Materials Sciences and Applications, 2011, 2, 299-306
doi:10.4236/msa.2011.25039 Published Online May 2011 (
Copyright © 2011 SciRes. MSA
Ultrafast Nonlinear Optical and Optical Limiting
Properties of Phthalocyanine Thin Films Studied
Using Z-Scan
Soma Venugopal Rao, Puliparambil Thilakan Anusha, Turaga Shuvan Prashant, Debasis Swain,
Surya P. Tewari
Advanced Centre of Research in High Energy Materials (ACRHEM) University of Hyderabad, Hyderabad, India.
Received January 20th, 2011; revised March 10th, 2011; accepted March 20th, 2011.
We have investigated the nonlinear optical properties, optical limiting thresholds, and figures of merits for five different
phthalocyanine thin films, achieved through doping in PMMA polymer, using the Z-scan technique at 800 nm with 2 ps
laser pulse excitation. From the open-aperture Z-scan data we derived that these molecules exhibit strong two photon
absorption (2PA) with the nonlinear absorption coefficients in the range of 15 - 200 cm/GW. We have also estimated
the sign and magnitude of real part of third order nonlinearity through the closed aperture data. Preliminary femtose-
cond pump- probe data suggests that the lifetimes of excited states are in the sub-100 ps regime for all the molecules in
film form. Our studies provide concrete evidence that these phthalocyanines are prospective candidates for mul-
ti-photon imaging and optical limiting applications.
Keywords: Z-Scan, Phthalocyanines, Thin Films, Picosecond, Figures of Merit
1. Introduction
Novel moieties with strong two-photon (2PA) and three-
photon absorption (3PA) crossections/coefficients are
attractive for potential applications in the fields of pho-
tonics and bio-medicine [1-4]. Studies on a variety of
molecules with strong 2PA/3PA have been established to
be relevant in fluorescence spectroscopy, 3D imaging,
and lithography because of high spatial resolution achie-
ved through intensity dependent processes [3-7]. Addi-
tionally, they can be used for optical data storage and
optical limiting purposes. Several materials such as por-
phyrins and phthalocyanines that possess such properties
have been investigated extensively in recent times by
several groups, including ours [8-24]. Especially, phtha-
locyanines have been examined by chemists, material
scientists, and physicists alike since these are ubiquitous
materials in which an alteration in molecular configura-
tion allows the engineering of both linear and nonlinear
optical (NLO) properties for specific applications. These
organic polymeric systems contain conjugated π electron
structure and show large optical nonlinearities combined
with nonlinear response time in the femtosecond time-
scales [7,8]. Due to the large, ultrafast third-order non-
linearities and the ease with which one can derive several
new compounds through peripheral and axial substitu-
tions, these molecules have generated tremendous inter-
est in nonlinear optics [10-24]. The third order NLO
properties of phthalocyanines are closely dependent on
the central metal ion and other modifications to the core
and peripheral substitution. However, phthalocyanines
are hardly soluble in organic solvents and do not crys-
tallize easily in matrices. Moreover, the molecule aggre-
gation of phthalocyanines greatly influences the third
order nonlinearity, χ(3). To circumvent these problems
modification of phthalocyanines such as the introduction
of peripheral substitutions or attaching them to polymer
chains have been carried out. The latter approach has the
advantage that a high concentration of nonlinear chro-
mophores can be incorporated into the polymer system
without crystallization, phase separation, or formation of
concentration gradients.
Several earlier reports [10-24] on the optical and NLO
properties have been in various forms of thin films. We
have recently reported the NLO studies of novel phtha-
locyanines using continuous wave (cw), nanosecond (ns),
Ultrafast Nonlinear Optical and Optical Limiting Properties of Phthalocyanine Thin Films Studied Using Z-Scan
Copyright © 2011 SciRes. MSA
picosecond (ps), and femtosecond (fs) laser pulses [25-
33]. Our recent efforts have been towards understanding
the nonlinearities in all the time domains, albeit in solu-
tion form. We have also studied the optical limiting per-
formance of these molecules in thin films form (doped in
PMMA) and demonstrated strong limiting properties in
the cw regime. Our comprehensive studies strongly sug-
gest that these are potential molecules with strong nonli-
nearities and figures of merit in each time domain. In this
paper we report two-photon absorption (2PA) and the
nonlinear refraction studies of phthalocyanine thin films
achieved using ps pulses. The standard Z-scan technique
was utilized to investigate the third order NLO properties
of the film. The knowledge of ultrashort pulse nonlinear
refractive index aids in deciding materials appropriate for
optical switching applications. Herein we present the
results of our investigations on the NLO properties of
five films of phthalocyanines studied using 2 ps pulses.
