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Nonlinear optical studies of newly synthesized polythiophenes containing pyridine and 1, 3, 4-oxadiazole units.

INTRODUCTION

Nonlinear optics has drawn increasing attention from researchers because of its potential applications in optical switching, optical data storage, optical communications, and eye and sensor protection (1-3). The emphasis has been on identifying new materials those possess lame third-order nonlinear optical properties with ultra fast response times. In recent times, there has been an enormous interest in organic materials possessing strong [pi]-electron delocalization which determines the strength of third-order nonlinearity. An important advantage of using organic materials is the fact that their molecular structure can be modified, there by altering optical properties. Among the organic materials, conjugated polymers possess a unique combination of electronic and optical properties. In recent years, variations in molecular structure have yielded conjugated polymers with excellent solubility, adequate thermal stability, and desirable optical properties. The possibility of making systematic structural modification at the molecular level has made these materials attractive candidates for photonic device applications (4), (5).

Conjugated polymers such as polyacetlycnes, polydiacetylenes, polyphenylenevinylenes, etc. were extensively studied for their third-order nonlinear optical properties (2), (6). In recent times, polythiophenes, a very versatile class of conjugated polymer, have been widely investigated because of their good optical stability, easy process-ability and large third-order nonlinear optical properties (7), (8). In Polythiophenes, nonlinear optical properties can be tuned by attaching an alkyl chain to the thiophene ring, which enhances the delocalization of [pi]-electrons in the polymer.

Recently Cassano et al. (9) have showed that, by a proper choice of the side chains in a series of dialkoxy substituted poly(p-phenylenevinylene), it is possible to enhance the third-order nonlinear optical coefficients. They also reported a new strategy for tuning the linear and nonlinear optical properties of soluble derivatives of fluorinated poly(p-phenylenevinylene) copolymers based on the effect of the simultaneous presence of electron-acceptor and electron-donor substituted aromatic rings in the conjugated backbone (10). Using similar design concepts, we have synthesized a polythiophene containing 1, 3, 4-oxadiazole and pyridine as electron-accepting units and 3, 4-didecyloxythiophene as electron-donating units along the polymer chain. In this article, we present the experimental measurements of third-order nonlinear optical properties of the newly synthesized polythiophene carried out using the single beam Z-scan Technique.

EXPERIMENT

Single beam Z-scan technique (11). was employed to measure the third-order optical nonlinearities of the copolymer samples prepared. This technique enables simultaneous measurement of nonlinear refraction (NLR) and non-linear absorption (NLA). Basically, in this technique a Gaussian laser beam is focused, using a lens, on the cuvette containing the liquid sample. The cuvette is translated across the focal region and changes in the far-field intensity pattern are monitored. The experiments were performed using a Q-switched, frequency doubled Nd: YAG laser (Spectra-Physics GCR170) which produces 7ns pulses at 532 nm and at a pulse repetition rate of 10 Hz. The laser beam was focused by using a lens of 25 cm focal length. The laser beam waist at the focused spot was estimated to be 18.9 [micro]m and the corresponding Ray-leigh length is 2.11 mm. The Z-scan measurements were carried out using a cuvette of I mm thickness, which is less than the Rayleigh length. Hence, the thin sample approximation is valid. The Z-scan experiment was performed at an input peak-intensity of 0.478 GW/[cm.sup.2]. The nonlinear transmission of the copolymer, with and without the aperture in front of the detector was measured in the far-field using Laser Probe Rj-7620 Energy Meter with Pyroelectric detectors. The optical power limiting experiments were performed by keeping the cuvette containing copolymer solution, at the focus of the laser beam and measuring the transmitted laser energy at various input laser energies.

The synthesis and characterization of the conjugated copolymer used in this study have been reported elsewhere (12). Figure I shows the general structure of the copolymer. The UV-Visible absorption spectrum of the copolymer dissolved in tetrahydrofuran (THF). was recorded using a Fiber Optic Spectrometer (Ocean Optics) and is shown in Fig. 2. For the Z-scan experiments polymer solution of concentration 5 X [10.sup.-4] mol/L was used.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Z-SCAN RESULTS AND DISCUSSION

