The quest for manipulation of magnetization on ultrafast timescales faces many technological challenges. Successful achievement thereof could shed light on novel fundamental phenomena, such as inertial magnetization dynamics, as well as accelerate technological advancements towards higher information processing rates. One of the recent approaches towards this end concerns excitation of magnetization dynamics via laser-induced picosecond acoustic pulses, which has given birth to the field of ultrafast magneto-acoustics. Considerable progress has been made in the field from an experimental point of view, as well as from the perspective of theoretical modelling. In this talk, we aim to review some of the aforementioned progress and propose the frequency dependent cooperativity parameter (strong coupling regime) to measure the efficiency of resonantly enhanced phonon-magnon interactions in the GHz-to-THz frequency range.
While most ultrafast time-resolved optical pump-probe experiments in magnetic materials reveal the spatially homogeneous magnetization dynamics of ferromagnetic resonance (FMR), here we explore the magneto-elastic generation of GHz-to-THz frequency spin waves (exchange magnons). Using analytical magnon oscillator equations, we apply time-domain and frequency-domain approaches to quantify the results of ultrafast time-resolved optical pump-probe experiments in free-standing ferromagnetic thin films. Simulations show excellent agreement with the experiment, provide acoustic and magnetic (Gilbert) damping constants and highlight the role of symmetry-based selection rules in phonon-magnon interactions. The analysis is extended to hybrid multilayer structures to explore the limits of resonant phonon-magnon interactions up to THz frequencies.
We present a simple experimental approach to generating and detecting surface-propagating magneto-elastic waves. Using the ultrafast optical transient grating geometry, we drive in-plane propagating surface acoustic waves which couple to, and resonantly drive, magnetization precession in thin magnetic films. The optical approach provides for the real-time detection of both elastic wave transients as well as the tightly coupled magnetization precession in independent detection channels and thus reveals the tight coupling between the two when an appropriate magnetic field is applied. We discuss the experimental geometry and resulting linear magneto-elastic responses. We briefly touch upon nonlinear magnetoelastic properties, which is the focus of our current work.
In order to investigate the ultrafast dynamics of free carriers generated in bulk dielectrics by intense femtosecond laser pulses we have designed a setup for ultrafast time-resolved imaging Mach-Zehnder interferometry. The application of the 2D-Fourier-transform technique allows us to accurately reconstruct the actual laser-induced phase shifts and transmission changes for the probe pulses, which provide the properties of free carriers. Interferometric measurements in high-purity fused silica clearly demonstrate that the dominant ionization mechanism for intensities below 10 TW/cm2 is multiphoton ionization.
Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern (Newton fringes). Optically sharp surfaces occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials (Si, graphite, Au, Al).
The formation of well-defined craters is a general feature of laser ablation with ultrashort laser pulses, indicative of a sharp ablation threshold. Results of a microscopic characterization of ablation craters on semiconductors after irradiation with single intense ultrashort laser pulses are presented.
Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern. Optically sharp surface occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials.
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