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CHAPTER I

INTRODUCTION TO CRYSTAL GROWTH AND NONLINEAR OPTICS

1.1 INTRODUCTION

The three dimensional repetitive arrangement of atoms is called crystal. In single

crystals, the periodicity extends throughout the material. In poly crystalline substances, the

periodicity is interrupted at grain boundaries and does not extent throughout crystal (Mullin

.J.W 2001 et al). Hence, a large single crystal is more useful than a polycrystalline material

for studying the physical and chemical properties of solids. So, it is important to convert the

polycrystalline materials into single crystal by using various crystal growth techniques. Due

to modern society's demand for improved telecommunications and high speed data

processing, photonics -- the use of light to acquire, store, process, and transmit data -- has

become an active field of research. The design of devices that utilize photons instead of

electrons in the transmission of information has created a need for new materials with unique

optical properties. In order to meet this increasing demand of new material with good optical

properties, identifying and growth of new optical crystals with good crystalline nature has

become an interesting branch in crystal growth.

Furthermore, we need power full, long-lasting lasers in the blue and green region of

wavelengths for laser television and medical applications. The green lasers have many

advantages over red lasers. It has more than fifty times brighter than red laser and due to this

the green laser can be seen from miles away. Thus it can be used in advanced high-tech

weapons for aiming purposes. Moreover, the green laser pointers have a shorter wavelength

(532 nm) than the red laser (650 nm) and the green laser beam can be seen under dark

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conditions unlike the red laser which can be seen only on the landing surfaces. Due to this

fact, the green lasers are highly useful for astronomers as sky pointers. Green has the further

advantage that the human eye is most sensitive to green light, due to the fact that we (and

other animals) have more green-detecting cones in our retinas than any other color.

Most of the commercially available green lasers are based on the diode pumped solid

state frequency- doubled (DPSSFD) laser technology. For the past several decades,

researchers and several industries were trying to develop the laser diodes based on the

compound semiconductors such as Gallium nitride (GaN) and Indium Gallium nitride

(InGaN) especially in the blue and green region of wavelengths (Nakamura et al 1996).

However, it is very difficult to grow bulk crystal of these materials with reasonable quality.

Moreover, for the preparation of epitaxial thin films of these materials on substrates, we need

highly sophisticated and expensive techniques like molecular beam epitaxy (MBE) and metal

organic chemical vapour deposition (MOCVD). Apart from the growth aspects, the relatively

low power and limited wavelength range restricts their use in important applications.

Therefore, laser sources based on SHG is the better choice for the applications requiring

higher powers or longer wavelengths (> 400 nm). As a consequence, the green laser

technology is still depends on the nonlinear optical phenomena such as frequency doubling.

In the DPSSFD lasers, a NLO crystal must be placed to halves the wavelength of the

solid state laser. In the today’s market, inorganic NLO crystals of Potassium Titanyl

Phosphate (KTP), Lithium triborate (LBO) are used as frequency doublers. For example, the

KTP crystal is used to generate green laser at 532 nm by halving the wavelength of Nd:YAG

laser of 1064 nm. Moreover, other inorganic crystals like KDP, ADP, and LiNbO3, are also the

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commercially available nonlinear optical materials increasingly being used for frequency

doubling of Nd: YAG lasers to generate green and blue lasers.

The organic materials are superior to inorganic materials both in the speed of

response and in the magnitude of the third-order effect. Hence, for new organic NLO

materials for which single crystal specimen are not available, it is necessary to grow single

crystal specimens of high optical quality. Most widely encountered organic crystals for this

type of application are urea, POM, MHBA, NMBA and recently DAST etc (Chemla et al

1987). Due to the technological importance of these organic nonlinear optical crystals, the

need for high quality organic crystal has grown dramatically in the last decades (Zyss et al

1985). Also, large size single crystals are very much essential for device fabrication (Brice

1986). There are various methods are available for the growth of organic single crystals.

Generally the organic crystals have been grown by solution growth like slow cooling and

slow evaporation techniques and melt growth.

1.2 CHOOSING A METHOD FOR CRYSTAL GROWTH

The procedure and condition for growing single crystals can be selected on the basis

of the physical and chemical characteristics of the crystallizing substance. If the

physicochemical processes involved in crystallization are taken into account, the optimum

condition can be established. These conditions pertain to the phase composition of the

feedstock, its chemical purity and form (powder, pellets, ingot), nature of the crystallization

atmosphere, the material and shape of the container, the growth rate, the temperature

gradients and shape of the crystallization front, the degree of stabilization of the growing

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condition, and the way in which crystallization begins (spontaneous nucleation or

crystallization on a seed) (Chernov 1984).

