Appl Phys B (2009) 96: 85–94DOI 10.1007/s00340-009-3573-1

One- and two-photon-pumped luminescence studies on DASTand UDAST organic dye molecules

K. Kumar · R.N. Rai · S.B. Rai

Received: 29 November 2008 / Revised version: 17 April 2009 / Published online: 15 May 2009© Springer-Verlag 2009

Abstract The Optical studies on chemically synthesizedDAST (4-N, N-dimethylamino-4′-N′-methyl-stilbazoliumtosylate) and UDAST (containing urea as additive) sam-ples have been carried out in solid and solution phases.Both the solid samples show intense one-photon fluores-cence in the 550–850-nm region when pumped with wave-length lower than 550 nm. Solid DAST shows two broademission bands centered at 589 nm (intense) and 721 nm(weak). Solid UDAST sample also exhibits two bands cen-tered at 618 and 740 nm but opposite pattern in inten-sity. The emission band at 721 nm was supposed to arisedue to the intermolecular charge transfer (ICT) from onechromophore (stilbazolium) to another (tosylate). The ef-fect of urea on the fluorescence emission is discussed. Insolution phase both the samples show only single fluores-cence band (band width ∼75 nm) at 611 nm peak wave-length. Also, the DAST and UDAST in solid/solution phaseshow two-photon-pumped (TPP) upconversion emission on1.06 µm pumping. Further, DAST was doped into a sol–gel-derived glass matrix, and in this glass a new emissionband has been observed at 402-nm on 266-nm pump wave-length.

PACS 32.80.Wr · 33.20.Kf

K. Kumar · S.B. Rai (�)Department of Physics, BHU, Varanasi 221005, Indiae-mail: sbrai49@yahoo.co.inFax: +91-542-2369889

R.N. RaiDepartment of Chemistry, BHU, Varanasi 221005, India

1 Introduction

Organic nonlinear optical (NLO) materials are expectedto play a major role in optical information processing,harmonic generations, telecommunications, optical storage,two photon pumped lasers, etc. [1, 2]. Recent and ongo-ing advances in molecular synthesis chemistry and materialsengineering have made organic materials highly attractivefor photonic applications [3]. 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) is one of the promis-ing NLO materials, having large phase-matched nonlinearoptical coefficient of deff ∼ 290 pm/V [4, 5] compared toinorganic nonlinear crystals (maximum deff ∼ 35 pm/V) atthe telecommunication wavelength (1.5 µm) and generateswideband (0.1–1.5 THz) radiations [6].

A general approach for synthesizing NLO materials hastwo steps: first, the synthesis of extended conjugated sys-tems with donor and acceptor groups having large polar-izability and, second, arranging molecules in macroscopicnoncentrosymmetric space group [7]. The noncentrosym-metry is required for second-order nonlinear optical andelectro-optic applications, and to achieve this, one has toovercome the strong dipole–dipole intermolecular interac-tions that provide motivating force for centrosymmetricspace group formation during crystallization [8–11]. DASThas noncentrosymmetric packing [10], which is induced bycoulomb intermolecular interaction, and fulfills the basic re-quirements to be used for optical applications. Good opti-cal quality, mechanical stability, low dielectric constant, andhigh nonlinearity makes DAST crystals promising candi-dates for high-speed modulation and frequency mixing ap-plications, including generation and detection of terahertz(THz) radiations [12, 13].

The fabrication of infrared wavelength pumped upcon-version lasers has current interest, and DAST has shown

86 K. Kumar et al.

promise for the two-photon-pumped (TPP) upconversionlaser [14]. The TPP lasing offers distinct merits and someof these are: (i) frequency upconversion without the restric-tion of phase matching; (ii) possible use of a long interac-tion length; and (iii) minimization of local thermal damagebecause of weaker absorption. The two main requirementsfor TPP lasing are to maintain high concentration of thedye and high optical quality of the sample. A typical opti-cally pumped organic laser [14–18] consists of a conductinghost material and the low concentration of a laser dye/salt.The pump energy is either absorbed by the host materialor directly by the dye itself (e.g., Rodamine-6G doped inmethanol [19]).