2. Experimental Details
Figure 1 depicts the schematic of the experimental set up
used in the present study. Z-scan measurements were
performed using an amplified Ti:sapphire laser system
(LEGEND, Coherent) delivering nearly transform-limited
pulses of ~2 ps with a repetition rate of 1 kHz at 800 nm.
The pulse duration was confirmed using an intensity
autocorrelation experiment at 800 nm using a BBO crys-
tal in non-collinear geometry. The amplifier was seeded
with ~15 fs (55 - 60 nm FWHM) pulses from an oscilla-
tor (MICRA, Coherent, 1 W average power, 80 MHz
repetition rate, 800 nm central wavelength). Laser pulses
with typically 2 - 5 μJ energy were used for the experi-
ments. The beam was focused using 200 mm focal length
lens into the sample. The beam waist (ωo) estimated was
20 ± 2 μm with a Raleigh range of 1.6 ± 0.3 mm. The
transmittance changes of the sample placed on the trans-
lation stage (Newport, ILS250PP), which was controlled
by Newport ESP 300 Motion Controller, were measured
with a sensitive power meter in the far-field. An aperture
was placed in front of detector for closed aperture scans.
The power meter and the translational stage were inter-
faced to the computer. We established that the pulse en-
ergies remained low to avoid contribution from higher
order nonlinearities. The experiments were repeated more
than once and the best data were used for obtaining the
nonlinear optical coefficients from the best fits. These mo-
lecules are highly soluble in chloroform in which com-
mercially available PMMA (poly methyl methacrylate)
was dissolved and the resulting solution was processed
into thin films. Such films, upon curing, produced cross
linked polymeric system in which phthalocyanine deriva-
tive is covalently linked. Typically the concentration of
phthalocyanines in polymer was ~1% by weight (of
PMMA) though we could put in a maximum of ~5% by
weight. PMMA concentration was ~10% by weight in
3. Results and Discussion
The IUPAC nomenclature for the compounds is shown in
Table 1. The complete details of synthesis of these mole-
Figure 1. Experimental set up used for open aperture and closed aperture Z-scans. NDF stands for neutral density filters and
PM stands for power meter.
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Table 1. Nomenclature of compounds used in the present
Sample Nomenclature
PC1 2,3,9,10,16,17,23,24-octakis-(heptyloxy)phthalocyanine
PC2 2,3,9,10,16,17,23,24-octakis-(heptyloxy)phthalocyanine
PC3 (2-(3-(Butane-1,4-dioic acid)-9(10), 16(17),
23(24)-tri tert-butyl phthalocyanine Zinc(II)
PC4 2(3),9(10),16(17),23(24) tetra tert-butyl phthalocyanine
PC5 2(3), 9(10),16(17),23(24) tetra tert-butyl Zinc
cules, their molecular structure, and their detailed absor-
ption spectra are reported in our earlier publications
[28-33]. All the molecules were purified prior to thin
film fabrication achieved through spin coating. The mo-
lecules were dissolved in a sonicated crystal clear solu-
tion of PMMA in chloroform. The solution was then
transferred to clean microscopic slides and spin coated to
achieve films of various thicknesses. The typical thick-
ness of the films measured using a profilometer were in
the 8 - 12 μm range. The absorption coefficients of all the
films were estimated to be in the range of 104 m1 for all
the films. Linear absorption spectra of typical PC2 and
PC3 thin films are shown in Figure 2. The slight broa-
dening and splitting observed in Q band (arising from
electronic transitions) and broadening of Soret band in
the thin film absorption spectra, compared to solutions,
could be ascribed to the intermolecular interaction and
molecular distortion/deformation [34,35]. In addition
there is also the possibility of aggregation in solid state
thereby slightly modifying the energy level configuration.
Figures 3(a) to 3(e) show the open aperture data (open
stars) of phthalocyanine thin films PC1 to PC5 recorded
at 800 nm with 2 ps pulses and input peak intensities in
the range of 200 GW/cm2. The presence of valley in
normalized transmittance in open aperture (OA) scans
indicates strong reverse saturation absorption (RSA) at
these peak intensities. Inset of Figure 3(a) shows the
typical closed aperture data for PC1 film. The peak fol-
lowed by valley in the normalized transmittance data
clearly suggests that the sample possesses negative type
of nonlinearity and self-defocusing behavior. Similar
behavior for n2 was observed for other thin films also.
The n2 values obtained from the fits to data for all the
films were in the range of (2 - 15) × 1013 cm2/W. In or-
der to extract the information of nonlinear refraction
alone, the experimental closed aperture Z-scan data was
divided by the open aperture data. This greatly eliminat-
ed the influence of nonlinear absorption on the non- li-
near refraction data.
Figure 2. UV-visible absorption spectra of PC2 (top, red
color) and PC3 (bottom, green color) doped in PMMA.
