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Optics & Laser Technology 40 (2008) 373–378

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Z-scan studies and optical limiting of nanosecond laser pulsesin neutral red dye

Mathew Georgea, C.I. Muneeraa,�, C.P. Singhb, K.S. Bindrab, S.M. Oakb

aDepartment of Physics, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala 695581, IndiabUltrafast Studies Section, Raja Ramanna Centre for Advanced Technology, Indore, India

Received 31 August 2006; received in revised form 15 June 2007; accepted 28 June 2007

Available online 30 August 2007

Abstract

The nonlinear optical absorption, refraction and optical limiting behaviour of an organic dye, neutral red, were investigated under

excitation with nanosecond laser pulses at 532 nm. The nonlinear optical responses of the material were studied both in solution and solid

film, made in methanol and polyvinyl alcohol, respectively, using single-beam Z-scan technique. The open aperture Z-scans of the

solution samples displayed a switch over from saturable absorption to enhanced absorption with increase in input intensity. Theoretical

fit to the experimental data indicated that the dominant mechanism of nonlinear absorption is two-photon absorption. The closed

aperture Z-scans of both the samples denoted positive nonlinearity, which was three orders larger in magnitude in solid film, compared

with that in solution. The results of optical limiting experiments revealed that neutral red exhibited strong optical limiting of nanosecond

laser pulses with a threshold lower than that of C60 in toluene.

r 2007 Elsevier Ltd. All rights reserved.

PACS: 42.70.�a; 42.65.�k

Keywords: Nonlinear optical absorption; Neutral red; Optical limiting

1. Introduction

In the recent past, rapid technological advancements inoptics have placed great demand on the development ofnonlinear optical (NLO) materials suitable for photonicdevices [1,2]. Numerous organic chromophores exhibitextremely large and fast nonlinearities, much better thanthose observed in inorganic crystals. In addition, due to theversatility of organic synthesis, their NLO properties canbe custom-tailored for a specific application and aretherefore, a much better choice for uses in NLO applica-tions [3,4]. One of the important applications of thesematerials is in optical limiting (OL) devices used to protecteyes and sensors against damage by exposure to suddenhigh-intensity light, which still remains a challengingproblem. Optical limiters can transmit low-intensity laserpulses effectively and attenuate high-intensity laser pulses

e front matter r 2007 Elsevier Ltd. All rights reserved.

tlastec.2007.06.008

ing author. Tel.: +91471 2337478.

ess: drcimuneera@hotmail.com (C.I. Muneera).

strongly by using the NLO properties of the materials. Theselection of efficient materials is still the key point foroptical power limiters and it has led to the study ofmaterials that exhibit strong nonlinear absorption [5,6].Nonlinear absorption in dyes could be due to reversesaturable absorption (RSA), two-photon absorption (TPA)and saturable absorption (SA) depending on the changein the absorption (increase or decrease) with increase inintensity. Both TPA and RSA lead to increase inabsorption in the sample with increase in intensity. For agiven organic sample at a fixed wavelength, either TPA orRSA is the dominant mechanism leading to increase inabsorption with intensity. RSA is observed when excitedstate absorption (ESA) is greater than the ground stateabsorption, which results in decrease in the transmissionthrough the medium with increase in the input intensity.However, for SA, the transmission through the mediumincreases with increase in the input intensity. Switch overfrom RSA to SA with increase in the input intensity isobserved in various materials under nano- and picosecond

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N

N

H

CH3

NH2N(CH3)2

Fig. 1. Molecular structure of neutral red dye molecule.

400 500 600 7000.0

0.2

0.4

0.6

Absorb

ance (

a.u

.)

Wavelength (nm)

Fig. 2. Absorption spectra of neutral red dye solid film (solid line) and

solution (dashed line) samples.

D2D1

D3

+z-z

BSSL

BSA

Fig. 3. Schematic diagram of Z-scan setup: D1—reference photodiode,

D2—open aperture photodiode, D3—closed aperture photodiode, BS—

beam splitter, A—aperture, L—lens, S—sample.

