P. Paramasivam - Final Thesis.pdf

STUDIES ON THE GROWTH AND CHARACTERIZATION

OF SOME OPTICAL CRYSTALS

Thesis of the research work submitted to

Bharathidasan University, Thiruchirappalli in partial fulfillment

of the requirements for the award of the degree of

DOCTOR OF PHILOSOPHY

IN

PHYSICS

Submitted by

P.PARAMASIVAM

Under the Supervision of

Dr. C. RAMACHANDRA RAJA, Ph.D., Associate Professor in Physics

POSTGRADUATE & RESEARCH DEPARTMENT OF PHYSICS

GOVERNMENT ARTS COLLEGE (AUTONOMOUS)

KUMBAKONAM - 612001

TAMIL NADU, INDIA

FEBRUARY 2012

Dr. C. Ramachandra Raja, Ph.D.,

Associate Professor in Physics,

Department of Physics,

Government Arts College (Autonomous), Phone : +91 4364221751

Kumbakonam 612001, CelL: +91 9976696277

Tamil Nadu, India. Email: [email protected]

CERTIFICATE

This is to certify that the thesis entitled STUDIES ON THE GROWTH

AND CHARACTERIZATION OF SOME OPTICAL CRYSTALS submitted by

Mr. P.PARAMASIVAM is a bonafide record of the research work done by him

during the period of study from 2004 to 2011 under my supervision in the Department

of Physics, Government Arts College (Autonomous), Kumbakonam and that the thesis

has not previously formed the basis for the award of any Degree, Diploma,

Associateship, Fellowship or any other similar title. This thesis represents an

independent work on the part of candidate.

Kumbakonam C. Ramachandra Raja

(Research Supervisor)

Mr. P.Paramasivam,

Research Scholar (Part - Time),

Department of Physics,

Government Arts College (Autonomous),

Kumbakonam 612001,

Tamil Nadu, India.

DECLARATION

I hereby declare that the work presented in this thesis entitled STUDIES

ON THE GROWTH AND CHARACTERISATION OF SOME OPTICAL

CRYSTALS has been originally carried out by me under the guidance and

supervision of Dr.C.Ramachandra Raja, Associate Professor, Department of

Physics, Government Arts College (Autonomous), Kumbakonam. This work has not

been submitted either in whole or in part for any other Degree or Diploma at any

Universities or Research Institutes.

Kumbakonam P.Paramasivam

ACKNOWLEDGEMENT

The author deeply expresses his wholehearted gratitude to his respectful guide

and supervisor Dr. C. RAMACHANDRA RAJA, Associate Professor, Department

of Physics, Government Arts College (Autonomous), Kumbakonam, India for his

effective guidance, continuous encouragement, and who has had a profound influence

to complete the research and thesis work. This thesis shall always bear testimony to

my respect and gratitude towards my mentor.

The author expresses his sincere gratitude to Dr.J.Govindhadas, Principal,

Government Arts College, Kumbakonam, India and extends his profound thanks to

Dr.K.C.Srinivasan, Head of the Department of Physics, Government Arts College,

Kumbakonam, India for providing this opportunity.

The author is deeply thankful to Dr.R.Jayavel, Director, Department of Nano

Technology, Anna University, Chennai, India, Dr.R.Mohan Kumar, Professor,

Presidency College, Chennai, India, Dr.N.Vijayan, Scientist, NPL, New Delhi, India,

Dr.V.Manivannan, Addl. Director (CRD), PRIST University, Thanjavur,

Prof.R.S.Sundararajan, Department of Physics, Govt. Arts College, Kumbakonam,

India, Mr.B.Vijayabhaskaran, Assistant Professor of Physics, Anjalai Ammal-

Mahalingam Engineering College, Kovilvenni, Tiruvarur, India and Dr.A.Antony

Joseph, Assistant Professor of Physics, Annai Enggineering college, Kumbakonam,

India for their valuable suggestions, fruitful discussions and immense help at various

phases of the research.

iv

The author records his immense gratitude to Dr.P.K.Das, Professor, IPC, IISc,

Bangalore, India for providing the opportunity to do the NLO study. The author also

gratefully acknowledges the valuable help extended by the authorities of SAIF, IIT,

Chennai, India, ICP, CECRI, Karaikudi, India and ACIC, St. Josephs College,

Tiruchirappalli, India to carryout the desired studies.

The author expresses his deep sense of gratitude to Dr.M.Arivazhagan,

Assistant Professor of Physics, A.A. Government Arts College, Musiri, India,

Dr.I.Vethapothakar, Assistant Professor of Physics, Anna University of Technology,

Thiruchirappalli, Mr.M.Chitravel, Assistant Professor of Chemistry, T.R.P. Engg.

College, Thiruchirappalli, India, Ms.D.Trixy Nimmy Priscilla, Lecturer, Department

of Physics, Anjalai Ammal-Mahalingam Engineering College, Kovilvenni, Tiruvarur,

India for their consistent support throughout this work.

The author is also thankful to Prof. M.Arulanandasamy Department of

English, Anjalai Ammal-Mahalingam Engineering College, Kovilvenni, Tiruvarur and

Dr.P.Arangamsamy, Head of the Department of English, periyar Maniammai

University, Thanjavur for their careful revision and proof- reading of the text at every

stage of its preparation.

The author extends his sincere thanks to all the teaching and non teaching staff

members of the Department of Physics, Government Arts College, Kumbakonam for

their continued support.

v

Lastly, and most importantly, the author wants to thank his parents, wife and

sons without whom this work could not have been accomplished successfully. Their

support, even at the cost of their personal comfort and needs, is worth the whole

world. The author also thanks his entire extended family and friends for having

provided a loving environment for him during the whole course of his research work.

The Author

vi

TABLE OF CONTENTS

Chapter No Title Page No

Preface xii

List of Publications xvi

Conferences / Seminars xvii

List of Tables xviii

List of Figures xix

List of Symbols xxi

List of Abbreviations xxii

1 INTRODUCTION TO CRYSTAL GROWTH AN OVERVIEW

1.1 INTRODUCTION 1

1.2. NUCLEATION 3

1.2.1. Kinds of Nucleation 4

1.2.2. Classical Theory of Nucleation 5

1.2.3. Kinetic Theory of Nucleation 5

1.3. STABILITY OF NUCLEUS 6

1.4. ENERGY FORMATION OF SPHERICAL NUCLEUS

7

1.5. SUPERSATURATION AND ITS EXPRESSION 10

1.6. CLASSIFICATION OF CRYSTAL GROWTH 11

1.6.1. Growth from Melt 12

1.6.2. Growth from Vapour 15

1.6.3. Growth from Solution 17

1.7. GEL GROWTH 18

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1.8. HYDROTHERMAL GROWTH 19

1.9. FLUX GROWTH 19

1.10. LOW TEMPERATURE SOLUTION GROWTH 20

1.10.1. Slow Cooling Method 21

1.10.2. Temperature Gradient Method 22

1.10.3. Slow Evaporation Method 22

1.11. CRITERIA FOR OPTIMIZING SOLUTION GROWTH

23

1.11.1. Material Purification 23

1.11.2. Solvent Selection 24

1.11.3. Solubility 24

1.11.4. Solution Preparation and Crystal Growth 25

1.11.5. Crystal Habit 25

1.12 ADVANTAGES OF LOW TEMPERATURE SOLUTION GROWTH TECHNIQUE

26

2 AN OVERVIEW OF OPTICAL MATERIALS

2.1. INTRODUCTION 28

2.2 IMPORTANCE OF CRYSTALS AS OPTICAL MATERIALS

30

2.3 NONLINEAR OPTICAL MATERIALS 31

2.4. THEORETICAL EXPLANATION OF NONLINEAR OPTICS

33

2.5. VARIOUS TYPES OF NLO EFFECTS 36

2.5.1. Second Harmonic Generation 37

2.5.2. Sum Frequency Generation 38

2.5.3. Difference Frequency Generation 39

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2.5.4. Optical Parametric Generation 39

