Optical limiting and nonlinear optical studies of ferrocenyl substituted calixarenes

Chemical Physics Letters 616–617 (2014) 189–195

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Optical limiting and nonlinear optical studies of ferrocenyl substituted calixarenes C.P. Singh a,∗ , Rekha Sharma b , V. Shukla a , P. Khundrakpam a , Rajneesh Misra b , K.S. Bindra c , Rama Chari a a

Laser Physics Applications Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India Department of Chemistry, Indian Institute of Technology, Indore 452017, India c Solid State Laser Division, Raja Ramanna Centre for Advanced Technology, Indore 452013, India b

a r t i c l e

i n f o

Article history: Received 20 July 2014 In final form 14 October 2014 Available online 22 October 2014

a b s t r a c t Optical limiting behavior of three ferrocenyl substituted calixarenes (FSCs) where ferrocenyl group is attached via ethynyl(a), meta-(b) and para-(c) ethynyl phenyl spacers studied with nanosecond pulses at 532 nm is in the order c > a > b. While, with femtosecond laser pulses at 800 nm strength of nonlinearity is in the order c > b > a. Largest nonlinear refraction (n2 = 9.5 × 10−16 cm2 /W) is estimated for the FSC c. Nonlinear absorption in these FSCs is attributed to three-photon absorption. Optical limiting behavior with nanosecond laser pulses is mainly due to thermal nonlinearity and reverse saturable absorption, whereas, with femtosecond laser pulses mainly intrinsic electronic nonlinearity contributes. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Study of nonlinear optical properties of materials is an area of continued research interest due to understanding fundamental physical phenomena involved in the properties and using them for device applications in optical information processing, limiting, switching, etc. [1–6]. Among the materials, organic moieties are particularly interesting due to amenability to tailoring of their properties during the synthesis in a very systematic and reproducible way [5–7]. Over the years calixarene based molecular systems have gained substantial attention in the field of nonlinear optics [8–12]. Calixarenes are versatile host molecules to incorporate into numerous elaborate structures. The calix[4]arene (calixarene) scaffold can be easily functionalized at the parapositions of the phenolic units resulting in donor-acceptor system. Highly non-symmetric structure of functionalized calixarenes makes them interesting for frequency doubling studies. The cone conformation of calixarenes leads to better pre-organization towards second-order nonlinear optical effects [10–12]. Third order nonlinearity is interesting towards optical limiting behavior. Optical limiters are important for protection of human eyes and sensitive optical instruments from intense laser radiation [13]. Limited studies have been performed on calixarenes as optical limiting material. Optical limiting of nanosecond laser pulses at

∗ Corresponding author. E-mail address: [email protected] (C.P. Singh). http://dx.doi.org/10.1016/j.cplett.2014.10.030 0009-2614/© 2014 Elsevier B.V. All rights reserved.

532 nm have been reported in tetrathiacalixarene [14] and calixarene bridged metal phthalocyanines [15]. Ferrocene, one of the most widely studied organometallic compound shows strong nonlinear optical behavior [16]. Recently, ferrocenyl substituted metal porphyrins have been shows to exhibit strong optical limiting behavior [17]. Hence, ferrocene group is incorporated into the calixarene scaffold, as it is expected to show improved nonlinear optical properties. Recently, ferrocenyl substituted calixarenes (FSCs) with varying spacers were synthesized and characterized in detail [18]. Solubility of FSCs is better than ferrocenyl substituted metal porphyrins. For the first time to our knowledge, in this letter, optical limiting and nonlinear optical studies of FSCs with nanosecond and femtosecond laser pulses at 532 and 800 nm, respectively have been presented. We have studied three FSCs and their structures are shown in Figure 1. In FSC a, ferrocenyl group is attached via ethynyl spacers, whereas in FSCs b and c it is attached by meta-ethynyl phenyl spacer and para-ethynyl phenyl spacers, respectively. FSC c shows highest nonlinearity and superior optical limiting compared to FSCs a and b. Optical limiting behavior with nanosecond laser pulses is mainly due to reverse saturable absorption (RSA). Overall, optical limiting performance of FSCs is comparable to fullerene C60 and ferrocenyl substituted porphyrins [17], particularly, FSC c is a better performer. However, with femtosecond laser pulses, three-photon absorption is shown to be the dominant phenomenon instead of two-photon absorption. Closed aperture Z-scan experiments performed on FSCs exhibit peak followed by valley profile with nanosecond laser pulses while the experiments with