M. George et al. / Optics & Laser Technology 40 (2008) 373–378374

pulse excitations [7]. At very high fluences, the ESA thatcauses RSA also gets saturated. Recently, transition fromSA to RSA behaviour has been reported in platinumnanoparticles [8] and rhodamine B [9] under nanosecondpulse excitation and in monolayer-protected gold, silverand gold–silver alloy nanoclusters [10] under picosecondpulse excitation. Different reasoning has been given in theanalysis of the results.

In this paper, we present the results of our investigationson the nonlinear absorption, nonlinear refraction and OLresponse of the organic chromophore, neutral red (NR)under nanopulsed laser light irradiation at 532 nm. NR(3-amino-7-dimethylamino-2-methyl phenazine; molecularstructure shown in Fig. 1) is a low-cost organic dye whichfinds many applications in biology [11]. Z-scan experimentsand nonlinear transmission measurements were carried outto investigate the NLO properties and OL behaviour,respectively, of the dye chromophore. The NR solutionsamples displayed a switch over from SA to enhancedabsorption with intensity. The dominant mechanismresponsible for optical nonlinearity (increase in theabsorption with increase in intensity) in NR samples isshown to be two-photon absorption. NR solid film samplessignified a positive optical nonlinearity, three orders largerin magnitude than the solution samples. Moreover, neutralred exhibited strong OL of nanosecond laser pulses with athreshold lower than that of C60 in toluene.

2. Experimental

The solution samples of the dye, NR (M/s NiceChemicals, Cochin, India) were prepared in methanol(spectroscopic grade). The solid films were fabricated usingaqueous polyvinyl alcohol (PVA) as matrix, following theguest–host method [12,13]. The former was contained in a1mm quartz cuvette, and had a refractive index of 1.33,while the latter, with 80 mm thickness, had a refractiveindex of 1.4.

Fig. 2 shows the linear absorption spectra of solid andsolution samples. The linear transmission of the solidsample used was 1% and that of the solution sample was20% at 532 nm. The linear absorption maxima (lmax) ofsolution (461 nm) and solid film (470 nm) samples corre-spond to the neutral form of the dye. But, in the case ofsolution, a second absorption band appears around531 nm, which, according to the literature, is due to thepresence of the cationic form of the dye (NRH+) [11].

The nonlinear absorption and refraction in NR dye wereinvestigated using single-beam Z-scan technique. Z-scanmethod is a simple and effective tool for accuratedetermination of nonlinear absorption as well as fornonlinear refraction and its sign. Several modifications tothis technique are also employed for the measurement ofNLO parameters [14–20]. Z-scan measurements are sensi-tive to the homogeneity of the sample. The homogeneity ofthe samples was assured by performing transmissionmeasurements at various parts of the film. The Z-scanexperiments were conducted employing frequency doubledpulsed Nd:YAG laser light (532 nm, 30 ns) with a repetitionrate of 1Hz. The sample remained stable even afterexposure to laser pulses for a long period of time. Thestandard Z-scan set up [21], shown in Fig. 3, was used forthe measurement of NLO coefficients. The transmittance ofthe incident laser beam through an aperture in the far fieldwas measured as a function of the sample position withrespect to the focal plane. This formed a closed aperturescan. The experiment was performed at fixed input laserenergy. For the open aperture scan, a laser beam splitterwas kept before the aperture and this laser beam was

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-12 -8 -4 0 4 8 12

0.7

0.8

0.9

1.0

1.1

1.2

1.3

No

rma

lize

d T

ran

sm

iss

ion

Z (cm)

Fig. 5. Open aperture Z-scan experimental data (squares) of NR solution

of 20% linear transmission at 532 nm at I0 ¼ 80MW/cm2. Solid line shows

theoretical fit to the experimental data.

