1. Second-Order Nonlinear Optical Characteristics of Nanoscale Self-Assembled Multilayer Films J. R. Heflin R. M. Davis H. W. Gibson G. Indebetouw H. Marand Ph. D. Thesis Defense by Patrick J. Neyman June 16, 2004 © Patrick Neyman: patrickneyman.com

2. Preface This presentation is a combination of my Ph. D. defense presentation and supplemental slides for further clarification The supplemental slides are those which I developed for conference presentations Thank you for watching -Patrick Neyman © Patrick Neyman: patrickneyman.com

3.  Nonlinear Optics (NLO) Introduction  Device Applications using NLO Molecules  Measurment Techniques  Molecules for NLO Applications  ISAM Film Properties  Thick ISAM Films  Absorption of Second Harmonic  Hybrid Covalent / Ionic Fabrication Method  Thermal Stability  Temporal Stability  Electro-Optic Coefficient Measurement Outline © Patrick Neyman: patrickneyman.com

4. Linear (classical) Optics Optical electric field induces polarization of the molecules Induced polarization is linearly proportional to the electric field molecular dipole moment: macroscopic polarization field: The index of refraction of a material is given by: At high intensities, the linear relationship between & no longer holds © Patrick Neyman: patrickneyman.com

5. Nonlinear Optics (NLO) The polarization may be expanded in a Taylor series: For an anisotropic medium, the polarization field is given by: and the dipole moment is given by: © Patrick Neyman: patrickneyman.com

6. Second-Order NLO Applications When both an optical and a dc field are applied along one axis: P (2) (t)=   {E  cos(  t) + E 0 } 2 =   {½E  2 cos(2  t) + 2E  E 0 cos(  t) + ½E  2 + E 0 2 } Three different modes of oscillation: ½E  2 cos(2  t)Second Harmonic Generation (SHG) 2E  E 0 cos(  t)Electro-Optic Effect (  n) ½E  2 + E 0 2 Optical Rectification  Consider applied optical and DC fields with amplitudes E  and E 0 © Patrick Neyman: patrickneyman.com

7. Noncentrosymmetry Required for Second-Order NLO Response The second-order polarization field strength is given by: If the medium is centrosymmetric, it must possess inversion symmetry, which means the following relationship must hold: which suggests that These relations can hold only when  (2) = 0 © Patrick Neyman: patrickneyman.com

8.  Nonlinear Optics (NLO) Introduction  Device Applications using NLO Molecules  Measurment Techniques  Molecules for NLO Applications  ISAM Film Properties  Thick ISAM Films  Absorption of Second Harmonic  Hybrid Covalent / Ionic Fabrication Method  Thermal Stability  Temporal Stability  Electro-Optic Coefficient Measurement Outline © Patrick Neyman: patrickneyman.com

9. Second-Order NLO Applications For polarization at frequency  the displacement field is Refractive index is dependent upon the applied electric field strength © Patrick Neyman: patrickneyman.com

10. Application Requirements Sufficient asymmetry and conjugation along z-axis –measured as r 33 or Target film thickness = 1  m Thermal stability Temporal stability © Patrick Neyman: patrickneyman.com

11.  Nonlinear Optics (NLO) Introduction  Device Applications using NLO Molecules  Measurment Techniques  Molecules for NLO Applications  ISAM Film Properties  Thick ISAM Films  Absorption of Second Harmonic  Hybrid Covalent / Ionic Fabrication Method  Thermal Stability  Temporal Stability  Electro-Optic Coefficient Measurement Outline © Patrick Neyman: patrickneyman.com

12. Experimental Apparatus © Patrick Neyman: patrickneyman.com

13. Beam Propagation in Sample © Patrick Neyman: patrickneyman.com

14. Longitudinal Intensity Profile of Fundamental Beam SHG intensity scan of the beam along the z-axis at focus Beam travels ~ 1.7 mm within the sample The “focus length” is ~ 3.7 mm. © Patrick Neyman: patrickneyman.com

15. Spatial Intensity Profile of Fundamental Beam Intensity scan in the x-y plane at the focus of a f = 450 mm The beam waist radius is approximately 50  m. Vertical, y (  m) Horizontal, x (  m) = waste radius (  m) © Patrick Neyman: patrickneyman.com

