1. Introduction

The development of novel platforms for integrated photonics, characterized by a strong versatility in terms of applications, is of prime importance for the realization of emerging photonics applications in the fields of sensing, metrology, quantum communications and quantum computing [1,2].

Several technologies for photonic integrated circuits (PICs) have been proposed and developed in the last decades. Silicon photonics emerged among the first, exploiting the advanced CMOS manufacturing techniques optimized for microelectronics. Nowadays, Silicon-on-Insulator (SOI) based devices are spreading in the commercial world (see Ref. [3] and references therein). State of the art SOI devices achieve propagation losses of less than $1$ dB/cm in the telecom C-Band, offering highly efficient passive components [4–6]. Active components are implemented with two approaches: phase-shifters based on p-n junctions are realized directly on the Si waveguides, while photodetectors and light sources require hetero-integration of germanium or III-V compound semiconductors [7–11]. The SOI platform is functional in the spectral range limited to wavelengths longer than $1.1~\mu$m (energy bandgap $E_g\sim 1.1$ eV) for linear operations and is characterized by significant excited-carrier (EC) and two-photon absorption (TPA) up to $2.2~\mu$m ($\sim E_g/2$) in nonlinear operations.

Silicon Nitride (SiN) has been introduced as an alternative dielectric platform for integrated photonics [12]. It is transparent at wavelengths from $400$ nm to $7.0 \mu$m (up to $3.0 \mu$m if embedded in silica), while very recent developments report on operation in the ultra-violet region [13]. Passive components show performance comparable to the SOI counterparts, with propagation losses down to $<0.1$ dB/cm over the whole operational range [14–16]. Due to its dielectric nature, phase-shifter in SiN technology can relay only to the relatively slower thermo-optical modulation of the refractive index. Nevertheless, due to the large optical bandgap of $E_g >4$ eV, SiN does not suffer EC or TPA losses, while nonlinear optical generation can still be significant owing to appreciable third-order nonlinearities. Stoichiometric silicon nitride (Si$_{3}$N$_{4}$) offers superior optical quality, however, the film thicknesses should be kept to less than $200$ nm to avoid film cracking due to the large tensile stress. This limitation can be overcome using sophisticated techniques, such as photonic Damascene [17] and multilayered TriPleX [15,18,19] processes.

An alternative approach to reduce film stress consists in introducing oxygen into the SiN material, by depositing directly a silicon oxynitride (SiON) film [20–23]. The refractive index of SiON can be continuously tuned between 1.45 (SiO$_{2}$) and 2.00 (Si$_{3}$N$_{4}$) by controlling the relative content of O and N in the film. The material loss can be lower than that of SiN in the visible and ultra-violet regions [24], meantime maintaining a very low stress for film thicknesses of up to few micrometers. As a drawback, the reduction of the material refractive index causes an increase in the device footprint, as well as a weakening of the thermo-optical and nonlinear optical characteristics of the guiding components [25].

In this work, we present a novel, low-loss, photonic platform based on SiON channel waveguides, capable to manage a wide range of VIS/NIR wavelengths for both linear and nonlinear optics applications. The core material consists in a relatively high refractive index SiON (1.66 at 850nm wavelength), which enables small footprint PIC designs with propagation losses $\sim 1.5$ dB/cm, improvable by at least a factor of two. The material shows an optical bandgap of 4.0 eV with a relatively strong optical nonlinearity of $1.3\pm 0.6 \times 10^{-19}$ m$^2$/W close to the TPA absorption edge. This, combined with the possibility to remove locally the cladding without damaging the waveguide, offers large versatility to engineer the waveguide dispersion for applications such as nonlinear Four-Wave Mixing (FWM) in ring resonator devices. Our SiON platform has the potential to be further developed for the monolithic integration of all necessary functionalities – photon sources, light manipulation circuits and photon detection (recently proven in Ref. [26]) – on a single chip, operating at room temperature, for classical and quantum applications.

