Organic Crystals for Nonlinear Optics

Organic Crystals for Nonlinear Optics The fast-growing development of optical devices for many modern applications has promoted the search for new highly nonlinear materials, suitable for fast and efficient processing of optical signals. Some organic crystals exhibit optical nonlinearities and a laser-damage threshold potentially superior to their presently available inorganic counterparts. This article briefly reviews the properties and main nonlinear optics (NLO) applications of these organic materials (OM), as well as the techniques for screening, purifying, and growing them in the form of bulk single crystals.

1. Nonlinear Optical Properties in Organic Materials NLO materials are characterized by contributions to the electric dipole moment per unit volume, called electric polarization (P ), which depend on higher powers of the oscillating electric field (E ) of the radiation that is being propagated through them. This polarization is associated with the electron cloud of the atoms in the solid, which can respond to the frequencies of the light-wave electric field. For a nonconducting, nonmagnetic material, Maxwell’s relations lead to a differential wave equation, which gives E as a function of P (Yariv et al. 1984). In order to integrate this differential equation, which involves second-order space and time derivatives of E and a second-order time derivative of P , a model of dielectric solid must be set up for calculating P and its derivatives. One possible model expresses P as a power series of E : PlP

jχ(") :

!

E jχ(#): E E jχ($): E E E j…

(1)

where P is the overall polarization in the presence of E , P is the intrinsic polarization, and χ(n) is the nth order ! susceptibility coefficient. These χ(n) coefficients are tensors of the (nj1)th rank and express a measure of the binding stiffness of the electrons in the molecular framework of an organic molecule; the less stiff the binding, the larger the value of P . Further, the odd terms of χ(n) contribute to P in all materials, while the even terms are only found, because of symmetry requirements, in crystals having noncentrosymmetric space groups. Linear optics is represented by χ("), while the higherorder χ(n) (which are of interest only when exceptionally large) represent nonlinear optics. In practice, only second- and third-order NLO effects are considered. The high values of χ(#) are associated with the linear-electrooptic (LEO) and second-harmonic-gen-

eration (SHG) effects. The LEO (or ‘‘Pokels’’) effect, on which modulators and switches are based, consists of a change of the refractive index when a lowfrequency (with respect to optical frequencies) electric field is applied. It is often measured through the birefringence induced by E . In the SHG effect, the light frequency (ω) is doubled (ωjω l 2ω) during the propagation of light through the crystal. An important feature of SHG is the phase-matching condition (Dalven 1980), whose physical meaning is a space resonance of the interacting light waves with frequencies ω and 2ω. The attainment of this condition, which maximizes intensity and efficiency of SHG, is limited by inhomogeneities (of any origin) in the crystal lattice. Third-order nonlinearity includes effects such as third-harmonic generation (THG; ωjωjω l 3ω) and self-focusing (ωkωjω l ω). Current interest in NLO materials is being driven by their potential in the fabrication of devices (rectifiers, amplifiers, new-laser-frequency generators, frequency mixers, parametric oscillators, etc.) to be employed in developing technologies such as optical computing and data storage and fiber communications. Among NLO materials, some organic solids have χ(#) and χ($) values that are larger than those of the best NLO inorganic materials, typically LiNbO and KDP. These OM are of interest for application $because, in spite of their softness and low melting point, they present very high optical damage thresholds, well over 500 MW cmV#, compared with 100 MW cmV# or less, typical of most inorganic materials (Chemla and Zyss 1987).

2. NLO Organic Materials A number of OM of interest for NLO applications, today available as single crystals of different good optical quality, is listed in Table 1. For each of them the acronym (or the commercial name) and the chemical denomination are given. The NLO behavior in OM depends on the molecular structure of the crystals, where the molecules, which are bound together by weak dispersion forces, tend to preserve their individuality to a large extent. However, the particular structure of the organic molecules and their arrangement in the lattice give rise to a net charge transfer over an extended range in particular crystallographic directions. This, in turn, will eventually produce an internal (static) electric field (E ). The NLO properties in OM are the response of the!crystal (through E ) to the external oscillating electric field (E ) ! of the propagating light. As an example, consider how LEO and SHG effects can be accounted for in terms of their dependence on χ(") and χ(#). In this simplified analysis, vector and tensor indices are ruled out. For a plane polarized 1