2.5.5. Linear Electro Optic Effect 40

2.5.6. Optical Rectification 40

2.6. NONLINEAR OPTICAL MATERIALS 40

2.7. DEVELOPMENT OF NLO MATERIALS 41

2.7.1. Organic Crystals 42

2.7.2. Semi-Organic Crystals 44

2.7.3. Inorganic Crystals 45

2.8. SQUARIC ACID, L-PROLINE, GLYCINE AND THIOCYANATE BASED OPTICAL CRYSTALS

46

2.9. SCOPE OF THE RESEARCH WORK 50

3 CHARACTERIZATION TECHNIQUES


3.2. SINGLE CRYSTAL XRD STUDIES 53

3.2.1. Principle of X-ray diffraction 53

3.2.2. Sample Selection and Preparation 55

3.2.3. Sample Mounting 55

3.2.4. Sample Centering 55

3.3 POWDER X-RAY DIFFRACTION STUDIES 57

3.3.1. X-ray Powder Diffractometer 58

3.4. FT-IR SPECTRAL ANALYSIS 60

3.4.1. Preparation of Liquid Sample 64

3.4.2. Preparation of Solid Sample 64

3.5 NUCLEAR MAGNETIC RESONANCE ANALYSIS 65

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3.5.1 Introduction 65

3.5.2 NMR Spectroscopy - Principle 66

3.5.3 Nuclear spins 66

3.5.4 NMR Spectrometer - Construction 68

3.5.5 NMR Spectrometer - Working 69

3.5.6 Applications of NMR Spectroscopy 69

3.6. UV-Vis-NIR SPECTROSCOPY 70

3.7. THERMAL STUDIES 72

3.7.1. Differential Thermal Analysis 74

3.7.2. Thermogravimetry Analysis 75

3.8. KURTZ POWDER METHOD 77

3.8.1. Introduction 77

3.8.2. Experimental Procedure 77

4 SYNTHESIS, GROWTH AND CHARACTERIZATION OF A NEW NONLINEAR OPTICAL MATERIAL: 4-PHENYLPYRIDINIUM HYDROGEN SQUARATE (4PHS)


4.2. EXPERIMENTAL PROCEDURE 81

4.3. CHARACTERIZATION STUDIES

4.3.1. Single Crystal X-RD Analysis 82

4.3.2. FT-IR Spectral Analysis 83

4.3.3. Nuclear magnetic resonance 86

4.3.4 Optical transmission spectrum analysis 88

4.3.5. Second Harmonic Generation Analysis 89

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4.3.6. Thermal Analysis 90

4.4. CONCLUSION 91

5 GROWTH AND CHARACTERIZATION OF A NEW NONLINEAR OPTICAL CRYSTAL: GHS



5.3. CHARACTERIZATION STUDIES 96


5.3.2. Powder XRD Analysis 97


5.3.4. Optical Transmission Spectrum Analysis 101

5.3.5. Nuclear magnetic resonance 102


5.3.7 Thermal Analysis 106

5.4. CONCLUSION 107

6 CRYSTALLIZATION AND CHARACTERIZATION OF A NEW NONLINEAR OPTICAL CRYSTAL: LPS



6.3. CHARACTERIZATION STUDIES 111






6.4. CONCLUSION 117

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7 GROWTH AND CHARACTERIZATION OF CADMIUM MANGANESE THIOCYANATE (CMTC) CRYSTAL



7.3. CHARACTERIZATION 121





7.4. CONCLUSION 126

8 GROWTH AND CHARACTERIZATION OF ZINC MANGANESE THIOCYANATE (ZMTC) CRYSTAL



8.3. CHARACTERIZATION STUDIES





8.4. CONCLUSION 134

9 SUMMARY AND SUGGESTIONS FOR FUTURE WORK

9.1. SUMMARY 135

9.2. SUGGESTIONS FOR FUTURE WORK 137

REFERENCES 139

ANNEXURE

xii

PREFACE

During the last decades the growth of single crystals has assumed enormous

importance for both academic research and technology. Atomic arrays that are

periodic in three dimensions with repeated distances are called single crystals. It is

clearly more difficult to prepare single crystals than poly-crystalline material and extra

effort is justified because of the outstanding advantages of single crystals. Nonlinear

optical materials are gaining attention due to their enormous applications in

telecommunication activities such as optical computing, laser remote control, optical

modulators, data processing, color display and medical diagnostic. Both organic

materials and inorganic materials were used for research work. Second harmonic

generation is a nonlinear optical process in which photons interacting with a nonlinear

material are effectively combined to form new photons with twice the energy and

therefore, twice the frequency and half the wavelength of the initial photons. In the

present research work, the optical property arises due to donor and acceptor groups at

the opposite ends of the molecule which produces dipolar structure. It has been long

recognized that the electronic structure and the strength of donor and acceptor groups

are responsible for achieving optical properties.

The thesis comprises of nine chapters. The first chapter is an over view of

crystal growth and nonlinear optical phenomenon. An overview of optical materials is

discussed in the second chapter. The third chapter describing the different

characterization techniques involved in this thesis.

xiii

The fourth chapter deals with growth and characterization of

4-phenylpyridinium hydrogen squarate (4PHS) crystal. Single crystals of 4PHS have

been successfully synthesized by slow evaporation solution growth method. The

measurements from the single crystal XRD indicates that the crystal belongs to

monoclinic crystal system and its unit cell parameters have been determined. The

vibrational frequencies have been reported using FTIR technique. The presence of

carbon and protons has been confirmed from the 13C and 1H NMR analyses. It is

found that the crystal is transparent in the range of wavelength 240-2000 nm. The UV

transparency cut-off wavelength of 4PHS crystal occurs at 240 nm. The relative SHG

efficiency has been determined by Kurtz powder technique and found to be five times

greater than that of KDP. The presence of SHG exhibits the NLO property of the

grown crystal. The sharp endothermic peak at around 2600C is assigned as the melting

point of 4PHS crystal.

The fifth chapter presents the growth and characterization of glycinium

hydrogen squarate (GHS) crystal. Single crystals of glycinium hydrogen squarate

were grown by adopting the slow evaporation solution growth method using de-

ionized water as solvent at room temperature. From the single crystal XRD and

Powder XRD measurements, it is observed that the crystal belongs to monoclinic

system. The functional groups were confirmed by FTIR technique. The material has

extended its transmission greater than 90% for light with incident wavelengths from

390-1100 nm. The UV cut-off wavelength of GHS crystal occurs at 342 nm. The

chemical structure has been confirmed from 1H and 13C-NMR analysis. The relative

efficiency of SHG has been determined from Kurtz powder technique and found to be

xiv

17% of that of KDP. From the DTA/TGA curve, it is observed that the material is

stable upto 1500C, which denotes the melting point of the substance.

The sixth chapter describes the growth and characterization of L-Proline

succinate (LPS) crystal. A new non-linear optical crystal with an interesting

hydrogen bonding network that holds together the L-Proline and succinic acid

molecules was synthesized. The grown crystals were characterized by different

instrumental techniques. The dimension of the grown crystal is 8x5x7mm3. The

single crystal XRD studies proved that the grown LPS crystals belong to monoclinic

system. The presence of the functional groups of the grown crystal was confirmed by

FTIR analysis. From the UV-Vis-NIR spectrum, it is seen that the transmission is

greater than 90% for light with incident wavelengths from 204-1100 nm. The UV

transparency cut-off wavelength of LPS crystal occurs at 204 nm. The SHG study

shows that its relative efficiency which was determined by Kurtz powder technique is

found to be 23% of that of KDP crystal. The DTA and TGA studies reveal that the

crystal is thermally stable upto 1600C.

The seventh chapter deals with the cadmium manganese thiocyanate (CMTC)

crystal. A new optical crystal CMTC has been successfully synthesized and grown by

slow evaporation solution growth method at room temperature. The dimension of the

grown crystal is 30x20x30 mm3. From the XRD measurements, it has been proved

that the crystal is of tetragonal crystallographic system. The presence of functional

groups was confirmed by the FTIR techniques. The optical behaviour has been studied

using UV-Vis-NIR analysis and found that the crystal transparency is in the range

xv

from 380 to 1170nm, which highlights its prospects of application in opto-electronic

devices. The UV cut-off wavelength of the grown crystal is 380nm. The thermal

behaviour of CMTC crystal was studied by TGA/DTA analysis which confirms the

melting point of the crystal at 4300C

The eighth chapter deals about the zinc manganese thiocyanate (ZMTC)

crystal. Single crystals of ZMTC have been conveniently grown by slow evaporation

at room temperature. The measurements from the single crystal XRD indicates that

the crystal belongs to tetragonal crystal system and its unit cell parameters have been

determined. It is seen that the crystallographic data agree well in comparison with the

results of X-ray powder diffraction pattern. The absorption bands assigned to the

particular vibrations have been predicted by FTIR technique. From the recorded UV-

Vis-NIR spectrum it is observed that the crystal is transparent in the wavelength range

380-1193 nm and the UV transparency cut-off wavelength is found to occur at 380

nm. The exceptional thermal stability of ZMTC crystal is much higher than the

inorganic molecular crystals which were determined by TGA-DTA investigations.

The crystal is thermally stable upto 8060C.

The chapter nine describes the summary of the present investigation and

suggestions for the future work.

xvi

LIST OF PUBLICATIONS

1. Synthesis, growth and characterization of a new nonlinear optical material:

4-Phenylpyridinium hydrogen squarate (4PHS),

C. Ramachandra Raja, P. Paramasivam and N. Vijayan,

Spectrochimica Acta A, 69 (2008) 1146 1149.

2. Synthesis, growth and characterization of cadmium manganese thiocyanate

(CMTC) crystal

P. Paramasivam and C. Ramachandra Raja

Spectrochimica Acta A, 79 (2011) 1109 1111

3. Synthesis, growth and characterization of zinc manganese thiocyanate crystal

P. Paramasivam, M. Arivazhagan and C. Ramachandra Raja.

Indian Journal of Pure & Applied Physics, Vol.49 (June 2011) 394 397.

