Nonlinear optical properties of castor oil Introduction Experimental ...

XXIX ENFMC - Annals of Optics 2006
Nonlinear optical properties of castor oil
R. F. Souza 1* , M. A. R. C. Alencar 2 , M. R. Meneghetti 3 , J. M. Hickmann 2
1 Departamento de Eletrônica, Centro Federal de Educação Tecnológica de Alagoas, Maceió,
AL, 57000-000, Brazil
2 Instituto de Física, Universidade Federal de Alagoas, Maceió, AL, 57072-970, Brazil
3 Instituto de Química e Biotecnologia, Universidade Federal de Alagoas, Maceió, AL, 57072-
970, Brazil
* rogerio@loqnl.ufal.br
Abstract
We report on the investigation of the nonlinear optical properties of castor oil using the Z-scan
technique. Nonlinear refraction and absorption measurements were performed for two different
laser wavelengths, 514 and 810 nm, in the CW regime, and for 810 nm in femtosecond regime.
The CW results showed that castor oil has a large negative nonlinear refractive index n 2 = -3.2 x
10 -8 cm 2 /W for the laser wavelength at 514 nm, and this value was about one order of magnitude
smaller for excitation tuned at 810 nm. The negative sign and the slow response of this
nonlinearity indicate the thermal origin of this effect. In the femtosecond regime, it was not
possible to observe a typical Z-scan curve for castor oil. The nonlinear absorption was absent for
all experimental configurations.
Introduction
Organic materials have been exploited as a basis for photonic devices due to their large optical nonlinearities
and fast response, which can be attributed to light-induced molecule reorientation, incoherent electronic
excitation and thermal lens effect [1]. Besides their intrinsic nonlinear optical properties, organic materials have
been used in the development of colloidal structures containing nanoparticles.
Among the myriad of organic materials, the castor oil is of particular interest because of its several industrial
applications, such as a key raw-material for the production of several commodities like paints, polymers,
lubricants, etc. In terms of molecular constitution, castor oil is a mixture of triglycerides, predominantly derived
from a unsaturated and hydroxylated C:18 fatty acid, the ricinoleic acid. Recently, we showed that castor oil is a
very good stabilizer for colloidal systems containing gold nanoparticles and that these systems present a larger
spatial self-phase modulation effect [2]. However, a more accurate measurement of the nonlinear refractive
index and absorption of castor oil have not been performed yet.
In this work, we measured the nonlinear optical properties of castor oil and investigated its origin using the Z-
scan technique [3]. The measurements were performed for two different excitation wavelengths, 514 nm and
810 nm, in the CW regime, and for 810 nm in femtosecond regime. The origin of the observed large nonlinearity
of this oil is also discussed.
Experimental Setup
In the Z-scan technique the transmittance of a tightly focused Gaussian beam through a finite aperture in the far
field is measured as a function of the sample position z with respect to the focal plane. At each position, the
sample experiences a different light intensity. The nonlinear refraction of the sample causes a spatial beam
broadening or narrowing in the far field and thus modifies the fraction of light that passes through the aperture
as the sample position is changed. A typical peak-valley (valley-peak) transmittance curve is obtained when the
nonlinear refractive index of the medium is negative (positive). In the limit where the sample can be considered
thin, compared to the beam Rayleigh length (L< z 0 ), it is possible from the peak to valley variation of the
measured transmittance curve to evaluate the maximum nonlinear phase-shift and hence, knowing the incident
laser power, to obtain the nonlinear refractive index n 2 [3,4]. Removing the aperture in the far field it is possible

