Third Order Optical Nonlinearities and Spectral Characteristics of Methylene Blue

We have investigated third order nonlinear optical properties and spectral characteristics of methylene blue dye in both polymer and liquid mixtures. The spectral characteristics of the dye is studied by recording the absorption and fluorescence spectra of the dye doped in poly(methylmethacrylate) modified with additive n-butyl acetate(nBA) and the dye in MMA and nBA (liquid mixture). The spectral results of the dye doped polymer rod are compared with dye in liquid Mixture. The nonlinear measurements of the dye in liquid and polymer medium were performed using CW He-Ne laser of wavelength 632.8 nm by employing z-scan technique. The dye methylene blue showed a negative nonlinear refractive index.


Introduction
The protection of sensitive optical equipment has been the focus of much attention in recent times.Of great importance is the protection of human eye from potentially harmful intense beams.A large number of compounds have been synthesized to realize nonlinear susceptibilities far larger than the inorganic optical material [1].In this paper we report the synthesis, characterization and nonlinear optical properties of methylene blue dye doped polymer and compare it with dye doped monomer.Nonlinear optical properties of polymer solution was studied by means of a z-scan set-up.Nonlinear optical effects can be employed for the design and performance of optical limiter.It has been showed that optical limiting can be used for the protection of eyes and sensors from intense lasers [2].
Solid matrix used as lasers get rid of many of the common problems associated with static or flowing liquid systems.The most frequently used polymeric material is polymethylmethacrylate (PMMA) [3].Review of literature showed most of the work on dye-doped polymers was done with rhodamine dyes [4] and pyromethane dyes [5].Some work was reported on coumarin dyes [3] and on dye IR140 [6].
In this paper, the fabrication of dye methylene blue doped polymer rods and films, its spectral parameters and the study of nonlinear refractive index under the He-Ne laser excitation are reported.The properties of the dyes in liquid medium are compared with that in the solid matrix.as additive.The dye doped polymer film (DDP) and rod are synthesized using thermal bulk free radical polymerization [7] of dye concentration 0.05 × 10 -3 M. The internal optical qualities of polymer rods and film are checked by passing the laser beam of 5 mW He-Ne laser (632.8 nm) through these rods.No dispersion or distortion of the He-Ne laser beam was observed.

Spectral Characteristics, Life Time, Quantum Yield
The UV-VIS absorption and fluorescence spectrum of the dye in liquid medium and the solid matrix (PMMA + nBA) was obtained using Hitachi U2000 spectrophotometer and Hitachi F2000 spectrofluorometer respectively.These spectra are shown in Figures 2 and 3.The fluorescence lifetime is studied by using the picosecond time correlated single photon counting technique and employing analytical deconvocation methods.Quantum yields [8] are calculated using Rhodamine 6 G in ethanol as the fluorescence standard with refractive index and differential absorption correction.The spectral parameters such as absorption peak wavelength, molar extinc- tion coefficient (€), band width ‫ﬠ∆(‬ 1/2 ), oscillator strength (F), fluorescence peak wavelength, full width at half maximum (FWHM), Stoke's shift of the dyes and the calculated lifetime ‫ح(‬ f ) quantum yield (Ø f ), radiative (K r ) and non-radiative (Knr) decay constants of the dyes are shown in Table 1.

Nonlinear Studies
The closed z-scan [9]   parts of the nonlinear susceptibility, but also the sign of real part [9].The radius of the beam waist (ωo) was 75µm with a Rayleigh range of 27.91 mm.Care is taken that the absorber is not saturated.The film is more valuable in application than the solution and the discrepancy between films and solutions will greatly affect the behavior of molecules, which will have an obvious influence on their nonlinear properties.

Results
The analysis z scan curve of methylene blue showed that dye has a negative (self-defocusing) nonlinearity.The peak followed by a valley -normalized transmittance curve obtained from the closed z scan data, indicates the sign of refraction nonlinearity is negative, i.e., Self defocusing effect.The self defocusing effect is due to local variation of refractive index with temperature.The defocusing effect of the dye in liquid medium and in polymer film is shown in Figures 5 and 6, is attributed to a thermal nonlinearity resulting from absorption of 632.8 nm.The localized absorption of tightly focused beam propagating through the absorbing dye medium produces a spatial distribution of temperature in the dye solution and, consequently, a spatial variation of refractive index that act as a thermal lens resulting in phase distortion of the propagating beam.The z-scan signature for the dye in liquid medium gives the value of the transmission from peak to valley (T p-v ) as 0.0468, which shows an index change of <∆n o >= -1.907 × 10 -6 .This value of <∆n o > corresponds to the value of  = -1.05159× 10 -12 m 2 /W.The z-scan signature for the dye Methylene blue in solid matrix (polymer film) gives the value of the transmission from peak to valley (T p-v ) as 0.0537, which gives an index change of -1.362 × 10 -6 .This gives  = -1.20295× 10 -12 m 2 /W

Figure 2 .
Figure 2. Absorption spectra of dye Methylene blue in PMMA + nBA and MMA + nBA.

Figure 3 .
Figure 3. Fluorescence spectra of dye Methylene blue in PMMA + nBA and MMA + nBA.

Figure 6 .
Figure 6.Measured Z scan of the dye methylene blue doped polymer film.The solid line is the calculated result with  0 = -0.13.