Growth and Characterization of Nonlinear Optical...

Growth and Characterization of Nonlinear Optical

Borate Crystals: CBO, SBO and BBO

International School of Photonics

A dissertation submitted to

Cochin University of Science and Technology, Cochin, Kerala

towards the partial fulfillment of award of degree of

Master of Technology in Optoelectronics and Laser Technology

by

Pimpalwadkar Anand Darshan

under the guidance of

Work carried out at

Crystal Growth Laboratory Laser Materials Development and Devices Division

Raja Ramanna Centre for Advanced Technology, Indore

Dr. Rajeev Bhatt

Scientific Officer F

LMDDD, RRCAT, Indore

Dr. A K Karnal

Scientific Officer H

LMDDD, RRCAT, Indore

i

ACKNOWLEDGEMENT

Firstly, I am highly grateful to Dr. P. K. Gupta, Head, Laser Materials Development and

Devices Division (LMDDD), RRCAT for giving me the opportunity to work in the Crystal

Growth Lab. as a trainee. It has been truly an enlightening experience for me to work in the lab

at RRCAT. I am also grateful to Sh. P.K. Kush, Chairman, Project Placement Committee

for selecting me to carry out the project work at RRCAT.

I felt delighted that Dr. A.K. Karnal, Head, Crystal Growth Laboratory, LMDDD, accepted

me as his student for guiding me for my M.Tech. project work. I am highly obliged for his

constant inspiration, able guidance. I express my deep sense of acknowledgement to Dr. Rajeev

Bhatt, for the encouragement, support and guidance. I am thankful to Dr. Indranil Bhaumik

for his help and valuable suggestions throughout the project work.

I am thankful to Mr. M. Soharab and Mr. Amit Saxena for their significant contributions in

performing the experiments for the best results, having interesting discussions and for the many

light moments. I wish to acknowledge Dr. Gurvinderjit Singh and Mr. Prem Kumar, for

helping me to carry out the XRD experiments. I am grateful to Mr. B.K. Sajith, Mr. S.R.

Bagade and Mr. S.M. Sharma for setting up systems and troubleshooting. I would like to

thank all the members of Laser Materials Development & Devices Division (LMDDD) for

providing healthy working environment during the course of this project work.

I avail this opportunity to express my deep sense of gratitude towards Dr. M Kailasnath,

Director, ISP for allowing me to carry out project work at RRCAT Indore. The

acknowledgement is incomplete without mentioning Dr. P Radhakrishnan and Dr. V P N

Nampoori who will be constant source of inspiration for me throughout my life. I consider

myself lucky to have attended their lectures. I am also thankful to the all faculties and staff of

International School of Photonics for their support and concern.

I am thankful to Praveen, Durgesh, P. Rameshbabu for their help throughout the year in

carrying out experiments.

ii

ABSTRACT

Laser sources in UV and deep UV regions are scarce. There is high demand of efficient,

compact solid state laser sources in these regions for various applications. Borate materials

are being considered having potential to fulfill this demand. Owing to their

noncentrosymmetric structures, high nlo coefficients, high laser damage thresholds and

transparency till deep UV range make them suitable candidates to generate UV laser through

second and higher harmonic generations.

In this work, borate materials belonging to three different anionic groups were studied and

single crystals were grown for characterization. Cesium triborate and strontium tetraborate

were grown with Czochralski technique and top seeded solution growth was implemented to

grow single crystals of β-barium borate. XRD measurements were carried out on grown

crystals to confirm single phase. Samples were cut from these crystals and transmission spectra

was recorded. Refractive indices in different crystallographic directions were obtained for

different wavelengths. Sellmeier coefficients and birefringence were calculated from this data.

Thermo optic coefficients of refractive index were also determined for these crystals.

Temperature dependence of refractive index for strontium tetraborate was determined for the

first time and unlike most of the borate materials it possessed positive thermo optic coefficient

of refractive index. Conoscopic pattern of β-barium borate confirmed its negative uniaxiality.

iii

CONTENTS

LIST OF FIGURES .................................................................................................................. v

1. INTRODUCTION ............................................................................................................. 1

1.1 What is a Crystal? ............................................................................................................. 1

1.2 Importance of Crystals ...................................................................................................... 2

1.3 Crystal Growth .................................................................................................................. 3

1.3.1 Supersaturated Solution ............................................................................................. 5

1.3.2 Supercooled Solution ................................................................................................. 6

1.3.3 Nucleation .................................................................................................................. 6

1.3.4 Critical Nucleus Size for a Spherical Embryo ........................................................... 7

1.4 Crystal Growth Techniques .............................................................................................. 9

1.4.1 Growth from Melt ...................................................................................................... 9

1.4.1.1 Czochralski Method ................................................................................................ 9

1.4.1.2 Kyropoulos Technique .......................................................................................... 10

1.4.1.3 Optical Floating Zone ........................................................................................... 11

1.4.3 Growth from Solution .............................................................................................. 12

1.4.3.1 Low Temperature Solution Growth ...................................................................... 13

1.4.3.2 Flux Growth .......................................................................................................... 13

2. LITERATURE SURVEY ............................................................................................... 15

2.1 Motive for Project Work ................................................................................................. 15

2.2 What are Borates? ........................................................................................................... 17

2.3 Important Anionic Groups in Borates ............................................................................. 18

2.4 Transparency of Borates in UV and deep UV Regions .................................................. 19

2.5 Cesium Triborate (CBO) ................................................................................................ 20

2.5.1 Structure of CBO ..................................................................................................... 21

2.5.2 Phase Diagram of CBO ............................................................................................ 22

2.5.3 CBO Crystal Growth History ................................................................................... 23

iv

2.6 Strontium Tetraborate (SBO) ......................................................................................... 25

2.6.2 SBO Crystal Growth History ................................................................................... 26

2.7 Barium Borate (BBO) ..................................................................................................... 27

3. CRYSTAL GROWTH .................................................................................................... 31

3.1 Growth of Cesium Triborate (CBO) ............................................................................... 31

3.1.1 Resistive Furnace Setup ........................................................................................... 31

3.1.2 Charge Synthesis ...................................................................................................... 33

3.1.3 Calculations .............................................................................................................. 33

3.1.4 Cleaning of crucible ................................................................................................. 34

3.1.5 Formation of Precursor ............................................................................................ 34

3.1.6 Growth Process ........................................................................................................ 35

3.2 Growth of Strontium Tetraborate (SBO) ........................................................................ 38

3.2.1 Charge Synthesis ...................................................................................................... 38

3.2.2 Growth Process ........................................................................................................ 38

3.3 Growth of Barium Borate (BBO) ................................................................................... 41

3.3.1 Charge Synthesis and Growth Process ..................................................................... 41

4. OPTICAL CHARACTERIZATIONS .......................................................................... 47

4.1 Refractive Index Measurement ....................................................................................... 47

4.2 Optical Transmission Measurement ............................................................................... 52

4.3 Study of Conoscopic Pattern .......................................................................................... 54

5. RESULTS AND DISCUSSION ..................................................................................... 56

6. CONCLUSIONS & FUTURE SCOPE ......................................................................... 59

v

LIST OF FIGURES

Figure 1.1 Single crystal vs polycrystal..................................................................................... 2

Figure 1.2 Phase diagram of water ............................................................................................ 4

Figure 1.3 Transition from metastable to stable state ................................................................ 4

Figure 1.4 Change in free surface energy vs radius of embryo ................................................. 8

Figure 1.5 Czochralski method ................................................................................................ 10

Figure 1.6 Kyropoulos method ................................................................................................ 11

Figure 1.7 Optical float zone method ...................................................................................... 12

Figure 2.1 Basic structure units of borates (a) BO3 (b) B3O6 (c) B3O7 ................................... 18

Figure 2.2 Nonplanar (B3O7 )5- anionic group ......................................................................... 20

Figure 2.3 Unit cell of CBO crystal ......................................................................................... 21

Figure 2.4 Phase diagram of Cs2O – B2O3 system .................................................................. 22

Figure 2.5 Unit cell of SBO crystal ......................................................................................... 25

Figure 2.6 Unit cell structure of BBO ..................................................................................... 27

Figure 3.1 Schematic diagram of experimental setup for CBO growth .................................. 31

Figure 3.2 Temperature profile of the furnace ........................................................................ 32

Figure 3.3 Meniscus region in Czochralski technique ............................................................ 36

Figure 3.4 XRD pattern of CBO .............................................................................................. 36

Figure 3.5 Different growth rates and grown CBO crystal inset grown inset ......................... 37

Figure 3.6 Different growth rates and grown SBO crystal inset ............................................. 39

Figure 3.7 XRD pattern of SBO .............................................................................................. 40

Figure 3.8 Schematic of experimental setup ........................................................................... 41

Figure 3.9 XRD pattern of BBO .............................................................................................. 42

Figure 4.1 Schematic of prism coupling technique ................................................................. 48

Figure 4.2 Light intensity at detector vs angle of incidence (taken as position) ..................... 48

Figure 4.3 Sellmeier fitted curves for (a) CBO, (b) SBO and (c) BBO crystals at 300C ........ 49

Figure 4.4 Temperature dependent refractive index of (a) CBO, (b) SBO and (c) BBO

crystals ...................................................................................................................................... 50

Figure 4.5 Transmission spectra of (a) CBO (b) SBO and (c) BBO ....................................... 52

vi

Figure 4.6 Determination of band gap of BBO crystal ........................................................... 53

Figure 4.7 Conscopy pattern of c-cut BBO sample ................................................................. 55

Introduction

1

1. INTRODUCTION

1.1 What is a Crystal?

The word ‘crystal’ originates from the Ancient Greek word ‘krustallos’ which means ice. The

enchanting appearance of crystals through their colours, transparency, shapes and healing

powers as believed by some have always fascinated humans from the beginning of the Stone

Age. Ancient Sanskrit texts like Ratnashastra, Ratnapariksha, Ratna Dipika consist valuable

information about ratnas or gems which are sophistically grown crystals [1]. The shiningly

bright crystals were predominantly used as ornaments and some others were part of the

alchemists’ study who were trying to develop a universal medicine or philosopher’s stone [2].