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a

b

Fe

Fe

O

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Fe

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F Fe

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Figure 1. Supramoleculer structures and linear absorption spectra of the FSCs a, b, and c. The inset shows the standard energy level diagram.

femtosecond laser pulses exhibit valley followed by peak profile, which are indicative of negative and positive nonlinearity, respectively. Origin of nonlinearity is attributed to thermal nonlinearity and intrinsic electronic nonlinearity for studies with nanosecond and femtosecond lasers, respectively. Optical limiting performance of FSCs with femtosecond laser pulses is also interesting and the performance is comparable to the recently reported Fe3 O4 -Ag nanocomposites [19] and functionalized carbon nanotubes [20]. 2. Experimental method Optical limiting studies were performed with frequency doubled Q-switched Nd:YAG laser generating ∼30 nanosecond (FWHM) laser pulses at 532 nm wavelength. To measure optical nonlinear characteristics of a medium, standard Z-scan experimental setup was used. In the setup, collimated laser beam was focused using a 20 cm focal length lens on to the sample kept on a translational stage along the beam propagation direction. Spot size of the beam at the focus (ω0 ) was ∼50 ␮m resulting in Rayleigh range of ∼1.5 cm for the setup. Transmitted beam through the sample was detected at two places, one through an aperture placed in the far field to account for closed-aperture Z-scan profile and the other using a beam splitter before the aperture to collect the whole

transmitted beam through the sample to account for the open aperture Z-scan profile. The open and the closed aperture Z-scans were simultaneously measured in the setup [21]. A small portion of the incident beam was used as reference for normalization. Samples were held in a 5 mm path length quartz cell. For optical limiting studies, sample was kept at the focus of the laser beam and the input laser energy was varied using neutral density filters. For femtosecond experiments, light pulses were employed from Ti:sapphire laser system consisting amplifier (Titan, Quantronix) seeded by oscillator (Tsunami, Spectra Physics) delivering ∼100 femtosecond (FWHM) laser pulses centered around 800 nm wavelength operating at 1 kHz repetition rate. In this case, incident beam was focused by a 30 cm focal length lens. Spot size of the beam at the focus (ω0 ) was ∼60 ␮m resulting in Rayleigh range of ∼1.4 cm for the setup. Rayleigh range of the setups were much larger than the sample thickness leading to thin sample approximation. Concentration of FSCs was kept 1 mM in toluene. 3. Results and discussion Linear absorption spectra of the ferrocenyl substituted calixarenes (FSCs) are shown in Figure 1. The spectra were recorded at lower concentration of 10 ␮M to avoid saturation of the detector in

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(a) Output energy (μJ)

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Output energy (μJ)

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20

Input energy (μJ) Figure 2. Variation of output energy with input energy for FSCs a (squares), b (triangles), and c (circles) measured with nanosecond laser pulses at 532 nm for (a) Geometry (i) and (b) Geometry (ii). Solid lines are guide to eye.

the UV region. Strong absorption band in the range 250–350 nm is due to ␲ → ␲* transition and the weak absorption band appearing between 400 and 500 nm is due to d–d transition of ferrocene [18]. The samples exhibit negligible linear absorption beyond 550 nm wavelength. Optical limiting measurements with nanosecond laser pulses were performed in two geometries. In Geometry (i), the transmitted laser beam was collected though 90% transmission aperture placed in the far field before the detector. In Geometry (ii), the whole transmitted beam was collected on the detector to exclude the contribution of intensity induced refractive index variation resulting in self-focusing/de-focusing. Figure 2(a) and (b) shows variation of output energy with input energy on passing through FSCs dissolved in toluene for Geometries (i) and (ii), respectively. In both the geometries, FSCs show deviation from linear behavior which is indicative of optical limiting characteristic. Strength of deviation from the linear behavior is in the order c > a > b. Deviation from linear behavior is more in the case of Geometry (i) compared to Geometry (ii) due to additional nonlinear phenomena contributing in the former geometry. For example, in Geometry (i) the output energies for FSCs a, b and c are 21.5, 28.5 and 7.5 ␮J respectively, for 150 ␮J input energy, leading to respective transmission values of 14.3%, 19% and 5%. For 50 ␮J input energy output energies are 11.5, 14.2 and 4.4 ␮J for FSCs a, b and c, respectively, leading to transmission values of 23%, 28.4% and 8.9% respectively. However, the linear transmission values exhibited by FSCs a, b, and c were 57%, 68% and 35% respectively, at 532 nm for 1 mM concentration solution, giving rise to ground state absorption cross-section values of 1.9 × 10−18 cm2 , 1.3 × 10−18 cm2 and 3.5 × 10−18 cm2 , respectively At 150 ␮J input energy, the transmission values for FSCs a, b, and c are 4, 3.5

