Measurement of Nonlinearity and Spectral Study of a Laser Dye

Rapid technological developments in optics have placed great demand on the development of nonlinear optical (NLO) materials [1]. The use of solid state dye lasers gets rid of many of the common problems associated with static or flowing liquid systems. The most frequently used polymeric material is polymethylmethacrylate (PMMA) [2]. The low solubility of dye in PMMA causes limitations, which is overcome by introducing modifying additives. A review of literature showed that most of the work on dye-doped polymers has been done with Rhodamine dyes [3] and pyromethane dyes. Some works are reported on coumarin dyes [2]. The study of nonlinear refractive index on dye IR140 [4], has been reported. No work has been reported on the study of nonlinear refractive index of the dye Victoria blue. The physical origin of nonlinear refraction can be electronic, molecular, electrostrictive or thermal. Historically, the first optical limiter was based on thermal mechanism with a cw laser [5]. Recently, thermal effects have been shown to be efficient even with nanosecond pulses [6,7]. In this paper, we have studied the spectral characteristics and nonlinear properties of the dye Victoria blue in both the polymer and the corresponding monomer compositions in detail.


Introduction
Rapid technological developments in optics have placed great demand on the development of nonlinear optical (NLO) materials [1]. The use of solid state dye lasers gets rid of many of the common problems associated with static or flowing liquid systems. The most frequently used polymeric material is polymethylmethacrylate (PMMA) [2]. The low solubility of dye in PMMA causes limitations, which is overcome by introducing modifying additives. A review of literature showed that most of the work on dye-doped polymers has been done with Rhodamine dyes [3] and pyromethane dyes. Some works are reported on coumarin dyes [2]. The study of nonlinear refractive index on dye IR140 [4], has been reported. No work has been reported on the study of nonlinear refractive index of the dye Victoria blue. The physical origin of nonlinear refraction can be electronic, molecular, electrostrictive or thermal. Historically, the first optical limiter was based on thermal mechanism with a cw laser [5]. Recently, thermal effects have been shown to be efficient even with nanosecond pulses [6,7] . In this paper, we have studied the spectral characteristics and nonlinear properties of the dye Victoria blue in both the polymer and the corresponding monomer compositions in detail.

Experimental Procedure
The laser dye, Victoria blue obtained from Exciton Inc., USA was chosen for this study. Thin layer chromatography (TLC) test confirms the absence of any impurities in this dye. Methyl methacrylate (MMA) (Lancaster) was used as monomer. Initial MMA compositions were cleared of foreign inclusions. Spectroscopic grade n-butyl acetate (nBA) purchased from Merck (India) was chosen as an additive because it combines good solubility for victoria blue dye.

Synthesis of dye-doped polymer rod and film
The dye-doped polymer rod of dye concentration 0.05 × 10 -3 M was synthesized by thermal bulk free radical polymerization method [8].
MMA and nBA were taken in the ratio 4:1(v/v). Known weight of dye was dissolved in this mixture. 3 g of 2,2-azobis (isobutyronitrile) per litre of MMA solution was used as an initiator for polymerization. The solution was put into a polymerization tube and kept in the nitrogen atmosphere. Bulk polymerization was carried out in a temperature controlled water bath and the temperature was maintained at 35°C for 2 days, at 40°C for another 2 days and at 50°C for 7 days. The polymerized rod was removed by breaking the glass tube. Rod (length 3 cm: diameter 1cm) obtained, was cut, ground and polished to optical quality required. The internal optical qualities of polymer rod were checked by passing the He-Ne laser beam of 5mW through the rod. No dispersion or distortion of the He-Ne laser beam was observed. The dye doped polymer thin film of concentration 0.05 × 10 -3 M was prepared by pouring the viscous dye solution with initiator mixture on to a glass slide placed inside a glass enclosure, kept at a temperature of 40°C in temperature controlled water bath.

Spectral characteristics, quantum yield
The spectral properties of the dye are studied by recording the absorption and fluorescence spectra of dye in MMA, in nBA, in a mixture of MMA and nBA (liquid medium) and in the solid matrix (PMMA modified with nBA) using Hitachi U2000 spectrophotometer and Hitachi F2000 spectrofluorometer respectively. The fluorescence spectra were corrected using quinine sulphate in 0.1N H 2 SO 4 and fluorescein in 0.1N NaOH [9]. Care was taken to record all spectra under identical conditions. These spectra are shown in Figures 1 and 2. Quantum yields [10] were calculated using Rhodamine 6G (Rh 6G) in ethanol as the fluorescence standard with refractive index and differential absorption correction. The absorbance of Rh 6G in ethanol at 500nm is 0.66 and the reported quantum yield of the dye Rh 6G is 0.95 [11]. The spectral parameters such as absorption peak wavelength (peak ‫,)ג‬ molar extinction coefficient (€), bandwidth ‫ﬠ∆(‬ 1/2 ), oscillator strength (F),fluorescence peak wavelength (peak ‫,)ג‬ full width at half maximum (FWHM), fluorescence quantum yield (Ø f ), Stoke's shift of the dye were calculated and are shown in Table 1.

