1

1. INTRODUCTION

The author introduces the topic of research in this chapter.

1.1 Importance of Single Crystals

Crystals have been admired by man from ancient times because of their

beauty. Crystallization of salt is mentioned in a Chinese reprint of 2700 BC. Crystals

have fascinated men and women for many thousands of years. Naturally occuring

hard gem stone crystals were priced along the gold antiquity. The scientific approach

to crystal growth was born during early 17th

centuary when Kepler (1611) correlated

the morphology and structure, followed by Nicolous Steno who explained the origin

of a variety of external forms exhibited by natural quartz crystals in terms of different

growth rates in different crystallographic directions [1, 2]. The work carried out

during the 9th

century laid a firm foundation for the modern scientific and

technological developments in crystal growth.

Crystals are the pillars of modern technology. Crystals play a vital role in

electronic industry, photovoltaic solar cells, fibre optic communications, detecting

instruments, sintillators and in space technology. Integrated micro-electronics and

opto-electronics, necessitate improved crystal growth technology for large diameter

silicon, GaAs and InP in combination with optimized defect and property control on

submicron scale. Laser fusion technology depends on high power laser crystals and

oxide crystals.

Crystal growth is an inter disciplinary subject covering physics, chemistry,

mineralogy, metallurgy, materials science, crystallography, etc. In the recent years,

direct visualization at atomic resolution of nucleic acid and variety of proteins with

2

which it interacts is possible by growing single crystals. Crystallography is concerned

with the nature of the regular atomic arrangements within the crystal.

Crystallographers had made remarkable studies about the crystal before the discovery

of X-ray by crystals. However, only after that, it became possible to know about the

internal arrangement of atoms in the crystals, in a more developed way. As there was

a remarkable achievement in the study of internal atomic arrangements it leads to the

study of more physical properties. This interest shifted from the study of natural

crystals to the laboratory grown crystals. The significance of crystal growth [3] to

electrical engineering, chemistry and physics is illustrated in Figure 1.

Modern technology requires physicists, chemists, electrical engineers,

metallurgists and crystal growers to assist each other at many levels. Crystal growth is

a vital and fundamental part of materials science and engineering, since crystals of

suitable size and perfection are required for fundamental data acquisition and for

practical devices such as detectors, integrated circuits and for other applications.

Figure 1: Significance of crystal growth to electrical

engineering, chemistry and physics

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Crystal growth is an important field of material science, which involves

controlled phase transformation. A single crystal consists of atomic arrays that are

periodic in three dimensions with equal repeated distances in a given direction.

Generally, matter exists in three states, namely, solids, liquids and gases (see

Figure 2). The solid state materials can be further classified as single crystals, poly

crystals and amorphous materials depending upon the arrangement of constituent

molecules, atoms or ions. An ideal crystal is one in which the surroundings of any

atom would be exactly the same as the surroundings of every similar atom. Real

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

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

crystals. The uniformity of single crystals can allow the transmission without a

scattering of electro magnetic waves. Since, for the past few decades, one could see

that there are a lot of developments in science and technology-especially, in the fields

of electronics, fibre – optic communication and lasers. The vacuum tubes in electronic

equipment have become obsolete and have already been replaced by transistors,

integrated circuits and microprocessors. We could achieve this development due to

the availability of single crystals like silicon, germanium and gallium arsenide. Also,

with the invention of nonlinear optical properties in some single crystals, we can have

tunable lasers. Since there is a vast market for solid state devices in the fields of

computers, telecommunication, etc, effort has been made in recent years on producing

larger size single crystals [4, 5] .

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Figure 2: Classification of materials

The importance of crystals extends daily for things such as frequency

controlled oscillators made up of quartz, polarizers by calcite and sodium nitrate,

quartz, Rochelle salt and ADP as transducers, diamond for grinding, potassium

chloride and anthracene as radiation detectors. On emerging into the field of

optoelectronics, germanium and silicon play a vital role in the transistors, Gallium

arsenide and indium phosphide as tunnel-diodes and also magnetic devices (garnets),

strain gauges (silicon), ultrasonic amplifiers (CdS), masers and lasers (ruby, GaAs,

Solids

Matter

Fluids

Crystalline Amorphous Quasi crystalline Liquids Gases

Marco (bulk) crystals

crystals

Micro crystals Nanocrystals

Single crystals Polycrystals

Twins Multiplex

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calcium tungstate), lenses (fluorite), etc. Over million pounds of synthetic quartz are

produced annually for a variety of applications ranging from optical components due

to its transparency to precise time and frequency oscillators based on its piezo electric

properties [6].

Crystals are subdivided into macro, micro, and nano crystals. Macro crystals

are ordered crystals of mm (= 10-3

m and above) size. They are visible (bulk single

crystals). Micro crystals are microscopically small crystals. Nano crystals are crystals

of dimensions = 10-7

and below.

Inorganic crystals like KDP, ADP, KTP and β-BaB2O4 are the best nonlinear

optical materials increasingly being used for the second harmonic generation,

frequency doubling of Nd-YAG laser and also in electro-optical applications. The

super ionic crystals like silicates, germanates, phosphates, and tungstates built of

octahetra and tetrahetra form a major group of fast ionic conductors after the

discovery of three dimensional ionic conductor NASICON [7].

Organic crystals show a complex range of phase behaviour, photo and thermal

stability, solubility and morphology. The rapid development of optical

communication system has led to demand for nonlinear optical materials of high

structural and optical quality. The most widely encountered crystals for this type of

application are urea, MNA, PDM, etc [8]. The perfect organic crystals should have

high efficiency, low absorption edge (cut-off wavelength) and high damage threshold.

There are a number of properties, particularly relevant to crystal growth, which are

common to many organic materials. First intermolecular forces are comparatively

weak, being predominantly Van der Waals forces or permanent dipole-dipole

6

interactions. Due to the technological importance of these nonlinear crystals, the need

for high quality organic crystals has grown dramatically in the last few decades [9].

The growth of large single crystals, from aqueous solution is of interest for

essentially two reasons. First, there is a growing body of applications in the area of

high-power laser technology where such solution grown crystals are required. To

date, depending on the material, desired crystals have been either entirely unavailable

or else unavailable only at very high cost. Second, research into this area of crystal

growth and the corresponding in-depth examination of several key systems provides

fundamental case studies generating theory and technology, applicable to all of

solution crystal growth, including new aqueous growth systems and high temperature

solution growth as well [10].

Since an understanding of the various crystal growth methods is very much

essential for the growth of nonlinear optical and dielectric single crystals, the

materials of choice for this investigation, the author discusses briefly in the following

sections the fundamentals of the various methods of growing single crystals.

1.2 Classification of Crystal Growth

There are four major categories of crystal growth methods which are:

1) Solid state growth → processes involving solid-solid phase transition.

2) Vapour phase growth→ processes involving vapour-solid phase transition.

3) Solution growth→ processes involving liquid-solid phase transition.

4) Melt growth→ processes involving liquid-solid phase transition.

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Solution and melt growth methods are treated separately because solution

growth methods differ much from methods used for pure melt growth. For additional

knowledge on the growth of single crystals, it is suggested to refer the books by

Buckley [11], Pamplin (edn.) [12], Hartman [13], Mullin [14], Sangwal [15], Byrappa

and Ohachi [16], etc.

