�
CHAPTER I
INTRODUCTION TO CRYSTAL GROWTH AND NONLINEAR OPTICS
1.1 INTRODUCTION
The three dimensional repetitive arrangement of atoms is called crystal. In single
crystals, the periodicity extends throughout the material. In poly crystalline substances, the
periodicity is interrupted at grain boundaries and does not extent throughout crystal (Mullin
.J.W 2001 et al). Hence, a large single crystal is more useful than a polycrystalline material
for studying the physical and chemical properties of solids. So, it is important to convert the
polycrystalline materials into single crystal by using various crystal growth techniques. Due
to modern society's demand for improved telecommunications and high speed data
processing, photonics -- the use of light to acquire, store, process, and transmit data -- has
become an active field of research. The design of devices that utilize photons instead of
electrons in the transmission of information has created a need for new materials with unique
optical properties. In order to meet this increasing demand of new material with good optical
properties, identifying and growth of new optical crystals with good crystalline nature has
become an interesting branch in crystal growth.
Furthermore, we need power full, long-lasting lasers in the blue and green region of
wavelengths for laser television and medical applications. The green lasers have many
advantages over red lasers. It has more than fifty times brighter than red laser and due to this
the green laser can be seen from miles away. Thus it can be used in advanced high-tech
weapons for aiming purposes. Moreover, the green laser pointers have a shorter wavelength
(532 nm) than the red laser (650 nm) and the green laser beam can be seen under dark
�
conditions unlike the red laser which can be seen only on the landing surfaces. Due to this
fact, the green lasers are highly useful for astronomers as sky pointers. Green has the further
advantage that the human eye is most sensitive to green light, due to the fact that we (and
other animals) have more green-detecting cones in our retinas than any other color.
Most of the commercially available green lasers are based on the diode pumped solid
state frequency- doubled (DPSSFD) laser technology. For the past several decades,
researchers and several industries were trying to develop the laser diodes based on the
compound semiconductors such as Gallium nitride (GaN) and Indium Gallium nitride
(InGaN) especially in the blue and green region of wavelengths (Nakamura et al 1996).
However, it is very difficult to grow bulk crystal of these materials with reasonable quality.
Moreover, for the preparation of epitaxial thin films of these materials on substrates, we need
highly sophisticated and expensive techniques like molecular beam epitaxy (MBE) and metal
organic chemical vapour deposition (MOCVD). Apart from the growth aspects, the relatively
low power and limited wavelength range restricts their use in important applications.
Therefore, laser sources based on SHG is the better choice for the applications requiring
higher powers or longer wavelengths (> 400 nm). As a consequence, the green laser
technology is still depends on the nonlinear optical phenomena such as frequency doubling.
In the DPSSFD lasers, a NLO crystal must be placed to halves the wavelength of the
solid state laser. In the today’s market, inorganic NLO crystals of Potassium Titanyl
Phosphate (KTP), Lithium triborate (LBO) are used as frequency doublers. For example, the
KTP crystal is used to generate green laser at 532 nm by halving the wavelength of Nd:YAG
laser of 1064 nm. Moreover, other inorganic crystals like KDP, ADP, and LiNbO3, are also the
�
commercially available nonlinear optical materials increasingly being used for frequency
doubling of Nd: YAG lasers to generate green and blue lasers.
The organic materials are superior to inorganic materials both in the speed of
response and in the magnitude of the third-order effect. Hence, for new organic NLO
materials for which single crystal specimen are not available, it is necessary to grow single
crystal specimens of high optical quality. Most widely encountered organic crystals for this
type of application are urea, POM, MHBA, NMBA and recently DAST etc (Chemla et al
1987). Due to the technological importance of these organic nonlinear optical crystals, the
need for high quality organic crystal has grown dramatically in the last decades (Zyss et al
1985). Also, large size single crystals are very much essential for device fabrication (Brice
1986). There are various methods are available for the growth of organic single crystals.
Generally the organic crystals have been grown by solution growth like slow cooling and
slow evaporation techniques and melt growth.
1.2 CHOOSING A METHOD FOR CRYSTAL GROWTH
The procedure and condition for growing single crystals can be selected on the basis
of the physical and chemical characteristics of the crystallizing substance. If the
physicochemical processes involved in crystallization are taken into account, the optimum
condition can be established. These conditions pertain to the phase composition of the
feedstock, its chemical purity and form (powder, pellets, ingot), nature of the crystallization
atmosphere, the material and shape of the container, the growth rate, the temperature
gradients and shape of the crystallization front, the degree of stabilization of the growing
�
condition, and the way in which crystallization begins (spontaneous nucleation or
crystallization on a seed) (Chernov 1984).
