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Abstract
Here we realize the first reported integrated photonic devices fabricated using sputtered niobium-tantalum oxide films. Sputtered niobium-tantalum oxide films are highly promising for integrated photonics as they are scalable to high volume manufacturing, possess high refractive index, and are transparent in the ultraviolet through near infrared wavelength range. At a wavelength near 1550 nm, we observe propagation losses as low as 0.47 dB/cm in waveguides and ring resonators with resonator quality factors as high as 860,000. We also characterize the nonlinear performance of these films and find a Kerr coefficient (n2) of 1.2 ( ± 0.2) × 10−18 m2/W. With this high Kerr coefficient we demonstrate optical parametric oscillation in a ring resonator and supercontinuum generation in a waveguide.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As the field of integrated photonics continues to mature the need for new material platforms is essential for increasing chip complexity. Specifically, chip complexity is constrained by the combined impacts of refractive index contrast and propagation loss. Higher refractive index contrast allows for tighter waveguide bends and thus denser device integration. Whereas propagation loss constrains the achievable propagation distance and thus maximum chip area. Thus the combined impacts of refractive index contrast and propagation loss determine the maximum number of devices that can be integrated into a single chip [1]. Notably, while loss is critical in considering the maximum complexity, there is a limit at which further lowering the loss no longer impacts the maximum complexity as it is instead constrained by the maximum available chip area. To this end, propagation losses in silicon nitride ($\mathrm {Si_3N_4}$) waveguides are already sufficiently low to allow for maximum chip areas limited only by other fabrication constraints (i.e. film stresses, reticle size, wafer size etc.). Thus increasing achievable chip complexity relies primarily on discovering new materials with higher refractive index contrast than silicon nitride while maintaining sufficiently low loss.
Furthermore, in a highly complex photonic integrated circuit (PIC) it is unlikely that a single material platform will have the most desirable properties for each component of the circuit and therefore heterogeneous integration is also highly desirable. There have been numerous demonstrations of heterogeneous integration to achieve superior performance to single material platforms. Transfer printing is one method that has been used for this type of heterogeneous integration [2]. However, transfer printing is a highly complex fabrication technique and alignment between layers is typically limited to the micron scale [3]. Another technique is to deposit films directly on top of previous layers using low temperature plasma-enhanced chemical vapor deposition (PECVD) [4,5]. This represents a much simpler fabrication process and allows for nanometer scale alignment between layers when using electron-beam lithography (EBL). The primary constraint for this process is deposition temperature, where films that can be deposited at low-temperatures provide increased compatibility with underlying layers that can be damaged or have their material properties changed in the presence of high temperatures.
Sputtering is an alternative low-temperature and highly back-end compatible deposition technique, which is ideal for heterogeneous integration. Ion-beam sputtered films have been used to fabricate high quality tantalum pentoxide ($\mathrm {Ta_2O_5}$) [6–8] and titanium dioxide ($\mathrm {TiO_2}$) [9–11] waveguides. Integrated devices based on tantalum pentoxide films have been shown to be low loss [7] and a promising platform for nonlinear optics. However, in some cases low-loss films required relatively high temperature annealing ($\mathrm {\sim 600^{o} C}$), which introduces some limitations to back-end-of-the-line (BEOL) compatibility [8]. Furthermore, tantalum pentoxide possesses a refractive index (2.09 at a wavelength of 1550 nm) only slightly larger than silicon nitride (1.99 at a wavelength of 1550 nm) and thus does not allow for significantly increasing device density over that achievable with silicon nitride. Similarly, ion-beam sputtered titanium dioxide waveguides have been identified as a promising alternative to silicon nitride waveguides as they provide a much higher refractive index (2.31 at a wavelength of 1550 nm) and thereby index contrast ($\mathrm {\Delta n = \frac {n_H - n_L}{n_L}}$) with silicon dioxide (58% as compared to 36% for $\mathrm {Si_3N_4}$) and like $\mathrm {Ta_2O_5}$, the films are deposited at lower temperatures, making the devices more compatible with BEOL processing. Additionally, the nonlinear refractive index of amorphous $\mathrm {TiO_2}$ ($2.3 \times 10^{-18} \frac {m^{2}}{W}$ [11]) has shown values $\sim$10x greater than that of $\mathrm {Si_3N_4}$ ($2.5 \times 10^{-19} \frac {m^{2}}{W}$ [12]). However, despite the low temperature deposition of $\mathrm {TiO_2}$, stability issues with fabrication processes makes deposition a challenge and necessitate an extremely slow ion-beam sputtering technique that requires about 8 hours or more for deposition of $\sim$300 nm films with waveguides of modest propagation losses in the 3 dB/cm range [11]. Increasing the film thickness to $\sim$460 nm for dispersion engineering purposes requires even longer deposition times and increases the propagation losses in the 5.4 dB/cm range [9]. Although the increase in index contrast provides a path toward increased optical component density, the limitations with the existing fabrication techniques and propagation loss has prevented $\mathrm {TiO_2}$ from being a viable alternative to $\mathrm {Si_3N_4}$.