Organic Crystals for Nonlinear Optics Table 1 NLO organic materials. Acronym or commercial name

Acronym or commercial name

Chemical denomination

NPP NPAN

p-NA m-NA BNA CNA m-DNB (also MDB) m-CNB

2-methyl-4-nitroaniline 2-methyl-4-nitro-Nmethylaniline para-nitroaniline meta-nitroaniline 2-bromo-4-nitroaniline 2-chloro-4-nitroaniline meta-dinitro-benzene meta-chloro-nitro-benzene

m-BNB

meta-bromo-nitro-benzene

DAN

4-( N,N ’-dimethyl-amino)3-acetamido-nitro-benzene m-dihydroxy-benzene carbonyl-diamide

HMT, urotropine (hexamine) ABP

MNA MNMA

α-resorcinol urea

NMU MAP

benzyl MANS POM PNP COAMP BAMP

LAP AMA

N-methyl-urea methyl-(2,4-dinitrophenyl)amino-2-propanate

m-AP α-AP

light, which passes through a NLO crystal with an internal electric field E , assume that the time de! by pendent part of E is given E l Eω sin ωt

(2)

E and E are supposed, for simplicity, to have the same ! From Eqns. (1) and (2), the polarization to direction. the second order will be given by P l P jP jP ! " #

(3)

P l ε ( χ(")E jχ(#)E #j" χ(#)Eω#) ! ! ! ! # P l ε χ eff Eω sin ωt " ! P l k "ε χ(#)Eω# cos 2ωt # # !

(4)

in which

(5) (6)

and where, in Eqn. (5), χeff l χ(")j2χ(#)E

!

(7)

is an effective susceptibility, in terms of which one obtains an effective refractive index n l N1jχeff

(8)

The LEO effect consists of the dependence of n on E , ! where E can also be seen as the resultant of the ! 2

Chemical denomination N-(4-nitrophenyl)-()-prolinol nitro-4-phenyl-N(methyl-cyano-methyl)-amine diphenyl-α,β-diketone para-dimethylamino-β-nitrostyrene methyl-3-nitro-4-pyridine-1-oxide 2-(N-prolinol)-5-nitropyridine 2-cyclo-octylamino-5-nitropyridine 2-(α-methyl-benzil-amino)5-nitro-pyridine hexamethylene-tetraamine 4-amino-benzophenone -arginine-phosphate R-(j)-N-methylidene-gem(carbethoxy-phenyl)amide of 4-N-dimethylaminobenzylidene cyanacetic acid 3-amino-phenol 2-amino-phenol

internal electric field plus an externally applied electric field able to modulate the refractive index. The SHG effect consists of the generation, because of P in Eqn. (6), of a second harmonic of frequency 2ω. #In other words, the fundamental light wave, which propagates in a NLO crystal with frequency ω, will generate a radiation with frequency 2ω, which propagates as well. This process may also be referred to as a ‘‘mixing’’ of the incident electric field E with itself, leading to a component (P ) of the polarization P at # 2ω instead of ω. Interesting (actual and potential) applications have been reported (Chemla and Zyss 1987, Badan et al. 1987, Penn et al. 1991) based on: (i) LEO effect, for urea, NMU, POM, mNA, MNA; (ii) SHG effect, for urea, NMU, MBAMP, POM, MAP, DAN, NPP; (iii) THG effect, for LAP, COANP. Regarding points (i) and (ii), applications are possible only if the optical absorption spectra of the materials are such that the new harmonic frequencies which are generated will not be reabsorbed. A further requirement for NLO crystals, in view of their application, is their ‘‘high optical quality.’’ This means a low and uniform density of both impurities (dopants) and structural (lattice) defects. Nonuniformities would bring about optical inhomogeneities, namely local variations in the refractive index and hence distortions of the optical beam with reduction of device performances (i.e., low signal-to-noise ratios). The optical quality is also to be accounted for the final crystal surfaces. For a review on how to attain an