The basic common principle in all these methods is that a nucleus is first

formed, and it grows into single crystal by organizing and assembling ions or

molecules with specific interactions and bonding, so that the process is slow and

multiple nucleation is minimized. Crystal growth process and size of the grown

crystal differ widely and are determined by the characteristics of the material. An

efficient process is the one, which produces crystals adequate for their use at

minimum cost. The growth method is essential because it suggests the possible

impurity and other defect concentrations. Choosing the best method to grow a given

material depends on material characteristics.

1.2.1 Solid state growth

Solid state growth technique can be considered as the conversion of a

polycrystalline material into a single crystal by causing the grain boundaries to be

swept through and pushed out of the material due to atomic diffusion. But, this is very

slow at ordinary temperatures and is only rarely used.

1.2.2 Vapour phase growth

Vapour phase growth methods are the processes involving vapour-solid phase

transition. In these, the source material to be crystallized is provided from the vapour

phase. Physical vapour deposition and chemical vapour deposition are the two widely

8

used techniques. Deposition from the vapour phase is mostly used for the fabrication

of thin layers of metal, insulator and semiconductor materials. The main advantages

of these methods are that they do not involve the contacting of the growing surface

with a liquid or solid phase, thus avoiding numerous potential problems during and

after the growth process. Single crystals with high purity can be grown from the

vapour by sublimation and chemical vapour deposition.

Crystals of high purity can also be grown from vapour phase by sublimation,

condensation and sputtering of elemental materials. Molecular beam techniques have

also been applied recently to crystal growth problems. The most frequently used

method for the growth of bulk crystals from vapour phase utilizes chemical transport

reaction in which a reversible reaction is used to transport the source material as a

volatile species to the crystallization region. Finding a suitable transporting agent is a

formidable problem in this technique. It is rarely possible to grow large crystals

because of multinucleation. This growth is mainly sub-divided into (i) Physical

Vapour Transport (PVT) (ii) Chemical Vapour Transport (CVT). In PVT, no carrier

gas is used, but, in CVT, carrier i.e., transporting gas is used to carry the material

from source zone to the growth zone [17]. The crystals of Al2O3, CdS, ZnSe, GaP and

GaAs are grown from vapour phase. The commercial importance of vapour growth is

in the production of thin layers by Chemical Vapour Deposition (CVD).

1.2.3 Solution growth

Solution growth method is an ancient crystal growth method permitting crystal

growth at a temperature well below melting point. Material which decomposes on

heating and / or which exhibit any structural transformation while cooling from the

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melting point can be grown by low temperature solution growth if suitable solvents

are available. The supersaturation may be attained by evaporation of the solvent, by

cooling the solution or by a transport process in which the solute is made to flow from

a hotter to a colder region. In high temperature solution growth, the constituents of the

material to be crystallized are dissolved in a suitable solvent and crystallization occurs

as the solution becomes critically supersaturated.

When the crystal is in dynamic equilibrium with its parent phase, the free

energy is at a minimum and no growth will occur. For crystal growth to occur, this

equilibrium must be disturbed by a change of a correct sign in temperature, pressure,

chemical potential, electrochemical potential or strain. The system may then release

energy to its surroundings to compensate for the decrease in entropy occasioned by

the ordering of atoms in the crystal and evolution of heat of crystallization. Normally,

just one of these parameters is held minimally away from its equilibrium value to

provide a driving force for the growth of crystal. The solution growth method is used

to grow the crystals which have high solubility and have variation in solubility with

temperature [18]. There are two methods in solution growth depending upon the

solvents and the solubility of the solute. They are:

1. High temperature solution growth (flux growth, hydrothermal growth) and

2. Low temperature solution growth.

In the present study, we used only the low temperature aqueous solution

growth method for the growth of single (sample) crystals. So, we do not present here

the details of various methods used for the growth of single crystals. However, some

details of low temperature solution growth methods are provided in Chapter 2.

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High temperature solution growth

Flux growth

In this method, a solid is used as the solvent instead of liquid and the growth

process takes place well below the melting temperature [19] of the solute. The flux

growth is preferably used for the following reasons:

1. The material melts incongruently,

2. The melting point of the material is too high,

3. The material is non-stoichiometric at its melting point due to a high vapour

pressure of one or more constituents,

4. Better quality crystals can be grown by this method and

5. A destructive phase transition is present closer to the melting point.

Hydrothermal growth

A number of metals, metal oxides and other compounds, practically insoluble

in water upto its boiling point, show an appreciable solubility when the temperature

and pressure are increased well above 100 ⁰C and 1 atmosphere respectively. Growth

is usually carried out in steel autoclaves with gold or silver linings. The liquids from

which the process starts are usually alkaline aqueous solutions. Pressure is typically in

the range of hundreds or thousands of atmosphere. The requirements of high pressure

presents practical difficulties and there are only few crystals of good quality and large

size grown by this technique [20]. Quartz is the crystal grown industrially by this

technique.

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1.2.4 Melt growth

This is the most important method of crystal growth. 80% of the global

requirement of crystals is grown by this method. Melt growth is the process of

crystallization by fusion and resolidification of the pure material. In this technique,

apart from possible contamination from crucible material and surrounding

atmosphere, no impurities are introduced in the growth process and the rate of growth

is normally much higher than the other methods [21]. In principle, all materials can be

grown into a single crystal from the melt, provided they melt congruently, they do not

decompose before melting and they do not undergo a phase transition between the

melting and room temperature. The melt growth can be classified as follows.

(i) Bridgman – Stockbarger technique

(ii) Czochralski technique

(iii) Kyropoulos technique

(iv) Zone melting technique

(v) Verneuil technique

The important feature of Bridgman technique is the steady motion of a

freezing solid liquid interface along an ingot which is mounted either vertically or

horizontally. The material is melted in a vertical cylindrical container. The container

is lowered slowly from the hot zone of the furnace into the cold zone [22].

Crystallization begins at the tip of the container by forming a nucleus and continues to

grow from that nucleus. One of the constraints of this technique is the choice of the

crucible. The crucible should not contaminate the melt. The crystal should not adhere

to the crucible as this also can introduce excessive strains during cooling. This

technique cannot be used for materials which decompose before melting. This

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technique is best suited for low melting point materials. Germanium, gallium arsenide

and such other materials expand on solidification and hence this method is not useful

to grow such crystals. By this technique, we can grow AgBr, AgCl, CaF2, PbS, etc.

In Czochralski method, the material is taken in a crucible and is kept in a

furnace. By controlling the furnace temperature, the material is melted [23]. A seed

crystal is lowered to touch the molten charge which has been maintained at its melting

point. When the temperature of the seed is maintained very low compared to the

temperature of the melt, by suitable water cooling arrangement, the molten charge in

contact with the seed will solidify on the seed. Then the seed is pulled with

simultaneous rotation of the seed rod and the crucible in order to grow perfect single

crystals. Liquid Encapsulated Czochralski abbreviated as LEC technique makes it

possible to grow single crystals of materials which consist of components that

produce high vapour pressure at the melting point. This refined method of

Czochralski technique is widely adopted to grow the III-V compound semiconductors.

In Kyropoulous technique, the crystal is grown in larger diameter. From the

larger diameter crystal, we can make windows, prisms, lenses and other optical

components. As in the Czochralski method, here also the seed is brought into contact

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

allowed to melt and a short narrow neck is grown. After this, the vertical motion of

the seed is stopped and growth proceeds by decreasing the power into the melt. The

major use of this method is for alkali halides to make optical components.