4. Crystallization and characterization of a new nonlinear optical crystal:

L-Proline succinate (LPS)

P.Paramasivam and C. Ramachandra Raja.

Journal of Crystallization Process and Technology, 2(2012) 21-25

Paper under review

1. Synthesis, growth and characterization of a new nonlinear optical crystal:

Glycinium hydrogen squarate (GHS)

P.Paramasivam and C. Ramachandra Raja.

Spectrochimica Acta A

xvii

CONFERENCES / SEMINARS

1. Symposium on nonlinear optical crystals and modelling in crystal growth,

February 28-March1, 2005, Department of Physics, Anna University, Chennai.

2. Growth and Characterization of a new nonlinear optical material:

4-Phenylpyridinium hydrogen squarate (4PHS)

C. Ramachandra Raja, P.Paramasivam and N. Vijayan

18th AGM, Materials Research Society of India, Theme Symposium on

Materials for Energy Generation, Conservation and Storage, February 12-14,

2007. National Physics Laboratory, New Delhi, India.

3. Synthesis, Growth and Characterization of a new nonlinear optical crystal:

Glycinium hydrogen squarate (GHS) crystal.

C. Ramachandra Raja and P.Paramasivam

International Conference on Advances in Engineering and Technology 2011,

May 27th & 28th 2011, E.G.S. Pillay Engineering College, Nagapattinam.

xviii

LIST OF TABLES

Table No Title Page No

2.1 Optical effects of nonlinear optical materials 35

4.1 Assignments of FT-IR bands observed for 4PHS crystal 86

5.1 Comparative statement of glycine, squaric acid and GHS 97

5.2 Cell parameters of GHS crystal 98

5.3 Powder XRD data of GHS crystal 99

5.4 FT-IR spectral assignments of GHS crystal 100

5.5 Chemical shift assignments of proton of GHS crystal 103

5.6 Chemical shift assignments of carbon of GHS crystal 104

6.1 FT-IR spectral assignments of LPS crystal 114

7.1 FT-IR spectral assignments of CMTC crystal 124

8.1 FT-IR spectral assignments of ZMTC crystal 131

9.1 Comparative statement of the grown crystals 136

xix

LIST OF FIGURES

Figure No Title Page No

1.1 Free energy diagram 08

2.1 Schematic diagram of SHG 37

2.2 Schematic diagram of sum frequency generation 38

2.3 Schematic diagram of difference frequency generation 39

2.4 Schematic diagram of optical parametric generator 40

3.1 Experimental setup for single crystal X- ray diffractometer 56

3.2 Schematic diagram of Guinier geometry 59

3.3 Schematic diagram of FT-IR spectrometer 63

3.4 Nuclear spin 67

3.5 Schematic diagram of NMR spectrometer 68

3.6 Schematic diagram of TGA equipment 76

3.7 Experimental setup for SHG efficiency measurement 78

4.1 Photograph of 4PHS single crystal 82

4.2 FT-IR spectrum of 4PHS crystal 85

4.3 Indication of NMR spectra analysis of 4PHS crystal 88

4.4 UV-Vis-NIR spectrum of 4PHS crystal 89

4.5 TGA / DTA curve of 4PHS crystal 91

5.1 Photograph of GHS single crystal 96

5.2 Powder X-Ray Diffraction of GHS crystal 99

xx

Figure No

Title

Page No

5.3 FT-IR Spectrum of GHS crystal 101

5.4 UV-Vis-NIR spectrum of GHS crystal 102

5.5 1H NMR spectrum of GHS crystal 104

5.5 13C NMR spectrum of GHS crystal 105

5.6 TGA/DTA curve of GHS crystal 107

6.1 Photograph of LPS single crystal 111

6.2 FT-IR spectrum of LPS crystal 113

6.3 UV-Vis-NIR spectrum of LPS crystal 115

6.4 TGA / DTA curve of LPS crystal 117

7.1 Photograph of CMTC crystal 121

7.2 FT-IR spectrum of CMTC crystal 123

7.3 UV-Vis-NIR spectrum of CMTC crystal 125

7.4 TGA / DTA curve of CMTC crystal 126

8.1 Photograph of ZMTC single crystal 129

8.2 FT-IR spectrum of ZMTC crystal 131

8.3 Optical transmission spectrum analysis 132

8.4 TGA / DTA curve of ZMTC crystal 134

xxi

LIST OF SYMBOLS

Symbols Descriptions

Angstrom Unit

G Gibbs free energy change

GV Volume excess Free energy

GS Surface excess Free energy

surface energy change per unit area

E Electric field vector

P Polarization

Linear susceptibility

2 , 3 Non linear susceptibilities

0 Permittivity of free space

OC Degree Celsius

m micrometer

nm nanometer

Frequency of incident radiation

a, b and c Cell parameters

, and Interfacial angles

cm-1 per centimeter

ns nanosecond

mJ/pulse milli Joule per pulse

MHz Mega hertz

Wavelength

xxii

LIST OF ABBREVIATIONS

Abbreviations Descriptions

NLO Nonlinear Optics

SHG Second Harmonic Generation

4PHS 4- Phenylpyridinium Hydrogen Squarate

GHS Glycinium Hydrogen Squarate

LPS L- Proline Succinate

CMTC Cadmium Manganese Thiocyanate

ZMTC Zinc Manganese Thiocyanate

AR Analytical Reagent

XRD X-ray Diffraction

UV-Vis-NIR Ultra Violet- Visible- Near Infra Red

FT-IR Fourier Transform Infrared

TGA Thermo Gravimetric Analysis

DTA Differential Thermal Analysis

KDP Potassium dihydrogen orthophosphate

Nd:YAG Neodymium: Yttrium Aluminium Garnet

CHAPTER 1

INTRODUCTION TO CRYSTAL GROWTH - AN OVERVIEW

1.1. INTRODUCTION

A short history of observations on the shapes of snow crystals in ancient

China was summarized by Kepler in 1611. During 16th 19th centuries, quartz

to sapphire crystals was used as gems and precious stones. The largest event

that showed the importance of the crystals was the invention of transistor. In

the 20th century, contributions of crystal growth in the fabrication of the

electronic and optical devices have thrown more light on the importance of

crystals. Crystal growth is an interdisciplinary subject covering physics,

chemistry, material science, electrical engineering, mineralogy, metallurgy etc.

Nowadays, crystals are produced artificially to satisfy the needs of jewelers,

science and technology.

In the past few decades, there has been a growing interest in crystal

growth process, particularly in view of the increasing demand for materials for

technological applications [1-3]. New materials are the life blood of solid state

research and device technology. New materials are not usually discovered by

device engineers or solid state theorists; they are mostly grown by crystal

growers.

An ideal crystal is one, in which the surroundings of any atom would be

exactly the same as the surroundings of every similar atom. Real crystals are

2

finite and contain defects. However, single crystals are solids in the most

uniform condition that can be attained and this is the basis for most of the uses

of these crystals. The uniformity of single crystals can allow the transmission

without the scattering of electromagnetic waves. The strong influence of single

crystals in the present day technology led to the recent development and

advancement in the fields of semiconductors, solid state lasers, ultrasonic

amplifiers, infrared detectors, transducers, nonlinear optic, piezoelectric,

photosensitive materials, thin films and computer industries.

All these developments could be achieved due to the availability of

single crystals like silicon, germanium, gallium arsenide and also with the

invention of nonlinear optical properties in some inorganic, semi-organic and

organic crystals. The desired physical phenomena for the fabrication of

devices are exhibited only by certain single crystals. Hence in order to achieve

high performance, good quality single crystals are needed.

Therefore, researchers worldwide have always been in the search of

new materials through their single crystal growth. The methods of growing

crystals are very wide and mainly dictated by the characteristics of the material

and its size [4-5]. In this chapter, the fundamentals of the various methods to

grow quality single crystals and, in particular, the solution growth method is

discussed.

3

1.2. NUCLEATION

Nucleation is an important event in crystal growth. A comprehensive

study on the growth of crystals should start from an understanding of

nucleation process [6]. Nucleation is the physical reaction which occurs when

components in a solution start to precipitate out forming nuclei which attracts

more precipitate. In a supersaturated or super-cooled system when a few atoms

or molecules join together, a change in energy takes place in the process of

formation of the cluster. The cluster of such atoms or molecules is termed

embryo. An embryo may grow or disintegrate and disappear completely. If

the embryo grows to a particular size, critical size known as critical-nucleus,

then greater is the possibility for the nucleus to grow into a crystal. There are

four stages involved in the formation of stable nucleus:

(a) The first stage is the development of supersaturation:

Supersaturation may be attained due to a chemical reaction, changes

in temperature, pressure or any other physical or chemical condition.

