Nonvolatile optical control of interlayer stacking order in 1T-TaS₂

Nonvolatile optical manipulation of material properties on demand is a highly sought-after feature in the advancement of future optoelectronic applications. Here, we unravel the nature of the single-laser-pulse induced hidden state in 1T-TaS₂ by systematically investigating the electronic structure evolution and the pulse-pair control throughout the reversible transition cycle. Our data indicate a mixed-stacking state involving two similarly low-energy interlayer orders, which is manifested as the charge density wave phase disruption. Furthermore, we elucidate distinct mechanisms underlying the bidirectional transformations — the ultrafast formation of the hidden state is initiated by a coherent phonon which triggers a competition between interlayer stacking orders, while its recovery is governed by the progressive domain evolution. Our work highlights the deterministic role of the competing interlayer orders in the nonvolatile phase transition in 1T-TaS₂, establishing all-optical engineering of stacking orders in low-dimensional materials as a viable strategy for achieving desirable nonvolatile electronic devices.