In zone melting technique, a liquid zone is created by melting a small amount

of material in a relatively larger long solid charge or ingot. It is then made to traverse

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through a part or the whole of the charge. It is a more advantageous method than the

other methods due to the removal or addition of impurities from or to the crystal as

the crystal is growing. In this method, the rate of zone movement depends on the

orientation of the two solids binding the liquid zone as well as the thickness and

temperature of the zone. [24].

In the Verneuil technique, a fine dry powder of the material to be grown is

showered through a wire mesh and allowed to fall through the oxy-hydrogen flame.

The powder melts and a film of liquid is formed on the top of the seed crystal,

maintained on a pedestal at the bottom of the flame [25]. This freezes progressively as

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

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

diameter. By this method, ruby crystals are grown for use in jeweled bearing and

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

1.3 Introduction to Nonlinear Optics

Nonlinear optical (NLO) effects are analyzed by considering the response of

the dielectric material at the atomic level to the electric fields of an intense light beam.

The propagation of a wave through a material produces changes in the spatial and

temporal distribution of electrical charges as the electrons and atoms interact with the

electromagnetic fields of the wave. The main effect of the forces exerted by the field

on the charged particles is displacement of the valence electrons from their normal

orbits. This perturbation creates electric dipoles whose macroscopic manifestation is

the polarization [26].

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In linear materials, the response is always proportional to the stimulus. The

induced polarization is proportional to the field and the susceptibility is independent

of the field. In practice, this is always the case at low fields. However at high fields,

the polarization stops being proportional to the field and hence the susceptibility starts

depending on the field.

It is called Nonlinear Optics (NLO) because, at high intensity, the graph

representing the dependence of optical polarization on the light field amplitude has

curvature and deviates from straight line. When a string is bowed with much force or

a wind instrument is blown hard, many overtones may be generated; similar thing

happens to the electrons in matter when they are violently excited by high intensity

light; overtones of light are created. This has the dramatic effect that a red light beam

may be changed to a UV beam with twice or thrice the frequency or one half or one

third of the wavelength.

1.4 Theory of Nonlinear Optics

Nonlinear materials exhibit optical responses when their optical properties are

field dependent. When a light wave propogates through an optical medium, the

oscillating electromagnetic field exerts a polarizing force on all the electron

comprising the medium. The induced oscillation of the charges in the medium is

propotional to the electric field of the light. This means that the response is linear. The

nonlinear response in light can be observed only with a very strong source of light

like laser. It is now possible to generate harmonics of light frequencies:

,E.εD→→→

= ……. (1.1)

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Where →

ε is the permittivity tensor and →

E is the electric field. It is useful

to write the permittivity tensor

,εεε 0

→→

= ……. (1.2)

ε0 is the permittivity of free space and εr is called relative permittivity. The dielectric

displacement can also be written as

→→→

+= PEεD 0 ……. (1.3)

→

P is the electric polarization (electric displacement density). In general,

from the above equation, the electric polarization can be written as

→→→

−= E1).ε(εP r0

→→

= E .χ ε 0 ……. (1.4)

→→→

= E .χ/εP 0 ……. (1.5)

Where →

χ is called the electric susceptibility tensor.

In the case of crystalline media, →

P and →

E are not necessarily parallel [27].

The polarization must then be expressed as an expansion of the type

K+++=→→→→→→→→→→

EE.EχE.EχE.χ/εP(3)(2)(1)

0 ……. (1.6)

Where (3)(2)(1)

χ,χ,χ→→→

are linear, quadratic (2nd

– order nonlinear), and cubic

(3rd

– order nonlinear) susceptibility tensors, respectively. The expansion is often

written as the sum of two terms

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NLL

PPP→→→

+=

……. (1.7)

Where the linear polarization is

→→→

= E.χεP(1)

0 ……. (1.8)

The remainders is the nonlinear polarization and is given by

K++=→→→→→→→→

EE.EχεE.EχεP(3)

0

(2)

0

NL

……. (1.9)

If the electric field →

E is a sum of n monochromatic plane waves, i.e

( )

−

→→

∑=

→→

=

ωtri.ki

eωE(t)En

1i

1

……. (1.10)

Fourier transformation of P yields

( ) ( ) ( ) ( ) K+++=→→→→

ωPωPωPωP321

……. (1.11.a)

Where

( ) ( ) ( )ωE.ωχεωP(1)

0

(1) →→→

= ……. (1.11.b)

( ) ( ) ( ) ( )jiji

(2)

0

(2)

ωEωE:ωωωχεωP→→→→

+== ……. (1.11.c)

( ) ( ) ( ) ( ) ( )kjikji

(3)

0

(3)

ωEωEωE:ωωωωχεωP→→→→→

++== ……. (1.11.d)

and so on. Due to conservation of energy, the output frequency has to be the sum of

the input frequencies, where both positive and negative frequencies are allowed.

In many literatures, a standard notation is used to write Equation (1.11). For

example, the component i of (2)

P→

(ω) in equation (1.11.c) is written as

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( ) ω2

j

ω1

j213

(2)

ijk0

(ωω3 EEω,ω;ω-χEP =

Where the negative sign designates output frequency, and ω1 + ω2 = ω3. The

linear susceptibility tensor is responsible for refraction and absorption in a material,

while higher-order tensors are responsible for nonlinear (field intensity dependent)

refraction and absorption, as well as other nonlinear optical phenomena such as

harmonic generation, phase conjugation, and frequency mixing. For example, second

harmonic generation (SHG) arises from the term ( ) ω

k

ω

j

(2)

ijk0

2ω

i EEω,2ωωχεP −= while

third harmonic generation (THG) comes from the term

( ) ω

l

ω

k

ω

j

(3)

ijk0

3ω

i EEEω,3ωωχεP −=

All materials have nonzero third-order nonlinear susceptibility

→ (3)

χ and

exhibit third-order nonlinear effects. For example, DC Kerr and optical Kerr effects,

self focusing or self-defocusing, which leads to the self phase modulation, etc. For

Optical Kerr effect, the refractive index change is propotional to the optical intensity:

∆n = n21,

Where n2 is called the nonlinear optical Kerr index.

1.5 Nonlinear Optical Crystalline Materials

Coherent radiation at a few discrete frequencies can be produced by laser

devices as in solid-state lasers or with narrow range of tunability as in dye laser. Many

applications require frequencies that are not readily available from such laser sources.

The most effective way for converting a fundamental laser frequency to other

frequencies, either to higher or to lower frequencies, is harmonic generation or

parametric oscillation in a non centrosymmetric crystalline medium [28]. Now, after

18

years of research with NLO materials, it is possible to cover almost continuously the

range from 170 nm to 18 µm. As a result, further extension of applications to the

ultraviolet (UV) and far-infrared regions will be possible. However, materials

limitations are significantly slowing the development of required optical devices.

