Fig. 1: Polymorphic transition in MoTe₂ induced by high-field THz pulses.

a Schematic energy landscape and top view of lattice structures of 2H-MoTe₂ (left), the intermediate state (middle), and 1\({T}^{{\prime} }\)-MoTe₂ (right). E_barrier is the potential barrier between the 2H and 1\({T}^{{\prime} }\)-phase. b Cross-sectional schematic illustration of a 2H-MoTe₂ crystal spanning an insulating gap between deposited gold strips, which serve as a THz field enhancement structure. The monolayer 2H-MoTe₂ crystal is encapsulated between top and bottom h-BN. THz pulses are incident from the bottom side of the fused silica substrate. c Field strength calculation of the THz enhancement structure. Numerical simulation results showing THz field enhancement by a factor of 20–50 in significant regions in and near the gap between the gold strips. The enhancement factor is defined as the ratio between the actual and incident electric field. d Optical micrograph of a monolayer sample, including a 15-nm-thick top h-BN (white dashed line), a monolayer 2H-MoTe₂ (green dashed line), and a 5-nm-thick bottom h-BN (blue dashed line), all spanning a 1.8-μm insulating gap (dark horizontal line) between the top and bottom gold strips. e Raman spectra of monolayer MoTe₂ after successive THz pulse irradiation with free-space field amplitudes of 270 kV/cm. The A_g mode of the 1\({T}^{{\prime} }\) phase sets in after 5 THz pulses and dominates upon further irradiation. By contrast, the A_1g and E_2g modes of the 2H phase are strongly reduced. The reduction of the overall Raman intensity in monolayer samples may be related to compromised crystallinity after the irradiation. The Raman measurements were conducted with a 1-μm-diameter laser spot and were therefore averaged over regions that had been subjected to differently enhanced THz field strengths. The two dashed lines are aligned with the A_g and A_1g mode of the 1\({T}^{{\prime} }\) and 2H-phase, respectively.