Crystals were seen as mesmerizing work of nature when physicists and mathematicians were

studying their internal structure which ultimately bestowed them their external appearance.

The crystal is a solid formed through three dimensional periodic array of identical building

blocks of a material (atoms, molecules or ions) which does not contain imperfections and

impurities that may accidently be induced in the structure [3].

In single crystals, distribution of basic constituents like atoms is continuous throughout the

crystal without any discontinuous boundaries. Perfect single crystals are rare in nature and they

have to be grown in laboratory in well controlled atmosphere. Before defining polycrystals, it

is important to know about crystallites or grains which are referred as microscopic crystals or

often single crystals which vary in their size and orientation. When such crystallites of different

size and orientation are aggregated to form a solid which is discontinuous at grain boundaries

is referred as polycrystal.

Introduction

2

Figure 1.1 Single crystal vs polycrystal

1.2 Importance of Crystals

“… Today, man lives on the boundary between the Iron Age and a new Materials Age.” Nobel

laureate Sir George Thomson [4] in 1937 made us aware of the upcoming material revolution

which is still accelerating. Technology demands efficient materials for its implementation. Best

example is growth of electronics industry witnessed by advancement of semiconductor

materials like silicon, germanium, gallium arsenide, etc. Optoelectronics and laser technology

is boosted with new optical materials especially nonlinear optical materials like lithium niobate,

lithium tantalate, KDP, etc. It is difficult to imagine the photonic industry without crystals. Dr.

Tadahiro Sekimoto, a Japanese businessman and scientist has accurately put forward the

necessity of developing novel materials for advancement of technology as, who dominates

materials dominates technology [5]. The necessity of rapid development of new materials has

established material science as an emerging discipline. As crystal is a basic building block of a

solid state material, growing newer and better crystals from existing and future techniques is

inevitably important. Some of the recently worked upon crystals like photonic crystals and

quasi crystals are gaining their ground.

Invention of lasers in 1960 and first demonstration of a nonlinear effect as second harmonic

generation by Franken in following year marked the importance of crystals in this regime. It

was considered difficult to make short wavelength lasers [6] until certain nonlinear optical

materials found capable of emitting at low wavelength like blue lasers or UV lasers which is

revolutionizing optical memory density concepts for storing data. Optical parametric

amplification process now finding its suitability in various communication applications.

Introduction

3

Nonlinear optical crystal in electro optic system can be used to intensity modulate data and

demodulate it using optical detector.

1.3 Crystal Growth

Matter in the universe is present in different states or phases in which it can attain maximum

stability. A system always prefers most stable state through energy transfer with surroundings.

Different phases a matter can exist at particular pressure and temperature can be explained by

a phase diagram. Each phase has chemical potential (Δμ) or energy associated with it which is

used as work done while phase transition. Chemical potential is defined as amount of work

done in order to change number of particles in a phase by unity [7]. Two phases are in

equilibrium when their chemical potentials equal. Consider two phases of a matter as A and B,

with potential energies 𝛥𝜇𝐴 and 𝛥𝜇𝐵 respectively. These phases will be in equilibrium when

𝛥𝜇𝐴(𝑃, 𝑇) = 𝛥𝜇𝐵 (𝑃, 𝑇)

If there arises difference in chemical potentials of these phases, phenomena of mass transport

takes place which continues until both phases acquire equilibrium again. This imbalance of

chemical potentials works as the driving force for phase transition [8].

𝛥𝜇 ∗ 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡𝑒𝑑 = 𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒

It is important to note that only fraction of energy is available to be converted into work termed

as Gibbs free energy and remaining energy is called bound energy.

Gibbs free energy is an important concept to understand the thermodynamics of crystal growth,

especially concept of nucleation and determining critical size of nuclei which finally grows into

a crystal. From thermodynamic point of view Gibbs free energy establishes relation between

enthalpy (H) and entropy (S) of the system through temperature (T) as,

𝐺 = 𝐻 − 𝑇𝑆

It is a measure of stability of a phase. Lesser the free energy more stable the phase would be.

For a phase transition above equation can be rewritten as,

𝛥𝐺 = 𝛥𝐻 − 𝑇𝛥𝑆

Introduction

4

where 𝛥𝐺, 𝛥𝐻 and 𝛥𝑆 represents change in free energy, enthalpy and entropy of the system

respectively while transition from one phase to another.

Formation of crystal is controlled phase transformation from solid, liquid or gaseous phase into

solid state. Crystal phase favoured at high pressure and low temperature [8].

Figure 1.2 Phase diagram of water

Along the line separating two phases, both phases are in equilibrium due to equality of their

chemical potentials at corresponding values of pressure and temperature. This balance can be

disturbed through change in pressure or temperature in one of the phases which makes transition

to other phase to attain stability. In crystal growth technique such a state is obtained through

supersaturation or supercooling of the initial available phase often referred as metastable state

or mother phase. Energy associated with metastable state is intermediate of stable and unstable

states. Growth is a result of highly one directional flow of substituents from one phase to

another which is obtained as transition from metastable to stable state.

Figure 1.3 Transition from metastable to stable state

Introduction

5

In figure 1.3, net growth from A to B is not obtained at equilibrium temperature as transition

from A to B is equal to transition from B to A. But at lower temperature, if B has lower free

energy than A, net growth from A to B is observed as transition from B to A puts higher energy

barrier to overcome [9].

1.3.1 Supersaturated Solution

A solvent at given temperature can dissolve a suitable solute up to a certain equilibrium

concentration of solute. After the equilibrium concentration is reached, the solute is not further

soluble in solvent. Such a solution is called as saturated solution which is in thermodynamic

equilibrium. Generally, solubility can be increased with increase in temperature. More quantity

of solute can be made to dissolve in the solvent at higher temperature than its actual equilibrium

concentration. When this solution is allowed to cool down slowly without agitation or

fluctuations, it creates favourable conditions for nucleation through which crystal can be grown

eventually. Such a solution is termed as supersaturated solution which is in thermodynamic

non equilibrium. Saturation 𝑆 is ratio of present concentration of solute to its equilibrium

concentration.

𝑆 =[𝐶]

[𝐶∗]

If 𝑆 > 1, solution is supersaturated.

Introduction

6

1.3.2 Supercooled Solution

A liquid in its pure (without contamination) form can maintain liquid state even below the

freezing point due to absence of nucleation sites. Such a state is a metastable state which is

favourable for crystallization through provision of nucleation centre. Water freezes at 0o C. But

supercooled water can maintain liquid state even at low temperature like -40oC. Actual freezing

point of water is recently calculated as -48o C [10].

1.3.3 Nucleation

One of the definitions of nucleation states that it is a process of generating within metastable

mother phase initial fragments of new and more stable phase, capable of developing

spontaneously into gross fragments of stable phase [11].

The theory of formation of a nucleus (aggregating centre) was put forward by Volmer and

Gibbs. According to this theory fluctuations caused internally or externally in supersaturated or

supercooled systems, make few atoms or molecules to join and form clusters in the new

developing phase. Such cluster is often called as embryo or nucleation centre, etc. As it is a

phase transition, change in free energy takes place in this process. Not all clusters grow as a

stable nucleation centre but dissolve back into metastable mother phase as change in free energy

is not higher enough for phase transition to solid phase. In fact, probability of a cluster or

embryo growing into a stable nucleus which can support further growth increases with increase

in change in free energy associated with phase transition.

There always exists minimum size of embryo termed as critical nucleus, which can sustain in

mother phase and finally grow into product phase as crystal. Nucleation can occur in two ways.

If the embryo is formed within the mother phase without any external interference, it is termed

as spontaneous or homogeneous nucleation. On the other hand if embryo is formed around an

impurity or artificially induced nucleation site into metastable phase it is known as

heterogeneous nucleation. Use of seed in crystal growth is an example of such nucleation

process.

Introduction

7

1.3.4 Critical Nucleus Size for a Spherical Embryo

Change in Gibbs free energy is contributed through surface excess free energy (ΔGS) between

surface of particle and bulk of particles as well as volume excess free energy (ΔGV) between

bulk of particles and solute in solution.

𝛥𝐺 = 𝛥𝐺𝑆 + 𝛥𝐺𝑉

𝛥𝐺𝑆 for a spherical embryo of radius r is given as,

𝛥𝐺𝑆 = 4𝜋𝑟2𝜎

where 𝜎 is called interfacial tension between developing crystalline surface and mother phase.

𝛥𝐺𝑉 has negative value as,

𝛥𝐺𝑉 = −4

3𝜋𝑟3𝛥𝐺𝜇

where 𝛥𝐺𝜇 is free energy change of transformation per unit volume.