2. Photonic platform

The photonic platform we introduce is based on the use of high-index SiON for the core material and SiO$_{2}$ claddings, resulting in a relatively large core/cladding index contrast of $\sim 15\%$. In addition, the channel waveguide is encapsulated between two thin films of Si$_{3}$N$_{4}$. These last act as an etch-stop barrier during the wet chemical etching, which is used to open windows in the waveguide’s top SiO$_{2}$ cladding in specific locations on the chip, without the risk to underetch the bottom SiO$_{2}$ cladding. The removal of top SiO$_{2}$ is an optional process, which can be used for different purposes, for example, a chemical functionalization of the waveguide surface for sensing applications [22,27–30] or, as it will be discussed in Section 5, in cases when the refractive index dispersion of waveguides should be engineered for non-linear optics applications [31–36]. Furthermore, a conformal thin film of Si$_{3}$N$_{4}$ is expected to reduce the sidewall roughness of SiON waveguides, thus improving the propagation loss [37].

The fabrication process of the proposed platform is schematically described in Fig. 1. Starting from a 6 inch silicon wafer, first a $1.7~\mu$m silicon oxide has been grown by wet thermal oxidation at $975~^{\circ }$C to form the bottom cladding. On top of it, a $50$ nm thick Si$_{3}$N$_{4}$ is deposited in a Low-pressure chemical vapor deposition (LPCVD) furnace at $770~^{\circ }$C, followed by a $500$ nm SiON film deposition in a plasma-enhanced chemical vapor deposition (PECVD) chamber using SiH$_{4}$, N$_{2}$O and NH$_{3}$ gas precursors. Next, the photonic devices were defined by photoresist patterning using an i-line stepper lithography, and the pattern transferred to the SiON/Si$_{3}$N$_{4}$ layers using reactive ion etching (RIE). Then, the SiON waveguides were thermally treated at $1050~^{\circ }$C for 90 min in a N$_{2}$ atmosphere to allow the release of residual H$_{2}$ and the improvement of the optical properties of the SiON film. Next, a second deposition of $50$ nm Si$_{3}$N$_{4}$ was performed, followed by a deposition of LPCVD borophosphosilicate glass (BPSG) and PECVD SiO$_{2}$ films at $640~^{\circ }$C and $300~^{\circ }$C, respectively, to form the top cladding of a total of $1.6~\mu$m.

 

Fig. 1. Sketch of the fabrication process of the high-index SiON photonic platform. (1) The Si wafer with the bottom SiO$_{2}$ cladding and the Si$_{3}$N$_{4}$ film, (2) deposition of the SiON core material, (3) lithography and etching of waveguides, (4) deposition of the top Si$_{3}$N$_{4}$ film, (5) deposition of the top borophosphosilicate glass cladding (BPSG), (6) sputtering of the TiN film, (7) lithography and etching of the TiN resistors, (8) deposition of an oxidation protective film and (9) optional local opening of the waveguides top cladding.

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In a next step, a multi-stack of 150 nm TiN and 1200 nm of Al was sputtered, patterned, and etched with RIE to allow for the realization of metal lines, contact pads and micro-heaters for the thermo-optical tuning of photonic components. The top Al film was selectively removed on top of the micro-heaters, via wet chemical etching, in order to realize efficient TiN micro-resistances with a sheet resistance of $5~\Omega$/sq. The wafer was covered with 500 nm PECVD SiO$_{2}$ protective film, which was then removed from pad positions to allow for external electrical contact.

Finally, the chip boundaries and the waveguide facets were defined by RIE of the dielectric multilayer, with an additional $140~\mu$m deep etch into the Si substrate through a Bosch process in order to ease the butt-coupling between optical fibers and waveguides. Figure 2(a) shows the SEM cross-sectional micrograph of the core of a $1300$ nm $\times$ $500$ nm SiON waveguide. All the different films, including the surrounding $50$ nm-thick Si$_{3}$N$_{4}$, the bottom and top claddings as well as the Si substrate are clearly visible.

 

Fig. 2. (a) Cross-sectional SEM micrograph of a $1300~nm\times 500$ nm SiON waveguide. The different materials of the multilayer composing the platform are labelled on the figure. (b) Dispersions for the real refractive index and the extinction coefficient of the SiON core material. The wavelength range of study for this work is highlighted in grey. Bottom: electromagnetic field distribution of the (c) TE0 and (d) TE1 modes of the waveguide showed in (a), with an indication of the simulated mode loss towards the silicon substrate.