The Open aperture Z-scan (i.e. without aperture in front of the detector) was performed to measure the nonlinear absorption in the copolymer sample, which is related to imaginary part of third-order optical susceptibility [[chi].sup.(3)]. Figure 3 shows the open aperture Z-scan curve of the copolymer sample which is symmetric with respect to the focus indicating intensity dependent absorption. The absorption may be clue to two photon absorption (TPA), excited state absorption (ESA), reverse saturable absorption (RSA), etc. Nonlinear absorption of nanosecond pulses can be understood using the five level model (13), (14) shown in Fig. 4. The relevant energy levels are the singlet levels [S.sub.0], [S.sub.1] and [S.sub.2] and the triplet levels are [T.sub.1] and [T.sub.2]. Each of these states contains number of vibrational levels. When the molecule is excited by the laser pulse, electrons are initially excited from lowest vibrational level of [S.sub.0] to upper vibrational levels of [S.sub.1],where they relax in picoseconds by nonradiative decay. In nanosecond time scale, singlet transition does not deplete the population of [S.sub.1] level appreciably, since atoms excited to [S.sub.2] decay to [S.sub.1] itself within picoseconds. From [S.sub.1] electrons are transferred to [T.sub.1] via intersystem crossing (ISC), from where transitions to [T.sub.2] occur. The process ([S.sub.1][left arrow] [S.sub.0]) is known as ground state absorption. The two process ([S.sub.2] [left arrow] [S.sub.1]) and ([T.sub.2] [left arrow] [T.sub.1]) are known as ESA (Excited state absorption), and if the absorption cross-sections are larger than that of the ground state the process is called RSA (reverse saturable absorption). With excitation of laser pulses on the nanosecond scale, which is true in our case, triplet-triplet transitions are expected to make significant contribution to nonlinear absorption.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The excited state absorption cross section [[sigma].sub.exc], can be measured using the normalized energy transmission of open aperture Z-scan (i.e. without aperture.) (15-18). The change in the intensity of the laser beam as it propagates through the sample is given by

[dI/dZ] = - [alpha]I - [[sigma].sub.exc]N(t), (1)

[dN/dt] = [[alpha]I/h[omega]], (2)

where I is the intensity, Z is the sample position, N is the density of charges populated in the excited state, [omega] is the angular frequency of the laser and [alpha] is linear absorption. By combining Eqs. 1 and 2 yield

[dI/dZ] = - [alpha]I - [[[sigma].sub.exc][alpha]I/h[omega]][[integral].sub.-[infinity].sup.t]I(t')dt', (3)

Solving the above equation for the fluence and integrating over spatial extent of the beam, gives the normalized energy transmission for open aperture and is given by (17),

T = In(1 + [[q.sub.0]/[1 + [x.sup.2]]])/([[q.sub.0]/[1 + [x.sup.2]]]), (4)

where x = [z/[z.sub.0]], z is the distance of the sample from the focus, [z.sub.0] is the Rayleigh length given by the formula [z.sub.0] = [2[pi][w.sub.0.sup.2]/[lambda]] (k is the wavelength and [w.sub.0] is the beam waist at the focus) and [q.sub.0] is given by the equation (15-17),

[q.sub.0] = [[[sigma].sub.exc][alpha][F.sub.o](r = 0)[L.sub.eff]/2h[omega]], (5)

where [alpha] is the linear absorption coefficient, [L.sub.eff] = [1 - exp(-[alpha]L)]/[alpha], [omega] is the angular frequency of the laser and [F.sub.o] is the on-axis fluence at the focus which is related to the incident energy [E.sub.total] by,

[F.sub.o] = [2[E.sub.total]/[pi][[omega].sub.o.sup.2]], (6)

The value of excited state absorption cross section [[sigma].sub.exc]' of the copolymer was obtained by fitting the open aperture data to the Eq. 4. The linear absorption spectra of the copolymer in the Fig. 2 show that the absorption edge is close to experimental wavelength 532 nm. The small absorption tail at 532 nm gives the linear absorption coefficient [alpha] = 0.2943 [cm.sup.-1], for the copolymer. The ground state absorption cross section, [[sigma].sub.g], was calculated using the relation,

[alpha] = [[sigma].sub.g][N.sub.a]C, (7)

where [N.sub.a] is the Avogadro's number and C is the concentration in mol/L.

The measured values of ground state and excited state absorption cross sections of the copolymer were 9.77 X [10.sup.-19] [cm.sup.2] and 6.42 X [10.sup.-18] [cm.sup.2] respectively. The larger value of [[sigma].sub.exc], as compared to [[sigma].sub.g], of the copolymer indicates that the operating nonlinear mechanism is reverse saturable absorption (RSA) (15). To confirm this we have measured [[beta].sub.eff] as a function of the on-axis input laser intensity [I.sub.o]. As seen from Fig. 5. [[beta].sub.eff] decreases on increasing [I.sub.o] which is a signature of RSA (19). In contrast, for two photon absorption [[beta].sub.eff] is known to be independent of [I.sub.o] (20). However, Hein et al. (21) have reported decrease of [[beta].sub.eff], with increasing [I.sub.o] for the thiophene oligomers, where they attributed to saturation of instantaneous two-photon absorption.