Nonlinear studies
The closed z-scan set up is shown in Figure 3. A He-Ne laser (632.8 nm, power 10 mW) was used as the light source. The dye doped polymer film was used as sample of thickness 1mm. The transmittance of the nonlinear medium through a finite aperture in the far field as a function of the sample position z was measured by using a positive lens having a focal length f=4 cm. The results obtained are compared with the dye in nBA. The liquid was filled in the sample cell with a path length of 1mm.The beam waist (ω 0 ) of He-Ne laser was calculated using the formula ω 0 ‫ג22.1=‬ f/d (d is the diameter of the aperture in front of the focal lens, f is the focal length of the convex lens used in the set up) and is found to be 75 μm with a Rayleigh range of 27.91 mm. The transmitted energy was measured using PMT (RCA 931) connected to an oscilloscope (OS768A) in a far field. Care was taken that PMT does not get saturated.
As the monomer (MMA) does not introduce any nonlinearity, the experiment was performed only for the dye in nBA. Figures 4 and  5 show the z-scan of the dye victoria blue in nBA and thin polymer film respectively. Figure 6 shows the optical limiting curves obtained with a He-Ne laser input of wavelength 632.8 nm for victoria blue in solvent nBA. The samples show very good optical limiting behavior arising from nonlinear refraction. The output power rises initially with increase in input power, but after a certain threshold value the samples start defocusing the beam resulting in a greater part of the beam crosssection being cut off by the aperture. Thus the transmittance recorded by the photo detector remained reasonably constant showing a plateau region.

Results
The absorption and fluorescence spectral profiles of the dye in liquid and in solid media are identical. This indicates that at low concentrations, the dye exists only in its monomeric form inside a polymer matrix and that at such low concentration there is a given amount of free volume within the polymeric matrix in which the dye molecules distribute in a way to what happens in a diluted liquid solution of the dye [12].
The peak wavelength of absorption and fluorescence of victoria blue in the solid matrix shows a blue shift from that in the liquid media. These changes may be attributed to the structure of dye. The spectral changes of the dye in the solid matrix and liquid media may also be due to the changes in the dielectric constant of the respective media.
The absorption bandwidth and the fluorescence bandwidth of the dye victoria blue are broader in solid matrix than in liquid medium. The spectral characteristics of the lasing dye in polymer depend on the intermolecular interaction between the dye molecule and macromolecules.
The Stoke's shift of the dye in solid matrix is more than that in the liquid medium. The Stoke's shift of the dye molecule in the solid matrix depends on the difference in the dipole moments between the   Analysis of nonlinearity in the dye victoria blue shows that the dye has a negative (self-defocusing) nonlinearity. Self-defocusing effect is due to local variation of refractive index with temperature. A pre-focal transmittance minimum (valley) followed by a post-focal transmittance maximum (peak) in the z-scan experiment is a signature of positive nonlinearity. The z-scan signature for the dye victoria blue in nBA gives the value of the transmission from peak to valley (∆T p-v ) is 0.19, which shows an index change of <∆n o >=-5.067 × 10 -6 , calculated using the formula, ∆Ø o =k ∆n o L eff , where k is the wave vector, L eff= ( 1-e -α L )/α , with L the sample length and α the linear absorption coefficient. This value of <∆n o > corresponds to the value of nonlinear coefficient of refractive index γ=-4.47 × 10 -12 m 2 /W, found using ∆n o =γ I 0 with I 0 being the onaxis irradiance at focus of the lens. The results are compared with dye doped polymer film which shows the value of transmission from peak to valley (∆T p-v ) as 0.201 giving an index change of < ∆n o >=-5.22 × 10 -6 . This value of <∆n o > corresponds to the value of γ=-4.61 × 10 -12 m 2 /W. The optical nonlinearity in this dye may be due to laser heating induced nonlinear effect. A laser beam, while passing through an absorbing media, induces temperature and density gradients that change the refractive index profile. This intensity-induced localized change in the refractive index results in a lensing effect on the optical beam. The value of (∆T p-v ) has increased for the dye doped polymer film when compared to the dye in nBA. This may be due to the heat dissipation in liquid is fast when compared to the polymer film. And it is this heat inducing a hike in (∆T p-v ) value in polymer film.

Conclusion
The spectral characteristics of dye victoria blue in liquid and solid environments have been studied in detail. Nonlinear studies show that the Victoria blue show a negative nonlinear refractive index. This dye doped polymer can be utilized in optical limiting, low-power degenerate four wave mixing, data storage and gating applications.
Nonlinear refraction in the membrane is used to obtain optical limiting behaviour in an aperture-limited geometry. The origin of optical nonlinearity observed in the CW regime is attributed to the thermal variation of local refractive index in the medium.