Here we investigate niobium tantalum oxide as a platform for complex linear and nonlinear integrated photonics. Niobium tantalum oxide (NbTaOx) is a new material platform for integrated photonics, which was developed by VIAVI Solutions for use in multi-layer thin-film optical filters. It has been used for decades as an environmentally stable and robust, high-index thin film material in many high volume products. It is an insulator with a transparency window extending from about 350 nm to >5000 nm. Such properties make it free from nonlinear loss mechanisms like two-photon absorption (TPA) for C-band wavelengths. Notably, the addition of niobium significantly increases the refractive index over tantalum pentoxide to around 2.17 at 1550 nm. This allows for higher index contrast waveguides than silicon nitride (50% as compared to 36% for $\mathrm {Si_3N_4}$ [12]) with smaller minimum bend radii to increase device density and smaller effective areas to enhance nonlinear processes. Additionally, unlike the previous work on tantalum pentoxide and titanium dioxide integrated photonics, which leverage an extremely slow ion beam sputtered deposition process, niobium tantalum oxide is deposited using pulsed-DC magnetron sputtering, with 800 nm films being deposited in $\sim$30 minutes. Using such films we demonstrate propagation losses as low as 0.47 dB/cm and resonator quality factors as high as 860,000, without any annealing steps in highly confined $\mathrm {NbTaOx}$ waveguides. Furthermore, we measure a Kerr coefficient ($\mathrm {n_2}$) of $1.2 (\pm 0.2) \times 10^{-18}$ $\mathrm {m^{2}/W}$, and demonstrate optical parametric oscillation and supercontinuum generation in this platform.
2. Film characterization, waveguide design, and fabrication
The refractive index and extinction coefficient values for the $\mathrm {NbTaOx}$ films were characterized using a J.A. Woollam variable angle spectroscopic ellipsometer. The data is shown in Fig. 1. Similar to both $\mathrm {Si_3N_4}$ [13] (400-2350 nm) and $\mathrm {Ta_2O_5}$ [7] (300-8000 nm), $\mathrm {NbTaOx}$ has a broad transparency making it an effective platform for wavelengths spanning from the ultraviolet to the infrared (350-5000 nm). We anticipate the transparency window extends as far as $\mathrm {Ta_2O_5}$, but are limited by our ellipsometry measurement. The refractive index near the central telecommunication wavelength of 1550 nm is shown in Fig. 1(b). The high refractive index of 2.17 at 1550 nm provides increased index contrast with $\mathrm {SiO_2}$ compared to both $\mathrm {Si_3N_4}$ and $\mathrm {Ta_2O_5}$. The increased index contrast creates the potential for smaller bend radii and higher mode confinement, which prove to be of vital importance for dense linear photonic networks and nonlinear optical enhancement. This transparent part of the refractive index spectrum is fit to a Cauchy function and used to simulate the waveguide effective index and dispersion of the $\mathrm {NbTaOx}$ waveguides.
Fig. 1. a) Refractive index and extinction coefficient for $\mathrm {NbTaOx}$ from $\lambda$ = 250 nm to 5000 nm. b) Refractive index from $\lambda$ = 600 nm to 1800 nm.
Since we are interested in investigating both the linear and nonlinear optical properties of NbTaO$\mathrm {_x}$, we design our waveguides to have a low anomalous group-velocity dispersion (GVD) to support a broad four-wave mixing bandwidth. Four-wave mixing is an extremely useful third-order nonlinear phenomenon of great interest in integrated photonics, with diverse applications including all-optical switching [14–16], parametric amplification [17], optical parametric oscillation [18], optical frequency comb generation [19–21], the generation of correlated photons [22,23], as well as many more. In addition to these demonstrations, low power four-wave mixing can be used to measure the Kerr coefficient of a material platform with high accuracy [24]. To fulfill the phase matching condition of the four-wave mixing process, dispersion engineering [25] is used to design our waveguides to have anomalous dispersion for C-band wavelengths.
