Electrically switchable ferron upconversion in a van der Waals ferroelectric

Abstract

Nonlinear phononics provides a powerful ultrafast route to control lattice excitations, enabling access to hidden quantum orders, phononic computing, and quantum transduction. However, dynamic control of anharmonic phonon interactions remains limited, as these interactions are typically fixed by the equilibrium crystal lattice and lack external tunability. Emergent ferrons in ferroelectrics, which are collective oscillations of the spontaneous electric polarization, may offer a promising platform to overcome this limitation by combining intrinsic phononic nonlinearity with direct electrical control of the ferroelectric order parameter. Here we report electrically controllable nonlinear ferron upconversion in the van der Waals ferroelectric NbOI2. We show that resonant THz excitation of a 3.1 THz ferron drives coherent upconversion to a 7.0 THz optical phonon. Using two-dimensional THz spectroscopy, we directly resolve off-diagonal coupling features and establish the nonlinear upconversion pathway. Supported by first-principles calculations and analytical modeling, we identify the microscopic origin as a cubic anharmonic lattice coupling. Importantly, in situ electric-field switching enables nonvolatile control of both the ferron dynamics and the associated upconversion process. The phase reversal and hysteretic behavior across the coercive fields establish that the ferron-mediated nonlinear phononic interaction is strongly dependent on the underlying ferroelectric order parameter. These results introduce ferron upconversion as a new and universal regime of nonlinear phononics in ferroelectrics and establish an electrically programmable platform for coherent lattice control, paving the way for ferronic information processing and quantum phononic transduction.

Figures (4)

• Figure 1: Coherent THz excitation of ferron and optical phonon modes in vdW ferroelectric NbOI2. (a) Schematic of the THz pump–optical probe experiment used to phononic excitations in NbOI2, with Nb (green), I (blue), and O (red). Arrows indicate the calculated atomic displacement patterns of the A-symmetry 3.1 THz ferron mode and the B-symmetry 7.0 THz optical phonon mode. (b) Time-resolved optical reflectivity change under THz excitation with the field aligned along the ferroelectric axis, revealing coherent oscillations of the ferron mode at 3.1 THz. (c) Time-resolved polarization rotation signal showing coherent oscillations at both the ferron mode (3.1 THz) and a higher-frequency optical phonon at 7.0 THz. The latter lies outside the spectral bandwidth of the THz pump, indicating a nonlinear excitation pathway.
• Figure 2: Polarization and fluence dependence of THz-driven coherent phonon excitations in NbOI2. (a) Polarization-resolved time-domain traces of the THz-induced optical polarization rotation measured for different probe polarization angles relative to the crystal axes. (b) Extracted amplitude of the ferron mode (3.1 THz) and higher frequency mode (7.0 THz) as a function of probe polarization angle, revealing distinct symmetry-dependent detection responses. (c,d) THz field dependence of the Fourier-transformed peak amplitudes for the (c) 3.1 THz ferron mode and (d) 7.0 THz optical phonon mode. The ferron mode exhibits approximately linear scaling with field ($\gamma\approx 1$), consistent with direct resonant excitation. The 7.0 THz mode exhibits a sublinear dependence indicative of nonlinear phononic coupling.
• Figure 3: Two-dimensional THz spectroscopy revealing the ferron and longitudinal optical phonon coupling in NbOI2. (a) Schematic of the two-dimensional THz spectroscopy. Two single-cycle THz pump pulses generated via optical rectification in a DSTMS crystal are separated by an inter-pump delay $\tau$ and and focused collinearly onto the NbOI2 sample. The induced polarization rotation of an 800 nm probe pulse is recorded as a function of the probe delay time $t$ and inter-pump delay $\tau$. (b) Double THz pump-induced optical polarization rotation plotted as a function of $t$ and $\tau$. (c) Two-dimensional fast Fourier transform (2D FFT) of the nonlinear signal as a function of detection frequency $f_\text{det}$ and excitation frequency $f_\text{exc}$. A pronounced off-diagonal feature appears at $f_\text{det}= 7.0$ THz when the sample is driven at $f_\text{exc}= 3.1$ THz, unambiguously revealing nonlinear coupling between the ferron and the higher frequency optical phonon. (d) Calculated lattice energy landscape as a function of the ferron mode amplitude $Q_3$ for several values of the higher frequency optical phonon mode amplitude $Q_7$. The energy for zero displacement is taken as the reference point.
• Figure 4: In situ electric-field control of ferron upconversion in NbOI2. (a) Schematic of the device used to apply a static electric field to the NbOI2 flake. The inset shows an optical image of the device with graphene electrodes separated by 25 $\mu m$. The electric field is applied along the polar b-axis. (b) Time-domain traces of the THz pump-induced optical polarization rotation change ($\Delta\theta$) measured at 0 kV/cm before and after electrical polarization switching. Reversing the ferroelectric polarization leads to a clear inversion of the ferron oscillation phase. (c) Fourier-transformed spectra of the ferron at 3.1 THz and the optical phonon at 7.0 THz at 0 kV/cm before and after polarization reversal. (d) Electric-field dependence of complex amplitude of upconverted 7 THz mode (left axis), together with the simultaneously recorded source-drain current I$\textsubscript{sd}$ (right axis). The sweep direction is indicated by the black arrows. The complex amplitude exhibits a hysteretic response that follows the ferroelectric polarization switching, evidencing nonvolatile electrical control of the ferron upconversion process.