The selection of the technique may be made on the basis of growth kinetics and
requirements, such as size, shape and purity even though more than one technique can be
employed for growing single crystals of a given material. Crystal growth involves phase
transformation to solid phase from supersaturated mother phase. Diffusion of growth units
occurring at the growth site and they orderly arranged in the lattice. In the following sections,
various important techniques of crystal growth are discussed.
1.3 METHODS OF CRYSTAL GROWTH
The crystallization process of solids, liquids and vapor undergo phase transformation
into solid form. The growth techniques are classified on the basis of phase transformations.
Solid to Solid Phase - Growth from Solid
Liquid to Solid Phase - Growth from Liquid (melt/solution)
Vapour to Solid Phase - Growth from Vapour
In general, crystal growth is the conversion of a polycrystalline piece of material into a single
crystal by causing the grain boundaries to be swept through and pushed out the crystal
(Buckley 1951, Mullin 1976). Crystal growths from liquid falls into four categories namely
melt growth, flux growth, hydrothermal and low temperature solution growth. There are
number of growth techniques in each category.
�
1.3.1 Growth from melt
Crystal growth from the melt is the fastest among the growth methods as its rate does
not depend on the mass transport processes. Melt growth can be applied to solids which can
be melted and crystallized by the change in phase from liquid to solid. In this method, apart
from possible contamination from crucible material and the surrounding atmosphere, no
impurities are introduced into the growing crystal.
The methods of growing single crystal from the melt includes Kyropoulos, Verneuil,
Bridgman – Stockbarger, floating zone, Czochralski and zone melting methods. Among these
methods Bridgman-Stockbarger and Czochralski methods are widely used for variety of
materials Viz., from oxide to semiconductors and organic to inorganic materials.
1.3.1.1 Bridgman – Stockbarger technique
The essential feature of this method is the steady motion of a freezing solid-liquid
interface along an ingot which is mounted either horizontally or vertically. Either the whole
charge is melted initially called normal freezing or a molten zone is established namely zone
melting. The motion of the interface can be achieved in two ways. One can traverse a muffle
furnace over the charge or the charge through the furnace. In this method, the temperature
gradient of the furnace plays an important role in getting single crystal. The furnace
normally consists of two zones, the upper zone held at a temperature, slightly above the
melting point of the material to be grown (hot zone) and the lower zone at a temperature just
below the melting point (cold zone). The crucible is made of quartz (or) glass and has a
pointed lower end. This is filled with the material to be grown and the crucible is lowered
very slowly. The shape of the container material plays a major role in getting single
�
nucleation. The formation of just one nucleus is more probable if the super cooled volume
(Hurle et al 1967) is very small. Thus, traditionally crucibles have tapered tips. As the
crucible is lowered, the first nucleated seed grown as more melt cools below its melting
point. The small crystals appeared that may undergo geometric selection and one crystal
remains, which increases in size until it fully occupies the cross section of the container.
With an appropriate shape of crucible, one can grow a single crystal with controlled
orientation.
1.3.1.2 Czochralski pulling technique
Growth of single crystal from the melt using the crystal pulling process named after
its inventor J.Czochralski. In this technique, the material to be grown is taken in a vertical
crucible and placed in a resistively heated furnace. It is melted by heating above the melting
point of the charge material. By keeping the top surface of the melt is just barely above the
melting temperature, the melt is contacted with a small seed crystal of specified orientation.
Following the thermal equilibrium, the seed crystal, which is attached to the pull rod is
slowly withdrawn, usually that rod is called as seed rod. As the heat from the melt flows up
to the seed, the melt surface cools and the crystals begins to grow. With proper adjustment
of the temperature, melt contact with seed is preserved during pulling. If the pulling rate is
higher than the growing rate of the crystal then the contact of the melt surface with seed will
be disconnected. If the growing rate is higher than the pulling rate, we can grow a crystal
with increasing diameter of the crystal. Hence, the diameter of the growing crystal can be
adjusted, by changing the temperature as well as pulling rate. A vast amount of research and
development works by many authors particularly those working in the field of electronic and
�
optic materials has developed the simple Czochralski pulling to a sophisticated technology
(Teal and Little (1950), Van Uitert et al (1961), Nassau and Broyer (1962)). Of many crystal
growth methods in use today, one method, which can produce crystals weighing from several
grams to many kilograms, is the crystal pulling technique.
