CHAPTER I

GROWTH TECHNIQUES AND NONLINEAR OPTICS

1.1 CRYSTAL GROWTH PHENOMENA

1.1.1 Introduction

Solids can be broadly classified as either crystalline or

noncrystalline. In a crystal the atoms are arranged in a periodic manner in all

three directions whereas in a noncrystalline solid the arrangement is random. A

crystalline solid can either be a single crystal or polycrystalline. In the case of

single crystal the entire solid consists of only one crystal. Polycrystal is an

aggregate of many small crystals separated by well defined boundaries.

Crystals are the pillars of modern technology. Modern

technologies based on optoelectronics, acousto-optics etc. have exploited the

versatile properties of crystals. The rapid advances in these branches of

technologies have been made possible due to the availability of a variety of

crystalline materials. In other words the development of the crystalline materials

is the backbone of modern technologies. Progress in crystal growth and epitaxy

technology is highly demanded in view of its essential role in the development of

several important areas such as production of high efficiency photovoltaic cells

and detectors for alternative energy and the fabrication of bright long-lifetime

light emitting diodes, for saving energy by wide use in illumination and trace

lights. The success of laser fusion energy depends on the timely development of

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high-power laser crystals and radiation-resistant frequency multiplying crystals of

oxide compounds. Furthermore, the great and wide potential of oxide

superconductors with high transition temperatures has not been explored so far

because proper crystal growth and materials technology development has been

neglected [1].

Single crystal forms the foundation for modern technology. The

ordered array of atoms in repeated groups, showing characteristic symmetry

elements, by which entire block of the material is built is called single crystal.

Single crystal growth helps study many physical properties of solids and effects

of grain boundaries. The grain boundaries present in the crystal and the part

played by imperfection are helpful in determining the physical and chemical

properties of solids [2].

Nonlinear optic (NLO) is a new frontier of science and

technology playing a major role in the emerging era of photonics. Photonics

involves the application of photons for information and image processing and is

branded to be the technology of the 21st century wherein nonlinear optical

processes have applications in the vital functions such as frequency conversion

and optical switching [3,4]. Search for new materials with enhanced NLO

properties has increased considerably over the recent years as a result of

potentially wide range of applications in optical communication and computation.

Efforts have been made on amino acid with organic and inorganic complexes, in

order to improve the chemical stability, laser damage threshold, and linear and

non-linear optical properties. The importance of amino acid for NLO application

lies on the fact that almost all amino acids contain an asymmetric carbon atom

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and crystallize in non-centro symmetric space group. In solid state, many amino

acids contain a depronated carboxylic acid group (COO-) and protonated amino

groups (NH3+

). This dipolar nature provides peculiar physical and chemical

properties to amino acid, thus making them ideal candidate for NLO application.

Among the materials producing NLO effects, particularly the

second harmonic generation (SHG), the organic materials have been identified to

be of considerable importance owing to their synthetic flexibility to design and

produce many novel materials [5]. Semi-organic NLO crystals have also

attracted attention because they have been proposed as a new approach for

materials with fascinating NLO properties which have the combined properties of

both inorganic and organic crystals, like high damage threshold, wide

transparency range, less deliquescence and high non- linear coeffcients which

make them suitable for device fabrication [6]. Examples of a series of studies on

semiorganic amino acid compounds are: L-Proline Zinc Chloride (LPZ) [7], L-

Arginine Phosphate (LAP) [8], L-Arginine Hydrobromide (L-AHBr) [9], L-

Histidine Nitrate (LHN) [10], L-Arginine Hydrochloride (L-HCl) [11], and

Glycine Sodium Nitrate (GSN) [12].

1.1.2 Nucleation

Nucleation is an important phenomenon in crystal growth and is

the precursor of crystal growth and of the overall crystallization process. The

condition of supersaturation alone is not sufficient for a system to begin

crystallization. Before crystals can grow, there must exist in the solution a

number of minute solid bodies known as centers of crystallization, seeds,

embryos or nuclei.

