Third-order nonlinear optical response of push–pull azobenzene polymers

Chemical Physics Letters 554 (2012) 107–112

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Third-order nonlinear optical response of push–pull azobenzene polymers I. Papagiannouli a,b, K. Iliopoulos c, D. Gindre c, B. Sahraoui c,⇑, O. Krupka d, V. Smokal d, A. Kolendo d, S. Couris a,b,⇑ a

Department of Physics, University of Patras, 26504 Patras, Greece Institute of Chemical Engineering and High Temperature Chemical Processes (ICE-HT), Foundation for Research and Technology-Hellas (FORTH), 26504 Patras, Greece c LUNAM Université, Université d’Angers, CNRS UMR 6200, Laboratoire MOLTECH-Anjou, France d Kyiv Taras Shevchenko National University, 60 Volodymyrska, 01033 Kyiv, Ukraine b

a r t i c l e

i n f o

Article history: Received 28 July 2012 In final form 3 October 2012 Available online 12 October 2012

a b s t r a c t The nonlinear optical response of a series of azo-containing side-chain polymers is investigated using Zscan technique, employing 35 ps and 4 ns laser pulses, at 532 nm. The systems were found to exhibit strong nonlinear optical response, dominated by nonlinear refraction. In all cases, the nonlinear absorption and refraction have been determined and are compared with those of disperse red 1 considered as reference. The corresponding third-order susceptibilities v(3) were determined to be as large as 107 and 105 esu under ps and ns laser excitation, respectively. Finally, the results are discussed and compared with other reported data. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In the direction of synthesizing novel materials exhibiting high nonlinear optical response, compounds consisting of an electron donating and electron accepting group connected with a p-conjugated chain are of great interest. In many cases the presence of electron donating and accepting groups has been reported to result to a significant increase of the nonlinearities [1–5]. The azobenzenes are a category of materials with very interesting chemical/optical and in particular nonlinear optical properties, which can satisfy the prerequisites for a variety of optoelectronic applications. One of their most important features is that they can exist in two isomeric forms (trans and cis), while the ratio of the two isomers can be modified by proper irradiation and can be detected by the modification of their absorption spectra [6]. Moreover, photoinduced birefringence and dichroism are well known to take place in such systems [7,8]. It has also been demonstrated that two-photon absorption processes can be employed in order to photo-orient azobenzene molecules [9]. Because of their interesting properties the azobenzenes can be useful in a variety of applications including optical data storage, surface relief gratings, all optical switching, etc. [10–13]. For the aforementioned reasons the second-order (after introducing the necessary non-centrosymmetry to the systems) and third-order nonlinearities of azobenzenes are widely investigated ⇑ Corresponding authors at: Institute of Chemical Engineering and High Temperature Chemical Processes (ICE-HT), Foundation for Research and Technology-Hellas (FORTH), 26504 Patras, Greece. Fax: +30 2610 965223 (S. Couris). E-mail addresses: [email protected] (B. Sahraoui), couris@iceht. forth.gr (S. Couris). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.007

[14–17]. In a previous Letter [14], we have investigated the nonlinear optical response of the systems S1–S3 (see Figure 1) by means of Second Harmonic Generation (SHG) and Third Harmonic Generation (THG) techniques. In this Letter, as a physical continuation of our previous findings the nonlinearity of the same azobenzene/ polymer systems is investigated by means of the Z-scan technique employing picosecond and nanosecond excitation. The present investigation is expected to provide useful information about the transient and electronic nonlinear optical responses and shed light about the physical origins of the nonlinearity of these systems. So, as far as it concerns the ps measurements, it is expected that different mechanisms will contribute to the observed nonlinearity, compared to our previously published results in Ref. [14], as it is well known that THG and Z-scan techniques can detect different origin nonlinear optical contributions [18]. More specifically, the former technique detects only the electronic contribution to the nonlinear response, while the latter apart from the electronic contribution can detect other, slower in general mechanisms, such as the molecular orientation, redistribution, etc. On the other hand, by performing measurements under ns laser excitation regime the transient response of the systems is investigated. Comparison between the responses exhibited at the two time regimes can provide an estimation of the importance of each contribution to the overall nonlinear optical response of the azobenzene systems.