𝛥𝐺 = 4𝜋𝑟2𝜎 −4

3𝜋𝑟3𝛥𝐺𝜇

Maximum change in Gibbs free energy can be obtained by differentiation above equation with

respect to r and equating it to zero to get value of optimum r as,

𝑟∗ =2𝜎

3𝛥𝐺𝜇

𝑟∗ is the critical nucleus. A nucleus formed with lesser size than critical value does not sustain

growth and dissolves back. Free energy change at critical nucleus is given as

𝛥𝐺∗ =16𝜋𝜎3

3𝛥𝐺𝜇2

Size of the crystal is well affected by number of nucleation sites. Fewer nucleation sites result

into larger crystal size whereas many available nucleation sites result into smaller average sized

crystal.

Introduction

8

Figure 1.4 Change in free surface energy vs radius of embryo

Introduction

9

1.4 Crystal Growth Techniques

Based on the mother phase from which crystal is grown, crystal growth can be categorised on

the basis of phase transformation as

Solid to solid (Precipitation)

Liquid to solid (Solution and melt)

Vapour to solid (Sublimation)

1.4.1 Growth from Melt

Crystal growth from melt is directional solidification from the melt. It is the fastest process as

the growth rate does not depend upon mass transport process. Large single crystals of material

which melts congruently are obtained through this technique. Melted material is slowly cooled

down to obtain crystalline matter. Crystals of metals, semiconductors, laser materials can be

obtained by using melt technique.

Melt growth technique can be further classified into normal freezing and zone growth methods.

Normal freezing technique covers Bridgman and Czochralski methods whereas zone growth

covers optical and electrical floating zone methods.

1.4.1.1 Czochralski Method

Jan Czochralski accidently invented this method of crystal growing when he dipped nib of his

ink pen in molten tin pot instead of inkpot. Crystal growth in this method is accomplished with

the help of seed crystal. The molten charge is kept in crucible at temperature just above its

melting point. The seed crystal mounted on a rod is suspended vertically downwards. The seed

and charge are not in contact at the start of the process.

To initiate the process of crystal growth, rod is dipped into melt charge which starts nucleation

and then slowly pulled upwards to avoid remelting. One important characteristic of Czochralski

method lies in simultaneous pulling and rotating the seed crystal to maintain homogeneity in

grown crystal. This method requires controlled atmosphere in inert chamber. A typical

cylindrical ingot is obtained as final product of this method. This method is extensively used to

Introduction

10

obtain silicon ingots in semiconductor industries for production of silicon wafer. This method

cannot be used for materials with high vapour pressure.

Figure 1.5 Czochralski method

1.4.1.2 Kyropoulos Technique

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

method, here also the seed is brought into contact with the melt but not pulled 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 by slow cooling of melt. The major use of

this method is growth of alkali halides to make optical components.

The process involved in Kyropoulos technique is depicted in following figure which shows

three important steps followed, melting of precursor, dipping of seed and obtaining crystal

through slow cooling.

Introduction

11

Figure 1.6 Kyropoulos method

1.4.1.3 Optical Floating Zone

Floating zone technique was invented by W G Pfann in 1951. The word floating signifies no

need of crucible to hold the charge and zone is the small portion of charge which is actually

made to melt and grows as a single crystal with assistance from seed. The charge used in this

technique is a polycrystalline rod instead of a melt one.

Polycrystalline rod of the material is passed vertically downwards through a heating zone which

starts melting of some part of the rod coming under influence of heating field. A seed crystal is

made to contact with melt region which initiates nucleation and growth of single crystal. At the

interfacial boundary between solid and liquid phases impurity diffuse more into liquid phase

than solid phase which results into highly pure single crystal. An example of such naturally

purified sample can be observed in case of icebergs in ocean which are free from salt.

Float zone techniques can be classified with respect to method of heating the rod in heat zone.

Induction coil, resistance, optical heating are some of the examples.

In optical floating zone technique high power halogen or xenon lamps along with ellipsoidal

mirrors are used which focus the heat generated from lamps at the rod.

Introduction

12

Figure 1.7 Optical float zone method

1.4.3 Growth from Solution

Crystals of all materials cannot be made from their melts especially those which do not melt

congruently or decomposes before melting or having very high melting point. For such

materials solution growth technique is adapted. For efficiently growing crystal from solution

growth the material should be moderately soluble in the given solvent, system should be less

viscous and non-volatile.

Solution growth is one of the oldest crystal growing technique whose basics lie in preparing

supersaturated solution using material to be crystallized and suitable solvent to give rise to

nucleation followed by growth. This is achieved either by slow evaporation of solvent or slow

cooling of solution [11]. Solution growth is further subdivided into high temperature solution

growth or flux growth and low temperature solution growth.

Introduction

13

Materials having moderate to high solubility from room temperature to 100oC at atmospheric

pressure can be grown by low temperature solution growth which may take weeks, months

sometimes years [12].

1.4.3.1 Low Temperature Solution Growth

This technique is mostly used for materials which are unstable and undergo phase

transformation at elevated temperatures. The alternative growth methods are solution growth at

room temperature or near to room temperature, close to equilibrium conditions. Material which

suffers from decomposition at or below its melting point, having moderate to high solubility in

the temperature range ambient to 100° C can be grown by the low temperature solution growth

method.

1.4.3.2 Flux Growth

Flux growth is popular name for high temperature solution growth where material is dissolved

into a suitable solvent called flux which has lower melting point. Flux can be defined as a liquid

reaction medium that dissolves reactants but do not participate in the reaction. This method is

very slow and crystals grown are often smaller in size. Typical solvents or molten salt fluxes

are PbO, Na2O, KF, NaF, SnF2, B2O3, etc.

Introduction

14

References

1. Krishnamurthy R. (1992). Gemmology in Ancient India. Indian Journal of History of

Science, 27 (3).

2. Scheel H. J. and Fukuda T. (2003). Crystal Growth Technology. John Wiley and Sons Ltd.

3. Kittel C. (2005). Introduction to Solid State Physics. John Wiley and Sons, Inc.

4. Elwell D. and Scheel H. J. (2011). Crystal Growth from High Temperature Solutions.

Academic Press.

5. Byrappa K. and Ohachi T. (2003). Crystal Growth Technology. William Andrew

Publishing.

6. Silfvast, W. T. (2004). Laser Fundamentals. Cambridge University Press.

7. Markov I. (2003). Crystal Growth for Beginners. World Scientific.

8. Hurley D. T. J. (1993). Handbook of Crystal Growth. Vol. 1, Elsevier.

9. Mullin J. W. (2001). Crystallization. 4th Ed, Butterwoth-Heineman.

10. Moore E. B. and Molinero V. (2011). Structural Tranformation in Supercooled Water

Controls the Crystallization Rate of Ice. Nature, 479, 506-508.

11. Sivakumar S. (2012). Cognition on Some Nonlinear Optical Crystals. Ph.D. Thesis, Periyar

University, India.

12. Sureshkumar P. (2011). Investigation on the Growth and Properties of Nonlinear Optical

Crystals. Ph.D. Thesis, Anna University, India.

Literature Survey

15

2. LITERATURE SURVEY

2.1 Motive for Project Work

Unlike IR and red solid state laser sources, blue and ultraviolet lasers are scarce. This fact is

well elaborated by W. Silfvast, a renowned physicist in the field of gas lasers. In his book Laser

Fundamentals, considered as one of the authoritative books in learning lasers, he quotes,

Considering strong wavelength dependence of stimulated emission cross section (𝜎𝑢𝑙), it is

difficult to make short wavelength lasers [1].

Possibility of lasing in a medium is decided by growth factor, also known as gain of active

medium, given as,

𝜎𝑢𝑙(𝜈)∆𝑁𝑢𝑙𝐿𝑠𝑎𝑡

𝜎𝑢𝑙(𝜈) = stimulated emission cross section

∆𝑁𝑢𝑙 = population difference between upper and lower laser level

𝐿𝑠𝑎𝑡 = saturation length of active medium

For both homogeneous and Doppler broadening, value of 𝜎𝑢𝑙(𝜈) is proportional to the factor,

𝜆𝑢𝑙2𝐴𝑢𝑙

Δ𝜈

𝜆𝑢𝑙 = emission wavelength

𝐴𝑢𝑙 = radiative transition probability from upper to lower laser level

Δ𝜈 = emission bandwidth

This factor clearly indicates how predominantly gain depends on the emission wavelength

required. Moving towards lower wavelength regions of electromagnetic spectrum to obtain high

energy lasing, gain of the medium naturally reduces. Transition probability is inversely

proportional to upper laser level lifetime so as emission bandwidth. UV laser active media

Literature Survey

16

generally have short upper laser level lifetime demanding more pumping [2] as well as

increasing denominator factor Δ𝜈 which invariably reduces gain.

Currently available UV laser sources are limited as rare-gas halide excimer lasers such as XeF

(353 nm), KrF (248 nm), ArF (193 nm), XeCl (308 nm) which are capable of producing high

average output power. GaN and InGaN based diode lasers are capable of producing near UV

laser emission. These lasers carry certain disadvantages like excimer lasers suffer from

bulkiness, demand of regular maintenance, use of corrosive gases, high voltage gaseous

discharges, etc. Disadvantage of diode lasers include divergence and short coherent lengths.

Subject to these shortcomings, new UV and deep UV laser sources are still in high demand for

medical surgeries, optical data storage devices to increase memory density, optical

communications and other industrial applications. 158 nm and 193 nm coherent light sources

are specifically required for semiconductor photolithography. Micromachining and material

processing also needs deep UV laser sources as high energy photon can induce bond braking

process in many materials. Need of UV and deep UV coherent light sources is well discussed

in review articles by Sasaki et al. [2] and R. Arun Kumar [3].