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Figure 1.1 Classification of Nucleation

Nucleation may occur spontaneously or it may be induced

artificially and they are usually referred to as homogeneous and heterogeneous

nucleations respectively. As shown in Figure 1.1, the term primary is used for

both homogeneous and heterogeneous nucleation even in systems that do not

contain crystalline matter. On the other hand, during secondary nucleation, nuclei

are often generated in the vicinity of the crystals present in the supersaturated

system. This process involves the dislodgement of the nuclei from the parent

crystal at supersaturation in which primary homogeneous nucleation cannot

occur. When a supersaturated solution is disturbed by agitation, friction or

mechanical stimulus in the presence of the crystalline substance of the solute,

embryos are formed at the surfaces of the parent crystal. These embryos give rise

to secondary nucleation. If the nuclei form homogeneously in the interior of the

phase, it is called homogeneous nucleation. If the nuclei form heterogeneously

around ions, impurity molecules or on dust particles, on surfaces or at structural

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singularities such as imperfections or dislocations, it is known as heterogeneous

nucleation [13]. Further, the creation of a new phase in the homogeneous

solution, demands for the expenditure of certain quantity of energy. The total

Gibbs free energy change, �G of the embryo between the two phases associated

with this process is then given as:

�G = �Gs + �Gv; (1.1)

where �Gs is the surface free energy and �Gv is the volume free energy. For a

spherical nucleus of radius r,

�G = 4�r2� + (4/3)�r

3�Gv: (1.2)

where � is the interfacial tension and �Gv is the free energy change

per unit volume and is a negative quantity. The quantities �G, �Gs, and �Gv are

represented in Figure 1.2. Since surface free energy increases with r2 and the

volume free energy decreases with r3, the total net free energy change increases

with increase in size, attains a maximum and decreases with further increase in

the size of the 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

dr

Gd∆�������������������������������������������������������������(1.3)

and the expression for critical radius is given by

r* = −2� / �Gv. (1.4)

Substituting the values of r* in the above equation, the free energy change

associated with the critical nucleus is,

�G*

= 16��3

/ 3�G2

v (1.5)

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The number of molecules in the critical nucleus is expressed as:

i*= (4/3)��(r

*)3 (1.6)

The interfacial tension (r*)

3 has been determined experimentally.

Figure 1.2 Change in free energy due to the formation of nucleus

1.2 CRYSTAL GROWTH TECHNIQUES

Crystal growth is a highly complex phase change phenomenon.

The phase change may occur from the solid, liquid or vapour state. With regard

to the phase transitions, the crystal growth methods are broadly classified into

four main categories [14-17].

i ) Solid growth (solid solid)

ii) Vapour growth (vapour solid)

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iii) Melt growth (liquid solid)

iv) Solution growth (liquid solid)

1.2.1 Solid Growth Techniques

In solid growth technique, single crystals are developed by the

preferential growth of a polycrystalline mass. This can be achieved by straining

the material and subsequent annealing. Large crystals of several materials,

especially metals have been grown by this method [18]. The main advantage of

solid growth method is that this technique permits the growth at low temperatures

without the presence of additional component. But as the growth takes place in

the solid, density of sites for nucleation is high and it is difficult to control

nucleation.

1. 2.2 Growth from Vapour

Vapour growth techniques can be adopted for the growth of

materials which lack a suitable solvent and sublime before melting at normal

pressure. Vapour growth methods have been employed to produce bulk crystals

and to prepare thin layers on crystals with a high degree of purity. Growth from

vapour phase may generally be subdivided into

i) Physical vapour transport

ii) Chemical vapour transport.

i) Physical Vapour Transport (PVT)

In PVT technique the crystal is grown from its own vapours and

this method does not involve any extraneous compound formation or reaction.

The PVT methods are limited to materials having an appreciable vapour pressure

at attainable temperatures. There are two types of techniques employed in

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physical vapour transport process; sublimation-condensation and sputtering. The

first method involves sublimation of the charge at the high temperature end of the

furnace, followed by the condensation at the colder end [19, 20]. Sputtering

techniques are preferred to low vapour-pressure substances and mainly this

method has been used to prepare thin films rather than discrete crystals. The

principal advantage of this technique is that film growth can be possible at lower

temperature than in ordinary sublimation-condensation growth. The PVT

techniques are used to prepare a variety of crystals [21-23] and for the production

of epitaxial films [24, 25].

ii) Chemical Vapour Transport (CVT)

Chemical vapour transport technique involves a chemical reaction

between the source material to be crystallized and a transporting agent. The

material to be crystallized is converted into one or more gaseous product, which

either diffuses to the colder end or gets transported by a transporting (carrier) gas.

