Multiphysics simulation of all-optical switching and sequential magneto-optic readout in an integrated Si$_3$N$_4$/GdFeCo waveguide
Year: 2026
Type: preprint
Abstract: In an integrated all-optical magnetic device, a guided pulse writes a magnetic state and a later optical signal reads it out. We study both steps for an 8 nm GdFeCo film inserted across a Si$_3$N$_4$ waveguide. A three-dimensional dispersive finite-difference time-domain calculation gives the energy absorbed from an 800 nm pump pulse. This energy drives a four-temperature model and calibrated stochastic two-sublattice Landau-Lifshitz-Bloch dynamics. After relaxation, a separate 532 nm probe is propagated through the fixed magnetic state for the optical readout. At the reference pump fluence, the model reverses 0.859 of the film from either saturated state. The switched fraction remains within 0.81-0.87 over a 2.4-fold range of incident fluence. A threshold rule applied to the local absorption map reproduces the five three-dimensional simulation points to within $8\times10^{-4}$, showing that, within the model, incomplete reversal results from non-uniform guided-mode absorption. When the film starts from zero magnetization, thermal noise orients the recovered local moments at random; over 16 realizations, $0.502 \pm 0.004$ of the cells end with a negative $x$ component of the FeCo moment. Among the simulated states, the probe transmission, reflection, and absorption fractions vary by at most 0.02. The magnetic Faraday and Kerr rotations are much more sensitive and become about 12 and 130 times smaller, respectively, in the stochastically recovered state than in a saturated film. The reflected signal also contains a magnetization-independent geometric background, which can be removed by subtracting a reference signal. These results link the switching efficiency to the guided-mode absorption profile and predict polarization-based readout of the simulated magnetic states. The model assumes a homogeneous film and neglects spatial exchange stiffness and subsequent domain coarsening.
Source: arXiv (Cornell University)
Institution: Padjadjaran University
Language: English
Subfield: Condensed Matter Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funder: Universitas Padjadjaran
OpenAlex ID: W7204153617