Borates have gain importance as popular nonlinear optical (NLO) materials due to their ability

to produce UV and deep UV laser through cascaded sum frequency generation using IR lasers

by second, third and higher harmonic generations [2]. These materials satisfy important

conditions to be a NLO crystal as high NLO coefficient, moderate birefringence, large laser

damage threshold, etc.

Literature Survey

17

2.2 What are Borates?

Oxyanions is a term used to identify anions containing oxygen with generalised chemical

formula (𝐴𝑥𝑂𝑦)𝑧− where 𝐴 is an element. Borates are oxyanions with boron. Boron atom is

capable of existing either in three fold or four fold symmetries. This property allows numerous

borate structure types to be formed such as (𝐵𝑂3)3−, (𝐵𝑂4)5−, (𝐵2𝑂5)4−, (𝐵3𝑂6)3−,

(𝐵3𝑂7)5−, (𝐵3𝑂9)9−, (𝐵5𝑂10)5−, etc. Borate crystals known today contain one of such borate

anionic group as their basic structural units extensively covered in a review article by Chen et

al. who developed a theory called anionic group theory during 1968 - 1974 [4].

Anionic group theory was one of the first successful theoretical models in calculating second

harmonic generation (SHG) coefficients in NLO crystals. The theory establishes relation

between macroscopic NLO properties of a material and its microscopic structure [5]. Based on

anionic group theory 𝐵𝑎𝐵2𝑂4 (𝐵𝐵𝑂) was established as an excellent UV NLO crystal followed

by 𝐿𝑖𝐵3𝑂5 (𝐿𝐵𝑂), 𝐶𝑠𝐵3𝑂5(𝐶𝐵𝑂), 𝐿𝑖𝐶𝑠𝐵6𝑂10(𝐶𝐿𝐵𝑂), 𝐾2𝐴𝑙2𝐵2𝑂7(𝐾𝐴𝐵𝑂), 𝑆𝑟𝐵4𝑂7(𝑆𝐵𝑂) as

well as rare earth based borate crystals.

Higher value of second order susceptibility (𝜒(2)) is alone not sufficient to establish a crystal

as a good NLO crystal. Along with nonlinear parameters optical properties such as

birefringence, absorption edge, walk off angle, laser damage threshold are also equally

important. Borates fit well in all such criteria. Moreover, 36% of all borate structures reported

yet are noncentrosymmetric [3]. Before BBO only two crystals were reported in UV spectral

range, KB5 (𝐾𝐵5𝑂8. 4𝐻2𝑂) the first ever crystal developed for potential UV light generation

and Urea ((𝑁𝐻2)𝐶𝑂). But both of them suffered from certain disadvantages like urea was

much sensitive to moisture and KB5 in spite of having desirable short absorption edge had

smaller value of SHG coefficient, making them ‘weak’ UV NLO crystals [5].

Chen and co-authors in their book Nonlinear Optical Borate Crystals clearly mention three

advantages beside large SHG coefficient that made them search for new UV NLO crystals

based on anionic group theory [5],

Literature Survey

18

1. Transparency of most of the borate crystals in UV and deep UV regions

2. Various structure types available in borate compounds

3. High laser damage threshold

2.3 Important Anionic Groups in Borates Among various possible anionic groups as basic structural units in borate crystals, following

three have gained more importance as most of the UV NLO crystals can be constructed from

them,

1. Trigonal anionic group (𝐵𝑂3)3−

2. Planar ring anionic group (𝐵3𝑂6)3−

3. Nonplanar ring anionic group (𝐵3𝑂7)5−

(a) (b) (c)

Figure 2.1 Basic structure units of borates (a) BO3 (b) B3O6 (c) B3O7

(solid circles represent boron atoms and hollow circles represent oxygen atoms)

NLO coefficient 𝜒(2) is largest in crystals where basic unit is (𝐵3𝑂6)3− owing to their planar

hexagonal structure followed by crystals from (𝐵3𝑂7)5− and (𝐵𝑂3)3− groups. But (𝐵3𝑂7)5−

group has shorter UV absorption edge (160 nm to 170 nm) making it ideal group for deep UV

NLO materials [4].

Literature Survey

19

Some of the important borate crystals have been categorised according to their structural units

in following table,

Table 2.1 Borate crystals according to their structural units

Borate crystals Basic structure unit

SBBO, KBBF BO3

BBO B3O6

CBO, LBO, CLBO B3O7

SBO B3O9

2.4 Transparency of Borates in UV and deep UV Regions

Transparency of a material is decided by its energy band gap which can be attributed to the

strength and overlapping of bonds inside the material. Stronger the bond more will be the

energy to transfer electrons into conduction band and hence larger energy band gap (Eg).

Another important parameter which decides Eg is electronegativity of atoms (specifically ions)

taking part in bond formation as far as ionic bonds are considered. Greater the electronegativity

difference between ions, stronger will be the bond and larger Eg is obtained [6].

When a photon of energy hν is incident on a material, it can excite valence band electrons into

conduction band only if hν is greater than or equal to Eg, else material will not respond to

incoming photon. In other words the material will be transparent to the particular wavelength

of light incident upon it.

A material with band gap between 3.1 eV and 1.77 eV can be excited by a photon with

wavelength between 400 nm to 700 nm, which is visible region of the electromagnetic

spectrum. If band gap is greater than 3.1 eV, the material will be transparent to visible range of

spectrum as well as IR wavelengths. Generally a material will be transparent in UV and deep

UV region (200 nm), if the energy band gap is greater than 6.2 eV.

Borates have wide band gap energies owing to larger difference in electronegativities [4] of

boron (2.04) and oxygen (3.44) ions in different borate groups which make stronger B-O bonds.

Literature Survey

20

It is the reason borates show transparency in UV and deep UV region. Following are typical

values of Eg for some of the borate crystals [7].

Table 2.2 Energy band gap values of some borate crystals

Borates Energy band gap (Eg) in eV

BBO 6.28

CBO 7.3

LBO 8.01

SBO 8-10

2.5 Cesium Triborate (CBO)

Cesium triborate bears chemical formula 𝐶𝑠2𝑂. 3𝐵2𝑂3 or 𝐶𝑠𝐵3𝑂5. Basic structural unit of CBO

is nonplanar (𝐵3𝑂7)5− anionic group which is slightly different from the planar (𝐵3𝑂6)3− with

one of the boron atom is changed in structural coordination from trigonal to tetrahedral. This

change accounts for deforming planar ring (𝐵3𝑂6)3− group to slightly nonplanar (𝐵3𝑂7)5−

group and shifts absorption edge towards lower wavelengths as a result of weakened 𝜋 -

conjugated orbital system compared to (𝐵3𝑂6)3− group [2].

Figure 2.2 Nonplanar (B3O7 )5- anionic group

Literature Survey

21

2.5.1 Structure of CBO

The Crystal structure of CBO is orthorhombic which implies 𝑎 ≠ 𝑏 ≠ 𝑐 and 𝛼 = 𝛽 = 𝛾 = 900

with space group P212121 [8]. 𝐵3𝑂7 anionic group consists two 𝐵𝑂3 triangular and one 𝐵𝑂4

tetrahedral in such a way that four exo-rings oxygen atoms are available which can attach to

neighbouring 𝐵3𝑂7 groups as shown in figure 2.2. CBO structure is formed with such

continuous spiral chains of 𝐵3𝑂7 groups with Cs ion placed in interstices [5].

Unit cell dimensions of CBO crystal are calculated as, a = 6.213 Å, b = 8.521 Å, c = 9.170 Å

and α = β = γ = 900 [8].

Figure 2.3 Unit cell of CBO crystal

Literature Survey

22

2.5.2 Phase Diagram of CBO

Phase diagram of binary system 𝐶𝑠2𝑂 − 𝐵2𝑂3 has been reported by various research groups

over the years [8, 9, 10, 11]. Phase diagram confirms existence of various congruently (𝐶𝑠𝐵𝑂2,

𝐶𝑠𝐵3𝑂5, 𝐶𝑠𝐵5𝑂8, 𝐶𝑠𝐵9𝑂14 ) as well as incongruently (𝐶𝑠2𝐵4𝑂7, 𝐶𝑠3𝐵7𝑂12, 𝐶𝑠3𝐵13𝑂21)

melting phases of CBO. Reported melting point of 𝐶𝑠𝐵3𝑂5 varies slightly from each other [9,

11, 12]. Most recently studied phase diagram is depicted in following figure and reported

melting point is 8350 C [11].

Figure 2.4 Phase diagram of Cs2O – B2O3 system

Literature Survey

23

2.5.3 CBO Crystal Growth History

Cesium oxide (𝐶𝑠2𝑂) - Boron oxide (𝐵2𝑂3) binary system was studied by J. Krogh Moe and

proposed the existence of congruently melting compound 𝐶𝑠𝐵3𝑂5 or 𝐶𝑠2𝑂. 3𝐵2𝑂3 in 1958 [8].

In1974, crystal structure of cesium triborate (CBO) was studied and unit cell dimensions were

calculated [8].

Anionic group theory developed by Chen et al. was utilized to calculate SHG coefficient of

CBO crystal Kinetic studies were performed by Marlor et al. in 1975 on CBO [5].

No report on single crystal growth of CBO came until 1993 when Wu et al. successfully grew

CBO single crystal through stoichiometric melt and measured its optical properties. Wide

transmission range (170 nm to 3000 nm), large NLO coefficient, high laser damage threshold

and type I, II phase matching for SHG and THG of 1.064um Nd:YAG made them to conclude

CBO as new NLO material [8].

