M. Savoini

Self‐Assembled Organic Microfibers for Nonlinear Optics

While highly desired in integrated optical circuits, multiresponsive and tunable nonlinear optical (NLO) active 1D (sub)wavelength scale superstructures from organic materials are rarely reported due to the strong tendency of organic molecules to self-assembly in centrosymmetric modes. Here a solution-processed assembly approach is reported to generate non-centrosymmetric single-crystalline organic microfibers with a cumulative dipole moment for anisotropic combined second- and third-order NLO.

Nanoscale Confinement of All-Optical Magnetic Switching in TbFeCo - Competition with Nanoscale Heterogeneity

Single femtosecond optical laser pulses, of sufficient intensity, are demonstrated to reverse magnetization in a process known as all-optical switching. Gold two-wire antennas are placed on the all-optical switching film TbFeCo. These structures are resonant with the optical field, and they create a field enhancement in the near-field which confines the area where optical switching can occur. The magnetic switching that occurs around and below the antenna is imaged using resonant X-ray holography and magnetic circular dichroism. The results not only show the feasibility of controllable switching with antenna assistance but also demonstrate the highly inhomogeneous nature of the switching process, which is attributed to the process depending on the materials heterogeneity.

Nanoscale sub-100 picosecond all-optical magnetization switching in GdFeCo microstructures.

Ultrafast magnetization reversal driven by femtosecond laser pulses has been shown to be a promising way to write information. Seeking to improve the recording density has raised intriguing fundamental questions about the feasibility of combining ultrafast temporal resolution with sub-wavelength spatial resolution for magnetic recording. Here we report on the experimental demonstration of nanoscale sub-100 ps all-optical magnetization switching, providing a path to sub-wavelength magnetic recording. Using computational methods, we reveal the feasibility of nanoscale magnetic switching even for an unfocused laser pulse. This effect is achieved by structuring the sample such that the laser pulse, via both refraction and interference, focuses onto a localized region of the structure, the position of which can be controlled by the structural design. Time-resolved photo-emission electron microscopy studies reveal that nanoscale magnetic switching employing such focusing can be pushed to the sub-100 ps regime.Abstract The recently discovered magnetization reversal driven solely by a femtosecond laser pulse hasbeen shown to be a promising way to record information at record breaking speeds. Seeking toimprove the recording density has raised intriguing fundamental question about the feasibility tocombine the ultrafast temporal with sub-wavelength spatial resolution of magnetic recording. Herewe report about the rst experimental demonstration of sub-di raction and sub-100 ps all-opticalmagnetic switching. Using computational methods we reveal the feasibility of sub-di raction mag-netic switching even for an unfocused incoming laser pulse. This e ect is achieved via structuringthe sample such that the laser pulse experiences a passive wavefront shaping as it couples andpropagates inside the magnetic structure. Time-resolved studies with the help of photo-emissionelectron microscopy clearly reveal that the sub-wavelength switching with the help of the passivewave-front shaping can be pushed into sub-100 ps regime.

Ultrafast time-resolved magneto-optical imaging of all-optical switching in GdFeCo with femtosecond time-resolution and a μm spatial-resolution

We developed an ultrafast time-resolved magneto-optical (MO) imaging system with several millidegree resolution of light polarization angle, 100 fs time-resolution, and a micrometer spatial resolution. A CCD camera with about 10(6) pixels is used for detection and MO images with an absolute angle of the light polarization are acquired by the rotating analyzer method. By optimizing the analysis procedure with a least square method and the help of graphical processor units, this novel system significantly improves the speed for MO imaging, allowing to obtain a MO map of a sample within 15 s. To demonstrate the strength of the technique, we applied the method in a pump-and-probe experiment of all-optical switching in a GdFeCo sample in which we were able to detect temporal evolution of the MO images with sub-picosecond resolution.

All-optical subdiffraction multilevel data encoding onto azo-polymeric thin films

By exploiting photoinduced reorientation in azo-polymer thin films, we demonstrate all-optical polarization-encoded information storage with a scanning near-field optical microscope. In the writing routine, five-level bits are created by associating different bit values to different birefringence directions, induced in the polymer after illumination with linearly polarized light. The reading routine is then performed by implementing polarization-modulation techniques on the same near-field microscope in order to measure the encoded birefringence direction.