16. Maker fringe equation: For films with l<<l c : For a reference film compared to quartz: For a film compared to the reference: Quartz Measurement © Patrick Neyman: patrickneyman.com

17. Interference Fringe Pattern Signal increases with increased incident angle below 60  due to – decreased reflective loss of the p-polarized light – increased path length – increased coupling with the  (2) tensor l c = 21  m, typical for glass © Patrick Neyman: patrickneyman.com

18. Tilt Angle Measurements 0 100 200 300 400 500 600 700 -100 -80 -60 -40 -20 0 20 40 60 80 100 Polarizer Angle I(2  ) © Patrick Neyman: patrickneyman.com

19.  Nonlinear Optics (NLO) Introduction  Device Applications using NLO Molecules  Measurment Techniques  Molecules for NLO Applications  ISAM Film Properties  Thick ISAM Films  Absorption of Second Harmonic  Hybrid Covalent / Ionic Fabrication Method  Thermal Stability  Temporal Stability  Electro-Optic Coefficient Measurement Outline © Patrick Neyman: patrickneyman.com

20. Organic Chromophores 154DEA-TCVAB 133DMA-DCVS 52DMA-NS 47Disperse Red 1 37NB-DMAA 12DMNA  0 (10 -30 cm 5 /esu) StructureChromophore long conjugation length strong electron donors and acceptors diametric positioning of donors and acceptors  To have large molecular second order NLO responses (  ), organic molecules need: N is number density F is local field factor  is tilt angle away from polar axis where:  © Patrick Neyman: patrickneyman.com

21. Chromophores for NLO Note that some chromophores absorb the second-harmonic (532 nm) This can be a significant effect in thick films © Patrick Neyman: patrickneyman.com

22. Polymers for ISAM Films NLO active polyanions: PCBS, Poly S-119 NLO inactive polycation: PAH © Patrick Neyman: patrickneyman.com

23. Polymers for ISAM Films PAH Poly S-119 Nine repeat units of Poly S-119, nineteen of PAH Modeled in vacuo using ChemDraw 3D © Patrick Neyman: patrickneyman.com

24.  Nonlinear Optics (NLO) Introduction  Device Applications using NLO Molecules  Measurment Techniques  Molecules for NLO Applications  ISAM Film Properties  Thick ISAM Films  Absorption of Second Harmonic  Hybrid Covalent / Ionic Fabrication Method  Thermal Stability  Temporal Stability  Electro-Optic Coefficient Measurement Outline © Patrick Neyman: patrickneyman.com

25. ISAM Films C ∞ symmetry Formation time < 45 sec Layer thickness ~ 1-10 nm Homogeneous Physically robust Temporal stability: 6+ yr Thermal stability: >150  C © Patrick Neyman: patrickneyman.com

26. ISAM Film Formation Immersion in oppositely charged aqueous solutions May repeat indefinitely Structural control at molecular level G. Decher et al. Makromol. Chem., Makromol. Symp. 46, 321 (1991); Thin Solid Films 210/211, 831 (1992). © Patrick Neyman: patrickneyman.com

27. Interfaces are “fuzzy” rather than discrete resulting in a periodically varying density of each material Interpenetration may occur over several monolayers ISAM Film Formation © Patrick Neyman: patrickneyman.com

28. Second Harmonic Intensity (I 2  ) Scales Quadratically Fundamental Intensity (I  )   (ISAM)    (quartz) J. R. Heflin et al. SPIE Proc. 3147, 10 (1997); App. Phys. Lett. 74, 495 (1999). © Patrick Neyman: patrickneyman.com

29. In general:, for l << l c, as here: Chromophore orientation same for all layers Quadratic Growth of SHG with Film Thickness J. R. Heflin et al. SPIE Proc. 3147, 10 (1997); App. Phys. Lett. 74, 495 (1999). © Patrick Neyman: patrickneyman.com

30. Regions of  potential about an ionic endgroup with radius a, from the Debye-Hückel approximation, may be written as where the Debye length  - -1 is the distance at which  is reduced by 1/e, and is given by Effect of Solution Counter Ion Concentration © Patrick Neyman: patrickneyman.com