Next successful growth of CBO was reported by Kagebayasghi et al. using Kyropoulos

technique in 1999. Through 𝐶𝑠2𝑂 enriched melt they obtained 45*41*44 mm3 CBO crystal free

of inclusions and cracks. Optical transparency range reported was wider than earlier as 167 nm

to 3400 nm [12].

Czochralski technique was used in 1999 by Peizhen et al. to grow single crystal of CBO. Axial

gradient of 60oC/cm with pulling rate of 8mm/day and 10rpm they obtained crystal of

dimensions 20mm diameter and 30mm height [13, 14]. On account of high viscosity contributed

by 𝐵2𝑂3 and volatility of 𝐶𝑠2𝑂, efforts were directed to grow CBO using TSSG (top seeded

solution growth).

Yoshimura et al. grew CBO single crystal with TSSG method in 2003 and successfully

demonstrated 355 nm light generation using grown crystal [15].

Literature Survey

24

During 2004, Saji et al. used various 𝐶𝑠2𝑂 and 𝐵2𝑂3 enriched solutions to grow CBO through

TSSG method with the help of phase diagram of 𝐶𝑠2𝑂 - 𝐵2𝑂3 binary system. They also

measured viscosity and volatility of the melt with respect to varying 𝐵2𝑂3 concentration [11].

Chang et al. reported seed submerged growth technique to obtain 65*44*49 mm3 single crystal

of CBO in 2004. They reported that similar morphology was existed with different seed

orientations [16].

The most recent paper on CBO growth appeared in November 2012 from Liu et al. who grew

47*45*41 mm3 size of CBO through TSSG method using 𝐶𝑠2𝑂 - 𝐵2𝑂3 – 𝑀𝑜𝑂3 system with

𝑀𝑜𝑂3 as flux [17].

Literature Survey

25

2.6 Strontium Tetraborate (SBO)

Strontium tetraborate SrB4O7 is another promising nonlinear optical borate crystal belonging to

nonplanar (B3O9)9- anionic group with all three boron atoms in six membered ring forming

tetrahedral with neighboring oxygen atoms shown in figure 2.6. This anionic group is one of

the least explored for borate crystals. Chen and co-authors stated no report of the crystal

containing this group [5]. But besides SBO, PbB4O7 (PBO) and EuB4O7 are the only reported

borates where all boron atoms form tetrahedral structure [18].

Figure 2.5 Unit cell of SBO crystal

The crystal structure of SBO was studied by Krogh-Moe in 1964 and found it to be

orthorhombic with lattice parameters as a = 10.711Å, b = 4.427Å, and c = 4.235Å [19].

Literature Survey

26

2.6.2 SBO Crystal Growth History

In 1993, Oseledchik et al. [20] implemented Czochralski method to grow SBO crystal for the

first time. Characteristics like excellent transparency range extending till vacuum UV (130 nm)

with high laser damage threshold, nonlinear optical coefficient, non hygroscopicity were

reported. SBO was found unable for SHG due to the absence of phase matching resulting from

small birefringence.

Comparitive studies between PBO and SBO were performed by Oseledchik et al. in 1996 [21].

In spite of absence of phase matching, non coherent SHG was observed in both crystals owing

to their high coherence length.

Pan et al. [22] grew SBO crystal using Kyropoulos technique and reported absorption edge in

vacuum UV below 120 nm. Non coherent SHG was observed using 1064 nm Nd:YAG laser as

source, producing green light at output.

SBO crystal was demonstrated to convert femtosecond UV pulses down to 125 nm with the use

of non phase matched SHG method by Petrov et al. [23].

Komatsu et al. [24] in 2005 reported to have grown pure and Samarium (Sm) doped SBO to

study transparency, dielectric properties and potential applications. Europium (Eu) doped SBO

was grown by Aleksandrovsky et al. [25] in 2006 to study various properties. Both groups used

Czochralski method to grow doped SBO crystals.

Atuchin et al. [26] studied electronic band structure of Czochralski grown SBO crystal. Zaitsev

et al. [27] observed partially ordered domain structure in SBO crystal for the first time.

Morphology and twinning in SBO crystal has been discussed by Zaitsev et al. [28] recently.

The idea to undertake growth of SBO was inspired by its vacuum UV transmission [21, 22]. As

per our knowledge, temperature dependent refractive index of SBO has been calculated for the

first time and it is shown that thermal coefficient of refractive index is positive. Recent studies

about SBO are focused on its potential applications. We tried to explore thermoluminescence

in SBO which can lead for its use in detectors for detecting xray, gamma ray radiation.

Literature Survey

27

2.7 Barium Borate (BBO)

Barium Borate (β-BaB2O4) is one of the most widely used nonlinear optical crystals well known

for its deep UV applications, high laser damage threshold, high NLO coefficient, etc. BBO is

categorized under planar anionic (B3O6)3- group with hexagonal crystal system and space group

R3C [5]. Lattice parameters are calculated as a = b = 12.531 Å, c = 12.721 Å and γ = β = 900,

γ = 1200.

It exists in two phases based on temperature of formation namely α-BBO and β-BBO where the

later one is low temperature form. Melting point of BBO is around 10900C and α-β transition

occurs at 9250C [5]. Hence flux growth technique is most common to grow single crystals of

BBO using number of fluxes such as Na2O, K2O, NaF, KF, NaCl, etc.

Figure 2.6 Unit cell structure of BBO

Literature Survey

28

References

1. Sifvast, W. T. (2004). Laser Fundamentals. Cambridge University Press.

2. Sasaki, T., Mori, Y., Yoshimura, M., Yap, Y. K., & Kamimura, T. (2000). Recent

Development of Nonlinear Optical Borate Crystals : Key Materials for Generation of

Visible and UV Light. Materials Science and Engineering, 30, 1-54.

3. Kumar, R. A. (2012). Borate Crystals for Nonlinear Optical and Laser Appications : A

Review. Journal of Chemistry, 2013, 6.

4. Chen, C., Wu, Y., & Li, R. (1990). The Development of New NLO Crystals in the Borate

Series. Journal of Crystal Growth, 99, 790-798.

5. Chen, C., Sasaki, T., Li, R., Wu, Y., Lin, Z., Mori, Y., Hu, Z., Wang, J., Uda, S., Yoshimura,

M., & Kaneda, Y. (2012). Nonlinear Optical Borate Crystals Principles and Applications.

Wiley-VCH Verlag & Co. KGaA.

6. Duffey, J. A. (2001). Ultravioet Transparency of Glass : A Chemical Approach in terms of

Band Theory, Polarisability and Electronegativity. Phys. Chem. Glasses, 42(3), 151-157.

7. He, R., Lin, Z. S., Zheng, T., Huang, H., & Chen, C. T. (2012). Energy Band Gap

Enginerring in Borate Ultraviolet Nonlinear Optical Crystals : ab initio Studies. Journal of

Physics : Condensed Matter, 24(145503), 6.

8. Wu, Y., Sasaki, T., Tang, H., & Chen, C. (1993). CsB3O5 : A New Nonlinear Optical

Crystal. Appl. Phys. Lett., 62(21), 21.

9. Kaplun, A., & Meshalkin, A. (2000). Phase Equilibria in the Binary Systems Li2O-B2O3

and Cs2O-B2O3. Journal of Crystal Growth, 209, 890-894.

10. Penin, N., Touboul, M., & Nowogrocki, G. (2003). New Form of the Cs2O-B2O3 Phase

Diagram. Journal of Crystal Growth, 256, 334-340.

11. Saji, T., Hisaminato, N., Nishioka, M., Yoshimura, M., Mori, Y., & Sasaki, T. (2005).

Growth of Nonlinear Optical Crystal CsB3O5 from Self-Flux Solution. Journal of Crystal

Growth, 274, 183-190.

12. Kagebayashi, Y., Mori, Y., & Sasaki, T. (1999). Crystal Growth of Cesium Triborate

CsB3O5 by Kyropoulos Technique. Bull. Matter. Sci., 22(6), 971-973.

13. Fu, F., Wang, J., Hu, Z., Wu, Y., Yin, S., & Xu, Z. (1997). Growth and Properties of

Ultraviolet Nonlinear Optical Crystal Cesium Triborate.

Literature Survey

29

14. Wu, Y. (1997). Crystal Growth and Nonlinear Optical Properties of Cesium Triborate. In

T. Sasaki (Ed.), Proceedings of International Symposium on Laser and Nonlinear Optical

Materials, (pp. 120-125). Singapore.

15. Yoshimura, M., Mori, Y., Hu, Z., & Sasaki, T. (2004). Growth and Characterization of

Nonlinear Optical Borate Crystals CsLiB6O10, CsB3O5, and BaAlBO3F2. Optical Materials,

26, 421-423.

16. Cheng, F., Fu, P., Wu, Y., Chen, G., Xu, Z., & Chen, C. (2005). Growth of Large CsB3O5

Crystals. Journal of Crystal Growth, 277, 298-302.

17. Liu, S., Zhang, G., Feng, K., & Wu, Y. (2013). Growth, Thermophysical and Dielectric

Properties of the Nonlinear Optical Crystal CsB3O5. Journal of Crystal Growth, 364, 46-

50.

18. Zaitsev, A., Aleksandrovskii, A., Zamkov, A., Sysoev, A. (2006). Nonlinear Optical,

Piezoelectric, and Acoustic Properties of SrB4O7. Inorganic Materials, 42(12), 1360-1362.

19. Krogh-Moe, J. (1964). Crystal Structure of Strontium Diborate, SrO.2B2O3. Acta Chemica

Scandinavica, 18, 2055-2066.

