XXIX ENFMC
- Annals of Optics
2006
Dynamics nonlinear optical properties in indocyanine green
solutions
D. C. J. Rezende, L. De Boni, C. R. Mendonça
Departamento de Física e Ciência dos Materiais, Instituto de Física de São Carlos,
Caixa Postal 369, 13560-970 São Carlos, SP, Brasil
[email protected]
Abstract
Dynamic nonlinearities of Indocyanine Green solutions were studied by different Z-scan
techniques. The excited singlet and triplet cross-sections of this molecule and the intersystem
crossing times were obtained solving numerically rate equations.
Introduction
Indocyanine green (ICG) is an organic dye with application in several distinct areas. In lasers, it is used
as the active medium [1] and as a saturable absorber [2]. In medicine, ICG has been mainly used for diagnosis
[3] and photo-dynamic therapy (PDT) of cancer [4,5]. Regarding photonics, it can be applied in optical limiting
devices [6] due to its efficient reverse saturated absorption (RSA). The results obtained here have shown that the
excited singlet and triplet states cross sections of this molecule are extremely high with respect to the ground
state one, describing a RSA with a fast intersystem crossing time.
Experimental Setup
In this work we studied the dynamic of the nonlinear absorption of ICG (Fig. 1) diluted in water and
dimethyl sulfoxid (DMSO). We used two configuration of the Z-scan technique to obtain the singlet and triplet
states dynamics, independently. One configuration is the traditional single pulse Z-scan technique [7] that
consists in translating the sample through the focal plane of a Gaussian beam and monitoring the changes in the
intensity in the far-field as a function of the sample-focal position. The other configuration is the pulse train
(PTZ-scan) Z-scan technique [8], which is able to verify the nonlinear dynamics in the nanosecond time scale.
The experimental apparatus is similar to the traditional Z-scan. In this case, the sample is also moved through the
focal plane, and several pulse trains are acquired and the amplitudes of each pulse are normalized to the one
obtained when the sample is far from the focus. The excitation of the sample was made at 532 nm, being the
second harmonic of a Nd:YAG Q-switched and mode-locked laser (Antares Quantronix). The characteristic laser
pulse delivered from Antares contains a pulse train with about 20 pulses, separated by 13 ns and with 100 ps of
FWHM. This pulse configuration was used in PTZ-scan. For the single pulse Z-scan technique, the pulse train
pass through a Pockels cell placed between two crossed polarizes, which is used to extracted a single pulse
envelope.
Figure 1: Indocyanine green structure.
XXIX ENFMC
- Annals of Optics
2006
In order to measure the nonlinear properties of ICG, we used a concentration of approximately
1.2x1017molecules/cm3. The UV-Vis absorption spectrum was obtained with a Cary-17A spectrometer at room
temperature, with a concentration six times more diluted that used in the nonlinear measurements.
Results and Discussions
Figure 2 shows the UV-Vis absorption spectrum of ICG diluted in water (solid line) and in DMSO
(dash line) for a concentration of the 2x1016molecules/cm3. It presents a strong band around 800 nm related
π → π * transition. This band changes its intensity with increasing the temperature, as showed in the inset. For
temperatures lower than 60oC, this optical process is reversible. However, for temperatures higher than 60oC it
does not recovery the initial absorption, which is probably associated with sample degradation. Due to this
thermal behavior, we have just carried out the nonlinear measurements at room temperature.
0.8
0.6
Absorbance
Absorbance
1.2
20
30
40
50
60
60
60
0.4
0.2
0.0
Water
DMSO
Sample
temperature (ºC)
600
800
1000
Wavelenght (nm)
0.4
0.0
600
800
Wavelenght (nm)
1000
Figure 2: Absorption spectrum of indocyanine green in water (solid line) and in DMSO
(dashed line). The inset shows the absorption changes with the sample heating in water
solvent.
Employing the single pulse Z-scan technique we obtained the normalized transmittance (TN) as a
function of pulse irradiance for both samples, as shown in Fig. 3. Each point in this figure is the minimum value
extracted from a single open-aperture Z-scan measurement (insets in Fig. 3). The solid lines represent the fittings
using a three singlet energy level diagram, that provides the absorption cross-sections σ 12water = (13 ± 1)× 10 −17 cm 2
and σ 12DMSO = (12 ± 1) × 10 −17 cm 2 . We can also observe a saturation effect of the TN. This behavior is due to the
accumulation of molecules in the singlet excited state and to the depleting of the ground state. In the ICG diluted
in DMSO, the saturation occurs for low intensities due to its fluorescence time (500 ps) [9], that is 25 times
higher than that of the ICG- water (20 ps). In this way, saturation intensity of the first excited state for ICGDMSO is lower than for ICG-water. This implies that for the same pulse irradiance, more population is
accumulated in singlet excited state to ICG-DMSO, which causes a premature saturation of RSA effect. With
more molecules excited in the first state, more transitions can occur to second excited state, that present a small
absorption cross-section. This process can be visualized in the change of the slope on the fitting in Fig. 3(b),
around 3 GW/cm2, or also in the focal position z=0 in the inset of the same figure, where the TN present a
discontinuously in comparison with the inset of Fig. 3(a).
