (c) Measured thermal susceptibilities of many resonances of a 100 GHz FSR microresonator from wafer 3. A moving average of the obtained values is superimposed in yellow. The estimated limits of the thermal susceptiblity in the case of complete absorption of the dropped power are shown as dashed lines. The inset shows an example of the simulated heat distribution for a uniformly heated waveguide core. (d) Measured intrinsic linewidths κ0/2π and their moving average in yellow, corresponding to the thermal susceptibilities shown in (c). The estimated absorption rate limits are shown in red. The positions of modal crossings leading to local deviations of the resonance properties are indicated in gray. (e), (f) Intrinsic loss rates measured for uncladded devices fabricated with and without 15 s BHF dip, as well as the estimated lower limit of their absorption loss rates. The expected spectral position of hydrogen-related overtone absorptions are shown in gray.

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We measure χth for quasi-TE polarized resonances of under-coupled 100 GHz FSR microresonators with 1.5 μm wide waveguides between 1460 nm and 1570 nm. As shown in Fig. 3(c), thermal susceptibilities around 80 MHz/mW are found for a fully cladded sample from wafer 3. With the estimated upper limits of χth, these values translate into a possible range for the absorption loss rate of up to κabs/2π20  MHz±2  MHz between 1500 nm and 1540 nm and κabs/2π9  MHz±2  MHz at the border of the measurement range. This result matches well with the residual, non-scattering losses found through the resonance doublet analysis for the quasi-TM modes. Figures 3(e) and 3(f) reveal similar absorption loss rates for uncladded samples from wafers 5 and 6, for which only a lower limit of κabs/2π (in the case of perfect top surface cooling) can be estimated.

The measurement range covers spectral regions (1500 nm to 1540 nm) where hydrogen impurity-related absorption peaks are expected, as indicated in Figs. 3(e) and 3(f) [36,56]. While in general the intrinsic loss rates κ0/2π (blue crosses) exhibit spectral variation, only in Fig. 3(e) is a slight increase of κabs/2π commensurate with the central Si-H-related overtone observed. We conclude that the absorption losses in our samples, especially in the best performing without top cladding, are dominated by a broadband absorbing species rather than the usually inculpated hydrogen impurities. Moreover, we find that the excess losses caused by the LTO cladding do not seem to be of absorptive nature, as the absorption loss rates of cladded and uncladded samples are very similar. In summary, absorption loss rates of 20  MHz were found in the 1.5 μm wide waveguides, accounting for almost half the propagation losses. For wider waveguides, a further reduction of the scattering loss contribution, and consequently a higher fraction of absorption losses in the total loss budget, is expected.

6. MATERIAL ANALYSIS

Overtones of the optically active modes of Si-H and N-H bonds are the usual suspects for impurity-related absorption losses in Si3N4 waveguides [36,56]. The wavelength-independent absorption loss rate that was found in the previous section brings this common knowledge into question for the here presented samples. Transition metal ions are an important class of impurities in the context of optical fibers, causing broadband absorption even at ppm-level concentrations [57]. Due to their efficient electronic trapping, such impurities are also well known in CMOS fabrication technology [58], e.g., in the context of solar cells [59], but, to the best of our knowledge, have never been considered in integrated photonics.

The precise measurement of ppm-level transition metal impurity concentrations is challenging, and previously their concentration in low pressure chemical vapor deposition (LPCVD) Si3N4 thin films has been measured using vapor phase decomposition and x-ray fluorescence [60]. Here, we use glow discharge mass spectroscopy (GDMS) to analyze the concentration of common transition metals in samples of unprocessed SiO2 and Si3N4 thin films. GDMS uses argon ions created in a cathode discharge plasma to sputter etch the sample surface. The ejected products are subsequently atomized in the glow discharge plasma, before entering a mass spectrometer. The technique offers impurity detection limits in the ppb range and does not suffer from matrix effects [61].

Indeed, we measure concentrations between 0.12  ppmwt for transition metals, such as Cr, Fe, and Cu, in all thin films that form the optical waveguide, even before processing. A detailed overview can be found in Table 2 in Supplement 1. Typical processing steps such as dry etching and high-temperature anneals can also introduce impurities into the device, as well as cause their diffusive redistribution. To test the impurity levels in a final device and corroborate our findings, secondary ion mass spectroscopy (SIMS) is performed on fabricated samples to obtain quantified concentration profiles of the most prominent transition metal and hydrogen impurities. SIMS uses a localized ion beam to atomize material from the film stack, which is subsequently analyzed in a mass spectrometer. A disadvantage of SIMS is the so-called matrix effects, which relate to the interaction between the ion beam and the matrix material, causing varying impurity extraction efficiency for different materials.

Figure 4 shows the results obtained for a fully cladded sample from wafer 3. Measurement details and further data for other samples are found in Supplement 1. Neither chromium nor iron could be detected in concentrations above the respective detection limits, but a copper concentration of 1018  atoms/cm3 (10  ppmwt) is found in the Si3N4 core area. Moreover, as shown in Fig. 3 in Supplement 1, hydrogen and chlorine impurities are found in concentrations of 5×1020  atoms/cm3 (5000  ppmwt), respectively, ×1019  atoms/cm3 (100  ppmwt).

 figure: Fig. 4.

Fig. 4. Concentration profile of common transition metal impurities in fully SiO2-cladded Si3N4 sample. Secondary ion mass spectroscopy (SIMS) allows to locally probe the metal concentration profile, as shown in the inset. The matrix raw ion counts of Si and N indicate the material layer composed of the top (LTO) and bottom (wet thermal oxide) cladding layers and the LPCVD Si3N4 in between (gray background). The profiling is performed for copper, iron, and chromium impurities, out of which the detected signal levels of iron and chromium are below the detection limit. A copper concentration of 1018  atoms/cm3 is measured within the Si3N4 layer.