Circular Dichroism Probed by Two-Photon Fluorescence Microscopy in Enantiopure Chiral Polyfluorene Thin Films

Two-photon fluorescence scanning confocal microscopy sensitive to circular dichroism with a diffraction-limited resolution well below 500 nm is demonstrated. With this method, the spatial variation of the circular dichroism of thermally annealed chiral polyfluorene thin films has been imaged. We observed circular dichroism associated with submicrometer-sized domains showing helicoidally twisted macromolecular organization. Domains with opposite chiroptical properties, corresponding to left- or right-handed molecular organization, coexist in the film. Our results are consistent with those obtained by one-photon imaging and illustrate the potential of two-photon imaging for use in studying helical macromolecular organization.

Optical energy optimization at the nanoscale by near-field interference

Employing plasmonic antennas for subdiffraction focusing of light on recording media requires to take into account the complete structure of the medium, including dielectric cover layers. We find, with finite difference time domain simulations, that optical energy transfer to the magnetic recording layer is most efficient for an off-resonant antenna. Furthermore, we show that the focal spot in the magnetic film is well below the diffraction limit, making nanoscale all-optical magnetic data recording achievable.

Optical excitation of thin magnetic layers in multilayer structures

To the Editor — Ultrafast laser-induced demagnetization is often ascribed to the absorption of light by electrons in the magnetic medium, with subsequent redistribution of energy and angular momentum1. Recently, an alternative mechanism was introduced, namely, a superdiffusive spin transport between adjacent layers2. Eschenlohr et al. claim to validate this mechanism1. In an optically excited Au/Ni layered structure, they found that the demagnetization of Ni can be up to 80%. It was calculated that light absorption in the magnetic layer is negligible compared with the absorption in the gold capping layer. Thus they conclude: “In strong contrast to existing knowledge we find that direct optical excitation is not a precondition for ultrafast demagnetization”. Knowledge of the exact absorption profile of light in multilayered thin films is crucial to disentangle thermal and non-thermal phenomena, such as in ref. 1. In this Correspondence, a comprehensive approach is given for absorption profile calculations for multilayer structures. In particular, we show that a crucial error was made in the calculations in ref. 1, which led to a misinterpretation of the experimental results, and a conclusion that cannot be substantiated with the presented experiment. The light intensity I(z) at position z in a material is defined by the Poynting vector (equation 24 in ref. 3), and is given by

Near-field circular polarization probed by chiral polyfluorene

We demonstrate that a high degree of circular polarization can be delivered to the near field (NF) of an aperture at the apex of hollow-pyramid probes for scanning optical microscopy. This result is achieved by analyzing the dichroic properties of an annealed thin polymer film containing a chiral polyfluorene derivative, placed in close proximity to the optical probe. We also prove that the degree of circular polarization in the probe NF does not depend in a significant way on the shape of the aperture, at variance with the far-field behavior. These results demonstrate the feasibility of nano-optics applications exploiting circularly polarized NFs.

Ultrafast Formation of a Charge Density Wave State in 1T-TaS_{2}: Observation at Nanometer Scales Using Time-Resolved X-Ray Diffraction.

C. Laulhé, 2, ∗ T. Huber, G. Lantz, 3 A. Ferrer, S.O. Mariager, S. Grübel, J. Rittmann, J.A. Johnson, V. Esposito, A. Lübcke, † L. Huber, M. Kubli, M. Savoini, V.L.R. Jacques, L. Cario, B. Corraze, E. Janod, G. Ingold, P. Beaud, S.L. Johnson, and S. Ravy Synchrotron SOLEIL, L’Orme des Merisiers, Saint Aubin BP 48, F-91192 Gif-sur-Yvette, France Université Paris-Saclay (Univ. Paris-Sud), F-91405 Orsay Cedex, France Institute for Quantum Electronics, Physics Department, ETH Zurich, CH-8093 Zurich, Switzerland Laboratoire de Physique des Solides, Université Paris-Sud, CNRS, UMR 8502, F-91405 Orsay, France Swiss Light Source, Paul Scherrer Institute, CH-5232, Villigen, Switzerland Institut des Matériaux Jean Rouxel UMR 6502, Université de Nantes, 2 rue de la Houssinière, F-44322 Nantes, France (Dated: September 29, 2018)