Several research groups in the world are involved in thedevelopment of DAST for various applications using differ-ent crystal growth techniques [20–24]. The growth of high-quality DAST crystals with noncentrosymmetry has been abig challenge. Even though it has become possible to syn-thesize DAST crystal, but their optical properties are notyet completely explored. In this work, we have synthesizedDAST powder and have grown two kinds of crystals fromthat: One as pure DAST crystal, while the other containingurea as an additive (UDAST, U refers to urea). The purposeof using urea as an additive is two-fold. Firstly, the urea iswell known for good harmonic generation property, and sec-ondly, the urea is transparent up to 190 nm. The DAST be-comes opaque below 600 nm. The combination of the twocompounds is expected to shift the DAST cut of wavelengthto lower wavelength and lead to the structural modificationswith newer properties. It is well recognized that the prepara-tion of binary organic material helps in modifying the mate-rial properties [25]. The results concerning the effect of sol-vent on crystal morphology and the synthesis of pure DASThave been already reported earlier [24]. In this article theresults of optical measurements on DAST and UDAST arereported. These results are expected to be useful speciallyfor visible upconversion fluorescence using 1.06-µm pump-ing wavelength.

2 Experimental

2.1 Synthesis and crystal growth

DAST material was synthesized [24] by the reaction ofequimolar ratio of 4-picoline and methyl toluenesulphonatefor an hour and the product was treated with 4-N,N-dimethylaminobenzaldehyde in equivalent molar ratio inpresence of piperidine catalyst. The synthesis of DAST wascarried out inside the glove box in nitrogen environment;however the purification of DAST material was done by re-crystallizing from methanol below 60◦C. On the other hand,urea (U) received from Aldrich, Germany (99.9%) was used

Fig. 1 Photograph of single crystals of UDAST showing plate’s mor-phology. The average crystal size is ∼3 mm (width) × 10 mm (length).The visible grid lines behind the crystal infer transparency of the crys-tal

as such. The DAST crystal was grown from saturated so-lution of DAST in methanol. The crystals of DAST alongwith urea were however grown from the saturated solutionof methanol containing 0.8 mole % of DAST and 0.2 mole %of urea. In both cases the temperature lowering techniquewas adopted in growing the crystals using the temperaturecontrolled water bath.

The self-nucleated single crystals of DAST/UDAST(∼3 × 10 mm2) were grown. The grown crystals of UDASTshow change in shape and color than that of DAST. The sol-ubility of DAST in methanol was found to be enhanced inpresence of 0.2 mole % urea. The nucleation took place at asingle point in supersaturated methanol solution of UDAST,and the crystals having the plate’s morphology were har-vested (Fig. 1). The appreciable growth of crystal was ob-served in two dimensions. In third dimension the observedgrowth is nominal, and therefore, the morphology of crystalappears like a plate.

2.2 Characterization

To check the purity of DAST, presence of urea in DAST,and the single crystalline nature of UDAST crystal, pow-der X-ray diffraction (XRD) patterns were recorded using acomputerized X-ray diffractometer (Scintage, USA).

The bulk samples were sieved into powder form (parti-cle size in the micrometer range) for optical measurements.

One- and two-photon-pumped luminescence studies on DAST and UDAST organic dye molecules 87

Fig. 2 X-ray diffractionpatterns of (a) sieved DAST,(b) sieved UDAST, (c) UDASTsingle crystal, and (d) Urea

The optical absorption spectra of DAST and UDAST crys-tals were taken in 200–1000-nm region using UV–VIS–NIR (JASCO, Japan) spectrophotometer and in 200–980-nm region using QE65000 spectrophotometer (Ocean Op-tics, USA). FTIR spectra (Perkin Elmer, Spectrum RX1) ofthe samples were taken in the 4000–400-cm−1 region with2-cm−1 resolution. Fluorescence and upconversion lumines-cence spectra of DAST/UDAST were recorded using thefundamental (1064 nm), second (532 nm), third (355 nm),and fourth (266 nm) harmonics of a pulsed Nd: YAG laser(Spitlight 600, Innolas, Germany). To record the fluores-cence spectra, two spectrometers iHR320 (Horiba JobinYvon, USA), equipped with a Hamamatsu R928P photoncounting PMT, and QE65000, equipped with TE cooledCCD, were used. Methanol, Polyethylene Glycol (PEG),and water were used as solvents to record the spectra ofDAST/UDAST in solution phase.

