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Nonlinear Optics of Devices SURFACE MEDIATED NONLINEAR OPTIC EFFECTS IN LIQUID CRYSTALS by JESSICA M. MERLIN Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Thesis Advisor: Kenneth D. Singer, Ph.D. DEPARTMENT OF PHYSICS CASE WESTERN RESERVE UNIVERSITY MAY, 2007 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the dissertation of ______________________________________________________ candidate for the Ph.D. degree *. (signed)_______________________________________________ (chair of the committee) ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________ ________________________________________________ (date) _______________________ *We also certify that written approval has been obtained for any proprietary material contained therein. To my parents iv Table of Contents Chapter 1 Introduction ......................................................................................................................1 1.1 The Liquid Crystal Phases .......................................................................................2 1.2 Liquid Crystal Applications.....................................................................................6 1.2.1 Liquid Crystal Displays ...............................................................................6 1.2.2 Temperature Sensing...................................................................................8 1.2.3 Photorefraction, Spatial Light Modulation and Optical Limiting................9 1.3 Thesis Organization...............................................................................................12 1.4 References..............................................................................................................14 Chapter 2 Theory & Background .......................................................................................................16 2.1 Nematic Liquid Crystal Basics ..............................................................................16 2.1.1 Order Parameter.........................................................................................18 2.1.2 Bulk Properties...........................................................................................19 2.1.3 Surface Properties......................................................................................21 2.2 Nonlinear Optical Properties..................................................................................24 2.2.1 Friedericksz Transition..............................................................................24 2.2.2 Adiabatic Following of Light.....................................................................27 2.2.3 Mechanism of the Photorefractive Effect ..................................................29 2.2.4 The Photorefractive Effect in Liquid Crystals: Bulk Considerations...................................................................................31 2.3 Surface Mediated Effects.......................................................................................35 2.3.1 Photorefractive Measurements...................................................................36 2.3.2 Electronic Measurements...........................................................................40 2.3.3 Other Interesting Photorefractive Results..................................................45 2.3.4 Optical Switching Applications .................................................................48 2.4 Device Applications...............................................................................................49 2.5 Summary and Conclusions ....................................................................................50 2.6 References..............................................................................................................51 Chapter 3 Experimental Methods.......................................................................................................58 3.1 Materials and Sample Geometry............................................................................58 3.1.1 Sample Geometry.......................................................................................58 3.1.2 Materials ....................................................................................................59 3.1.3 Properties of PVK-TNF .............................................................................63 3.1.4 C60 Doped Liquid Crystal Cells .................................................................64 3.2 Experimental Apparatus.........................................................................................65 3.2.1 Friedericksz Transition Measurement........................................................65 3.2.2 Gate Measurement .....................................................................................67 3.2.3 Current Transient Measurement ................................................................69 3.2.4 Optical Limiting Measurement..................................................................70 v 3.3 Summary................................................................................................................70 3.4 References..............................................................................................................72 Chapter 4 Experimental Results: PVA ...............................................................................................73 4.1 Friedericksz Transition Shift..................................................................................74 4.2 Optical Gate Effect ................................................................................................77 4.3 Intensity Dependence of the Friedericksz Transition ............................................79 4.4 Optical Limiting.....................................................................................................81 4.5 Conclusions............................................................................................................83 4.6 References..............................................................................................................84 Chapter 5 Experimental Results: PVK ...............................................................................................86 5.1 Demonstration of the Surface Effect......................................................................87 5.2 Polarity Dependence ..............................................................................................89 5.3 Intensity Dependence.............................................................................................93 5.4 Thickness Dependence...........................................................................................97 5.4.1 Cell Thickness Dependence.......................................................................97 5.4.2 Alignment Layer Thickness Dependence ................................................100 5.5 Electronic Measurements.....................................................................................102 5.5.1 Capacitance-Voltage Measurements........................................................103 5.5.2 Frequency Dependence of the Friedericksz Transition Threshold ..........103 5.6 Optical limiting and optical switching.................................................................104 5.7 Discussion & Conclusions ...................................................................................105 5.8 References............................................................................................................107 Chapter 6 Summary of Results and Proposed Model, Comparison to the Literature, Discussion & Conclusions, and Future Outlook ....................................................................................108 6.1 Summary of Results and Proposed Model for cells with PVK-TNF...................108 6.2 Summary of Results and Proposed Model for cells with PVA............................114 6.3 Disscussion and Comparison to the Literature ...................................................116 6.4 Side by Side Comparison of cells made with PVK-TNF and PVA.....................120 6.5 Future Outlook.....................................................................................................123 6.6 Discussion and Conclusions ................................................................................125 6.7 References............................................................................................................127 vi List of Tables Chapter 2 Table 2.1 Diffraction characteristics for different combinations of liquid crystals and alignment layers..................................................................................................37 Table 2.2 Two-Beam coupling figures of merit for various cell types. The range of experimental condition for two-beam coupling suggests that a more direct approach is necessary to evaluate the mechanism as well as the device parameters...........................................................................................................49 Chapter 3 Table 3.1 Some material characteristics of 5CB.................................................................60 Table 3.2 Some material properties of PVK.......................................................................62 vii List of Figures Chapter 1 Figure 1.1 Graphical depiction of the degree
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