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Preface

List of Acronyms

List of Principal Symbols and Operators

Chapter 1 Overview

1.1 Introduction

1.2 From Columnar to Sculptured Thin Films

1.2.1 Columnar thin films

1.2.2 Growth mechanics

1.2.3 CTFs as dielectric continuums

1.2.4 Primitive STFs with nematic morphology

1.2.5 Chiral STFs

1.2.6 Sculptured thin films

1.3 Time-Harmonic Electromagnetic Fields

1.3.1 Linear constitutive relations

1.3.2 Electromagnetic wave propagation

1.3.3 Structure-property relationships

1.4 Optical Applications of STFs

1.4.1 Optical filters

1.4.2 Optical fluid sensors

1.4.3 Displays

1.4.4 Optical interconnects

1.4.5 Optical pulse-shapers

1.4.6 Biochips and biosensors

1.4.7 STFs with transverse architectures

1.5 Other Applications

1.6 Prognostications

Chapter 2 History of Thin-Film Morphology

2.1 Synoptic view

2.2 Early history (pre-1940)

2.3 Intermediate history (1940-1970)

2.4 Recent history (1970-2004)

2.5 Low-adatom-mobility morphology

Chapter 3 PVD Methods for STFs

3.1 Important Factors for STF Deposition

3.2 STF Deposition Methods

3.2.1 Thermal evaporation

3.2.2 Sputtering

3.2.3 Bombardment-enhanced evaporation

3.2.4 Ion-beam methods

3.3 Desirable Future Developments

Chapter 4 Engineering of Thin-Film Morphology

4.1 Continuum of Morphology

4.1.1 Variation of energy of bombarding ions

4.1.2 Variation of vapor incidence angle

4.1.3 Variation of substrate rotation velocity and vapor flux density

4.1.4 Toward a quantitative and evolutionary SZM

4.2 From Concepts to Quantification

4.2.1 Quantitative analysis of morphology

4.2.2 Computer simulation of morphology

4.2.2.1 Geometric models

4.2.2.2 Continuum models

4.2.2.3 Ballistic aggregation models

4.2.2.4 Molecular dynamics models

4.3 Matchstick Morphology

4.3.1 Dense arrays of parallel columns

4.3.2 Arrays of separated parallel columns

4.4 Anisotropy Due to Atomic-Level Self-Shadowing

4.5 Dynamic Self-Shadowing

Chapter 5 Speculations on STF Morphology

5.1 Deposition on nonplanar substrates

5.2 Controlled low-energy bombardment

5.3 Self-shadowing, again!

5.4 Distribution functions

Chapter 6 Macroscopic Electromagnetism

6.1 Macroscopic Maxwell Postulates

6.1.1 Microphysics route

6.1.2 Spacetime route

6.1.3 Time-harmonic Maxwell postulates

6.2 Constitutive Relations

6.2.1 Linear dielectric materials

6.2.2 Linear bianisotropic materials

6.3 Constitutive Relations of STFs

6.3.1 Single-section STFs

6.3.2 Multisection STFs

6.4 From the Nanostructure to the Continuum

6.4.1 Local homogenization

6.4.2 Nominal model

6.4.3 Practical bianisotropy in STFs

6.5 Dielectric STFs

6.5.1 Relative permittivity dyadic

6.5.2 Nominal model

Chapter 7 Optics of Columnar Thin Films

7.1 Electromagnetic Fundamentals

7.1.1 MODE

7.1.2 Exact analytical solution

7.1.3 Propagation in the morphologically significant plane

7.2 Reflection and Transmission

7.2.1 Incident, reflected, and transmitted plane waves

7.2.2 Boundary value problem

7.3 Normal Incidence

7.3.1 Wave plates

7.3.2 Multilayers

7.3.3 Morphology and optics

Chapter 8 Optics of Sculptured Nematic Thin Films

8.1 Electromagnetic Fundamentals

8.1.1 MODE

8.1.2 Matrizant

8.1.3 Matrix polynomial expansion technique

8.1.4 Piecewise uniform approximation technique

8.1.5 Propagation in the morphologically significant plane

8.1.6 Axial propagation

8.2 Reflection and Transmission

8.2.1 Normal incidence

8.2.2 Rugate filters

8.2.3 Morphology and optics

Chapter 9 Optics of Chiral STFs

9.1 Electromagnetic Fundamentals

9.1.1 MODE

9.1.2 Oseen transformation

9.1.3 Matrizants

9.2 Transfer Matrix

9.2.1 Axial propagation

9.2.2 Nonaxial propagation

9.2.3 Numerical methods

9.3 Reflection and Transmission

9.3.1 Incident, reflected, and transmitted plane waves

9.3.2 Boundary value problem

9.3.3 Bragg phenomenons

9.3.4 Circular Bragg phenomenon

9.3.5 Circular Borrmann effect

9.3.6 Excitation by finite-sized sources

9.4 Normal Incidence

9.4.1 Lorentz model of permittivity

9.4.2 Remittances

9.4.3 Coupled-wave expressions

9.4.4 Dichroisms

9.4.5 Optical rotation

9.4.6 Axial propagation

9.5 Chiral STF Half-Space

9.5.1 Planewave reflectances

9.5.2 Pulse bleeding

9.6 Morphology and Optics

Chapter 10 Optical Applications of Chiral STFs

10.1 Optical Filters

10.1.1 Circular polarization filters

10.1.2 Bandstop filters and laser mirrors

10.1.3 Bandpass filters

10.1.4 Polarization-discriminatory handedness inverter

10.1.5 Narrow bandpass filters