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and 7 times less compared to their respective linear transmission values. Optical limiting performance of FSCs is comparable, rather better (FSC c) than the measurements reported on a modified calix[4]arene, which, was shown to limit energy of a nanosecond laser pulses at 532 nm to 9 ␮J energy [14]. Overall, optical limiting performance of FSCs is comparable to fullerene C60 and ferrocenyl substituted porphyrins reported recently with the same setup [17], particularly FSC c is the better performer. In the case of Geometry (ii), output energies for FSCs a, b and c are 37, 55 and 10 ␮J, respectively for 150 ␮J input energy, leading to respective transmission values of 24.7%, 36.7% and 6.7%. For 50 ␮J input energy, output energies are 16.5, 24 and 6.5 for FSCs a, b and c respectively, leading to transmission values of 33%, 48% and 13%, respectively. Decrease in the transmission from the linear transmission values are 2.3, 1.9 and 5.2 times at 150 ␮J input energy for FSCs a, b, and c, respectively. Optical limiting experiments were also performed, keeping linear transmission of FSCs same (∼70%) at 532 nm. FSC a and b show comparable limiting performance while FSC c still trends to superior performance. Decrease in the transmission in geometry (ii) can be attributed to Reverse Saturable Absorption (RSA). RSA is observed when the excited state absorption cross-section is larger than the ground state absorption cross-section. RSA is one of the commonly reported phenomena responsible for optical limiting behavior observed with nanosecond laser pulses in many organic dyes and molecules namely porphyrins [17,22], phthalocyanines [22,23], fullerenes, [24] etc. In the case of FSCs, it can be understood by the standard energy level diagram (see Figure 1 inset). The nanosecond laser pulses at 532 nm excite molecules from the ground state (S0 ) to the first excited singlet state (S1 ). From where the excited molecules make transition within the pulse duration to the first triplet state (T1 ) due to fast intersystem crossing time. Larger absorption cross-section ( T ) of the T1 state compared to the ground state absorption cross-section ( G ) would result in increase in the deviation in the output energy from the linear behavior (decrease in the transmission) with increase in the input energy [6,23]. Exact estimation of the excited state absorption cross-section requires rate equation analysis considering different excited states of the energy level diagram. Due to unavailability of the material parameters, such as lifetimes of different states, as a rough estimate, simplified two state model can be considered [13,17]. From Geometry (ii) measurements, estimated excited state absorption cross-section values using this model for FSCs a, b and c are 5.3 × 10−18 cm2 , 3.6 × 10−18 cm2 and 9.9 × 10−18 cm2 respectively. Observed deviation in the output energy from the linear behavior is more for Geometry (i) compared to Geometry (ii). In other words transmission decreases more in the case of Geometry (i) compared to Geometry (ii) with increase in the input energy resulting in superior optical limiting action in Geometry (i). Larger decrease in the transmission in the Geometry (i) could be due to nonlinear refraction or scattering, which are excluded in the Geometry (ii) measurements. To clarify the contribution of nonlinear scattering and nonlinear refraction in the optical limiting behavior of FSCs, we performed Z-scan experiments with nanosecond laser pulses at 532 nm. The open and the closed aperture Z-scan measurements are shown in Figure 3. The closed aperture Z-scan profiles for FSCs exhibit peak followed by a valley profile, indicative of negative nonlinearity. Pure toluene does not exhibit any signature in the Z-scan profile. Strength of nonlinearity is proportional to the change in the transmission from peak to valley (Tp–v ). Tp–v values for FSCs a, b and c are 0.35, 0.25 and 0.47 respectively. Hence nonlinearity is also in the order c > a > b, which is consistent with the optical limiting measurements. Thermal nonlinearity can be assigned as the responsible mechanism contributing in the closed

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1.2

Normalized transmission

(a) 1.0

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Z (mm) Figure 3. Z-scan measurements with nanosecond laser pulses at 532 nm at ∼100 ␮J input energy for (a) FSC a, (b) FSC b, and (c) FSC c. Circles and squares represent the open and the closed aperture Z-scan profiles.