The advantage of this method is that the crystal can be observed as it grows and
adjustment in temperature and pulling rate can be made whenever needed. Also, it has high
growth rate so large size crystals of semiconductors like Si, Ge, SiGe, GaAs, and InP are
widely grown by this method (Brice et al 1973a).
1.3.1.3 Heat transfer modes in crystal and melt (Solid – liquid interface)
An important role is played in this connection by the convective transport in the melt,
which may differ depending on the growing method i.e., Bridgman Stockbarger and
Czochralski method.
The melt crystallizes successively at the crystal-melt interface. The heat transfer in
the interface involves various growth variables such as diameter of the crystal, pulling rate,
crystal-melt interface shape, temperature gradient and its symmetry. But the crystal-melt
interface directly influences the crystal perfection and impurity distribution throughout cross
section (Feigelson et al 1980).
In this case, when the growth proceeds normal to the interface and stray crystallites
will grow in outward direction and does not harm the major portion of the crystal. On the
contrary, if the interface shape is concave to the liquid, stray crystals can grow in and harm
�
the growth of single crystal (Dutta et al 1994).The shape of the solid - liquid interface
become more concave if
(i) the growth rate is increased, or
(ii) the thickness of the crucible wall increased, or
(iii) the thermal conductivity of the crucible is decreased.
The temperature of the molten and homogenous source material is adjusted to the
slightly above the melting point. After thermal equilibrium is achieved, the seed crystal is
made in contact with the melt and withdrawn at a rate that gives a desired crystal diameter.
In case of Bridgman, the ampoule restricts the size of the crystal. As the melt solidifies and
the crystal is pulled, the latent heat of fusion is transferred to the crystal. The heat is
transported from the crystal-melt interface to the growing crystal. To control and maintain
the solidification rate depends on the supply of thermal energy to the melt, the removal of the
latent heat of solidification from the crystal and other associated heat losses from the system.
In the case of Bridgman method, heat is also lost from the crystal diameter which is
achieved by maintaining the solidification isotherm in a vertical position intersecting the
meniscus at the point where the isotherm becomes perpendicular to the melt surface. The
following parameter contributes to maintaining this condition.
The pulling rate Gp, the rate of melt level drown Gm, the heat fluxes gain and loss, the
crystal rotation rate Ws, and the crucible rotation rate Wc. By assuming the solid-liquid
interface as flat, no radial and axial temperature gradient in the melt then the maximum
pulling rate [Gp] max is given by
[Gp] max = - (Kc / (�H) ρc) α (dT / dZ)c – (dT/dZ)m (1.1)
�
where ‘Kc’ is the thermal conductivity of the crystal, ‘ρc’ is the crystal density, and
(dT/dZ)c and (dT/dZ)m are the temperature gradients in the crystal and the melt respectively
at the crystal-melt interface. The negative sign in equation (1.1) accounts for the fact that
dT/dZ is a negative quantity for the usual co-ordinate system in which ‘Z’ is zero at the
interface and increases positively along the crystal length.
In equation (1.1), [Gp]max depends only on the crystal temperature gradient at the
interface; however, the temperature gradient is a very complex function of puller geometry
and ambient conditions. In the case of Czochralski, crystal growth rates compared with the
theoretical one may result from the effect of temperature fluctuations in the melt that occur
near the crystal-melt interface. Subsequent decreases in temperature increases the
solidification rate, leading to an increase in the crystal ingot diameter. To maintain the
crystal diameter, the pulling rate at the instant must be increased.
As the diameter of the growing crystal is increased, the maximum pulling decreases
because the heat loss is proportional to the surface area of the crystal ingot, which increase
only linearly with the diameter. But, the heat gain is proportional to the volume being
crystallized, which increases as the square of the ingot radius.
1.3.2 Growth from solution
Crystal growth from solution is extremely popular in the production of many
technologically important crystals. The solution growth technique is mostly used to grow
good, transparent, nonlinear and ferroelectric crystals. Depending upon the solvent and
solubility, the solution growth method is classified into two groups as
�
(i) Low temperature solution growth
(ii) High temperature solution growth
1.3.2.1 Low temperature solution growth
In this method, solutions are prepared by dissolving a compound in solvents which
are in liquid state at ambient temperature. The growth of crystals by low temperature solution
growth involves weeks, months and sometimes years. Materials having moderate to high
solubility in temperature range, ambient to 100°C at atmospheric pressure can be grown by
low-temperature solution method. The mechanism of crystallization from solution is
governed, in addition to other factors, by the interaction of ions or molecules of the solute
and the solvent which is based on the solubility of substance on the thermodynamical
parameters of the process; temperature, pressure and solvent concentration (Chernov 1984).
