Browsing by Author "Kaivola, M."

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  • Aberration-insensitive microscopy using optical field-correlation imaging
    (2019-06-01) Ilina, E.; Nyman, M.; Švagždyte, I.; Chekurov, N.; Kaivola, M.; Setälä, T.; Shevchenko, A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The possibility to reduce the effect of optical aberrations has been proposed in several publications on classical ghost imaging. The two-armed ghost-imaging systems make use of spatially incoherent illumination and point-by-point scanned intensity-correlation measurements in the arms. In this work, we introduce a novel ghostlike imaging method that uses a Mach-Zehnder interferometer and is based on optical-field interference instead of intensity correlations. The method allows us to obtain sharp images of microscopic objects even in the presence of severe aberrations that completely destroy the intensity-based image. Furthermore, pure phase objects can be imaged with micrometer-scale resolution in the presence of strong aberrations, which has not been demonstrated previously with a correlation-based imaging technique. In the setup, we use a light-emitting diode source and an ordinary camera as the only light detector. The imaging approach that we put forward in this work may find significant applications in advanced optical microscopy, optical coherence tomography, and a variety of interferometric sensors and detectors.
  • Bifacial Metasurface with Quadrupole Optical Response
    (2015-08) Shevchenko, A.; Kivijärvi, V.; Grahn, P.; Kaivola, M.; Lindfors, K.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We design, fabricate, and characterize a metasurface, whose multipole optical response depends significantly on the illumination direction. The metasurface is composed of gold-nanodisc dimers embedded in glass. In spite of their nanoscale size, the dimers exhibit a dominating electric-current-quadrupole response in a wide range of wavelengths around 700 nm when illuminated from one side, and a primarily electric-dipole response when illuminated from the opposite side. This leads to two consequences. First, the reflection coefficient of the metasurface considerably differs for the two sides of illumination. Second, quadrupole excitation results in a significant local enhancement of both electric and magnetic fields around the dimers. Our experimental spectroscopic data are in good agreement with simulations obtained using a multipole expansion model.
  • Collision effects in velocity-selective optical pumping of sodium
    (1983-08-01) Aminoff, C. G.; Javanainen, J.; Kaivola, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report on a quantitative experimental investigation of velocity-changing collisions by means of velocity-selective optical pumping (VSOP). We have calculated the VSOP line shape for an atom with hyperfine structure with the use of two phenomenological kernels for the collision effects: the Keilson-Storer kernel, and a two-term kernel consisting of a broad Keilson-Storer part and a narrower Gaussian component. Corrections were included to account for the finite absorption in the sample and the backward reflection of the pumping beam. The experiments were carried out in sodium vapor with neon as the perturber gas. The D1 line of sodium was used for optical pumping, and the orientation of the ground state was detected. Free parameters of the theory were determined by fitting the predicted line shapes to experimental curves. The Keilson-Storer kernel proved unsatisfactory, but the two-term kernel reproduced well the observed line shapes over the entire collision profiles in the neon pressure range 0-57 mtorr. In an independent experiment using rapidly modulated VSOP we also measured directly the cross section of velocity-changing collisions: =(1.13±0.10)×10-14 cm2. The large weight obtained for the narrow Gaussian from the fits, as well as the collision cross section which is three times as large as the cross section deduced from tabulated gas kinetic radii, may indicate the presence of collisions with relatively small velocity changes in addition to hard-sphere encounters.
  • Decomposition of the Point-Dipole Field into Homogeneous and Evanescent Parts
    (1999-01-01) Setälä, T.; Kaivola, M.; Friberg, A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In near-field optics the resolution and sensitivity of measurements depend on the abundance of evanescent waves in relation to propagating waves. The electromagnetic field propagator is related to the scalar spherical wave, for which the Weyl expansion is a half-space representation containing both evanescent and homogeneous plane waves. Making use of these results, we decompose the dyadic free-space Green function into its evanescent and homogeneous parts and show that some approaches put forward in the literature are inconsistent with this formulation. We express the results in a form that is suitable for numerical computation and illustrate the field decomposition for a point dipole in some typical cases.
  • Electric dipole-free interaction of visible light with pairs of subwavelength-size silver particles
    (2012-07-13) Grahn, P.; Shevchenko, A.; Kaivola, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In subwavelength-sized particles, light-induced multipole moments of orders higher than the electric dipole are usually negligibly small, which allows for the light-matter interaction to be accurately treated within the electric dipole approximation. In this work we show that in a specially designed meta-atom, a disc metadimer, the electric quadrupole and magnetic dipole can be the only excitable multipoles. This condition is achieved in a narrow but tunable spectral range of visible light both for individual metadimers and for a periodic array of such particles. The electromagnetic fields scattered by the metadimers fundamentally differ from those created by electric dipoles. A metamaterial composed of such metadimers will therefore exhibit unusual optical properties.
  • Emission control with metallic hole arrays
    (2011) Moerland, R. J.; Kuipers, L.; Kaivola, M.
     A4 Artikkeli konferenssijulkaisussa
    Adding holes in a periodic arrangement to metallic thin films greatly affects the optical properties of the metal film. The compound structure can exert a large influence on electromagnetic fields that interact with the hole array. Many parameters affect the actual response of the hole arrays to electromagnetic fields, such as the periodicity, the size of the holes and their shape. Here, we will show by calculation that the angular emission and lifetime of emitters, embedded in hole arrays comprised of rectangular holes with varying aspect ratio, depend strongly on the hole aspect ratio. Specifically, changing the aspect ratio of the holes leads to a large variety in far-field emission patterns and a more than 10-fold changes in decay rate of a single emitter placed in the central hole of such an array.