10.1.6 Ultranarrow bandstop filters

10.1.7 Solc filters

10.2 Optical Sensors

10.3 Optical Emitters

10.4 Tuning and Bandwidth Control

Appendix: Dyads and Dyadics

Bibliography

Index

Preface

Picking up a polished sample of ulexite, one of us thought that the parallel fibrous microstructure of this mineral was not unlike the matchstick morphology of columnar thin films shown to him by the other some years earlier. A telephone conversation led to a brainstorming session from which the mathematical concept of sculptured thin films--STFs for short--began to emerge. That was in 1992. A 1966 paper of J.M. Niuewenhuizen and H.B. Haanstra provided the initial morphological bedrock, which was brought into optical focus by a 1989 paper of T. Motohiro and Y. Taga. Four years after our initial presentation in 1994 at a conference in France, and three years after our definitive presentation of the STF concept at Penn State, we stumbled upon a 1959 paper of N.O. Young and J. Kowal that provided an antecedent confirming the STF concept, which ties morphology and optics together.

Imagine a mass of isolated parallel matchsticks, all stuck on their lower ends to a glassy substrate. This arrangement describes columnar thin films. Imagine further that a still-life of the matchstick arrangement were painted by Salvador Dali: all the matchsticks--still isolated, still parallel to each other, and still propped upward on the substrate--were depicted not straight but bent in some fanciful form. The imaginary Daliesque painting describes STFs. The matchsticks of cross-section diameter between 10 and 300 nm comprise clusters of sizes between 1 and 3 nm. Thus, the columns of an STF are shaped nanowires, and STFs can be considered as constituting a class of nanoengineered materials.

The optical response of a columnar thin film is like that of an orthorhombic crystal. Upon passage through a columnar thin film, the vibration ellipse of a plane wave is rotated and its axial ratio altered. Most importantly, a columnar thin film for optical purposes is effectively homogeneous. Think of a stack of extremely thin slices of an orthorhombic crystal. This stack is equivalent to a single-section STF for optical purposes. A single-section STF is therefore functionally nonhomogeneous in the thickness direction, but homogeneous in any transverse plane. In addition, it is effectively anisotropic. A multisection STF is a stack of single-section STFs. Upon passage through an STF, the vibration ellipse of a plane wave is rotated and its axial ratio altered in a desired fashion that can be incorporated by design into the morphology of the STF. There are, of course, nonoptical applications of STFs, which lie outside the scope of this book.

Our aim here is to provide the reader a basic knowledge of the morphology and the optical response characteristics of STFs, which are nanoengineered by directional physical vapor deposition (PVD) onto substrates at oblique angles. While writing this book, our intent was not to simply compile and discuss the literature on STFs; in this day and age, that can be accomplished quite easily by anyone with the extensive electronic databases readily available. Rather, our intent was to lay the foundation for understanding thin-film morphology so that scientists and technologists can design and engineer STF materials and devices for future applications, in particular optical applications. As such, the focus of this book is to couple the most detailed knowledge of thin-film morphology--that includes the anisotropic, nanoscale clustering critical to STFs--with the response characteristics of optical STF devices.

We have consciously avoided developments that we consider either too primitive or irrelevant to the theme of this book. For instances: (i) STFs made of magnetic materials have drawn some slight attention, but reported investigations are too rudimentary to be included in this text; (ii) photonic crystals made by directional PVD are not STFs due to transverse nonhomogeneity at the wavelength scale; (iii) isotropic thin films are not STFs due to the absence of anisotropy; and (iv) organic STFs have not been included since only a couple of reports on polymeric STFs have been published to date. As research on STFs continues unabated, a complete treatment of all aspects of STFs is not possible at this time. We apologize if we left out a few topics dear to the reader's heart, but let us also note that we left out some parts of our own recent research on STFs.