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Based on these values, an exact derivation of the absorption losses induced by the impurities is difficult. Not only the values obtained by mass spectroscopy have significant error bars but also the absorbance of transition metal ions depends on their valence state, which is generally unknown. For copper only, the Cu2+ state is highly absorptive, and thus literature values on absorption per ppm impurity concentration range from 0.1 dB/km/ppm to several hundreds dB/km/ppm [57,62,63]. We note that for an impurity concentration of 110  ppmwt, as found for several transition metals in our samples, a value of 100 dB/km/ppm would equal to a significant broadband absorption loss rate of κabs/2π10100  MHz in the telecom C-band.

7. CONCLUSION

In summary, we have presented a novel photonic Damascene reflow process enabling the fabrication of ultra-smooth waveguides. Using two independent characterization techniques, we determined the scattering and absorption loss rates for tightly confining waveguide geometries, relevant for nonlinear photonics experiments. Our systematic study revealed a significant reduction of the scattering losses by the reflow process and identified the cladding oxide as one main limiting factor for the current devices. Mean resonator Q-factors in excess of 5×106 in tightly confining waveguides are obtained in a reproducible way. While scattering losses still represent the dominant loss contribution, also absorption losses are revealed to be significant in our samples. For the first time, we were able to relate such absorption losses in on-chip waveguides to the presence of transition metal ions. As scattering losses are likely to be reduced further through improved lithography and etching processes, this is an important finding also relevant to the application of Si3N4 waveguides in the visible range [26], where the absorption of most transition metal ions reached peak values [57]. Our results provide important insights into the loss origins of the widely used Si3N4 waveguide platform. Based on this understanding, future fabrication process improvements and new, advanced materials will enable on-chip microresonators for nonlinear applications with ultra-high Q-factors and propagation losses less than 1 dB/m.

Funding

Defense Advanced Research Projects Agency (DARPA) (HR0011-15-C-0055); Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (SNF) (161573).

Acknowledgment

Si3N4 microresonator samples were fabricated in the EPFL Center of MicroNanotechnology (CMi).

 

See Supplement 1 for supporting content.

REFERENCES

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8. V. Brasch, M. Geiselmann, T. Herr, G. Lihachev, M. H. P. Pfeiffer, M. L. Gorodetsky, and T. J. Kippenberg, “Photonic chip-based optical frequency comb using soliton Cherenkov radiation,” Science 351, 357–360 (2016). [CrossRef]  

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15. P. Trocha, D. Ganin, M. Karpov, M. H. P. Pfeiffer, A. Kordts, J. Krockenberger, S. Wolf, P. Marin-Palomo, C. Weimann, S. Randel, W. Freude, T. J. Kippenberg, and C. Koos, “Ultrafast optical ranging using microresonator soliton frequency combs,” arXiv: 1707.05969 (2017), pp. 1–9.

16. W. Liang, D. Eliyahu, V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, D. Seidel, and L. Maleki, “High spectral purity Kerr frequency comb radio frequency photonic oscillator,” Nat. Commun. 6, 7957 (2015). [CrossRef]  

17. E. Obrzud, M. Rainer, A. Harutyunyan, M. H. Anderson, M. Geiselmann, B. Chazelas, S. Kundermann, S. Lecomte, M. Cecconi, A. Ghedina, E. Molinari, F. Pepe, F. Wildi, F. Bouchy, T. J. Kippenberg, and T. Herr, “A microphotonic astrocomb,” arXiv: 1712.09526 (2017), pp. 1–7.

18. M.-G. Suh, X. Yi, Y.-H. Lai, S. Leifer, I. S. Grudinin, G. Vasisht, E. C. Martin, M. P. Fitzgerald, G. Doppmann, J. Wang, D. Mawet, S. B. Papp, S. A. Diddams, C. Beichman, and K. Vahala, “Searching for exoplanets using a microresonator astrocomb,” arXiv: 1801.05174 (2018), pp. 1–7.

19. D. T. Spencer, T. Drake, T. C. Briles, J. Stone, L. C. Sinclair, C. Fredrick, Q. Li, D. Westly, B. R. Ilic, A. Bluestone, N. Volet, T. Komljenovic, L. Chang, S. H. Lee, D. Y. Oh, M.-G. Suh, K. Y. Yang, M. H. P. Pfeiffer, T. J. Kippenberg, E. Norberg, L. Theogarajan, K. Vahala, N. R. Newbury, K. Srinivasan, J. E. Bowers, S. A. Diddams, and S. B. Papp, “An integrated-photonics optical-frequency synthesizer,” arXiv preprint: 1708.05228 (2017), pp. 27–29.

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22. V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, and L. Maleki, “Nonlinear optics and crystalline whispering gallery mode cavities,” Phys. Rev. Lett. 92, 043903 (2004). [CrossRef]  

23. T. J. Kippenberg, S. M. Spillane, D. K. Armani, and K. J. Vahala, “Fabrication and coupling to planar high-Q silica disk microcavities,” Appl. Phys. Lett. 83, 797–799 (2003). [CrossRef]  

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25. K. Y. Yang, D. Y. Oh, S. H. Lee, Q.-F. Yang, X. Yi, and K. Vahala, “Bridging ultra-high-Q devices and photonic circuits,” arXiv: 1702.05076 (2017), pp. 1–8.

