Ta₂O₅-on-LNOI Photonic Platform

Updated 5 December 2025

Ta₂O₅-on-LNOI is a multilayer integrated photonic platform that combines low-loss Ta₂O₅ waveguides with LNOI’s strong electro-optic and χ(2) properties.
It uses precision lithography, CMP, and ion-beam sputtering to achieve ultra-low insertion loss (<0.002 dB per crossing) and minimal crosstalk (<–62 dB) for complex photonic circuits.
The platform supports diverse applications like quantum photonics, neural network processors, and frequency comb generation by leveraging both χ(2) and χ(3) nonlinear effects.

The Ta₂O₅-on-LNOI integrated photonic platform comprises a monolithic, multilayer architecture in which thin-film tantalum pentoxide (Ta₂O₅) waveguides are deposited directly atop lithium-niobate-on-insulator (LNOI) substrates. This integrated photonics approach leverages the low-loss, high-index contrast of Ta₂O₅ for passive and nonlinear χ(3) devices, as well as the strong electro-optic and χ(2) properties of LiNbO₃. The platform enables full-wafer, lithographically aligned 3D photonic circuits, supporting ultra-low-loss waveguide crossings, highly efficient interlayer routing, and the co-integration of advanced nonlinear optical and electro-optic functionalities (Nan et al., 4 Dec 2025, Brodnik et al., 9 Sep 2025).

1. Material Stack and Layer Architecture

The substrate is an X-cut thin-film LiNbO₃ (TFLN) layer, typically with thickness $h_\mathrm{LN} = 300$ –600 nm on a thermally grown SiO₂ buffer layer ( $\sim$ 2–3 µm) atop a Si handle wafer. The lower cladding for both waveguide layers is SiO₂, deposited via plasma-enhanced CVD (PECVD) or inductively-coupled plasma CVD (ICPCVD) to achieve a total oxide thickness of up to 3 µm. The upper waveguide layer consists of Ta₂O₅ deposited by room-temperature ion-beam sputtering (IBS), with thickness $t_\mathrm{Ta₂O₅} = 300$ –570 nm.

Typical refractive indices at 1550 nm are $n_\mathrm{LN}\approx2.21$ , $n_\mathrm{Ta₂O₅}\approx2.12$ –2.03, and $n_\mathrm{SiO₂}\approx1.44$ . Waveguide cores are realized as ridges in both LN (bottom layer) and Ta₂O₅ (top layer), with air or SiO₂ as the final upper cladding.

The stack supports single-mode operation in both layers, with effective-index calculations via slab and rectangular waveguide approximations. The normalized frequency for single-mode cutoff is $V = (2\pi/\lambda)(h/2)\sqrt{n_1^2 - n_2^2} < \pi$ .

2. Waveguide, Crossing, and Coupling Design

Bottom layer LN ridge waveguides typically have width $w_\mathrm{LN}\approx1\,\mu$ m and height defined by the full TFLN thickness ( $h_\mathrm{LN}$ ), while top Ta₂O₅ waveguides have $w_\mathrm{Ta₂O₅}\approx1\,\mu$ m and $h_\mathrm{Ta₂O₅}=300$ –570 nm. The pitch of the LN array is 10 µm; for Ta₂O₅, 127 µm is standard.

Propagation modes are simulated using the finite-difference eigenmode (FDE) or finite-element method (FEM), and single-mode operation is verified by ensuring $V_x<\pi$ and $V_y<\pi$ in both width and height. The stack supports high optical confinement and is suitable for devices across visible to near-infrared wavelengths.

Interlayer coupling is achieved using vertical transitions mediated by CMP-rounded SiO₂ tapers or by adiabatic inverse tapers in both LN and Ta₂O₅ layers, with tapers from 2 µm to 150 nm across 250 µm length. The optical coupling efficiency is given by the field overlap integral:

$\eta = \frac{\left| \iint E_\mathrm{LN}(x, y) E^*_\mathrm{Ta₂O₅}(x, y) \, dx \, dy \right|^2}{\iint \left| E_\mathrm{LN} \right|^2 dx\,dy \cdot \iint \left| E_\mathrm{Ta₂O₅} \right|^2 dx\,dy}$

and empirically scales as $\eta(d) \simeq 1 - A \exp(-B d)$ , where $d$ is the interlayer separation.

3. Fabrication Processes and Integration Flow

The monolithic 3D fabrication does not require intermediate bonding and proceeds as follows:

Patterning and Etching of LN Layer: Electron-beam lithography (EBL) or photolithography defines the bottom waveguides and, optionally, poling electrodes for periodically poled LN (PPLN). LN is dry-etched (e.g., Ar ion-mill) to ~150–600 nm depth for ridge formation.
Oxide Deposition and Planarization: The entire structure is conformally coated with SiO₂ (PECVD/ICPCVD) up to 3 µm thickness. Chemical–mechanical polishing (CMP) yields ≪10 nm RMS flatness and smooth edge rounding (radius ≈1 µm).
Interlayer Coupler Definition: Windows in SiO₂ are etched to a controlled 1.5 µm depth, followed by further CMP to expose the underlying LN with a rounded profile ( $z(x) = z_0 - \alpha x^2$ ).
Ta₂O₅ Deposition and Patterning: IBS deposition directly onto the processed LNOI stack builds the Ta₂O₅ layer (thickness up to 570 nm) with <50 MPa residual stress. EBL patterns an alumina or Ti hardmask, which is transferred by reactive-ion etching (RIE).
Post-Processing: Thermal annealing at 500 °C for 12 h in N₂ reduces absorption and stabilizes refractive index. An optional 20 nm ALD SiO₂ capping layer suppresses photorefractive effects.