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

excellent optical quality. This means that for new materials, for which single crystal

specimens are not available, it is necessary to grow single crystal specimens of optical

quality. Thus in many cases the search for new and better nonlinear optical materials

is very largely a crystal growing effort. It is realized that the requirements on optical

quality for a useful nonlinear optical material are more stringent than even the most

exacting requirements on optical quality for materials used in linear optics. For a

device to succeed, it is vital that it meets a number of other criteria and these other

criteria should receive greater emphasis. The relevant issues include reliable crystal

growth techniques for availability, optical nonlinearity, birefringence, moderate to

high transparency and optical homogeneity for high conversion efficiency,

mechanical strength, chemical stability, polishing and coating technology for ease of

fabrication, low absorption, temperature phase matching band width, fracture

toughness, thermo-mechanical properties for high average power, damage threshold,

nonlinear absorption and brittleness index for lifetime and system capability.

KDP is an efficient angle-tuned dielectric medium for optical harmonic

generation in and near the visible region [29]. This material offers high transmission

through out the visible spectrum and meets the requirements for an optical

birefringence large enough to bracket its refractive index for even the extreme

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wavelength over which it is transparent. An additional advantage of KDP is its ability

to withstand repeated exposure to high power density laser radiation without inducing

strains and subsequent in homogeneities in the refractive index [30]. These

characteristics make KDP a desirable material for frequency doubling and mixing

experiments with many solid state and dye lasers with fundamental wavelengths

between 1060 and 525 nm.

KD2PO4 (DKDP) provides the same excellent conversion efficiency and

resistance to optical damage as KDP, but has the advantage of higher transmittance at

1060 nm. This characteristic is useful when harmonic generation is attempted with

high repetition rate, high average power lasers operating in the 1000 nm region.

The nonlinear effect observed in some crystals provides a means of obtaining

additional wavelengths from single frequency lasers. To generate an optical harmonic,

two conditions must be fulfilled. First, a nonlinear material must be selected such that

the necessary interaction between the incident electromagnetic wave and the material

occurs. Second, the crystal material must be oriented so that the laws of conservation

of energy and momentum are preserved. This condition can be achieved by matching

the velocities of the fundamental and second-harmonic waves propagating through the

crystal. The direction of propagation must be at an appropriate angle θ, with respect to

the crystal optic axis (angle tuning). Each combination of fundamental and second

harmonic requires a different polar angle to provide the necessary indices of

refraction. Alternatively changing the crystal temperature (temperature tuning) can, in

some materials achieve the same effect [31].

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1.6 Developments in NLO Materials

The emergence of new materials with superior quality is often responsible for

major advances in new technologies. New techniques applied to the fabrication of

ultra-pure silica glass that enabled the fabrication of fibers with ultra-low loss

provided the main stimulus to optical fiber communication. The recent emergence of

erbium doped glasses and the fabrication of fiber amplifies, another major milestone

in this area, enabled 50 gigabits per second transmission rates. Such high

amplification rates cannot be achieved with standard electronic amplifiers. The high

speed, high degree of parallelism of optics will lead gradually to optoelectronics

systems where an increasing number of functions will be implemented optically.

However, the development of photonic technology relies largely on the progress

achieved in fabricating new optical materials with better performance. In that respect,

materials with nonlinear optical (NLO) response are expected to play a major role in

enabling optoelectronic and photonic technologies.

1.6.1 Organic NLO crystals

The NLO properties of large organic molecules have been the subject of

extensive theoretical and experimental investigations during the past few decades and

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

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

hyperpolarizabilities compared to inorganic NLO materials. The low- temperature

solution growth technique is widely used for the growth of organic compounds to get

quality single crystals.

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The bulk size single crystals of L-alanine were grown by the slow evaporation

solution growth method at room temperature. Its perfection was evaluated by high-

resolution X-ray diffraction (HRXRD) analysis. The laser damage threshold was

measured and SHG behaviour was tested by a Q-switched Nd:YAG laser [32].

Glycinium oxalate (GOX) single crystals were grown by the slow cooling

solution growth method [33]. The hardness value was found to be higher than glycine.

The UV-Vis studies show that GOX crystals can be used for nonlinear applications.

The dielectric measurement indicates that the GOX crystals have domains of varying

sizes and varying relaxation time. The SHG output of GOX was 210 mV at given

pulse energy of 5 mJ/s and KDP was 240 mV.

Organic nonlinear optical crystal of guanidinium 4-aminobenzoate (GuAB)

has been grown by the slow evaporation solution growth technique. Optical properties

of the grown crystal have been studied by means of UV-Vis-NIR transmission and

absorption spectra in the wavelength range of 200 to 1000 nm. The refractive index

and band gap energy of the GuAB crystal are obtained as 1.68 and 3.73 eV

respectively. Mechanical hardness has been carried out on the grown crystal and the

material was found to be soft material category [34].

Single crystals of DL-alanine crystallizing in a non-centrosymmetric space

group were grown by the slow evaporation method [35]. It was thermally stable up to

280 ⁰C and optically transparent in the wavelength region of 220-1100 nm. The SHG

efficiency was found to be 1.7 times higher than that of standard KDP.

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L-asparagine thiourea monohydrate (LATM) single crystal has been grown.

From UV-Vis transmittance spectrum it was found that the material has wide optical

transparency in the entire visible region. The birefringence of the crystal in the visible

region was found to vary with the wavelength [36].

Organic nonlinear optical crystal of N-bromosuccinimide (NBS) was grown

by the slow cooling solution technique using methnol as the solvent [37]. UV-Vis

spectral studies reveal that it is transparent in the wavelength region 325-1100 nm.

The intrinsic defects could be understood from photoluminescence study. From SHG,

laser damage threshold and dielectric data, it is found to be an efficient material

compared to KDP crystal and the crystal is a good candidate for the NLO

applications.

L-valinium picrate (LVP) was grown by the slow evaporation method at room

temperature. The crystal is optically transparent in the wavelength range of

500-1000 nm, the band gap energy is found to be 2.24 eV, mechanically stable up to

50 gm and SHG efficiency is about 60 times greater than that of KDP. Owing to these

properties LVP could be a promising material for NLO applications [38].

L-alanine maleate (LALM) was synthesized and etching studies were carried

out using various etchants [39]. Mechanical behaviour was studied on {011}, and the

hardness values are found to be comparable with pure L-alanine. It is stable up to

162.2 ⁰C. From UV-Vis spectrum the lower cutoff was found to be as low as 320 nm

and from SHG data the crystal was found to be 1.2 times more NLO active than that

of KDP crystal.

23

Undoped and thiourea-doped γ-glycine salts were synthesized and single

crystals of the synthesized salts were grown at ambient temperature by slow

evaporation solution technique (SEST) [40]. The values of SHG efficiency, density,

microhardness, dielectric constant, dielectric loss, and decomposition point are

observed to be altered when γ-glycine crystals are doped with thiourea. The UV cutoff

wavelength is found to be less for thiourea doped γ-glycine crystal (compared to pure

γ-glycine crystal), suitable for NLO devices.

Crystals of benzophenone were grown rapidly by the low temperature solution

growth technique at room temperature. The structural, optical and qualitative NLO

efficiency properties were analysed [41]. The microhardness values were found to be

high. From the UV-Vis spectrum the sample of benzophenone was found to be 95 %

transparent and the fundamental groups were identified by the FTIR analysis.