26. A. Rahim, E. Ryckeboer, A. Z. Subramanian, S. Clemmen, B. Kuyken, A. Dhakal, A. Raza, A. Hermans, M. Muneeb, S. Dhoore, Y. Li, U. Dave, P. Bienstman, N. Le Thomas, G. Roelkens, D. Van Thourhout, P. Helin, S. Severi, X. Rottenberg, and R. Baets, “Expanding the silicon photonics portfolio with silicon nitride photonic integrated circuits,” J. Lightwave Technol. 35, 639–649 (2017). [CrossRef]  

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35. M. H. P. Pfeiffer, A. Kordts, V. Brasch, M. Zervas, M. Geiselmann, J. D. Jost, and T. J. Kippenberg, “Photonic damascene process for integrated high-Q microresonator based nonlinear photonics,” Optica 3, 20–25 (2016). [CrossRef]  

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References

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  1. M. Belt, J. Bovington, R. Moreira, J. F. Bauters, M. J. R. Heck, J. S. Barton, J. E. Bowers, and D. J. Blumenthal, “Sidewall gratings in ultra-low-loss Si3N4 planar waveguides,” Opt. Express 21, 1181–1188 (2013).
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  6. T. J. Kippenberg, R. Holzwarth, and S. A. Diddams, “Microresonator-based optical frequency combs,” Science 332, 555–559 (2011).
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  7. T. Herr, V. Brasch, J. D. Jost, C. Y. Wang, N. M. Kondratiev, M. L. Gorodetsky, and T. J. Kippenberg, “Temporal solitons in optical microresonators,” Nat. Photonics 8, 145–152 (2014).
    [Crossref]
  8. V. Brasch, M. Geiselmann, T. Herr, G. Lihachev, M. H. P. Pfeiffer, M. L. Gorodetsky, and T. J. Kippenberg, “Photonic chip-based optical frequency comb using soliton Cherenkov radiation,” Science 351, 357–360 (2016).
    [Crossref]
  9. J. D. Jost, T. Herr, C. Lecaplain, V. Brasch, M. H. P. Pfeiffer, and T. J. Kippenberg, “Counting the cycles of light using a self-referenced optical microresonator,” Optica 2, 706–711 (2015).
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  16. W. Liang, D. Eliyahu, V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, D. Seidel, and L. Maleki, “High spectral purity Kerr frequency comb radio frequency photonic oscillator,” Nat. Commun. 6, 7957 (2015).
    [Crossref]
  17. E. Obrzud, M. Rainer, A. Harutyunyan, M. H. Anderson, M. Geiselmann, B. Chazelas, S. Kundermann, S. Lecomte, M. Cecconi, A. Ghedina, E. Molinari, F. Pepe, F. Wildi, F. Bouchy, T. J. Kippenberg, and T. Herr, “A microphotonic astrocomb,” arXiv: 1712.09526 (2017), pp. 1–7.
  18. M.-G. Suh, X. Yi, Y.-H. Lai, S. Leifer, I. S. Grudinin, G. Vasisht, E. C. Martin, M. P. Fitzgerald, G. Doppmann, J. Wang, D. Mawet, S. B. Papp, S. A. Diddams, C. Beichman, and K. Vahala, “Searching for exoplanets using a microresonator astrocomb,” arXiv: 1801.05174 (2018), pp. 1–7.
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    [Crossref]
  22. V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, and L. Maleki, “Nonlinear optics and crystalline whispering gallery mode cavities,” Phys. Rev. Lett. 92, 043903 (2004).
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    [Crossref]
  24. A. A. Savchenkov, V. S. Ilchenko, A. B. Matsko, and L. Maleki, “Kilohertz optical resonances in dielectric crystal cavities,” Phys. Rev. A 70, 1–4 (2004).
    [Crossref]
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2017 (6)

V. Brasch, E. Lucas, J. D. Jost, M. Geiselmann, and T. J. Kippenberg, “Self-referencing of an on-chip soliton Kerr frequency comb without external broadening,” Light Sci. Appl. 6, e16202 (2017).
[Crossref]

P. Marin-Palomo, J. N. Kemal, M. Karpov, A. Kordts, J. Pfeifle, M. H. Pfeiffer, P. Trocha, S. Wolf, V. Brasch, M. H. Anderson, R. Rosenberger, K. Vijayan, W. Freude, T. J. Kippenberg, and C. Koos, “Microresonator-based solitons for massively parallel coherent optical communications,” Nature 546, 274–279 (2017).
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A. Rahim, E. Ryckeboer, A. Z. Subramanian, S. Clemmen, B. Kuyken, A. Dhakal, A. Raza, A. Hermans, M. Muneeb, S. Dhoore, Y. Li, U. Dave, P. Bienstman, N. Le Thomas, G. Roelkens, D. Van Thourhout, P. Helin, S. Severi, X. Rottenberg, and R. Baets, “Expanding the silicon photonics portfolio with silicon nitride photonic integrated circuits,” J. Lightwave Technol. 35, 639–649 (2017).
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X. Ji, F. A. S. Barbosa, S. P. Roberts, A. Dutt, J. Cardenas, Y. Okawachi, A. Bryant, A. L. Gaeta, and M. Lipson, “Ultra-low-loss on-chip resonators with sub-milliwatt parametric oscillation threshold,” Optica 4, 619–623 (2017).
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L. Chang, M. H. P. Pfeiffer, N. Volet, M. Zervas, J. D. Peters, C. L. Manganelli, E. J. Stanton, Y. Li, T. J. Kippenberg, and J. E. Bowers, “Heterogeneous integration of lithium niobate and silicon nitride waveguides for wafer-scale photonic integrated circuits on silicon,” Opt. Lett. 42, 803–806 (2017).
[Crossref]