20. Oseledchik, Yu., Prosvirnin, A., Starshenko, V., Osadchuk, V., Pisarevsky, A., Belokrys,

S., Korol, A., Svitanko, N., Selevich, T., Krikunov, S. (1994). Crystal Growth and

Properties of Strontium Tetraborate. Journal of Crystal Growth, 135, 373-376.

21. Oseledchik, Yu., Prosvirnin, A., Pisarevskiy, A., Starshenko, V., Osadchuk, V., Osadchuk,

V., Belokrys, S., Svitanko, N., Korol, A., Krikunov, S., Selevich, T. (1995). New Nonlinear

Optical Crystals: Strontium and Lead Tetraborates. Optical Materials, 4, 669-674.

22. Pan, F., Shen G., Wang, R., Wang, X., Shen, D. (2002). Growth, Characterization and

Nonlinear Optical Properties of SrB4O7 Crystals. Journal of Crystal Growth, 241, 108-114.

23. Petrov, V., Noack, F., Shen, D., Pan, F., Shen, G., Wang, X., Komatsu, R., Alex, V. (2004). Application of the Nonlinear Cystal SrB4O7 for Ultrafast Diagnostics Converting to

Wavelengths as short as 125 nm. Optics Letters, 29(4), 373.

24. Komatsu, R., Kawano, H., Oumaru, Z., Shinoda, K., Petrov, V. (2005). Growth of

Transparent SrB4O7 Single Crystal and its New Applications. Journal of Crystal Growth,

275, 843-847.

Literature Survey

30

25. Aleksandrovsk, A., Malakhovskii, A., Zabluda, V., Zaitsev, A., Zamkov, A. (2006). Optical

and Magneto-Optical Spectra of Europium-Doped Strontium Tetraborate Single Crystals.

Journal of Physics and Chemistry of Solids, 67, 1908-1912.

26. Atuchin, V., Kesler, V., Zaitsev, A., Molokeev, M., Aleksandrovsky, A., Kuzubov, A.,

Ignatova N. (2013). Electronic Structure of α-SrB4O7: Experiment and Theory. Journal of

Physics: Condensed Matter, 25, 085503.

27. Zaitsev, A., Aleksandrovsky, A., Vasiliev, A., Zamkov, A. (2008). Domain Structure in

Strontium Tetraborate Single Crystal. Journal of Crystal Growth, 310, 1-4.

28. Zaitsev, A., Radionov, N., Cherepakhin, A., Vasiliev, A., Zamkov, A. (2015). Morphology

of the Polar Twin Structure in Czochralski Grown α-SrB4O7 Crystals. Jouurnal of Crystal

Growth, 416, 17-20.

Crystal Growth

31

3. CRYSTAL GROWTH

3.1 Growth of Cesium Triborate (CBO)

It was decided to implement Czochralski method to grow single crystals of CBO as it was rarely

implemented before. Growth of CBO crystal was carried out with single zone resistive heating

furnace. Eurotherm PID programmable controller was used for temperature control. Pulling and

rotation control system with data logging software, developed in house was used.

3.1.1 Resistive Furnace Setup

Resistive furnace with low resistance (0.546 Ω) kanthal 1 (melting point 15000C) coil was used

to attain desired temperature which was measured using a thermocouple. Eurotherm 902P was

used as temperature controller. Figure 3.1 shows furnace setup for actual growth of crystal. The

furnace was well insulated with silicon wool to avoid heat loss. Silicon wool is carcinogenic

hence safety precautions were followed while handling it.

Figure 3.1 Schematic diagram of experimental setup for CBO growth

Crystal Growth

32

Unlike three or five zone furnaces temperature distribution in single zone furnace is not uniform

hence temperature profiling is essential to calculate the axial and if required radial temperature

gradient. Following is the temperature profile curve of the furnace.

Figure 3.2 Temperature profile of the furnace

Kanthal furnace without connections Silicon wool wrapped furnace with muffle

Crystal Growth

33

3.1.2 Charge Synthesis

Cesium triborate was prepared through reaction between cesium carbonate and boron oxide or

boric power as a liquid state chemical reaction. Cesium carbonate (𝐶𝑠2𝐶𝑂3) and boron oxide

(𝐵2𝑂3) were taken as reactants in purest (99.99%) form which forms CBO as,

𝐶𝑠2𝐶𝑂3 + 3𝐵2𝑂3

∆→ 2𝐶𝑠𝐵3𝑂5 + 𝐶𝑂2 ↑

Cesium carbonate is highly hygroscopic chemical which needs extra precaution at the time of

weighing to avoid inclusion of moisture. Handheld halogen lamp was used to keep surroundings

warmer, while taking it out of bottle as well as during weighing. It was immediately transferred

to a clean platinum crucible and kept in an oven around 1000 C. Boron oxide was weighed under

normal ambient conditions and transferred in a clean airtight plastic bottle. Preliminary safety

precautions were followed throughout the process as cesium carbonate is toxic and boron oxide

is irritant for skin.

3.1.3 Calculations

Reactants Formula weight (g/mol)

Cs2CO3 325.8198

B2O3 3 * 69.6021 = 208.8606

Products Formula weight (g/mol)

CsB3O5 2 * 245.3354 = 490.6709

CO2 44.0095

490.6709 gm of 𝐶𝑠𝐵3𝑂5 contains 1 mol of it, hence 1 gm of cesium triborate contains

(1

490.6709) = 0.0020380 mol of it.

Amount of Cs2CO3 required to obtain 1 gm of 𝐶𝑠𝐵3𝑂5 = 325 ∗ 0.0020380 = 0.664029 𝑔𝑚

Amount of 𝐵2𝑂3 required to obtain 1 gm of 𝐶𝑠𝐵3𝑂5 = 208.8606 ∗ 0.0020380 =

0.425663 𝑔𝑚.

Crystal Growth

34

Initial charge was prepared for 50 gm of CBO hence,

Amount of Cs2CO3 required = 50 ∗ 0.6640 = 33.2014 𝑔𝑚.

Amount of 𝐵2𝑂3 required = 50 ∗ 0.4256 = 21.2831 𝑔𝑚.

3.1.4 Cleaning of crucible

One of the biggest disadvantages of Czochralski method for growth of single crystal is charge

contamination through the crucible that is not cleaned properly. To avoid the contamination

from platinum crucible, it was dipped in hot nitric acid (800 C) for half an hour and washed with

plenty of water. This procedure was repeated until platinum crucible became completely clean.

Such a crucible (3.5 cm in diameter and 4 cm in height) was used to transfer cesium carbonate.

3.1.5 Formation of Precursor

Melting point of cesium carbonate is 6100 C and it decomposes around 6000 C as,

Cs2CO3

∆→ 𝐶𝑠2𝑂 + 𝐶𝑂2 ↑

After cesium carbonate completely melted, boron oxide was added into melt 2 gm each time.

Melting point of boron oxide is 4500 C and it forms highly viscous melt. As a result CO2 fumes

were trapped under the viscous layer of melted boron oxide and highly violent frothing was

observed and extra care was demanded to avoid spilling the charge out of crucible. This

problem was avoided by implementing batch melting process in which chemicals were mixed

initially and later allowed to melt in batches of 2-3 gm each time. The temperature was kept

around 8900C. Once compete charge was melted, temperature was brought down to 8550C and

kept overnight for homogenization.

Crystal Growth

35

3.1.6 Growth Process

Czochralski technique was adopted to grow CBO crystal. As CBO had to be grown for the first

time at crystal growth lab, RRCAT, seed crystal was unavailable. Initial runs were tried with

platinum wire as a seed which is a standard practice followed in absence of seed crystal. Once

a crystal is grown with platinum wire which is not of superior quality or even a single crystal,

seeds with different orientations can be cut from the obtained crystal. Growth temperature when

seed is attached varies from platinum seeded growth. Different growth parameters were set for

different runs. A general practice follows cutting the crystal obtained from each run and portion

of it is used as a seed for next run which improves the quality of crystal during each run.

Following images show the grown polycrystalline disc and seeds obtained out of it.

Polycrystalline disc

After homogenization, temperature was reduced and kept around 50 C above the melting point

and melt was allowed to remain at same temperature for some time (15 – 20 minutes). Seed tied

to ceramic rod with platinum wire was then slowly dipped such that it should be at the centre

of crucible which increased chances of equal growth from all directions. After dipping, the seed

was pulled out 2 – 3 mm to form a meniscus. Meniscus region is considered the most important

regions in Czochralski technique. It is critical because the boundary between solid (crystal) and

liquid (melt), also called as interface, lies in this region where crystallization takes place through

releasing heat, as it is an exothermic reaction.

Circled seeds were used during

next runs

Disc taken out

of crucible

Crystal Growth

36

Phase diagram of 𝐶𝑠2𝑂 − 𝐵2𝑂3 system confirms congruent melting of CBO at 8350 C at 75%

mol of 𝐵2𝑂3. According to phase diagram, no other phase is formed near melting temperature.

It is important to crosscheck the phases formed in the crystal using x – ray diffraction (XRD)

technique and comparing it with JCPDS (The International Centre for Diffraction Data Sample)

database. Results were in accordance with JCPDS file number #740357 and lattice parameters

calculated using Qbasic software were in accordance with actual parameters as a = 6.2135 Å, b

= 8.5322 Å, c = 9.1841 Å and γ = β = γ = 900.

Following figure shows XRD pattern obtained from the polycrystalline disc shown earlier.

