Demonstration of on-chip all-optical switching of magnetization in integrated photonics

Abstract

Ultrafast all-optical magnetization switching (AOS) holds great promise for nextgeneration spintronic memory and hybrid spintronic-photonic systems. However, most implementations to date rely on bulky free-space optical setups, limiting scalability and practical integration. As a critical step toward integrated applications, we demonstrate single-pulse AOS within a silicon nitride (Si3N4) photonic integrated circuit. Using trains of femtosecond laser pulses guided through on-chip waveguides, we achieve deterministic toggle switching in a sub-micron out-of-plane Co/Gd Hall cross patterned directly atop the photonic waveguide. Electrical readout via the anomalous Hall effect reveals a switching contrast of up to 90% for 500 nm-wide devices. In larger Hall crosses, the contrast decreases and switching becomes stochastic, consistent with spatially non-uniform optical absorption as confirmed by finite-element simulations. This behavior is hypothetically attributed to domain wall relaxation and thermally assisted (de)pinning processes within partially switched regions. Our results highlight the critical role of device scaling in achieving robust on-chip AOS and establish a foundation for ultrafast, energy-efficient, and fully integrated spintronic-photonic platforms.

Figures (7)

• Figure 1: Schematic and microscopic characterization of the integrated spintronic–photonic device. (a) Conceptual illustration of the device architecture, showing a patterned Hall cross structure integrated atop a Si3N4 waveguide. Extended Si3N4 stages are included to ensure a planar device. (b,c) Fabrication details of the sensing window: (b) Optical microscope image highlighting the Si3N4 waveguide (red), the surrounding stages, and the sensing window. (c) Cross-sectional schematic of the sensing window, illustrating the selectively etched SiO2 cladding. (d) Atomic force microscopy (AFM) image showing topographical details of the final device structure, with a Hall cross arm width of 500 nm aligned to the integrated Si3N4 waveguide and stage. (e) Conceptual illustration of the experimental setup for on-chip all-optical switching (AOS) measurements. A beam of femtosecond laser pulses is focused onto an edge coupler (edge illumination) to inject light into the Si3N4 waveguide, or orthogonally onto the Hall cross surface (front illumination). Optical switching of the Hall cross located on top of the waveguide is electrically detected via the anomalous Hall effect (AHE). See Methods for further details. (f) Top view of the device (outline in red) imaged by a CCD camera, in the presence of high repetition rate edge illumination and absence of back light, which visualizes light leakage radiating from the waveguide, with intense scattering observed at the Hall cross section.
• Figure 2: Analyses of experimental and simulated data for all-optical magnetization switching. (a) Time trace of the normalized anomalous Hall effect (AHE) signal from an on-chip 500 nm Hall cross device. The signal is normalized to the signal swing of a full magnetization switch induced by an external out-of-plane magnetic field, $\vec{H}_{\text{ext}}$. After the dashed red line, $\vec{H}_{\text{ext}}$ is removed and the device is illuminated with a train of femtosecond (fs) laser pulses at a repetition rate of 1 Hz. (b) Magnified view of the green-shaded segment in (a), highlighting the periodic toggle switching of a specific Hall cross region induced by fs-laser pulses at 1 Hz. (c) Time trace of the normalized AHE signal showing partial magnetization reversal in a 1 $\mathrm{\upmu}$m Hall cross device under similar laser excitation conditions. (d) Normalized AHE signal measured near the switching threshold energy ($0.85 E_{0}$) in the smaller 500 nm Hall cross. At this threshold, fs-laser pulse illumination at 1 Hz causes intermittent switching, zero crossings, and compensated magnetic states. Minor first-order drift corrections were applied to the signals in (a) and (c); further details are provided in S.I. \ref{['section:driftCorr']}. (e) Finite-difference time-domain (FDTD) simulation of the spatially imbalanced absorption profile in the Co layer, illustrating the non-uniform energy distribution that contributes to the partial switching and thermal damage observed in (a–d). The absorption density is normalized to the optical power in the waveguide away from the Hall cross. Switchable regions, calculated using a simplified microscopic three-temperature model, are indicated for different optical input energies, $E_{\text{in}}$. (f) Conceptual schematic depictions of probable magnetic domain wall formation and pinning at the device edges: (f.1), for the 1 $\mathrm{\upmu}$m Hall cross, corresponding to (c); (f.2), for the 500 nm Hall cross, corresponding to (a), (b), and (d), consistent with the observed stochastic switching behavior.
• Figure 3: (a) Time traces of normalized AHE signals for the 1 $\mathrm{\upmu}$m Hall cross device under front illumination at a 1 Hz repetition rate. Three pulse energies are shown: $E_{0}$, which slightly exceeds the switching threshold to achieve full magnetization reversal, $0.81 E_{0}$, and $0.67 E_{0}$. (b) Conceptual illustration of domain wall dynamics under near-threshold illumination.
• Figure 4: Raw (blue) and drift-corrected (red) normalized anomalous Hall effect signal.
• Figure 5: Results of microscopic three-temperature model (M3TM) simulations for a Co/Gd system. (a) Temporal evolution of the electron ($T_{\mathrm{e}}$, blue) and phonon ($T_{\mathrm{p}}$, green) temperatures following optical excitation. The Gaussian pulse energy density (red) is derived from finite-difference time-domain (FDTD) simulations. Dashed lines indicate the Curie temperatures of cobalt ($T_{\mathrm{C,Co}} = 1388$ K) and gadolinium ($T_{\mathrm{C,Gd}} = 292$ K). (b) Corresponding relative magnetization dynamics of Co (blue) and Gd (red) on the same timescale, showing magnetization reversal in both layers with respect to their initial orientation.