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3. Linear properties

3.1 Material dispersion

The optical properties of the SiON films were characterized by variable-angle spectroscopic ellipsometry (VASE). The refractive index was modeled with the New Amorphous model based on the Forouhi-Bloomer dispersion equations [38]

Of particular interest for this work, the refractive index of SiON has been engineered to be higher than that of typical low-index SiON used in previous works [20,39,40]. This choice allows to design optical components with smaller footprint and larger optical mode confinement. Despite the relatively higher refractive index (1.66 at 850 nm), the estimated band-gap of $E_g \approx 4.0$ eV allows the photonic platform to be used without material absorption loss in the whole near-infrared and visible regions.

3.2 Propagation loss

Another important property to be optimized for a photonic platform is the propagation loss of waveguides, that can be attributed to several sources such as the material absorption, the radiative loss at bends, the loss towards the silicon substrate and the scattering due to surface roughness. The overall propagation loss per unit length can be characterized emulating the cutback method, by measuring the input/output power ratio of spiral-like waveguides of different lengths, as shown in Fig. 3.

 

Fig. 3. (a) Measurement of the propagation losses $\alpha _{pro}$ through the cutback method at $780$ nm wavelength for the TE polarization. Two different waveguide widths wg$_w=1.1~\mu$m (empty circles) and $1.3~\mu$m (full squares) show $\alpha _{pro}=1.8\pm 0.1$ dB/cm and $1.5\pm 0.1$ dB/cm, respectively. Errorbars and dotted lines represent the standard deviation of the experimental data and the fit, respectively. (b) Optical image of a $2.7$ cm long spiral waveguide under laser excitation.

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This characterization was realized for two different widths of waveguides, namely $1.1~\mu$m and $1.3~\mu$m, in the transverse-electric (TE) polarization. The analysis results a nearly flat response in the wavelength region 740 nm to 840 nm, with average propagation losses ($\alpha _{pro}$) of $1.8\pm 0.2$ dB/cm and $1.5\pm 0.2$ dB/cm for the two widths, and a coupling loss ($\alpha _{cpl}$) of $3.8\pm 0.3$ dB per waveguide facet.

Within the proposed platform, the lowest propagation loss achievable with ideally smooth waveguides sidewalls is given by the loss $\alpha _{sub}$ toward the silicon substrate. The designed waveguides support both the first order (TE0) and the second order (TE1) modes, as shown in Fig. 2(c) and (d). However, the TE1 mode has a lower index contrast and is characterized by a higher substrate loss of $\alpha _{sub}=0.95$ dB/cm. For what concerns the TE0 mode, the ultimate propagation loss limit is fixed by simulation at $\alpha _{sub}=0.17$ dB/cm, with $1700$ nm thickness of the bottom SiO$_{2}$ cladding. An increase of that thickness would allow to decrease, arbitrarily, the substrate loss. Yet, the proposed platform is intended to be monolithically integrated with silicon photodetectors into the substrate [26], which imposes a constrain for larger cladding thicknesses. The final $1700$ nm thickness was selected as a balance between the two complementary needs.

4. Non-linear properties

The third-order optical nonlinearities of the SiON photonic platform have been studied by exploiting the phenomenon of Self-Phase-Modulation (SPM) [41,42]. An intense laser pulse, which propagates in a nonlinear medium, induces a local variation of the refractive index due to strong light-matter interactions. This variation causes a phase shift between the spectral components of the pulse resulting in a modulation of the pulse spectrum. Consequently, by measuring the spectral broadening of an ultra-short pulse with known power, one can retrieve the non-linear index of refraction $n_2$ of the material. In particular, following the split-step method described in Ref. [43], one can simulate the expected SPM effect for a given set of parameters, including: the material’s $n_2$ coefficient, the waveguide geometry and the initial characteristics of the pulse. Then, by comparing the simulated results with the measured SPM spectra, the nonlinear index of the material can be estimated.