We perform finite-difference method (FDM) simulations using FIMMWAVE commercial software over a range of waveguide geometries. Similar to demonstrations in $\mathrm {Ta_2O_5}$ [6], we can take advantage of the sputter deposition process to get film thickness of >700 nm without the fabrication challenges associated with depositing thick low-pressure chemical vapor deposition $\mathrm {Si_3N_4}$ films [26]. The group velocity dispersion for various width and thickness combinations of the waveguides is shown in Fig. 2(a)-b. The results indicate that to achieve anomalous dispersion at 1550 nm wavelength, the waveguide thickness needs to be greater than 700 nm. We chose a thickness of 800 nm. This thickness results in anomalous dispersion in the transverse electric (TE)-like mode as well as the transverse magnetic (TM)-like mode.
Fig. 2. Group velocity dispersion for NbTaOx waveguides as a function of a) waveguide thickness and b) waveguide width.
Using a production-grade pulsed-DC magnetron sputtering process [27], we sputter 800 nm of NbTaO$\mathrm {_x}$ on the surface of a 100 mm silicon wafer thermally oxidized with 3 $\mu$m of silicon dioxide on the surface ($\mathrm {SiO_2}$). Films are characterized for refractive index using variable angle spectroscopic ellipsometry. Next a 200 nm etch mask layer of chromium is deposited using electron-beam evaporation. The waveguides are patterned using electron beam lithography with a 100 kV accelerating voltage and a field size of 520 $\mathrm {\mu m}$ x 520 $\mathrm {\mu m}$. The waveguide patterns are etched into the chromium layer using a metal inductively coupled plasma (ICP) reactive ion etcher (RIE). The $\mathrm {NbTaOx}$ layer is then etched using a $\mathrm {C_4F_6}$ based ICP oxide etch. After etching $\mathrm {NbTaOx}$, the chromium mask is removed using a wet etch and finally the devices are clad with 2 $\mathrm {\mu m}$ of $\mathrm {SiO_2}$ using plasma enhanced chemical vapor deposition (PECVD) at 350$\mathrm {{}^{\circ }C}$. Individual chips are finally diced out of the wafer.
3. Linear loss characterization
We begin by characterizing the linear propagation losses of the $\mathrm {NbTaOx}$ waveguides using the cut-back method. An erbium doped fiber amplifier (EDFA) amplified spontaneous emission (ASE) output is used as the source for the loss measurements. This broadband source is used to avoid potential measurement inaccuracies from waveguide Fabry-Perot effects. The output of the EDFA is sent through a polarization beamsplitter, which allows us to tune the polarization using a fiber paddle polarization controller. In these measurements, light was tuned to the TE-like mode. Light is coupled to the chip via a lensed fiber with a mode field diameter of 2.5 $\mu$m. We verify coupling to the fundamental mode through inspection of the waveguide output mode profile. The output of the waveguide is collected using an aspheric lens with a numerical aperture of 0.51 and the power is measured using a germanium free space detector. Waveguide propagation lengths ranging from 6 mm to 28 mm are measured. We test varying waveguide widths of 1.5 $\mu$m, 2.5 $\mu$m, and 3.5 $\mu$m in order to investigate the contribution of sidewall roughness to linear losses. The results are shown in Fig. 3(a). We extract the losses by fitting the measured data to a linear curve. Losses extracted from the cut-back method are found to be $\mathrm {\sim 0.62}$ dB/cm, $\mathrm {\sim 0.99}$ dB/cm, and $\mathrm {\sim 0.56}$ dB/cm for the waveguide widths of of 1.5 $\mu$m, 2.5 $\mu$m, and 3.5 $\mu$m, respectively.
Fig. 3. a) Cut-back measurements for 1.5 $\mu$m, 2.5 $\mu$m, and 3.5 $\mu$m waveguide widths. b) SEM images of waveguide surface/sidewall and cross section.