31. Thickness Controlled by Solution Parameters Constant deposition per bilayer Thickness controlled by pH or NaCl (~1 – 10 nm) © Patrick Neyman: patrickneyman.com

32. Consistency Along Surface Interference fringe data taken for 35 mm along the length of the film, at 0.5 mm intervals For timeliness, each datum was averaged over 10 counts, which is reflected in the roughness of the “surface” The signal remains constant along the length of the slide New technique using Mathematica 4.0 for analysis of several fringe data files to produce map of surface Complete surface map may be obtained in minutes rather than hours Prior to multi-axis stage control, mapping would take several weeks (different sample than that in example of previous technique) © Patrick Neyman: patrickneyman.com

33. Variation of Inactive Polycation pH Quadratic growth of SHG with film thickness Increased cation pH increases anion layer thickness Increased cation pH increases SHG © Patrick Neyman: patrickneyman.com

34. Variation of Inactive Polycation pH Increased pH dramatically increases bilayer thickness  (2) decreases due to increased thickness, despite: –Increased SHG –Decreased tilt angle PAH pH Tilt Angle Bilayer Thickness (nm)  (2) (10 -9 esu) 10 37  9.2 0.33 7 65  0.21 3.1 © Patrick Neyman: patrickneyman.com

35. Impact of Choice of Polycation PCBS with Poly(L-Lysine) or PDDA in place of PAH Chromophore deposition per bilayer constant for each film Some cations fail to exhibit bulk SHG © Patrick Neyman: patrickneyman.com

36. Thermal Stability Heat to Hold for 150 °C 18 hours Cool to room temp © Patrick Neyman: patrickneyman.com

37. Thermal Stability © Patrick Neyman: patrickneyman.com

38. Chromophore degradation accounts for loss in SHG after heating well beyond T g Thermal Stability © Patrick Neyman: patrickneyman.com

39. SHG Recovery Independent of Humidity Identical samples heated to 150 degrees to draw out moisture No difference in second harmonic intensity between cooling in nitrogen environment or cooling in air © Patrick Neyman: patrickneyman.com

40. Interface Effects The susceptibilities asymptotically approach a true value for the film Surface SHG and the lack of interpenetration for the first few layers causes the susceptibility to be artificially inflated The artificial inflation becomes negligible as film thickness is increased © Patrick Neyman: patrickneyman.com

41. The susceptibilities asymptotically approach a true value for the film Surface SHG and the lack of interpenetration for the first few layers causes the susceptibility to be artificially inflated The artificial inflation becomes negligible as film thickness is increased Interface Effects © Patrick Neyman: patrickneyman.com

42. Interface Effects One NLO bilayer of PCBS / PAH Variation of the number of buffer bilayers (PMMA / PAH) between NLO bilayer and substrate © Patrick Neyman: patrickneyman.com

43. Interface Effects One NLO bilayer of PCBS / PAH 20 buffer layers (PMMA / PAH) between glass and film Varying number of buffer bilayers between film and air © Patrick Neyman: patrickneyman.com

44. Interface Effects 5 buffer bilayers each side of NLO ISAM film versus no buffer layers “Artificially inflated” SHG at low number of layers due to effects at film-glass and film-air interfaces One NLO bilayer of PCBS / PAH Variation of the number of buffer bilayers (PMMA / PAH) between NLO bilayer and substrate © Patrick Neyman: patrickneyman.com

45. “Capping” Effect Capped: outer layer is NLO inactive material (PAH) Drop in SHG due to “capping” effect, where outermost chromophores are pulled away from the preferred direction © Patrick Neyman: patrickneyman.com

46.  Nonlinear Optics (NLO) Introduction  Device Applications using NLO Molecules  Measurment Techniques  Molecules for NLO Applications  ISAM Film Properties  Thick ISAM Films  Absorption of Second Harmonic  Hybrid Covalent / Ionic Fabrication Method  Thermal Stability  Temporal Stability  Electro-Optic Coefficient Measurement Outline © Patrick Neyman: patrickneyman.com