The STF concept has been resolutely mathematical from its emergence--to enable precise and predictable engineering of the vibration ellipse. Much experimental effort has been directed toward the realization of that goal. This we hope to have reflected in the 10 chapters of this book. Chapter 1 is a bird's eye view of the past, the present, and the future of STFs, and is thus a book within a book. It can be read either all by itself or as an introduction to the following chapters. Chapters 2 to 5 focus on the shaped-nanowire morphology of STFs at the 1- to 1000-nm length scales, with emphasis on ways to achieve the desired morphology through simple movements of the substrate during growth. Chapters 6 to 9 focus on the optical properties of STFs, the effect of morphology on the reflection and transmission characteristics, and the principles underlying STF devices such as filters, polarizers, sensors, and radiators. Mathematica[TM] programs are provided in the text and on the accompanying CD so that the presented formalisms can be easily put to use. We expect that this book will enable the reader to select conditions to grow STFs with distinct morphologies, to understand opportunities and limitations of the evolution of morphology, to solve electromagnetic equations in order to compute reflectances and transmittances, and eventually to engineering the morphology in order to fabricate optical STF devices with desirable polarization and bandwidth characteristics.

This book is aimed toward graduate students in optics at universities as well as toward practicing engineers in the optics industry. Expert researchers may find it useful in extending the STF concept and applications. Furthermore, we expect that the book is accessible to anyone who is interested in emerging nanotechnologies for optical devices as well as optics-based devices, provided he/she has taken typical undergraduate physics courses in optics and electromagnetism. Some knowledge of vectors, matrixes, calculus, and differential equations is also necessary.

And now to the pleasant duty of acknowledging our debts of gratitude to many fine colleagues and friends:

Over the years, we have benefited from collaborations with several leading researchers worldwide. For our work on STFs, we thank (in alphabetical order) Michael J. Brett (University of Alberta, Edmonton), Francesco Chiadini (Universita di Salerno), Robert W. Collins (University of Toledo), Tariq Gilani (Millersville University), Ian J. Hodgkinson (University of Otago), Mark W. Horn (Penn State), Martin W. McCall (Imperial College London), John A. Polo, Jr. (Edinboro University of Pennsylvania), Kevin Robbie (Queen's University, Kingston, Ontario), the late Werner S. Weiglhofer, and Qi hong Wu (University of Otago). Current and former students to whom we are indebted include Matthew D. Brubaker, Ryan J. Carey, Elif Ertekin, Craig Frankel, Joseph B. Geddes III, Thomas Gehrke, Ajay P. Giri, Robert Knepper, David P. Lewis, Mark W. Meredith, Jason T. Moyer, Steven F. Nagle, Frank Papa, Matthew D. Pickett, Wilfredo Otano, Randy C. Ross, Pablo I. Rovira, Ronnen A. Roy, Joseph A. Sherwin, Erik E. Steltz, Paul D. Sunal, Philip Swab, Vijayakumar C. Venugopal, Bangyi Yang, Joseph E. Yehoda, Howard S. Witham, Fei Wang, and Jianwei Wang.

We are grateful to Alvaro Gomez (Universidad de Cantabria) and Mark W. Horn (Penn State) for supplying important illustrations, and to Alberto Lopez Galindo (Universidad de Granada) for a sample of agate mineral. We thank Francisco Chiadini (Universita di Salerno), Didier Felbacq (Universite Montpellier II), Claes-Goran Granqvist (Uppsala Universitet), Tom G. Mackay (University of Edinburgh), and Walid Tabbara (Universite Paris VI) for helping us locate old publications. We are indebted to Thomas Gehrke (Intrinsic Semiconductor) for translating old German publications.

Appreciation is extended to Craig F. Bohren (Penn State) for several discussions on planar optics during the course of writing this book, to S.V. Krishnaswamy (Northrop Grumman) for joint research on the morphology of thin films, and to Juan-Manuel Garcia-Ruiz (Universidad de Granada) for a crucial collaboration on fractal morphology of materials. Also, we gratefully acknowledge our debt of gratitude to two prepublication reviewers.

We thank our colleagues, the late Werner S. Weiglhofer (University of Glasgow) and Francesco Costanzo (Penn State) for assistance in typesetting the book. Thanks are due to Joseph B. Geddes III, Natalya S. Lakhtakia, Paul D. Sunal, Fei Wang, and Jian Xu for carefully going through various drafts of the manuscript. Penn State authorities kindly granted us both sabbatical leaves of absence for a semester, during which period a large part of this book was written. We also thank Richard P. McNitt and Judith A. Todd for sustained support of our STF research for many years.

Rick Hermann and Sharon Streams at SPIE supported our book proposal from its inception, while Margaret Thayer shepherded the production of the book, for which we are very grateful. We take this opportunity to also thank the office-bearers of SPIE for sustaining the scholarly endeavors of not only us but of many other colleagues worldwide.

Without the stability brought in our lives by our respective spouses, Mercedes Lakhtakia and Linda Messier, and their unstinted encouragement, this book would have taken several more years to write. To them this book is affectionately dedicated.

Akhlesh Lakhtakia and Russell Messier
University Park, PA
October 2004