Figure 3.4 XRD pattern of CBO

Figure 3.3 Meniscus region in Czochralski technique

Crystal Growth

37

Following images show crystals obtained during various runs. Images are numbered in

increasing order of their quality in terms of transparency, facets, growth rate, etc.

Figure 3.5 Different growth rates and grown CBO crystal inset

grown inset

Crystal Growth

38

3.2 Growth of Strontium Tetraborate (SBO)

SBO was decided to grow for the first time at crystal growth laboratory at RRCAT like CBO.

3.2.1 Charge Synthesis

Strontium tetraborate was prepared from strontium carbonate (SrCO3) and boric acid (H3BO3)

taken as reactants in purest (99.99%) form. The melting point of SBO is around 9800C. The

liquid state reaction can be given as,

𝑆𝑟𝐶𝑂3 + 4𝐻3𝐵𝑂3

∆→ 𝑆𝑟𝐵4𝑂7 + 6𝐻2𝑂 + 𝐶𝑂2 ↑

Initially, 50 gm charge was prepared and reactants were weighed accordingly. Calculations are

similar to those of CBO discussed in previous section. Synthesized charge was well mixed and

put in air tight bottle. The charge was transferred into cleaned crucible kept around 10600C in

batches and allowed to melt completely. Batch melting approach was adopted to avoid froth

formation through violent eruption of CO2 gas trapped under viscous layer of boron oxide. Once

the charge was completely transferred into crucible and allowed to melt, temperature of the

furnace was set 200C above the melting point and kept overnight to obtain homogenized melt.

3.2.2 Growth Process

The same experimental setup was used to grow CBO and then SBO crystals. Due to

unavailability of seed crystal, SBO was crystallized on platinum wire initially. Crystallization

on wire was found to be more difficult compared to CBO. Once seed crystals were obtained,

same were used for next growth runs. The procedure followed was exactly same as that of

during the growth of CBO crystal except change in temperature (~ 9850C). Rotation was kept

about 7 – 8 rpm with pulling rate of 0.2 mm/hr. After 15 hours of growth run, crystal was

detached carefully from the melt and allowed to cool slowly (200C/hr) to avoid any cracks

resulting from rapid temperature change. Growth rate is elaborated in figure 3.6.

Crystal Growth

39

Following images show SBO crystals obtained during growth runs.

XRD was performed on finely ground SBO crystal to confirm phases developed. It was found

to be in accordance with JCPDS file # 712191. Lattice parameters were cross verified using

obtained diffraction angles (2θ) for corresponding (h k l) planes and feeding them into Qbasic

software program. Values of lattice parameters obtained were a = 4.2425 Å, b = 4.4354 Å, c =

10.70 Å. XRD pattern of SBO crystal is shown in figure 3.7.

Figure 3.6 Different growth rates and grown SBO crystal inset

Crystal Growth

40

Samples were cut from grown crystals for further characterizations shown in following image.

Figure 3.7 XRD pattern of SBO

Crystal Growth

41

3.3 Growth of Barium Borate (BBO)

Growth conditions for BBO are different than other borates as it requires high temperature

gradient and slow pulling which are commonly implemented in Czochralski technique and not

in solution growth. We used top seeded solution growth (TSSG) technique, referred as modified

Czochralski [1] to grow BBO crystals using Na2O as flux. High temperature gradient

(~ 120 C/cm) is required to grow good quality crystals. An indigenously built high gradient two

zone resistive furnace was used for the same shown below.

3.3.1 Charge Synthesis and Growth Process

Barium carbonate (BaCO3), sodium carbonate (Na2CO3) and boron oxide (B2O3) were taken in

purest (99.99%) form and weighed. Chemicals were put in air tight bottle and well mixed before

transferring into cleaned platinum crucible and directly melted at 10500C. Melt was

homogenized at 200C above saturation temperature and kept for 24 hours. An already grown

crystal was cut and used as a seed. Temperature of homogenized melt was further reduced and

kept at saturation temperature optimized by repeated seeding. Rotation rate of 4 rpm was

Figure 3.8 Schematic of experimental setup

Crystal Growth

42

maintained and cooling rate of 0.040C/h was employed. Crystal was pulled around 0.3 – 0.5

mm/day.

Following image show grown crystals

XRD was performed on finely ground BBO crystal to confirm phases developed. It was found

to be in accordance with JCPDS file #801489 Lattice parameters were cross verified using

obtained diffraction angles (2θ) for corresponding (h k l) planes and feeding them into Qbasic

software program. Values of lattice parameters obtained were a = b =12.5290 Å,

c = 12.7241 Å and α = β = 900 and γ = 1200. XRD pattern of BBO crystal is shown in figure

3.9

Figure 3.9 XRD pattern of BBO

Crystal Growth

43

Samples were cut from grown crystals for further characterizations shown in following image.

Crystal Growth

44

References

1. Bhatt, R., Ganesamoorthy S., Bhaumik, I., Karnal, A., Wadhawan, V. (2007). Growth Rate

Anisotropy and Absorption Studies on β-BaB2O4 Single Crystals Grown by the Top-seeded

Solution Growth Technique. Optical Materials 29, 801-805.

2. JCPDS file #740357

Crystal Growth

45

3. JCPDS file # 712191

Crystal Growth

46

4. JCPDS file #801489

Optical Characterization

47

4. OPTICAL CHARACTERIZATIONS

Optical characterizations like transmission measurement, refractive index measurement, and

study of conoscopic pattern were performed on processed samples obtained from grown

crystals. Samples were cut from the crystal, lapped and polished before measurements so as to

obtain reliable results.

4.1 Refractive Index Measurement

Refractive index of the sample measured over different wavelengths and temperature range is

useful in determining Sellmier’s coefficients as well as thermal coefficient. Calculation of

refractive index is a preliminary requirement to establish the usefulness of the crystal as a

nonlinear element in different optical and optoelectronic processes. Birefringence of the sample

can be evaluated by calculating the difference in refractive indices obtained through incidence

of orthogonally polarized laser lights.

The prism coupling technique was implemented to measure refractive index of samples. This

technique uses a prism of known refractive index (np) and sample is brought into contact using

pneumatic coupling head generally operated at pressure of five bar. Laser light is allowed to

fall on the sample through prism and reflected light is detected using a suitable detector. Light

with TE and TM polarization is used to calculate refractive indices along orthogonally polarized

directions. Basic principle used in this technique is to allow light to fall on the sample at critical

angle θc at which minimum light is reflected from the sample which results in sudden drop in

the detector output as shown in figure 4.2, referred as knee point. Refractive index can be

calculated at knee point as, 𝑛 = 𝑛𝑝𝑠𝑖𝑛𝜃𝑐.

Instrument used for refractive index measurement was a commercially available prism coupler

manufactured by Metricon Corp., UK with model 2010/M with five different laser wavelengths

407, 532, 828, 1064 and 1551 nm. High refractive index (n = 2.512) Rutile (TiO2) prism was

used for measurements.


48

Figure 4.2 Light intensity at detector vs angle of incidence (taken as position)

Temperature dependent refractive index was calculated in the range 30 - 1500C where both

sample and prism are heated through temperature controller. Temperature (T) dependent

Sellmeier equation was used to fit the data and coefficients A, B, C and D were calculated with

wavelength λ taken in µm.

𝑛2(𝜆, 𝑇) = 𝐴(𝑇) +𝐵(𝑇)

𝜆2 − 𝐶(𝑇)− 𝐷(𝑇)𝜆2

Figure 4.1 Schematic of prism coupling technique


49

(a) (b)

(c)

Comparing above graphs it can be concluded that birefringence value (Δn) in SBO is lesser

than that of BBO crystal. Phase matching is absent in SBO crystal owing to its smaller

birefringence (ΔnSBO ~ 0.0026) which limits its use for SHG.

Following table shows Sellmeier coefficients for CBO, SBO and BBO crystals calculated at

room temperature.

Figure 4.3 Sellmeier fitted curves for (a) CBO, (b) SBO and (c) BBO crystals at 300C


50

Table 4.1 Sellmeier coefficients for SBO and BBO crystals

Crystal Mode A B C D

CBO nx 2.3197 0.0353 -0.2314 0.0020

ny 2.3700 0.0495 -0.3771 0.0023

nz 2.4607 0.0117 0.0232 0.0128

SBO nx 2.9621 0.0373 -0.2262 0.0125

ny 2.9499 0.0463 -0.3242 0.0130

nz 2.9571 0.0313 -0.0204 0.0170

BBO no 2.6927 0.0400 -0.0818 -0.0028

ne 2.3517 0.0361 -0.2416 -0.0076

Following graphs show change in refractive index of SBO and BBO with respect to temperature

at given wavelength.

(a)

Figure 4.4 Temperature dependent refractive index of (a) CBO, (b) SBO and (c) BBO

crystals

(b)

(c)


51

Thermal coefficient of refractive index (𝑑𝑛

𝑑𝑇) can be calculated from above graphs over the range

of 300C – 1500C and tabulated in following table.

Table 4.2 Temperature dependence of refractive index

Wavelength (nm) (dn/dT) x 10-5 /0C

for CBO

(dn/dT) x 10-5 /0C for

SBO

(dn/dT) x 10-5 /0C

for BBO

532 -3.1177 5.0346 -2.4598

828 -3.5016 4.5459 -2.1200

1064 -4.5048 4.2564 -2.0677

1551 -3.6306 4.5903 -1.3600


52

4.2 Optical Transmission Measurement

Optical transmission spectra recorded over the range of wavelengths through a device called

spectrophotometer determines transparency and characteristic cut off wavelength of the crystal.