3 Results and discussion

3.1 Crystal growth and powder XRD studies

The purity of synthesized samples was confirmed by study-ing the DSC and powder XRD patterns. Even so, these twotechniques were adopted to study the composite nature ofUDAST crystals and its single crystalline nature. In the DSCcurve of UDAST a small peak was seen around 134◦C,which is probably indication of urea, the major peak how-ever was at the melting point of DAST, i.e., 265◦C. The

XRD patterns of different samples are shown in Fig. 2. TheXRD pattern recorded for UDAST crystal plate shows singleorientation (Fig. 2(c)). The crystal grows corresponding tothe plane present at angle 2θ = 33.2 degree in UDAST pow-der. The monoclinic phase of DAST has been confirmed bythe previous reports [20, 22]. The XRD pattern of UDASTshows two peaks at 22.64 and 24.84 two theta values that in-dicates the presence of urea in it. This indicates that a binarycomposite of DAST and urea has been formed. Due to thiscomposite nature, UDAST is expected to show an entirelydifferent optical behavior as compared to DAST. The crys-tallographic data for single and randomly oriented DASTcrystals are given elsewhere [20, 26].

3.2 Absorption spectra

The absorption spectra of solid DAST and UDAST havebeen recorded in 200–1000-nm region. Both samples havenegligible absorption between 700–1000 nm and show verystrong absorbance below 700 nm. A comparison of the twoabsorption curves (Fig. 3) reflects the effect of additionof urea in DAST. For DAST, the absorption of light startsnear 738 nm, whereas for UDAST, it starts at ∼610 nm.A shift of ∼130 nm towards the lower wavelength sideis observed for UDAST. Moreover, for UDAST, the in-crease in absorption is more gradual than for the caseof DAST. The absorption spectra of DAST and UDASTin solution phase have also been recorded between 200–980 nm. Figure 4 shows the absorption spectra of DAST

88 K. Kumar et al.

Fig. 3 Absorption spectra ofsolid phase (a) DAST and(b) UDAST. UDAST shows ashift in band to shorterwavelength

Fig. 4 Absorption spectra ofDAST in Polyethylene glycolsolution showing two bands atlow concentration. Fluctuationsin the curve (a) arise either dueto the high optical density ofsample or due to the vibration ofmolecule

in polyethylene glycol (PEG) for two different concentra-tions. The absorption spectrum of DAST at low concen-tration (9.756 × 10−4 mol/l) shows two distinctly sep-arated absorption bands, whereas at high concentration(60.972 × 10−4 mol/l) the two bands seem to overlap andgive impression of a single band. The observed fluctua-tions in the absorption spectrum of DAST, as shown inFig. 4a, might be due to the high optical density of the sam-ple.

3.3 FTIR spectra

The molecular and acoustic lattice vibrations significantlycontribute to the electro-optic effect and also affect the opti-cal properties of the molecule. The contribution of these lat-tice vibrations have been termed by different names by dif-ferent authors [27, 28]. We have recorded the FTIR spectraof DAST and UDAST in the 1800–400-cm−1 region (Fig. 5)in order to assign vibrational bands and study the effect of

One- and two-photon-pumped luminescence studies on DAST and UDAST organic dye molecules 89

Fig. 5 Fourier transforminfrared spectra of DAST andUDAST samples

urea in the complex formation. UDAST sample shows peakshifts, and this shift would be due to the attachment of ureamolecule with DAST. Some of the peaks that are observedin pure DAST spectra are either disappeared or suppressedin the sample synthesized in presence of urea, e.g., vibra-tional peak at 1227 cm−1 due to phenolic C–O stretch inpure DAST completely disappears in UDAST sample. Thischange leads to the structural modification in presence ofurea. The following bands are found in the IR spectra ofDAST sample [29]:

1646 cm−1: C=C stretch1587–1528 cm−1: aromatic ring skeletal (C–C=C–C) inplane stretching vibration1370–1318 cm−1: CH3 deformation and C–N stretch1180–1165 cm−1: symmetric stretching mode of SO2 group1030–1010 cm−1: olefinic C–H bond834–818 cm−1: bending mode of aromatic hydrocarbon(C–H)688–685 cm−1: cis orientation of the substitute at theolefinic double bond

Additional bands are observed in UDAST at 1689, 1478, and610 cm−1.

3.4 Fluorescence spectra of red phase DAST/UDAST

Fluorescence measurements on red phase (centrosymmetricspace group) DAST/UDAST samples have been made withone- and two-photon excitations. Freshly synthesized sam-ples are found in yellow phase (noncentrosymmetric spacegroup), but, due to the exposure in open atmosphere, sam-ples turn out into red phase (centrosymmetric space group)

and lose its second harmonic generation (SHG) property butshow strong luminescent property.