Organic Crystals for Nonlinear Optics

Figure 1 Solution growth apparatus for growing POM crystals (cooling technique); (a) stirrer; (b) rotating seed holder; (c) temperature sensor connected to a temperature regulator; (d) heating resistor driven by the regulator; (e) seeds; (f ) solution (solvents: acetonitrilejmethylacetatej 1,2dichloroethane); (g) thermostated liquid (water) in a 20liter tank (after Badan et al. 1987).

initial amount of impurities in the polycrystalline source material (charge). Purification is commonly carried out by recrystallization in an appropriate solvent, selected on the basis of a higher solubility of the impurities as compared to the solubility of the substance to be purified. When a too-high level of impurities persists after several recrystallizations, other purification processes can be used, such as zone refining (ZR) and sublimation. By ZR, it is often possible to purify the charge down to 1 ppm, provided that no decomposition occurs during melting. Purification by sublimation is best achieved by gradient sublimation, a procedure by which the substance to be purified is placed in the hot end of a glass tube and the sublimation is carried out either in a flow of inert gas or under vacuum. Impurity analyses are usually performed by gas chromatography (GC), high-performance liquid chromatography (HPLC) and by combining GC and mass spectrometry (Karl 1980).

5. Growth of Bulk Crystals

optical finish of NLO crystal surfaces see, for example, Penn et al. (1991).

Bulk crystals of OM have been grown from solution, melt, and vapor phases. Although in principle the growth techniques are the same as those developed for inorganic materials, nonetheless differences do exist, as will be pointed out below.

3. The Identification of NLO Organic Materials

5.1 Crystal Growth from Solution

With OM, a great number of crystalline structures (natural or man-made by molecular tayloring) are to be considered, which are potentially useful for NLO applications. However, the development of a bulk crystal growth technology is expensive and timeconsuming, therefore potentially interesting NLO structures must be preliminarily detected by available screening methods. The Kurz powder technique (Kurtz and Perry 1968) is useful for screening polycrystalline powders to observe second-order nonlinearities and estimate the intensity of the second-harmonic radiation. Other screening techniques, such as electric-field-induced SHG (EFISHG) and the so-called ‘‘solvatochromic method’’ are reviewed by Penn et al. (1991), together with a number of theoretical methods based on quantum mechanical calculations. Reported values of NLO properties of inorganic and organic materials are tabulated by Dmitriev et al. (1991).

For general aspects of solution growth, see Growth from Solutions. For NLO OM, for which solution growth is the most employed growth technique, see Badan et al. (1987) and Penn et al. (1991). Solution growth is carried out by placing one or more crystallographically oriented seeds in a saturated solution, then supersaturating the seed region in such a way as to control the growth process. At any temperature, there is a range of small supersaturations (metastable region) within which nucleation can be kept under control. Beyond this range, at greater supersaturations (labile region), the growth process becomes uncontrollable because of spontaneous nucleation. As reported so far for NLO organic crystals, supersaturation control can be variously achieved by (i) slowly lowering the temperature (cooling techniques); (ii) increasing the solute concentration through solvent evaporation at constant temperature (solŠent eŠaporation techniques); or (iii) imposing appropriate temperature gradients to the solution (temperature-gradient techniques).

4. Purification of Polycrystalline Source Materials Organic crystals for NLO applications require very low impurity levels, not exceeding 10–100 ppm. The impurity concentration in bulk crystals depends on the

(a) Cooling techniques. A saturated solution is slowly cooled in a growth vessel in which one or 3

Organic Crystals for Nonlinear Optics Table 2 NLO organic crystals. Crystal POM AMA MAP MBANP m-CNB ABP PNP MNA m-NA α-resorcinol m-AP BNA

Growing techniques

Crystal

Growing techniques

SCT, SSET SCT SCT, BS SCT SCT SCT SCT SSET, CPVT SSET, BS, CPVT SSET SSET, STGT SSET

CNA HMT COAMP urea NMU m-DNB NPP MNMA benzyl m-BNB (m-CNB)x(m-BNB)1Vx MANS p-NA

SSET SSET, CPVT, SOPVT STGT SCT, SOPVT SCT, SOPVT(VLS) BS BS BS CZ KP KP CPVT CPVT

Solution growth: cooling techniques (SCT), solvent-evaporation techniques (SSET), temperature-gradient techniques (STGT); vapor growth: CPVT, SOPVT; melt growth: BS, KP, CZ.