At the cold end, the reaction is reversed so that the gaseous product decomposes

to deposit the parent material, liberating the transporting agent which diffuses to

the hotter end and again reacts with the charge. The commercial importance of

vapour growth is in the production of thin layer by chemical vapour deposition

[26-30].

1.2.3 Melt Growth Technique

Melt growth is the process of crystallization by fusion and

resolidification of the pure material. It is the fastest of all crystal growth methods

and is widely used for the preparation of large single crystals. Melt growth

methods are limited to materials which melt congruently and have an

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experimentally viable vapour pressure at its melting point. This method requires

only simple systems. The material to be grown is melted and after that it may

progressively be cooled to yield the crystalline matter. This method has been

generally employed for the growth of metals, semiconductors, and laser host

crystals. Single crystals with high degree of perfection and purity can be obtained

by this method. Usually melt growth methods are grouped into two categories.

1) Normal freezing method

a) Bridgman technique

b) Czochralski technique

2) Zone-growth method

a) Zone melting method

b) Floating zone method

There are two versions for Bridgman's method; Horizontal Bridgman

method (Chalmer's technique) and Vertical Bridgman method (Bridgman-

Stockbarger technique). In these techniques, directional solidification is obtained

by slowly withdrawing a boat containing molten material through a temperature

gradient [31, 32]. The Bridgman technique is most frequently applied for the

growth of metals, semiconductors and alkaline earth halides [33-35]. But this

method cannot be used for materials having high melting point and high

expansion coefficient. Czochralski method is the most powerful method for

growing single crystals and is basically a crystal pulling system. The advantage

of this method over the Bridgman method is that it can accommodate the volume

expansion associated with the solidification. Czochralski method has gained wide

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recognition particularly in growing single crystals of semiconductors like silicon

[36] and other materials [37, 38].

Zone melting is mainly considered as a purification technique.

However, it may be used as a method for the growth of single crystals. In this

method, a zone or part of the solid material is melted and this molten zone travels

together with the heating elements. The advantage of zone melting is that it offers

a relatively simple way of producing doped crystals containing deliberately

admixed additives in a given concentration in uniform distribution [39]. Floating

zone technique developed by Keck and Golay [40] is a variant of the zone

melting technique in which no crucible is used. This method is especially suitable

for the preparation of high purity silicon and germanium.

1.2.4 Growth from Solution

Another method of growth of crystals is the precipitation

technique from solution. In this method, the crystals are prepared from a solution

at a temperature well below its melting point. This may help to grow crystals

even at room temperature, and it will turn out to be more advantageous [41].

Here the crystallisation takes place from the critically supersaturated solution.

The supersaturation may be achieved by lowering the temperature of the solution

or by slow evaporation or by giving continuous supply of materials to

compensate for the material that precipitates out. The present work utilises this

method. The solution growth methods are classified according to the temperature

range and the nature of the solvents used. The main methods commonly used in

this process are:

i. High temperature solution growth.

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ii. Hydrothermal growth

iii. Slow cooling or slow evaporation method

iv. Gel growth

1.3 HIGH TEMPERATURE SOLUTION GROWTH

High temperature solution growth includes a number of related

techniques [42]. The flux method and liquid phase epitaxy are the two widely

used methods [43, 44]. A high temperature solvent, which reduces the melting

temperature of the solute, is referred as flux [45]. The main advantage of the flux

growth is the reduction of high temperature. The materials to be crystallised are

dissolved in proper solvent at a temperature slightly above the saturation

temperature; slow cooling of the container allows the growth of crystals. Slow

cooling of the flux is also effective in obtaining slightly bigger crystals.

1.4 Hydrothermal Growth

This is a well known and widely used technique to grow crystals

of certain class or species of materials, which are insoluble in water at standard

temperature and pressure. This is more imitative to the natural growth of certain

important minerals. Almost all metals and oxides show an appreciable increase

in solubility due to the increase in temperature and pressure. It can be treated as

aqueous solution growth at elevated temperature and pressure. Autoclaves with

gold or silver linings are usually utilised for the growth purpose. The hot

saturated solution is directed towards the upper (colder) part, where it becomes

cold and supersaturated and hence the growth takes place. The solution simply

acts as a transporting agent for the solid phase. Synthetic quartz crystals are

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grown by this technique [46]. The natural process of crystallisation beneath the

earth resembles this growth technique.