[FIGURE 5 OMITTED]

To determine the sign and magnitude of nonlinear refraction, closed aperture Z-scan was performed by placing an aperture in front of the detector. Figure 6 shows the closed aperture Z-scan curve of the copolymer. The peak-valley shape of the curve indicates negative nonlinear refraction. The nonlinear refractive index [gamma]([m.sup.2]/w) is given by the formula (11),

[FIGURE 6 OMITTED]

[gamma] = [[DELTA][[phi].sub.o][lambda]/2[pi][L.sub.eff][I.sub.o]]([m.sup.2]/W), (8)

where, [[DELTA][[phi].sub.o] is the is the on-axis phase change given by the equation,

[DELTA][[phi].sub.o] = [[DELTA][T.sub.p-v]/0.406[(1 - S).sup.0.25]] for [DELTA][absolute value of [[phi].sub.o]][less than or equal to][pi], (9)

where, [DELTA][T.sub.p-v] is the peak to valley transmittance difference and S is the linear aperture transmittance, which is equal to 0.5 in our experiments.

The nonlinear refractive index [n.sub.2] (in esu) is related to [gamma]([m.sup.2])/w) by,

[n.sub.2](esu) = (c[n.sub.o]/40[pi])[gamma]([m.sup.2]/w), (10)

The normalized transmittance for the closed aperture Z-scan conditions is given by (11),

T(z) = 1 + [4x[DELTA][[phi].sub.o]/[([x.sup.2] + 9])([x.sup.2] + 1)]] - [2[DELTA][[psi].sub.o]([x.sup.2] + 3)/[([x.sup.2] + 9)([x.sup.2] + 1)]], (11)

where [DELTA][[psi].sub.o] is the on-axis phase shift due to nonlinear absorption and is given by

[DELTA][[psi].sub.o] = [1/2][[beta].sub.eff][I.sub.0][L.sub.eff].

The closed aperture Z-scan data also includes the contribution from nonlinear absorption. To extract the pure nonlinear refraction part, following Sheik-Bahae et al. (11), we have computed the value of the closed aperture data by the open aperture data. Figure 7 shows the resulting curve corresponding to pure nonlinear refraction. The normalized transmittance for pure nonlinear refraction is given by (11),

T(z) = 1 + [4x[DELTA][[phi].sub.o]/[([x.sup.2] + 9)([x.sup.2] + 1)]] (12)

[FIGURE 7 OMITTED]

The nonlinear refractive index [n.sub.2] (esu), and nonlinear absorption coefficient [beta], are related to the real and imaginary part of third-order nonlinear optical susceptibility through the equations,

Re[[chi].sup.(3)] = 2[n.sub.o.sup.2]c[[epsilon].sub.o][n.sub.2](esu) (13)

Im[[chi].sup.(3)] = [n.sub.o.sup.2]c[[epsilon].sub.o][lambda][beta]/2[pi]], (14)

where [n.sub.0] is the linear refractive index, [[epsilon].sub.0] is the permittivity of free space and c is velocity of light in vacuum.

In [phi]-conjugated polymers, electrons can move in large molecular orbitals, which result from the linear superposition of the carbon [P.sub.z] atomic orbitals, leading to high [[chi].sub.(3)] (9). The copolymer studied in our experiments consists of alternating electron donating (alkoxy pendant at 3,4 position) and electron withdrawing groups (1,3,4-oxadiazole and pyridine) in the chain. Thus the copolymer has the groups forming an donor-acceptor configuration which is essential to exhibit large third-order nonlinear optical properties. The substitution of the alkoxy group not only enhances the delocalization [pi]-electrons in the copolymer, but also acts as a solubilizing group. The third-order non-linearity in the copolymer arises due to the high [pi]-electron density along the polymer backbone which is delocalized. The coupling factor [rho] is defined as the ratio of imaginary part to real part of the third-order nonlinear susceptibility.