The selection of the technique may be made on the basis of growth kinetics and

requirements, such as size, shape and purity even though more than one technique can be

employed for growing single crystals of a given material. Crystal growth involves phase

transformation to solid phase from supersaturated mother phase. Diffusion of growth units

occurring at the growth site and they orderly arranged in the lattice. In the following sections,

various important techniques of crystal growth are discussed.

1.3 METHODS OF CRYSTAL GROWTH

The crystallization process of solids, liquids and vapor undergo phase transformation

into solid form. The growth techniques are classified on the basis of phase transformations.

Solid to Solid Phase - Growth from Solid

Liquid to Solid Phase - Growth from Liquid (melt/solution)

Vapour to Solid Phase - Growth from Vapour

In general, crystal growth is the conversion of a polycrystalline piece of material into a single

crystal by causing the grain boundaries to be swept through and pushed out the crystal

(Buckley 1951, Mullin 1976). Crystal growths from liquid falls into four categories namely

melt growth, flux growth, hydrothermal and low temperature solution growth. There are

number of growth techniques in each category.

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1.3.1 Growth from melt

Crystal growth from the melt is the fastest among the growth methods as its rate does

not depend on the mass transport processes. Melt growth can be applied to solids which can

be melted and crystallized by the change in phase from liquid to solid. In this method, apart

from possible contamination from crucible material and the surrounding atmosphere, no

impurities are introduced into the growing crystal.

The methods of growing single crystal from the melt includes Kyropoulos, Verneuil,

Bridgman – Stockbarger, floating zone, Czochralski and zone melting methods. Among these

methods Bridgman-Stockbarger and Czochralski methods are widely used for variety of

materials Viz., from oxide to semiconductors and organic to inorganic materials.

1.3.1.1 Bridgman – Stockbarger technique

The essential feature of this method is the steady motion of a freezing solid-liquid

interface along an ingot which is mounted either horizontally or vertically. Either the whole

charge is melted initially called normal freezing or a molten zone is established namely zone

melting. The motion of the interface can be achieved in two ways. One can traverse a muffle

furnace over the charge or the charge through the furnace. In this method, the temperature

gradient of the furnace plays an important role in getting single crystal. The furnace

normally consists of two zones, the upper zone held at a temperature, slightly above the

melting point of the material to be grown (hot zone) and the lower zone at a temperature just

below the melting point (cold zone). The crucible is made of quartz (or) glass and has a

pointed lower end. This is filled with the material to be grown and the crucible is lowered

very slowly. The shape of the container material plays a major role in getting single

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nucleation. The formation of just one nucleus is more probable if the super cooled volume

(Hurle et al 1967) is very small. Thus, traditionally crucibles have tapered tips. As the

crucible is lowered, the first nucleated seed grown as more melt cools below its melting

point. The small crystals appeared that may undergo geometric selection and one crystal

remains, which increases in size until it fully occupies the cross section of the container.

With an appropriate shape of crucible, one can grow a single crystal with controlled

orientation.

1.3.1.2 Czochralski pulling technique

Growth of single crystal from the melt using the crystal pulling process named after

its inventor J.Czochralski. In this technique, the material to be grown is taken in a vertical

crucible and placed in a resistively heated furnace. It is melted by heating above the melting

point of the charge material. By keeping the top surface of the melt is just barely above the

melting temperature, the melt is contacted with a small seed crystal of specified orientation.

Following the thermal equilibrium, the seed crystal, which is attached to the pull rod is

slowly withdrawn, usually that rod is called as seed rod. As the heat from the melt flows up

to the seed, the melt surface cools and the crystals begins to grow. With proper adjustment

of the temperature, melt contact with seed is preserved during pulling. If the pulling rate is

higher than the growing rate of the crystal then the contact of the melt surface with seed will

be disconnected. If the growing rate is higher than the pulling rate, we can grow a crystal

with increasing diameter of the crystal. Hence, the diameter of the growing crystal can be

adjusted, by changing the temperature as well as pulling rate. A vast amount of research and

development works by many authors particularly those working in the field of electronic and

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optic materials has developed the simple Czochralski pulling to a sophisticated technology

(Teal and Little (1950), Van Uitert et al (1961), Nassau and Broyer (1962)). Of many crystal

growth methods in use today, one method, which can produce crystals weighing from several

grams to many kilograms, is the crystal pulling technique.