CA and OA data for all the films were fitted using fol-
lowing equations [36]:
 
 
 
  
,1In1 ,0ed
TzSq z
 
 
 
22 2
,1 π,0
lnln 1exp2,0expd
 
With q0 = βLeffI0,
, β is the 2PA
coefficient and α3 is the 3PA coefficient, I00 is the peak
intensity, Z is the sample position, 2
z is the
Ray- leigh range; ω0 is the beam waist at the focal point
(Z = 0), λ is the laser wavelength; effective path lengths
in the sample of length L for 2PA, 3PA is given as
1e 1e
, .
eff eff
From all the fits to experimental data it is evident that
2PA is the dominant mechanism for the observed RSA
kind of behavior. 2PA absorption coefficients estimated
from the fits were in the range of 15 - 200 cm/GW. 3PA
fits are also shown in the figures for comparison and un-
doubtedly the fits are off from the experimental data. All
the samples were checked for presence of any nonlinear
absorption losses due to scattering. The transmitted light
was devoid of any scattering in the far-field indicating
Current Distortion Evaluation in Traction 4Q Constant Switching Frequency Converters
Copyright © 2011 SciRes. MSA
Figure 3. OA Z-scan curves of PC1 to PC5 recorded with peak intensity of 200 GW/cm2. Open stars represent the experi-
mental data while the solid lines are fits. Red (dotted) and blue (solid) lines represent theoretical fit for 3PA and 2PA, respec-
tively. Inset of (a) shows CA scan recoded at same intensities.
that 2PA indeed was the main mechanism contributing to
nonlinear absorption. The values of nonlinear coeffici-
ents presented here are accurate within 15% and the er-
rors arise due to uncertainties in the estimation of spot
size at focus and peak intensities, fitting procedures, ca-
libration of neutral density filters etc. The χ(3) values,
~1010 e.s.u., obtained from our films are comparable to
one of the highest reported nonlinearity for Aluminum
phthalocyanine films [21]. The best n2 value was obtained
for PC3 (1.44 × 1016 m
2/W) is also comparable to the
values obtained in self-assembled multilayer films con-
taining tetrasulfonated iron phthalocyanine [20]. Howev-
er, in their case due to the multi-layer nature of films the
π-electron molecular orbits of macrocycles in the aggre-
gates probably coupled with neighbors so strongly
enough to enlarge the conjugation system, thus, changing
the electronic structure of the phthalocyanine molecule
and improving the third-order optical response. Ma et al.
[16] reported χ(3) values of 1010 e.s.u. for their tita-
nylphthalocyanine films which is again comparable to
the values obtained in present study. Furthermore we
observed strong 2PA in our molecules which find poten-
tial applications in bio-imaging and good optical limiting
Whilst the solution studies of these molecules, excited
with fs pulses exhibited, strong 3PA in alkyl phthalocya-
nines (PC4 and PC5) [29], an amalgamation of excited
state absorption, 2PA, and 3PA in alkoxy phthalocya-
nines (PC1 and PC2) [28], and saturable absorption be-
havior with ps pulses in the asymmetric phthalocyanine
(PC3) [31,32] all the films studies obviously pointed to
the presence of strong 2PA. This could be explained with
the changes in absorption spectra, and thereby the energy
level configuration, corresponding to solutions and thin
films combined with the aspect of peak intensities used
for these studies. With the broadening of Soret band,
there is a possibility of direct two-photon state existence
for the molecule when excited with 800 nm photon in the
bulk unlike in solutions. PC3 in solution depicted SA
(due to small absorption at 800 nm) when excited with
lower peak intensities but immediately switched to RSA
with increasing peak intensities [31,32]. PC1 and PC2
also demonstrated similar behavior of RSA with increas-
ing peak intensities. The nonlinear absorption in such
molecules is indeed complex phenomena involving sev-
eral excited state mechanisms such as 2PA/3PA induced
excited state absorption, pure 2PA/3PA, single photon
induced excited state absorption etc. [37-41]. Further
detailed wavelength and intensity dependent studies are
in progress to elucidate the complete nonlinear absorp-
Ultrafast Nonlinear Optical and Optical Limiting Properties of Phthalocyanine Thin Films Studied Using Z-Scan
Copyright © 2011 SciRes. MSA
tion behavior in these materials.
We have assessed the merit factors W and T for all the
thin films studied and defined as
where λ is the wavelength and Isat is the light intensity at
which the nonlinear refractive index saturates. Ideally W
must be > 1 and T ought to be < 1 for practical device
applications. In the present case W is certainly > 1 for all
the films while T is also > 1. We strongly feel that these
robust molecules are good for multi-photon absorption
based applications due to the presence of strong 2PA.