Solubility of the material in a solvent decides the amount of the material, which is
available for the growth and hence defines the total size limit. If the solubility is too high, it
is difficult to grow bulk single crystals and too small a solubility restricts the size and growth
rate of the crystals. Solubility gradient is another important parameter, which dictates the
growth procedure. Neither a flat nor a steep solubility curve will enable the growth of bulk
crystals from solution; while the level of supersaturation could not be varied by reducing the
temperature in the former, even a small fluctuation in the temperature will affect the
supersaturation to growth of good quality bulk crystals in both cases. If the solubility
gradient is very small, slow evaporation of the solvent is the other option for crystal growth
to maintain the supersaturation in the solution.
�
Growth of crystals from solution is mainly a diffusion-controlled process; the
medium must be viscous to enable faster transference of the growth units from the mother
solution by diffusion. Hence, solvent with less viscosity is preferable. Supersaturation is an
important parameter for the solution growth process. The crystal grows by the accession of
the solute in the solution; as a degree of supersaturation is maintained. The solubility data at
various temperatures is essential to determine the level of supersaturation. Hence, the
solubility of the solute in the chosen solvent must be determined before starting the growth
process. Low temperature solution growth can be subdivided into the following methods.
a. Slow cooling method
b. Slow evaporation method
c. Temperature gradient method
Among these three low temperature solution growth methods, slow cooling and slow
evaporation methods are widely used for crystal growth of variety of material.
1.3.2.1a Slow Cooling method
Generally, the slow cooling method is used to grow bulk single crystals from
solution. In this method of growth, the supersaturation can be achieved by cooling the
temperature of the solution at low cooling rates using a temperature controller. The
crystallization process is carried out in such as way that the temperature dependence of the
solute concentration moves towards the metastable region along the saturation curve in the
direction of lower solubility. As the volume of the crystallizer and the solute concentration in
it are finite, the system needs systematic cooling to acquire supersaturation. The
supersaturation of the solution provides the necessary driving force to initiate the growth on
�
the surface of the suspended seed crystal in the solution. Moreover, in the slow cooling
method, growth proceeds in the seed crystal until the solution remains in the metastable
region. As a consequence, the width of the metastable region defines the size of the crystal to
be grown.
1.3.2.1b Slow Evaporation method
In this method of crystallization, the supersaturation of the solution can be achieved
by evaporating the solvent from the saturated solution. The continuous evaporation of
solvents causes the reduction of solution volume at constant temperature. Unlike to slow
cooling method, the spontaneous nucleation and growth can be observed in the solution
through the continuous evaporation of solvent at constant temperature. Generally this method
of growth can be employed to grow good quality seed crystals. The rate of evaporation of
solvents are mainly depends on the vapour pressure of the solvent. Moreover, the evaporation
of the solvent can be controlled by placing the crystallizers in a constant temperature bath
and thus the growth process can controlled up to some extent although the nucleation is
spontaneous.
1.3.2.2 High temperature solution growth
High temperature solution growth can be further classified into two major categories.
The first one is growth from single component systems and the second one is that from the
multi-components. In the single component method, only the chemical component forming
the crystal is present in the growth system, while in the multi-component method, more than
�
one component is added to the growth system. The primary reason for this addition is to
reduce the crystallization temperature.
This reduction in the crystallization temperature is necessary if the material to be
crystallized has an incongruent melting behavior, that is, materials which decompose before
melting so that crystallization from the melt results in some other phase. Also the reduction
in the crystallization is necessary for the materials which undergo a phase transformation
thus results in severe strain or even fracture. Also, for material which have a very high vapor
pressure at high temperature. The two main categories are hydro thermal growth and flux
growth.
1.4 PURIFICATION PROCESSES
Impurities in organic materials are of considerable importance, not only because of
their influence on the physical and chemical properties on the resulting crystals, but also they
can play a dominant role in controlling the crystal growth behavior. In the later case,
impurities can modify crystal morphology and growth rates as well as altering the stability of
growth through constitutional supercooling. Therefore, the most essential feature of growth
of high optical quality crystal is the material and the solution purification (Catesby 1972).
