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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6 �
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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