  • Functional optical metamaterials employing spatial dispersion and absorption
    (2014) Shevchenko, Andriy; Grahn, P.; Kaivola, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Generation of light in spatially dispersive materials
    (2017-04-04) Nyman, M.; Kivijärvi, V.; Shevchenko, A.; Kaivola, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Spatial dispersion makes optical properties of materials depend on the direction of light propagation. The effect can be applied to control optical emission by sources embedded in such media. We propose a method to determine the radiation pattern of essentially any emitter located in a general spatially dispersive and optically anisotropic medium. The method is based on a decomposition of the source into waves of electric current, each creating optical plane waves whose properties are determined by the wave parameters, the refractive index, and impedance. The method is computationally fast and very accurate even in strongly spatially dispersive plasmonic metamaterials. In particular, we observe large modification of dipole emission in a bifacial and a diffraction-compensating metamaterial. The method is applicable to a large variety of nanostructured materials and, therefore, we believe that it can find numerous applications in nano-optics.
  • Interaction of metamaterials with optical beams
    (2015) Kivijärvi, Ville; Nyman, M.; Karrila, A.; Grahn, P.; Shevchenko, A.; Kaivola, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We develop a general theoretical approach to describing the interaction of metamaterials with optical beams. The metamaterials are allowed to be anisotropic, chiral, noncentrosymmetric, and spatially dispersive. Unlike plane waves, beams can change their field distributions upon interaction with metamaterials, which can reveal new optical effects. Our method is based on a vector form of the angular spectrum representation and a technique to calculate the wave parameters for all required directions of wave propagation. Applying the method to various metamaterial designs, we discover a new optical phenomenon: the conversion of light polarization by spatial dispersion. Because of this phenomenon, the refractive index and impedance cannot be introduced for many metamaterial designs. In such cases, we propose an alternative approach to treating the beam–metamaterial interaction. This work takes a step forward in describing optical metamaterials by moving from unphysical plane waves to realistic optical beams.
  • Internally twisted non-centrosymmetric optical metamaterials
    (2014) Grahn, Patrick; Shevchenko, A.; Kaivola, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Internally twisted spatially dispersive optical metamaterials
    (2014) Shevchenko, A.; Grahn, P.; Kaivola, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
  • Multifrequency Bessel beams with adjustable group velocity and longitudinal acceleration in free space
    (2022-03-01) Hildén, P.; Ilina, E.; Kaivola, M.; Shevchenko, A.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The group velocity of an optical beam in free space is usually considered to be equal to the speed of light in vacuum. However, it has been recently realized that, by structuring the beam's angular and temporal spectra, one can achieve well pronounced and controlled subluminal and superluminal propagation. In this work, we consider multifrequency Bessel beams that are known to propagate without divergence and show a variety of possibilities to adjust the group velocity of the beam by means of designed angular dispersion. We present several examples of multifrequency Bessel beams with negative and arbitrary positive group velocities, as well as longitudinally accelerating beams and beams with periodically oscillating local group velocities. The results of these studies can be of interest to scientists working in the fields of optical beam engineering, light amplitude and intensity interferometry, ultrafast optics, and optical tweezers.
  • Pulsed Standing-Wave Mirror for Neutral Atoms and Molecules
    (2000-05-29) Ryytty, P.; Kaivola, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Reflection of neutral atoms and molecules by a pulsed standing wave with a duration on the order of nanoseconds is studied. It is shown that, with a suitable choice of the laser parameter values, each period of the standing-wave pattern functions as an independent mirror, thus providing a novel way to manipulate large samples of neutral gas-phase particles even with a single laser pulse. At moderate field intensities, the pulsed standing-wave mirror would be directly applicable, e.g., for the manipulation of buffer-gas cooled molecules.
  • Shaping single emitter emission with metallic hole arrays: strong focusing of dipolar radiation
    (2013) Moerland, R.J.; Eguiluz, L.; Kaivola, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Nanoscale plasmonic structures allow for control of the emission of single emitters, such as fluorescent molecules and quantum dots, enabling phenomena such as lifetime reduction, emission redirection and color sorting of photons. We present single emitter emission tailored with arrays of holes of heterogeneous size, perforated in a gold film. With spatial control of the local amplitude and phase of the electromagnetic field radiated by the emitter, a desired near- or far-field distribution of the electromagnetic waves can be obtained. This control is established by varying the aspect ratio of the individual holes and the periodicity of the array surrounding the emitter. As an example showing the versatility of the technique, we present the strong focusing of the radiation of a highly divergent dipole source, for both p- and s-polarized waves.
  • Theoretical description of bifacial optical nanomaterials
    (2013) Grahn, P.; Shevchenko, A.; Kaivola, M.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In general, optical nanomaterials composed of noncentrosymmetric nanoscatterers are bifacial, meaning that two counter-propagating waves inside the material behave differently. Thus far a practical theory for the description of such materials has been missing. Herein, we present a theory that connects the design of the bifacial nanomaterial’s “atoms” with the refractive index and wave impedance of the medium. We also introduce generalized Fresnel coefficients and investigate the role of electromagnetic multipoles on the bifaciality. We find that in any material two counter-propagating waves must experience the same refractive index, but their impedances can differ. The model is demonstrated in practice by the design of a nanomaterial slab with one of its facets being optically reflective, while the other being totally non-reflective.