The nonlinear refractive index recorded was also high for
these molecules compared to some the recently reported
molecules of interest and furthermore the excitation wave-
length was non-resonant. However, supplementary wave-
length dependent studies will reveal the actual potency of
these molecules for optical switching based applications
which is strongly dependent on n2.
Optical limiting is a phenomena observed when the
transmission of a medium decreases with increasing in-
put laser intensity (or fluence). An effective optical limi-
ter will have low limiting threshold, high optical damage
threshold and stability, fast response time, high linear
transmittance throughout the sensor band width, optical
clarity and robustness. One of the major mechanisms
involved is reverse saturable absorption (RSA), usually
observed with nanosecond pulse excitation. OL can be
achieved through various nonlinear optical mechanisms
such as multi-photon absorption (MPA), excited state
absorption (ESA), free carrier absorption (FCA), self-
focusing, self-defocusing, nonlinear scattering, photo-re-
fraction etc. Coupling two or more of these mechanisms
has also causes OL like self de-focusing along with MPA.
In the case of Pc thin films, it is observed that sample
exhibits strong two-photon absorption and therefore is
responsible for the limiting reported here.
Figure 4 shows a typical optical limiting curve depicting
the sample transmission as a function of input laser fluence
(i.e. energy per area). The input laser energy density was
evaluated using the relation
4ln2 π
where Ein is the input laser pulse energy and ω(z) is the
beam radius (HW1/e2M). It is evident that under strong
irradiance the curve departs from Beer’s law leading to
optical power limiting in the thin film. The limiting thre-
sholds evaluated from the data and fits were in the 0.1 -
0.6 mJ/cm2 range. Table 2 summarizes the nonlinear
coefficients of all the films extracted from the present
study. The high nonlinearity of PC3 compared to other
phthalocyanines can be ascribed to the asymmetry in
structure compared to other symmetrical phthalocyanines
and is consistent with our earlier results [28-33]. Metal
phthalocyanine films exhibited higher nonlinearities
when compared to free base phthalocyanine films for
both alkyl and alkoxy cases whereas alkoxy phthalocya-
nine films displayed higher nonlinearity compare to alkyl
phthalocyanine films.
We are also investigating the response time of these
Figure 4. Optical limiting cure recorded for PC3 in PMMA.
Blue (solid) line is the fit for 2PA while red (dotted) line is
the fit for 3PA.
Table 2. Summary of the nonlinear coefficients of phthalocyanine thin films extracted from the present study.
Sample n2 (m2/W)
× 1017
n2 (e.s.u)
× 1010
Re|χ(3)| (m2/V2)
× 1019
α2 (m/W)
× 1010
Im|χ(3)| (m2/V2)
× 1019
|χ(3)| (m2/V2)
× 1019
|χ(3)| (e.s.u)
× 1011
Figure of
Merit W
Figure of
Merit T
PC1 4.30 1.53 5.06 6.00 4.50 6.78 4.86 3.7 11.2 0.164
PC2 6.94 2.47 8.17 18.0 13.5 15.8 11.3 7.3 20.7 0.137
PC3 14.4 5.12 17.0 20.0 15.0 22.6 16.2 2.6 11.1 0.112
PC4 2.18 0.78 2.57 6.00 4.50 5.18 3.71 2.3 22.0 0.131
PC5 2.30 0.82 2.71 1.50 1.13 2.93 2.10 3.1 5.2 0.550
Ultrafast Nonlinear Optical and Optical Limiting Properties of Phthalocyanine Thin Films Studied Using Z-Scan
Copyright © 2011 SciRes. MSA
films using ps and fs pulse excitation. Our initial fs
pump-probe studies offered a testimony for the ultrafast
response time with the decay observed in the sub-100 ps
time scale [42]. Several recent reports too predict the
lifetimes in a number of phthalocyanine thin films to be
in the ps time domain [43-46]. Our objective is to obtain
a complete perception of how these molecules respond to
cw, ns, ps, and fs pulse excitation in both solution and
thin film form by evaluating the nonlinear coefficients,
response times, and figures of merit to entirely categorize
and utilize their potential in each of these time domains.
4. Conclusions
We have investigated the nonlinear optical properties of
five different phthalocyanine thin films at 800 nm with 2
ps pulses. All the samples were found to possess good n2
values when compared to some of the recently reported
values for similar molecules. The sign of the nonlinearity
was confirmed to be negative. Open aperture studies
demonstrated strong two photon absorption to be respon-
sible for the nonlinear absorption. Figures of merit have
been evaluated for these films and the data suggests these
molecules are excellent candidates for multi-photon im-
aging and optical limiting applications.
5. Acknowledgments
We are thankful to Dr. L. Giribabu, IICT, Hyderabad,
India for generously providing the phthalocyanine sam-
ples in large quantity and for fruitful discussions. We
also acknowledge DRDO for financial support.
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