The cut-back measurements results indicate no obvious relationship between waveguide width and linear loss value. Since no size-dependent loss trend is observed, we expect that the losses due to sidewall interaction may not be increasing over this range of waveguide sizes. Additionally, coupling efficiency variations between the fabricated waveguides of varying lengths adds noise to the cut-back method and therefore waveguide-width dependent linear propagation losses may not be observable using this method. For this reason, to further characterize the linear losses, we fabricate and measure the response of ring resonators. Because the dimensions of our waveguides support high order modes, the ring resonators are designed using Euler bend ring resonator design employed by Ji et al to ensure coupling to the fundamental mode [28]. The ring resonator is designed to have a bend radius that varies across the round trip length, where the largest bend radius occurs in the coupling region. This type of design suppresses the coupling to higher-order modes in the coupling region. In our design, the effective ring radius is 135 $\mu$m with a maximum bending radius of 890 $\mu$m and a minimum bending radius of 80 $\mu$m. The resonance spectrum is measured by sweeping a Santec tunable laser source (TSL 550) over a 100 nm bandwidth. The output is collected using a fiber collimator and sent to a Thorlabs PDB450C photodetector. The RF output of the photodetector is then sent to a Tektronix digital phosphor oscilloscope. Based on the sweep rate of the Santec laser and the sampling rate of the oscilloscope, the time signal can be converted to wavelength. The spectrum of the TE-like resonances for a ring resonator of each of the three waveguide widths are shown in Fig. 4(b). Using the resonant wavelengths from Fig. 4(b), the longitudinal wave vector, $\beta$, can be extracted as a function of wavelength. The group velocity dispersion based on the resonator spectrum can then be estimated by fitting $\beta$ versus optical frequency using a Taylor series expansion centered at 1550 nm. Using this process, the resonator spectrums show the 1.5 $\mu$m width waveguide has a GVD of 225($\pm$150) ps/nm/km, the 2.5 $\mu$m width waveguide has a GVD of 150($\pm$130) ps/nm/km, and the 3.5 $\mu$m width waveguide has a GVD of 60($\pm$120) ps/nm/km for the fundamental TE-like modes. The error bounds represent the 95 % confidence window of a least squares fit. Our simulated GVD values of 130, 30, and -10 ps/nm/km for waveguides widths of 1.5, 2.5 and 3.5 $\mu$m respectively, fall within the range of values for GVD extracted for the ring resonator spectrum, which verifies the consistency between the measured resonator spectrums and the ellipsometry data and simulated dispersion. Figure 4(c) from top to bottom shows a zoomed in resonance for devices with 800 nm x 1.5 $\mu$m dimensions and a 500 nm waveguide to resonator gap, 800 nm x 2.5 $\mu$m dimensions and a 400 nm waveguide to resonator gap, and 800 nm x 3.5 $\mu$m dimensions and a 400 nm waveguide to resonator gap. The resonance in the center panel of Fig. 4(c) (800 nm x 2.5 $\mu$m) has a full-width half maximum (FWHM) of 246 MHz corresponding to a loaded quality factor of 784,000. The quality factor can be used to extract the round-trip propagation losses of the resonator. This is done by first calculating the intrinsic quality factor using the equation below [29]:
Fig. 4. a) SEM image of Euler bend ring resonator and 250 nm gap coupling region. b) Resonant spectrum for 1.5 $\mu$m, 2.5 $\mu$m, and 3.5 $\mu$m waveguide width ring resonators. c) Lorentzian fit of a 1.5, 2.5, and 3.5 $\mu$m width resonance with intrinsic quality factor and full-width half maximum labeled.
In this equation, the $\mathrm {T_0}$ is the fraction of power transmitted at the resonant wavelength with respect to the power transmitted off resonance. For the resonance shown in Fig. 4(c), the intrinsic quality factor is 860,000. The propagation losses based on the intrinsic quality factor can be written as [29]:
Using a group index ($\mathrm {n_g}$) value of 2.29, the resonant wavelength of 1554 nm, and the intrinsic quality factor of 860,000, the propagation losses are calculated to be 0.47 dB/cm. This value matches closely to the values extracted from the cutback method from Fig. 3(a). The other two waveguides widths, 3.5 $\mu$m and 1.5 $\mu$m, were found to have lower intrinsic quality factors than the 2.5 $\mu$m width, with peak intrinsic quality factors of 600,000 and 430,000, corresponding to 0.7 dB/cm and 1 dB/cm propagation losses respectively. Again, there is no clear relationship between the losses and the waveguide width. We believe that our quality factors are being limited by the quality of the $\mathrm {SiO_2}$ cladding and not by the waveguide material. Further optimization of our deposition process is needed to ensure conformal filling of the resonator gaps to decrease losses in the coupling region and to reveal clearer quality factor versus waveguide width trend.
