Despite their partial ionic nature, many layered diatomic crystals avoid internal electric polarization by forming a centrosymmetric lattice at their optimal van-der-Waals stacking. Here, we report a stable ferroelectric order emerging at the interface between two naturally-grown flakes of hexagonal-boron-nitride, which are stacked together in a metastable non-centrosymmetric parallel orientation. We observe alternating domains of inverted normal polarization, caused by a lateral shift of one lattice site between the domains. Reversible polarization switching coupled to lateral sliding is achieved by scanning a biased tip above the surface. Our calculations trace the origin of the phenomenon to a subtle interplay between charge redistribution and ionic displacement, and provide intuitive insights to explore the interfacial polarization and its unique “slidetronics” switching mechanism.

Striking a metallic nanostructure with a short and intense pulse of light excites a complex out‐of‐equilibrium distribution of electrons that rapidly interact and lose their mutual coherent motion. Due to the highly nonlinear dynamics, the photo‐excited nanostructures can generate energetic photons beyond the spectrum of the incident beam, where the shortest pulse duration is traditionally expected to induce the greatest nonlinear emission. Here, these photo‐induced extreme ultrafast dynamics are coherently controlled by spectrally shaping a sub‐10 fs pulse within the timescale of coherent plasmon excitations. Contrary to the common perception, it is shown that stretching the pulse to match its internal phase with the plasmon‐resonance increases the second‐order nonlinear emission by >25%. The enhancement is observed only when shaping extreme‐ultrashort pulses (<20 fs), thus signifying the coherent electronic nature as a crucial source of the effect. A detailed theoretical framework that reveals the optimal pulse shapes for enhanced nonlinear emission regarding the nanostructures’ plasmonic‐resonances is provided. The demonstrated truly‐coherent plasma control paves the way to engineer rapid out‐of‐equilibrium response in solids state systems and light‐harvesting applications.

Striking a metallic nanostructure with a short and intense pulse of light excites a complex out-of-equilibrium distribution of electrons that rapidly interact and lose their mutual coherent motion. Due to the highly nonlinear dynamics, the photo-excited nanostructures may further emit energetic photons beyond the spectrum of the incident beam, where the shortest pulse duration is traditionally expected to induce the greatest nonlinear emission. Here, we coherently control these photo-induced extreme ultrafast dynamics by spectrally shaping a sub-10 fs pulse within the timescale of coherent plasmon excitations. Contrary to the common perception, we show that stretching the pulse to match its internal phase with the plasmon-resonance increases the second-order nonlinear emission by > 25%. The enhancement is observed only when shaping extreme-ultrashort pulses (< 20 fs), thus signifying the coherent electronic nature as a crucial source of the effect. We provide a detailed theoretical framework that reveals the optimal pulse shapes for enhanced nonlinear emission regarding the nanostructures’ plasmonic-resonances. The demonstrated truly-coherent plasma control paves the way to engineer rapid out-of-equilibrium response in solids state systems and light-harvesting applications.

We experimentally coherent control the extreme ultrafast excitation in plasmonic nanostructures within their coherence lifetime. We predict and demonstrate a significant enhancement of the nonlinearity greater than the nonlinearity induced by a maximally compressed pulse.