1.5 LOW TEMPERATURE SOLUTION GROWTH

Growth of crystals from aqueous solution is one of the ancient

methods of crystal growth. The method of crystal growth from low temperature

aqueous solutions is extremely popular in the production of many technologically

important crystals. Materials having moderate to high solubility in temperature

range, ambient to 100oC at atmospheric pressure can be grown by low-

temperature solution method. The mechanism of crystallization from solutions is

governed, in addition to other factors, by the interaction of ions or molecules of

the solute and the solvent which is based on the solubility of substance on the

thermodynamical parameters of the process; temperature, pressure and solvent

concentration [47].

1.5.1 Solution, Solubility and Supersolubility

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

is the component, which is present in a smaller quantity. Solubility of the material

in a solvent decides the amount of the material which is available for the growth

and hence defines the total size limit. Solubility gradient is another important

parameter, which dictates the growth procedure. Neither a flat nor a steep

solubility curve will enable the growth of bulk crystals from solution, while the

level of supersaturation could not be varied by reducing the temperature in the

former. Even a small fluctuation in temperature will affect the supersaturation in

the growth of good quality bulk crystals in both cases. If the solubility gradient is

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very small, slow evaporation of the solvent is the best option for crystal growth in

order to maintain a constant supersaturation in the solution.

Growth of crystals from solution is mainly a diffusion-controlled

process. The medium must be viscous enough to enable faster transference of the

growth units from the bulk solution by diffusion. Hence, a solvent with less

viscosity is preferable. Supersaturation is an important parameter for the solution

growth process. The crystal grows by the access of the solute in the solution

where the degree of supersaturation is maintained. The solubility data at various

temperatures are essential to determine the level of supersaturation. Hence, the

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

the growth process. The diagram is divided into three zones, which are termed as

region I, region II and region III. Region I corresponds to the under saturated

zone, where crystallization is impossible. This region is thermodynamically

stable. The region II between the super solubility curve and the solubility curve is

termed as metastable zone where spontaneous crystallization is improbable.

Seeded crystal growth can be achieved in this region. The unstable or labile zone

occurs at region III where the spontaneous nucleation is more probable (Figure

1.3). Many crystals are grown for basic and advanced research having

technological applications from low temperature solution growth. This method is

executable not only for water soluble materials, but also for insoluble materials

which can be brought into solution by forming complexes.

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Figure 1.3 Meirs solubility curve

Low temperature solution growth can be further subdivided as

1. Slow cooling method;

2. Slow evaporation method;

3. Temperature gradient method.

1.5.2 Crystallization by Slow Cooling

This is one of the best suited methods of growing bulk single

crystals. In this method, supersaturation is attained by a change in temperature

usually throughout the whole crystallizer. The crystallization process is carried

out in such a way that the point on the temperature dependence of the

concentration moves into the metastable region along the saturation curve in the

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direction of lower solubility. Since the volume of the crystallizer is finite and the

amount of substance placed in it is limited, the supersaturation requires

systematic cooling. It is achieved by using a thermostated crystallizer and

volume of the crystallizer is selected based on the desired size of the crystals and

the temperature dependence of the solubility of the substance. The temperature at

which such crystallization can begin is usually within the range of 45 o

C to 75 o

C

and the lower limit of cooling is the room temperature.

1.5.3 Crystallization by Solvent Evaporation

In this method, an excess of a given solute is established by

utilizing the difference between rates of evaporation of the solvent and the solute.

In contrast to the cooling method, in which the total mass of the system remains

constant, in the solvent evaporation method, the solution loses particles which are

weakly bound to other components, and, therefore, the volume of the solution

decreases.

In almost all cases, the vapour pressure of the solvent above the

solution is higher than the vapour pressure of the solute and, therefore, the

solvent evaporates more rapidly and the solution becomes supersaturated.

Usually, it is suffcient to allow the vapour formed above the solution to escape

freely into the atmosphere. This is the oldest method of crystal growth and

technically, it is very simple. Typical growth conditions involve temperature

stabilization of about ±0.005o

C and the rate of evaporation of a few mm3/hr.

1.5.4 Solvent Selection

In solution growth, suitable choice of solvent is necessary. The

solvent must be chosen, taking into account the following factors:

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1. good solubility of the given solute;

2. good temperature co-efficient of solute solubility;

3. less viscosity;

4. less volatility;

5. less corrosion and non-toxicity;

6. small vapour pressure; and

7. cost advantage.