aperture Z-scan. Focused laser beam generates heat in the medium, resulting in the refractive index variation across the spatial profile of the beam leading to lensing effect. A negative lensing effect generates peak followed by a valley profile in the closed aperture Z-scan [21]. If nonlinear scattering were the dominant process contributing in the nonlinear optical process in FSCs, only a dip was expected in the closed aperture Z-scan profile. However, the experimental data exhibits peak-valley profile indicating negligible contribution from nonlinear scattering [3]. The open aperture Z-scan profiles shown in Figure 3 indicate a clear dip in FSC c but not clearly observable in FSCs a and b which, supports better optical limiting performance of FSC c. Z-scan experiments and optical limiting results obtained for FSCs with nanosecond laser pulses indicate that thermal nonlinearity and RSA are the governing physical phenomenon in these

nonlinear optical measurements. Both the processes are fluence dependent and initiated by single photon absorption at 532 nm, as the samples exhibit significant linear absorption at the excitation wavelength. With nanosecond laser pulses, fluence is high, while with the much shorter pulses (femtosecond), instead of fluence, intensity is very high. For example for nanosecond laser pulses fluence is about ∼2 J/cm2 and corresponding intensity is ∼40 MW/cm2 , where as for femtosecond laser pulses fluence is ∼10 mJ/cm2 and corresponding intensity is ∼65 GW/cm2 . Intrinsic nonlinearity and two photon absorption are intensity dependent phenomenon. However, to study two-photon absorption one need to excite the sample away (red shifted) from single photon absorption wavelength. Therefore, for nonresonant excitation, we used Ti:sapphire laser wavelength (∼800 nm), where FSCs exhibit negligible linear absorption. With femtosecond pulse duration of the laser, contribution of intensity dependent effects in the medium can be measured. Hence, to get insight about the electronic nonlinearity of FSCs we performed Z-scan experiments with femtosecond laser pulses at 800 nm. Peak intensity becomes very high (∼100 GW/cm2 ) with femtosecond laser pulses and the optical nonlinearity of the solvent also starts contributing in the nonlinearity measurement [25]. Z-scan measurements performed with femtosecond laser pulses at 800 nm are shown in Figure 4. It is evident from Figure 4 (squares) that all the samples including solvent show valley peak profile indicative of positive nonlinearity. At the excitation wavelength, samples have no single photon absorption hence, the observed nonlinearity can be attributed to pure intrinsic effects due to anharmonic motion of the bound electrons. Tp–v values for FSCs a, b, c and toluene are 0.38, 0.42, 0.65 and 0.38 respectively. It is apparent from Figure 4 that FSC c shows largest nonlinearity followed by FSC b. There is no change in the nonlinearity in the case of FSC a compared to the solvent. The open aperture Z-scan profiles shown in Figure 4 (circles), exhibit no signature of nonlinear absorption except the FSC c. To estimate the nonlinear refraction coefficient and nonlinear absorption ShekhBahae formalism is followed, according to that phase  and intensity I at the exit face of the sample are given by [21] d = kn dz 

(1)

dI = −ˇI 2 dz 

(2)

where n = n2 I is the change in the index of refraction and n2 is the intrinsic electronic nonlinear refraction, z is the propagation depth in the sample, ˇ is the two-photon absorption coefficients (TPA) of the medium, k is the wave vector in free space. Looking at the absorption spectra there is no linear absorption at the excitation wavelength hence, the same is not taken into account. Intensity at input position of the sample is taken as Gaussian in space and time by [21] Iin (r, z, t) = Io

 ω 2 0 ω(z)

 exp



−

t2 2



 exp

−2r 2 ω2 (z)

 (3)

2

where ω(z) = ω0 1 + (z/z0 ) , z0 is Rayleigh range given by ω02 /, ω0 is half width at 1/e2 point in the intensity and  is half width at 1/e point in the intensity profile. The intensity at the exit face of the sample is obtained from Eq. (2) as Iex (r, t, z) =

Iin (r, t, z) 1 + ˇIin (r, t, z)L

(4)

where L is the path length of the sample. For the open aperture Z-scan the energy transmission through the sample for Gaussian

C.P. Singh et al. / Chemical Physics Letters 616–617 (2014) 189–195

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Figure 4. Z-scan measurements with femtosecond laser pulses at 800 nm at 65 GW/cm2 peak intensity for (a) FSC a, (b) FSC b, (c) FSC c and (d) toluene. Circles and squares are open and closed aperture Z-scan data points. Solid lines are fit to the experimental data.

