Browsing by Author "Friberg, Ari T."

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  • Bulk-wave propagation and non-diffracting waves in anisotropic elastic media
    (1997) Salo, Janne
     Helsinki University of Technology | Master's thesis
  • Characterization of polarization fluctuations in random electromagnetic beams
    (2009) Setälä, Tero; Shevchenko, Andriy; Kaivola, Matti; Friberg, Ari T.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Two stationary, partially polarized electromagnetic beams with equal degrees of polarization may exhibit completely different time evolutions of the instantaneous polarization state. In this work, we derive a statistical quantity that describes the rate at which the field intensity in the beam, on average, is redistributed between the beam's polarization state at any time and the state orthogonal to it. This method allows one to treat the dynamical properties of the polarization fluctuations both theoretically and experimentally. We demonstrate the method by applying it to important special cases, such as fields obeying Gaussian statistics, black-body radiation pencils and depolarized laser beams. We also prove that a geometric approach introduced earlier is closely connected with the present model.
  • Coherent and partially coherent beams in optical systems
    (1986) Turunen, Jari
     Helsinki University of Technology | Licentiate thesis
  • Connection between electric and magnetic coherence in free electromagnetic fields
    (2005) Jouttenus, Teppo; Setälä, Tero; Kaivola, Matti; Friberg, Ari T.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We introduce quantitative measures for the description of the electric and magnetic coherence in a stationary, random electromagnetic field at two points, in a volume, and in the Fourier space. These quantities are applied to free electromagnetic fields, and several theorems regarding the relationship between the two types of coherences in such fields are established. Fields which are statistically homogeneous, and those which, in addition, are statistically isotropic are considered separately. Furthermore, the connection between the electric and magnetic coherence is exemplified for some specific statistically homogeneous fields.
  • Conoscopic interferometry of wafers for surface-acoustic wave devices
    (1997) Äyräs, Pekka; Friberg, Ari T.; Kaivola, Matti; Salomaa, Martti M.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We show that in interpreting the conoscopic interference fringes, one should exercise care in employing approximate expressions which fail for certain crystal cuts. In this paper, we study 64°- and 128°-rotated Y-cut and Z-cut LiNbO3 wafers. We show that the error made in using the approximate formulae for the samples is more than 25% and that one has to use exact formulae in order to attain quantitative agreement with the experimental data.
  • Decomposition of the point-dipole field into homogeneous and evanescent parts
    (1999) Setälä, Tero; Kaivola, Matti; Friberg, Ari T.
     School of Science | 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.
  • Degree of polarization for optical near fields
    (2002) Setälä, Tero; Shevchenko, Andriy; Kaivola, Matti; Friberg, Ari T.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We investigate an extension to the concept of degree of polarization that applies to arbitrary electromagnetic fields, i.e., fields whose wave fronts are not necessarily planar. The approach makes use of generalized spectral Stokes parameters that appear as coefficients, when the full 3×3 spectral coherence matrix is expanded in terms of the Gell-Mann matrices. By defining the degree of polarization in terms of these parameters in a manner analogous to the conventional planar-field case, we are led to a formula that consists of scalar invariants of the spectral coherence matrix only. We show that attractive physical insight is gained by expressing the three-dimensional degree of polarization explicitly with the help of the correlations between the three orthogonal spectral components of the electric field. Furthermore, we discuss the fundamental differences in characterizing the polarization state of a field by employing either the two- or the three-dimensional coherence-matrix formalism. The extension of the concept of the degree of polarization to include electromagnetic fields having structures of arbitrary form is expected to be particularly useful, for example, in near-field optics.
  • Degree of polarization in near fields of thermal sources: effects of surface waves
    (2002-03-25) Setälä, Tero; Kaivola, Matti; Friberg, Ari T.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We introduce the concept of degree of polarization for electromagnetic near fields. The approach is based on the generalized Stokes parameters that appear as expansion coefficients of the 3×3 coherence matrix in terms of the Gell-Mann matrices. The formalism is applied to optical near fields of thermally fluctuating half-space sources with particular interest in fields that are strongly polarized owing to resonant surface plasmons or phonons. This novel method is particularly useful when assessing the full vectorial characteristics of random evanescent fields, e.g., for near-field spectroscopy and polarization microscopy.