2. Experimental The third-order nonlinear optical properties of the three azobenzene side chain polymeric systems shown in Figure 1 were investigated by means of the Z-scan technique [19]. Since the

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CH3 H2C

R O

O OCH3

CH3

CH3

C CH2

C CH2

C O

n

M

R

where R=

m

OCH3

O

O

C O

S

CH3 N

post-focal peak or a pre-focal peak followed by a post-focal valley. The former case corresponds to self-focusing (c0 > 0) while the latter to self-defocusing (c0 < 0) behavior of the sample, respectively. From the difference of the normalized transmission DTpv between the peak and the valley of the ‘divided’ Z-scan, the nonlinear refractive parameter c0 can be easily deduced. The third-order susceptibility is a complex number, and its imaginary part is related to the two-photon absorption process, while its real part to the nonlinear refraction. As a result, the absorption and the refraction index of a material under intense laser radiation can be expressed, including the intensity-dependent terms, as:

CH3

a ¼ a0 þ bI

N

n ¼ n0 þ c0 I

N

where a0 ðcm Þ and n0 are the linear terms, IðW=cm Þ is the incident radiation intensity, while the two-photon absorption parameter b (cm/W) and the nonlinear refractive parameter c0 (cm2/W) are related to the corresponding third-order nonlinear susceptibility v(3) by the following equations [20]:

ð1Þ ð2Þ 2

1

N

N

N

N

N N

N

N

N

NO2

NO2

1

2

3

Figure 1. Investigated azo-benzene polymers.

technique has been described in details elsewhere [20], here only a brief description will be presented. Briefly, the Z-scan technique is based on the measurement of the transmission of a sample when it is irradiated by a focused Gaussian laser beam and its position is scanned along the beam direction (z-axis) through the focal plane. As the sample experiences different laser intensities at different positions, the recordings of the transmission as a function of the z coordinate provide information about the nonlinear effects present. The technique allows the determination of both the real and imaginary parts of the third-order susceptibility v(3) from one single measurement, by performing simultaneously two different kinds of transmission measurements, the so-called ‘open-aperture’ (OA) and ‘closed-aperture’ (CA) Z-scans. This is achieved by dividing the transmitted through the sample laser beam by means of a beam splitter placed just after the sample. In particular, during the former transmission measurement (i.e. the OA), the totally transmitted light through the sample is collected and detected for each position of the sample, providing information about the nonlinear absorption of the sample and allowing the determination of the nonlinear absorption parameter b, while during the latter transmission measurement (i.e. the CA), only a part of the laser beam is collected after it has passed through an small circular aperture positioned in the far-field, in front of the detector. The shape of the OA Z-scan recordings can exhibit either a transmission minimum (b > 0) or a transmission maximum (b < 0), corresponding to reverse saturable absorption (RSA) or to saturable absorption (SA) behavior, respectively. In the case where the nonlinear absorption is relatively weak, then the nonlinear refraction can be simply obtained, by dividing the CA recording by the corresponding OA one, the resulting curve called ‘divided’ Z-scan, exhibiting a pre-focal valley followed by a

Im

vð3Þ ðesuÞ ¼

107 c2 n20 b 96p2 x

ð3Þ

Re

vð3Þ ðesuÞ ¼

106 cn20 0 c 480p2

ð4Þ

where c is the speed of light in cm/s and x is the fundamental frequency given in cycles s1. In that context, Z-scan measurements have been carried out in this Letter employing a 35 ps mode-locked Nd:YAG laser having a Gaussian profile and a 4 ns Q-switched Nd:YAG laser having a tophat beam profile, both operating at 10 Hz repetition rate. In the case of ps measurements, the nonlinear optical parameters b and c0 have been determined according to the procedure described in details in references [19,20]. So, the b values have been deduced by fitting the OA Z-scan recording using the following equation:

T ¼ pffiffiffiffi

Z

1

b I0 Leff pð1þz 2 =z2 Þ

þ1 1

  b I0 Leff 2 ln 1 þ expðt Þ dt 1 þ z2 =z20

ð5Þ

0

where T is the normalized transmittance, I0 is the on-axis irradiance at the focus and Leff is the effective path length of the sample defined as Leff ¼ 1  expða0 LÞ=a0 . The values of the nonlinear refractive parameter c0 have been determined from the slopes of the linear best fits of the DTpv values plotted versus the incident laser energy through the following relation:

T ¼1þ

4 DU0 ðz=z0 Þ ½ðz=z0 Þ2 þ 9½ðz=z0 Þ2 þ 1

ð6Þ

where DU0 ¼ k I0 Leff c0 is the on-axis nonlinear phase shift at the focus. In the case of the ns laser excitation, where the laser beam had a top-hat profile, a different procedure has been followed for the determination of the nonlinear optical parameters b and c0 , as suggested by Zhao and Palffy-Muhoray in Refs. [21,22]. According to this procedure, the imaginary and the real phase shifts, due to nonlinear absorption and nonlinear refraction, respectively, (i.e. the parameters W and Uo), were employed, which under the thin-sample approximation ðL h z0 Þ are connected to the nonlinear optical parameters b and c0 with the following equations:

