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Second-order polarization techniques for the characterization of thin films

Second-order polarization techniques for the characterization of thin films. Martti Kauranen Institute of Physics Tampere University of Technology Finland. Where is Tampere?. TAMPERE. Second-order nonlinear optics. Second-harmonic response Spatial symmetry symmetry group of material

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Second-order polarization techniques for the characterization of thin films

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  1. Second-order polarization techniques for the characterization of thin films Martti Kauranen Institute of Physics Tampere University of Technology Finland

  2. Where is Tampere? TAMPERE COST MP0604, Ancona, October 26, 2007

  3. Second-order nonlinear optics • Second-harmonic response • Spatial symmetry • symmetry group of material • Centrosymmetry • second-order processes electric-dipole-forbidden • probes of surfaces and thin films COST MP0604, Ancona, October 26, 2007

  4. Multipole interactions weak • Hamiltonian • Second-order • Magnetic and quadrupole tensors • symmetry properties are different from those of the electric-dipole tensor axial 4th rank axial 4th rank electric-dipole-forbidden effects can occur COST MP0604, Ancona, October 26, 2007

  5. Chirality and optical activity • Chiral molecules • two mirror-image forms (enantiomers) • noncentrosymmetric • biological material • pharmacological molecules • Optical activity • optical effects due to chirality • optical rotation • circular dichroism • interference between electric and magnetic contributions • reverse sign between the enantiomers COST MP0604, Ancona, October 26, 2007

  6. Chirality and nonlinear optics • Second-order materials • low molecular symmetry • electric-dipole response in isotropic materials • magnetic response in centrosymmetric materials • New probes of chirality • optical activity in the nonlinear response • sensitivity to surfaces and thin films • small amount of material • electric-dipole-allowed probes • relative magnitude ~1 high macroscopic symmetry COST MP0604, Ancona, October 26, 2007

  7. Interesting quantities • Susceptibility tensor • symmetry group • structural features • strength of nonlinear response • Molecular ordering • degree of orientation q COST MP0604, Ancona, October 26, 2007

  8. Thin-film characterization • Fundamental field • Second-harmonic field • In-plane isotropy no ED COST MP0604, Ancona, October 26, 2007

  9. Chiral film with in-plane isotropy SH intensity rotation angle of quarter waveplate (degrees) • Polyisocyanide COST MP0604, Ancona, October 26, 2007

  10. Magnetic contributions? SH intensity rotation angle of WP (degrees) • Polyisocyanide • evidence through complete tensor analysis • Polythiophene • direct evidence COST MP0604, Ancona, October 26, 2007

  11. Anisotropic achiral film SH intensity rotation angle of quarter waveplate (degrees) • 2-docosylamine-5-nitropyridine (DCANP) COST MP0604, Ancona, October 26, 2007

  12. Chiral and anisotropic film • Thiohelicene • chiral • Langmuir-Blodgett film • columnar aggregates • in-plane anisotropy • Symmetry group • in-plane anisotropy • chiral • eight susceptibility components • arbitrary in-plane axes COST MP0604, Ancona, October 26, 2007

  13. Measurements • Independent measurements • vary azimuth f • detect s- and p-polarized signals • Recovery of susceptibility tensor • regression based data analysis • comparison of theoretical models • statistical indicators f, g, h for each signal COST MP0604, Ancona, October 26, 2007

  14. Experimental results symmetry group is actually residuals tensor components COST MP0604, Ancona, October 26, 2007

  15. Absolute probes of chirality? • Normal incidence required • no difference between s and p • Circular polarizations required • insensitivity to anisotropy • Problem • even tight focusing does not produce strong longitudinal polarization components poor coupling with surface nonlinearity COST MP0604, Ancona, October 26, 2007

  16. Radial and azimuthal polarization • Cylindrical symmetry • insensitive to anisotropy • Focusing • longitudinal components in focus • coupling to surface nonlinearity • Beams with handedness • sensitive to chirality? • Combine different types of beams • radial and circular polarization COST MP0604, Ancona, October 26, 2007

  17. NLO microscopy of nanodots* SHG THG • Cylindrical gold nanodots** • SHG forbidden for ideal samples at normal incidence • THG allowed bright particles depend on polarization *Jeff Squier, CSM **Tapio Niemi, ORC, TUT COST MP0604, Ancona, October 26, 2007

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