It is known that the choice of solvent provides some control over

crystal habits and this effect depends on the interaction of the surface of the

crystal as it grows and the solvent molecules. Sometimes this is suffcient to result

either in the precipitation of a new crystalline phase or in habit modifications

which were observed on adding impurities [48].

1.5.5 Preparation of Solutions

Preparation of the solution to grow the desired crystal is an

important stage in solution growth. The solution is saturated as per the available

solubility diagram (accurate solubility-temperature data). The saturated solution

is filtered using the filter paper. The filtered solution is transferred into the

growth beaker and placed in the Constant Temperature Bath (CTB). The desired

supersaturation required is obtained by just lowering the temperature. Extreme

care is to be taken to avoid under saturation, which results in the dissolution of

seed crystal. Similarly high supersaturation is also to be avoided in order to

prevent the formation of spurious nucleation. The growth vessel is hermetically

sealed in order to avoid the evaporation of the solvent. The solution is tested for

saturation by suspending small test seed crystal in the solution. If the system is

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not in equilibrium, the seed crystal either dissolves or the solute will crystallize

on the seed. By adjusting the temperature, the necessary equilibrium condition is

achieved and the test seed crystal is removed and a fresh seed crystal is

introduced for crystal growth.

1.5.6 Seed Preparation

The quality of the grown crystal very much depends on the quality

of the seed crystal used. Small seed crystals can be obtained by spontaneous

nucleation in the labile region of the supersaturated solution. The seed used to

grow a large uniform crystal must be a single crystal free of inclusions, cracks,

block boundaries, sharp cleaved edges, twinning and any other obvious defects. It

should be of minimum size, compatible with other requirements. When larger

crystals of the same material are already available, they can be cut in the required

orientation to fabricate the seed crystal. Since the growth rate of the crystal

depends on the crystallographic orientation, the seed crystal must be cut in such a

way that it has larger cross-section in the fast growing direction.

1.5.7 Cooling Rate

To obtain the required supersaturation, which is the driving force

for the growth of crystal, the temperature of the growth solution is lowered. The

cooling rate is to be programmed according to the growth rate of the crystals. A

large cooling rate changes the solubility beyond metastable limit and fluctuations

in the supersaturation may encourage inclusions. A proper balance between the

temperature lowering rate and growth rate will yield a good quality crystal.

1.5.8 Harvesting of the Grown Crystals

The extraction of a crystal from its mother liquor requires

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enormous care because any damage may destroy completely the scientific value

of the crystal or even fracture it altogether. If a crystal is extracted from a solution

kept close to room temperature, it can be simply dried by means of filter paper.

Filter paper must be used to rub the surface since the majority of crystals

prepared from low temperature solutions are easily scratched. The surface of a

carelessly treated crystal may immediately acquire many defects. The quality of

the harvested crystal depends on

1. Purity of the starting material;

2. Quality of the seed crystal and

3. Cooling rate employed.

1.6 GEL GROWTH

Gel growth is an alternative technique to solution growth with

controlled diffusion and the growth process is free from convection. This

technique has gained considerable importance due to its simplicity and

effectiveness in growing single crystals of certain compounds.

Gel technique is a simple and elegant method of growing single

crystals under controlled growth and at room temperatures. Here, solutions of

two suitable compounds, which give rise to the required insoluble crystalline

substance by mere chemical reaction between them, are allowed to diffuse into

the gel medium and chemically react as follows:

AX + BY AB +XY

where AX and BY are the solutions of two compounds, AB is the insoluble

substance and XY is the waste product. This method can be useful for substances

having very high solubility [49].

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The gel medium prevents turbulence and helps the formation of

good crystals by providing a frame work of nucleation sites. Moreover, the

convection is absent in gel growth experiments.

1.7 NONLINEAR OPTICS

Nonlinear optics is given increasing attention due to its wide

application in the area of laser technology, optical communication and data

storage technology [50]. Nonlinear optics is completely a new effect in which

light of one wavelength is transformed to light of another wavelength. The

creation of light of new wavelength can be best understood, by studying the

electrons in nonlinear crystal. Electrons in a nonlinear crystal are bound in

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 spring’s restoring force increases linearly with the electron displacement

from its equilibrium position. The electric field in a light wave passing through

the crystal exerts a force on the electrons and pulls them away from their

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

their equilibrium position at the frequency of this electronic field. According to

the fundamental law of physics an oscillation change will radiate at its frequency

of oscillation, hence these electrons in the crystal "generate" light at the

frequency of the original light wave.