(b) The second stage is the generation of embryo:

The formation of embryo may be either homogeneous (the atoms or

molecules build themselves in the interior of the parent system) or

heterogeneous (the molecules build up on dust particles or on the

surface of the container or any other imperfections).

(c) The third stage is the growth of the embryo from the unstable critical

state to stable state.

4

(d) The fourth stage is the relaxation process, where, the texture of the

new born nucleus changes.

1.2.1. Kinds of Nucleation

Nucleation is broadly classified into two types. These two types are

primary and secondary nucleation. The former occurs either spontaneously or

induced artificially.

The primary nucleation is further divided into homogeneous and

heterogeneous nucleation. The spontaneous formation of crystalline nuclei

within the interior of parent phase is called homogeneous nucleation. The

formation of nuclei in the bulk of supersaturated system is a comparatively rare

occurrence; it gives the basic principles for understanding the numerous

processes in science and technology as well as in nature where phase

transitions are involved. On the other hand, if the nuclei form heterogeneously

around ions, impurity molecules or on dust particles or on the surface of the

container or at structural singularities such as dislocation or imperfection, it is

called heterogeneous nucleation.

If the nuclei are generated in the vicinity of crystals present in

supersaturated system, then this phenomenon is often referred to as secondary

nucleation [7]. Nucleation can often be induced by external influence like

agitation, mechanical shock, friction, spark, extreme pressure, electric and

magnetic fields, UV - rays, X - rays, gamma rays and so on.

5

1.2.2. Classical Theory of Nucleation

The formation of the crystal nuclei is a difficult and complex process,

because the constituent atoms or molecules in the system have to be oriented

into a fixed lattice. In practice, a number of atoms or molecules may come

together to form an ordinary cluster of molecules known as embryo. The

energetic considerations show that this embryo is likely to re-dissolve unless it

reaches a certain critical size. If it does not dissolve it means that the assembly

is stable under the prevailing conditions.

1.2.3 Kinetic Theory of Nucleation

The main aim of the nucleation theory is to calculate the rate of

nucleation. Rate of nucleation is nothing but the number of critical nuclei

formed per unit time per unit volume. In kinetic theory, nucleation is treated as

the chain reaction of monomolecular addition to the cluster and ultimately

reaching macroscopic dimensions.

Two monomers collide with one another to form a dimer. A monomer

joins with a dimer to form a trimer. This reaction builds a cluster having

i-molecules known as i-mer. As the time increases, the size distribution in the

embryos changes and larger ones increases in size. As the size attains a critical

size Aj*, further growth into macroscopic size is guaranteed, and there is also a

possibility for the reverse reaction i.e., the decay of a cluster into monomers.

6

The reaction is represented as follows:

A1 + A1 A2

A2 + A1 A3

Ai-1 + A1 Ai

Ai + A1 Ai+1

Aj-1 + A1 Aj*

1.3. STABILITY OF NUCLEUS

The total free energy of a crystal in equilibrium with its surrounding at

constant temperature and pressure would be a minimum for a given volume [7].

Since the volume free energy per unit volume is a constant, then

ai i = minimum 1.1

where ai - area of ith face and

i - surface energy per unit area

7

1.4. ENERGY FORMATION OF SPHERICAL NUCLEUS

Energy is quite essential for the creation of a new phase. When a droplet

nucleus forms due to supersaturation of vapour, certain quantity of energy is

spent in the creation of a new phase. The free energy change associated with

the formation of a nucleus can be written as

G = GS + GV 1.2

G can be represented as a combination of surface excess free energy

(GS) and volume excess free energy (GV). For the spherical nucleus,

G = 4 r2 + 4/3 r3Gv 1.3

Where Gv is the free energy change per unit volume which is a

negative quantity and is the surface energy change per unit area. The

quantities G, GS and GV are represented in Fig. 1.1.

The surface excess free energy increases with r2 and the volume

excess free energy GV decreases with r3. So, the net free energy change

increases with the increase in size, attains the maximum and then decreases for

further increase in the size of nucleus.

The size corresponding to the maximum free energy change is called

critical nucleus. The radius of the critical nucleus is obtained by setting the

condition,

8

Fig. 1.1.

Free energy diagram

Surface Term GS

GV Volume Term

G*

G

r* Radius

9

i.e. 0=dr

Gd

when r = r*(radius of critical nucleus)

r* = -2GV

1.4

The free energy change associated with the formation of critical nucleus

can be estimated by substituting equation 1.4 in equation 1.3.

G* = 163 / 3 Gv2 1.5

In terms of r* the above equation can be written as,

G* = 4/3

G* = 1/3 S. 1.6

Where, S is the surface area of the critical nucleus. The crucial

parameter between a growing crystal and the surrounding mother liquid is the

interfacial tension (). Interfacial tension is a measurement of the excess energy

present at an interface arising from the imbalance of forces between molecules

at an interface (gas/liquid, liquid/liquid, gas/solid, and liquid/solid). This

complex parameter can be determined by conducting the nucleation

experiments. The significant nucleation parameters have been estimated for

10

KTP and LAP crystals, which are grown from high temperature and low

temperature solutions respectively [8-9].

Though the present phase is at constant temperature and pressure, there

will be variation in the energies of the molecules. The molecules having higher

energies temporarily favour the formation of the nucleus. The rate of nucleation

can be given by Arrhenius reaction [10] which is a velocity equation since the

nucleation process is basically a thermally activated process. The nucleation

rate J is given by

=

KT*G AexpJ 1.7

where, A- pre-exponential constant

K- Boltzmann constant

T- absolute temperature

1.5. SUPERSATURATION AND ITS EXPRESSION

The concentration of the solution is more than the equilibrium

concentration is called super saturation. In supersaturation the solution has

exceeded its solubility limit. In order to grow crystals, the solution must be

supersaturated; the concentration of the solution is more than the equilibrium

concentration. Usually the concentration is defined as the mass of the solute

11

dissolved in one litre of the solvent. Supersaturation is the driving force, which

controls the rate of crystal growth.

The driving force *CCC =

where C is the concentration of the dissolved substance

C* is the solubility limit

The supersaturation ratio (S) is defined as the ratio between the

concentration of the dissolved substance and the solubility limit.

*CCS =

1.6. CLASSIFICATION OF CRYSTAL GROWTH

Crystal growth is a controlled phase transformation, either from solid or

liquid or gaseous phase to solid phase. The choice of a particular method for

growing a desired single crystal critically depends on the physical and chemical

properties of the substances. The consistency in the characteristics of devices

fabricated from the crystals depends mainly on the homogeneity and defect

present in the crystals. Thus, the process of producing single crystals, from

homogeneous media with directional properties, attracts more attention and

gains more importance than any other process. The method of crystal growth

may range from a small inexpensive technique to a complex sophisticated

technique. The basic methods of growing single crystals are:

12

(a) growth from melt

(b) growth from vapour

(c) growth from solution

The basic methods to grow single crystals have been discussed in detail by

several authors [2, 11-12]. In the solid growth of crystals, the important factor

is conversion of a polycrystalline piece of a material into a single crystal by

causing the grain boundaries to sweep through and pushed out of the crystal

[13]. The basic methods of growing single crystals are explained below.

1.6.1. Growth from Melt

A gas is cooled until it becomes a liquid, which is then cooled further

until it becomes a solid. Polycrystalline solids are typically produced by this

method unless special techniques are employed. In any case, the temperature

must be controlled carefully. Knowledge of how crystals grow from the melt

and the effects of the various factors which may influence crystal growth is a

potentially important tool in interpreting textural and chemical features and

crystallization histories of igneous rocks.

The first detailed study of crystal-growth phenomena was explained by

Tamman (1899), who measured the rate of crystal growth from a melt. He

found that the rate is zero at the liquid state, increases to a maximum, and then

decreases with decrease in temperature. Later in the year 1931, Volmer and

Marder developed a simple theory to explain this relationship. Depending on

13

the thermal characteristics, the following techniques are employed for the

crystal growth:

(a) Czochralski technique

(b) Bridgman technique

(c) Kyropoulos technique

(d) Zone melting technique

(e) Verneuil technique

Large crystals can be grown rapidly from the liquid elements using a

popular method invented in 1918 by the Polish scientist Jan Czochralski [14].

One attaches a seed crystal to the bottom of a vertical arm such that the seed is

barely in contact with the material at the surface of the melt.

The arm is raised slowly, and a crystal grows underneath at the interface

between the crystal and the melt. Usually the crystal is rotated slowly, so that

inhomogeneities in the liquid are not replicated in the crystal. Large diameter

crystals of silicon are grown in this way for use as computer chips. Based on

measurements of the weight of the crystal during the pulling process, computer

controlled apparatus can vary the pulling rate to produce any desired diameter.

Crystal pulling is the least expensive way to grow large amounts of pure

crystal. Synthetic sapphire crystals can be pulled from molten alumina. Special

care is required to grow binary and other multi-component crystals; the

temperature must be precisely controlled because such crystals may be grown

14

only at a single, extremely high temperature. The melt has a tendency to be

inhomogeneous, since the two liquids may try to separate by gravity.