47. Thick ISAM Films: 250-bl PCBS Absorbance @ 362 nm is effective thickness Thickness at 1.30 Absorbance is 580 ± 20 nm = 1.1×10 -10 esu © Patrick Neyman: patrickneyman.com

48. SHG Intensity, SHG-Absorbing a)Non-SHG-absorbing film, l c = 1  m (red, solid line), Film with l c = 1  m and a 2  = 5.0  m -1 (blue, dotted line) b)Non-absorbing approximation (green, long-dashed) SHG-Absorbing approximation (purple, short-dashed) © Patrick Neyman: patrickneyman.com

49. SHG Absorption Non-SHG-absorbing film, l c = 10  m (red, solid line), Film with l c = 10  m and a 2  = 0.1  m -1 (blue, dotted line) Absorption may hinder prediction of electro-optic response at telecommunication wavelengths © Patrick Neyman: patrickneyman.com

50. SHG Conversion Efficiency Non-SHG-absorbing film (red, solid line), Film with 1.0 absorbance at the SHG wavelength (blue, dashed line) Absorption may hinder prediction of electro-optic response at telecommunication wavelengths SHG Conversion Efficiency (%)  kL/2 © Patrick Neyman: patrickneyman.com

51. SHG Conversion Efficiency SHG Conversion Efficiency (%)  kL/2 Non-SHG-absorbing film (red, solid line), Film with 1.0 absorbance at the SHG wavelength (blue, dashed line) Thin, mildly-absorbing films may be accurately characterized © Patrick Neyman: patrickneyman.com

52. Thick ISAM Films: 200-bl Poly S-119 1064-nm data follows expected curve for the exhibited SHG absorbance Absorbance @ 480 nm is effective thickness (2.5 ~ 750 nm) Correction obtained via Excel using approximation: where © Patrick Neyman: patrickneyman.com

53. Thick ISAM Films: 200-bl Poly S-119 1064-nm data follows expected curve for the exhibited SHG absorbance 1200-nm data unaffected by SHG absorption Absorbance @ 480 nm is effective thickness Thickness at 2.53 Absorbance is 745 ± 30 nm = 5.8×10 -10 esu,= 3.3×10 -10 esu © Patrick Neyman: patrickneyman.com

54. Shortcoming of Polymer-Polymer Films  (2) should not vary with film thickness Not all chromophores contribute to SHG © Patrick Neyman: patrickneyman.com

55. Summary of Polymer-Polymer Films Thickness grows linearly with number of bilayers Thickness > 750 nm achievable SHG grows quadratically with number of bilayers Increased counterion concentration results in –Increased SHG per bilayer –Increased thickness per bilayer –Decreased  (2)  Increase in thickness outweighs increase in SHG  More loopy polymer conformation results in –Increased chromophore adsorption into “fuzzy” interfaces –Increased thickness of each monolayer –Decreased net polar order © Patrick Neyman: patrickneyman.com

56.  Nonlinear Optics (NLO) Introduction  Device Applications using NLO Molecules  Measurment Techniques  Molecules for NLO Applications  ISAM Film Properties  Thick ISAM Films  Absorption of Second Harmonic  Hybrid Covalent / Ionic Fabrication Method  Thermal Stability  Temporal Stability  Electro-Optic Coefficient Measurement Outline © Patrick Neyman: patrickneyman.com

57. Incorporation of Monomer Chromophores Chromophores between layers of PAH Significant reduction in film thickness Significant increase in net polar order © Patrick Neyman: patrickneyman.com

58. Hybrid Ionic / Covalent Assembly Triazine covalently bonds with non-protonated amines Covalent bonding occurs above pK a of PAH (~9) Ionic bonding of sulfonates with protonated amines below pK a © Patrick Neyman: patrickneyman.com

59. Resonantly Enhanced  (2) SHG measured at various wavelengths in absorbing region of Procion Red Normalized absorbance spectrum shown as green line  (2) expected to increase with increased absorbance © Patrick Neyman: patrickneyman.com