Purification of a material may be carried out by the following methods:
(i) Recrystallization
(ii) Sublimation
(iii) Melt phase zone refining
Recrystallization can be applied to the soluble materials, and the only real problem being the
choice of a suitable solvent phase from which the recrystallization takes place. The choice of
�
solvent is made such that at room temperature the material has little or no solubility, but at or
near the solvent boiling point the material is readily soluble. The recrystallization has to be
carried out two or three times to enhance the purity of the material. The solvent for such
recrystallization should be distilled to prevent contamination (Han et al 1989).
The technique described above uses the presence of a solvent phase to achieve the
desired purification. The presence of such solvent can lead to problems in the later stages of
purification or crystal growth. The removal of solvents is often most effectively achieved by
the use of vacuum sublimation. This process is additionally a highly efficient purification
technique, removing both highly volatile and non-volatile impurities from a material. The
technique, of course, depends on the material having a reasonable vapour pressure without
which the separation is both time consuming and inefficient (Mc Ardle et al 1974). The
solvent and volatile impurities can be trapped out and the non volatile impurities remain in
the experimental tube can be dropped out. This process can be repeated again to achieve
better purification.
The zone refining technique is a reliable and efficient technique for achieving ultra
pure material. The procedure is based on the principles of fractional crystallization, which
utilizes the difference in impurity concentration in the liquid and solid phases. The practical
aspects of zone refining are based on the automation of the repetitive passage of a zone (or
zones) of molten material from one end of a solid charge to the other. The resulting
distribution of impurities after zone refining depends on the density of the impurities. Due to
this density variation, the impurities will collect at the end portion of the zone purified
material. Hence, only the centre section of the ingot can be used for crystal growth. Zone
�
refining cannot be used for materials which are unstable in the melt or in the solid state near
the melting point.
1.5 INTRODUCTION TO NONLINEAR OPTICS
The beginning of the field of nonlinear optics is often taken to be the discovery of
second-harmonic generation in quartz crystal by Franken et al (1961), shortly after the
demonstration of the first working laser by Mainman (1960). Nonlinear optical phenomena
are “nonlinear” in the sense that they occur when the response of a material system to an
applied optical field depends in a nonlinear manner upon the strength of the optical field
(Boyd 1992).
Many of the materials used for photonics are non-linear optical (NLO) materials,
which mean that they interact with light in such a way that the light changes the properties of
the material, which, in turn, changes the properties of the light. The propagation of a wave
through a material produces changes in the spatial and temporal distribution of electrical
charges as electron and atoms react to the electromagnetic fields of the wave. The effect of
the forces exerted by the fields on the charged particles is a displacement of the valence
electron from their normal orbits. This change develops electric dipoles whose macroscopic
manifestation is the polarization (Laud 1991 & Gupta et al 1993).
In the case of conventional optics (i-e., linear optics), when a light beam interact with
nonlinear medium, the medium gets polarized and the relation between E and P is essentially
linear. Its polarizability P can be expressed as
P=�0 � E (1.2)
�
where the constant of proportionality � is known as the linear susceptibility, �0 is permittivity
of free space and E is the electric field strength of electromagnetic wave. However, when the
high intensity light beam like laser beam propagates through any dielectric medium, the
polarization no longer remains dependent linearly on E but also depends upon the higher
powers of E. If we include the higher order terms, then we write
P =�����χ �(1) E���χ �(2) E2
���χ(3) E3��������� (1.3)
This equation is also be written as
P = P (1) + P (2) + P (3) +…….. (1.4)
The relations are basically tensor relations but for simplicity we can use them in scalar form.
It is evident that higher order terms are important only for higher values of electric field
strength of light beam. The terms χ � (1),� χ � (2), χ� (3) are the second and third order nonlinear
susceptibilities respectively.
This can give rise to a number of interesting effects such as frequency conversion, in
which light of one color (frequency) is transformed into light of a different color upon
passing through the NLO material. NLO materials are used for making photonic devices such
as optical switches, optical memories, and logic gates.
One of the primary requirements for a nonlinear crystal is that it should have
excellent optical quality. The relevant properties for nonlinear optics are optical nonlinearity,
large birefringence, moderate to high transparency and optical homogeneity for high
conversion efficiency, high mechanical strength, chemical stability, polishing and coating
technology for ease of fabrication, high damage threshold, fracture toughness and thermo –
�
mechanical properties for high average power and reliable crystal growth techniques for
availability.