1.6.2. Inorganic NLO crystals

The search for novel crystals with nonlinear optical properties is still a

challenge for scientists. To fulfill the ”molecular engineering” of nonlinear optical

crystals, two theoretical models suitable respectively for the studies of the absorption

edge and birefringence of a nonlinear optical crystal have been set up [42]. The

following parameters are critically important for an NLO crystal: (i) nonlinear optical

coefficients χijk (ii) birefringence, (iii) absorption edge on the UV side for the UV and

VU-Vis crystals, (iv) damage threshold, (v) optical homogeneity and (vi) physic-

chemical stability and mechanical properties. As a useful ultraviolet (UV) NLO

material, K[B5O6(OH)4].2H2O(KB5) is the first NLO crystal discovered in the series

of borates [43]. After that various borate crystals including (β-BaB2O4) (BBO),

24

LiB3O5(LBO), Sr2B2Be2O7 (SBBO), BiB3O6(BiBo) and the latest Ca4LnO(BO3)3

(CLnOB, where Ln = Gd, La, Y) have been studied as promising NLO crystals. The

family of various borate crystals thus plays a very important role in the field of

nonlinear optics [44].

The dependence of tapering angle θ and micromorphology of tapered faces of

KDP on the concentration of Fe3+

and Cr3+

impurities at various supersaturations have

been reported [45]. The second order nonlinear optical (NLO) properties of doped

lithium niobate (LN) crystals (abbreviated as M:LN, where M=Mg2+

, Zn2+

and In3+

respectively). It was observed that the second order NLO response of doped LN

crystals decreases with increasing dopant concentration in the crystal [46].

Ga and Ce doped KTP (potassium titanyl phosphate) crystals were grown by

flux method. KTP has wide applications as waveguides, electro-optical and periodic

poling structures. In this, KTP crystals should possess low conductivity. By doping

the KTP with Ga or Ce, it was found that the conductivity of KTP crystals is reduced

[47].

A high quality cesium lithium borate (CLBO) crystal was obtained with

dimensions of 146×132×18 mm3

by the Kyropoulos method [48]. Centimeter-sized

single crystals of TI3PbBr5 were grown using Bridgman-Stockbarger method. This

compound undergoes phase transition at 237 ⁰C [49]. The spectroscopic properties

and second harmonic generation tests suggest that it is a potential material for middle

infrared nonlinear optics. Enhancement of crystalline perfection by organic dopants in

ZTS, ADP and KHP crystals were investigated using HRXRD and SEM [50].

25

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

of KDP crystal. The absorption edge of the crystal was found at 360 nm [51]. The

structures of the non-centrosymmetric borate chlorides Ba2TB4O9Cl (T=Al, Ga) have

been determined [52]. The second harmonic generation (SHG) efficiency (deff) for a

powder sample of Ba2GaB4O9Cl was found to be 0.95 relative to a KH2PO4 standard.

K5Nd(MoO4)4 crystals with different Yb3+

concentrations were grown using

Czochralski technique [53]. Room temperature absorption spectra were recorded and

assigned on the basis of Dieke’s diagram for Nd3+

ion; the standard Judd-Ofelt theory

has been used to analyse the spectra. Increase of Yb3+

concentration leads to variation

of the corresponding Judd-Ofelt intensity parameters. Significant contribution of the

Yb-Nd energy transfer into the formation of the Nd3+

absorption spectra causes the

observed changes. After the illumination of the crystals with CW Nd:YAG laser

changes show good correlation with the content of Yb ions.

The incorporation of Nd2+

ions into ADP crystals produces some stress and

results in very low angle grain boundaries. Addition of Ni2+

increases the thermal

stability of ADP. The dielectric measurements revealed that ADP-1% Ni crystal is

comparatively better than pure ADP for electro-optic modulation, second harmonic

generation (SHG), microelectronic industries because the dielectric constant of grown

ADP-1% Ni crystal at higher frequency is lower than the pure ADP [54].

Crystals of new compound, diammoniun tetrachloromanganate (II)

monohydrate, were grown by slow evaporation solution growth method at room

temperature [55] and are characterized through thermogravimetric, low temperature

differential scanning calorimetric methods and Fourier transform infrared

26

spectroscopy. The elemental analysis and the thermal studies confirm the

stoichiometry of the compound. The thermal anomalies observed in differential

scanning calorimetric curve at -9.8 ⁰C and -20.4 ⁰C in the cooling cycle indicate a

first order transition. The phase transition is attributed to the gradual ordering of NH4+

and MnCl42-

ions at low temperatures. The infrared spectrum of the compound

characterizes the various chemical bonding and water molecules in the compound.

RbNd(WO4)2 single crystals with different concentrations of Yb3+

have been

grown using the top seeded solution growth. Room-temperature absorption spectra

were assigned on the basis of the Diek’s diagram for Nd3+

ion and analysed by means

of the standard Judd-ofelt theory. It was shown that the increase of the Yb3+

concentration and corresponding decrease of the Nd3+

concentration leads to the

increase of the corresponding Judd-Ofelt intensity parameters, which suggests

significant contribution of the Yb-Nd energy transfer in the formation of the

absorption spectra [56].

Crystals of LaPO4 and LaPO4 phosphor doped with Eu rare-earth ions were

grown using solid state synthesis method [57]. The phase purity has been verified by

XRD, SEM, EDAX and FTIR spectral analyses. The XRD data indicate that the peak

positions do not change with the substitution of La by Eu into monazite type LaPO4

lattice. The PL intensity is very high therefore LaPO4: Eu phosphors can be easily

applied in various types of lamp and display.

Pure and allyl thiourea doped KDP crystals were grown from the solution by

employing slow evaporation of the solvent. Increasing the doping levels in the KDP

crystal decreases the values of electrical parameters, viz. εr, tan δ and σac which in turn

27

improve the optical transparency. Thus these crystals are useful for photonic and

electro-optic device fabrication [58].

Morenosite added with glycine crystals were grown by the free evaporation

method and were found to be optically transparent in the wavelength range

210-1100 nm, NLO active, mechanically soft and exhibit normal dielectric behavior.

The conductivity was found to be due to proton transport and the doping morenosite

with glycine resulted in the discovery of promising NLO active and low-εr value

dielectric materials [59].

1.6.3. Semiorganic NLO crystals

The search for new frequency conversion materials over the past decades has

led to the discovery of many organic NLO materials with high nonlinear

susceptibilities. The approach of combining the high nonlinear optical coefficients of

the organic molecules with excellent physical properties of the inorganics has been

found to be overwhelmingly successful in the recent past. 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 [60,

61, 62].

High quality bulk single crystals of novel NLO semiorganic crystals L-

arginine tetrafluoroborate (L-AFB) and L-histidine tetrafluoroborate (L-HFB)

measuring 78x50x35 mm3 have been grown by temperature lowering methods. The

useful transmission range of (L-AFB) extends from 198 to 900 nm, which makes it

valuable for applications that require blue-green light [63].

28

Tris(thiourea)zinc sulphate was synthesized and the growth rate along the

(100) plane is higher at pH = 4.17 than at pH=3.8 [64]. The UV-Vis spectrum shows

that it has a good optical transmittance in the entire visible region and it is a potential

candidate for optoelectronics. The birefringence value was found to be higher than

KDP. The material has a good thermal and mechanical stabilities.

The L-lysine monohydrochloride dihydrate crystals were grown by the slow

evaporation solution growth technique and it was found to be NLO material having a

short cut-off wavelength within UV region [65].

The growth and characterization of a new nonlinear metal-organic crystal,

potassium thiourea chloride (PTC), reported to have a good optical transmission in the

entire visible region, which is an essential requirement for a nonlinear crystal [66].

