Monolithic 3D integration of tantalum pentoxide photonics on arbitrary substrates (2509.08092v1)

Abstract: The photonics landscape encompasses a wide scope of material platforms, each optimized for specific functionalities, yet no platform meets the demands of all current and evolving photonic applications. While combining integrated photonics materials enhances overall capability - such as unifying nonlinear optics, low-loss passive devices, and electro-optics - material and process compatibility remains a major challenge. We introduce full-wafer, monolithic 3D integration of tantalum pentoxide (Ta$_2$O$_5$, hereafter tantala) photonics onto arbitrary substrates, which we explore here with thin-film lithium niobate (LN) on silicon. Tantala's unique properties, importantly room-temperature deposition, low-temperature annealing, and low stress in thick films optimized for phase matching, make it well suited for monolithic 3D integration without compromising substrate performance or compatibility. We demonstrate low-loss, high-quality-factor microresonators and nanophotonics in tantala, robust quasi-phase-matching in poled LN waveguides, and efficient 3D interlayer routing. This enables us to demonstrate a rich palette of nonlinear frequency conversion processes, including $\chi^{(3)}$ optical parametric oscillation (OPO) and soliton microcomb generation in tantala microresonators and photonic-crystal resonators, $\chi^{(2)}$ second-harmonic generation (SHG) in periodically poled LN, and combinations thereof. Monolithic 3D integration with tantala opens a new paradigm for scalable, multifunctional photonic systems, enabling visible, near-IR, and nonlinear operation into existing photonic infrastructure.

• The paper introduces a monolithic 3D integration process for Ta2O5 photonics, reducing process complexity and enabling wafer-scale device fabrication.
• The paper demonstrates high quality factors (up to 5×10^6) and low interlayer routing loss (<0.2 dB), highlighting robust passive device performance.
• The paper validates efficient nonlinear optical processes including SHG, FWM, and OPO, proving the platform’s versatility for multifunctional photonics.

Monolithic 3D Integration of Tantala Photonics on Arbitrary Substrates

Introduction and Motivation

The paper presents a comprehensive paper of monolithic three-dimensional (3D) integration of tantalum pentoxide (Ta$_2$O$_5$, tantala) photonics onto arbitrary substrates, with a focus on thin-film lithium niobate (LN) on silicon. The motivation stems from the need to combine complementary photonic functionalities—such as nonlinear optics, low-loss passive devices, and electro-optics—within a unified, scalable platform. Existing heterogeneous integration approaches, while effective for certain applications, suffer from process complexity and limited scalability due to recurring customization requirements. Monolithic integration, by contrast, offers a streamlined, wafer-scale process that enables vertical stacking and direct interfacing of distinct photonic materials, facilitating multifunctional device architectures.

Monolithic Tantala-LN Platform: Process and Architecture

The integration process leverages room-temperature ion-beam sputtering (IBS) of tantala, which is compatible with a wide range of substrates due to its low residual stress and low-temperature annealing requirements. The workflow begins with patterning and etching the LN layer, followed by oxide deposition and planarization, and subsequent tantala deposition and patterning. This sequence enables precise alignment and low-loss interfaces between the upper tantala and lower LN layers.

Figure 1: Direct deposition of tantala onto arbitrary substrates, SEM cross-section of tantala-LN, waveguide configurations, refractive index profiles, and device palette for monolithic 3D integration.

The platform supports diverse waveguide designs, including air-clad tantala for high-index contrast in the visible, and metal integration for poling electrodes and electro-optic control. The complementary optical properties of tantala and LN—broadband transparency from UV to SWIR and comparable refractive indices—enable efficient mode matching and nonlinear device engineering.

Fabrication and Passive Device Performance

The fabrication process is detailed, including EBL for electrode patterning, atomic layer deposition for interlayers, argon ion milling for LN etching, and CMP for oxide planarization. Tantala is deposited to a thickness of 570 nm, followed by alumina hardmask patterning and fluorene-based plasma etching. A conformal SiO$_2$ layer and annealing mitigate photorefractive effects.

Figure 2: Process flow for monolithic 3D integration, measured intrinsic $Q_i$ for microresonators, interlayer taper schematic and mode profiles, SEM of tantala taper, and measured taper losses.