M. H. Pfeiffer, J. Liu, M. Geiselmann, and T. J. Kippenberg, “Coupling ideality of integrated planar high-Q microresonators,” Phys. Rev. Appl. 7, 1–9 (2017).
[Crossref]

2016 (6)

2015 (3)

2014 (3)

D. T. Spencer, J. F. Bauters, M. J. R. Heck, and J. E. Bowers, “Integrated waveguide coupled Si3N4 resonators in the ultrahigh-Q regime,” Optica 1, 153–157 (2014).
[Crossref]

T. Herr, V. Brasch, J. D. Jost, C. Y. Wang, N. M. Kondratiev, M. L. Gorodetsky, and T. J. Kippenberg, “Temporal solitons in optical microresonators,” Nat. Photonics 8, 145–152 (2014).
[Crossref]

F. Dell’Olio, T. Tatoli, C. Ciminelli, and M. N. Armenise, “Recent advances in miniaturized optical gyroscopes,” J. Eur. Opt. Soc. 9, 14013 (2014).
[Crossref]

2013 (4)

M. Belt, J. Bovington, R. Moreira, J. F. Bauters, M. J. R. Heck, J. S. Barton, J. E. Bowers, and D. J. Blumenthal, “Sidewall gratings in ultra-low-loss Si3N4 planar waveguides,” Opt. Express 21, 1181–1188 (2013).
[Crossref]

D. J. Moss, R. Morandotti, A. L. Gaeta, and M. Lipson, “New CMOS-compatible platforms based on silicon nitride and Hydex for nonlinear optics,” Nat. Photonics 7, 597–607 (2013).
[Crossref]

P. T. Lin, V. Singh, L. Kimerling, and A. M. Agarwal, “Planar silicon nitride mid-infrared devices,” Appl. Phys. Lett. 102, 251121 (2013).
[Crossref]

Q. Li, A. A. Eftekhar, Z. Xia, and A. Adibi, “Unified approach to mode splitting and scattering loss in high-Q whispering-gallery-mode microresonators,” Phys. Rev. A 88, 033816 (2013).
[Crossref]

2012 (2)

H. Lee, T. Chen, J. Li, O. Painter, and K. J. Vahala, “Ultra-low-loss optical delay line on a silicon chip,” Nat. Commun. 3, 867 (2012).
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H. Lee, T. Chen, J. Li, K. Y. Yang, S. Jeon, O. Painter, and K. J. Vahala, “Chemically etched ultrahigh-Q wedge-resonator on a silicon chip,” Nat. Photonics 6, 369–373 (2012).
[Crossref]

2011 (2)

2010 (1)

2009 (1)

T. J. Kippenberg, A. Tchebotareva, J. Kalkman, A. Polman, and K. J. Vahala, “Purcell-factor-enhanced scattering from Si nanocrystals in an optical microcavity,” Phys. Rev. Lett. 103, 027406 (2009).
[Crossref]

2008 (2)

M. I. Ojovan, “Configurons: thermodynamic parameters and symmetry changes at glass transition,” Entropy 10, 334–364 (2008).
[Crossref]

A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, and J. L. O’Brien, “Silica-on-silicon waveguide quantum circuits,” Science 320, 646–649 (2008).
[Crossref]

2007 (1)

P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, “Optical frequency comb generation from a monolithic microresonator,” Nature 450, 1214–1217 (2007).
[Crossref]

2006 (1)

C. G. Poulton, C. Koos, M. Fujii, S. Member, A. Pfrang, and T. Schimmel, “Radiation modes and roughness loss in high index-constrast waveguides,” IEEE J. Sel. Top. Quantum Electron. 12, 1306–1321 (2006).
[Crossref]

2005 (1)

2004 (6)

T. Carmon, L. Yang, and K. Vahala, “Dynamical thermal behavior and thermal self-stability of microcavities,” Opt. Express 12, 4742–4750 (2004).
[Crossref]

D. Macdonald and L. J. Geerligs, “Recombination activity of interstitial iron and other transition metal point defects in p- and n-type crystalline silicon,” Appl. Phys. Lett. 85, 4061–4063 (2004).
[Crossref]

H. Rokhsari, S. M. Spillane, and K. J. Vahala, “Loss characterization in microcavities using the thermal bistability effect,” Appl. Phys. Lett. 85, 3029–3031 (2004).
[Crossref]

M. Borselli, K. Srinivasan, P. E. Barclay, and O. Painter, “Rayleigh scattering, mode coupling, and optical loss in silicon microdisks,” Appl. Phys. Lett. 85, 3693–3695 (2004).
[Crossref]

A. A. Savchenkov, V. S. Ilchenko, A. B. Matsko, and L. Maleki, “Kilohertz optical resonances in dielectric crystal cavities,” Phys. Rev. A 70, 1–4 (2004).
[Crossref]

V. S. Ilchenko, A. A. Savchenkov, A. B. Matsko, and L. Maleki, “Nonlinear optics and crystalline whispering gallery mode cavities,” Phys. Rev. Lett. 92, 043903 (2004).
[Crossref]

2003 (2)

T. J. Kippenberg, S. M. Spillane, D. K. Armani, and K. J. Vahala, “Fabrication and coupling to planar high-Q silica disk microcavities,” Appl. Phys. Lett. 83, 797–799 (2003).
[Crossref]