The Bridgman method [15-16] is also widely used for growing large

single crystals. The molten material is put into a crucible, often of silica, which

has a cylindrical shape with a conical lower end. Heaters maintain the molten

state. As the crucible is slowly lowered into a cooler region, a crystal starts

growing in the conical tip. The crucible is lowered at a rate that matches the

growth of the crystal, so that the temperature at the interface between crystal

and melt is always same. The rate of moving the crucible depends on the

temperature and the material. Then, the entire molten material in the crucible

grows into a single large crystal. One disadvantage of this method is that,

impurities are pushed out of the crystal during growth. A layer of impurities

grows at the interface between melt and solid as this surface moves up the melt,

and the impurities become concentrated in the higher part of the crystal.

In Kyropoulos technique, the crystal is grown in a large diameter. As in

the Czochralski method, here also the seed is brought in contact with the melt

and is not raised much during the growth, i.e. part of the seed is allowed to melt

and a short narrow neck is grown. After this, the vertical motion of the seed is

stopped and growth proceeds. The major use of this method is the growth of

alkali halides to make optical components.

15

Zone refining was developed by William Gardner Pfann [17] in Bell

Labs as a method to prepare high purity materials for manufacturing transistors.

In the zone melting technique, the feed material is formed into a mass by heat

and pressure then the seed is attached to one end. A small molten zone is

maintained by surface tension between the seed and the feed. The zone is

slowly moved towards the feed. Single crystal is obtained over the seed. This

method is applied to materials having large surface tension. The main reasons

for the impact of zone refining process to modern electronic industry are the

simplicity of the process, the capability to produce a variety of organic and

inorganic materials of extreme high purity, and to produce dislocation free

crystal with a low defect density.

In the Verneuil technique, a fine dry powder of size 1-20 microns of the

material to be grown is shaken through the wire mesh and allowed to fall

through the oxy-hydrogen flame. The powder melts and a film of liquid is

formed on the top of the seed crystal. This freezes progressively as the seed

crystal is slowly lowered. The art of the method is to balance the rate of charge

feed and the rate of lowering of the seed to maintain a constant growth rate and

diameter. This technique is widely used for the growth of synthetic gems.

1.6.2. Growth from Vapour

Crystals can be grown from vapour when the molecules of the gas attach

themselves to a surface and move into the crystal arrangement. Several

16

important conditions must be met for this to occur. At constant temperature and

equilibrium conditions, the average number of molecules in the gas and solid

states is constant; molecules leave the gas and attach to the surface at the same

rate that they leave the surface to become gas molecules [18].

For crystals to grow, the gas solid chemical system must be in a non-

equilibrium state such that there are too many gaseous molecules for the

conditions of pressure and temperature. This state is called supersaturation.

Molecules are more prone to leave the gas than to rejoin it, so they get

deposited on the surface of the container. Supersaturation can be induced by

maintaining the crystal at a lower temperature than the gas. A critical stage in

the growth of a crystal is seeding, in which a small piece of crystal of proper

structure and orientation, called a seed, is introduced into the container. The

gas molecules find the seed a more favourable surface than the walls and

preferentially deposit there. Once the molecule is on the surface of the seed, it

wanders around this surface to find the preferred site for attachment. Growth

proceeds as one molecule at a time and one layer at a time. The process is slow;

it takes days to grow a small crystal. The advantage of vapour growth is that

very pure crystals can be grown by this method, while the disadvantage is that

it is slow.

Most clouds in the atmosphere are ice crystals that form by vapour

growth from water molecules. In the laboratory, vapour growth is usually

accomplished by sending a supersaturated gas over a seed crystal. Quite often a

17

chemical reaction at the surface is needed to deposit the atoms. Crystals of

silicon can be grown by moving chlorosilane (SiCl4) and hydrogen (H2) over a

seed crystal of silicon. Hydrogen acts as the buffer gas by controlling the

temperature and rate of flow. The molecules dissociate on the surface in a

chemical reaction that forms hydrogen chloride (HCl) molecules. Hydrogen

chloride molecules leave the surface, while silicon atoms remain to grow into a

crystal. Binary crystals such as gallium arsenide (GaAs) are grown by a similar

method.

1.6.3. Growth from Solution

The essential technique which produces large single crystals suitable for

lot of applications at minimum cost is vital for research and commercial

purpose. The selection of growth method is also important because it suggests

the possible impurity and other defect concentrations to improve the physical

and chemical properties of the material.

The crystal growth from solution falls into:

(a) gel growth

(b) flux growth

(c) hydrothermal growth

(d) low temperature solution growth

18

Growth of crystals from solution is an important process that can be

used in laboratory, industry, research and development. In order to grow good

quality single crystals by solution growth, the material should have high

solubility and variation in solubility with temperature. The viscosity of the

solvent solute system should be low [19]. The materials used for the growth of

crystal must not be a flammable one. Another aspect to consider is that the

container and stirrer should be non-reactive with material.

Among the various methods, growth from solution at low temperature

occupies a prominent place owing to its versatility, simplicity and used to

produce technically important crystals. Growth from solution at low

temperature occurs close to equilibrium conditions and hence good quality bulk

single crystals of utmost perfection can be grown easily.

1.7. GEL GROWTH

It is an alternative technique to solution growth with controlled diffusion

and the growth process is free from convection. Gel is a two component system

of a semisolid rich in liquid and inert in nature. The material, which

decomposes before melting, can be grown in this medium by counter diffusing

two suitable reactants. Crystals with dimensions of several millimeters can be

grown in a period of 3 to 4 weeks. The crystals grown by this technique have

high degree of perfection and fewer defects since the growth takes place at

room temperature.

19

1.8. HYDROTHERMAL GROWTH

Hydrothermal implies conditions of high pressure as well as high

temperature. Substances like calcite, quartz is considered to be insoluble in

water but at high temperature and pressure, these substances are soluble. This

method of crystal growth at high temperature and pressure is known as

hydrothermal method. Temperatures are typically in the range of 400C to

600C and the pressure involved is high.

Growth is usually carried out in steel autoclaves with gold or silver

linings. Depending on the pressure the autoclaves are grouped into low,

medium and high-pressure autoclaves. The concentration gradient required to

produce growth is provided by a temperature difference between the nutrient

and growth areas. The requirement of high pressure presents practical

difficulties and there are only a few crystals of good quality and large

dimensions are grown by this technique. Quartz is the outstanding example of

industrial hydrothermal crystallization. One serious disadvantage of this

technique is the frequent incorporation of OH-

ions into the crystal, which

makes them unsuitable for many applications.

1.9. FLUX GROWTH

In this method of crystal growth, the components of the desired

substance are dissolved in a solvent (flux). The method is particularly suitable

for crystals needing to be free from thermal strain and it takes place in a

20

crucible made of non reactive metals. Crucibles are normally sealed in

evacuated quartz ampoules or reactions take place in controlled atmosphere

furnaces. A saturated solution is prepared by keeping the constituents of the

desired crystal and the flux at a temperature slightly above the saturation

temperature long enough to form a complete solution. Then the crucible is

cooled in order to cause the desired crystal to precipitate. Nucleation happens

in the cooler part of the crucible. A disadvantage is that most flux method

syntheses produce relatively small crystals.

1.10. LOW TEMPERATURE SOLUTION GROWTH

In the present investigation, the low temperature solution growth

technique is employed and the fundamentals of the same are given below:

Solubility and supersaturation are the two important parameters for the

solution growth process. Solubility is defined as the maximum amount of

substance dissolved in a particular solvent at a given temperature. Before

starting the solution growth process, the solubility of the solute must be

determined by dissolving the solute in the solvent at a constant temperature

with continuous stirring. Solubility of the substance increases with increase in

temperature for most of the materials. Either by cooling or evaporating the

solvent, the solution attains its supersaturation. The solution is said to be in

supersaturated state, if the concentration of the solution is greater than the

equilibrium concentration.

21

When the starting materials are unstable at high temperatures, low

temperature solution growth is the most widely used method for the growth of

crystals [20]. The supersaturation is achieved either by temperature lowering

or by solvent evaporation. This method is widely used to grow bulk crystals,

from materials, which have high solubility and have variation in solubility with

temperature [21-22].

Growth of crystals from solution at room temperature has many

advantages over other growth methods. But the rate of crystallization is slow in

this method. Since growth is carried out at room temperature, the structural

imperfections in the grown crystals are relatively low [23]. The ambient

temperature of growth, the pH of the solution and the presence of deliberately

added impurities are the essential additional parameters that determine the rate

of growth and morphology of the crystal. Low temperature solution growth

(LTSG) can be subdivided into the following categories:

(a) slow cooling method

(b) slow evaporation method

(c) temperature gradient method

1.10.1. Slow Cooling Method

Slow cooling method is one in which the solution is allowed to cool to a

lower temperature in order to achieve supersaturated solution and the

temperature of the solution is reduced in small steps. By doing so, the solution

22

which is just saturated at the initial temperature will become a supersaturated

solution. Once supersaturation is achieved, growth of single crystal is possible.