  • Degree of polarization in near fields of thermal sources: effects of surface waves
    (2002) Setälä, Tero; Kaivola, Matti; Friberg, Ari T.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We introduce the concept of degree of polarization for electromagnetic near fields. The approach is based on the generalized Stokes parameters that appear as expansion coefficients of the 3×3 coherence matrix in terms of the Gell-Mann matrices. The formalism is applied to optical near fields of thermally fluctuating half-space sources with particular interest in fields that are strongly polarized owing to resonant surface plasmons or phonons. This novel method is particularly useful when assessing the full vectorial characteristics of random evanescent fields, e.g., for near-field spectroscopy and polarization microscopy.
  • Geometric phase in beating of light waves
    (2019-08-14) Hannonen, Antti; Saastamoinen, Kimmo; Leppanen, Lasse-Petteri; Koivurova, Matias; Shevchenko, Andriy; Friberg, Ari T.; Setala, Tero
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Beating is a simple physical phenomenon known for long in the context of sound waves but remained surprisingly unexplored for light waves. When two monochromatic optical beams of different frequencies and states of polarization interfere, the polarization state of the superposition field exhibits temporal periodic variation-polarization beating. In this work, we reveal a foundational and elegant phase structure underlying such polarization beating. We show that the phase difference over a single beating period decomposes into the Pancharatnam-Berry geometric phase and a dynamical phase of which the former depends exclusively on the intensities and polarization states of the interfering beams whereas the sum of the phases is determined solely by the beam frequencies. Varying the intensity and polarization characteristics of the beams, the relative contributions of the geometric and dynamical phases can be adjusted. The geometric phase inherent in polarization beating is governed by a compact expression containing only the Stokes parameters of the interfering waves and can alternatively be obtained from the individual beam intensities and the amplitude of the intensity beats. We demonstrate both approaches experimentally by using an interferometer with a fast detector and a specific polarimetric arrangement. Polarization beating has a unique character that the geometric and dynamical phases are entangled, i.e. variation in one unavoidably leads to a change in the other. Our work expands geometric phases into a new domain and offers important novel insight into the role of polarization in interference of electromagnetic waves.
  • Holografinen lasersäteen pyyhkäisymenetelmä
    (1985) Kajanto, Markus
     Helsinki University of Technology | Master's thesis
  • Kuituoptinen lämpötilanmittaus
    (1985) Kajanto, Isko
     Helsinki University of Technology | Master's thesis
  • Lasersäteen holografinen poikkeutusmenetelmä
    (1984) Rantala, Esko
     Helsinki University of Technology | Master's thesis
  • Optisen paikkakoherenssin mittaus ja käyttö
    (1989) Tervonen, Eero
     Helsinki University of Technology | Master's thesis
  • Optimization of holographic optical systems
    (1988) Fagerholm, Juha
     Helsinki University of Technology | Licentiate thesis
  • Polarization dependent beaming properties of a plasmonic lattice laser
    (2021-06) Asamoah, Benjamin O.; Partanen, Henri; Mohamed, Sughra; Heikkinen, Janne; Halder, Atri; Koivurova, Matias; Nečada, Marek; Set, Tero; Turunen, Jari; Friberg, Ari T.; Hakala, Tommi K.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We study beaming properties of laser light produced by a plasmonic lattice overlaid with organic fluorescent molecules. The crossover from spontaneous emission regime to stimulated emission regime is observed in response to increasing pump fluence. This transition is accompanied by a strong reduction of beam divergence and emission linewidth due to increased degree of spatial and temporal coherence, respectively. The feedback for the lasing signal is shown to be mainly one-dimensional due to the dipolar nature of the surface lattice resonance. Consequently, the beaming properties along x and y directions are drastically different. From the measurements, we obtain the M 2 value along both principal directions of the square lattice as a function of the pump fluence. Our work provides the first detailed analysis of the beam quality in plasmonic lattice lasers and reveals the underlying physical origin of the observed strong polarization dependent asymmetry of the lasing signal.