more seeds are located on a seed supporter, which also acts, in the solution bulk, as a rotating stirrer. The growth vessel is usually placed in a thermostatic bath. Typical growth apparatuses for NLO organic crystals have been described for POM, AMA, and MAP (Badan et al. 1987; Penn et al. 1991). An example of a growth apparatus is shown in Fig. 1. Some NLO organic crystals grown by cooling techniques are listed in Table 2. When applying these techniques to NLO organic crystals, an appropriate solution circulation (stirring) is needed to minimize supersaturation and temperature nonuniformities ahead of the growing interface. This will favor compositional and structural homogeneity (i.e., a good optical quality) in the final crystal.

(b) SolŠent-eŠaporation techniques. These techniques, which are based on a controlled supersaturation increase by means of a programmed solvent evaporation under isothermal conditions, are suitable for NLO organic crystals with narrow thermal-stability ranges and\or with unfavorable slopes on the solubility curve for growth by cooling techniques. A typical growth apparatus is given in Fig. 2. A drawback of these techniques is, however, that when the solvent evaporation proceeds, the concentration of impurities in the solution tends to increase with time. This favors not only a nonuniform distribution of impurities in the final crystal, but also the formation of structural defects (e.g., growth striations), because of the interaction of impurities with the growth mechanisms on an atomic scale. Structural defects can also be the result of spurious heterogeneous nucleations, likely to occur as the supersaturation increases with solvent evaporation. 4

(c) Temperature-gradient techniques. A temperature gradient is applied to a saturated solution in which the solute (i.e., the source material) is present as a solid phase. The higher temperature, which is chosen to be the saturation temperature, is imposed on the source region, usually the bottom of a vertically positioned glass tube. When a seed is located in the colder end of the tube it will start growing, since, at any temperature below the saturation one, the solution is supersaturated. When growing NLO crystals, great care should be taken to work far from the ‘‘labile’’ region. This means using temperature gradients not exceeding 1–2 mC cmV". NLO organic crystals successfully grown by these techniques are COAMP and α-AP.

(d ) General remarks on solution growth. Solution growth is advantageous because, due to the low temperatures involved, costly and complex growth apparatuses are usually avoided. Further, it permits the growth of crystals of NLO OM which decompose at (or below) the melting point; in fact, decomposition makes melt and vapor growth unpractical. However, a drawback of solution growth is the easy incorporation of impurities, including solvent traces, in the crystal lattice, with possible formation of compositional and structural nonuniformities, detrimental to NLO device performances. Impurities can also affect the growth rate through adsorption on the growing interfaces and modification of growth mechanisms. High concentrations of impurities can bring about changes in crystal habit unfavorable to crystal processing in view of NLO device fabrication. Solution growth of NLO organic crystals is further limited by the availability of solvents with sufficient purity, correct slope in the ‘‘solubility-versus-temperature’’ curve (solubility curve) and ability to impart a low viscosity to the solution. This last requirement,

Organic Crystals for Nonlinear Optics

(a)

(b) (c) (d)

Figure 2 Solution growth apparatus based on solvent evaporation (after Brice 1986) suitable for NLO organic crystals; (a) water-cooled pipe on which the solvent is let to condense and drip outside of the growth vessel; (b) seed in holder; (c) crystal; (d) heater.

which comes from the need of a fast removal of supersaturation nonuniformities ahead of the growing interfaces, is particularly strict for NLO organic crystals. Finally, solution growth is only feasible if a crystal displays sufficiently high kinetical barriers at the growing interface. In fact, crystals being grown from solution dissipate their crystallization heat only through the solution, whose temperature must therefore always be lower than that of the growing interface. This means that constitutional supercooling is always present and its destabilizing effects are avoided only when the interface kinetics are slow enough to prevent instability from developing during growth. When screening OM for potential NLO applications, this aspect should not be neglected. However, since atomically flat faces are the slowest-growing ones, solution-grown crystals are always faceted, which is advantageous with NLO crystals, as facets help process crystals in device fabrication.