The main disadvantage of slow cooling method is the need to use a range of

temperature. The temperature at which such crystallization can begin is usually

within the range 45-75C and the lower limit of cooling is the room

temperature. Wide range of temperature may not be desirable because the

properties of the grown crystal may vary with temperature. Even though this

method has technical difficulty of requiring a programmable temperature

control, it is widely used with great success. The crystals produced by this

method are small and possess unpredictable shape.

1.10.2. Temperature Gradient Method

This method involves the transport of the materials from hot region to a

cooler region, where the solution is supersaturated and the crystal grows. The

advantages of this method are that the crystal is grown at fixed temperature,

this method is insensitive to changes in temperature (provided both the source

and the growing crystal undergo the same change) and the cost of the basic

materials are low. On the other hand, small changes in temperature difference

between the source and the crystal zones have a large effect on the growth rate.

1.10.3. Slow Evaporation Method

In this process the temperature of the solution is not changed, but the

solution is allowed to evaporate slowly. When the solvent begins to evaporate,

23

the concentration of solute is increased and, therefore, supersaturation is

achieved. The advantage of using this method is that the crystals grow at a

fixed temperature. This method can effectively be used for materials having

very low temperature coefficient of solubility. But inadequacies of the

temperature control system still have a major effect on the growth rate. In

order to control the temperature of the system, constant temperature bath can

be used. In spite of some of the disadvantages, this method is simple and

convenient to grow bulk single crystals.

1.11. CRITERIA FOR OPTIMIZING SOLUTION GROWTH

The growth of good quality single crystals requires optimized

conditions; this may be achieved with the help of the following criteria:

(a) material purification

(b) solvent selection

(c) solubility

(d) solution preparation

(e) crystal habit

1.11.1. Material Purification

Availability of the material with highest purity is an essential

requirement for success in crystal growth. The impurity included into crystal

lattice may lead to the formation of flaws and defects. Some times, impurities

24

may slowdown the crystallization process. To harvest good quality crystals,

material purification is a must. A careful repetitive use of standard purification

methods of re-crystallization followed by filtration of the solution would

increase the level of purity.

1.11.2. Solvent Selection

Solution is a homogeneous mixture of a solute in a solvent. Solute is the

component present in a smaller quantity. For a given solute, there may be

different solvents. Apart from high purity starting materials, solution growth

requires a good solvent. The solvent must be chosen taking into account the

following factors:

high solubility for the given solute

low viscosity

low volatility

low corrosion

low cost

high purity

1.11.3. Solubility

Solubility is an important parameter which dictates the growth

procedure. If the solubility is too high, it is difficult to grow bulk crystals and

too low solubility restricts the size and growth of bulk crystals. Hence

25

solubility of the solute in the chosen solvent must be determined before starting

the growth process [24].

1.11.4. Solution Preparation and Crystal Growth

After selecting the desirable solvent with high purity solute to be

crystallized, the next important part is preparation of the saturated solution. To

prepare a saturated solution, it is necessary to have an accurate solubility-

temperature data of the material. The saturated solution at a given temperature

is placed in the constant temperature bath. Wattman filter papers are used for

solution filtration. The filtered solution is transferred to crystal growth vessel

and the vessel is sealed by polythene paper in which 1520 holes are made for

slow evaporation. Then the crystallization is allowed to take place by slow

evaporation at room temperature or at a higher temperature in a constant

temperature bath. As a result of slow evaporation of solvent, the excess of

solute which has got deposited in the crystal growth vessel results in the

formation of crystals.

1.11.5. Crystal Habit

The growth of a crystal at approximately equivalent rates along all the

directions is a prerequisite for its accurate characterization. This will result in a

large bulk crystal. Such large crystals should also be devoid of dislocation and

other defects. These imperfections become isolated into defective regions

surrounded by large volumes of high perfection. In the crystals the

26

imperfections grow as needles or plates, the growth dislocations propagate

along the principle growth directions and the crystals remain imperfect [20].

Change of habit in such crystals which naturally grow as needles or plates can

be achieved by any one of the following ways:

changing the temperature of the growth

changing the pH of the solution

adding a habit modifying agent

changing the solvent

Achievement of the above parameters is of great industrial importance,

where such morphological changes are induced during crystallization to yield

crystals with better perfection and packing characteristics.

1.12. ADVANTAGES OF LOW TEMPERATURE SOLUTION

GROWTH TECHNIQUE

Low temperature solution growth is utilized for crystal growth due to its

simplicity and versatility. Following are the important advantages of using low

temperature solution growth technique:

(a) simple growth apparatus

(b) growth of strain and dislocation free crystals

(c) permits the growth of prismatic crystals by varying the growth

conditions

27

(d) this is the only method which can be used for substances that undergo

decomposition before melting

Following are the disadvantages of this technique:

(a) the growth substance should not react with solvent

(b) this method is applicable for substances fairly soluble in a solvent

(c) inclusions of solvent may present in the grown crystal

(d) growth rate of this method is low

28

CHAPTER 2

AN OVERVIEW OF OPTICAL MATERIALS

2.1. INTRODUCTION

An optical material is one which is transparent to light or to infrared,

ultraviolet, or X-ray radiation, such as glass, certain single crystals,

polycrystalline materials, and plastics. All substances used in the construction

of devices or instruments whose function is to alter or control electromagnetic

radiation in the ultraviolet, visible, or infrared spectral regions. Optical

materials are fabricated into optical elements such as lenses, mirrors, windows,

prisms, polarizers, detectors, and modulators. These materials serve to refract,

reflect, transmit, disperse, polarize, detect, and transform light. The term

light refers here not only to visible light but also to radiation in the adjoining

ultraviolet and infrared spectral regions. At the microscopic level, atoms and

their electronic configurations in the material interact with the electromagnetic

radiation (photons) to determine the material's macroscopic optical properties

such as transmission and refraction. These optical properties are functions of

the wavelength of the incident light, the temperature of the material, the applied

pressure on the material, and in certain instances the external electric and

magnetic fields applied to the material.

The ability to focus the optical field to deeply sub-wavelength

dimensions opens the door to an entirely new class of photonic devices. If one

29

could combine the imaging powers of X-ray wavelengths with the economy

and maturity of visible light sources, one could greatly broaden the practical

engineering toolbox. Imagine focusing visible photons to spatial dimensions

less than ten nanometers. By doing so, electron beam microscopy is

immediately displaced by optical microscopy, replacing expensive electron

beam sources with inexpensive visible lasers. Beyond simple economics,

though, this achievement would extend the range of nanometer scale

microscopy to living biological samples and highly insulating surfaces.

There is a wide range of substances that are useful as optical materials.

Most optical elements are fabricated from glass, crystalline materials,

polymers, or plastic materials. In the choice of a material, the most important

properties are often the degree of transparency and the refractive index, along

with each property's spectral dependency. The uniformity of the material,

temperature limits, hygroscopicity, chemical resistivity, and availability of

suitable coatings should also be considered. Fused silica, which transmits to

about 180 nm, is well suited for the lithography in the ultraviolet region.

However, the crystalline material calcium fluoride, which transmits into the

ultraviolet region to about 140 nm, outperforms any glass in printing

microchips using fluorine excimer lasers. Deep-ultraviolet applications of

fused-silica glasses include high-energy lasers, spacecraft windows, blanks for

large astronomical mirrors, optical imaging, and cancer detection using

ultraviolet-laser-induced autofluorescence.

30

The need for an inexpensive, unbreakable lens that could be easily mass-

produced precipitated the introduction of plastic optics in the mid-1930s.

Although the variety of plastics suitable for precision optics is limited

compared to glass or crystalline materials, plastics are often preferred when

difficult or unusual shapes, lightweight elements, or economical mass-

production techniques are required. The softness, inhomogeneity, and

susceptibility to abrasion intrinsic to plastics often restrict their application.

Haze (which is the light scattering due to microscopic defects) and

birefringence (resulting from stresses) are inherent to plastics. Plastics also

exhibit large variations in the refractive index with changes in temperature.

Shrinkage resulting during the processing must be considered.

2.2 IMPORTANCE OF CRYSTALS AS OPTICAL MATERIALS

Although most of the early improvements in optical devices were due to

advancements in the production of glasses, the crystalline state has taken on

increasing importance. Synthetic crystal-growing techniques have made

available single crystals such as lithium fluoride (of special value in the

ultraviolet region, since it transmits at wavelengths down to about 120 nm),

calcium fluoride, and potassium bromide (useful as a prism at wavelengths up

to about 25 m in the infrared). Many alkali-halide crystals are important

because they transmit into the far-infrared. Single crystals are indispensable for

transforming, amplifying, and modulating light. Birefringent crystals serve as

31

retarders, or wave plates, which are used to convert the polarization state of the

light. In many cases, it is desirable that the crystals not only be birefringent, but

also behave nonlinearly when exposed to very large fields such as those

generated by intense laser beams. A few examples of such nonlinear crystals

are ammonium dihydrogen phosphate (ADP), potassium dihydrogen phosphate

(KDP), beta barium borate (BBO), lithium borate (LBO), and potassium titanyl

phosphate (KTP).