The advantage of this method is that the crystal can be observed as it grows and

adjustment in temperature and pulling rate can be made whenever needed. Also, it has high

growth rate so large size crystals of semiconductors like Si, Ge, SiGe, GaAs, and InP are

widely grown by this method (Brice et al 1973a).

1.3.1.3 Heat transfer modes in crystal and melt (Solid – liquid interface)

An important role is played in this connection by the convective transport in the melt,

which may differ depending on the growing method i.e., Bridgman Stockbarger and

Czochralski method.

The melt crystallizes successively at the crystal-melt interface. The heat transfer in

the interface involves various growth variables such as diameter of the crystal, pulling rate,

crystal-melt interface shape, temperature gradient and its symmetry. But the crystal-melt

interface directly influences the crystal perfection and impurity distribution throughout cross

section (Feigelson et al 1980).

In this case, when the growth proceeds normal to the interface and stray crystallites

will grow in outward direction and does not harm the major portion of the crystal. On the

contrary, if the interface shape is concave to the liquid, stray crystals can grow in and harm

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the growth of single crystal (Dutta et al 1994).The shape of the solid - liquid interface

become more concave if

(i) the growth rate is increased, or

(ii) the thickness of the crucible wall increased, or

(iii) the thermal conductivity of the crucible is decreased.

The temperature of the molten and homogenous source material is adjusted to the

slightly above the melting point. After thermal equilibrium is achieved, the seed crystal is

made in contact with the melt and withdrawn at a rate that gives a desired crystal diameter.

In case of Bridgman, the ampoule restricts the size of the crystal. As the melt solidifies and

the crystal is pulled, the latent heat of fusion is transferred to the crystal. The heat is

transported from the crystal-melt interface to the growing crystal. To control and maintain

the solidification rate depends on the supply of thermal energy to the melt, the removal of the

latent heat of solidification from the crystal and other associated heat losses from the system.

In the case of Bridgman method, heat is also lost from the crystal diameter which is

achieved by maintaining the solidification isotherm in a vertical position intersecting the

meniscus at the point where the isotherm becomes perpendicular to the melt surface. The

following parameter contributes to maintaining this condition.

The pulling rate Gp, the rate of melt level drown Gm, the heat fluxes gain and loss, the

crystal rotation rate Ws, and the crucible rotation rate Wc. By assuming the solid-liquid

interface as flat, no radial and axial temperature gradient in the melt then the maximum

pulling rate [Gp] max is given by

[Gp] max = - (Kc / (�H) ρc) α (dT / dZ)c – (dT/dZ)m (1.1)

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where ‘Kc’ is the thermal conductivity of the crystal, ‘ρc’ is the crystal density, and

(dT/dZ)c and (dT/dZ)m are the temperature gradients in the crystal and the melt respectively

at the crystal-melt interface. The negative sign in equation (1.1) accounts for the fact that

dT/dZ is a negative quantity for the usual co-ordinate system in which ‘Z’ is zero at the

interface and increases positively along the crystal length.

In equation (1.1), [Gp]max depends only on the crystal temperature gradient at the

interface; however, the temperature gradient is a very complex function of puller geometry

and ambient conditions. In the case of Czochralski, crystal growth rates compared with the

theoretical one may result from the effect of temperature fluctuations in the melt that occur

near the crystal-melt interface. Subsequent decreases in temperature increases the

solidification rate, leading to an increase in the crystal ingot diameter. To maintain the

crystal diameter, the pulling rate at the instant must be increased.

As the diameter of the growing crystal is increased, the maximum pulling decreases

because the heat loss is proportional to the surface area of the crystal ingot, which increase

only linearly with the diameter. But, the heat gain is proportional to the volume being

crystallized, which increases as the square of the ingot radius.