XXIX ENFMC - Annals of Optics 2006
to perform nonlinear absorption measurements. Such Z-scan traces are expected to be symmetric with respect to
the focal point (z = 0) where they exhibit a minimum transmittance in the case of nonlinear absorption
(multiphoton absorption) and a maximum for the saturation absorption. For media exhibiting both nonlinear
refraction and absorption properties, a closed aperture Z-scan measurement is sensitive to both effects. Dividing
the closed aperture data by the open aperture one yields a Z-scan trace typical of a purely refractive nonlinearity
[3].
The experimental setup used to measure the nonlinear refractive index and absorption of castor oil in this work
is depicted in the Figure 1. A CW Argon laser (Ti:sapphire laser) operating at 514 nm (810 nm) was used as a
light source. The laser beam was modulated by a chopper and focused onto the sample by a convergent lens of
7.5 cm focal length. The sample consisted of a 1 mm quartz cuvette within castor oil. It was mounted on a
translation stage and moved around the lens focus (z = 0) by a computer controlled stepper motor. The light
transmittance was then measured by a closed-aperture photodetector as a function of the sample position. The
detected signal was amplified by a lock-in amplifier and then processed by a computer. Nonlinear absorption
measurements were performed using the same experimental setup but using an opened aperture configuration. In
femtosecond regime, the Ti:sapphire laser was operated mode locked, producing pulses of 200 fs, at a repetition
rate of 76 MHz. Using a pulse selector, the light beam was modulated. In this case, the pulse repetition rate was
reduced to the range of 100 Hz to 10 kHz.
Figure 1: Experimental setup for optical nonlinearity measurements using Z-scan technique.
Results and Discussions
The linear UV-VIS absorption spectrum of castor oil is presented in Figure 2. As can be seen in this figure, this
oil presents a gap in the UV-blue region and it is almost transparent in the near infrared region. The laser
wavelengths used in our experiments are also indicated in this figure. Note that, the linear absorption coefficient
of castor oil is larger at 514 nm than at 810 nm.
Absorption coefficient (cm -1 )
6
5
4
3
2
1
514 nm
0
300 400 500 600 700 800
Wavelength (nm)
810 nm
Figure 2: Linear absorption spectrum of castor oil.

XXIX ENFMC - Annals of Optics 2006
Figure 3 shows the typical normalized transmittance, closed aperture, as a function of the sample position, for
CW laser excitation tuned at 514 nm. From the observed transmittance variation, the castor oil nonlinear
refractive index was evaluated as n 2 = -3.2 x 10 -8 cm 2 /W. Although this value is smaller than other materials,
such as photorefractive crystals and Chinese tea, this nonlinearity is much larger than several organic liquids as
DMSO and CS 2 , for example.
1,3
Normalized Intensity
1,2
1,1
1,0
0,9
0,8
0,7
-8 -4 0 4 8
1,3
z/z 0
Figure 3: Z-scan curve for castor oil at 514 nm in the CW regime, P laser = 10 mW.
A typical Z-scan curve obtained for CW laser excitation tuned at 810 nm is presented in Figure 4. In this case,
the measured nonlinear refractive index was much smaller than it was in the former case, n 2 = - 3.3 x 10 -9
cm 2 /W. This happens because the energy of the excitation at 514 nm is nearer to the gap energy of the castor oil,
and hence it is expected that the linear absorption and the nonlinear properties of this medium to be larger than it
is when the laser is tuned far from the resonance.
Normalized Intensity
1,2
1,1
1,0
0,9
0,8
0,7
-8 -4 0 4 8
z/z 0
Figure 4: Z-scan curve for castor oil at 810 nm in the CW regime, P laser = 177 mW.
When the laser was operated in femtosecond regime, no Z-scan curve was obtained in the range of 100 Hz to 10
kHz repetition rate. This result indicates that the slow nonlinear effects, such thermal lens, give the most
important contributions for the large nonlinearity that this medium presents.
We also performed the z-scan measurements with the opened aperture configuration. In this case, nonlinear
absorption was not observed at any experimental conditions exploited in this work.

XXIX ENFMC - Annals of Optics 2006
Conclusions
We have carried out an investigation of the nonlinearity optical properties of castor oil using the Z-scan
technique for two excitation wavelengths, 514nm and 810 nm, in the CW regime, and for 810 nm in the
femtosecond regime. Large nonlinear refractive indexes were obtained for both wavelengths, in the CW
regime. However, in the femtosecond regime, it was not possible to observe a typical Z-scan curve for
castor oil. This result suggests that this nonlinearity is mainly due to thermal effects. The nonlinear
absorption was absent for all experimental configurations.
Acknowledgements
The authors thank the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico
(CNPq), Fundação de Amparo à Pesquisa do Estado de Alagoas (FAPEAL), Coordenação de Aperfeiçoamento
de Pessoal de Nível Superior (CAPES), Nanofoton network, Nanoanálise e Diagnóstico network, and
Financiadora de Estudos e Projetos (FINEP) for financial support.
References
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Interscience, New York (1991).
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Spatial Self-Phase Modulation in Castor Oil Enhanced by Gold Nanoparticles, Proceedings of SPIE 6103,
6 (2006).
[3] M. Sheik-Bahae, A. A. Said, T-H Wei, D. J. Hagan and E. W. Van Stryland, Sensitive Measurement of
Optical Nonlinearities Using a Single Beam, IEEE J. Quantum Electron. 26, 760 (1990).
[4] I. P. Nikolakakos, A. Major, J. S. Aitchison and P. W. E. Smith, Broadband Characterization of the
Nonlinear Optical Properties of Common Reference Materials, IEEE J. of Sel. Top. In Quantum Electron.
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