5.2 Crystal Growth from the Melt The growth from the melt of NLO organic crystals has been reported in a few cases, in which the Bridgman– Stockbarger (BS), Kyropulos (KP), and Czolchraski (CZ) methods were used. Though in principle these methods are the same as described for inorganic materials (see Crystal Growth from the Melt), they are somewhat modified to account for the much lower

Figure 3 A modification of the BS apparatus used by Badan et al. (1987) for growing MAP and NPP crystals: (a) ampoule; (b) O-ring; (c) melt heater; (d) polycrystal; (e) molten zone; (f ) monocrystal; (g) crystallization heater (after Badan et al. 1987).

thermal conductivity of the OM. This makes the growth of optical-quality crystals possible only at very low growth rates, typically 10V' cm sV", compared with 10V%–10V# cm sV" for inorganic materials. However, with respect to inorganic crystals, the much lower melting point of the OM often offers the advantage that transparent glass-made growth apparatuses can be used, in which the growth process can be easily followed by direct viewing. The BS techniques, also often used for NLO organic crystals, consist of the slow motion of an ampoule (crucible), which contains the charge to be crystallized, along a temperature profile in such a way as to maintain a steep temperature gradient at the growing interface. The ampoule is usually moved vertically downwards. The charge is completely melted in the high-temperature region and begins to crystallize in the tapered end of the ampoule as this enters the low-temperature 5

Organic Crystals for Nonlinear Optics

Motor

Vp z

(a)

H1 90 °C T0

Effusion hole

H2

(b)

Figure 4 Simplified Kyropoulos variant suitable for organic NLO crystals: (a) glass beaker containing undercooled melt and growing crystal; (b) ventilator; (c) heating resistor; (d) temperature sensor; (e) temperature controller and regulator (after Klapper 1994).

region below the melting point. The tapered end of the ampoule is sometimes provided with a capillary to favor a single nucleation. The steep temperature gradient at the growing interface is necessary to prevent undesired constitutional supercooling. An improved modification of the BS apparatus, especially developed for NLO organic crystals, makes use of a molten-zone configuration as shown in Fig. 3. By the BS technique some reasonably good opticalquality crystals could be produced (see Table 2). However, optical quality is often degraded because of the contact of crystal and melt with the crucible walls, which favors the incorporation of impurities and the formation of lattice strains during post-growth cooling. The KP technique, as developed for inorganic materials, consists in vertically dipping an oriented seed in a molten charge of the material to be grown. The seed, which is attached to a vertical shaft, rotates during growth. The growth process is activated and controlled by the slow cooling down of the molten charge, i.e., the melt is kept undercooled with respect to the seed by dissipating the latent heat of crystallization through the melt. When applied to NLO organic crystals, the KP techniques do not usually make use of rotating oriented seeds, but the seed is simply kept immersed in an undercooled melt by means of a thread to which it is attached (Fig. 4). In spite of its simplicity, the method has proved suitable for growing good optical-quality crystals (see Table 2). Since no contact occurs between crystals and crucible walls, the density of structural defects is generally low. The CZ technique is similar to the KP one, but differs in that the crystal is pulled out of the melt 6

H3

94 °C T 91 °C s

(c) T Vacuum

Vacuum

Figure 5 SOPVT facility for growing urea crystals: Šp, pulling rate, typically 1 mm per day; effusion hole: 0.3 mm diameter, wall thickness 1.5 mm, outside vacuum of about 0.04 mbar; H1, H2, H3: controlled temperature zones; shaded areas of the perforated ampoule: urea source (c) and crystal (a); vapor space (b) (after Zuccalli et al. (1996).

vertically upwards while counterrotated with respect to the crucible. The latent heat of crystallization, in comparison with the KP method, is lost to the ambient through the growing crystal and its gaseous surroundings, usually an inert gas. It is potentially the best melt growth technique because of its ability to control at the same time many growth parameters, namely, crystallographic orientation (via seed), thermal field, pulling and rotation rates, and thus, to some extent, allowing control of the fluid-dynamic regime, and hence the shape and stability of the growing interface. Further, the dislocation density can be kept low by ‘‘necking’’ procedures, well known in the CZ-growth of inorganic crystals. All these potentials so far have not been exploited for NLO organic crystals. The main limitations of the method, when applied to OM, are strictly related to the specific chemico-physical properties of the materials, in particular the low values of solid–liquid surface energy and thermal conduc-

Organic Crystals for Nonlinear Optics tivity. The tendency of many OM to sublime and\or decompose when approaching the melting point poses further problems. The best example of applications of the CZ technique to NLO organic materials is probably the growth of benzil crystals.

