Single-crystal lithium niobate is an appealing material platform for on-chip photonic elements, such as electro-optic modulators, due to the fact that a voltage applied along the crystal’s z-axis induces a particularly strong change in refractive index. For integration, lithium niobate has to be cut into very thin slices with the cutting plane suitably oriented to take advantage of this anisotropy. Ghoname et al. have used lithium niobate cut perpendicularly to the crystal’s z-axis (“z-cut”) to great advantage by spiralling a long 2.2 mm waveguide grating into a tiny 120 x 120 μm² area and creating a fast optical modulator in the process. The use of the z-cut lithium niobate-on-insulator platform allows optical waveguides to have bendy paths on-chip while maintaining the strong electro-optic effect of lithium niobate. This is unlike the ordinary situation in x-cut chips where devices like modulators generally need to be straight and long (typically millimetres). By coiling a waveguide up like this, the resulting spiral Bragg grating modulator achieves high speed (25 GHz) operation while using very little of the chip’s real estate. This design contributes to optical integration goals in an important nonlinear optical material.

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