3.4.1 One-photon fluorescence

Solid phase The absorption spectra of DAST and UDASTshow significant absorbance at wavelengths <550 nm(Fig. 3). With any excitation wavelength <550 nm, bothsamples give strong and broad one-photon luminescenceranging from 550 to 880 nm. Three pumping wavelengths,532, 355, and 266 nm from an Nd: YAG laser have beenused to record the fluorescence spectra and in every casewe found the same pattern of emissions. Figure 6a showsthe 532-nm wavelength excited emission spectra of solidphase DAST and UDAST samples. Both samples give twobroad emission bands but inverse in intensity and shift inpeak wavelengths. The DAST sample gives two emissionbands having peak centers at 589 and 721 nm. Whereasthe UDAST sample gives emission bands with peak cen-ters at 618 and 740 nm. The absorption of 532 nm light is sostrong that DAST sample looks yellow and UDAST looksdeep red in color without use of excitation filter. These emis-sions are the electronic transitions from the excited level tothe ground level involving vibrational levels. The first band(589–618 nm) is expected to occur from the diethylaminogroup present in DAST/UDAST. The second band at longerwavelength side is supposed to appear from another chro-mophore (sulphonic group) through molecule exhibitingtwisted intermolecular charge transfer (TICT) process [30].The TICT molecule on electronic excitation forms a moder-ately nonpolar state, and this nonpolar excited state under-goes an intermolecular transfer of an electron from donor

90 K. Kumar et al.

Fig. 6 Comparison in emissionspectra of DAST and UDASTwith 532-nm excitation (a) insolid phase and (b) in methanolsolution

(diethylamino group) to an acceptor (sulphonyl group) con-nected through flexible single bond about which rotation isfree. In our case only solid samples give emission at longerwavelength side, and molecular rotation is not possible inthis case. Here electron transfer occurs between two mole-cules through ionic interaction between ≡N+ and –SO−

3groups, but molecule does not undergo twisting. In UDASTsample, larger intensity at 740 nm is due to the presenceof urea, which increases the charge transfer efficiency fromone chromophore to another. The two peaks in this case aremuch broader and red shifted.

Solution phase Fluorescence measurements of DAST andUDAST samples in solution phase show only one emissionband at ∼611 nm when pumped with any wavelength from

532, 355, and 266 nm. Figure 6b shows emission spectra ofDAST and UDAST in methanol solution. The concentrationdependence shows small blue shift with dilution. The pos-sible reason for occurrence of a single peak in liquid phaseis that in liquid media two chromophores that constitute thewhole DAST/UDAST molecule got ionized and hence theenergy transfer probability vanishes. The effect of differentsolvents on the fluorescence efficiency has also been stud-ied. Four solvents viz. water, methanol, acetone, and poly-ethylene glycol (PEG) were used. Figure 7 compares thefluorescence intensity of the DAST in three solvents. Thehighest intensity is observed for PEG solvent and is almostten times stronger than in water. The possible explanation ofthis lies with dependence of fluorescence intensity on the po-

One- and two-photon-pumped luminescence studies on DAST and UDAST organic dye molecules 91

Fig. 7 Comparison influorescence emission intensityof DAST in different solvents

Fig. 8 Figure compares theemission spectra ofDAST-doped silica glass withsieved solid sample. The insetshows the enlarge portion of thesolid curve in 300–500-nmregion

larity of the medium. Higher is the polarity of the mediumlarger is the nonradiative relaxation probability and hencesmaller will be the emission intensity.

DAST doped in glass To study the fluorescence propertiesof DAST/UDAST in solid dilute form, DAST was dopedinto a silica glass host through sol–gel procedure. The ab-

sorption spectrum of this glass shows two absorption bands,as is observed in the case of liquid phase at low concentra-tion. But excitation of glass with UV light (266 nm in ourcase) results a new emission band at 402 nm along with theband at 598 nm. Figure 8 compares the emission spectra ofDAST-doped glass with that of DAST powder. No emissionin blue region is observed for the case of DAST powder. The

