Journal of Crystal Growth 225 (2001) 470–473

Growth of vanillin crystals for second harmonic generation(SHG) applications in the near-IR wavelength region

O.P. Singha, Y.P. Singha, Namwar Singha, N.B. Singhb,*aChemistry Department, K.N. Post Graduate College, Gyanpur, Sant Ravidas Nagar, UP, India

bScience and Technology Center, ESSS, Northrop Grumman Corporation, ATL-3D14, 1212 Winterson Road, Linthicum,

MD 21090, USA

Abstract

Vanillin also known as 4-hydroxy 3-methoxy benzaldehyde, is an excellent candidate for second harmonic generationfor ultra-violet and near infrared wavelength region because of its higher effective conversion efficiency. We have grown

centimeter sized single crystals from solution by lowering the temperature of the bath. We used a mixture of methanol,chloroform as the solvents and a temperature region of 358C– 408C was used for growing the crystals. A qualitativecomparative study was performed for its effective conversion efficiency to compare vanillin with m. nitroaniline (m.NA)

a well known organic crystal. We observed that the output intensity for second harmonic conversion was very high forvanillin in comparison to any other commercially available crystal. # 2001 Published by Elsevier Science B.V.

PACS: 42.70.M; 81.10.Dn

Keywords: A1. Recrystallization; B2. Nonlinear optical materials

1. Background

The field of non-linear optics has traditionallybeen in hands of physicists and electrical engineerssince the past three decades. The potentialapplications of this field have not been realizeddue to lack of development of non-linear opticalmaterials. In the past 15 years many organic andinorganic materials have been developed [1–4] tocover ultra-violet, near and far- infrared wave-length regions. The non-linear optical coefficient

[5] depends on the nature of electronic environ-ment, crystal symmetry and exact nature of theinteracting field component. Organic moleculescontaining conjugate systems have been studiedbecause of the possibility of highly enhancedelectronic non-linear optical polarization re-sponses. A necessary condition for the crystal tohave large macroscopic second order electricaldipole susceptibility is that a crystal with a spacegroup is from the noncentrosymmetric class. Wehave carried out extensive work on synthesis,purification, crystal growth, fabrication and char-acterization of nitro and -amino compounds.Because of the presence of –NO2 or –NH2 groupin the aromatic rings the transparency of thesecompounds is limited to approximately 0.40 mm.

*Corresponding author. Tel.: +1-410-765-1590; fax: +1-

410-765-7652.

E-mail address: narsingh b singh@md.northgrum.com

(N.B. Singh).

0022-0248/01/$ - see front matter # 2001 Published by Elsevier Science B.V.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 9 1 8 - 6

Consequently, these compounds do not transmitin the UV wavelength region. Urea is an importantmaterial, which transmits in the UV range, but hasa very low non-linear optical coefficient. Becauseof this reason we carried out experiment to studythe crystal growth of Vanillin, also known as 4-hydroxy 3-methoxy benzaldehyde (HMB). Theresults of solubility and crystal growth arepresented in this paper. A qualitative comparativestudy was performed for its effective conversionefficiency to compare vanillin with m.NA crystals.

2. Experiment

We used as supplied vanillin from the vendor.The material was listed for 99% purity. It wasfurther purified by fractional recrystallizationusing a mixture of acetone and water as solvent.

Crystal growth was carried out in a mannersimilar to that described in Ref. [2] by the solutiongrowth method. It involved solubility measure-ment and then the selection of the best solvent forgrowth. All solubility measurements were taken ina glass ampoule. Vanillin and the solvent wereweighed in an ampoule, which was sealed andplaced in a water bath. The ampoule wascontinuously vibrated to enhance mixing. Thetemperature of the bath during solubility wasraised at the rate of 0.5K/h. Several solvents suchas water, methanol and chloroform were used inthis study.

We used methanol-chloroform co-solvent togrow the vanillin crystals. The solubility of vanillinin the methanol-chloroform co-solvent reported inRef. [2] was observed to be suitable for thetemperature range of 308C–358C.

Lattice parameters were measured by usingPhilips automated powder diffractometer. Bulkhomogeneity was evaluated by an imaging videosystem. The comparative SHG tests were takenusing a laser at 1.06 mm wavelength.