J. H. Jang, W. Zhao, J. W. Bae, D. Selvanathan, S. L. Rommel, I. Adesida, A. Lepore, M. Kwakernaak, and J. H. Abeles, “Direct measurement of nanoscale sidewall roughness of optical waveguides using an atomic force microscope,” Appl. Phys. Lett. 83, 4116–4118 (2003).
[Crossref]

2000 (3)

G. Vereecke, M. Schaekers, K. Verstraete, S. Arnauts, M. M. Heyns, and W. Plante, “Quantitative analysis of trace metals in silicon nitride films by a vapor phase decomposition/solution collection approach,” J. Electrochem. Soc. 147, 1499–1501 (2000).
[Crossref]

R. Germann, H. W. M. Salemink, R. Beyeler, G. L. Bona, F. Horst, I. Massarek, and B. J. Offrein, “Silicon oxynitride layers for optical waveguide applications,” J. Electrochem. Soc. 147, 2237–2241 (2000).
[Crossref]

M. L. Gorodetsky, A. D. Pryamikov, and V. S. Ilchenko, “Rayleigh scattering in high-Q microspheres,” J. Opt. Soc. Am. B 17, 1051–1057 (2000).
[Crossref]

1996 (1)

1994 (1)

F. P. Payne and J. P. R. Lacey, “A theoretical analysis of scattering loss from planar optical waveguides,” Opt. Quantum Electron. 26, 977–986 (1994).
[Crossref]

1987 (1)

1986 (1)

W. W. Harrison, K. R. Hess, R. K. Marcus, and F. L. King, “Glow discharge mass spectrometry,” Anal. Chem. 58, 341–356 (1986).
[Crossref]

1981 (1)

Y. Ohishi, S. Mitachi, and T. Kanamori, “Impurity absorption losses in the infrared region due to 3D transition elements in fluoride glass,” Jpn. J. Appl. Phys. 20, L787–L788 (1981).
[Crossref]

1974 (1)

P. C. Schultz, “Optical absorption of the transition elements in vitreous silica,” J. Am. Ceram. Soc. 57, 309–313 (1974).
[Crossref]

1973 (1)

G. R. Newns, P. Pantelis, J. L. Wilson, R. W. J. Uffen, and R. Worthington, “Absorption losses in glasses and glass fibre waveguides,” Opto-electronics 5, 289–296 (1973).
[Crossref]

1966 (1)

K. Kao and G. Hockham, “Dielectric-fibre surface waveguides for optical frequencies,” Proc. Inst. Electr. Eng. 113, 1151–1158 (1966).
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Abeles, J. H.

J. H. Jang, W. Zhao, J. W. Bae, D. Selvanathan, S. L. Rommel, I. Adesida, A. Lepore, M. Kwakernaak, and J. H. Abeles, “Direct measurement of nanoscale sidewall roughness of optical waveguides using an atomic force microscope,” Appl. Phys. Lett. 83, 4116–4118 (2003).
[Crossref]

Adesida, I.

J. H. Jang, W. Zhao, J. W. Bae, D. Selvanathan, S. L. Rommel, I. Adesida, A. Lepore, M. Kwakernaak, and J. H. Abeles, “Direct measurement of nanoscale sidewall roughness of optical waveguides using an atomic force microscope,” Appl. Phys. Lett. 83, 4116–4118 (2003).
[Crossref]

Adibi, A.

Q. Li, A. A. Eftekhar, Z. Xia, and A. Adibi, “Unified approach to mode splitting and scattering loss in high-Q whispering-gallery-mode microresonators,” Phys. Rev. A 88, 033816 (2013).
[Crossref]

Agarwal, A. M.

P. T. Lin, V. Singh, L. Kimerling, and A. M. Agarwal, “Planar silicon nitride mid-infrared devices,” Appl. Phys. Lett. 102, 251121 (2013).
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Al Noman, A.

Anderson, M. H.

P. Marin-Palomo, J. N. Kemal, M. Karpov, A. Kordts, J. Pfeifle, M. H. Pfeiffer, P. Trocha, S. Wolf, V. Brasch, M. H. Anderson, R. Rosenberger, K. Vijayan, W. Freude, T. J. Kippenberg, and C. Koos, “Microresonator-based solitons for massively parallel coherent optical communications,” Nature 546, 274–279 (2017).
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E. Obrzud, M. Rainer, A. Harutyunyan, M. H. Anderson, M. Geiselmann, B. Chazelas, S. Kundermann, S. Lecomte, M. Cecconi, A. Ghedina, E. Molinari, F. Pepe, F. Wildi, F. Bouchy, T. J. Kippenberg, and T. Herr, “A microphotonic astrocomb,” arXiv: 1712.09526 (2017), pp. 1–7.

Arcizet, O.

P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, “Optical frequency comb generation from a monolithic microresonator,” Nature 450, 1214–1217 (2007).
[Crossref]

Armani, D. K.

T. J. Kippenberg, S. M. Spillane, D. K. Armani, and K. J. Vahala, “Fabrication and coupling to planar high-Q silica disk microcavities,” Appl. Phys. Lett. 83, 797–799 (2003).
[Crossref]

Armenise, M. N.

F. Dell’Olio, T. Tatoli, C. Ciminelli, and M. N. Armenise, “Recent advances in miniaturized optical gyroscopes,” J. Eur. Opt. Soc. 9, 14013 (2014).
[Crossref]

Arnauts, S.