1.3.2 Growth from solution

Crystal growth from solution is extremely popular in the production of many

technologically important crystals. The solution growth technique is mostly used to grow

good, transparent, nonlinear and ferroelectric crystals. Depending upon the solvent and

solubility, the solution growth method is classified into two groups as

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(i) Low temperature solution growth

(ii) High temperature solution growth

1.3.2.1 Low temperature solution growth

In this method, solutions are prepared by dissolving a compound in solvents which

are in liquid state at ambient temperature. The growth of crystals by low temperature solution

growth involves weeks, months and sometimes years. Materials having moderate to high

solubility in temperature range, ambient to 100°C at atmospheric pressure can be grown by

low-temperature solution method. The mechanism of crystallization from solution 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 (Chernov 1984).

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. If the solubility is too high, it

is difficult to grow bulk single crystals and too small a solubility restricts the size and growth

rate of the crystals. 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 the temperature will affect the

supersaturation to growth of good quality bulk crystals in both cases. If the solubility

gradient is very small, slow evaporation of the solvent is the other option for crystal growth

to maintain the supersaturation in the solution.

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Growth of crystals from solution is mainly a diffusion-controlled process; the

medium must be viscous to enable faster transference of the growth units from the mother

solution by diffusion. Hence, solvent with less viscosity is preferable. Supersaturation is an

important parameter for the solution growth process. The crystal grows by the accession of

the solute in the solution; as a degree of supersaturation is maintained. The solubility data at

various temperatures is 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. Low temperature solution growth can be subdivided into the following methods.

a. Slow cooling method

b. Slow evaporation method

c. Temperature gradient method

Among these three low temperature solution growth methods, slow cooling and slow

evaporation methods are widely used for crystal growth of variety of material.

1.3.2.1a Slow Cooling method

Generally, the slow cooling method is used to grow bulk single crystals from

solution. In this method of growth, the supersaturation can be achieved by cooling the

temperature of the solution at low cooling rates using a temperature controller. The

crystallization process is carried out in such as way that the temperature dependence of the

solute concentration moves towards the metastable region along the saturation curve in the

direction of lower solubility. As the volume of the crystallizer and the solute concentration in

it are finite, the system needs systematic cooling to acquire supersaturation. The

supersaturation of the solution provides the necessary driving force to initiate the growth on

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the surface of the suspended seed crystal in the solution. Moreover, in the slow cooling

method, growth proceeds in the seed crystal until the solution remains in the metastable

region. As a consequence, the width of the metastable region defines the size of the crystal to

be grown.

1.3.2.1b Slow Evaporation method

In this method of crystallization, the supersaturation of the solution can be achieved

by evaporating the solvent from the saturated solution. The continuous evaporation of

solvents causes the reduction of solution volume at constant temperature. Unlike to slow

cooling method, the spontaneous nucleation and growth can be observed in the solution

through the continuous evaporation of solvent at constant temperature. Generally this method

of growth can be employed to grow good quality seed crystals. The rate of evaporation of

solvents are mainly depends on the vapour pressure of the solvent. Moreover, the evaporation

of the solvent can be controlled by placing the crystallizers in a constant temperature bath

and thus the growth process can controlled up to some extent although the nucleation is

spontaneous.

1.3.2.2 High temperature solution growth

High temperature solution growth can be further classified into two major categories.

The first one is growth from single component systems and the second one is that from the

multi-components. In the single component method, only the chemical component forming

the crystal is present in the growth system, while in the multi-component method, more than

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one component is added to the growth system. The primary reason for this addition is to

reduce the crystallization temperature.

This reduction in the crystallization temperature is necessary if the material to be

crystallized has an incongruent melting behavior, that is, materials which decompose before

melting so that crystallization from the melt results in some other phase. Also the reduction

in the crystallization is necessary for the materials which undergo a phase transformation

thus results in severe strain or even fracture. Also, for material which have a very high vapor

pressure at high temperature. The two main categories are hydro thermal growth and flux

growth.

1.4 PURIFICATION PROCESSES

Impurities in organic materials are of considerable importance, not only because of

their influence on the physical and chemical properties on the resulting crystals, but also they

can play a dominant role in controlling the crystal growth behavior. In the later case,

impurities can modify crystal morphology and growth rates as well as altering the stability of

growth through constitutional supercooling. Therefore, the most essential feature of growth

of high optical quality crystal is the material and the solution purification (Catesby 1972).