4. Kerr nonlinearity characterization
Next we investigate the nonlinear capabilities of this material platform by first measuring the Kerr coefficient. The Kerr coefficient is extracted through the measurement of the maximum conversion efficiency of the degenerate four-wave mixing process at low pump powers. Four-wave mixing is a third-order nonlinear process where two pump photons are annihilated and converted into a signal and idler. In this process both energy and momentum are conserved, where the conservation of momentum is known as the phase matching condition. Small values of group velocity dispersion are required for large four-wave mixing phase-matching bandwidths, due to the anomalous dispersion offsetting the nonlinear phase shift for high pump powers. In our experimental process, both signal and pump photons are input to the waveguide. The pump wavelength is kept constant at 1554 nm, whereas the signal is swept to find the peak conversion efficiency. The pump is amplified using a Keopsys erbium doped fiber amplifier (EDFA) capable of outputting 2 W, both the pump and signal are tuned to the TE-like mode using fiber paddle polarization controllers, and they are combined using a wavelength division multiplexer before being sent to the chip via lensed fiber. The light is collected at the output using an aspheric lens and coupled back into fiber through a collimator before being measured on a Yokogawa optical spectrum analyzer (OSA). On-chip input power is estimated by measuring the output power in free-space and accounting for output coupling and on-chip propagation losses. Figure 5(a) shows the spectrum of the four-wave mixing process for a pump power of 55 mW and waveguide propagation length of 2.8 cm, while the signal wavelength is swept from 1570 nm to 1630 nm. The peak conversion efficiency, which is defined as $\frac {P_{idler,out}}{P_{signal,in}}$, was measured to be -48.4 dB. Peak conversion efficiency corresponds to the wavelength detuning where the phase matching condition of the four-wave mixing process is achieved. Although the low value of group velocity dispersion of our waveguide results in only small changes to the conversion efficiency over the measured bandwidth, perfect phase matching and peak conversion efficiency will only be achieved at one specific wavelength detuning from the pump. Using this peak conversion efficiency value, the Kerr coefficient is then calculated using [24]:
Fig. 5. a) FWM spectrum as signal wavelength is swept. b) Conversion efficiency as a function of idler wavelength with simulation data confirming our simulated waveguide dispersion. c) Experimental setup for four-wave mixing bandwidth measurements.
In this equation $\mathrm {n_2}$ is the nonlinear refractive index or Kerr coefficient, $\mathrm {A_{eff}}$ is the waveguide effective area, $\mathrm {L_{eff}}$ is the effective propagation length, and $\mathrm {P_{pump}}$ is the on-chip pump power at the input of the waveguide. The calculated Kerr coefficient is ($\mathrm {n_2}$) of $1.2 (\pm 0.2) \times 10^{-18}$ $\mathrm {m^{2}/W}$. This value was consistent across five different measured waveguides with propagation lengths of both 2 and 2.8 cm. The reported value is the average of these five measurements and the error bound is the standard deviation. The $\mathrm {n_2}$ value was consistent across $\sim$10 nm of pump wavelength detuning, and we are operating very far from the band edge and therefore the $\mathrm {n_2}$ is consistent across our small range of C-band wavelengths. This is more than four times higher than the Kerr coefficient for silicon nitride [12], and indicates that $\mathrm {NbTaOx}$ is a promising material for Kerr nonlinear optics.
5. Optical parametric oscillation and Kerr frequency comb generation
The high Q resonators and exceptional Kerr nonlinearity described in previous sections indicate that $\mathrm {NbTaOx}$ is a promising platform for optical parametric oscillation and frequency comb generation. In Kerr nonlinear platforms, optical parametric oscillation occurs due to spontaneous four-wave mixing in a cavity. The most common cavity is a ring resonator. Similar to laser oscillation, parametric oscillation occurs when the parametric gain exceeds the round trip losses of the cavity. As the input power is increased passed the oscillation threshold, the four-wave mixing process cascades and a spectrum of equidistant lines form a frequency comb. Kerr frequency combs have many applications including sensing/spectroscopy [30], generation of low-noise microwaves [31], optical communications [32], and many others. As a result of the many applications, Kerr frequency combs have been demonstrated in many material platforms including crystalline silicon [33], $\mathrm {Si_3N_4}$ [21], $\mathrm {Ta_2O_5}$ [8], $\mathrm {AlGaAs}$ [19], AlN [34], and SiON:D [20].