5.3 Crystal Growth from the Vapor Phase Vapor growth techniques are based on physical vapor transport (PVT) and chemical vapor transport (CVT). In both cases, a source material is vaporized at a source temperature (Ts) and then, under a suitable temperature gradient, the vapors are moved into a crystallization zone at a growth temperature Tc ( Ts), where they become supersaturated and crystallize. For a general introduction to vapor growth see Bulk Crystals : Vapor Growth. So far, NLO-OM have only been grown by PVT, where vaporization occurs by the sublimation of a source material without any recourse to heterogeneous chemical reactions as in CVT. The various PVT techniques are sometimes referred to as ‘‘sublimation’’ techniques. Their application to OM was first reviewed by Bradley (1963). Regarding NLO materials, some account is given by Chemla and Zyss (1987) and Penn et al. (1991). The growth apparatuses so far reported for NLO organic crystals are either for closed (C) and or semi-open (SO) PVT techniques. In C-PVT, the sublimed vapors move into the growth region under either vacuum or a reduced pressure of an inert gas. Care must be taken to keep the growing temperature Tc ( Ts) sensibly constant at the growing interface. This is usually obtained by moving the growth vessel (ampoule) along the temperature profile of the furnace with a pulling rate approximately equal and opposite to the advancement rate of the growing interface (pulling techniques). As an alternative, the ampoule is kept fixed while the temperature profile is electronically varied with an appropriate time rate (cooling techniques). The C-PVT method appears suitable for growing thin platelike organic crystals, but in some cases bulk NLO crystals could be obtained, such as MANS (interesting, though centrosymmetric, for its high third-order nonlinearity), p-NA, m-NA, MNA and HMT (see Table 2). If volatile impurities tend to concentrate ahead of the growing interface, then diffusional barriers to vapor mass transport are created, which severely limit the growth process. In these cases, the SOPVT is advantageous over CPVT. In fact, these barriers are lowered by reducing the overall pressure in the growth vessel through a leak in the vessel walls, close to the growth region (Fig. 5). This leak, usually a small orifice drilled in the glass wall and vented to vacuum, should be small enough to allow 1% vapor loss to the ambient, by effusion rather than by convection,

during the entire run. By this technique, the mass transport can be increased even by orders of magnitudes compared to CPVT. SO-PVT has been successfully employed for growing urea crystals of high structural quality (Fig. 6) and HMT. The application of SO-PVT to the growth of NMU (Zha et al. 1997) evidenced a vapor–liquid–solid (VLS) transition, probably due to the formation of a metastable NMU layer on the growing interface. The presence of a liquid layer allows growth of large-size crystals (up to 10 cm$) with good optical quality, at a growth rate much faster than usually observed even in SOPVT processes. Compared to solution growth, PVT growth yields purer crystals (especially by SO-PVT), free from solvent traces. Compared to melt growth, the advantages of PVT are the lower growth temperatures, which means that many undesired, thermally activated processes (solid-state diffusion of impurities, structural defect formation, wall contamination, etc.) become less effective. Further, materials which decompose at the melting point are less prone to decompose at lower temperatures, which makes vapor growth a possible alternative. Drawbacks are, however, the easy spurious nucleation, which is facilitated by the small solid–vapor surface energies of NLO OM, and the slow growth rates.

6. Structural Defects in NLO Organic Crystals Structural defects include all deviations from the periodic (lattice) arrangement of atoms, molecules or ions, which arise during growth, post-growth and device processing. Certain defects appear during growth in association with impurities (e.g., growth striae). The structural defects have not been investigated in detail in NLO organic crystals. It is, however, well known that their effects on NLO properties such as LEO, phase-matched SHG, optical damage threshold, and others are detrimental to subsequent device performances. The structural defects in NLO organic crystals are similar to those observed in inorganic crystals. They can be categorized as follows: (i) Point defects, which include lattice vacancies, (self )-interstitials, wrongly oriented molecules, defect-impurity associates, etc. Note, however, that organic crystals are not likely to have ‘‘pure’’ Frenkel and Schottky defects, owing to their molecular structure and the molecule size. Partial ‘‘holes’’ less than one molecule in size are in fact often observed. (ii) Line defects as dislocations, the most frequent defect in NLO organic crystals. (iii) Planar defects: these include high- and low-angle grain boundaries, always associated with lattice distortions, twin boundaries, growth-sector boundaries, and stacking faults. (iv) Volume defects, as bubbles (in melt grown crystals) and precipitates due to solvent traces 7