1.7.1 Theoretical Explanation of Nonlinear Optics

The explanation of nonlinear effects lies in the way in which a

beam of light propagates through a solid. The nuclei and associated electrons of

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the atoms in the solid form an electric dipole. 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 intensity of

the incident radiation increases the relationship between irradiance and amplitude

of vibration, which becomes nonlinear resulting in the generation of harmonic in

the frequency of radiation emitted by the oscillating dipoles. Thus frequency

doubling or Second Harmonic Generation (SHG) and indeed higher order

frequency effect occur as the incident intensity is increased. In a nonlinear

medium the induced polarization is a nonlinear function of the applied field. A

medium exhibiting SHG is a crystal composed of molecules with asymmetric

charge distributions arranged in the crystal 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.

P = �0 � E (1.7)

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 field and the susceptibility becomes field dependent. Therefore, this

nonlinear response is expressed by writing the induced polarization as a power

series in the field.

P = �0

{� (1)

E + � (2)

E. E + � (3)

E. E. E + . . .} (1.8)

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In nonlinear terms, product of two or more oscillating fields gives

oscillation at combination of frequencies and therefore the above equation can be

expressed in terms of frequency as:

P (-�0

) = �0

{� (1)

(-�0; �

1 ). E (�

0) + �

(2)

(-�0; �

1, �

2 ). E�

1. �

2 +

� (3)

(-�0; �

1, �

2, �

3 ). E�

1. �

2. �

3 +. . . . } (1.9)

where �(2)

, �(3)

…. are the nonlinear susceptibilities of the medium. �(1)

is the

linear term responsible for the 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 bi-stability. Hence the induced

polarization is capable of multiplying the fundamental frequency to second, third

and even higher harmonics. The coefficients of �(1)

, �(2)

and �(3)

give rise to certain

optical effects.

If the molecule or crystal is centro-symmetric then �(2)

= 0. If a

field +E is applied to the molecule (or medium), equation 1.9 predicts that the

polarization induced by the first nonlinear term is predicted to be +E2

, yet if the

medium is centro-symmetric the polarization should be –E2

. This contradiction

can only be resolved if �(2)

= 0 in centro-symmetric media.

If the same argument is used for the next higher order term, +E

produces polarization +E3

and – E produces – E3

, so that � (3)

is the first non-zero

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nonlinear term in centro-symmetric media. In second harmonic generation, the

two input wavelengths are the same

2�1= �

2 (or) (�

1 = 2 �

2 ).

During this process, a polarization wave with the second harmonic

frequency 2�1

is produced. The refractive index, n1

is defined by the phase

velocity and wavelength of the medium. The energy of the polarization wave is

transferred to the electromagnetic wave at a frequency �2

.The phase velocity and

wavelength of this electromagnetic wave are determined by n2

, the refractive

index of the doubled frequency. To obtain high conversion efficiency, the vectors

of input beams generated are to be matched.

�K = 2�

� (n1-n2)

where �K represents the phase–mismatching. The phase–mismatch can be

obtained by angle tilting, temperature tilting or other methods. Hence, to select a

nonlinear optical crystal, for a frequency conversion process, the necessary

criterion is to obtain high conversion efficiency. The conversion efficiency � can

be expressed as:

�PL2 (deffSin � K L / � K L)

2

where deff

is the effective nonlinear coefficient, L is the crystal length, P is the

input power density and �K is the phase – mismatching. In general, higher power

density, longer crystal, large nonlinear coefficients and smaller phase

mismatching will result in higher conversion efficiency. Also, the input power

density is to be lower than the damage threshold of the crystal.

