Mikko Kataja

Ultrasensitive and label-free molecular level detection enabled by light phase control in magnetoplasmonic nanoantennas

Systems allowing label-free molecular detection are expected to have enormous impact on biochemical sciences. Research focuses on materials and technologies based on exploiting localized surface plasmon resonances in metallic nanostructures. The reason for this focused attention is their suitability for single molecule sensing, arising from intrinsically nanoscopic sensing volume, and the high sensitivity to the local environment. Here we propose an alternative route, which enables radically improved sensitivity compared torecently reported plasmon-based sensors. Such high sensitivity is achieved by exploiting the control of the phase of light in magnetoplasmonic nanoantennas. We demonstrate a manifold improvement of refractometric sensing figure-of-merit. Most remarkably, we show a raw surface sensitivity (i.e., without applying fitting procedures) of two orders of magnitude higher than the current values reported for nanoplasmonic sensors. Such sensitivity corresponds to a mass of ~0.8 ag per nanoantenna of polyamide-6.6 (n=1.51), which is representative for a large variety of polymers, peptides and proteins.

Surface lattice resonances and magneto-optical response in magnetic nanoparticle arrays

Structuring metallic and magnetic materials on subwavelength scales allows for extreme confinement and a versatile design of electromagnetic field modes. This may be used, for example, to enhance magneto-optical responses, to control plasmonic systems using a magnetic field, or to tailor magneto-optical properties of individual nanostructures. Here we show that periodic rectangular arrays of magnetic nanoparticles display surface plasmon modes in which the two directions of the lattice are coupled by the magnetic field-controllable spin–orbit coupling in the nanoparticles. When breaking the symmetry of the lattice, we find that the optical response shows Fano-type surface lattice resonances whose frequency is determined by the periodicity orthogonal to the polarization of the incident field. In striking contrast, the magneto-optical Kerr response is controlled by the period in the parallel direction. The spectral separation of the response for longitudinal and orthogonal excitations provides versatile tuning of narrow and intense magneto-optical resonances.

Polarizability and magnetoplasmonic properties of magnetic general nanoellipsoids

An approach to compute the polarizability tensor of magnetic nanoparticles having general ellipsoidal shape is presented. We find a surprisingly excellent quantitative agreement between calculated and experimental magneto-optical spectra measured in the polar Kerr configuration from nickel nanodisks of large size (exceeding 100 nm) with circular and elliptical shape. In spite of its approximations and simplicity, the formalism presented here captures the essential physics of the interplay between magneto-optical activity and the plasmonic resonance of the individual particle. The results highlight the key role of the dynamic depolarization effects to account for the magneto-optical properties of plasmonic nanostructures.

Anisotropic Nanoantenna-Based Magnetoplasmonic Crystals for Highly Enhanced and Tunable Magneto-Optical Activity

We present a novel concept of a magnetically tunable plasmonic crystal based on the excitation of Fano lattice surface modes in periodic arrays of magnetic and optically anisotropic nanoantennas. We show how coherent diffractive far-field coupling between elliptical nickel nanoantennas is governed by the two in-plane, orthogonal and spectrally detuned plasmonic responses of the individual building block, one directly induced by the incident radiation and the other induced by the application of an external magnetic field. The consequent excitation of magnetic field-induced Fano lattice surface modes leads to highly tunable and amplified magneto-optical effects as compared to a continuous film or metasurfaces made of disordered noninteracting magnetoplasmonic anisotropic nanoantennas. The concepts presented here can be exploited to design novel magnetoplasmonic sensors based on coupled localized plasmonic resonances, and nanoscale metamaterials for precise control and magnetically driven tunability of light polarization states.

Hybrid plasmonic lattices with tunable magneto-optical activity.

We report on the optical and magneto-optical response of hybrid plasmonic lattices that consist of pure nickel and gold nanoparticles in a checkerboard arrangement. Diffractive far-field coupling between the individual emitters of the lattices results in the excitation of two orthogonal surface lattice resonance modes. Local analyses of the radiation fields indicate that both the nickel and gold nanoparticles contribute to these collective resonances and, thereby, to the magneto-optical activity of the hybrid arrays. The strong effect of noble metal nanoparticles on the magneto-optical response of hybrid lattices opens up new avenues for the realization of sensitive and tunable magneto-plasmonic nanostructures.