60. Procion Red pH Variation PAH layer thickness increases as pH is increased above pK a Interpenetration increases with pH due to electrostatic screening Reactivity of PR triazine with PAH amine increases with pH PR pH PAH pH Bilayer Thickness (nm) 10.510 4.3 10.5 7 0.52 7 7 0.55 10.5 4.5 0.34 7 4.5<0.3 PR / PAH Growth of SHG with number bilayers indicates bulk  (2) effect PAH pH 10:  (2) zzz  0.5  10 -9 e.s.u. PAH pH 7, 4.5:  (2) zzz  1.1  10 -9 e.s.u.  0.6  (2) zzz (quartz) Procion Red has low molecular hyperpolarizability Minimal reactivity of PR triazine with PAH amine below pK a No bulk polar order observed (only interface effects) Increase in absorption with film thickness due to ionic bonding Angew. Chem. 41 (2002), p3236 PR pH PAH pH Bilayer Thickness (nm) 10.510 4.3 10.5 7 0.52 10.5 4.5 0.34 PR pH PAH pH Bilayer Thickness (nm) 7 7 0.55 7 4.5<0.3 © Patrick Neyman: patrickneyman.com

61. Procion Red Structure Procion Red MX-5B Modeled in vacuo using ChemDraw 3D © Patrick Neyman: patrickneyman.com

62. Procion Brown Structure Procion Brown MX-GRN Modeled in vacuo using ChemDraw 3D © Patrick Neyman: patrickneyman.com

63. Procion Brown NaCl Variation Procion Brown / PAH at pH 10.5 / 7 Peak absorbance grows linearly with number of bilayers Addition of NaCl increases amount of adsorbed chromophores © Patrick Neyman: patrickneyman.com

64. Procion Brown NaCl Variation SHG grows quadratically with number of bilayers Addition of NaCl increases SHG © Patrick Neyman: patrickneyman.com

65. Procion Brown NaCl Variation Maximum benefit of NaCl in 0.25 - 0.50 M region © Patrick Neyman: patrickneyman.com

66. Procion Brown NaCl Variation Tilt angle measured relative to substrate normal (z-direction) 0.50 M yields best chromophore orientation © Patrick Neyman: patrickneyman.com

67. Procion Brown NaCl Variation NaCl (M) peak Abs per bilayer ± 0.0003 Bilayer thickness (nm) ±0.05 nm / bl (a.u.) ± 5% refractive index @ 532 nm Tilt Angle ± 4º, 1º (10 -9 esu) ± 10% (10 -9 esu) ± 12%, 10% 00.00100.261.21.5642.8º1730 0.100.00140.381.91.7140.8º1941 0.250.00220.744.31.8539.1º2256 0.500.00290.955.51.7738.3º2256 1.000.00401.326.21.8139.2º1845 0.50 M NaCl yields best susceptibility and thickness No change in chromophore concentration above 0.25 M 0.50 M and 0.0 M chosen for comparison studies © Patrick Neyman: patrickneyman.com

68. Rendition of Adsorption Surface Mixture of both covalent bonding possibilities for Procion Brown Increased NaCl → Increased contour -- like surface of spaghetti Decreased average tilt angle due to physical restriction Contraction of network further restricts chromophores © Patrick Neyman: patrickneyman.com

69.  Nonlinear Optics (NLO) Introduction  Device Applications using NLO Molecules  Measurment Techniques  Molecules for NLO Applications  ISAM Film Properties  Thick ISAM Films  Absorption of Second Harmonic  Hybrid Covalent / Ionic Fabrication Method  Thermal Stability  Temporal Stability  Electro-Optic Coefficient Measurement Outline © Patrick Neyman: patrickneyman.com

70. Thermal Stability: Procion Brown (0.5 M NaCl) Heat to 85 °C, hold for 36 hours Heat to 150 °C, hold for 24 hours SHG reduced with temperature -- no permanent loss of SHG © Patrick Neyman: patrickneyman.com

71. Thermal Stability: Procion Brown (0.0 M NaCl) 0.0 M NaCl Procion Brown stable at 85 °C, not at 150 °C Loss in SHG does not correspond with loss in absorbance Reorientation of the chromophores away from preferred direction © Patrick Neyman: patrickneyman.com

72. Thermal Stability: Procion Red 0.0 M NaCl Procion Red not stable at 100 °C 40% Loss in SHG, 7% loss in absorbance Reorientation of the chromophores away from preferred direction © Patrick Neyman: patrickneyman.com

