Introduction to Nonlinear Optical Effects and Materials T.P.Radhakrishnan School of Chemistry, University of Hyderabad Hyderabad 500 046, India tprsc@uohyd.ernet.in http://chemistry.uohyd.ernet.in/~tpr/ This file is available at http://chemistry.uohyd.ernet.in/~ch521/ Nonlinear Optical Processes and Materials Linear optical processes Reflection, Refraction, Absorption No change in properties of medium or light Nonlinear optical processes Electro-optic effect (Pockel's effect) - (2) (-;,0) Electro-optic modulators Frequency doubling - (2) (-2;,) Harmonic generation Frequency mixing - (2) (-0;a,b) Parametric amplifiers Frequency tripling Deep UV conversion - (3) (-3;,,) Nonlinear Optical Materials Inorganic crystals (KDP, LiNbO3) Molecular Solids Conjugated Polymers P Nonlinear polarisation Linear polarisation E P P E E time time Bulk Polarisation Pi = ij(1) Ej + ijk(2) Ej Ek + ijkl(3) Ej Ek El + …. Molecular Polarisation i = ij Ej + ijk Ej Ek + ijkl Ej Ek El + …. First hyperpolarisability Centrosymmetric systems E -E, P -P (2) , = 0 Noncentrosymmetric systems E -E, P P' (2) , 0 Second Harmonic Generation (SHG) E sin (t) P(2) sin2(t) { 1 - cos(2t)} Quadratic Nonlinear Optical (NLO) Effect : Second harmonic generation (Frequency doubling) 1064 nm (infra red laser) 532 nm (green laser) Symmetry condition : No centre of inversion in the material Resonance enhancement of SHG : DE ~ hν or 2hν Unsymmetric polarisation of a donor-acceptor substituted benzene D- A+ 1 D A 2 A- D+ 3 2-level 3e 2 2 = 2m f. D. DE 2 2 2 [( DE - 2h ) ( DE - (h ) )] 3e2h2 f. D 0 = 3 2m DE Oriented Gas Model (2)IJK = NfI(2) fJ() fK() bIJK Ng bIJK = {cos Ii(s) cos Ii(s) cos Ii(s)} ijk ijk s=1 Design of Noncentrosymmetric Molecular Materials Molecular Crystals H-bonding Alkyl Chain Effect Cancelling µ Steric Effect Salt Formation Chirality Langmuir-Blodgett Films Poled Thin Films, Polymers Host-Guest Systems and Intercalation H-bonding Urea - 8 atom molecule 8 H-bonds per molecule ! Melting point = 136oC Hygroscopic H-bond O O- Vanishing Ground Vanishing Ground State Dipole Moment (and Steric Factor)D donor group .01 D O- State Dipole Moment 4.01 D D+ NO 2 O- NO 2 CH 3 A- A acceptor group D > 0 Centrosymmetric + + + N N O- O N N O- O- O- O- 4.24 D 4.01 D NO 2 D+ D donor group + + N D < 0 N NO 2 P212121 Noncentrosymme Pmna Centrosymmetric CH 3 D > 0 + N N O- O- + + N N SHG = 13 U O- 4.24 D NO 2 3-methyl-4-nitropyridine -1-oxide (POM) O- Nonentrosymmetric NO 2 Pmna CH 3 P212121 Noncentrosymmetric Organic Salts Molecular dipoles of a typical covalent compound + + + + + + + + Molecular dipoles and counterions in an organic salt (CH3)2N + N H3C - SO3 CH3 Cc (noncentrosymmetric) SHG = 1000 U Chirality Chiral object lacks Sn symmetry Eg. S1 , S2 i Crystal of pure enantiomer has to be noncentrosymmetric Pure Enantiomer Crystal with no inversion symmetry Chirality and H-bonding H3C H COOCH3 NH NO2 (2,4-dinitrophenyl-Lalanine methyl ester (MAP) P21 space group SHG = 10 U NO2 Chirality and extended H-bonding H N NO2 CH2OH N-(4-nitrophenyl)-Lprolinol (NPP) P21 space group SHG = 150 U Organic Molecules with Large Second Order Nonlinearities Molecule Powder SHG / U Melting Point / o (1 U ~ 3 KDP) C O OMe H N O2N * 10 81 40 114 80 131 115 166 150 116 NO2 MAP NH2 NO2 mNA NH2 CH3 NO2 MNA N NH-CO-CH 3 NO 2 DAN CH 2OH * N NO 2 NPP Kurtz-Perry Powder SHG Measurement 1064 nm M F532 532 nm Mono F1064 S PC Osc 1064 nm : Nd:YAG laser F532 : green filter F1064 : interference (ir) filter M : concave mirror S : microcrystalline sample Mono : monochromator Osc : oscilloscope Electric Field Induced Second Harmonic Generation (EFISHG) Cell Design Electric Field Induced Second Harmonic Generation (EFISHG) Refractive Index variation n = n0 + n2I Pockel’s effect (2nd order effect) Refractive index varies with applied dc field Kerr effect (3rd order effect) Refractive index varies with incident laser intensity Kerr gate Cross Polarizer Polarizer Laser No light (3) crystal Laser 2 Cross Polarizer Polarizer Laser 1 Light (3) crystal