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1.7.2 Nonlinear Optical Materials

Nonlinear optical (NLO) materials play a major role in nonlinear

optics and in particular they have a great impact on information technology and

industrial applications. In the last decade, however, this effort has also brought its

fruits in applied aspects of nonlinear optics. This can be essentially traced to the

improvement of the performances of the NLO materials. The understanding of

the nonlinear polarization mechanisms and their relation to the structural

characteristics of the materials has been considerably improved. The new

development of techniques for the fabrication and growth of artificial materials

has dramatically contributed to this evolution. The aim is to develop materials

presenting large non-linearities and satisfying at the same time all the

technological requirements for applications such as wide transparency range, fast

response, and high damage threshold. But in addition to the processability,

adaptability and interfacing with other materials as well as improvements in

nonlinear effects in devices, led the way to the study of new NLO effects and the

introduction of new concepts. Optical solitons, optical switching and memory by

NLO effects, which depend on light intensity, are expected to result in the

realization of pivotal optical devices in Optical Fibre Communication (OFC) and

optical computing which make the maximum use of light characteristics, such as

parallel and spatial processing capabilities and high speed. The goal is to find and

develop materials presenting large non-linearities and satisfying at the same time

not only all the technological requirements for applications such as wide

transparency range, fast response, high damage threshold; but also processability,

adaptability and interfacing with other materials.

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1.7.3 Classification of Nonlinear Optical Crystal

On the basis of three types of cohesive forces that bind the charges

and polarization together, the NLO materials can be classified into the following

cases:

(i). Inorganic crystals

(ii). Organic crystals

(iii). Semiorganic crystals.

1.7.3 (i) Organic NLO Crystals

Recently, organic compounds with delocalized conjugated

p-electrons have gained much attention because of their large NLO properties and

quick response. Organometallic and coordination complexes materials exhibit

novel NLO behavior. Second order NLO materials have the ability to double the

frequency of incident light and have important commercial applications. Typical

NLO molecules must have a dipole and be polarizable. In practice, conjugated

molecules with donor and acceptor groups on opposite ends of a conjugated chain

are often used. Second order NLO materials must also have the correct alignment

of molecules in the solid state. This is necessary to avoid the individual molecular

dipoles pairing up and effectively cancelling each other out. Amino acid crystals

such as L-threonine, L-alanine, L-phenylalanine, L-arginine have been grown by

slow evaporation and temperature lowering methods from aqueous solution, and

reported [51- 54]. Optical properties of L-alanine single crystals was reported by

Misoguti et al [55]. Also Banfi et al [56] have grown high optical quality organic

crystal N-(4-nitrophenyl)-L-prolinol (NPP) in methanol solution starting from

toluene nucleated seeds.

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1.7.3 (ii) Inorganic NLO Materials

Nonlinear optical materials will be the key elements for future

photonic technologies based on the fact that photons are capable of processing

information with the speed of light. The search for new and efficient materials in

which to carry out nonlinear optical processes has been very active since SHG

was first observed in single crystal quartz by Franken and co-workers in 1961. In

the beginning, studies were concentrated on inorganic materials such as quartz,

potassium dihydrogen phosphate (KDP), lithium niobate (LiNbO3), and its

analogues, potassium titanyl phosphate (KTP) and its analogues, beta barium

borate [57] and semiconductors such as cadmium sulfide, selenium, and

tellurium. Many of these materials have been successfully used in commercial

frequency doublers, mixers and parametric generators to provide coherent laser

radiation with high frequency conversion efficiency in the new region of the

spectrum inaccessible by other nonlinear crystal conventional sources.

1.7.3 (iii) Semiorganic NLO Single Crystals

Presently, inorganic and organic materials are being replaced by

semi-organics. They share the properties of both organic and inorganic materials.

Recent interest is concentrated on metal complexes of organic compounds owing

to their large non-linearity [58]. The approach of combining the high nonlinear

optical coefficients of the organic molecules with the excellent physical

properties of the inorganics has been found to be overwhelmingly successful in

the recent search. Hence, recent search is concentrated on semiorganic materials

due to their large nonlinearity, high resistance to laser induced damage, low

angular sensitivity and good mechanical hardness [59-60]. The � conjucated

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network, in organic system with large nonlinearity, has significant absorption in

the visible region. Hence for the Second Harmonic Generation (SHG), in the blue

– near – UV region, more transparent and less extensively delocalized organics

like urea or its analogs have been considered.