(a)
0.8
- Annals of Optics
1.0
0.8
0.6
-4
0.6
-2
0
2
4
Z (c m )
DMSO
(b)
0.9
0.8
0.7
1.0
0.9
0.8
1
2
Water
(a)
0.9
0.8
0.7
DMSO
(b)
1.0
0.9
0.8
0.7
-4
0
2006
1.0
Normalized Transmittance
Water
Normalized Transmittance
Normalized Transmittance
1.0
No rm a lize d Tra n s m itt an ce
XXIX ENFMC
3
-2
2
Irradiance (GW/cm )
0
Z (cm)
2
4
0.7
4
5
Fig. 3: Normalized transmittance as function of pulse
irradiance for ICG in water (a) and DMSO (b).
0
5
10
Pulse Number
15
20
Fig. 4: Normalized transmittance along the complete
Q-switch envelope for ICG in water (a) and DMSO
(b).
In the Fig. 4 we show the results obtained using the Z-scan with pulse train (PTZ-scan) [8]. In this
technique the long duration of the Q-switch envelope (about 200 ns) and the 13 ns time interval between
consecutive pulses is employed to map the evolution of the nonlinearity. These results reveal an accumulative
process due to the triplet state population and can be explained considering a five-energy-level diagram. When a
pulse of the envelope excites the molecule to the level S1, it can be promoted to a second excited state, return to
the ground state or decay to the excited triplet state T1 (intersystem crossing process). With the arrival of the next
pulses from the same envelope, additive contributions to the optical nonlinearity can appear due to the
population build up in triplet state. The experiment results obtained here demonstrated that both samples exhibit
RSA due to the higher absorption cross section relative to the ground state ( σ water = (9 ± 1) × 10 −17 cm 2 and
σ DMSO = (7 ± 1)×10−17 cm2 ). The intersystem crossing times determined using PTZ-scan were 1 ns and 9 ns for
34
34
water and DMSO, respectively, in agreement with the literature [9,10]. The fittings are represented by lines in
Fig 4.
Conclusions
In conclusion, the results obtained with both techniques have shown that the excited singlet and triplet
cross-sections of ICG in both solvents are extremely high in relation to the ground state. If compared with de
ground state, the singlet excited state present a value 50 times larger for water and 75 times larger for DMSO. In
triplet state, this difference is lower than the singlet excited state one. In this case, the values are 33 and 45 times
larger for water and DMSO, respectively. The high singlet and triplet excited states cross section and a high
population in the triplet state makes ICG a good candidate for photodynamic therapy and optical limiting
devices.
Acknowledgements
The authors thank the FAPESP, CNPq e CAPES.
References
[1]
[2]
[3]
[4]
B. Pierce, R. Birge, IEEE J. Quantum Electron. QE-18 (1992) 114.
S. Oda, Y. Segawa, P. H. Kim, S. Namba, N. Kodama, Jpn. J. Appl. Phys. 228 (1989) L1977.
I. J. Fox, W. H. Wood, Mayo Clin. Proc. 35 (1996) 732.
W. Bäumler, C. Ables, S. Karrer, T. Wei, H. Messmann, M. Landthaler, R. M. Szeimies, Br. J. Cancer 80
(1999) 360.
[5] S. Fickweiler, R. M. Szeimies, W. Bäumler, P. Steinbach, S. Karrer, A. E. Goetz, C. Abels, F. Hofstädter,
M. Landthaler, J. Photochem. Photobiol. B: Biol. 38 (1997) 178.
[6]
J. W. Perry, in: H. S. Nalwa, S. Miyata (Eds.), Nonlinear Optics of Organic Molecules and Polymers,
CRC Press, New York, 1997, p. 813.
XXIX ENFMC
- Annals of Optics
2006
[7] M. Sheik-Bahae, A. A. Said, T. Wei, D. J. Hagan, D. J. Hagan, D. J. Van Stryland: IEEE J. Quantum
Electron. QE-26, 760 (1990).
[8] L. Misoguti, C. R. Mendonça, S. C. Zilio: Appl. Phys. Lett. 74, 1531 (1999).
[9] S. Reindl, A. Penzkofer, S. H. Gong, M. Landthaler, R. M. Szeimies, C. Abels, W. Bäumler, J. Photochem.
Photobiol. A: Chem. 105 (1997) 65.
[10] H. Gratz, A. Penzkofer, C. Abels, R. M. Szeimies, M. Landthaler, W. Bäumler, J. Photochem. Photobiol.
A: Chem. 128 (1999) 101.