The SPM measurements were obtained using a mode-locked Ti:Sapphire laser, tunable in the wavelength range 720 nm-840 nm, with a $3$-dB pulse-width of $0.2$ nm (2 ps) and a repetition rate of 82 MHz. The laser is directly injected into the SiON waveguide using a lensed glass optical fiber. The transmitted pulse is collected with a second identical fiber at the waveguide output and analyzed in an Optical Spectrum Analyzer with $0.04$ nm spectral resolution and sensitivity of −60 dBm.

The pulse-broadening experiment was performed for different input powers, ranging from 0 dBm to −20 dBm measured at the output of the waveguide, in order to verify the intensity dependence of SPM. Considering that the non-linear effects on the input pulse may occur also in the injection optics, composed of lenses and fibers, it is important to attenuate the power injected into the waveguides after the external optics. Therefore, the variation of the power coupled to the waveguide was realized by moving the input-fiber away from the waveguide’s facet, to decrease the coupling efficiency. Figure 4 shows an example of the experimental data, obtained for two different pulses with central wavelengths at $780$ nm and $840$ nm.

 

Fig. 4. Intensity-dependent spectral broadening of an ultra-short laser pulse induced by SPM effect in SiON waveguide for two different pulse wavelengths centered at (a) $780$ nm and (b) $840$ nm. The spectra are normalized to their peak powers. The difference in the input pulses lineshapes (the darkest blue lines) at the two central wavelengths is attributed to laser and table optics prior to interaction in the waveguides.

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In order to estimate the unknown Kerr nonlinearity $n_2$ of the SiON material, we have performed numerical simulations which transform an input pulse spectrum into a broadened one. Having the knowledge of the waveguide’s length and propagation loss, of the effective mode area (obtained by numerical simulations) and of the effective refractive index, the nonlinear Kerr coefficient can be then estimated according to the following procedure. For each set of measurements, the lowest-power signal is taken as the reference input pulse-shape. Then, for each power, a set of expected output signals are simulated with the split-step method for different values of $n_2$ (Fig. 5(a)). The final value of $n_2$ is thus estimated as the one that minimizes the spectral difference between the experimental data and the numerical solution (Fig. 5(b)). The main source of error in the estimation of $n_2$ within this approach is given by the error $\sigma _{\alpha _{tot}}$ of the total waveguide loss, given by the sum of propagation and coupling losses $\alpha _{tot} = \alpha _{pro}\cdot L + \alpha _{cpl}$, imposed in the numerical model. The uncertainty of the estimated $n_2$ was evaluated by simulating the spectra at three values of total waveguide loss: $\alpha _{tot}$ and $\alpha _{tot}\pm \sigma _{\alpha _{tot}}$, and by taking the difference $\delta n_2=(n_2^\textrm {max}-n_2^\textrm {min})/2$ as the estimated error of $n_2$.

 

Fig. 5. Representation of the numerical fitting procedure to estimate the value of $n_2$ at wavelength of $840$ nm. (a) Starting from the input pulse lineshape (dashed line), a set of output pulse lineshape was simulated for increasing values of the $n_2$ (blue to red, colorbar in units of $10^{-20}$ m$^2$/W), in order to match the experimental lineshape (black solid line). (b) Sum of the square residuals between the theoretical and the measured lineshapes. The minimum position corresponds to the optimal value of $n_2$ that gives the best fit to the experimental data. The $n_2$ estimations are obtained for three different values of the waveguides loss (propagation and coupling): $\alpha _{tot}$, $\alpha _{tot}+\sigma _{\alpha _{tot}}$ and $\alpha _{tot}-\sigma _{\alpha _{tot}}$ (red, blue and yellow, respectively), in order to project the error on the measured loss to the error of estimated $n_2$.