Rubidium bis-DL-malato borate (RBMB) was synthesized [67], TG-DTA

studies reveal that the material starts melting at 230 ⁰C thereby withstand the high

temperatures encountered in laser experiments, the lower cutoff is found to be as low

as 230 nm, allowing for frequency conversion down to UV-region. SHG emission was

confirmed by modified Kurtz and Perry powder method.

Single crystals of L- alanine sodium nitrate (LASN) crystals were grown by

the slow evaporation solution growth method from aqueous solution. Optical

assessment shows that it has a large transmission window, and it may be used for

frequency doubling and other NLO applications. The powder SHG efficiency of

LASN single crystals is 2 times greater than that of KDP. LASN crystals have high

damage threshold values [68].

29

L-proline cadmium chloride monohydrate single crystals were grown from

aqueous solution at room temperature [69]. UV-cutoff wavelength of 235 nm

indicates that this material is a potential candidate for generating blue-violet light

using a diode laser. The SHG efficiency is found to be superior to KDP crystals. It is

thermally stable upto 200 ⁰C and a moderately softer substance. Mechanical strength

is required if the crystal is to be used in devices.

L-alanine alaninium nitrate (LAAN), single crystals were grown by the slow

evaporation growth technique at room temperature. The TG-DTA studies establish

that the compound undergoes no phase transition and is stable upto its melting point

(i.e.) 149 ⁰C. Further it is found to be an NLO material having a short cut-off

wavelength within the UV region [70].

The growth and characterization of nickel mercury thiocyanate [71] have been

reported. It has good relative second harmonic generation efficiency and it confirms

the nonlinear optical property of the crystal.

Pure and allyl thiourea doped KDP crystals were grown from the solution by

employing slow evaporation of the solvent. Increasing the doping levels in the KDP

crystal decreases the values of electrical parameters, viz. εr, tan δ and σac which in turn

improves the optical transparency. Thus these crystals are useful for photonic and

electro-optic device fabrication [72].

A potential semi organic material tri-allylthiourea cadmium chloride (ATCC)

was synthesized by the slow evaporation technique. Thermal analysis reveals that the

ATCC crystal is stable upto 200 ⁰C. Mechanical strength was obtained and SEM and

30

AFM have been employed to investigate the surface and growth morphology of the

grown crystal [73].

Bulk single crystals of pure, L-arginine and glycine doped ammonium

dihydrogen orthophosphate (ADP) single crystals were grown by the conventional

and SR methods [74]. The zone width decreases as the temperature increases in the

case of both pure and doped crystals. TG investigation indicated that the grown

crystals are stable upto 200 ⁰C . The AC conductivity increased with frequency, and a

reverse trend was observed for the AC resistivity. Lower dielectric loss was observed

for the doped ADP crystal grown by SR method than that grown by conventional

found to have good crystalline perfection and low density of defects.

1.6.4. L-arginine derivative crystals

Organic nonlinear materials gain importance over ionic materials because of

their large polarizability and wide transmission window. L-arginine acetate (LAA) is

one of the new organic nonlinear optical crystals with relatively better nonlinear

properties than KDP. It crystallizes in a monoclinic structure with space group P21.

Because of its superior properties, LAA is expected to replace KDP, especially in the

laser fusion experiments. Influence of dopants on the growth and properties of LAA

has been investigated by several researchers. The present research work is on pure and

acids (one inorganic and two organic) doped LAA single crystals.

An interesting class of materials receiving wider attention in recent past

includes, the analogs of aminoacids like L-arginine, L-histidine, L-alanine, etc.

Among organic crystals of NLO applications, amino acids display specific features of

interest [75] such as (i) molecular chirality, which secures acentric crystallographic

31

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 favour crystal hardness. Further, amino acids can be used as the base

for synthesizing organic-inorganic compounds like L-arginine phosphate and

derivatives.

L-arginine phosphate monohydrate, (H2N2)+ CNH(CH2)3 H(NH3)

+COO

-

H2PO4-.H20, abbreviated as LAP is a nonlinear optical (NLO) material first introduced

by Chinese material scientists in 1983 [76]. LAP crystals are usually grown from

aqueous solution by the temperature lowering technique. LAP crystals caught the

attention of many researchers because of their high nonlinearity, wide transmission

range (220-1950 nm), high conversion efficiency (=38.9 %) and high damage

threshold [77,78]. It has been reported [79] that the synthesis and growth of

deuterated LAP (DLAP) crystals, and experiments of higher harmonic generation.

L-arginine dihydrogen phosphate (LADP), another analog of LAP was grown

by slow solvent evaporation technique. Owing to its good transparency, chemical

stability, dipolar strength, L-arginine diphosphate seems to be a promising material

for NLO applications [80].

Mixed crystals of LAHCl and LAHBr were grown and estimated the damage

threshold of LAHCl, LAHBr and LAHClBr as about 27.72, 16.37 and 29.84 GW/cm2

at 1064 nm respectively [81].

Optical, mechanical and thermal studies of nonlinear optical crystal L-arginine

acetate (LAA) were presented [82]. LAA had a wide optical transmission window

32

between 220 and 1500 nm. SHG efficiency is comparable with that of KDP.

Hardeness of LAA is anisotropic in nature. Young’smodulus were determined from

microhardness measurement.

Single crystals of pure and Cu2+

and Mg2+

doped L-arginine acetate (LAA)

were grown by Gulam Mohamed et al [83] using the slow evaporation method. It is

observed that both Cu2+

and Mg2+

dopants have increased the percentage of

transmission in LAA. Investigation on the nucleation studies of L-arginine acetate

single crystals was also reported [84].

L-arginine trifluoroacetate (LATF) was grown from the aqueous solution by

employing the temperature lowering method. It was reported that the optical damage

threshold of LATF at 1064 nm is higher than that of LAP and KDP [85].

L-arginine acetate (LAA) single crystals were grown by employing the low

temperature solution growth technique. LAA has its lower UV cut-off wavelength at

240 nm and hence it is suitable for frequency conversion applications [86].

The stability of saturated LAP solution as a function of supercooling rate by

observing the metastable zone width at different cooling rates using a polythermal

method. Crystal growth kinetics has been investigated as a function of supersaturation

[87]. Single crystals of L-arginine maleate were grown by the slow evaporation of the

saturated aqueous solution at 30 ⁰C. The UV-Vis-NIR transmission spectrum shows

that L-arginine maleate has lower cut-off at 300 nm [88]. Studies on the growth and

characterization of L-argininium formate (LAF) single crystals have been carried

out [89].

33

L-arginine trifluroacetate (LATF) single crystals were grown by the slow

evaporation technique along with determining nucleation parameters, solubility and

metastable zonewidth. With promising structural, optical and thermal properties of

LATF, this potential crystal can be used for NLO device applications [90]. Nucleation

growth mechanism and defects of nonlinear optical crystals of L-arg. CF3COOH have

been studied. AFM study demonstrated that the crystal surface grows by 2D

nucleation growth mechanism. Two dimensional nuclei frequently appear at the wider

step terraces. Trigonal deep pits are probably formed during the process of the steps

surrounding the impurities. Hollow cavities on the large slopes of the hillocks have

quadrate shape and they can fetch in extra stress which will affect the arrangement of

the lattice and further lead to structural [91].