pulses can be obtained by integrating above Eq. (4) over space and time as [21]. T (z) =

1 √ ˇIin (0, 0, z)L 



+∞

−∞

ln[1 + ˇIin (0, 0, z)L exp(−x2 )]dx

(5)

For the closed aperture Z-scan, total phase change and electric field is evaluated at the exit face of the sample by solving Eqs. (1) and (2). The field at the aperture placed in the far field is determined numerically by the Huygens–Fresnel propagation integral. The normalized transmission through aperture is estimated numerically by solving Eq. (11) of Ref. [19]. The n2 may be used as a parameter for fitting the closed aperture Z-scan data. Experimental results presented in Figure 4 were performed at 65 GW/cm2 peak intensity. From fitting of the data shown in Figure 4 (solid lines) with the above procedure, n2 values obtained are 5.4 × 10−16 , 6.0 × 10−16 and 9.5 × 10−16 cm2 /W for FSCs a, b and c respectively. n2 value of 5.4 × 10−16 cm2 /W obtained for FSC a shows a good fit for toluene also. Nonlinearity for toluene is in agreement with the values reported in the literature [25]. Fitting of the closed aperture Z-scan experimental data is not very good in the wings (away from focal regime) which, may be due to slight deviation of the laser beam profile from the theoretical Gaussian profile (Eq. (3)). However, important part in the data is the Tp–v which provides information about magnitude of nonlinear refraction. A small dip was observed in the open aperture Z-scan profile of FSC c. Two photon absorption coefficient (ˇ) value of 7.4 × 10−12 cm/W was considered in Eq. (5) to fit the data for FSC c. The closed aperture Z-scan measurements presented in Figure 4 exclusively provide information about nonlinear refraction part of

the nonlinearity which is related to the real part of ␹(3) (Re␹(3) ) and also indicate about the origin of the nonlinearity. Since there is no or negligible open aperture Z-scan signal at these intensities, it is an indirect indication that there is no contribution from intensity induced excited state nonlinear absorption. To understand the nonlinear absorption in the calixarenes, experiments were performed at higher intensity so that appreciable change in the open aperture Z-scan profiles can be detected. Figure 5 shows open aperture Z-scan profiles recorded at 255 GW/cm2 peak intensity. Corresponding closed aperture Zscan profiles are not shown here because, at this intensity no clear valley-peak profiles, rather complicated profiles were obtained due to larger phase change and the theory is no longer valid for such change. Here the discussion is on nonlinear absorption part which is related to the imaginary part of ␹(3) (Im␹(3) ). Experimentally measured open-aperture profiles for FSCs a, b and c along with the solvent are shown in Figure 5 (squares). Change in the normalized transmission at valley position (Tv ) is proportional to the magnitude of nonlinear absorption. Minimum transmission at Z = 0 is due to maximum nonlinear absorption in the medium due to highest intensity at this position. Largest change in the normalized transmission is observed for FSC c followed by FSC b while, for the FSC a and toluene the magnitude of change is almost similar. Looking at the linear absorption spectra, dip in the open aperture Z-scan profile can be indicative of intensity induced nonlinear absorption due to TPA (ˇ) or three photon absorption (˛3 ). To clarify, the experimental data was fitted with Eq. (5) by varying ˇ values in such a way that the depth of dip (normalized transmission at Z = 0) matches. ˇ values of 4.0 × 10−12 , 5.0 × 10−12 and 7.4 × 10−12 cm/W were obtained for FSCs a, b and c respectively, as shown in Figure 5 (dashed lines).

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1.0 Normalized transmission

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Figure 5. Open aperture Z-scan profiles with femtosecond laser pulses at 800 nm for (a) FSC a, (b) FSC b, (c) FSC c and (d) toluene. Squares are experimental data points, dashed and solid lines are fit to the experimental data using two photon absorption and three photon absorption coefficients respectively.