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The described numerical SPM simulations method is based on the experimental inputs, which account for the geometrical dimensions and linear optical properties of the fabricated waveguides. In order to verify the robustness of our approach, we applied this method to study a set of four waveguides. In particular, a pair of waveguides was studied for two different nominal widths of 1100 nm and 1300 nm, as described in Table 1. The validation test, reported in Fig. 6(a), shows that at a fixed wavelength the estimations of $n_2$ are independent on the waveguide’s geometry, within error, and therefore confirms the reliability of our analysis. The results are also consistent with previous works [25,44,45], indicating that the $n_2$ values of our SiON material are in between the expected values for pure SiO$_{2}$ and pure Si$_{3}$N$_{4}$. The estimated nonlinear index corresponds to a nonlinear coefficient $\gamma$ of $1.5$ m$^{-1}$W$^{-1}$ at 780nm wavelength and $0.7$ m$^{-1}$W$^{-1}$ at 840nm wavelength for the $1.1 \mu$m wide waveguides.

 

Fig. 6. (a) Results of $n_2$ estimation for different geometries of waveguides (see Table 1) at two different laser wavelengths: $780$ nm (blue) and $840$ nm (orange). The implemented analysis method allows to estimate $n_2$ values which are independent on the waveguide geometry, ensuring that the geometrical properties of the waveguides are weighted properly in the data analysis. (b) The measured spectral dispersion of the nonlinear refractive index $n_2$. An increasing trend towards shorter wavelengths is clearly visible. Errobars represent the half-interval $\delta n_2=(n_2^\textrm {max}-n_2^\textrm {min})/2$ between the minimum and maximum values of $n_2$ estimated, for each waveguide, with different assumed waveguides loss as shown in Fig. 5.

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Table 1. Geometrical dimensions and estimated $n_2$ coefficients (in units of $10^{-20}$ m$^2$/W), at two different pump wavelengths, for the different investigated devices.

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In the following, we have selected one of the 1100 nm wide waveguides (sample A) and performed a spectral analysis of the variation of $n_2$ with wavelength. Figure 6(b) shows that the Kerr coefficient strongly increases while reducing the pump wavelength from Near-Infrared to Visible-Red region. This behavior is in accordance with the theoretical model that foresees a maximum in the nonlinear index located close to the TPA edge at $E_g/2$ [45], corresponding to a wavelength of about $\lambda _\textrm {TPA} \approx 310$ nm for our SiON platform.

5. Dispersion-engineered SiON ring resonators for generation of correlated photon pairs

The knowledge of the linear and non-linear properties of the developed SiON photonic platform makes it possible to engineer the modal refractive index, $n_\textrm {eff}(\omega )$, of the waveguide to match particular applications. Nonlinear schemes of generation of non-classical states of NIR photons often relay on SiN integrated microphotonic devices [33,36,46,47]. In this section, we describe our approach for the engineering of SiON-based ring resonator devices for on-chip generation of photon pairs via Spontaneous Four Wave Mixing (SFWM).

The dependence of $n_\textrm {eff}$ on the light angular frequency leads to the dispersive nature of the mode’s propagation constant $\beta (\omega )= n_\textrm {eff}(\omega )\cdot \omega /c$. For nonlinear wave interactions, of particular interest are the first two orders $\beta _1$ and $\beta _2$ of the Taylor expansion, representing the group index $n_g$ and the Group Velocity dispersion (GVD):

In a ring resonator, the group velocity $\beta _1$ defines the spectral separation $\delta _c$ between successive resonant modes – the free-spectral range (FSR) – following the relation $\delta _c=1/(\beta _1 L_c)$, where $L_c$ is the physical length of the cavity. A flat dispersion of $\beta _1(\omega )$ over a range of frequencies provides energy-equidistant cavity resonances, which is essential to fulfill the energy conservation requirement, for example, in nonlinear FWM and frequency comb generation experiments [36,46], where the pump, the signal and the idler should satisfy the relation $\omega _i = 2\omega _p - \omega _s$.

The parameter $\beta _2$ describes how the different spectral components of a propagating pulse travel, and in either case $\beta _2>0$ (normal dispersion) or $\beta _2<0$ (anomalous dispersion) result in temporal broadening of the pulse. The case of $\beta _2=0$ at some frequency – the zero dispersion frequency (ZDF) – is of particular interest for nonlinear optical applications because around the ZDF different spectral components experience largely reduced second-order dispersion. In SFWM experiments with microring resonators around ZDF the spectral spread of $\delta _c$ is minimal and the flatness of $n_g$ provides with larger nonlinear generation bandwidth [43,48].