Among organic crystals of NLO applications, amino acids display

specific features of interest [61] such as, (i) molecular chirality, which secures

acentric crystallographic structures, (ii) absence of strongly conjugated bonds

leading to high transparency ranges in the visible and UV spectral regions and

(iii) zwitter-ionic nature of the molecule, which favours crystal hardness. Further

amino acids can be used as a basis for synthesizing organic-inorganic compounds

like L-arginine phosphate and its derivatives. L-arginine phosphate monohydrate

(LAP) is a potential nonlinear optical (NLO) material first introduced by Chinese

in 1983.

1.8 LITERATURE SURVEY

L-histidine salts can display higher NLO properties due to the

presence of imidazole group in addition to amino-carboxylate. Among the

L-histidine analogs, the low temperature solution grown L-Histidine TetraFluoro

Borate (LHFB){[(C3N

2H

4)CH

2CH(NH

3)(CO

2)]

+

BF4:HFB} is a promising NLO

material and has better NLO properties than LAP. Reena Ittyachen and Sagayaraj

[62] studied the growth of L-Histidine Bromide (LHBr), a semiorganic NLO

material with molecular formula C6H

12N

3O

3Br, by slow evaporation technique.

L-histidine diphosphate with molecular formula C6H

15N

3O

10P

2 is a new

semiorganic NLO crystal, which possesses good transparency, dipolar strength

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and is regarded as a promising material for NLO applications [63]. Single crystals

of L-tyrosine hydrobromide with molecular formula C6H12NO3Br which is found

to have SHG efficiency 1.2 times that of KDP, has been reported by Narayana

Moolya and Dharmaprakash [64]. L-histidinium perchlorate single crystals have

been grown by solvent evaporation method at room temperature [65].

Several new salts of L-histidine were explored due to the ability of

the imidazole moiety to act as a proton donor, a proton acceptor and a

nucleophilic agent. The salts of L-histidine find wide applications in

optoelectronics and photonics devices. As a result very good semiorganic

nonlinear optical materials such as L-histidine acetate [66], L-Histidine

Hydrofluoride dihydrate (LHHF) [67], L-Histidine nitrates [68], L-histidine L-

aspartate monohydrate[69], L-histidine bromide [70], L-histidine hydrochloride

[71], L-histidine trifluroacetate(L-HTFA) [72] and L-Histidinium Maleate [73]

are some of the good examples which proved very suitable materials for NLO

applications.

1.9 SCOPE OF THE THESIS

The thesis contains the observations and results of the growth and

characterization of L-Histidine Barium Chloride, L-Histidine Sodium Sulphate,

L-Histidine Sodium Nitrate, L-Histidine Sodium Chloride and L-Alanine Sodium

Sulphate crystals.

Chapter-1 deals with the general introduction to nucleation and

techniques of crystal growth methods for the growth of various types of crystals.

A discussion on the nonlinear optics is also presented in this chapter.

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The details of the various experimental techniques employed for

the present study are discussed in chapter-2.

Chapter-3 gives an account of the growth and structural

characterization of LHBC, LHSS, LHSN and LHSC single crystals. The

crystalline nature of the grown single crystals was confirmed by the powder X-

ray diffraction analysis. The sharp well defined Bragg’s peaks confirmed the

crystalline nature of the synthesized materials. The peaks were indexed using

least square method. The lattice parameters of the single crystals were obtained

using single crystal XRD analysis.

Chapter-4 deals with the investigation of the thermal parameters

such as the thermal stability and melting point of the grown crystals.

Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA)

have been carried out and are discussed. The sharpness of the endothermic peaks

shows the good degree of crystallinity of the sample.

Chapter-5 gives a clear picture about the spectral and mechanical

studies of LHBC, LHSS, LHSN and LHSC crystals. The presence of functional

groups was identified through Fourier Transform Infrared Spectroscopy (FTIR).

The transmission in the ultraviolet region and IR region shows that these crystals

are useful for NLO activity. The mechanical properties of the grown crystals

have been studied using Vicker’s micro hardness test. The second harmonic

generation behaviours of the single crystals were tested by Kurtz-Perry powder

technique. The second Harmonic signal, generated in the crystal was confirmed

from the emission of green radiation by the crystals.

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Chapter-6 describes the growth and characterization of L-Alanine

Sodium Sulphate single crystal. The grown crystal was subjected to different

analyses like the structural, thermal, electrical, optical and mechanical properties

of the crystals suitable for nonlinear applications. Also, it gives a comparative

assessment on the prospects of the two single crystals LASS and LHSS.

Last chapter summaries the previous discussions and provides the

conclusion.

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