Some of the practical applications of nonlinear optical materials are Second
Harmonic Generation (SHG), Sum Frequency Generation (SFG), Difference Frequency
Generation (DFG), electro-optic modulation and Optical Parametric Oscillation (OPO).
However, typically no more than one of these generations will be present with any
appreciable intensity in the radiation generated by the nonlinear optical interaction. The
reason for this behavior is that the nonlinear polarization can efficiently produce an output
signal only if a certain phase-matching condition is satisfied and usually this condition
cannot be satisfied for more than one frequency component of the nonlinear polarization. As
mentioned in the beginning of this chapter, among these NLO properties, SHG or frequency
doubling is one of the significant processes which are highly useful to generate blue and
green laser by DPSSFD laser technology.
�
1.5.1 Second harmonic generation (SHG)
SHG has been a subject of much interest and study since Franken et al (1961) first
observed frequency doubling in quartz. SHG is the conversion of coherent light of frequency
ω into light of frequency 2ω. Frequency changing occurs due to the materials ability to
change its refractive index and thus altering the frequency of the light passing through it.
This phenomenon of frequency altering by a medium when an electric field is applied is
called the Pockels effect (Boyd 1992). As an example, the conversion of the commercial
infrared laser (wavelength of 1064 nm), to green light (wavelength of 532 nm), when passed
�
through one of these second order nonlinear media. Here a laser beam whose electric field
strength is represented as
E (t) = E e-i�t + c.c (1.5)
and this is incident upon a crystal for which the second–order susceptibility χ (2) is non-zero.
The nonlinear polarization created in such a crystal is given as
P (2) = χ (2) E2 (t) (1.6)
where χ (2) is the second order nonlinear susceptibility. When the higher order terms are zero,
the material is said to be linear. A nonlinear material has non-zero higher order
susceptibilities. The specific classes of nonlinear materials that are discussed in this thesis are
the second order nonlinear materials or the ones that have a non-zero χ (2). This can only be
obtained from nonlinear noncentrosymmetric materials.
1.5.2 Phase matching
Phase matching means that the wave generating the polarization and the generated
waves (the three interacting waves) are in phase over the interaction region, so the
microscopic contributions of the generated polarization of each individual dipole in the
crystal can interfere constructively, adding up to a macroscopic field. Only after this
constructive interference the nonlinear effect can be observed. To achieve phase-matching,
the phase velocity of the generated waves (while traveling through the crystal) should equal
the phase velocity of the pump wave in a parametric process. This can be achieved in a
birefringent crystal which has different indices of refraction along the different crystal axes.
In birefringnent crystals, waves with different wavelength can travel at the same speed (so in
�
phase) when their polarization directions are along different crystal axes. To fulfill phase
matching, the generated waves and the applied wave must have different polarizations to
control their propagation velocities.
Certain asymmetric crystals are birefringnent (doubly refracting) because through
them, light can travel at two different velocities, described as ordinary (o) and extra ordinary
(e). These velocities actually vary with propagation direction and polarization as well as with
wavelength. In certain direction the ordinary fundamental light travels at exactly the same
velocity as extra ordinary harmonic light. When this happens, the SHG is greatly enhanced
and the system is said to be phase matched.
Two types of phase matching are possible according to the polarization of the � and
2� beams. Type I phase matching (n°�= n e 2�) or vice versa arises from a wave of one type
(either ordinary or extra ordinary) and type II phase matching [(1/2 (n°2� + n e �) = n° 2� or n e
2�] arises from a combination of the ordinary and extra ordinary waves. If only one type of
(�) wave is present (as when the electric field is polarized along or perpendicular to the
dielectric axis) then only type I phase matching can occur.
1.5.3 Organic nonlinear optical materials
Nonlinear optical (NLO) crystals with high conversion efficiencies for second
harmonic generation and transparent in the visible and ultraviolet ranges are required for
numerous device applications. Within the last decade much progress has been made in the
development of these NLO materials having large nonlinear optical coefficient. Organic
materials are in increasing demand, as they are better candidates for NLO and electro-optic
device application than those of inorganic materials (Allen 1989).
�
The superiority of organic NLO materials results from their versatility and possibility
of tailoring them for a particular device application. Organic NLO materials have high
nonlinear figure-of-merit for frequency conversion, high laser damage threshold and fast
optical response time as compared with inorganic NLO materials (Stephens et al 2003).
