Ultra-fast light-guided control of magnetization at the nanometer-length scale is key to achieving competitive bit sizes in next-generation data storage technology. Researchers from the Max Born Institute in Berlin and from the large-scale facility Elettra in Trieste, Italy, have successfully demonstrated the ultra-fast emergence of all-optical switching by generating a nanometer-scale grating through the interference of two pulses in the extreme ultraviolet spectral range.
The physics of optically driven magnetization dynamics on the femtosecond timescale is of great importance for two main reasons: first, for a deeper understanding of the fundamental mechanisms of non-equilibrium, ultrafast spin dynamics, and second, for its possible application in the next generation of information technology with a vision to meet the need for both faster and more energy-efficient data storage devices.
All-optical switching (AOS) is one of the most interesting and promising mechanisms for this endeavour, where the magnetization state between two directions can be reversed with a single femtosecond laser pulse, serving as “0s” and “1s”. Although the understanding of the temporal control of AOS has advanced rapidly, the knowledge about nanoscale ultrafast transport phenomena important for the realization of all-optical magnetic reversal in technological applications has been limited due to the wavelength limitations of optical radiation. An elegant way to overcome these limitations is to reduce the wavelengths to the extreme ultraviolet (XUV) spectral range in transient lattice experiments. This technique is based on the interference of two XUV rays leading to a nanoscale excitation pattern and was developed at the EIS-Timer beamline of the free electron laser (FEL) FERMI in Trieste, Italy.
Now, researchers from the Max-Born-Institute, Berlin and the FEL facility FERMI have generated a transient magnetic lattice (TMG) with a periodicity of ΛTMG = 87 nm in a ferrimagnetic GdFe alloy sample. The spatial evolution of the magnetization grating was examined by diffracting a time-delayed, third XUV pulse tuned to the Gd N edge at a wavelength of 8.3 nm (150 eV). Since AOS shows a strong nonlinear response to the excitation, characteristic symmetry changes of the evolving magnetic lattice, different from the initial sinusoidal excitation pattern, are expected. This information is encoded directly in the diffraction pattern: in the case of a linear magnetization response to the excitation and no AOS, a sinusoidal TMG is induced and the second diffraction order is suppressed. However, when AOS occurs, the shape of the grating changes, allowing for a pronounced second-order diffraction intensity. In other words, the researchers identified the intensity ratio between the second and first order (R21) as a fingerprint observable for AOS in diffraction experiments.
In the above image, a) and b) show the temporal evolution of the diffracted first and second order intensities, respectively. The researchers found similar decay times of τRE,first = (81 ± 7) ps and τRE,second = (90 ± 24) ps, consistent with lateral heat diffusion rates of the nanoscale lattices. c), shows the ratio R21 as a function of excitation fluidity at a constant pump-probe delay of 50 ps. For a low fluence below the threshold of AOS, the research team observed a constant and small value of R21 of about 1%. However, by increasing the excitation, R21 shows a steady increase to ~8%, providing the first evidence for AOS on the nanometer length scale. The ratio R21 as a function of time is shown in d) for two selected excitation influences. For the larger fluence (red circles), R21 shows an increased and constant ratio of about 6% over the measured time interval of 150 ps, indicating a stable magnetic structure, interpreted as optically inverted domains, i.e. AOS. Finally, the researchers were able to confirm their observations by using complementary all-optical measurements in real space using time-resolved Faraday microscopy.
In the future transient schedule experiments with significantly smaller periodicities up to
The research was published in Nano letters†
Kelvin Yao et al, All-Optical Switching at the Nanometer Scale Excited and Probes with Femtosecond Extreme Ultraviolet Pulses, Nano letters (2022). DOI: 10.1021/acs.nanolet.2c01060
Provided by Max Born Institute
Quote: Full optical switching at the nanometer scale (2022, June 15) retrieved June 15, 2022 from https://phys.org/news/2022-06-all-optical-nanometer-scale.html
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