3. Results and discussion

The as supplied vanillin was not suitable forcrystal growth. When dissolved in water, the

solubility was very low. When the temperaturewas raised, it was observed that it turned brownindicating that vanillin decomposes (Fig. 1) at hightemperatures in presence of moisture. Vanillin hasvery low solubility in pure water requires a largetemperature differential for crystallization. Sincevanillin has almost identical density (1.08 g/cm3),to that of water fibrous needles were suspended inthe bulk of solution. In spite of slow cooling andslow growth we could not get suitable crystalsfrom water. Therefore, it can be concluded thatwater is not a good solvent for growing largecrystals of vanillin. The solubility of vanillin inmethanol, chloroform and several mixtures wasmeasured [2] and the results are reported in Fig. 2.Even faster cooling showed thick needles (Fig. 3)indicating that this solvent mixture is suitable forcrystal growth. The effect of the solvent onmorphology of the vanillin crystal is very interest-ing. When vanillin crystals were grown fromwater, long fibrous needles crystallized where asvery thick morphology is observed when methanoland chloroform are used as the solvent. Atransition is observed from needle to plate likemorphology when a mixture of methanol-acetoneor methanol-chloroform is used. X-ray measure-ments from the recrystallized materials showed

Fig. 1. Fibrous morphology of oxidized vanillin grown from

hot water.

O.P. Singh et al. / Journal of Crystal Growth 225 (2001) 470–473 471

that crystal belong to orthorhombic class and thelattice parameters are a=7.882 A, b=13.974 A,and c=13.549 A.

A large single crystal shown in Fig. 4 was grownfrom methanol-chloroform (1 : 1) by lowering thetemperature at the rate of 0.5K/h from 358C. Themorphology of the crystal shows that vanillin is

very anisotropic. From the crystal size it is alsoclear that vanillin crystal grew much faster in[0 0 1] direction compared to other orientation. Weobserved growth steps very similar to that reportedin Ref. [2]. Large growth steps were observed,which occasionally spread over the surface. Fromthe surface step impurity and bubble trapping wasexpected, however, we did not observe any void orbubble in the crystal. The crystal was free fromvisual defects and water white transparent. Thethickness was approximately 0.5 cm. Second har-monic efficiency was evaluated by comparing thebeam intensity of 1.06 mm diode laser through theidentical thickness of vanillin and m.NA crystals.Intensity of 0.53 mm second harmonic beamthrough vanillin was much brighter than m.NAindicating that vanillin has a higher efficiency thanm.NA crystals. Also, vanillin transmission cuts offat 0.26 mm and can therefore, be used below thecut-off wavelength of -nitro and -aniline com-pounds which generally cut-off at a higherwavelength.

4. Summary

Centimeter size crystals of vanillin were grownfrom a methanol-chloroform co-solvent by low-ering the temperature of the bath. Conditions wereoptimized for a temperature region of 358C– 408Cfor growing the crystals. A qualitative comparativestudy was performed for its effective conversion

Fig. 2. Solubility of vanillin in chloroform-methanol mixture

[2].

Fig. 3. Long needles of vanillin grown from methanol.

Fig. 4. Vanillin crystal grown from a methanol-chloroform

co-solvent.

O.P. Singh et al. / Journal of Crystal Growth 225 (2001) 470–473472

efficiency to compare vanillin with m.NA.We observed that the output intensity of thesecond harmonic signal was higher for vanillin incomparison to m.NA for an identical length ofcrystal.

Acknowledgements

The authors thank Dr. R.D. Upadhyay, Princi-ple K.N. Government Post Graduate College forproviding research facilities.

References

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[3] N.B. Singh, R. Mazelsky, M.E. Glicksman, Mater. Lett. 7

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[4] N.B. Singh, T. Henningsen, R. Mazelsky, Namwar Singh,

Mater. Lett. 7 (1989) 401.

[5] K.D. Singer, A.F. Garito, J. Chem. Phys. 75 (1981) 3572.

O.P. Singh et al. / Journal of Crystal Growth 225 (2001) 470–473 473