X-cut lithium niobate
A lithium niobate wafer orientation in which the crystal's optic (Z) axis lies in the plane of the surface, so coplanar electrodes beside the waveguide can drive the strongest electro-optic coefficient for TE-polarized light. The workhorse orientation of thin-film lithium niobate modulators.
Lithium niobate wafers are named by which crystal axis sticks out of the surface. An X-cut wafer has the crystallographic X axis as its surface normal, which puts the optic axis (Z) in the plane of the chip. That geometric fact decides how a modulator must be built.
Why the orientation matters. The Pockels effect in lithium niobate is dominated by the coefficient, about 30 pm/V, which requires the applied electric field and the optical polarization both to point along Z. On an X-cut chip with the waveguide running along Y, a TE mode is polarized in-plane, along Z. Electrodes placed on either side of the waveguide produce an in-plane horizontal field, also along Z. Everything lines up, and the electrodes sit safely a few microns to the side of the ridge, where their metal cannot absorb the guided light. No buffer layer is needed between metal and waveguide.
The free push-pull. In an X-cut Mach-Zehnder modulator, the two arms are placed on opposite sides of a central signal electrode. The same drive field points along +Z at one arm and along Z at the other, so a single RF signal produces equal and opposite phase shifts. Chirp-free push-pull operation, which halves the drive voltage, comes from the geometry for free. This is the main reason nearly every thin-film lithium niobate modulator is X-cut.
X-cut versus Z-cut:
| X-cut | Z-cut | |
|---|---|---|
| Drive field direction | In-plane, from coplanar side electrodes | Vertical, from an electrode above the waveguide |
| Buffer layer | Not required | SiO₂ buffer needed under the top electrode; brings DC drift and charge effects |
| Push-pull in an MZM | Built into the geometry | Requires dual drive or domain inversion |
| Polarization used | TE | TM |
Numbers. Thin-film X-cut modulators on lithium niobate on insulator reach half-wave voltage-length products around 2 to 3 V·cm, and because the thin film lets the RF and optical waves travel at nearly the same velocity, electrodes can be centimeters long while electro-optic bandwidths beyond 100 GHz have been demonstrated at CMOS-compatible drive voltages.
Poling note. Ferroelectric domain inversion for quasi-phase-matching also acts along Z. On X-cut thin films the poling field can be applied between surface electrodes, in plane, which is how periodically poled waveguides for on-chip second-harmonic generation and parametric devices are made in the same platform.
References: Wang et al., Nature 562, 101 (2018); Zhu et al., Adv. Opt. Photon. 13, 242 (2021).