We performed numerical axisymmetric simulations based on the Finite Elements Method in order to develop proper geometries for a device operating at NIR wavelengths, supporting single-mode characteristics and showing an anomalous GVD over the spectral range of interest. The radius of the studied ring resonators was set to 25 $\mu$m. This choice is motivated by the necessity to keep the ring radius large enough to avoid radiative losses but small enough to provide with $\delta _c\sim 1.12$ THz ($2.4$ nm), in order to minimize the spectral overlap of pump pulses with more than one mode of the resonator around 800 nm of wavelength.

First, we investigated a conventional geometry, in which the waveguide of a slightly trapezoidal form is fully embedded within the SiO$_2$ cladding (Fig. 7(a)). The lateral boundaries of the waveguide have an inclination angle of $86^o$, which is the typical value according to our fabrication process. The height of the SiON waveguide was fixed to 500 nm. Simulations were performed by varying the ring waveguide width from 900 nm to 1700 nm and the azimuthal number $M$ of the fundamental radial mode from 270 to 360, covering a wavelength span of 200 nm around $\lambda =850$ nm. The obtained results for the TE0 mode showed that the GVD remains normal for all ring widths from 1100nm to 1300nm within the spectral range between 690 nm and 880 nm.

 

Fig. 7. (a) Sketch of the cross-sectional geometry of the SiON waveguide embedded in SiO$_2$ matrix. (b) The same geometry with the top SiO$_2$ cladding substituted with air. (c) The calculated GVD’s of TE0-modes for the respective geometries, showing that anomalous GVD can be achieved when the top SiO$_2$ cladding is substituted with air, with the ZDF depending on the waveguide width. (d) The calculated FSR of two rings evidences the sufficiently flat $\delta _c$-trend for the air-cladded devices.

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Next, we performed simulations of the same ring geometry substituting the top SiO$_2$ cladding with air (Fig. 7(b)). This new configuration turned to be particularly interesting since the GVD appeared to be much more sensitive to the variations of the waveguide width. We found that the GVDs of air-cladded resonators are already anomalous for smaller widths ($\sim 1100$ nm) and turn slowly into a normal one over 1350 nm. In Fig. 7(c) we compare the calculated ring resonators GVDs for 1100 nm (red) and 1300 nm (blue) waveguide widths in both configurations with top SiO$_2$ (dashed line) and air-claddings (continuous line). The zero-dispersion point (ZDP) is observed at around $\lambda \approx 708$ nm for the 1100 nm waveguide width and around $\lambda \approx 822$ nm for the 1300 nm-wide one. We note that it is possible to engineer the waveguide width in order to achieve the ZDP in an arbitrary position inside this interval (green shaded area), in order to optimize the generation at the wavelengths where the material nonlinearity is higher (reported in Fig. 6). The air-cladded ring shows sufficiently flat dispersion of the group index and, consequently, a stable FSR over a large span of wavelengths from 700 nm to 850 nm, depending on the waveguides width (Fig. 7(d)).

6. Conclusions

In this work, we have demonstrated a new silicon oxynitride-based integrated photonic platform for linear and nonlinear application in the VIS-NIR wavelength range. The fabricated devices show a low propagation loss $<1.8$ dB/cm, comparable to commercially available devices at $800$ nm wavelength and with ongoing improvement of at least a factor two. We have demonstrated that, despite the reduction of the refractive index with respect to SiN, the SiON waveguides preserve a relatively strong optical nonlinearity of $13 \times 10^{-20}$ m$^2$/W around the wavelength $780$ nm. Furthermore, thanks to the possibility to remove locally the cladding without damaging the waveguide, our platform allows for a larger versatility in engineering the waveguide dispersion. This enables to investigate one of the specific and peculiar applications of nonlinear FWM, by properly adjusting the group index and group velocity dispersion in order to enhance nonlinear photon pair generation in ring resonators. By combining the good linear properties, the promising optical nonlinearities for on-chip photon generation and our recently developed technology for on-chip photon detection [26], we envision the potential of this platform to achieve, in the near future, a full integration of photon generation sources, manipulation and detection on a single Silicon chip, operating at room temperature, for classical and quantum photonics applications.