We irradiate the surface of pure fused silica with few-cycle pulses (duration < 10 fs, central wavelength at 800 nm, repetition of 400 kHz and a pulse energy of ca. 1 µJ) focused at a quasi grating incidence and observe a permanent densification in the irradiated region. A translation of the sample in the direction of the laser results in the direct writing of surface waveguides. The eigen modes of such structures exhibit a pronounced sensitivity to the refractive index of the immediate environment which we exploit to demonstrate direct refractive index sensing as well as plasmonic sensing.
Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization because it is active during the optical excitation and does not rely on secondary mechanisms to dissipate angular momentum [1]. The experimental detection of the optically induced spin transfer (OISTR) is challenging, as it requires access to the element-specific transient density of states around the Fermi energy.
In our joint theoretical and experimental work, we show that OISTR from Pt to Co governs the ultrafast demagnetization dynamics of a CoPt alloy [2]. Furthermore, we demonstrate that the analysis of the transient helicity dependent absorption at the resonant M2,3 transition of Co in the extreme ultraviolet spectral range reveals detailed information on the transient spin-split density of states, which we can directly compare to our theoretical simulations employing time-dependent density functional theory.
The comparison between the theoretical and experimental data allows us to conclude that the laser-driven spin current originates from the available minority states above the Fermi level in conjunction with the electric field of the laser pulse, making this a general phenomenon in all multi-component magnetic systems.
A further all-optical study on different 3d transition metal alloys corroborates that the number of available states above the Fermi level drives OISTR, opening the possibility to control demagnetization on the fastest time scale by the engineering of the density of states.
References
[1] J. K. Dewhurst et al., Nano Lett., 18 (2018)
[2] F. Willems et al., submitted (2019)
The recent development of sources able to deliver laser pulses with a duration of a few optical cycles has created many opportunities for fundamental research. Few-cycle laser pulse sources are now commercially available and are able to deliver energetic pulses (tens of micro-joules) at a MHz repetition rate. With such an extremely short pulse duration (<10 fs FWHM at 800 nm), the amount of energy required to reach the breakdown threshold in dielectrics is minimal, thus suggesting that few-cycle laser pulses are a very promising tool for reducing the heat affected zone and therefore the amount of thermo-induced stress during and after irradiation in transparent materials. In this article, the potential relevance of few-cycle laser pulses for microprocessing fused silica is examined. In particular, we demonstrate the fabrication of optical microstructures in the volume as well as on the surface of undoped fused silica.
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