Microresonators fabricated on the tantala-LN platform exhibit intrinsic quality factors ($Q_i$) exceeding $5 \times 10^6$ for wide ring widths (normal dispersion) and up to $1 \times 10^6$ for narrow ring widths (near-zero dispersion). The $Q_i$ dependence on ring width is attributed to sidewall scattering losses, consistent with modal overlap theory. Interlayer routing is achieved via vertically coupled, collinear inverse tapers, with measured loss-per-transition below 0.2 dB at 1550 nm and 0.5 dB at 780 nm, confirming efficient broadband interlayer coupling.

Nonlinear Photonic Functionality

The platform enables a full palette of nonlinear optical processes, including $\chi^{(2)}$ second-harmonic generation (SHG) in periodically poled LN waveguides, $\chi^{(3)}$ four-wave mixing (FWM) and optical parametric oscillation (OPO) in tantala microresonators, and cascaded $\chi^{(3)}$-$\chi^{(2)}$ operation via interlayer routing.

Figure 3: SHG test devices, poling electrode schematic and domain inversion microscopy, SEM of poled LN waveguide, SHG gain spectra and phase matching, output spectrum for 970 nm to 485 nm conversion, and SHG power scaling for LN and tantala-LN devices.

SHG devices demonstrate geometry-normalized conversion efficiencies of 13,000 $\%$W$^{-1}$cm$^{-2}$ for LN, 2,200 $\%$W$^{-1}$cm$^{-2}$ for 485 nm tantala-LN, and 4,500 $\%$W$^{-1}$cm$^{-2}$ for 787 nm tantala-LN. The phase matching is controlled via poling pitch $\Lambda$, with measured gain spectra closely matching theoretical models, subject to wafer-level index variations.

Dispersion Engineering and Nanophotonic Designs

Dispersion engineering in tantala microresonators is achieved by varying waveguide thickness and ring width, enabling control over the $D_2$ group velocity dispersion (GVD) parameter. Near-zero dispersion designs support ultra-broadband OPO, while normal dispersion designs facilitate dark soliton microcomb generation.

Figure 4: Geometric dispersion engineering, photonic crystal design and mode split, integrated dispersion, and summary of nonlinear operations including OPO, soliton generation, SHG, and cascaded $\chi^{(3)}$-$\chi^{(2)}$ processes.

Nanophotonic photonic crystal resonators (PhCRs) are fabricated via sidewall modulations, enabling programmable phase matching and mode selectivity. Experimental results include octave-spanning wavelength conversion, dark pulse generation, and cascaded OPO-to-SHG in a single device, demonstrating the versatility of the monolithic platform.

Implications and Future Directions

The demonstrated monolithic 3D integration of tantala photonics on arbitrary substrates establishes a scalable route for multifunctional photonic systems, with direct relevance to metrology, quantum technologies, telecommunications, and visible-light applications. The platform's compatibility with foundry-scale processes and its ability to unite $\chi^{(2)}$ and $\chi^{(3)}$ nonlinearities in a single chip are significant for the development of compact, manufacturable sources, frequency converters, and modulators.

Key numerical results include:

• Intrinsic $Q_i$ exceeding $5 \times 10^6$ in tantala microresonators
• Interlayer routing loss below 0.2 dB per transition at telecom wavelengths
• SHG conversion efficiency up to 13,000 $\%$W$^{-1}$cm$^{-2}$ in LN devices
• Octave-spanning OPO and dark soliton microcomb generation in tantala

The approach is robust against substrate choice, enabling integration with emerging materials such as lithium tantalate and barium titanate. Future work may focus on further reducing propagation losses, expanding the nonlinear device palette, and integrating active components such as lasers and detectors via monolithic or hybrid strategies.

Conclusion

This work demonstrates a monolithic 3D integration process for tantala photonics on arbitrary substrates, validated with thin-film LN on silicon. The platform achieves low-loss, high-$Q$ passive devices, efficient nonlinear frequency conversion, and broadband interlayer routing. The results substantiate monolithic tantala-LN integration as a scalable, versatile foundation for next-generation photonic systems, with broad implications for nonlinear optics, quantum photonics, and integrated device manufacturing.