Purification of a material may be carried out by the following methods:

(i) Recrystallization

(ii) Sublimation

(iii) Melt phase zone refining

Recrystallization can be applied to the soluble materials, and the only real problem being the

choice of a suitable solvent phase from which the recrystallization takes place. The choice of

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solvent is made such that at room temperature the material has little or no solubility, but at or

near the solvent boiling point the material is readily soluble. The recrystallization has to be

carried out two or three times to enhance the purity of the material. The solvent for such

recrystallization should be distilled to prevent contamination (Han et al 1989).

The technique described above uses the presence of a solvent phase to achieve the

desired purification. The presence of such solvent can lead to problems in the later stages of

purification or crystal growth. The removal of solvents is often most effectively achieved by

the use of vacuum sublimation. This process is additionally a highly efficient purification

technique, removing both highly volatile and non-volatile impurities from a material. The

technique, of course, depends on the material having a reasonable vapour pressure without

which the separation is both time consuming and inefficient (Mc Ardle et al 1974). The

solvent and volatile impurities can be trapped out and the non volatile impurities remain in

the experimental tube can be dropped out. This process can be repeated again to achieve

better purification.

The zone refining technique is a reliable and efficient technique for achieving ultra

pure material. The procedure is based on the principles of fractional crystallization, which

utilizes the difference in impurity concentration in the liquid and solid phases. The practical

aspects of zone refining are based on the automation of the repetitive passage of a zone (or

zones) of molten material from one end of a solid charge to the other. The resulting

distribution of impurities after zone refining depends on the density of the impurities. Due to

this density variation, the impurities will collect at the end portion of the zone purified

material. Hence, only the centre section of the ingot can be used for crystal growth. Zone

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refining cannot be used for materials which are unstable in the melt or in the solid state near

the melting point.

1.5 INTRODUCTION TO NONLINEAR OPTICS

The beginning of the field of nonlinear optics is often taken to be the discovery of

second-harmonic generation in quartz crystal by Franken et al (1961), shortly after the

demonstration of the first working laser by Mainman (1960). Nonlinear optical phenomena

are “nonlinear” in the sense that they occur when the response of a material system to an

applied optical field depends in a nonlinear manner upon the strength of the optical field

(Boyd 1992).

Many of the materials used for photonics are non-linear optical (NLO) materials,

which mean that they interact with light in such a way that the light changes the properties of

the material, which, in turn, changes the properties of the light. The propagation of a wave

through a material produces changes in the spatial and temporal distribution of electrical

charges as electron and atoms react to the electromagnetic fields of the wave. The effect of

the forces exerted by the fields on the charged particles is a displacement of the valence

electron from their normal orbits. This change develops electric dipoles whose macroscopic

manifestation is the polarization (Laud 1991 & Gupta et al 1993).

In the case of conventional optics (i-e., linear optics), when a light beam interact with

nonlinear medium, the medium gets polarized and the relation between E and P is essentially

linear. Its polarizability P can be expressed as

P=�0 � E (1.2)

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where the constant of proportionality � is known as the linear susceptibility, �0 is permittivity

of free space and E is the electric field strength of electromagnetic wave. However, when the

high intensity light beam like laser beam propagates through any dielectric medium, the

polarization no longer remains dependent linearly on E but also depends upon the higher

powers of E. If we include the higher order terms, then we write

P =�����χ �(1) E���χ �(2) E2

���χ(3) E3��������� (1.3)

This equation is also be written as

P = P (1) + P (2) + P (3) +…….. (1.4)

The relations are basically tensor relations but for simplicity we can use them in scalar form.

It is evident that higher order terms are important only for higher values of electric field

strength of light beam. The terms χ � (1),� χ � (2), χ� (3) are the second and third order nonlinear

susceptibilities respectively.

This can give rise to a number of interesting effects such as frequency conversion, in

which light of one color (frequency) is transformed into light of a different color upon

passing through the NLO material. NLO materials are used for making photonic devices such

as optical switches, optical memories, and logic gates.

One of the primary requirements for a nonlinear crystal is that it should have

excellent optical quality. The relevant properties for nonlinear optics are optical nonlinearity,

large birefringence, moderate to high transparency and optical homogeneity for high

conversion efficiency, high mechanical strength, chemical stability, polishing and coating

technology for ease of fabrication, high damage threshold, fracture toughness and thermo –

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mechanical properties for high average power and reliable crystal growth techniques for

availability.

Some of the practical applications of nonlinear optical materials are Second

Harmonic Generation (SHG), Sum Frequency Generation (SFG), Difference Frequency

Generation (DFG), electro-optic modulation and Optical Parametric Oscillation (OPO).