-12 -8 0 4 8 12

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3N

orm

alized

Tra

nsm

issio

n

Z (cm)

-150-100 -50 0 50 100 150

0.5

0.6

0.7

0.8

0.9

1.0

Norm

aliz

ed T

ransm

issio

n

Z (cm)

-4

Fig. 6. Open aperture Z-scan experimental data (squares) of NR solution

of 20% linear transmission at 532 nm at I0 ¼ 130MW/cm2. Solid line

M. George et al. / Optics & Laser Technology 40 (2008) 373–378 375

collected by another detector. The open aperture scan issensitive only to nonlinear absorption, whereas, the closedaperture scan is sensitive to nonlinear absorption as well asrefraction. This method, used for the measurement ofoptical nonlinearities, particularly nonlinear refraction andnonlinear absorption, yields both the sign and themagnitude of the nonlinearity [21]. Both closed and openaperture Z-scan measurements were performed, simulta-neously. OL experiments were also preformed with NRsolution. The sample was kept at the focus of a converginglens (focal length 20 cm) and the output was collectedthrough a lens kept behind an aperture having 90%transmittance.

3. Results and discussions

Figs. 4–6 illustrate the open aperture Z-scan data, of NRsolution (linear transmission 20% at 532 nm) at peakincident laser intensities of 50, 80 and 130MW/cm2,respectively, the spot size of the beam being �35 mm. Thesample shows SA behaviour away from focus and RSAbehaviour near the focus in all the three figures. Similarbehaviour is reported in the literature for some organicmaterials [22–24]. Here, transmission at the focus decreaseswith increase in the intensity, while the peak transmissionremains the same. For example, at 50MW/cm2 peakintensity, the normalized transmission of the samplereaches 1.22 at peak positions, while it drops to 0.93 atthe focus as shown in the Fig. 4, and at 130MW/cm2 peakintensity, the normalized transmission at the peak is still1.22, while the dip transmission is 0.54 as shown in Fig. 6.We can see that in Fig. 4, the transmission peak occursat 71.25 cm Z-position, while in Fig. 6, it occurs at72.25 cm. This shift in the peak position in Fig. 6, withrespect to Fig. 4, is due to the larger intensity used in thecase of Fig. 6 that provides the required intensity forsaturation of transmission at a larger Z-position. To

-12 -8 -4 0 4 8 12

0.9

1.0

1.1

1.2

No

rmalized

Tra

nsm

issio

n

Z (cm)

Fig. 4. Open aperture Z-scan experimental data (squares) of NR solution

of 20% linear transmission at 532 nm at I0 ¼ 50MW/cm2. Solid line shows

theoretical fit to the experimental data.

shows theoretical fit to the experimental data. Dashed line shows

theoretical curve considering excited state absorption. Inset shows dashed

curve for large Z-values.

estimate saturation intensity and nonlinear absorptioncoefficient, experimental data were fitted with numericalsimulations [9]. For estimation of saturation intensity, thevariation of absorption coefficient with intensity in generalis given by

a ¼a0

1þ I=I s, (1)

where a0 is the low-intensity absorption coefficient, I is theincident laser intensity and IS is the saturation intensity.The RSA behaviour in dyes in general is explained either interms of ESA or TPA phenomena depending on theexcitation wavelength. As the excitation wavelength is nearthe edge of the absorption band, we can expect TPA to bethe predominant mechanism for nonlinear absorption. Atthis wavelength, excitation may be into the lowest level of

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-6 -4 -2 00.4

0.6

0.8

1.0

1.2

1.4

1.6

Norm

aliz

ed T

ransm

issio

n

Z (cm)

642

Fig. 7. Closed divided by open aperture Z-scan data of NR solution of

74% linear transmission at 532 nm at I0 ¼ 130MW/cm2. Solid line shows

theoretical fit to the experimental data.