Plasmon-induced demagnetization and magnetic switching in nickel nanoparticle arrays

We report on the manipulation of magnetization by femtosecond laser pulses in a periodic array of cylindrical nickel nanoparticles. By performing experiments at different wavelengths, we show that the excitation of collective surface plasmon resonances triggers demagnetization in zero field or magnetic switching in a small perpendicular field. Both magnetic effects are explained by plasmon-induced heating of the nickel nanoparticles to their Curie temperature. Model calculations confirm the strong correlation between the excitation of surface plasmon modes and laser-induced changes in magnetization.

Magneto-optical Kerr effect susceptometer for the analysis of magnetic domain wall dynamics

Domain wall dynamics in thin magnetic films with perpendicular and in-plane anisotropy is studied using a novel magneto-optical Kerr effect susceptometery method. The method allows for measurements of domain wall motion under ac field excitation and the analysis of dynamic modes as a function of driving frequency and magnetic field amplitude. Domain wall dynamics in the perpendicular anisotropy system, a Co/Pt multilayer, is characterized by thermally activated creep motion. For this dynamic mode, a polydispersivity exponent of β = 0.50 ± 0.03 is derived at small excitation energy, which is in excellent agreement with theoretical models. The dynamics of the other system, a Co wire with transverse uniaxial anisotropy, is dominated by viscous slide motion in a regular magnetic stripe pattern. Analytical expressions are derived for this magnetic configuration and by using these expressions, accurate values for the depinning field and the domain wall mobility are extracted from the susceptibility measurements.

Hybrid Ni/SiO2/Au dimer arrays for high-resolution refractive index sensing

Abstract We introduce a novel magnetoplasmonic sensor concept for sensitive detection of refractive index changes. The sensor consists of a periodic array of Ni/SiO2/Au dimer nanodisks. Combined effects of near-field interactions between the Ni and Au disks within the individual dimers and far-field diffractive coupling between the dimers of the array produce narrow linewidth features in the magneto-optical Faraday spectrum. We associate these features with the excitation of surface lattice resonances and show that they exhibit a spectral shift when the refractive index of the surrounding environment is varied. Because the resonances are sharp, refractive index changes are accurately detected by tracking the wavelength where the Faraday signal crosses 0. Compared to random distributions of pure Ni nanodisks or Ni/SiO2/Au dimers or periodic arrays of Ni nanodisks, the sensing figure of merit of the hybrid magnetoplasmonic array is more than one order of magnitude larger.

Coupling effects in plasmonic nanoparticle arrays: The weak and the strong coupling regime and the effects of spin-orbit coupling

Summary form only given. We study the spatial coherence properties of a system composed of periodic silver nanoparticle arrays covered with fluorescent organic molecule film [1]. The evolution of spatial coherence of the structure is investigated both in weak and strong coupling regimes by systematically varying the coupling strength between the localized molecular excitons and the collective, delocalized modes of the nanoparticle array known as surface lattice resonances (SLRs). In stark contrast to pure localized excitons, the high degree of spatial coherence is maintained in the strong coupling regime, even when the mode is very exciton-like (80%). The results of our recent luminescence experiments will be presented. The effects of spin-orbit coupling are studied in periodic rectangular arrays of magnetic Ni nanoparticles [2]. We observe SLR modes in which the two directions of the lattice are coupled by the magnetic-field-controllable spin-orbit coupling in the nanoparticles. When breaking the symmetry of the lattice, we find that the optical response shows Fano-type surface lattice resonances whose frequency is determined by the periodicity orthogonal to the polarization of the incident field. In striking contrast, the magneto-optical Kerr response is controlled by the period in the parallel direction. The spectral separation of the response for longitudinal and orthogonal excitations provides versatile tuning of narrow and intense magneto-optical resonances.

Magnetoplasmonic crystals based on anisotropic nanoantennas

By synergically combining experiments and simulations, we show how the excitation of lattice surface modes in ordered arrays of magnetic and optically-anisotropic nanoantennas leads to a highly enhanced and tunable Fano-like modulation of the magnetoplasmonic response.