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