L-arginine bis(trifluroacetate) crystals were grown by the temperature

lowering method from aqueous solution and morphology investigation reveals that the

crystal is a thin rhombohedron composed of quadragled and triangle faces. These

crystals possess a relatively large specific heat, and its thermal expansion co-efficients

are anisotropic. All the foregoing results suggest that LABTF crystal is a potential

candidate for NLO materials [92].

Single crystals of L-arginine acetate (LAA) were grown by the low

temperature solution growth technique [93]. The induction periods were measured at

various supersaturations and hence the interfacial energies were calculated. The

experimentally evaluated values of interfacial energies are found to be in good

agreement with theoretically predicated values.

34

LAA and LAO single crystals were grown by the slow cooling method from

aqueous solutions [94]. The dielectric parameters εr , tanδ and σac increase with

increase in temperature. Also it indicates that εr and tanδ values decrease whereas the

σac value increase with increase in frequency along both a-and c-directions and at all

temperatures. These results indicate that LAA and LAO are not only potential NLO

materials but also promising low εr value dielectric materials, expected to be useful in

the microelectronics industry.

L-arginine iodate single crystals were grown by the temperature lowering

method and also by the slow evaporation method at a constant temperature (30 ⁰C)

from its aqueous solution at pH value of 6. Among water, water-methonal, water-

ethonal and water-acetone, the solubility was found to be highest in water [95]. The

grown crystals were characterized by density measurement, X-ray powder diffraction

studies, UV-Vis spectral analysis laser induced damage threshold studies and

nonlinear optical study.

L-arginine hydrochloride monohydrate was synthesized and optical properties

were reported. The Z-scan measurement with 632.8 nm laser pulses revealed that

nonlinear refractive index of the crystal is in the range of 10-7

cm2/W. The measured

3rd

order nonlinear properties confirm its suitability for nonlinear optical devices such

as optical limiting and optical switching [96]. The dipole moment (µ), linear

polarizability ( )α , and first hyperpolarizability (βtot) of the asymmetric unit of

L-arginine phosphate (LAP) monohydrate crystal are investigated using the

supermolecule approach in combination with an interactive electrostatic polarization

scheme. The results suggest that the role of the crystal environment is to minimize the

35

effect of the intermolecular interactions in the electric properties. That is, µ and βtot

gain a more additive character in the presence of the field of the embedding charges.

This is specially marked for βtot [97].

Single crystals of nonlinear optical material L-arginine acetate (LAA) ,

spacegroup P21, were successfully grown for the first time by the temperature –

lowering method and also by the slow evaporation method at constant temperature

(30 ⁰C) from its aqueous solution with pH at 6 and dimension 21x15x3 mm3. Initially,

solubility tests were carried out for four solvents such as water and methanol, water

and ethanol, and water and acetone. Among the four solvents, the solubility of LAA

was found to be the highest in water, so crystallization of LAA was done from its

aqueous solution. Morphological analysis reveals that the crystal is a polyhedron with

16 developed faces with major face forms {100}, {001}, and {102} (pinacoids)

parallel to the polar axis. The grown crystals were characterized by chemical analysis,

density measurement, and X-ray powder diffraction studies. Infrared spectroscopy,

thermogravimetric analysis, and differential thermal analysis measurements were

performed to study the molecular vibration and thermal behaviour of LAA crystals.

Thermal analysis does not show any structural phase transition [98].

The crystal structure of L-arginine dinitrate, was undertaken to study

conformational aspects [99]. In the L-argininium dinitrate, the diprotonated

argininium molecule is linked by a strong O-H….O[2.653(7)Ǻ] hydrogen bond to the

nitrate anion. The single- bonded O atom of the carboxyl group exhibits a very

unusual cis conformation with respect to the α-amino N atom. Chelated three-centered

hydrogen bonds are observed in the case of the Nα

and Nє atoms with the nitrate

36

anions. The argininium molecules are connected by type A, B and D interactions

through nitrate anions.

A novel organic crystal, L-arginine 4-nitrophenolate 4-nitrophenol dihydrate

(LAAPP), synthesized and grown from aqueous solution. X-ray single crystal

diffraction shows that LAPP belongs to the monoclinic crystallographic system with

space group P21. FTIR and UV/Vis/NIR transmission spectra have been employed to

characterize the crystal. The computational calculation based on the density functional

theory at the B3LYP/6-31G (d,p) level has been used to compute the first-order

hyperpolarizability of LAPP relating to different molecular models. The morpholoy,

nonlinear characteristic and thermal stability of the crystal have also been investigated

[100].

Single crystals of LAA were grown by slow evaporation technique. Single

crystal XRD analysis confirmed that the crystals belong to monoclinic system with

space group P21. Fundamental parameters like plasma energy, Penn gap, Fermi

energy and electronic polirizability of the crystal have been calculated. The band gap

energy for the grown crystals are found to be 3.75 eV. The optical investigations show

a high value of both the extinction coefficient (K) and refractive index (n) indicating

high transparency of the crystal which confirms its suitability for optical switch

device fabrications. The frequency dependence of dielectric constant decreases with

increasing frequency at different temperatures [101].

The SHG efficiency of both the pure and Nd3+

doped LAA was found to be

higher than that of KDP. The thermal studies of pure LAA reveal that the

decomposition of pure LAA starts at 232.9 ⁰C. The role of dopant in the pure LAA

37

has marginal influence on the thermal properties of LAA. It can be noted that the

hardness of the crystal decreases with increasing load for both pure and doped

samples. The dielectric studies reveal that the low value of dielectric constant/

dielectric loss of the crystal at high frequency region. The photoconductivity studies

of both pure and doped LAA confirm the positive photoconductivity nature of the

sample [102].

Bulk crystals of LATF have been grown by slow-cooling technique. The

crystal is a polyhedron with nine developed facets, and the {101} facet is the most

prominent one. The dielectric constant decreases with the increasing frequency but

attains the saturation for frequencies larger than 100 kHz. The specific heat changes

little in the measured temperature range of 300.02-350.02 K. The thermomechanical

analysis shows that the crystal has lower expansion coefficients when compared to

many other NLO materials. The refractive indices measurements reveal that the

crystal has large values of refractive index and birefringence and is phase- matchable.

Apart from that, the crystal possesses a relative high optical damage threshold. Hence,

the aforesaid results make LATF crystal a good candidate for the NLO applications

[103].

Quality single crystals of pure and Cu2+

and Mg2+

doped LAA were grown by

slow evaporation technique. From UV-VIS-NIR studies these crystals possess

minimum absorption in the entire visible region. The doped one have lower cut off

wavelength. NLO studies reveal that doped crystals have increased efficiency [104].

For Cu2+

and Mg2+

doped LAA crystals show similar features as that of pure LAA but

38

there is a distinct shift in the decomposition temperature and this indicates the thermal

stability of these crystals.

L-arginine fluoride (LAF) is one of the potential semi-organic materials for

nonlinear optical applications. The range and percentage of transmission were found.

The decomposition temperature and weight loss of LAF during heating were

estimated [105].

Metal (Cu and Mg) doped single crystals of LADP were grown by slow

evaporation method in the period of 30-45 days. Owing to its good transparency,

chemical stability and dipolar strength, LADP crystal is a promising material for NLO

applications [106].