The experimental setup for frequency comb generation is similar to those previously described. In summary we use a continuous wave (CW) laser that is amplified with an EDFA and coupled to the waveguide via lensed fiber. The output light is collected and measured on our OSA. The results from this section come from a ring resonator with a 2.5 $\mu$m width and resonator gap of 300 nm (spectrum is shown in Fig. 4(b) center panel). The laser wavelength is slowly tuned onto resonance from the short wavelength side of the resonance because of the positive thermo-optic coefficient of this material. We first investigate the oscillation threshold by finding the minimum input power required for oscillation. Figure 6(a) shows the optical power in the first oscillating sideband as a function of input pump power. We find that our oscillation threshold is about 80 mW. This value is consistent with the equation for threshold power presented in [35], using the quality factor of 510,000 of this resonance and the extracted Kerr coefficient from section 4. The spectrum of the maximum coupled power of 275 mW is shown in Fig. 6(b). A frequency comb extending down to 1300 nm on the short wavelength side of the pump is shown. The comb line spacing and measured threshold power are consistent with the group velocity dispersion simulated in Fig. 2 for the fundamental TE-like mode. We anticipate a number of ways to further extend and fill the comb such as coupling more on-chip pump power, increasing our quality factors further, and reducing the size of our ring.
Fig. 6. a) Parametric oscillation measurement. b) Frequency comb spectrum for 275 mW pump power coupled to the bus waveguide. c) Experimental setup for frequency comb generation.
6. Supercontinuum generation
We further investigate the nonlinear capabilities of this platform through supercontinuum generation. A 100 MHz repetition rate Menlo Systems mode-locked laser is used as the seed laser for the supercontinuum generation. The laser pulse is centered at 1555 nm and has a pulse width of 3 ps. The input pulses are coupled to a straight waveguide with a 2.5 $\mu$m width via lensed fiber and propagate for 2.8 cm on chip. Figure 7 shows the output spectrum for different peak pulse powers. For the lowest peak power of 3 W, the spectrum spans only about 100 nm, whereas at the highest peak power of 37 W the spectrum broadens down to 1000 nm wavelength. We are somewhat limited in this measurement as our OSA only measures up to 1700 nm, but the broadening on the short wavelength side indicates more than an octave spanning supercontinuum is generated in our waveguides due to the dispersion engineering, low linear losses, and high Kerr nonlinearity of these waveguides.
Fig. 7. a) Output spectrum as a function of peak pulse power. b) Output spectrum for lowest and highest peak power.
7. Comparison to state-of-the-art
$\mathrm {NbTaOx}$ is compared to other integrated photonic platforms in Table 1. The parameters used in this comparison are the most basic indicators for a useful integrated photonic platform for both linear and nonlinear applications. These include the linear refractive index at the central C-band wavelength, linear propagation losses ($\alpha$), Kerr coefficient ($\mathrm {n_2}$), the range of wavelengths for which the material is transparent, and the deposition process. From a linear optics point of view, refractive index determines the minimum bending radius. This, in turn, influences the density of on-chip components for a given chip area and therefore the complexity of a chip. Additionally, refractive index influences the waveguide mode confinement, where a higher refractive index results in higher confinement. High confinement allows for dense components without unwanted cross talk and greater control over the group-velocity dispersion. Linear propagation losses significantly influence the utility of a material platform for linear optical application. It also relates to on-chip complexity, as it determines the amount of propagation that can occur before a signal needs amplification. Finally, the deposition process is an important consideration in terms of ease of fabrication. Dispersion engineering for broadband nonlinear interactions requires specific film thicknesses, which for some deposition processes (e.g. LPCVD $\mathrm {Si_3N_4}$) can require significant process engineering. Lastly, the specific deposition process can allow for BEOL compatible integration based on the temperature requirements. This table includes a wide range of material platforms with a variety of reported material parameters. Importantly, in general, linear refractive index, Kerr coefficient, and the deposition process for these materials are not tunable properties, whereas linear propagation losses can be significantly decreased through optimization of the fabrication process.