G. Vereecke, M. Schaekers, K. Verstraete, S. Arnauts, M. M. Heyns, and W. Plante, “Quantitative analysis of trace metals in silicon nitride films by a vapor phase decomposition/solution collection approach,” J. Electrochem. Soc. 147, 1499–1501 (2000).
[Crossref]

Bae, J. W.

J. H. Jang, W. Zhao, J. W. Bae, D. Selvanathan, S. L. Rommel, I. Adesida, A. Lepore, M. Kwakernaak, and J. H. Abeles, “Direct measurement of nanoscale sidewall roughness of optical waveguides using an atomic force microscope,” Appl. Phys. Lett. 83, 4116–4118 (2003).
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Baets, R.

Barbosa, F. A. S.

Barclay, P. E.

M. Borselli, K. Srinivasan, P. E. Barclay, and O. Painter, “Rayleigh scattering, mode coupling, and optical loss in silicon microdisks,” Appl. Phys. Lett. 85, 3693–3695 (2004).
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Barton, J. S.

Bauters, J. F.

Beichman, C.

M.-G. Suh, X. Yi, Y.-H. Lai, S. Leifer, I. S. Grudinin, G. Vasisht, E. C. Martin, M. P. Fitzgerald, G. Doppmann, J. Wang, D. Mawet, S. B. Papp, S. A. Diddams, C. Beichman, and K. Vahala, “Searching for exoplanets using a microresonator astrocomb,” arXiv: 1801.05174 (2018), pp. 1–7.

Belt, M.

Beyeler, R.

R. Germann, H. W. M. Salemink, R. Beyeler, G. L. Bona, F. Horst, I. Massarek, and B. J. Offrein, “Silicon oxynitride layers for optical waveguide applications,” J. Electrochem. Soc. 147, 2237–2241 (2000).
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Bienstman, P.

Billat, A.

C. Herkommer, A. Billat, H. Guo, D. Grassani, C. Zhang, M. H. P. Pfeiffer, C.-S. Bres, and T. J. Kippenberg, “Mid-infrared frequency comb generation with silicon nitride nano-photonic waveguides,” arXiv: 1704.02478 (2017), pp. 1–9.

Bluestone, A.

D. T. Spencer, T. Drake, T. C. Briles, J. Stone, L. C. Sinclair, C. Fredrick, Q. Li, D. Westly, B. R. Ilic, A. Bluestone, N. Volet, T. Komljenovic, L. Chang, S. H. Lee, D. Y. Oh, M.-G. Suh, K. Y. Yang, M. H. P. Pfeiffer, T. J. Kippenberg, E. Norberg, L. Theogarajan, K. Vahala, N. R. Newbury, K. Srinivasan, J. E. Bowers, S. A. Diddams, and S. B. Papp, “An integrated-photonics optical-frequency synthesizer,” arXiv preprint: 1708.05228 (2017), pp. 27–29.

Blumenthal, D. J.

Bogaerts, W.

A. Li, T. Van Vaerenbergh, P. De Heyn, P. Bienstman, and W. Bogaerts, “Backscattering in silicon microring resonators: a quantitative analysis,” Laser Photon. Rev. 10, 420–431 (2016).
[Crossref]

Bona, G. L.

R. Germann, H. W. M. Salemink, R. Beyeler, G. L. Bona, F. Horst, I. Massarek, and B. J. Offrein, “Silicon oxynitride layers for optical waveguide applications,” J. Electrochem. Soc. 147, 2237–2241 (2000).
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Borselli, M.

M. Borselli, T. Johnson, and O. Painter, “Beyond the Rayleigh scattering limit in high-Q silicon microdisks: theory and experiment,” Opt. Express 13, 1515–1530 (2005).
[Crossref]

M. Borselli, K. Srinivasan, P. E. Barclay, and O. Painter, “Rayleigh scattering, mode coupling, and optical loss in silicon microdisks,” Appl. Phys. Lett. 85, 3693–3695 (2004).
[Crossref]

Bouchy, F.

E. Obrzud, M. Rainer, A. Harutyunyan, M. H. Anderson, M. Geiselmann, B. Chazelas, S. Kundermann, S. Lecomte, M. Cecconi, A. Ghedina, E. Molinari, F. Pepe, F. Wildi, F. Bouchy, T. J. Kippenberg, and T. Herr, “A microphotonic astrocomb,” arXiv: 1712.09526 (2017), pp. 1–7.

Bovington, J.

Bowers, J. E.

Brasch, V.

V. Brasch, E. Lucas, J. D. Jost, M. Geiselmann, and T. J. Kippenberg, “Self-referencing of an on-chip soliton Kerr frequency comb without external broadening,” Light Sci. Appl. 6, e16202 (2017).
[Crossref]

P. Marin-Palomo, J. N. Kemal, M. Karpov, A. Kordts, J. Pfeifle, M. H. Pfeiffer, P. Trocha, S. Wolf, V. Brasch, M. H. Anderson, R. Rosenberger, K. Vijayan, W. Freude, T. J. Kippenberg, and C. Koos, “Microresonator-based solitons for massively parallel coherent optical communications,” Nature 546, 274–279 (2017).
[Crossref]

V. Brasch, M. Geiselmann, T. Herr, G. Lihachev, M. H. P. Pfeiffer, M. L. Gorodetsky, and T. J. Kippenberg, “Photonic chip-based optical frequency comb using soliton Cherenkov radiation,” Science 351, 357–360 (2016).
[Crossref]

M. H. P. Pfeiffer, A. Kordts, V. Brasch, M. Zervas, M. Geiselmann, J. D. Jost, and T. J. Kippenberg, “Photonic damascene process for integrated high-Q microresonator based nonlinear photonics,” Optica 3, 20–25 (2016).
[Crossref]