This sequence preserves CMOS foundry compatibility. Place-and-route capabilities (e.g., with PLACE lithography) and wafer-scale tolerances (±50 nm lateral, ±20 nm vertical) permit yield and scalability.

4. Performance Metrics: Loss, Crosstalk, Q, and Nonlinearities

The platform supports ultra-low-loss and near-zero-crosstalk waveguide crossings. The measured per-crossing insertion loss is $0.002 \pm 0.0005$  dB with crosstalk below $-62$  dB across 120 nm span ($1510$–$1630$ nm) (Nan et al., 4 Dec 2025). Cascaded measurements over 300 crossings show no statistical degradation. Table 1 summarizes representative metrics:

Platform (Device)	Wavelength (nm)	Insertion Loss (dB)	Crosstalk (dB)
Ta₂O₅-on-LNOI (crossing)	1510–1630	0.002 ± 0.0005	< –62

Ta₂O₅ microresonators demonstrate intrinsic $Q_i \sim 5\times10^6$ (4 µm width) with loss $\alpha \sim 0.04$  dB/cm at $1064$–$1600$ nm; narrower waveguides show $Q_i=0.7$ – $1\times10^6$ , $\alpha=0.2$  dB/cm (780–1064 nm) (Brodnik et al., 9 Sep 2025). Interlayer tapers enable $>90$ \% optical power transfer with $0.18$ dB loss at 1550 nm and $0.48$ dB at 780 nm.

Nonlinear photonic functions are demonstrated:

$\chi^{(2)}$ SHG in PPLN/LN: $η_\text{norm}=13,\!000$ \% W⁻¹cm⁻² for LNOI; $2200$–$4500$\% W⁻¹cm⁻² for Ta₂O₅-on-LNOI circuits.
$\chi^{(3)}$ OPO in Ta₂O₅ microresonators: octave-spanning oscillation (698–1572 nm with 968 nm pump), threshold $P_\text{th}\sim$ 10–12 mW.
Photonic crystal microresonators support dark-pulse microcombs and engineered group-velocity dispersion (GVD).

5. Device Demonstrations and System Integration

Demonstrated photonic components on the Ta₂O₅-on-LNOI platform include:

On-chip octave-spanning OPO and microcomb generation in Ta₂O₅ rings and photonic-chain resonators.
SHG and cascaded nonlinear operations via integration of PPLN and Ta₂O₅ waveguides with inverse tapers and 3D routing.
Ultra-dense waveguide grids for VLSI photonic switching networks, routers, and neural network cores, capitalizing on negligible crossing-induced loss.

Cascaded $\chi^{(3)}$ - $\chi^{(2)}$ architectures enable frequency conversion devices such as tunable sources (pump 1076 nm → OPO 1048 nm → SHG 524 nm) including sum-frequency byproducts.

Platform scalability has been demonstrated across 3″ wafers, supporting $>10\times10$  mm² chiplet arrays, compatible with standard foundry processes. Full-wafer monolithic fabrication avoids yield-limiting wafer bonding or transfer steps (Brodnik et al., 9 Sep 2025).

6. Comparative Analysis and Application Outlook

Waveguide crossing losses on Ta₂O₅-on-LNOI platforms ( $<0.002$  dB, crosstalk $< -$ 62 dB) outperform prior state-of-the-art crossings in Si/SiO₂ (IL $\approx$ 0.04–0.1 dB, CT $\approx$ –30 to –50 dB) and multilayer SiN/SiO₂ (IL $\approx$ 0.08 dB, CT $\approx$ –44 dB) by %%%%46$1630$47%%%% in loss and $>$ 10 dB in crosstalk suppression (Nan et al., 4 Dec 2025).

Potential applications include:

High-throughput, large-scale optical routers and buses.
Integrated photonic neural network processors.
Quantum photonic circuits (entangled photon pair generation and manipulation).
Multiwavelength frequency combs for metrology and spectroscopy.
Visible-to-SWIR signal processing in LiDAR and AR/VR.

This suggests that monolithic Ta₂O₅-on-LNOI 3D integration provides both the performance and fabrication scalability required for next-generation very-large-scale photonic integration (VLSPI), combining advanced nonlinear optical capabilities with industrial semiconductor process compatibility (Brodnik et al., 9 Sep 2025, Nan et al., 4 Dec 2025).