Journal of Quantum Informatio n Science, 2011, 1, 69-72
doi:10.4236/jqis.2011.12010 Published Online September 2011 (
Copyright © 2011 SciRes. JQIS
Third Order Optical Nonlinearities and Spectral
Characteristics of Methylene Blue
Velloli Sindhu Sukumaran1*, Alkondan Ramalingam2
1Central Institute of Plastics and Engineering Technology Guindy, Chennai, India
2Centre for Laser Technology, Department of Physics , Anna University, Chennai, India
E-mail: *
Received May 24, 2011; revised July 6, 2011; accepted July 26, 2011
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 tech-
nique. The dye methylene blue showed a negative nonlinear refractive index.
Keywords: Methylene Blue, Polymer, Solid Dye Laser, nBA, MMA, Spectral Characteristics, Nonlinear
Refractive Index, Polymer Thin Film
1. Introduction
The protection of sensitive optical equipment has been
the focus of much attention in recent times. Of great im-
portance is the protection of human eye from potentially
harmful intense beams. A large number of compounds
have been synthesized to realize nonlinear susceptibili-
ties 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 liq-
uid systems. The most frequently used polymeric mate-
rial 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.
2. Experimental Procedure
2.1. Synthesis of Dye-Doped Polymer Rods and
thin Films
Methylene blue, a phenothiazine dye, supplied by Exci-
ton, USA, is chosen for the study. The molecular struc-
ture of the dye is shown in Figure 1. Thin layer choro-
matography (TLC) test confirms the absence of any im-
purities in the dye. Methyl methacrylate (MMA) is used
as a monomer for synthesizing dye doped polymer film.
Initial MMA compositions are cleared of foreign inclu-
sions. Spectroscopic grade n-butyl acetate (nBA) is used
Figure 1. Molecular Structure of Methylene blue.
as additive. The dye doped polymer film (DDP) and rod
are synthesized using thermal bulk free radical polym-
erization [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 distor-
tion of the He-Ne laser beam was observed.
2.2. 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 spectropho-
tometer and Hitachi F2000 spectrofluorometer respec-
tively. 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 parame-
ters such as absorption peak wavelength, molar extinc-
Figure 2. Absorption spectra of dye Methylene blue in
PMMA + nBA and MMA + nBA.
tion coefficient (€), band width (∆ﬠ1/2), 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 (Øf), radiative (Kr)
and non-radiative (Knr) decay constants of the dyes are
shown in Table 1.
2.3. Nonlinear Studies
The closed z-scan [9] set up, developed by Sheik Bahae
et al. is used to characterize the nonlinear optical proper-
ties of the dye in different mixtures. It is based on inten-
sity dependent refractive index and includes variation of
refractive index as a function of incident beam irradiance
of the sample. The set up is shown in Figure 4. A Gaus-
sian beam from He-Ne laser (λ: 632.8 nm, power: 10
mW) was focused by a convex lens of focal length 20cm
and passed through the sample. The sample is scanned
through the beam, the far field profile shows intensity
variation across the beam profile, which is recorded
through an aperture using photo detector fed to the digi-
tal power meter (Field Master GS- coherent). The figure
provides not only the magnitude of real and imaginary
545 645 745
F luoresc ence intensity
Wavelength (nm)
PMMA mo d i fied
with nB A
Figure 3. Fluorescence spectra of dye Methylene blue in
PMMA + nBA and MMA + nBA.
Table 1. Spectral characteristics, quantum yield, lifetime, radiative and non-radiative decay constants of the dye Methylene
Absorption spectra Fluorescence spectra
Solvent/Medium Peak wavelength
in nm
104 L
mol–1 cm–1
Oscillator Strength
f 10–24 L mol–1 cm–2
Peak wavelength
in nm
M nm
shift cm–1
Distilled water 600 0.4 2797.20.4844 662 98 1560.9
PMMA modified with
Distilled water 555 0.352 2541.30.3873 588 107.5 1011.2
Copyright © 2011 SciRes. JQIS
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 valu-
able in application than the solution and the discrepancy
between films and solutions will greatly affect the be-
havior of molecules, which will have an obvious influ-
ence on their nonlinear properties.
3. 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 de-
focusing effect. The self defocusing effect is due to local
variation of refractive index with temperature. The defo-
cusing effect of the dye in liquid medium and in polymer
film is shown in Figures 5 and 6, is attributed to a ther-
mal nonlinearity resulting from absorption of 632.8 nm.
Figure 4. Z scan set-up.
-3 -2 -10123
Z/ Z0
Normali zed Transmi t tance
Figure 5. Measured z-scan of the dye Methylene blue. The
solid line is the calculated result with Øo = –0.95.
-3 -2 -1012
Normalised Transmittance
Figure 6. Measured Z scan of the dye methylene blue doped
polymer film. The solid line is the calculated result with
= –0.13.
The localized absorption of tightly focused beam pro-
pagating 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 (Tp-v) as 0.0468, which shows an index
change of <no>= –1.907 × 10–6. This value of <no>
corresponds to the value of = –1.05159 × 10–12 m2/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 (Tp-v) as 0.0537, which gives an in-
dex change of –1.362 × 10–6. This gives = –1.20295 ×
10–12 m2/W
4. Acknowledgements
Authors wish to acknowledge National Centre for Ultra
fast Processes, University of Madras for extending the
facilities available in their centre.
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