92 K. Kumar et al.

Fig. 9 Schematic representation of origin of two emission bands inDAST-doped silica glass with 266-nm excitation. E0, E1, and E2 rep-resent ground, first, and second excited states of DAST molecule

emission band at 402 nm most probably arises due to thetransition from the second excited state to the ground state.This transition is neither observed in DAST powder nor inliquid phase. In solid phase the distance between neighbor-ing molecules become so small, and, due to this small dis-tance, quenching in fluorescence occurs. The excited mole-cule in the second excited state relaxes nonradiatively to thefirst excited state, and only emission from the first excitedstate is observed. In liquid phase, absorption spectrum showtwo separated absorption bands, but emission from secondexcited state is not observed. The reason of this is that liq-uid phase has higher phonon frequency in comparison toglass, and, due to this higher phonon frequency, moleculesexcited to the second excited state prefer nonradiative re-laxation channel and relax to first excited state. In case ofDAST in silica host, decrease in average phonon frequencygives radiative transition probability from the second excitedstate to the ground state, and hence an emission at 402 nm isobserved. Figure 9 schematically represents mechanism ofthe emission. The study of DAST in solid host matrix seemsvery important to study the sensitizing behavior of DAST,

since DAST has very large absorption cross-section at UVregion and also high temperature stability.

3.4.2 Two-photon up-conversion fluorescence

The two photon spectra of DAST and UDAST were recordedusing a pulsed Nd: YAG laser at 1.06 µm. Both DASTand UDAST in solid form show very strong broad bandup-converted emissions in the red region. Pump power de-pendence of the upconversion intensity, Iup = P n

pump (whereIup is upconversion intensity, Ppump is pump power, and n

represents the number of photons involved in upconversionprocess) shows n = 1.8, i.e., the two-photon process. Asmentioned earlier, no linear absorption occurs in the spec-tral range from 700–1800 nm in DAST/UDAST. So it is con-cluded that absorption of simultaneous two photons (TPA) isresponsible for the up-conversion fluorescence. Figure 10(a)shows the upconverted fluorescence spectra of DAST andUDAST in solid phase. Two peaks are observed at 589 and731 nm in case of DAST and one at 758 nm in case ofUDAST. On comparison with single-photon fluorescence,we find that both contain same spectral patterns. Hence, theabsorption of two photons at 1.06 µm excites the sampleat 532 nm, and emission is observed from the first excitedelectronic level to the ground level. The solution phase ofDAST also shows upconverted emission at 614 nm (similarto peak observed in the case of one-photon process). Thesolvent effect shows that highest upconversion intensity isobserved for the case of PEG solvent. The upconversion las-ing in DAST has been reported in methanol solution [31].In our study DAST shows higher upconversion efficiency inPEG than in the methanol, so PEG may be a good mediumfor DAST to observe lasing emission. Figure 10(b) shows1.06-µm pumped upconversion spectrum of DAST in PEGsolution.

3.5 Conclusions

The transparent single crystals of UDAST have been grownusing solution growth technique, and the optical propertiesof DAST and UDAST have been studies and compared.The addition of urea has shifted the optical absorption band∼130 nm towards the shorter wavelength. The solid DASTand UDAST show very broad one-photon emission with twoemission bands. However, in solution phase both samplesshow only one emission band at 611 nm because in sol-vent the chromophores of DAST/UDAST get ionized dueto the weak coupling between the chromophores, and thusthe emission has been observed from only one chromophore.The studies of effect of solvent for the fluorescence intensityhave shown that the PEG is better among the common sol-vents like PEG, water, acetone, and methanol. Nonetheless,strong two-photon luminescence has been observed for both,

One- and two-photon-pumped luminescence studies on DAST and UDAST organic dye molecules 93

Fig. 10 Upconversion spectraof (a) DAST and UDAST insolid phase and (b) DAST insolution phase (polyethyleneglycol)

DAST and UDAST, with 1.06-µm pump wavelength. Fromthese studies we conclude that DAST/UDAST may be suit-able for getting upconversion emission and even lasing with1.06-µm pump wavelength. The apart from NLO propertiesof DAST, the studies reveal that the DAST and UDAST arethe promising materials for fluorescence properties. Further-more, the DAST-doped glass host matrix gives hope to use it

as a sensitizer to other visible luminescent organic/inorganicentities.

Acknowledgements Authors are grateful to Alexander von Hum-boldt Foundation, Germany, for donating Split Light 600, Nd:YAGlaser. Authors are also grateful to Prof. D.K. Rai for many valuablesuggestions.