  • Polarization of pulsed random beams
    (2011) Voipio, Timo
     Master's thesis
    Valon polarisaatiolla on monitahoinen matemaattinen rakenne sekä suuri määrä hyödyntämiskohteita eri tieteen ja tekniikan alueilla, kuten lähikenttäoptiikassa, valon ja aineen vuorovaikutuksessa sekä optisessa tiedonsiirrossa. Valonsäteiden polarisaatiota on tutkittu laajasti satunnaisten, tilastollisesti stationaaristen säteiden osalta, mutta tähän mennessä pulssitettujen säteiden osittaisen polarisaation kuvaamiseen ei ole kehitetty johdonmukaista, kattavaa teoriaa. Työn aluksi esitellään stationaaristen kenttien kuvaamiseen käytetty polarisaatioformalismi aikatasossa polarisaatiomatriisin ja taajuustasossa ristispektrimatriisin (crossspectral density matrix, CSD) avulla. Stationaaristen kenttien polarisaatioteorian kertaamisen jälkeen työn pääasiallisessa osuudessa kehitetään polarisaatiomatriisiformalismi ei-stationaarisille kentille aika- ja taajuustasoissa. Aika- ja taajuustason polarisaation välille johdetaan yhteys, jota analysoidaan mitattavissa olevien aika- ja taajuustasojen suureiden, polarisaatioasteen ja Stokesin parametrien, avulla. Valopulssin polarisaatioasteen ja -tilan muuttumista ajan ja taajuuden funktiona havainnollistetaan esimerkein. Yksi esimerkeistä kuvaa pulssia, jonka aikatason polarisaatioastetta ja -tilaa voidaan muokata halutulla tavalla pulssin pysyessä kuitenkin taajuustasossa täysin polaroituna. Työn viimeinen osuus esittelee optisen laitteiston, jolla voidaan muokata pulssien aikaprofiilia. Laitteisto perustuu peräjälkeen aseteltuihin dispersiivisiin optisiin elementteihin ja aikariippuviin vaihesuotimiin, jotka yhdessä muodostavat aika-avaruuden analogian paikka-avaruudessa toimivasta optisesta kuvantamisjärjestelmästä. Pulssin polarisaatioominaisuuksia voidaan muokata moninaisilla tavoilla kohdistamalla pulssin keskenään kohtisuoriin polarisaatiokomponentteihin toisistaan eroavat aikasuurennokset.
  • Polarization time and length for random optical beams
    (2008) Setälä, Tero; Shevchenko, Andriy; Kaivola, Matti; Friberg, Ari T.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We investigate the dynamics of the instantaneous polarization state of stationary, partially polarized random electromagnetic beamlike fields. An intensity-normalized correlation function of the instantaneous Poincaré vector is introduced for the characterization of the time evolution of the polarization state. This polarization correlation function enables us to define a polarization time and a polarization length over which the polarization state remains substantially unchanged. In the case of Gaussian statistics, the polarization correlation function is shown to assume a simple form in terms of the parameters employed to characterize partial coherence and partial polarization of electromagnetic fields. The formalism is demonstrated for a partially polarized, temporally Gaussian-correlated beam, and black-body radiation. The results are expected to find a range of applications in investigations of phenomena where polarization fluctuations of light play an important role.
  • Polarization time of unpolarized light
    (2017-01-20) Shevchenko, Andriy; Roussey, Matthieu; Friberg, Ari T.; Setälä, Tero
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Light produced by most natural and artificial sources is unpolarized or partially polarized. At any instant of time, however, such random light can be regarded as fully polarized, but the polarization state may vary drastically within short time intervals. This rate of change is another attribute that separates one unpolarized beam from another. Here, we study such polarization dynamics and, for the first time to our knowledge, measure the characteristic time, called the polarization time, in which the instantaneous polarization state stays essentially unaltered. The technique employs a polarization-sensitive Michelson interferometer and two-photon absorption detection valid for electromagnetic light, yielding superior femtosecond time resolution. We analyze two unpolarized light sources: amplified spontaneous emission from a fiber amplifier and a dual-wavelength laser source. The characterization of polarization dynamics can have significant applications in optical sensing, polarimetry, telecommunication, and astronomy, as well as in quantum and atom optics.
  • A Silicon-based fiber-optic temperature sensor
    (1987) Kajanto, Isko
     Helsinki University of Technology | Licentiate thesis