Extended Data Fig. 1: Devices for in-plane dielectric imaging of confined water.

a, Optical image of a representative device with channel height h ≈ 5 nm. The edges of the spacer layer are indicated by the white dashed lines. The large square region (pink) is the suspended SiN membrane. It has an approximately 20-μm-long rectangular slit in its centre (white) that connects nanochannels to a water reservoir at the back side of the device. b, Dark-field optical image of the same device. c, Zoom into its central region in which numerous parallel channels can be seen. d,e, Sketch of the device geometry: its cross-section (d) and top view (e) (not to scale). f, AFM topography image of the central region of the device. The black dashed lines indicate the position of the aperture. The white dashed curve indicates the edges of the hBN spacer crystal. Scale bars, 20 μm (a,b); 10 μm (c); 3 μm (f).