However, typically no more than one of these generations will be present with any

appreciable intensity in the radiation generated by the nonlinear optical interaction. The

reason for this behavior is that the nonlinear polarization can efficiently produce an output

signal only if a certain phase-matching condition is satisfied and usually this condition

cannot be satisfied for more than one frequency component of the nonlinear polarization. As

mentioned in the beginning of this chapter, among these NLO properties, SHG or frequency

doubling is one of the significant processes which are highly useful to generate blue and

green laser by DPSSFD laser technology.

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1.5.1 Second harmonic generation (SHG)

SHG has been a subject of much interest and study since Franken et al (1961) first

observed frequency doubling in quartz. SHG is the conversion of coherent light of frequency

ω into light of frequency 2ω. Frequency changing occurs due to the materials ability to

change its refractive index and thus altering the frequency of the light passing through it.

This phenomenon of frequency altering by a medium when an electric field is applied is

called the Pockels effect (Boyd 1992). As an example, the conversion of the commercial

infrared laser (wavelength of 1064 nm), to green light (wavelength of 532 nm), when passed

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through one of these second order nonlinear media. Here a laser beam whose electric field

strength is represented as

E (t) = E e-i�t + c.c (1.5)

and this is incident upon a crystal for which the second–order susceptibility χ (2) is non-zero.

The nonlinear polarization created in such a crystal is given as

P (2) = χ (2) E2 (t) (1.6)

where χ (2) is the second order nonlinear susceptibility. When the higher order terms are zero,

the material is said to be linear. A nonlinear material has non-zero higher order

susceptibilities. The specific classes of nonlinear materials that are discussed in this thesis are

the second order nonlinear materials or the ones that have a non-zero χ (2). This can only be

obtained from nonlinear noncentrosymmetric materials.

1.5.2 Phase matching

Phase matching means that the wave generating the polarization and the generated

waves (the three interacting waves) are in phase over the interaction region, so the

microscopic contributions of the generated polarization of each individual dipole in the

crystal can interfere constructively, adding up to a macroscopic field. Only after this

constructive interference the nonlinear effect can be observed. To achieve phase-matching,

the phase velocity of the generated waves (while traveling through the crystal) should equal

the phase velocity of the pump wave in a parametric process. This can be achieved in a

birefringent crystal which has different indices of refraction along the different crystal axes.

In birefringnent crystals, waves with different wavelength can travel at the same speed (so in

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phase) when their polarization directions are along different crystal axes. To fulfill phase

matching, the generated waves and the applied wave must have different polarizations to

control their propagation velocities.

Certain asymmetric crystals are birefringnent (doubly refracting) because through

them, light can travel at two different velocities, described as ordinary (o) and extra ordinary

(e). These velocities actually vary with propagation direction and polarization as well as with

wavelength. In certain direction the ordinary fundamental light travels at exactly the same

velocity as extra ordinary harmonic light. When this happens, the SHG is greatly enhanced

and the system is said to be phase matched.

Two types of phase matching are possible according to the polarization of the � and

2� beams. Type I phase matching (n°�= n e 2�) or vice versa arises from a wave of one type

(either ordinary or extra ordinary) and type II phase matching [(1/2 (n°2� + n e �) = n° 2� or n e

2�] arises from a combination of the ordinary and extra ordinary waves. If only one type of

(�) wave is present (as when the electric field is polarized along or perpendicular to the

dielectric axis) then only type I phase matching can occur.

1.5.3 Organic nonlinear optical materials

Nonlinear optical (NLO) crystals with high conversion efficiencies for second

harmonic generation and transparent in the visible and ultraviolet ranges are required for

numerous device applications. Within the last decade much progress has been made in the

development of these NLO materials having large nonlinear optical coefficient. Organic

materials are in increasing demand, as they are better candidates for NLO and electro-optic

device application than those of inorganic materials (Allen 1989).

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

of tailoring them for a particular device application. Organic NLO materials have high

nonlinear figure-of-merit for frequency conversion, high laser damage threshold and fast

optical response time as compared with inorganic NLO materials (Stephens et al 2003).

However there are some drawbacks with organic NLO materials which are explained below.