[rho] = [Im[[chi].sup.(3)]/[[chi].sub.R.sup.(3)]] = [[beta]/2k[n.sub.esu]] (15)

The coupling factor for the copolymer was found to be 0.264 is less than 1/3, this indicates that the nonlinearity is predominantly of electronic origin. The calculated values of nonlinear absorption coefficient, [[beta].sub.eff], nonlinear refractive index, [gamma] in [m.sup.2]/W ([n.sub.2] in esu), and the real and the imaginary parts of third-order nonlinear optical susceptibility %.sup.(3), were 18.93 cm/GW, 3.037 x [10.sup.-17] [[m.sub.2]/W] (-1.022 x [10.sup.-10] esu) 1.512 x [10.sup.-20][[m.sup.2]/[V.sup.2]] (-1.080 x [10.sup.-12] esu), and 0.399 x [10.sup.-20] [[m.sup.2]/[V.sup.2]] (0.285 x [10.sup.-12] esu), respectively, The real and imaginary part [[chi].sup.(3)] were converted to SI units using the relation.

[[chi].sup.(3)](SI) = [4[pi]/9] x [10.sup.-8][[chi].sup.(3)](cgs) (16)

The value of [n.sub.2], is nearly three orders of magnitude larger than that of thiophene oligomers obtained by Hein et al. (21). The value of [[beta].eff] is comparable with the value obtained by Cassano et al. (10). The value of third-order nonlinear optical susceptibility [[chi].sup.(3)], is comparable with the value of poly (3-dodeclyloxymethylthiophene) which is 5 x [10.sub.-12] esu obtained by Sasabe et al. (22).

OPTICAL POWER LIMITING STUDIES

Optical power limiting is a very important nonlinear optical property in the context of eye and sensor protection against intense light (3). An ideal optical limiter is perfectly transparent at light intensities below a threshold level. above which the transmitted intensity remains clamped at a constant value (2). The nonlinear mechanisms leading to optical power limiting include two photon absorption, free carrier absorption, reverse saturable absorption, nonlinear scattering, etc. The molecules exhibiting RSA generally have extremely fast response time. since it involves electronic transitions. The best known reverse saturable absorbers are fullerene ([C.sub.60]), porphyrin complexes, pthalocyanines etc., (2), (8), (9). Optical power limiting experiments were performed by keeping the cuvette, containing polymer solution at the focus fo the laser beam and measuring the transmitted energy for different input laser energies. Figure 8 shows the optical power limiting of the copolymer. The copolymer was found to exhibit good optical power limiting property for the nano-second laser pulses. The clamping levels of the copolymer were, ~34, ~48, and ~72 [micro]J respectively at concentrations of 5 x [10.sup.-4], 2.5 x [10.sup.-4], and 1.25 x [10.sup.-4] mol/L.The clamping levles of the copolymer decreased on increasing the concentration, this is because solutions with higher concentration posses more molecules per unit volume leading to more efficient absorption. We attribute the optical power limiting of the copolymer to reverse saturable absorption. Therefore, thiophene-based copolymer investigated by us is a promising candidate for making optical power limiting devices.

[FIGURE 8 OMITTED]

CONCLUSIONS

In summary, third-order nonlinear optical properties of a novel copolymer was investigated using the nanosecond single beam Z-scan technique. Z-scan results indicate that the copolymer exhibits self-defocusing effect or negative nonlinearity. The real and imaginary parts of third-order nonlinear optical susceptibility, [[chi].sup.(3)], were 1.512 x [10.sup.-20] [[m.sup.2]/[V.sup.2]] (-1.080 x [10.sup.-12] esu) and 0.399 x [10.sup.-20] esu) and 0.399 x [10.sup.-20] [[m.sup.2]/[V.sup.2]] (0.285 x [10.sup.-12]), respectively. The copolymer exhibits good optical power limiting of nanosecond laser pulses at 532 nm wavelength. The operating nonlinear mechanism leading to optical power limiting was found to be reverse saturable absorption. Large third-order nonlinear optical properties in the copolymer arise due to the strong delocalization of [pi]-electrons along the polymer chain. Hence, the copolymer investigated here seems to be promising material for making practical devices for photonics applications.

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P. Poornesh, (1) P.K. Hedge, (2) M.G. Manjunatha, (2) G. Umesh, (1) A.V. Adhikari (2)

(1) Department of Physics, National Institute of Technology Karnataka - Surathkal, Mangalore-575025, India

(2) Department of Chemistry, National Institute of Technology Karnataka - Surathkal, Mangalore-575025, India

Correspondence to: P. Poornesh; e-mail: poorneshp@gmail.com

Contract grant sponsor; Department of Information Technology (D.I.T.).

Govt, of India.

DOI 10.1002/pen.2l215

Published online in Wiley InterScience (www.interscience.wiley.com).

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Author:Poornesh, P.; Hedge, P.K.; Manjunatha, M.G.; Umesh, G.; Adhikari, A.V.
Publication:Polymer Engineering and Science
Article Type:Report
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Date:May 1, 2009
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