2.3 NONLINEAR OPTICAL MATERIALS

Optics is the study of interaction of electromagnetic radiation and

matter. Nonlinear optics is the study of the phenomena that occurs as a

consequence of the modification of optical properties of a material system by

the presence of light [25-26]. Nonlinear optics (NLO) has been an active field

of research since the late 1960s with the advent of lasers followed by the

demonstration of harmonic generation in quartz [27]. Nonlinear optics extends

the usefulness of lasers by increasing the number of wavelengths available.

Nonlinear optical material is the medium on which a laser beam interacts. After

the invention of laser, frequency conversion by nonlinear optical materials has

become an important and widely used technique.

Nonlinear optics is the study of the interaction of intense

electromagnetic field with materials to produce modified fields that are

32

different from the input field in phase and frequency. Nonlinear optics is

completely a new effect in which the light of one wavelength is transformed to

the light of another wavelength.

In a linear material, electrons are bound inside a potential well, which

acts like a spring, holding the electrons to lattice point in the crystal. If an

external force pulls an electron away from its equilibrium position the spring

pulls it back with a force proportional to the displacement. The springs

restoring force increases linearly with the electron displacement from its

equilibrium position. In an ordinary optical material, the electrons oscillate

about their equilibrium position at the same frequency of the electric field (E).

Hence, these electrons in the crystal generate light at the frequency of the

original light wave.

In the nonlinear material, if the light passing through the material is

intense enough, its electric field can pull the electrons so far that they reach the

end of their springs. The restoring force is no longer proportional to the

displacement and then it becomes nonlinear. The electrons are jerked back

rather than pulled back and they oscillate at frequencies other than the driving

frequency of the light wave. So, the electrons radiate at the new frequencies,

generating the new wavelength of light [28].

Nonlinear optics is now established as an alternative field to electronics

for the future photonic technologies. The fast-growing development in optical

33

fiber communication systems has stimulated the search for new, highly

nonlinear materials capable of fast and efficient processing of optical signals.

In recent years, many significant achievements have been realized in this field

because of the development of new nonlinear optical organic, semi-organic and

inorganic materials. Among the nonlinear crystals studied so-far, only a few

crystals satisfy the major requirements. For the development of new

technologies, the emergence of new nonlinear materials with superior quality is

needed.

2.4. THEORETICAL EXPLANATION OF NONLINEAR OPTICS

When a beam of electromagnetic radiation propagates through a solid,

the nuclei and associated electrons of the atoms create electric dipoles. The

electromagnetic radiation interacts with these dipoles causing them to oscillate,

which by the classical laws of electromagnetism, results in the dipoles

themselves acting as sources of electromagnetic radiation. If the amplitude of

vibration is small, the dipoles emit radiation of the same frequency as the

incident radiation.

As the intensity of the incident radiation increases, the relationship

between irradiance and amplitude of vibration becomes nonlinear resulting in

the generation of higher harmonics in the frequency of radiation emitted by the

oscillating dipoles. Thus, frequency doubling or second harmonic generation

34

(SHG) and, indeed, higher order frequency effects occur as the incident

intensity is increased.

In a nonlinear medium, the induced polarization is a nonlinear function

of the applied electric field. A medium exhibiting SHG is composed of

molecules with asymmetric charge distributions arranged in the medium in

such a way that a polar orientation is maintained throughout the crystal.

At very low fields, the induced polarization is directly proportional to

the electric field [3].

P = 0 . E 2.1

Where is the linear susceptibility of the material, E is the electric

field vector, 0 is the permittivity of free space.At high fields, polarization

becomes independent of the electric field and the susceptibility becomes field

dependent. Therefore, this nonlinear response is expressed by writing the

induced polarization as a power series in the fields.

P = 0 1 E + 2 E2 + 3 E3 + . 2.2

35

Table 2.1

Optical effects of linear and nonlinear optical materials

Order Susceptibility Optical Effects Applications

Linear effect

1 1 Refraction

Absorption

Transmission

Optical fibers

Colour Filter

Photolithography

Nonlinear effect

2 2 SHG

(=2) Frequency doubling

Frequency mixing

(1 2 =3) Optical parametric oscillations

Pockels effect

( + 0 =) Electro optical modulators

3 3 Four wave mixing Raman coherent spectroscopy

Phase gratings Real time holography

Kerr effect Ultra high speed optical gates

Optical amplitude Amplifiers, Choppers etc.

36

Where 2 , 3 .. are the nonlinear susceptibility of the medium. 1 is the

term responsible for materials linear optical properties like, refractive index,

dispersion, birefringence and absorption. 2 is the quadratic term which

describes second harmonic generation in non-centrosymmetric materials. 3 is

the cubic term responsible for third harmonic generation, stimulated Raman

scattering, phase conjugation and optical instability. Hence the induced

polarization is capable of multiplying the fundamental frequency to second,

third and even higher harmonics. The co-efficient of 1, 2, and 3 produces

certain optical effects, which are listed in Table 2.1.

2.5. VARIOUS TYPES OF NLO EFFECTS

Some nonlinear optical processes are familiar to physicists, chemists and

other scientists because they are in common use in the laboratories. Second

harmonic generation is a nonlinear optical process that results in the conversion

of an input optical wave into an output wave of twice as that of the input

frequency. The process occurs within a nonlinear medium, usually a crystal

(KDP-Potassium Dihydrogen Phosphate, KTP-Potassium Titanyl Phosphate,

etc.). Such frequency doubling processes are commonly used to produce green

light (532nm) using, a Nd:YAG (Neodymium:Yttrium Aluminum Garnet) laser

operating at 1064 nm [29]. Some of the NLO processes are given below:

(a) second harmonic generation

(b) sum frequency generation

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(c) difference frequency generation

(d) optical parametric generation

(e) linear electro optic effect or Pockels effect

(f) optical rectification

2.5.1 Second Harmonic Generation (SHG)

The process of transformation of light with frequency into light with

double frequency 2 and half the wavelength (Fig. 2.1) is referred to second

harmonic generation. The process is spontaneous and involves three photon

transitions. Second harmonic generation has been of practical interest ever

since after it was demonstrated because of its efficient conversion from

fundamental to second harmonic frequencies. This can be achieved by the

available powerful sources of coherent radiation at higher to unattainable

wavelengths [30].

Fig. 2.1

Schematic diagram of SHG

The most extensively studied conversion process of all has been the

doubling of the 1.064 m line obtained from the neodymium ion in various

NLO crystal

2

38

hosts. In particular, the doubling of the continuous wave Nd:YAG laser source

has recently been the subject of intensive study, because the laser light itself is

efficient and powerful so that the green light obtained by doubling is well

placed spectrally for detection by photomultipliers.

2.5.2. Sum Frequency Generation

It is a nonlinear optical process. Crystal materials with inversion

symmetry can exhibit nonlinearity. In such NLO materials the sum frequency

generation can occur. Fig. 2.2 illustrates the sum frequency generation.

1 + 2 = 3 2.3

When two electromagnetic waves with the frequency 1 and 2 interact

in a NLO medium, a nonlinear polarizability can be induced. The NLO

material generates an optical wave of frequency 3 which is equal to the sum of

the two input wave frequency 1 and 2. The energy of output wave is

represented in the equation 2.3.

Fig. 2.2

Schematic diagram of sum frequency generation

NLO crystal 1

2 3 = 1 +2

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2.5.3. Difference Frequency Generation

The process of difference-frequency generation is described by the

following equation 2.4.

1 - 2 = 3 2.4

Fig. 2.3

Schematic diagram of difference frequency generation

Fig. 2.3 illustrates the difference frequency generation. Here the

frequency of the generated wave is the difference of those of the input

frequencies.

2.5.4. Optical Parametric Generation

Optical parametric generation (Fig. 2.4) is an inverse process of sum

frequency generation and described by the following equation 2.5. It splits one

high-frequency photon (pumping wavelength p) into two low-frequency

photons (signal wavelength s and idler wavelength i)

s + i = p 2.5

NLO crystal 1

2 3 = 1 - 2

40

Fig. 2.4

Schematic diagram of optical parametric generation

2.5.5. Linear Electro Optic Effect

The Pockels effect is a linear change in the refractive index of a

medium in the presence of an external electric field. Here a dc field is applied

to a medium through which an optical wave propagates. The change in the

polarization due to the presence of these two interacting field components

effectively alters the refractive index of the medium.

2.5.6. Optical Rectification

The optical rectification is defined as the ability to induce a dc voltage

between the electrodes placed on the surface of the crystal when an intense

laser beam is directed into the crystal.