L-argininium dinitrate (LADN), a semiorganic nonlinear optical (NLO)

material have been successfully grown by slow evaporation technique. Good optical

quality single crystals with dimensions upto 28x1x1 mm3

were obtained. The optical

absorption spectrum shows that the absorption in LADN is nearly equal to zero in the

entire visible region. From the thermal studies the stability of the crystal is up to

130 ⁰C as the compound undergoes isomorphic transformation [107].

L-arginine diiodate with excellent transparency were grown with maximum

size of 20×10×10mm3 and the grown crystals were characterized by single crystal

XRD, FT-IR, FT-Raman, TGA-DTA, hardness study, and UV-vis- NIR studies. The

second harmonic generation (SHG) of the material was confirmed using Nd:YAG.

The L-arg. 2HIO3 has NLO efficiency 1.3 times higher than the KDP crystal. Laser

damage threshold studies revealed that the grown crystals possess high damage

39

threshold values and the crystal is a potential material for frequency conversion

applications [108].

Microhardness study of L-arginine hydrochlorobromomonohydrate

(LAHClBr) crystals was performed by Knoop and Vicker’s indentation methods on

the prismatic planes (100), (010) and (001). Both the values of Vickers and Knoop

microhardness showed that the cleavage plane (100) has the lowest value of hardness

number and as usual the lowest values of Young’s modulus. Young’s modulus

obtained from ultrasonic velocity measurement also supports the results of hardness

measurement. As a whole LAHClBr is a soft crystal. Hardness anisotropy of both first

order and second order is found to exist in this material and from the study of

orientation dependence of Knoop microhardness on (100) plane, (100),(010) is

identified as one of the slip system of this material [109].

The influence of mixed acids in the growth and characterized properties of a

new nonlinear optical material L-arginine formomaleate abbreviated as LAFM was

examined [110]. UV-Vis spectral study shows that LAFM is transparent down to 315

nm and its second harmonic generation efficiency is 1.2 times that of KDP.

Bulk single crystals of L-arginine tetrafluoroborate (L-AFB) a semiorganic

nonlinear optical material has been successfully grown from solution by the

temperature lowering method [111]. Large single crystals of L-AFB were grown with

dimensions 78x50x35 mm3 in eight weeks. Growth rate and effects of seed orientation

on morphologies of the crystals were studied. L-AFB crystals belong to a class of

organic-inorganic complexes in which the high optical nonlinearity of pure organic

compound is combined with the favourable mechanical and thermally stable

40

properties of an inorganic compound. Bulk single crystals of L-AFB are potential

materials for applications in blue-green wavelength region.

Optically good quality single crystal of L-argininium perchlorate (LARPCL) ,

a promising analog of LAP, was successfully grown by the slow solvent evaporation

technique at room temperature [112]. The moderate SHG efficiency, hardness value,

and encouraging dielectric properties of the crystal indicate the suitability of this

crystal for photonics device fabrication.

The optical transmission study of the lithium doped LAA crystal has good

optical transparency in the UV and visible region [113]. Studies on the rare earth

dopant lanthanam on LAA crystals confirmed that the crystals were nonlinear in

nature and metal substitution has enhanced the nonlinearity of the crystals. [114].

LAA crystals were developed and the effect of NaCl, KCl, glycine and urea,

added separately as impurities, on the electrical properties of the synthesized crystal

show that the organic impurities considered are able to reduce the electrical

parameters. In the case of NaCl and KCl, NaCl is able to increase while KCl is able to

decrease the electrical parameters even though the change is observed to be small. In

accordance with Miller rule, the lower value of dielectric constant is a suitable

parameter for the enhancement of second harmonic generation (SHG) coefficient. It is

already known [115] that LAA is promising low- εr value material. It is interesting to

note that the organic impurity addition leads to a reduction of dielectric constant for a

wide tempature range significantly and consequently leads to low εr value material,

which is gaining more importance nowadays in the microelectronics industry. Both

glycine and urea are found to be equally good in reducing the εr value. Oxygen

41

content of the impurity may be a considerable factor in choosing the impurity for

reducing the εr value.

1.7 Present Study

All the physical properties of crystals are governed by the nature of the atomic

arrangement within the crystal structure, and their chemical composition. The

physical properties can be directional or non-direction dependent. Optical properties

are an integral part of crystallography, because of their direct relation to the symmetry

and structure. Material scientists and device engineers need to know the degree of

perfection and purity of crystals to interpret structure dependent properties in order to

determine whether the material can be successfully employed in the equipments or

device fabrication.

It has been observed by many researchers that the undoped LAA single

crystals have some disadvantages over doped ones. In order to overcome the

disadvantages, variety of dopants such as organic and inorganic compounds have been

introduced in LAA crystals to achieve effective changes in the properties of the

crystals.

In accordance with Miller rule, the lower value of dielectric constant is a

suitable parameter for the enhancement of second harmonic generation (SHG)

coefficient. It is already known that LAA is promising low- εr value material. It is

interesting to note that the organic impurity addition leads to a reduction of dielectric

constant for a wide tempature range significantly and consequently leads to low εr

value material. which is gaining more importance nowadays in the microelectronics

industry. Both glycine and urea are found to be equally good in reducing the εr value.

42

Oxygen content of the impurity may be a considerable factor in choosing the impurity

for reducing the εr value [115].

The search and design of low εr value crystals are extremely important for

microelectronics industry. Organic and semiorganic NLO crystals formed with L-

arginine have been identified as potential candidates for replacing KDP in nonlinear

optical applications. Pure L-arginine acetate (LAA), pure L-arginine hydrochloride

(LAHCl), L-arginine oxalate, etc are promising NLO materials for device fabrication.

Considering the above, it can be understood that the acids namely formic acid,

hydrochloric acid, oxalic acid on addition to L-arginine acetate are also expected to

change the physical and chemical properties of LAA single crystals in a considerable

level. The features prompted a research programme to be carried out on the growth

and physical properties of pure and those acids added LAA single crystals.

The present investigation was aimed at:

1. Synthesizing the chosen materials for the growth of single crystals.

2. Identifying the crystal structure by single crystal and powder X-ray diffraction

analyses.

3. Chemically characterized by doing the CHNS and FTIR and EDAX analyses

on the grown crystals.

4. Optical studies are carried out by UV spectral analysis.

5. Characterizing the grown crystals by Kurtz powder NLO test.

6. Determining micro hardness values.

7. Measuring the dielectric constant, dielectric loss and AC conductivity of the

grown crystals.

43

We provide in this thesis a report of our present work. The thesis is divided

into five chapters with References, Resume of the Candidate and Appendixes are

cited at the end of the thesis. We have already introduced the topic which includes

crystals and crystal growth methods, NLO materials, aminoacid, and a review on

aminoacid NLO materials, in particular, L-arginine acetate (LAA) crystals. Chapter 2

gives a brief account of the growth of the samples crystals, the experimental setup of

the various instruments used for the analysis of structural, chemical composition and

the results and discussions of these characterizations are discussed in this chapter. The

third chapter provides the results and discussion of optical, thermal and mechanical

measurements made on the grown crystals. The instrumentation details of the analyses

are also given in this chapter. The fourth chapter provides the experimental methods

of electrical measurements; results and discussions of this characterization is also

discussed. Summary, conclusions and suggestions for the future work are dealt with in

the fifth chapter.