Ultrafast angular momentum transfer in multisublattice ferrimagnets.

Femtosecond laser pulses can be used to induce ultrafast changes of the magnetization in magnetic materials. However, one of the unsolved questions is that of conservation of the total angular momentum during the ultrafast demagnetization. Here we report the ultrafast transfer of angular momentum during the first hundred femtoseconds in ferrimagnetic Co0.8Gd0.2 and Co0.74Tb0.26 films. Using time-resolved X-ray magnetic circular dichroism allowed for time-resolved determination of spin and orbital momenta for each element. We report an ultrafast quenching of the magnetocrystalline anisotropy and show that at early times the demagnetization in ferrimagnetic alloys is driven by the local transfer of angular momenta between the two exchange-coupled sublattices while the total angular momentum stays constant. In Co0.74Tb0.26 we have observed a transfer of the total angular momentum to an external bath, which is delayed by ~150 fs.

Distinguishing the ultrafast dynamics of spin and orbital moments in solids.

For an isolated quantum particle, such as an electron, the orbital (L) and spin (S) magnetic moments can change provided that the total angular momentum of the particle is conserved. In condensed matter, an efficient transfer between L and S can occur owing to the spin-orbit interaction, which originates in the relativistic motion of electrons. Disentangling the absolute contributions of the orbital and spin angular momenta is challenging, however, as any transfer between the two occurs on femtosecond timescales. Here we investigate such phenomena by using ultrashort optical laser pulses to change the magnetization of a ferromagnetic film and then probe its dynamics with circularly polarized femtosecond X-ray pulses. Our measurements enable us to disentangle the spin and orbital components of the magnetic moment, revealing different dynamics for L and S. We highlight the important role played by the spin-orbit interaction in the ultrafast laser-induced demagnetization of ferromagnetic films, and show also that the magneto-crystalline anisotropy energy is an important quantity to consider in such processes. Our study provides insights into the dynamics in magnetic systems as well as perspectives for the ultrafast control of information in magnetic recording media.

Surface plasmon dynamics in arrays of subwavelength holes: the role of optical interband transitions.

Using femtosecond optical spectroscopy, we study the ultrafast dynamics of the surface plasmon polaritons in gold arrays of subwavelength holes. A large time dependent spectral broadening and shift of the surface plasmon resonances are reported. The experimental results are modeled by the diffraction of a transverse electromagnetic field through the nanostructure, taking into account both the electron dynamics near the interband transitions and the Drude-like conductivity of the metal. Our analysis, using either a theoretical or an experimentally determined dielectric function of gold, suggests that the losses propagation in plasmonic devices is strongly influenced by intrinsic and extrinsic electron scattering mechanisms.

Optical response of periodically modulated nanostructures near the interband transition threshold of noble metals.

We investigate the influence of the core d-electrons on the spectral optical response of arrays of sub-wavelength holes near the transition from the d-band to the Fermi level of noble metals (d?E(F)). Our model shows that, due to the dispersion of the metal dielectric function near d?E(F), the first order peaks in the enhanced spectral transmission shift nonlinearly as a function of the period of the nanostructure. In addition, we outline in that spectral region an apparent large resonance which does not depend on the geometrical parameters of the nanostructure. It is shown to correspond to the transparency window resulting from the spectral superposition of the large absorption associated to the core d-electrons and high reflectivity due to the conduction electrons. The analysis is performed for gold, copper and silver nanostructures.

Analytical model of the optical response of periodically structured metallic films.

In this paper we investigate the optical response of periodically structured metallic films constituted of sub-wavelength apertures. Our approach consists in studying the diffraction of transverse magnetic polarized electromagnetic waves by a one-dimensional grating. The method that we use is the Rigorous Coupled Waves Analysis allowing us to obtain an analytical model to calculate the diffraction efficiencies. The zero and first order terms allow determining the transmission, reflectivity and absorption of symmetric or asymmetric nanostructures surrounded either by identical or different dielectric media. For both type of nanostructures the spectral shape of the enhanced resonant transmission associated to surface plasmons displays a Fano profile. In the case of symmetric nanostructures, we study the conditions of formation of coupled surface plasmon-polaritons as well as their effect on the optical response of the modulated structure. For asymmetric nanostructures, we discuss the non-reciprocity of the reflectivity and we investigate the spectral dependency of the enhanced resonant transmission on the refractive index of the dielectric surrounding the metal film.