73. Thermal Stability: Poly S-119 Poly S-119 stable at 150 °C C. Figura, Ph. D. Thesis, VA Tech (1999) Loss in SHG above 150 °C corresponds with loss in absorbance Temperature-dependent reduction of SHG below 150 °C investigated by temperature dependence of absorbance © Patrick Neyman: patrickneyman.com

74. Thermal Stability: Poly S-119 Temperature-dependent loss of absorbance corresponds with trans-to-cis isomerization Langmuir 15 (1), (1999), p193-201. Trans-to-cis isomerization results in reduced conjugation T © Patrick Neyman: patrickneyman.com

75. Thermal Stability: Poly S-119 Trans-to-cis isomerization induced by UV exposure in right-hand figure Langmuir 15 (1), (1999), p193-201. In both cases: UV absorbance increases visible absorbance decreases T exposure time © Patrick Neyman: patrickneyman.com

76.  Nonlinear Optics (NLO) Introduction  Device Applications using NLO Molecules  Measurment Techniques  Molecules for NLO Applications  ISAM Film Properties  Thick ISAM Films  Absorption of Second Harmonic  Hybrid Covalent / Ionic Fabrication Method  Thermal Stability  Temporal Stability  Electro-Optic Coefficient Measurement Outline © Patrick Neyman: patrickneyman.com

77. Poly S-119 remains stable after >6.5 years PCBS remains stable after ~1.5 years Temporal Stability: Poly S-119, PCBS © Patrick Neyman: patrickneyman.com

78. Temporal Stability: Procion Red Procion Red films exhibit decrease in polar order © Patrick Neyman: patrickneyman.com

79. Temporal Stability: Procion Brown 0.5 M NaCl remains stable after >420 days 0.0 M NaCl exhibits increase in net polar order © Patrick Neyman: patrickneyman.com

80.  Nonlinear Optics (NLO) Introduction  Device Applications using NLO Molecules  Measurment Techniques  Molecules for NLO Applications  ISAM Film Properties  Thick ISAM Films  Absorption of Second Harmonic  Hybrid Covalent / Ionic Fabrication Method  Thermal Stability  Temporal Stability  Electro-Optic Coefficient Measurement Outline © Patrick Neyman: patrickneyman.com

81. Electro-Optic Measurements Al electrode ISAM film ITOGlass substrate Polarizer Analyzer Babinet- Soleil V Teng and Man electro-optic measurement (Appl. Phys. Lett. 56, 1734 (1990)) 1 kHz ac voltage between ITO and Al modulates s- and p-polarized refractive indices through r 33 and r 13, varying phase between s- and p-polarizations Modulation of intensity through crossed analyzer detected by lock-in amplifier 50-bilayer Procion Brown/PAH films with 0.5 M ionic strength have r 33 1/2 that of lithium niobate (30 pm/V) FilmDevicer 33 – r 13 (pm/V)r 33 (pm/V)Tilt Angle 0.5 M NaCl (not soaked) 18.614.3 41.9º 27.011.8 0.5 M NaCl (soaked) 18.214.2 42.5º 28.214.2 0.0 M NaCl (not soaked) 11.93.9 45.6º 22.04.3 © Patrick Neyman: patrickneyman.com

82. Conclusions  Thickness:750 nm film thickness achieved Quadratic scaling of SHG with thickness  Thermal Stability:Procion Brown stable at 150 °C for 24 hours after holding at 85 °C for 36 hours  Temporal Stability:No loss in Procion Brown SHG after 420 days  Electro-Optic Properties:r 33 of Procion Brown is ½ that of lithium niobate Significant Milestones Toward Application Requirements © Patrick Neyman: patrickneyman.com

83. Acknowledgments J. R. Heflin, Chair R. M. Davis H. W. Gibson G. Indebetouw H. Marand Presented to the committee on June 16, 2004 VPI Chemistry –H. W. Gibson –H. Wang VPI Chem-Eng –R. M. Davis –K. E. Van Cott VPI Physics –J. R. Heflin –C. Brands –C. Figura Luna Innovations –D. Marciu –M. Miller © Patrick Neyman: patrickneyman.com