In-plane optically tunable magnetic states in 2D materials via tailored femtosecond laser driving

Abstract

It is well established that light can control magnetism in matter, e.g. via the inverse Faraday effect or ultrafast demagnetization. However, such control is typically limited to magnetization transverse to light's polarization plane, or out-of-plane magnetism in 2D materials, while in-plane magnetic moments have remained largely unexplored. This is due to the difficulty of generating electronic orbital angular momentum components within light's polarization plane. Here we overcome this limitation, demonstrating complete three-dimensional, all-optical control of magnetism in 2D materials. Using first-principles simulations, we show that a tailored, two-color laser field can induce and steer magnetic moments in any direction with the relative angle between the laser polarizations playing a key parameter in coherent control. We analyze the physical mechanism of this process and show that it arises from a simultaneous breaking of time-reversal and spatial-inversion symmetries in the two-color laser. In-plane orbital moments are introduced via non-zero out-of-plane longitudinal photogalvanic currents enabled by broken inversion and mirror symmetries, while time-reversal symmetry breaking enables build-up of spin-rotation processes through spin-orbit coupling, translating the orbital moments to transient magnetism. Our findings demonstrate a full 3D coherent control scheme for transient magnetic states on femtosecond timescales driven by tailored lasers, and can be used to develop novel spectroscopies for magnetism, all-optical magnetic switching for ultrafast spintronics, and novel information storage capabilities.

Figures (4)

• Figure 1: Magnetism dynamics induced by two-color field in BiH. (a) Illustration of bichromatic laser field: a circularly polarized laser with frequency $\omega_1$ and a linearly polarized laser with frequency $\omega_2$ are superimposed, $\omega_2 = 2\omega_1$, with $\omega_1$ corresponding to 400 nm light. (b) structure of BiH, where purple atoms represent bismuth and blue atoms represent hydrogen. (c) Calculated induced magnetism in BiH induced by the two-color field. The external field is applied over a duration of 8.34 fs. It comprises a two-color pulse combining a circularly polarized fundamental field ($\omega_1$) and a linearly polarized second harmonic ($\omega_2 = 2\omega_1$) oriented along the $x$-axis. Laser field is set to an intensity of $10^{11}~\mathrm{W/m^2}$, with relative strength parameters $\delta_1 = \delta_2 = 1$.
• Figure 2: Coherent control of femtosecond laser-induced magnetism in BiH. (a) Dynamics of $S_y$ as a function of the relative polarization angle $\varphi$. The simulation employs a 6.67 fs two-color pulse ($\omega_2 = 2\omega_1$) with an intensity of $10^{11}~\mathrm{W/m^2}$. The relative strength parameters are $\delta_1 = \delta_2 = 1$, and the polarization vector of the second harmonic is defined as $\hat{\mathbf{e}}_{2}=(\cos\varphi \sin\theta, \sin\varphi \sin\theta, \cos\theta)$). (b) Dependence of net induced magnetism on $\varphi$. (c) Dynamics of spin under two-color pulse ($\omega_2 = 2\omega_1$) with a duration of 6.67 fs, the linearly polarized second harmonic oriented along the $x$-axis. The laser intensity is set to $10^{11}~\mathrm{W/m^2}$, with relative strength parameters $\delta_1 = 0.4$ and $\delta_2 = 1$. (d) Relation between induced magnetism and driving laser electric field amplitude indicating a highly nonlinear process.
• Figure 3: Physical mechanism of induced in-plane magnetism. (a) Femtosecond dynamics of out-of-plane photocurrents, $j_z(t)$, labeled with blue and $S_y(t)$ labeled with red, deeper color indicates stronger laser intensity ($10^{10}\sim10^{11}$ W/m$^2$), other parameters correspond to those in Fig. 1 (c). (b) Relation between amplitude of laser-induced $j_z$ and $S_y$ terms. (c) and (d) Induced magnetization dynamics and $j_z(t)$ (laser parameters correspond to those in in Fig. 1 (c)) in (c) BiH (mirror asymmetric) and (d) pristine flat bismuth (mirror symmetric).
• Figure 4: Induced in-plane magnetism in Transition metal dichalcogenides (TMDs) and Janus TMDs. Induced photocurrents (laser parameters correspond to those in Fig. 3(c) except the laser duration time is 10 fs) in (a) WTe$_2$ and (c) WSeTe (Janus TMD). Induced magnetism dynamics in (b) WTe$_2$ and (s) WSeTe.