Extended Data Fig. 5: Inversion symmetry breaking in ultrathin HZO via SHG.

a, Schematic of the SHG experimental setup, using a 960-nm pump and SHG intensity detected around 480 nm under tilt incidence, which is sensitive to out-of-plane polarization (Methods). NIR, near-infrared; 1D, one-dimensional; PBS, polarized beam splitter; Obj, objective; LP, BP and SP represent long-pass, band-pass and short-pass filters; DMSP, dichroic short-pass mirror; M1, M2 and M3 refer to mirrors; OPO, optical parametric oscillator. b, Schematic of the ten-cycle HZO islands probed by SHG (Methods); micrometre-sized islands enabled identification of specific HZO regions either poled with an electric field (applied by a PFM tip) or left as is. For these experiments, heavily doped (10¹⁹ cm⁻³) p-type Si substrates (p⁺⁺ Si) are used to serve as the bottom electrode. c, SHG spectrum on a ten-cycle HZO film, comparing poled versus unpoled SHG intensity. Spontaneous polarization is demonstrated by the presence of SHG—allowed only for inversion asymmetric systems—in unpoled ten-cycle HZO. This is consistent with PFM phase contrast in unpoled HZO regions (Fig. 2c), indicating elimination of the ‘wake-up’ effects for ferroelectricity in ultrathin HZO. The enhanced SHG contrast in poled films—possibly due to the electric field converting a small fraction of the film to the polar phase or aligning polar domains—indicates that the mechanism behind the SHG contrast is field-tunable. This field-enhanced SHG is consistent with ferroelectric origins and would probably eliminate SHG contrast from surface effects.