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

COST MP0604, Ancona, October 26, Where is Tampere? TAMPERE

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

COST MP0604, Ancona, October 26, 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, 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, Interesting quantities Susceptibility tensor –symmetry group –structural features –strength of nonlinear response Molecular ordering –degree of orientation   ijk for C 2 C 2 symmetry features zzz zxx zyy xxz=xzx yyz=yzy xyz=xzy yxz=yzx zxy=zyx - chirality chirality and anisotropy

COST MP0604, Ancona, October 26, Thin-film characterization Fundamental field Second-harmonic field In-plane isotropy achiral chiral no ED

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

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

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

COST MP0604, Ancona, October 26, 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, Measurements Independent measurements –vary azimuth  –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, Experimental results tensor components residuals symmetry group is actually

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