It can also help in detecting presence of dopants in the sample with a dip in recorded spectra at

their characteristic absorption wavelength.

Transmission measurements were carried out with the help of JASCO V-670 UV – VIS

spectrophotometer over the range 190 – 3200 nm. Transmission spectra of samples of CBO,

SBO and BBO is shown in the following figure.

(a) (b)

(c)

Figure 4.5 Transmission spectra of (a) CBO (b) SBO and (c) BBO


53

Inclusions throughout the crystal resulted in poor transparecy for CBO as compared to that of

SBO and BBO. Transparency in CBO signifacnatly reduced towards low wavelength regions

as scattering becomes dominant.

Transmission spectra alongwith refractive index data are useful to determine absorption

coefficient for the material as well as bandgap energy. Absorption coefiicient (α) can be

calculated as,

𝛼 =1

𝑑𝑙𝑛 [

(1 − 𝑅)2

𝑇]

where d is the thickness of sample, T is transmittance and R is reflectivity of the sample

calculated from refractive index data and Sellmeier’s equation as,

𝑅 = (𝑛 − 1

𝑛 + 1)

2

Absorption coefficient can be related to energy band gap of the material as,

𝛼 ∝ (ℎ𝜈 − 𝐸𝑔)12

where hν is photon energy. Direct band gap energy can be calculated from the relation between

α2 and hν shown in figure 4.6 for BBO crystal.

Figure 4.6 Determination of band gap of BBO crystal


54

From the above graph, band gap energy can be calculated as 6.2610 eV which is in well

accordance with reported value of 6.28 eV by Cheng et al. [1]. Abrupt rise in α2 with respect to

photon energy is result of photon absorption. Intercept of this line on energy axis which

necessarily means zero absorption, thus give value of energy band gap of the material. In case

of CBO, significant absorption was observed from visible range of spectra which was the result

of sample with inclusions. It was not possible to calculate energy band gap for SBO as it did

not exhibit significant absorption till 190 nm.

4.3 Study of Conoscopic Pattern

Conoscopy is an optical technique for a transparent specimen to make observations in a cone

of converging rays of light. For conoscopic observations and measurements a conoscope is

used. When diverging light rays travel through an anisotropic substance a conoscopic

interference pattern, a pattern of rings caused by optical interference, is observed. For uniaxial

crystal a plate if cut perpendicular to the optic axis, the pattern is in the form of concentric rings.

The shape and symmetry of the conoscopy pattern reveals the information about homogeneity

and strain in the crystal. Here to ascertain the quality of the grown crystals, a conoscopic study

was performed on a c-cut plate of BBO using an Olympus polarizing microscope in the

transmission mode.

Polarizing microscope consists of both a polarizer and an analyzer. A polarizer is positioned in

the light path somewhere before the specimen while an analyzer (a second polarizer) is placed

in the optical pathway between the objective rear aperture and the observation tubes or camera

port as shown in figure 4.6. The interaction of plane-polarized light with a birefrengent

specimen gives rise to image contrast to produce two individual wave components which are

polarized in mutually perpendicular planes. These components are termed the ordinary and the

extraordinary wave fronts whose velocities are different and vary with the propagation through

the specimen after which the light components are out of phase then pass through the analyzer

where they recombines with constructive and destructive interference.


55

Figure 4.7 Conscopy pattern of c-cut BBO sample

Concentric rings are evident for uniaxiality of the material and uniformity of rings account for

homogeneity of the sample. Conoscopic pattern for SBO could not be obtained as the

birefringence is very less.

References

1. Cheng, W., Huang, J., Lu, J. (1998). Electronic Structures and Nonlinear Optical

Coefficients of β-BaB2O4. Physical Review 57, 1527.

Results and Discussion

56

5. RESULTS AND DISCUSSION

Top seeded solution growth (TSSG) with self-flux (excess 𝐶𝑠2𝑂) or other suitable fluxes like

𝑁𝑎𝐹, 𝑉2𝑂5, 𝑀𝑜𝑂3 is the most preferred technique to grow CBO crystal [1]. First single crystal

of CBO was grown by Kyropoulos technique [2]. Growth of CBO using stoichiometric melt

involving Czochralski technique is considered difficult due to highly viscous 𝐵2𝑂3 and volatile

𝐶𝑠2𝑂 [1, 3]. A single paper is reported on growth of CBO using Czochralski technique [4] and

a conference paper [5], both reports describe the same CBO crystal having 20 mm diameter and

30 mm height at pulling rate of 8 mm/day at 600 C/cm gradient.

We opted to grow CBO crystal with Czochralski technique using a resistive furnace with an

indigenously built system at crystal growth lab, RRCAT. One of the major limitations working

with single zone resistive furnace was unavailability of getting high temperature gradient inside.

We could get 100 C/cm gradient with current setup after reaching certain height.

During initial runs when growth temperature was not optimised and platinum wire was used as

a seed, polycrystalline discs were obtained as result of fast and uncontrolled growth. Difficulty

was faced to nucleate and grow CBO using platinum wire, similar to CLBO [6]. Seeds were

melted during couple of runs due to low temperature gradient. After increasing the stand height

inside furnace, the problem of seed melting was rectified. Even high temperature gradient could

not solve problem of faster growth which most of times caused detachment of grown crystal

from the seed by hitting crucible wall. To deal with this problem, platinum crucible was placed

inside Zirconium crucible (melting point 18520 C) to obtain radial gradient but this resulted in

raising furnace temperature to reach optimum growth temperature.

Even after several runs continued for seven months we could not obtain good quality, inclusion

free single crystal of CBO.


57

BBO crystal was grown with well known Na2O flux with top seeded solution growth technique

discussed by Bhatt et al.[7]. Each run continued for 19 – 20 days and transparent single crystals

were obtained which were used for further characterization.

SBO crystal without having significance birefringence and SHG capabilities, was decided to

grow as it was primary attempt to obtain single crystal of SBO like CBO at RRCAT. Its

potential uses are still being investigated. One of the major applications being discussed is its

use as waveguide with lead tetraborate (LBO) due to large difference in refractive index [8].

SBO crystal was successfully grown and its thermal coefficient of refractive index was

calculated for the first time. It was observed that thermal coefficient of refractive index for SBO

was positive unlike most of the other borate materials.

Absorption studies could be performed only on BBO crystal as lower cut off wavelength for

the spectrophotometer used was 190 nm. Bandgap of BBO was calculated and found well in

accordance with the literature.

References

1. Chen, C., Sasaki, T., Li, R., Wu, Y., Lin, Z., Mori, Y., Hu, Z., Wang, J., Uda, S., Yoshimura,

M., & Kaneda, Y. (2012). Nonlinear Optical Borate Crystals Principles and Applications.

Wiley-VCH Verlag & Co. KGaA.

2. Kagebayashi, Y., Mori, Y., & Sasaki, T. (1999). Crystal Growth of Cesium Triborate

CsB3O5 by Kyropoulos Technique. Bull. Matter. Sci., 22(6), 971-973.

3. Saji, T., Hisaminato, N., Nishioka, M., Yoshimura, M., Mori, Y., & Sasaki, T. (2005).

Growth of Nonlinear Optical Crystal CsB3O5 from Self-Flux Solution. Journal of Crystal

Growth, 274, 183-190.

4. Fu, F., Wang, J., Hu, Z., Wu, Y., Yin, S., & Xu, Z. (1997). Growth and Properties of

Ultraviolet Nonlinear Optical Crystal Cesium Triborate.

5. Wu, Y. (1997). Crystal Growth and Nonlinear Optical Properties of Cesium Triborate. In

T. Sasaki (Ed.), Proceedings of International Symposium on Laser and Nonlinear Optical

Materials, (pp. 120-125). Singapore.

6. Karnal, A., Bhaumik, I., Ganesamoorthy, S., Bhatt, R., Saxena, A., Wadhawan, V., & Bhat,

H. (2007). Growth and Calorimetric Measurements on CsLiB6O10. Materials Letters, 61,

600-604.


58

7. Bhatt, R., Ganesamoorthy S., Bhaumik, I., Karnal, A., Wadhawan, V. (2007). Growth Rate

Anisotropy and Absorption Studies on β-BaB2O4 Single Crystals Grown by the Top-seeded

Solution Growth Technique. Optical Materials 29, 801-805.

8. Oseledchik, Yu., Prosvirnin, A., Pisarevskiy, A., Starshenko, V., Osadchuk, V., Osadchuk,

V., Belokrys, S., Svitanko, N., Korol, A., Krikunov, S., Selevich, T. (1995). New Nonlinear

Optical Crystals: Strontium and Lead Tetraborates. Optical Materials, 4, 669-674.

Conclusions and Future Scope

59

6. CONCLUSIONS & FUTURE SCOPE

Basics of crystals and different crystal growth techniques like melt growth (Czochralski,

Optical floating zone) and high temperature solution growth or flux growth were studied.

Single crystals of CBO, SBO and BBO were grown and characterized.

Good quality and inclusion free crystals of CBO were not obtained due to low temperature

gradient in the furnace.

Thermal coefficient of SBO was determined for the first time and found to be negative. It was

accounted for dominant polarizability over density changes with respect to temperature.

Growth of good quality CBO crystal implementing Czochralski technique can be taken up as

future work.

SBO crystals can be grown with suitable dopants and changes in its properties like refractive

index, thermoluminescence can be studied.

SBO grown over PBO substrate can be investigated as potential waveguide application.

Growth and Characterization of Nonlinear Optical...

Documents

Transcript of Growth and Characterization of Nonlinear Optical...