Ilya Razdolski

Ultrafast Transport of Laser-Excited Spin-Polarized Carriers in Au/Fe/MgO(001)

Hot carrier-induced spin dynamics is analyzed in epitaxial Au/Fe/MgO(001) by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au.

Resonant Enhancement of Second-Harmonic Generation in the Mid-Infrared Using Localized Surface Phonon Polaritons in Subdiffractional Nanostructures

We report on the strong enhancement of mid-infrared second-harmonic generation (SHG) from SiC nanopillars due to the resonant excitation of localized surface phonon polaritons within the Reststrahlen band. A strong dependence of the SHG enhancement upon the optical mode distribution was observed. One such mode, the monopole, exhibits an enhancement that is beyond what is anticipated from field localization and dispersion of the linear and nonlinear SiC optical properties. Comparing the results for the identical nanostructures made of 4H and 6H SiC polytypes, we demonstrate the interplay of localized surface phonon polaritons with zone-folded weak phonon modes of the anisotropic crystal. Tuning the monopole mode in and out of the region where the zone-folded phonon is excited in 6H-SiC, we observe a further prominent increase of the already enhanced SHG output when the two modes are coupled. Envisioning this interplay as one of the showcase features of mid-infrared nonlinear nanophononics, we discuss its prospects for the effective engineering of nonlinear-optical materials with desired properties in the infrared spectral range.

Nonlinear Surface Magnetoplasmonics in Kretschmann Multilayers

Nonlinear magnetoplasmonics aims to utilize plasmonic excitations to control the mechanisms and tailor the efficiencies of nonlinear light frequency conversion at the nanoscale. We investigate the mechanisms of magnetic second-harmonic generation in hybrid gold–cobalt–silver multilayer structures, which support propagating surface plasmon polaritons at both fundamental and second-harmonic frequencies. Using magneto-optical spectroscopy in Kretschmann geometry, we show that the huge magneto-optical modulation of the second-harmonic intensity is dominated by the excitation of surface plasmon polaritons at the second-harmonic frequency, as shown by tuning the optical wavelength over the spectral region of strong plasmonic dispersion. Our proof-of-principle experiment highlights bright prospects of nonlinear magnetoplasmonics and contributes to the general understanding of the nonlinear optics of magnetic surfaces and interfaces.

Second harmonic generation spectroscopy in the Reststrahl band of SiC using an infrared free-electron laser

The Reststrahl spectral region of silicon carbide has recently attracted much attention owing to its potential for mid-infrared nanophotonic applications based on surface phonon polaritons (SPhPs). Studies of optical phonon resonances responsible for surface polariton formation, however, have so far been limited to linear optics. In this Letter, we report the first nonlinear optical investigation of the Reststrahl region of SiC, employing an infrared free-electron laser to perform second harmonic generation (SHG) spectroscopy. We observe two distinct resonance features in the SHG spectra, one attributed to resonant enhancement of the nonlinear susceptibility χ(2) and the other due to a resonance in the Fresnel transmission. Our work clearly demonstrates high sensitivity of mid-infrared SHG to phonon-driven phenomena and opens a route to studying nonlinear effects in nanophotonic structures based on SPhPs.

Nanoscale interface confinement of ultrafast spin transfer torque driving non-uniform spin dynamics

Spintronics had a widespread impact over the past decades due to transferring information by spin rather than electric currents. Its further development requires miniaturization and reduction of characteristic timescales of spin dynamics combining the sub-nanometre spatial and femtosecond temporal ranges. These demands shift the focus of interest towards the fundamental open question of the interaction of femtosecond spin current (SC) pulses with a ferromagnet (FM). The spatio-temporal properties of the impulsive spin transfer torque exerted by ultrashort SC pulses on the FM open the time domain for probing non-uniform magnetization dynamics. Here we employ laser-generated ultrashort SC pulses for driving ultrafast spin dynamics in FM and analysing its transient local source. Transverse spins injected into FM excite inhomogeneous high-frequency spin dynamics up to 0.6 THz, indicating that the perturbation of the FM magnetization is confined to 2 nm.Ilya Razdolski, ∗ Alexandr Alekhin, Nikita Ilin, Jan P. Meyburg, Vladimir Roddatis, Detlef Diesing, Uwe Bovensiepen, and Alexey Melnikov † Physical Chemistry Dept., Fritz Haber Institute of Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany Faculty of Chemistry, University of Duisburg-Essen, Universitätsstr. 5, 45141 Essen, Germany Universität Göttingen, Institut für Materialphysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany Faculty of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen, Lotharstr. 1, 47057 Duisburg, Germany (Dated: February 15, 2018)

Vertical hybrid microcavity based on a polymer layer sandwiched between porous silicon photonic crystals

A vertical hybrid microcavity is fabricated by sandwiching a polymer layer between distributed Bragg reflectors (DBRs) composed of porous silicon photonic crystals. The DBRs are made by electrochemical etching of Si and consist of alternating porous Si layers of high and low porosity, the top DBR being a freestanding film. The hybrid microcavity demonstrates a deep microcavity mode placed within a 200 nm wide photonic band gap, and reveals a many-fold enhancement of the third-order nonlinearity of the microcavity layer. The fabrication technique employed is rather simple, enabling the use of a variety of functional materials as the microcavity spacer.

Analysis of the time-resolved magneto-optical Kerr effect for ultrafast magnetization dynamics in ferromagnetic thin films

We discuss fundamental aspects of laser-induced ultrafast demagnetization probed by the time-resolved magneto-optical Kerr effect (MOKE). Studying thin Fe films on MgO substrate in the absence of electronic transport, we demonstrate how to disentangle pump-induced variations of magnetization and magneto-optical coefficients. We provide a mathematical formalism for retrieving genuine laser-induced magnetization dynamics and discuss its applicability in real experimental situations. We further stress the importance of temporal resolution achieved in the experiments and argue that measurements of both time-resolved MOKE rotation and ellipticity are needed for the correct assessment of magnetization dynamics on sub-picosecond timescales. The framework developed here sheds light onto the details of the time-resolved MOKE technique and contributes to the understanding of the interplay between ultrafast laser-induced optical and magnetic effects.

Effects of crystal anisotropy on optical phonon resonances in midinfrared second harmonic response of SiC

We study the effects of crystal anisotropy on optical phonon resonances in the second harmonic generation (SHG) from silicon carbide (SiC) in its Reststrahl region. By comparing experiments and simulations for isotropic 3C-SiC and anisotropic 4H-SiC in two crystal cuts, we identify several pronounced effects in the nonlinear response which arise solely from the crystal anisotropy. Specifically, we demonstrate that the axial and planar transverse optical phonon resonances selectively and exclusively appear in the corresponding tensor elements of the nonlinear susceptibility, enabling observation of an intense SHG peak originating from a weak phonon mode due to zone-folding along the c-axis of 4H-SiC. Similarly, we identify an anisotropy factor

Second-Harmonic Generation from Critically Coupled Surface Phonon Polaritons

\zeta \equiv \epsilon_\perp/\epsilon_\parallel

Towards the nonlinear acousto-magneto-plasmonics

responsible for a steep enhancement of the transmitted fundamental fields at the axial longitudinal optical phonon frequency, resulting in strongly enhanced SHG. We develop a general recipe to extract all these features that is directly applicable to all wurtzite-structure polar dielectrics, where a very similar behavior is expected. Our model study illustrates the opportunities for utilizing the crystal anisotropy for selectively enhancing nonlinear-optical effects in polar dielectrics, which could potentially be extended to built-in anisotropy in artificially designed hybrid materials.