A. Kordts, M. H. P. Pfeiffer, H. Guo, V. Brasch, and T. J. Kippenberg, “Higher order mode suppression in high-Q anomalous dispersion SiN microresonators for temporal dissipative Kerr soliton formation,” Opt. Lett. 41, 452–455 (2016).
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J. D. Jost, T. Herr, C. Lecaplain, V. Brasch, M. H. P. Pfeiffer, and T. J. Kippenberg, “Counting the cycles of light using a self-referenced optical microresonator,” Optica 2, 706–711 (2015).
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T. Herr, V. Brasch, J. D. Jost, C. Y. Wang, N. M. Kondratiev, M. L. Gorodetsky, and T. J. Kippenberg, “Temporal solitons in optical microresonators,” Nat. Photonics 8, 145–152 (2014).
[Crossref]

Bres, C.-S.

C. Herkommer, A. Billat, H. Guo, D. Grassani, C. Zhang, M. H. P. Pfeiffer, C.-S. Bres, and T. J. Kippenberg, “Mid-infrared frequency comb generation with silicon nitride nano-photonic waveguides,” arXiv: 1704.02478 (2017), pp. 1–9.

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D. T. Spencer, T. Drake, T. C. Briles, J. Stone, L. C. Sinclair, C. Fredrick, Q. Li, D. Westly, B. R. Ilic, A. Bluestone, N. Volet, T. Komljenovic, L. Chang, S. H. Lee, D. Y. Oh, M.-G. Suh, K. Y. Yang, M. H. P. Pfeiffer, T. J. Kippenberg, E. Norberg, L. Theogarajan, K. Vahala, N. R. Newbury, K. Srinivasan, J. E. Bowers, S. A. Diddams, and S. B. Papp, “An integrated-photonics optical-frequency synthesizer,” arXiv preprint: 1708.05228 (2017), pp. 27–29.

Trocha, P.

P. Marin-Palomo, J. N. Kemal, M. Karpov, A. Kordts, J. Pfeifle, M. H. Pfeiffer, P. Trocha, S. Wolf, V. Brasch, M. H. Anderson, R. Rosenberger, K. Vijayan, W. Freude, T. J. Kippenberg, and C. Koos, “Microresonator-based solitons for massively parallel coherent optical communications,” Nature 546, 274–279 (2017).
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D. T. Spencer, T. Drake, T. C. Briles, J. Stone, L. C. Sinclair, C. Fredrick, Q. Li, D. Westly, B. R. Ilic, A. Bluestone, N. Volet, T. Komljenovic, L. Chang, S. H. Lee, D. Y. Oh, M.-G. Suh, K. Y. Yang, M. H. P. Pfeiffer, T. J. Kippenberg, E. Norberg, L. Theogarajan, K. Vahala, N. R. Newbury, K. Srinivasan, J. E. Bowers, S. A. Diddams, and S. B. Papp, “An integrated-photonics optical-frequency synthesizer,” arXiv preprint: 1708.05228 (2017), pp. 27–29.

K. Y. Yang, D. Y. Oh, S. H. Lee, Q.-F. Yang, X. Yi, and K. Vahala, “Bridging ultra-high-Q devices and photonic circuits,” arXiv: 1702.05076 (2017), pp. 1–8.

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T. Herr, V. Brasch, J. D. Jost, C. Y. Wang, N. M. Kondratiev, M. L. Gorodetsky, and T. J. Kippenberg, “Temporal solitons in optical microresonators,” Nat. Photonics 8, 145–152 (2014).
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P. Trocha, D. Ganin, M. Karpov, M. H. P. Pfeiffer, A. Kordts, J. Krockenberger, S. Wolf, P. Marin-Palomo, C. Weimann, S. Randel, W. Freude, T. J. Kippenberg, and C. Koos, “Ultrafast optical ranging using microresonator soliton frequency combs,” arXiv: 1707.05969 (2017), pp. 1–9.

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D. T. Spencer, T. Drake, T. C. Briles, J. Stone, L. C. Sinclair, C. Fredrick, Q. Li, D. Westly, B. R. Ilic, A. Bluestone, N. Volet, T. Komljenovic, L. Chang, S. H. Lee, D. Y. Oh, M.-G. Suh, K. Y. Yang, M. H. P. Pfeiffer, T. J. Kippenberg, E. Norberg, L. Theogarajan, K. Vahala, N. R. Newbury, K. Srinivasan, J. E. Bowers, S. A. Diddams, and S. B. Papp, “An integrated-photonics optical-frequency synthesizer,” arXiv preprint: 1708.05228 (2017), pp. 27–29.

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T. C. Briles, J. R. Stone, T. E. Drake, D. T. Spencer, C. Frederick, Q. Li, D. A. Westly, B. R. Illic, K. Srinivasan, S. A. Diddams, and S. B. Papp, “Kerr-microresonator solitons for accurate carrier-envelope-frequency stabilization,” arXiv: 1711.06251 (2017).

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P. Del’Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth, and T. J. Kippenberg, “Optical frequency comb generation from a monolithic microresonator,” Nature 450, 1214–1217 (2007).
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G. R. Newns, P. Pantelis, J. L. Wilson, R. W. J. Uffen, and R. Worthington, “Absorption losses in glasses and glass fibre waveguides,” Opto-electronics 5, 289–296 (1973).
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K. Y. Yang, D. Y. Oh, S. H. Lee, Q.-F. Yang, X. Yi, and K. Vahala, “Bridging ultra-high-Q devices and photonic circuits,” arXiv: 1702.05076 (2017), pp. 1–8.