However there are some drawbacks with organic NLO materials which are explained below.
A major obstacle to the development of many organic NLO materials and the exploitation of
their full potential has been the considerable difficulty associated with the growth of large
high quality single crystals of these materials (Halfpenny et al 1993). As with many other
organic solids, the intermolecular forces are comparatively weak, being predominantly van
der Waals or permanent dipole-dipole interactions. This typically results in low melting
points and relatively high vapour pressure. Mechanical properties are, in general, rather poor
with most organic solids being relatively soft. This can have important consequences for the
structural perfection of the crystals. Thermal instability is also common, with many organic
materials undergoing thermal decomposition at or below the melting point. These factors
together with the low thermal conductivities can pose substantial problems in crystal growth,
particularly from the melt.
These difficulties are the main challenges taken for this course of investigation. And,
attempts were made to subsidies or to some extent to eliminate these difficulties in growth
related problems, in the case of benzophenone and ethyl p-dimethylamino benzoate
(EDMAB) single crystals from various growth methods are explained in the following
chapters.
�
1.6 BENZOPHENONE
1.6.1 Material Introduction
Benzophenone (molecular formula: C13H10O) is one of the promising organic
nonlinear optical materials and it is a good alternate candidate for urea in all aspects (Wang
et al 2004b). Because in the field of nonlinear optics, urea is a well known organic material
for frequency conversion devices such as SHG and fifth harmonic generation and it was used
as the first organic optical parametric oscillator (Donaldson et al 1984). In spite of practical
application, urea has some undesirable properties such as mechanically soft and hygroscopic
in nature. As a result of hygroscopic nature urea cannot be exposed to normal atmosphere,
and for practical application, it is being used by immersing it in an index–matching fluid with
compatible chemical and optical characteristics (Rai et al 2002). From the literature, the
efforts made to resolve the problems associated with urea have not been very successful.
Benzophenone crystal is biaxial and it exhibits high (1.8 times of urea) second order
optical nonlinearity of urea with non hygroscopic in nature and mechanically hard. In
particular, the maximum value of effective nonlinear coefficient (deff) for type II phase
matching in the benzophenone crystal at a fundamental wavelength of 1064 nm is 2.195
pm/V and it is about 5.4 times than that of KDP (Urea: 3 times of KDP) (Wang et al 2004).
1.6.2 Structure and properties of benzophenone
Generally, molecules in which the -electron conjugated system was connected by a
carbonyl group appeared as good candidate materials for nonlinear optical applications. The
molecular structure of benzophenone is shown in the figure 1.1, where there are 24 atoms in
a single molecule. Benzophenone is an aromatic ketone which crystallizes in a stable
�
orthorhombic structure with non-centrosymmetric, space group P212121. The crystal structure
of the stable phase was first determined by Vul and Lobanova (1967). Single crystals of
benzophenone are optically active and it is the first organic molecular material to be
identified as polymorphic. It is a polar molecule and has a dipole moment of 2.98 Debye.
The unit cell contains four benzophenone molecules which has opposing dipole moment. The
unit cell dimension at room temperature is: a = 10.26 Å; b = 12.09 Å; c = 7.88 Å (Menard et
al 1973, & Roberts et al 1993). Numbers of works have been published on possible
correlations between the physical properties and the molecular packing geometry of single
crystals of benzophenone. Plastic deformation of the benzophenone was studied by
measuring the variation in the stress developed inside the crystal (Pethrick et al 1992).
Measurements of the dielectric and elastic properties of crystals of undefined defect structure
have been reported.
1.6.3 Literature on growth of benzophenone
To our knowledge only very few reports on the detailed growth kinetics of
benzophenone single crystals was available. Bleay et al (1978) have grown the
benzophenone single crystal by Czochralski pulling technique for the first time and analyzed
the grown-in dislocation by x-ray topography. In 1992, Tachibana et al have grown the
benzophenone single crystal by adopting the same experimental procedure as reported by
Bleay et al (1978), and calculated the burger vector using x-ray topography analysis. The
benzophenone single crystals were also grown in non aqueous solution by slow solvent
evaporation method and Bridgman method (Lewine 1976, Hooper et al 1980 & McArdle et
al 1987).