M. George et al. / Optics & Laser Technology 40 (2008) 373–378376

the first excited singlet state, and therefore, one can expectgreater localization of energy [25]. In this case, thepropagation of the laser beam through the medium canbe given by

dI

dz¼ �aI � bI2, (2)

where b is the TPA coefficient and z is the propagationdistance in the medium. In Figs. 4–6 solid lines showtheoretical fit to the experimental data using aboveequations, where b and Is have been used as fittingparameters to match the dip and peak, respectively, inthe experimental data. In all the figures, theoretical fits givesaturation intensity, Is ¼ 18MW/cm2 and TPA coefficient,b ¼ 110� 10�8 cm/W. The estimated b value in NRsolution is very large compared with recently reported bvalues in various dyes viz., Rubrene, Eosin, Pryridin1,Fluorescein 27, Rhodamine 6G and Rhodamine B [9,26].In Figs. 4–6 the open aperture Z-scan experimental dataand theoretically estimated curves match well, indicatingthat TPA is the mechanism responsible for the observednonlinear absorption.

Further, we tried to fit the experimental data with ESAto rule out its possibility. For ESA, rate equationsdescribed in Ref. [9] were solved to estimate the magnitudeof the ESA cross-section (s2). The transmission wasevaluated by considering the ground and ESA, and thesaturation intensity. In this case, s2 and b values were usedas the fitting parameters to match the transmission at focusin the experimental data. The already estimated value ofsaturation intensity, Is ¼ 18MW/cm2, was also used in theanalysis. The ground state absorption cross-section mea-sured from the linear transmission data is 9.2� 10�17 cm2.The dashed line shown in Fig. 6 was generated for the ESAcross-section, s2 ¼ 13.25� 10�17 cm2 and b ¼ 0, to matchthe dip in the transmission of the experimental data. Thetheoretically estimated complete normalized transmissioncurve does not appear within the experimentally usedZ-values. The complete dashed curve is shown in the insetof Fig. 6 and was generated with Z values much larger thanthat used in the experimental data. Obviously, the dashedcurve does not fit to the experimental data. The saturationof absorption is also included in the analysis, but no peak isobserved in the transmission curve. Therefore, we concludefrom our analysis that the dominant mechanism ofnonlinear absorption in neutral red dye is TPA. Similarconclusions were drawn in Ref. [9] in the case ofRhodamine B.

The observed large nonlinear absorption motivated us tostudy OL behaviour of the neutral red dye. OL studies aregenerally conducted on samples showing about 70% lineartransmission at the excitation wavelength. Before perform-ing OL experiments, we carried out Z-scan experiment withthis neutral red dye solution (74% linear transmission at532 nm). Fig. 7 illustrates the experimental data afterdividing the closed aperture Z scan by the open apertureZ-scan data. The figure shows clear pre-focal valley

followed by post-focal peak indicative of positive non-linearity. The experiment was performed at a peak intensityof 130MW/cm2. The nonlinear refractive index, n2 wasevaluated for the sample using the standard Z-scan analysis[21], according to which, the change in phase f is given by

dfdz¼ kn2I . (3)

The intensity at the input face of the sample is taken tobe Gaussian in space and time. The total amplitude andphase change at the end of the sample is obtained bysolving Eqs. (2) and (3). The electric field at the exit face ofthe sample is thus determined completely. The field at theaperture is then determined by the Huygens–Fresnelpropagation integral. The transmitted energy through theaperture placed in the far field is then calculated. Thetransmitted energy is normalized to unity for the sampleposition far away from the focus. The solid line in Fig. 7depicts the theoretical fit to the experimental data. Theestimated value of nonlinear refractive index, n2 of thesample was found to be 1.5� 10�12 cm2/W. The absoluteerror in the estimation of the n2 value is 750%. The errorarises due to error involved in the estimation of absoluteintensity. The absolute value of intensity is determined lessaccurately as it involves separate measurements of spotsize, pulse width and energy. However, the error in therelative n2 values is expected to be 75%. This is due to thefact that relative pulse energy measurements are estimatedto be accurate within 5%.Closed aperture Z-scan curve of NR solid film (Fig. 8)

also shows a clear pre-focal valley followed by a post-focalpeak, which implies that the nonlinear refractive index ofthe solid form of the dye in PVA matrix is also positive(n240), as exhibited by the dye solution in methanol. Theexperiment was performed at 2.53MW/cm2 peak intensity.Squares are experimental data points and the solid lineindicates theoretical fit to the experimental data. Theestimated value of n2 for the NR solid film was found to be

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-2 -1 0 1 20.2

0.4

0.6

0.8

1.0

1.2

Norm

aliz

ed T

ransm

issio

n

Z (cm)

Fig. 8. Closed aperture Z-scan curve of NR solid film; I0 ¼ 2.53MW/cm2.

