Ultrafast light-driven magnetization control on a nanometer length scale is critical for achieving competitive bit sizes in next-generation information encoded technology.
Scientists from the Max Born Institute in Berlin and the large-scale facility Elettra in Trieste, Italy, have conclusively demonstrated the superfast occurrence of all-optical switching by producing a nanometer-scale grating through the interference of two intense ultraviolet spectral pulses.
The physics of optically driven magnetization dynamics on the femtosecond time scale has piqued the attention of many researchers for two primary reasons: initially, acknowledging the underlying mechanism of non-equilibrium, ultrafast spin dynamics, and second, possible uses in the next generation of information systems to meet the demand for faster and more efficient energy data storage devices.
One of the most intriguing and appealing processes for this endeavor is all-optical switching (AOS), in which the magnetization state can be inserted between two directions with a single femtosecond laser pulse, serving as “0 seconds” and “1 second.”
While acceptance of AOS temporal control has advanced quickly, knowledge of ultrafast transport phenomena on the nanoscale, which is critical for realizing all-optical magnetic reversal in technological applications, has remained confined due to optical radiation wavelength constraints.
In transient grating experiments, one elegant way to overcome these constraints is to decrease the wavelengths to the severe ultraviolet (XUV) spectral range. This technique, which is based on the interference of two XUV beams, has been spearheaded at the EIS-Timer beamline of the FERMI free-electron laser (FEL) in Trieste, Italy.
Scientists from the Max-Born Institute in Berlin and the FEL facility FERMI have excited a transient magnetic grating (TMG) in a ferrimagnetic GdFe alloy sample with a periodicity of ΛTMG = 87 nm.
By diffracting a time-delayed third XUV pulse optimized to the Gd N-edge at 8.3 nm, the spatial transformation of the magnetization grating was investigated (150 eV). Since AOS has a highly nonlinear reaction to excitation, the emerging magnetic grating’s symmetry adjustments should be unique from the preliminary sinusoidal excitation pattern.
This data is encoded in the diffraction pattern: if the magnetization response to the excitation is linear and there is no AOS, a sinusoidal TMG is stimulated and the second diffraction order is suppressed.
If AOS takes place, however, the grating shape changes, enabling a more prominent second-order diffraction intensity. In other statements, the scientists outlined the intensity ratio between the second and first orders (R21) as a fingerprint noticeable for AOS in diffraction experiments.
In the figure above, (a) and (b) display the temporal development of the diffracted first and second-order intensities, correspondingly. The scientists discover comparable decay times of τRE, first = (81 ± 7) ps and τRE, second = (90 ± 24) ps, consistent with lateral heat diffusion rates of the nanoscale gratings. Here, (c) displays the ratio R21 as a function of the excitation fluence at a constant pump-probe delay of 50 ps.
The research group discovered a continuous and small value of R21 of around 1% for low fluence below the AOS threshold. However, as the excitation is increased, R21 shows a constant rise to 8%, offering the first proof for AOS on the nanometer length scale.
The (d) in the figure depicts the ratio R21 as a function of time for two different excitation fluences. R21 showcases an increased and consistent ratio of about 6% over the measured time interval of 150 ps for the larger fluence (red circles), indicating a stable magnetic structure, which is understood as optically reversed domains, i.e. AOS.
Eventually, the scientists were able to verify their findings by using time-resolved Faraday microscopy to perform complementary all-optical measurements in actual space.
Ultrafast lateral transport processes are anticipated to stabilize the excitation gradients within a few picoseconds in potential transient grating experiments with considerably smaller periodicities down to 20 nm, defining the fundamental spatial limits of AOS.
Yao, K., et al. (2022) All-Optical Switching on the Nanometer Scale Excited and Probed with Femtosecond Extreme Ultraviolet Pulses. Nano Letters. doi.org/10.1021/acs.nanolett.2c01060.