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Yang, Q.

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K. Y. Yang, D. Y. Oh, S. H. Lee, Q.-F. Yang, X. Yi, and K. Vahala, “Bridging ultra-high-Q devices and photonic circuits,” arXiv: 1702.05076 (2017), pp. 1–8.

Yi, X.

M.-G. Suh, Q. Yang, K. Yang, X. Yi, and K. J. Vahala, “Microresonator soliton dual-comb spectroscopy,” Science 354, 600–603 (2016).
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K. Y. Yang, D. Y. Oh, S. H. Lee, Q.-F. Yang, X. Yi, and K. Vahala, “Bridging ultra-high-Q devices and photonic circuits,” arXiv: 1702.05076 (2017), pp. 1–8.

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C. Herkommer, A. Billat, H. Guo, D. Grassani, C. Zhang, M. H. P. Pfeiffer, C.-S. Bres, and T. J. Kippenberg, “Mid-infrared frequency comb generation with silicon nitride nano-photonic waveguides,” arXiv: 1704.02478 (2017), pp. 1–9.

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J. H. Jang, W. Zhao, J. W. Bae, D. Selvanathan, S. L. Rommel, I. Adesida, A. Lepore, M. Kwakernaak, and J. H. Abeles, “Direct measurement of nanoscale sidewall roughness of optical waveguides using an atomic force microscope,” Appl. Phys. Lett. 83, 4116–4118 (2003).
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Supplementary Material (1)

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Figures (4)

Fig. 1.
Fig. 1. Nonlinear waveguide platforms and photonic Damascene process with reflow step. (a) Overview of nonlinear coefficients γ and attenuation α for different nonlinear waveguide platforms. Dashed lines indicate similar nonlinear performance based on a constant ratio of γ / α . (b) Schematic process flow of the photonic Damascene process highlighting the newly introduced preform reflow step, which consists of heating the substrate for sufficient time above its glass transition temperature. (c), (d) Final waveguide cross section from photonic Damascene process without and with reflow step. Rounding of the waveguide corners and an increased sidewall angle of 8° are observed after reflow.
Fig. 2.
Fig. 2. Systematic process comparison and resonance doublet analysis for 100 GHz FSR microresonators. (a) Resonance doublet with asymmetric linewidths fitted to extract the doublet asymmetry rate δ κ / 2 π and the resonance splitting γ / 2 π , in addition to the intrinsic linewidth κ 0 / 2 π . (b) Histogram showing the occurrence of κ 0 / 2 π values within 5 MHz bins for the quasi-TM mode of a 2 μm wide microresonator from wafer 3. The distribution is fitted using a Burr distribution (red) to extract the most probable linewidth value (red arrow). (c) Comparison of microresonator loss performance upon systematic variation of process parameters. (d) Comparison of intrinsic loss rates for quasi-TE and -TM modes for microresonators with different widths, fabricated with and without reflow process. Improvements through the reflow process are visible for tightly confining waveguides with widths of 1.5 μm and smaller. (e)–(h) Resonance doublet characteristics for 1.5 μm wide microresonators from different wafers. The mean coupling rate γ / 2 π and mean doublet asymmetry rate δ κ / 2 π are plotted for quasi-TM (e), (g) and quasi-TE (f), (h) polarized fundamental modes. Triangles indicate mean values based on less than three values. Linear correlations between the mean intrinsic loss rate and splitting or asymmetry rates are shown as dashed lines. The expected mean intrinsic loss rate for vanishing scattering is indicated.
Fig. 3.
Fig. 3. Determination of the absorption loss rate κ abs via thermal bistability spectroscopy of 100-GHz FSR microresonators with 1.5 μm wide waveguides. (a) Measurement and fit of a skewed resonance lineshape due to the transient heating-induced bistability. (b) Linear correlation of extracted resonance drags for different dropped powers revealing the resonance’s thermal susceptibility. (c) Measured thermal susceptibilities of many resonances of a 100 GHz FSR microresonator from wafer 3. A moving average of the obtained values is superimposed in yellow. The estimated limits of the thermal susceptiblity in the case of complete absorption of the dropped power are shown as dashed lines. The inset shows an example of the simulated heat distribution for a uniformly heated waveguide core. (d) Measured intrinsic linewidths κ 0 / 2 π and their moving average in yellow, corresponding to the thermal susceptibilities shown in (c). The estimated absorption rate limits are shown in red. The positions of modal crossings leading to local deviations of the resonance properties are indicated in gray. (e), (f) Intrinsic loss rates measured for uncladded devices fabricated with and without 15 s BHF dip, as well as the estimated lower limit of their absorption loss rates. The expected spectral position of hydrogen-related overtone absorptions are shown in gray.
Fig. 4.
Fig. 4. Concentration profile of common transition metal impurities in fully SiO 2 -cladded Si 3 N 4 sample. Secondary ion mass spectroscopy (SIMS) allows to locally probe the metal concentration profile, as shown in the inset. The matrix raw ion counts of Si and N indicate the material layer composed of the top (LTO) and bottom (wet thermal oxide) cladding layers and the LPCVD Si 3 N 4 in between (gray background). The profiling is performed for copper, iron, and chromium impurities, out of which the detected signal levels of iron and chromium are below the detection limit. A copper concentration of 10 18    atoms / cm 3 is measured within the Si 3 N 4 layer.

Equations (1)

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d a m d t = ( i Δ ω + κ 2 ) a m + i κ c 2 a m + δ m , C W κ ex s in

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