2.6. NONLINEAR OPTICAL MATERIALS

The search for new and efficient materials has been very active since

second harmonic generation (SHG) was first observed in single crystal quartz

[27]. The discovery of inorganic photorefractive crystals such as potassium

NLO crystal

p s

i

41

niobate (KNbO3), potassium dihydrogen phosphate (KH2PO4), barium titanate

(BaTiO3), lithium niobate (LiNbO3) and their optimization during the last thirty

five years have led to numerous demonstration of variety of optical

applications.

At the end of 1968, Kurtz and Perry SHG method was introduced and a

powdered sample is irradiated with a laser beam and scattered light is collected

and analyzed for its SHG efficiency. So, the stage was set for a rapid

introduction of new materials, both inorganic and organic [31]. For the optical

applications, a non linear material should have the following requirements [3]:

(a) wide optical transparency range

(b) ease of fabrication and high nonlinearity

(c) high laser damage threshold

(d) ability to process into crystals and thin films

(e) good environmental stability

(f) fast optical response time

(g) high mechanical and thermal stability

2.7. DEVELOPMENT OF NLO MATERIALS

In recent years, the extensive investigations carried out on NLO

materials have been very much helpful to identify different types of NLO

crystals. New techniques applied to the fabrication of ultra glass that enabled

the fabrication of fibers with ultra-low loss, provided the main stimulus to

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optical fiber communication. The recent emergence of Erbium doped glasses

and the fabrication of fiber amplifiers, another major milestone in this area,

enabled 50 gigabits per second transmission rates. Such high amplification

rates can not be achieved with standard electronic amplifiers. The high speed,

high degree of parallelism of optics will lead gradually to optoelectronic

systems where an increasing number of functions will be implemented

optically. In that respect, materials with a nonlinear optical (NLO) response are

expected to play a major role in enabling optoelectronics and photonic

technologies.

The nonlinear optical materials are broadly classified into:

(a) organic crystals

(b) semi-organic crystals

(c) inorganic crystals

2.7.1. Organic Crystals

The search for new NLO materials over the past two decades has

concentrated primarily on organic compounds because of their high

nonlinearity. Nonlinear organic crystals have proven to be interesting

candidates for a number of applications like, second harmonic generation,

frequency mixing, electro-optic modulation, optical parametric oscillation etc.

The superiority of organic NLO materials results from their versatility and the

possibility of tailoring them for a particular end use [3]. The NLO properties of

43

large organic molecules and polymers have been the subject of extensive

theoretical and experimental investigations during the past two decades and

have been investigated widely due to their high nonlinear optical properties,

rapid response in electro optic effect and large second or third order

polarizability.

Rosker and Tank [32] have reported that urea has been used in an optical

parametric oscillator to generate tunable radiation throughout the visible

region. Intrinsic absorption and phase matching considerations make urea

unsuitable for wavelengths greater than 1000nm. The efforts made to resolve

the problems associated with urea have not been successful. The newly grown

binary urea and m.nitrobenzoic acid (UNBA) crystal amplifier [33] is thermally

and mechanically harder than the crystal of the parent components.

Manivannan and Dhanuskodi [34] have grown a new organic crystal

3-[(1E)-N-ethylethanimidoyl]-4-hydroxy-6-methyl-2H-pyran-2-one and found

that its SHG efficiency is close to urea. Haja Hameed et al [35] have obtained

trans-4-(dimethylamino)-N-methyl-4-stilbazolium tosylate (DAST) crystals

and the crystal surfaces were analyzed with the help of optical and scanning

electron microscope.

Modified hippuric acid (HA) single crystals have been grown from

aqueous solution of acetone by doping with NaCl and KCl, with the vision to

improve the physicochemical properties of the sample [36]. A new nonlinear

optical organic single crystal 4-Phenylpyridinium hydrogen squarate (4PHS)

44

has been grown by Ramachandra Raja et al [37] and showed that the SHG

efficiency of the grown crystal is five times greater than that of KDP crystal.

L-alanine nitrate (LAAN) [38] an organic nonlinear optical material was grown

by slow evaporation method at room temperature from aqueous solution. The

transmission spectrum reveals that the crystal has a low UV cut-off wavelength

and has a good transmittance in the entire visible region.

2.7.2. Semi-Organic Crystals

The widest search for new compounds and crystals led to the

development of many amino acids based semi-organic single crystals. In

comparison with inorganic crystals, semi-organic crystals are less hygroscopic

and can be easily grown as single crystals. L-arginine phosphate monohydrate

(LAP) is one of the potential nonlinear optical crystals among the amino acid

based semi-organic materials. Monaco et al [39] synthesized LAP and its

chemical analogs are the strongly basic amino acid and various other acids. All

the compounds in this class contain an optically active carbon atom, and

therefore all of them form acentric crystals. All the crystals are optically biaxial

and several among them give second harmonic signals greater than quartz.

Different organic and inorganic acids were introduced into L-alanine

and L-hystidine and many new nonlinear optical materials were reported with a

better NLO efficiency compared to inorganic KDP crystals. LAP crystals are

usually grown from aqueous solution by temperature lowering technique. LAP

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crystals possess high nonlinearity, wide transmission range (220-1950nm), high

conversion efficiency (38.9%) and high laser damage threshold. Metal-organic

crystals form a new class of materials under semi-organics. Compared to

organic molecules, metal complexes offer a larger variety of structures, the

possibility of high environmental stability, and a diversity of electronic

properties by virtue of the coordinated metal center.

2.7.3. Inorganic Crystals

Inorganic materials are mostly ionic bonded and have high melting point

and high degree of chemical inertness. Investigations on nonlinear optical

phenomena in single crystals were initially focused on purely inorganic

materials such as quartz, lithium niobate (LiNbO3), potassium niobate

(KNbO3), potassium titanyl phosphate (KTiPO4), lithium iodate (LiIO3),

borates and semiconductor crystals.

Various borate crystals including -BaB2O4 (BBO), LiB3O5 (LBO),

have been reported as promising NLO crystals. The family of the various

borate crystals plays a very important role in the field of nonlinear optics [40].

Ravi et al [41] have reported the optimized growth condition of tetragonal

phase deuterated potassium dihydrogen phaspate (DKDP) with higher

deuterium concentration for growing large size crystals. D.Xue et al [42] have

studied the second order nonlinear optical properties of doped lithium

niobate(LN) crystals. It was observed that the second order NLO response of

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doped LN crystals decreases with increased doping concentration in the crystal.

Successful growth of a new NLO crystal Ca5(BO3)3 with UV cut-off 190nm

was presented by Guojun Chen et al [43]. Zhoubin Lin et al [44] have found

that the SHG efficiency of YCa9 (VO4)7 single crystal is 4.7 times as large as

that of KDP crystal. The structure and NLO efficiency of the non-

centrosymmetric borate chloride Ba2TB4O9Cl (T=Al, Ga) crystals have been

explained by Jacques Barbier [45].During the last few years, various borate

crystals like GdCa4O(BO3)3, YCa4O(BO3)3 and LaCa4O(BO3)3 have been

reported as promising NLO crystals.

2.8 SQUARIC ACID, L-PROLINE AND THIOCYANATE BASED

OPTICAL CRYSTALS

Squaric acid (3, 4 dihydroxy-3-cyclobutene-1-2 dione) was first

prepared by S. Cohen et al [46] has been the subject of great attention. It is a

chemically stable highly acidic colourless crystalline substance, which melts at

about 566 K with decomposition. The squaric acid (C4H2O4) at room

temperature consists of ordered layer of C4O4 groups [47-49]. Each C4O4 group

is linked by four O-H-O bonds to neighbouring molecules within the same

layer, thus forming a pseudo-two dimensional structure. The layers are held

together by Vander Waals forces. To our knowledge, most of the experimental

and theoretical investigations of squaric acid have been concentrated on

structural analysis. The structure of the following compounds

4-phenylpridinium betaine of squaric acid, (8-hydroxyquinolinium) squarate,

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4-phenylpridinium hydrogen squarate and 4-Dimethylaminopyridinium-1-

squarate belongs to monoclinic crystal system [50-53]. Our present work

focuses mainly on the growth and characterization of squaric acid and amino

acid based single crystals.

In squaric acid, the motion of the protons between the two equilibrium

sites in O-H-O creates an anion so that it acts as hydrogen donor which was

detected by 17O proton magnetic dipolar coupling measurement [54]. Squaric

acid when mixed with proton acceptor groups like NO2, NO, CN results in the

formation of dipole. This dipole is responsible for NLO activity of the

compound which is observed in non-centro symmetric crystals [55].

Vibrational spectral analysis of the nonlinear optical material, L-

prolinium tartrate (LPT) was carried out using NIR-FT-Raman and FT-IR

spectroscopy by Padmaja et al (2006). Also the single crystals of LPT were

grown by Martin Britto

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