Solid line shows theoretical fit to the experimental data.

0 2000 4000 600010

20

30

40

50

60

Tra

nsm

issi

on (

%)

Input fluence (mJ/cm2)

Fig. 9. Variation in transmission through NR solution of 74% linear

transmission at 532 nm with variation in input fluence.

0 2000 4000 60000

400

800

1200

1600

2000

Out

put f

luen

ce (

mJ/

cm2 )

Input fluence (mJ/cm2)

Fig. 10. Variation in output fluence with input fluence through NR

solution of 74% linear transmission at 532 nm. Solid line shown gives 74%

linear transmission of the sample.

M. George et al. / Optics & Laser Technology 40 (2008) 373–378 377

4.0� 10�9 cm2/W. The experimentally measured value ofn2 of the solid film was three orders of magnitude higherthan that of the solution form. It may be noted that theconcentration of NR dye in solution is �10�4mol/L, whilein solid film it is two orders of magnitude larger(�10�2mol/L). So, the larger value of n2 in solid form ofNR sample may be due to the greater concentration of NRdye molecules in the solid film compared with the solution.Pure PVA matrix shows no Z-scan signal. The Z-scansignal in the dye-doped solid matrix was thus attributed tothe neutral red dye molecules.

The OL response of NR solution with 74% lineartransmission is demonstrated in Fig. 9, which shows thebehaviour of the transmitted light intensity as a function ofthe input fluence. The measurements show a clear decreasein transmittance, when the incident fluence is increased. Ingeneral, the OL ability of a sample is evaluated in terms ofa limiting threshold (TH), which is defined as the incidentlaser fluence, at which the transmittance of the solution

falls to half its linear transmittance. The smaller the TH, thebetter would be the OL. We estimated TH of the sampleat T/T0 ¼ 0.5, where T0 is the linear transmittance. FromFig. 10 the estimated value of TH is 0.9 J/cm2. The OLthreshold value in NR solution is less than the reportedvalues for reference medium C60. For example, for C60 intoluene with 62% and 65% linear transmissions at 532 nm,reported TH are 1.6 J/cm2 [27] and 3.1 J/cm2 [28], respec-tively, and for C60 in chlorobenzene with 75% lineartransmission at 532 nm, TH is 2.7 J/cm2 [28]. The compar-ison also takes into account the effects of the differentsolvents used for C60 and NR. Even if the possible thermo-optic contribution of the solvents, to OL is considered,when nano-second pulsed laser is used for excitation, itwould be more pronounced for C60 in toluene andchlorobenzene than NR in methanol, since dn/dT oftoluene is greater than that of methanol [29,30].

4. Conclusions

In conclusion, the nonlinear absorption and refraction inthe organic dye, neutral red, both in solution and solid filmwere investigated using a single-beam Z-scan techniqueunder nanosecond laser pulse excitation at 532 nm. Openaperture Z-scans of the solution samples show increase intransmission away from focus and reduction in transmis-sion close to focus at this laser wavelength. Saturationintensity and nonlinear absorption coefficients were esti-mated by performing numerical fitting to the experimentaldata. It was found that the observed nonlinear absorptionis caused by a two-photon absorption process. Calculatedvalue of n2 of the solid film was three orders of magnitudehigher than that of the solution. The observed OL ofnanosecond laser pulses at 532 nm, showed that neutralred, which displayed a limiting threshold lower than that ofC60, is a good optical limiter. Moreover, the simultaneousoccurrence of several nonlinear processes in this dye,

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signifies the possibility of utilising it in photonic deviceapplications.

Acknowledgements

The help rendered by Kumari Geetha S., Louie Frobel P.G. and Suresh S. R. throughout the course of this work isgratefully acknowledged. We also thank Ms. P. Gaikwadfor providing help in extracting the experimental data.

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