TPA curves do not fit the experimental data well. Further, instead of ˇ, three photon absorption coefficient (˛3 ) was taken for fitting. In this case, Eq. (2) is modified to dI = −˛3 I 3 dz 

(6)

Intensity at the exit face of the sample is now given by Iex (r, z, t) =

Iin (r, z, t) 2 (r, z, t)L) (1 + 2˛3 Iin

(7)

1/2

and the transmission through the sample can be obtained as [26] T (z) =



1

√ 2 (0, 0, z)L  2˛3 Iin



+∞

 ln[

−∞

by two-photon absorption from ground to the first excited singlet state, and the subsequent excitation by single photon absorption from the first excited singlet state to the second excited singlet state. Pump-probe experiments are required to further probe the mechanism. The trend of variation in ˛3 value is same as that of ˇ and n2 for all the three FSCs. Magnitude of nonlinearity in FSCs is in the order c > b > a. Largest nonlinearity obtained in case of FSC c can be understood from the molecular structure, as it exhibits largest ␲ conjugation among all three donor–acceptor FSCs. meta-substitution disrupts the extended  conjugation in FSC b hence lowering its nonlinearity

 2 (0, 0, z)L exp(−2x2 ) + 1 + 2˛3 Iin

where ˛3 is used as the fitting parameter. Fitting of the experimental open aperture Z-scan data by three photon absorption coefficient using the Eq. (8) is shown in Figure 5 (solid line). ˛3 values 4.0 × 10−23 , 5.0 × 10−23 and 9.0 × 10−23 cm3 /W2 were obtained for FSCs a, b and c respectively. There is no change in ˛3 value obtained for toluene and FSC a. Our results on toluene are in line with recently reported Z-scan measurements on the solvent with femtosecond laser pulses [25]. Open aperture Z-scan profile does not fit well with two-photon absorption parameter however it fits well with three-photon absorption parameter. Second harmonic and third harmonic of the excitation laser (Ti:Sapphire) lies at ∼400 nm and ∼267 nm respectively. Twophoton absorption and three-photon absorption in a medium is expected due to linear absorption band around second and third harmonic wavelengths. Three photon absorption in FSCs could be a two step process also (2 + 1 photon absorption). The first excitation

2 (0, 0, z)L exp(−x2 )]dx(8) 2˛3 Iin

compared to the FSC c. Larger the conjugation (order c > b > a), larger the red shift in the absorption spectra and superior, is the nonlinearity. Variation of normalized transmission with input intensity using the open aperture Z-scan measurements is shown in Figure 6 for FSC a, b and c. At low intensity the transmission is high due to negligible linear absorption in the medium at the excitation wavelength. As the input intensity increases, the transmission tends to decrease due to intensity induced nonlinear absorption in the medium, mainly three photon absorption. Typical optical limiting characteristics were demonstrated by the calixarenes towards femtosecond laser pulses. FSC a, b and c exhibit ∼11%, ∼14% and ∼20% modulation in the transmission respectively, at 200 GW/cm2 while at 250 GW/cm2 intensity respective modulation becomes ∼14%, ∼17% and ∼23%. Optical limiting performance of calixarenes with femtosecond laser pulses is comparable or better than recently

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in the nonlinear absorption in FSCs as well as the solvent nonlinearity in case of the latter. Nonlinear refraction with nanosecond laser pulses is negative while with femtosecond laser pulses it is positive and the origin of nonlinearity is expected to be thermal and bound electronic, respectively. Further, the compounds were shown to exhibit promising optical limiting characteristics for both nano and femtosecond laser pulses.

1.0 Normalized transmission

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0.9

0.8 Acknowledgement

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Dr. H.S. Rawat, Head, LPAS is thankfully acknowledged for his kind support. 50

100

150

200

250

2

Intensity (GW/cm ) Figure 6. Variation of normalized transmission with input intensity for FSC a (squares), b (circles) and c (triangles).

reported Fe3 O4 -Ag nanocomposites [19] and multi-walled carbon nanotubes functionalized with porphyrin [20]. Strong nonlinear absorption exhibited by FSCs with femtosecond laser pulses make them a prospective candidate for optical limiting of ultrashort laser pulses. Moreover, two and three photon absorption based optical limiting behavior is particularly interesting compared to single photon absorption due to the reasons (i) no linear absorption at the excitation wavelength, makes the medium highly transparent at low intensity, (ii) fast temporal response, as two/three photon absorption phenomenon is instantaneous while the single photon based are due to excited state absorption (RSA) in which excited state lifetime/intersystem crossing rate make them slow. Since, calixarenes are potential candidates for donor–acceptor type compounds, hence offer greater flexibility in the synthesis for optimized nonlinear optical properties for device applications. 4. Conclusion In conclusion, optical limiting and nonlinear optical properties of ferrocenyl substituted calixarenes were studied with nano and femtosecond laser pulses using Z-scan technique. FSC c shows largest nonlinearity compared to FSCs a and b. Nonlinear absorption with nanosecond laser pulses is mainly due to RSA while with femtosecond laser pulses three-photon absorption plays main role

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