A major obstacle to the development of many organic NLO materials and the exploitation of

their full potential has been the considerable difficulty associated with the growth of large

high quality single crystals of these materials (Halfpenny et al 1993). As with many other

organic solids, the intermolecular forces are comparatively weak, being predominantly van

der Waals or permanent dipole-dipole interactions. This typically results in low melting

points and relatively high vapour pressure. Mechanical properties are, in general, rather poor

with most organic solids being relatively soft. This can have important consequences for the

structural perfection of the crystals. Thermal instability is also common, with many organic

materials undergoing thermal decomposition at or below the melting point. These factors

together with the low thermal conductivities can pose substantial problems in crystal growth,

particularly from the melt.

These difficulties are the main challenges taken for this course of investigation. And,

attempts were made to subsidies or to some extent to eliminate these difficulties in growth

related problems, in the case of benzophenone and ethyl p-dimethylamino benzoate

(EDMAB) single crystals from various growth methods are explained in the following

chapters.

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1.6 BENZOPHENONE

1.6.1 Material Introduction

Benzophenone (molecular formula: C13H10O) is one of the promising organic

nonlinear optical materials and it is a good alternate candidate for urea in all aspects (Wang

et al 2004b). Because in the field of nonlinear optics, urea is a well known organic material

for frequency conversion devices such as SHG and fifth harmonic generation and it was used

as the first organic optical parametric oscillator (Donaldson et al 1984). In spite of practical

application, urea has some undesirable properties such as mechanically soft and hygroscopic

in nature. As a result of hygroscopic nature urea cannot be exposed to normal atmosphere,

and for practical application, it is being used by immersing it in an index–matching fluid with

compatible chemical and optical characteristics (Rai et al 2002). From the literature, the

efforts made to resolve the problems associated with urea have not been very successful.

Benzophenone crystal is biaxial and it exhibits high (1.8 times of urea) second order

optical nonlinearity of urea with non hygroscopic in nature and mechanically hard. In

particular, the maximum value of effective nonlinear coefficient (deff) for type II phase

matching in the benzophenone crystal at a fundamental wavelength of 1064 nm is 2.195

pm/V and it is about 5.4 times than that of KDP (Urea: 3 times of KDP) (Wang et al 2004).

1.6.2 Structure and properties of benzophenone

Generally, molecules in which the -electron conjugated system was connected by a

carbonyl group appeared as good candidate materials for nonlinear optical applications. The

molecular structure of benzophenone is shown in the figure 1.1, where there are 24 atoms in

a single molecule. Benzophenone is an aromatic ketone which crystallizes in a stable

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orthorhombic structure with non-centrosymmetric, space group P212121. The crystal structure

of the stable phase was first determined by Vul and Lobanova (1967). Single crystals of

benzophenone are optically active and it is the first organic molecular material to be

identified as polymorphic. It is a polar molecule and has a dipole moment of 2.98 Debye.

The unit cell contains four benzophenone molecules which has opposing dipole moment. The

unit cell dimension at room temperature is: a = 10.26 Å; b = 12.09 Å; c = 7.88 Å (Menard et

al 1973, & Roberts et al 1993). Numbers of works have been published on possible

correlations between the physical properties and the molecular packing geometry of single

crystals of benzophenone. Plastic deformation of the benzophenone was studied by

measuring the variation in the stress developed inside the crystal (Pethrick et al 1992).

Measurements of the dielectric and elastic properties of crystals of undefined defect structure

have been reported.

1.6.3 Literature on growth of benzophenone

To our knowledge only very few reports on the detailed growth kinetics of

benzophenone single crystals was available. Bleay et al (1978) have grown the

benzophenone single crystal by Czochralski pulling technique for the first time and analyzed

the grown-in dislocation by x-ray topography. In 1992, Tachibana et al have grown the

benzophenone single crystal by adopting the same experimental procedure as reported by

Bleay et al (1978), and calculated the burger vector using x-ray topography analysis. The

benzophenone single crystals were also grown in non aqueous solution by slow solvent

evaporation method and Bridgman method (Lewine 1976, Hooper et al 1980 & McArdle et

al 1987).

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Figure 1.1 : Molecular structure of benzophenone

H2

H3

O

C1

C2

C3

C4

C5

C6

H4

H5

H6

C C'1

C'2C'3

C'4

C'5

C'6

H'2

H'3

H'4

H'5

H'6

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In 1992, Pethrick et al have grown the benzophenone crystal from super cooled melt

and analyzed the morphology, influence of plastic deformation on ultrasonic velocity and the

dislocations before and after plastic deformation by employing x-ray topography. Also they

have reported a detailed dielectric studies on the melt grown benzophenone crystals. A

theoretical analysis on the morphology of benzophenone and their experimental verification

were done by Roberts et al (1993).