Table 1. Optical Material Properties for Integrated Photonic Waveguides. Deposition process that are considered BEOL compatible are in bold in the Deposition column. (UFPLD - ultrafast pulsed laser depositon, LPCVD - low-pressure chemical vapor deposition, IBS - ion-beam sputtering, PECVD - plasma-enhanced chemical vapor deposition, DMS - DC magnetron sputtering, CZ process - Czochralski process, FZ process - float zone, HWCVD - hot wire chemical vapor deposition.)
Comparing $\mathrm {NbTaOx}$ to the other tantalum based sputtered material, $\mathrm {Ta_2O_5}$, it can be seen that $\mathrm {NbTaOx}$ has the advantage of a higher linear refractive index, while $\mathrm {Ta_2O_5}$ has lower propagation losses. However, the propagation losses reported for $\mathrm {Ta_2O_5}$ are for air-clad devices. We anticipate that our resonator losses are being limited by the lack of conformality of our $\mathrm {SiO_2}$ cladding and that with an improved cladding deposition process our losses would be closer to the state-of-the-art $\mathrm {Ta_2O_5}$ linear propagation losses. Also, the fabrication of the $\mathrm {Ta_2O_5}$ devices included an annealing step, which was not explored in our devices. The material with the lowest propagation losses is $\mathrm {Si_3N_4}$, which is over an order of magnitude lower than the other platforms listed. However, its linear refractive index is $\mathrm {\sim 10\%}$ smaller than $\mathrm {NbTaOx}$. This means that with improved linear loss performance, $\mathrm {NbTaOx}$ has the material characteristics to outperform $\mathrm {Si_3N_4}$ in dense and highly complex linear optical networks.
These same material properties as well as the Kerr coefficient ($\mathrm {n_2}$) and effective nonlinear parameter ($\gamma$) are also important material properties for nonlinear optical applications. From a nonlinear optical perspective, comparing $\mathrm {NbTaOx}$ to $\mathrm {Ta_2O_5}$ and $\mathrm {Si_3N_4}$, $\mathrm {NbTaOx}$ has the advantage of a higher Kerr coefficient and higher effective nonlinearity resulting from both its linear and nonlinear refractive indices. This means that with optimized fabrication and lower linear propagation losses, $\mathrm {NbTaOx}$ has the potential to outperform both of these platforms in nonlinear optical applications. Comparing $\mathrm {NbTaOx}$ to the highly nonlinear materials such as AlGaAs, c-Si, and a-Si:H, it can be seen that $\mathrm {NbTaOx}$ has the advantage in terms of a broad transparency window. This means that $\mathrm {NbTaOx}$ has utility over a broader spectrum of wavelengths and will not suffer from nonlinear loss mechanisms like two-photon absorption at C-band wavelengths. $\mathrm {NbTaOx}$ also has the advantage of being deposited through pulsed-DC magnetron sputtering rather than the low-pressure chemical vapor deposition process for $\mathrm {Si_3N_4}$. The sputtering deposition techniques provide the ability to deposit thick films required for the dispersion engineering that enables nonlinear phase matching without film cracking due to stress. Additionally, the low-temperature deposition of sputtered films opens up the possibility for 3-D integration with other integrated photonic platforms as well as with electronics.
8. Conclusion
In summary, we have shown that $\mathrm {NbTaOx}$ is an exciting new material platform for linear and nonlinear integrated photonics. We demonstrated low loss waveguides and high quality factor ring resonators, which enabled frequency comb generation and supercontinuum generation. To the best of our knowledge these results represent the first of their kind for this material platform. Its high linear and nonlinear refractive index, low propagation losses, and fast low temperature sputter deposition process make it a promising alternative to more mature integrated photonic platforms such as $\mathrm {Si_3N_4}$ and $\mathrm {Ta_2O_5}$.
Funding
VIAVI Solutions Inc.; National Eye Institute (R21EY031854).
Acknowledgments
The authors gratefully acknowledge Vince Genova from the CNF for process assistance. The authors also gratefully acknowledge Andy Hulse from VIAVI for facilitating the film deposition process. This work was performed in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant NNCI-2025233). The work reported here was partially carried out in the Nanofabrication Facility at the University of Delaware. This work was supported in part by the National Eye Institute (NEI) (R21EY031854). Research reported in this publication was supported by the NEI of the National Institutes of Health. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request. Ellipsometry data can be obtained through reasonable request through William Houck (bill.houck@viavisolutions.com).
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