�
Figure 1.1 : Molecular structure of benzophenone
H2
H3
O
C1
C2
C3
C4
C5
C6
H4
H5
H6
C C'1
C'2C'3
C'4
C'5
C'6
H'2
H'3
H'4
H'5
H'6
�
In 1992, Pethrick et al have grown the benzophenone crystal from super cooled melt
and analyzed the morphology, influence of plastic deformation on ultrasonic velocity and the
dislocations before and after plastic deformation by employing x-ray topography. Also they
have reported a detailed dielectric studies on the melt grown benzophenone crystals. A
theoretical analysis on the morphology of benzophenone and their experimental verification
were done by Roberts et al (1993).
The nonlinear optical properties of benzophenone and its derivatives have been
studied by many researchers using powder SHG method (Cockerham et al 1991, Terao et al
1990, Genbo et al 1993, & Lammers et al 2000). Their results show that several of these
compounds generate SHG signals stronger than that of urea (Vander Venden et al 1979).
Terao et al (1990), have synthesized several di-substituted benzophenone derivatives by
Friedal-Crafts process and measured their second order nonlinear optical susceptibilities.
They found that the SHG activity of 4-methoxy -4-nitrobenzophenone could be changed by
recrystallization solvent and recrystallization rate. They suggested that the difference in SHG
activity was attributed to polymorphism, based on results of x-ray analysis and differential
scanning calorimetry.
The relationship between crystal structures and nonlinear optical properties of
benzophenone was reported by Genbo et al (1993). The photo conversion ability of
benzophenone into benzophinacol by visible light, was investigated by Davydora et al (2002)
using Raman spectra. A two parameter Sellmeier fit for benzophenone was derived
(Lammers et al 2000) and the nonlinear optical co-efficient for benzophenone was
determined by Maker Fringe technique.
�
Moreover, the derivatives of benzophenone are attracted by many researchers due to
their large NLO susceptibilities and short cut-off wave lengths of about 400nm (Frazier .C.C
et al 1987, Lal .R.B et al 1997 & Jiang .M et al 1999). According to literature, 4-amino
benzophenone (4-ABP) is one of the best benzophenone derivatives which exhibit the SHG
efficiency of 360 times of ADP and 260 times of KDP (Frazier .C.C et al 1987 & Jiang .M et
al 1999). Therefore single crystals of 4-ABP are promising for generating green and blue
laser beams from Nd: YAG or other semiconductor diode lasers. Further, a new NLO single-
crystal 4-4' DMBP is found to be a promising candidate for NLO applications. Although the
structure of the 4-4' DMBP was reported by Biserka Kojic-Prodic et al. in 1990, the growth
of high quality transparent bulk size crystals and their physical properties have not yet been
studied in detail except few short papers (Graham .D.J et al 1982 & Anandhababu .G et al
2008).
1.7 Ethyl p-dimethylamino benzoate
1.7.1 Material introduction
Ethyl p-dimethylamino benzoate (EDMAB) is generally known as tertiary amines
which are mainly used as a part of self curing two part system for dental/medical
compositions comprising degradable copolymers which are suitable for use as root canal
sealants, root canal filling materials, dental restorative materials, implant materials, bone
cements and pulp capping materials (Jia Weitao et al 2004). Moreover, since the chemical
structure of the material has the – electron conjugated system attached with electron donor
and acceptor groups at opposite end, it could be applicable for nonlinear optical applications
(Nalwa .H.S et al 1997). However, no literature was found on the crystal structure, growth
�
and NLO properties of EDMAB. Hence, this investigation primarily aims to grow bulk single
crystal of EDMAB and to study its nonlinear optical properties for the possible fabrication of
nonlinear optical devices (Kalyana Sundar .J.K et al 2010, Natarajan .V et al 2011a &
Natarajan .V et al 2011).
1.8 Objective of the thesis
The purpose of this work is to understand the growth behavior of novel organic NLO
crystals such as benzophenone, two of its derivatives and EDMAB. The main objectives of
the thesis are as follows:
1. The first objective of the thesis is to study the direction dependent properties of
benzophenone crystals such as growth rate, laser damage threshold and
mechanical properties. Because, NLO properties are orientation dependent and
thus it is one of the important targets for acquiring innovative functions with
organic materials.
2. Understanding the growth mechanism of directional growth of benzophenone
crystals from melt by means of in-situ observation of growth process.
3. Comparatively analyze the growth aspects, structural and optical properties of
novel benzophenone derivatives such as 4-ABP and 4-4' DMBP.
4. Finally, identification of new NLO material with relatively high SHG efficiency.
In this aspect, investigate the growth aspects, structural and optical properties of
EDMAB single crystal.
The following chapters detail these objectives.