The nonlinear optical properties of benzophenone and its derivatives have been

studied by many researchers using powder SHG method (Cockerham et al 1991, Terao et al

1990, Genbo et al 1993, & Lammers et al 2000). Their results show that several of these

compounds generate SHG signals stronger than that of urea (Vander Venden et al 1979).

Terao et al (1990), have synthesized several di-substituted benzophenone derivatives by

Friedal-Crafts process and measured their second order nonlinear optical susceptibilities.

They found that the SHG activity of 4-methoxy -4-nitrobenzophenone could be changed by

recrystallization solvent and recrystallization rate. They suggested that the difference in SHG

activity was attributed to polymorphism, based on results of x-ray analysis and differential

scanning calorimetry.

The relationship between crystal structures and nonlinear optical properties of

benzophenone was reported by Genbo et al (1993). The photo conversion ability of

benzophenone into benzophinacol by visible light, was investigated by Davydora et al (2002)

using Raman spectra. A two parameter Sellmeier fit for benzophenone was derived

(Lammers et al 2000) and the nonlinear optical co-efficient for benzophenone was

determined by Maker Fringe technique.

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Moreover, the derivatives of benzophenone are attracted by many researchers due to

their large NLO susceptibilities and short cut-off wave lengths of about 400nm (Frazier .C.C

et al 1987, Lal .R.B et al 1997 & Jiang .M et al 1999). According to literature, 4-amino

benzophenone (4-ABP) is one of the best benzophenone derivatives which exhibit the SHG

efficiency of 360 times of ADP and 260 times of KDP (Frazier .C.C et al 1987 & Jiang .M et

al 1999). Therefore single crystals of 4-ABP are promising for generating green and blue

laser beams from Nd: YAG or other semiconductor diode lasers. Further, a new NLO single-

crystal 4-4' DMBP is found to be a promising candidate for NLO applications. Although the

structure of the 4-4' DMBP was reported by Biserka Kojic-Prodic et al. in 1990, the growth

of high quality transparent bulk size crystals and their physical properties have not yet been

studied in detail except few short papers (Graham .D.J et al 1982 & Anandhababu .G et al

2008).

1.7 Ethyl p-dimethylamino benzoate

1.7.1 Material introduction

Ethyl p-dimethylamino benzoate (EDMAB) is generally known as tertiary amines

which are mainly used as a part of self curing two part system for dental/medical

compositions comprising degradable copolymers which are suitable for use as root canal

sealants, root canal filling materials, dental restorative materials, implant materials, bone

cements and pulp capping materials (Jia Weitao et al 2004). Moreover, since the chemical

structure of the material has the – electron conjugated system attached with electron donor

and acceptor groups at opposite end, it could be applicable for nonlinear optical applications

(Nalwa .H.S et al 1997). However, no literature was found on the crystal structure, growth

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and NLO properties of EDMAB. Hence, this investigation primarily aims to grow bulk single

crystal of EDMAB and to study its nonlinear optical properties for the possible fabrication of

nonlinear optical devices (Kalyana Sundar .J.K et al 2010, Natarajan .V et al 2011a &

Natarajan .V et al 2011).

1.8 Objective of the thesis

The purpose of this work is to understand the growth behavior of novel organic NLO

crystals such as benzophenone, two of its derivatives and EDMAB. The main objectives of

the thesis are as follows:

1. The first objective of the thesis is to study the direction dependent properties of

benzophenone crystals such as growth rate, laser damage threshold and

mechanical properties. Because, NLO properties are orientation dependent and

thus it is one of the important targets for acquiring innovative functions with

organic materials.

2. Understanding the growth mechanism of directional growth of benzophenone

crystals from melt by means of in-situ observation of growth process.

3. Comparatively analyze the growth aspects, structural and optical properties of

novel benzophenone derivatives such as 4-ABP and 4-4' DMBP.

4. Finally, identification of new NLO material with relatively high SHG efficiency.

In this aspect, investigate the growth aspects, structural and optical properties of

EDMAB single crystal.

The following chapters detail these objectives.