94 K. Kumar et al.

References

1. P. Laveant, C. Medrano, B. Ruiz, P. Gunter, Chimia 57, 349(2003)

2. A. Schneider, M. Neis, M. Stillhart, B. Ruiz, R.U.A. Khan, P.Gunter, J. Opt. Soc. Am. B 23, 1822 (2006)

3. D.J. Williams, Nonlinear Optical Properties of Organic and Poly-meric Materials. ACS Symposium Series, vol. 233 (Am. Chem.Soc., Washington, 1983)

4. U. Meier, M. Bosch, C. Bosshard, P. Gunter, Synth. Met. 109, 19(2000)

5. S. Okada, H. Matsuda, H. Nakanishi, M. Kato, R. Muramatsu,Organic material for nonlinear optical use. Japanese patent,63048265JP, 1988

6. P.Y. Han, M. Tani, F. Pan, X.C. Zhang, Opt. Lett. 25, 675 (2000)7. G.R. Meredith, J. VanDusen, D.J. Williams, Macromolecules 15,

1385 (1982)8. S. Follonier, M. Fierz, I. Biaggio, U. Meier, C. Bosshard,

P. Gunter, J. Opt. Soc. Am. B 19, 1990 (2002)9. H. Umezawa, K. Tsuji, S. Okada, H. Oikawa, H. Matsuda, H.

Nakanishi, Opt. Mater. 21, 75 (2003)10. M.S. Wong, C. Bosshard, P. Gunter, Adv. Mater. 9, 837 (2004)11. G.S. He, C.F. Zhao, J.D. Bhawalkar, P.N. Prasad, Appl. Phys. Lett.

67, 3703 (1995)12. T. Taniuchi, S. Okada, H. Nakanishi, Electron. Lett. 40, 60 (2004)13. X.C. Zhang, X.F. Ma, Y. Jin, T.M. Lu, E.P. Boden, P.D. Phelps,

K.R. Stewart, C.P. Yakymyshyn, Appl. Phys. Lett. 61, 3080(1992)

14. F. Pan, M.S. Wong, C. Bosshard, P. Gunter, Adv. Mater. 8, 591(1996)

15. M. Berggren, A. Dodabalapur, R.E. Slusher, Z. Bao, Nature 389,466 (1997)

16. M. Lal, S. Pakatchi, G.S. He, K.S. Kim, P.N. Prasad, Chem. Mater.11, 3012 (1999)

17. C. Bosshard, M. Bosch, I. Liakatas, M. Jager, P. Gunter, in Non-linear Optical Effects and Materials. Springer Series in OpticalScience, vol. 72 (Springer, Berlin, 2000), p. 163

18. M. Baldo, M. Deutsch, P. Burrows, H. Gossenberger, M. Gersten-berg, V. Ban, S. Forrest, Adv. Mater. 10, 1505 (1998)

19. W. Demtroder, Laser Spectroscopy: Basic Concepts and Instru-mentation, 3rd edn. (Springer, Berlin, 2002)

20. R.N. Rai, C.W. Lan, J. Mater. Res. 17, 1587 (2002)21. N.A. van Dantzig, P.C.M. Planken, H.J. Bakker, Opt. Lett. 23, 466

(1998)22. S.R. Marder, J.W. Perry, W.P. Schaefer, Science 245, 626 (1989)23. J. Yabuzaki, Y. Takahashi, H. Adachi, Y. Mori, T. Sasaki, Bull.

Mater. Sci. 22, 11 (1999)24. R.N. Rai, J.M. Jeng, C.Y. Tai, C.W. Lan, J. Chin. Inst. Chem. Eng.

33, 461 (2002)25. J. Thomption, R.I.R. Blyth, M. Mazzeo, M. Anni, G. Gigli, R. Cin-

golani, Appl. Phys. Lett. 79, 560 (2001)26. S.R. Marder, J.W. Perry, C.P. Yakymyshy, Chem. Mater. 6, 1137

(1994)27. M. Del Zoppo, C. Castiglioni, G. Zerbi, Nonlinear Opt. 9, 73

(1995)28. S.H. Wemple, M. DiDomenico, Appl. Solid State Sci. 3, 263

(1972)29. W. Kemp, Organic Spectroscopy, 3rd edn. (Palgrave, New York,

1991)30. N. Sarkar, K. Das, D.N. Nath, K. Bhattacharya, Langmuir 10, 326

(1994)31. G.S. He, R. Signorini, P.N. Prasad, IEEE J. Quantum Electron. 34,

7 (1998)