Lithium niobate on insulator (LNOI)

Lithium niobate on insulator (LNOI), also called thin-film lithium niobate (TFLN), is a photonic platform consisting of a 300 – 700 nm film of single-crystal lithium niobate bonded to a thermally-oxidized silicon wafer. The high refractive-index contrast between the LiNbO3 film ($n \approx 2.21$) and the buried oxide ($n \approx 1.45$) provides strong optical confinement, enabling sub-wavelength-scale waveguide design — the same dense-integration regime that silicon photonics established for passive structures.

The key value proposition. Bulk lithium niobate is the workhorse material for telecom electro-optic modulators because of its large Pockels coefficient ($r_{33} = 30.8$ pm/V), but conventional Ti-diffused or proton-exchange LiNbO3 waveguides are large (5 – 10 μm) and weakly-confining, requiring multi-cm electrode lengths to reach reasonable $V_\pi$. LNOI waveguides are sub-micron, allowing the electric field to be concentrated in the optically active region. The $V_\pi \cdot L$ product drops from $\sim$ 20 V·cm in bulk LiNbO3 modulators to 2 – 5 V·cm in LNOI — a $\sim$ 5× improvement.

Comparison to other modulator platforms:

Fabrication. LNOI wafers are produced commercially (NanoLN, Soitec, NGK) by ion-slicing — implanting protons into a bulk LiNbO3 substrate at a controlled depth, bonding to an oxidized silicon handle wafer, then thermally splitting at the implanted layer to leave a thin LiNbO3 film. The resulting film is single-crystal with crystal orientation set by the parent boule (X-cut, Y-cut, or Z-cut depending on application).

Waveguides are then patterned via:

• Ridge etching — direct LiNbO3 etch using inductively-coupled plasma; produces low-loss ($< 0.1$ dB/cm) ridge waveguides
• Slab + loading — etch only a SiN or polymer overlay on top of unetched LiNbO3; lower-loss but weaker confinement
• Diffusion patterning — historical bulk technique, mostly obsolete in LNOI

Application areas as of 2024 – 2026.

• Telecom EO modulators: Mach-Zehnder modulators with $V_\pi < 2$ V at $> 100$ GHz bandwidth, displacing bulk LiNbO3 for highest-performance applications
• Microwave photonics: precision RF signal generation and analog photonic links
• Quantum photonics: efficient on-chip generation of entangled photons via parametric down-conversion in periodically-poled LNOI
• Frequency combs: high-Q LNOI microresonators for Kerr comb generation, sometimes with on-chip soliton pumping
• Hybrid platforms: LNOI bonded onto silicon photonic carriers for heterogeneous active+passive integration

Limitations. LiNbO3 is photorefractive — visible-wavelength absorption causes index changes that can degrade performance. Power handling at visible wavelengths is reduced. The platform is still emerging, with mature foundry process design kits only beginning to appear (Ligentec, NanoLN, HyperLight 2023-2026).

References: Boes et al., Status and Potential of Lithium Niobate on Insulator, IEEE J. Lightwave Tech. 2024; Wang et al., Nature 2018 (the breakthrough paper showing $V_\pi \cdot L = 2.7$ V·cm).