Organic Crystals for Nonlinear Optics For a review on defects and their characterization in OM see, e.g., Klapper (1991). Details on structural defects in NLO organic crystals are given by Chemla and Zyss (1987). See also: Crystal Growth from the Melt; Growth from Solutions; Bulk Crystals: Vapor Growth; Nonlinear Optics of Polymers; Photorefractive Polymers Bibliography

Figure 6 A sophisticated SOPVT facility for growing urea crystals: (a) transparent furnace and heaters; (b) evacuated glass cylinder; (c) ring-shaped urea charge on the walls of the growth cell (d); (e) growing crystal; (f ) optical glass window for in situ video camera viewing; (g) cold finger for favoring crystallization heat loss to the ambient; (h) bottom heater; effusion proceeds through the ground joint between crystal holder (pedestal) and growth cell (after Zha et al. 1995).

in solution-grown crystals; all lead to severe lattice distortions. (v) Mechanical defects, due to incorrect post-growth handling, including scratches and cracks.

8

Badan J, Hierle R, Perigaud A, Vidakovic P 1987 Growth and characterization of molecular crystals. In: Chemla D S, Zyss J (eds.) Nonlinear Optical Properties of Organic Molecules and Crystals. Academic Press, New York, Chap. II-4, pp. 291–356 Bradley R S 1963 Organic crystals. In: Gilman J J (ed.) The Art and Science of Growing Crystals. Wiley, New York, Chap. 3, pp. 55–61 Brice J C 1986 Crystal Growth Processes. Halstead, New York, Chap. 6, pp. 167–78 Chemla D S, Zyss J (eds.) 1987 Nonlinear Optical Properties of Organic Molecules and Crystals. Academic Press, New York Dalven R 1980 Introduction to Applied Solid State Physics. Plenum, New York, Chap. 9 Dmitriev V G, Gurzadyan G G, Nikogosyan D N 1991 Handbook of Nonlinear Optical Crystals. Springer, Berlin Karl N 1980 High purity organic molecular crystals. In: Freyhardt H C (ed.) Crystals: Growth, Properties and Applications. Springer, Berlin, Vol. 4, pp. 1–100 Klapper H 1991 X-ray topography of organic crystals. In: Freyhardt H C (ed.) Crystals: Growth, Properties and Applications. Springer, Berlin, Vol. 13, pp. 107–62 Klapper H 1994 Kristalle, die geerntet Werden, Die Waage. Z. der Gru$ nenthal GmbH, Aachen, Germany, Vol. 33, pp. 28–34 Kurtz S K, Perry T T 1968 A powder technique for the evaluation of nonlinear optical materials. J. Appl. Phys. 39 (8), 3798–813 Penn B G, Cardelino B H, Moore C E, Shields A W, Frazier D O 1991 Growth of bulk crystals of organic materials for nonlinear optical devices: an overview. Prog. Cryst. Growth Charact. 22, 19–51 Yariv A, Yeh P 1984 Optical WaŠes in Crystals. Wiley, New York Zha M, Franzosi P, Zanotti L, Zuccalli G, Paorici C, Capelletti R, Razzetti C 1995 Crystal growth and characterization of urea by physical vapour transport in semiopen cells. J. Cryst. Growth 146, 29–36 Zha M, Zanotti L, Zuccalli G, Ardoino M, Capelletti R, Paorici C 1997 Vapour-liquid-solid growth and characterization of Nmethylurea crystals. Cryst. Res. Technol. 32, 213–20 Zuccalli G, Zha M, Zanotti L, Paorici C 1996 Vapour growth of inorganic crystals by a semiopen Pizzarello (SOP) technique. In: Materials Science Forum. Transtech, Zurich, Vol. 203, pp. 35–8

C. Paorici

Organic Crystals for Nonlinear Optics

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