W ¼ bI0 Leff

ð7Þ

U0 ¼ 2pc0 I0 Leff =k

ð8Þ

I. Papagiannouli et al. / Chemical Physics Letters 554 (2012) 107–112

3. Nonlinear optical measurements In Figure 2, the UV–Vis optical absorption spectra of the S1, S2 and S3 investigated azopolymer systems are shown, obtained from thin-films of the azo-benzene polymers spin-deposited on glass substrates having a thickness of about 0.4–0.5 lm as determined from profilometer measurements. The details of the chemical synthesis of the azo-containing side-chain polymers and the corresponding characterizations have been reported elsewhere [14]. All spectra were exhibiting the characteristic absorption peaks, located in the visible region, corresponding to the p–p⁄ electronic transitions. The exact position and the intensity of these bands are generally greatly affected by the strength of the charge transfer interactions and consequently, on their turn, they can dramatically affect the nonlinear optical response, not only because the degree of resonance enhancement can be changed but by altering the efficiency of the trans/cis photoisomerization as well. So, the S1 azobenzene polymer (i.e. the disperse red 1 side-chain polymer) exhibited its maximum at 467 nm, while azobenzene polymer S3 maximum was found to be clearly red-shifted, located at 500 nm, as a result of its more intense charge transfer character. Oppositely, S2 azobenzene polymer maximum was found to be blue-shifted, located at 423 nm, due to its reduced charge transfer character compared to the two other azo-polymers. For the determination of the nonlinear optical properties of the azobenzene polymers, Z-scan measurements using different incident laser energies have been performed, using 532 nm ps and ns laser pulses. In both cases, the laser beam was focused by 20 cm focal length quartz lens onto the sample surface, while its energy was measured by means of a calibrated joule-meter. The ns and ps focused laser beam radii on the sample were measured using a CCD camera and were found to be 17 lm in both cases, while the Raleigh length was determined to 1.7 mm, i.e. much larger that the films’ thickness. In order to improve the quality of the measurements, since very low excitation energies were employed, each measurement point corresponds to the average of 100 laser shots. Measurements performed at the same and at different locations over the films’ surface have not shown significant variations suggesting that no surface modification has occurred as a result of the laser shots they received and that the films exhibited rather good spatial homogeneity. Most of the measurements were performed with the lasers operating at a repetition rate of 10 Hz, since measurements performed at lower repetition rates did not show

Absorbance (a.u.)

3

S1 S2 S3

2

1

0 300

400

500

600

700

800

900

1000

1100

Wavelength (nm) Figure 2. UV–Vis absorption spectra of the studied azo-benzene polymers thin films. The vertical line shows the wavelength where laser excitation occurred.

109

any significant differences, suggesting that thermal effects were negligible under the present experimental conditions. The duration of the laser pulses plays a very important role concerning the physical processes occurring during laser excitation. In fact, depending upon the laser pulse duration different contributions corresponding to different physical mechanisms can contribute to the observed nonlinearities, each one exhibiting a characteristic response time (e.g. electronic response, molecular orientation, vibrational contributions, population re-distribution, thermal effects, transient nonlinearity). In that respect, the use of picosecond and nanosecond excitation, allowed the determination of the electronic and the transient nonlinear response of the azobenzene polymers, respectively. As an example, in Figure 3 some representative OA and ‘divided’ Z-scans of the systems are presented, obtained under 532 nm, 35 ps (see e.g. Figure 3a, c and e) and 4 ns (see e.g. Figure 3b, d and f) laser excitation, respectively. The laser energies used were 40 and 4 nJ, respectively, corresponding to incident laser intensities of 0.3 GW/cm2 and 0.05 MW/cm2, respectively. As can be seen, all the systems were found to exhibit a pre-focal peak followed by a post-focal valley, indicative of self-defocusing behavior corresponding to negative sign nonlinear refraction (i.e. Rev(3) or c0 < 0) when excited with either picosecond or nanosecond laser pulses in the visible. The corresponding OA Z-scan recordings were found to exhibit a transmission maximum when the samples attained the focal level, suggesting saturable absorption like behavior (SA) corresponding to negative sign nonlinear absorption (i.e. Imv(3) or b < 0). However in the case of the S2 system the OA scans were flat, indicative of negligible nonlinear absorption under both ps and ns laser pulses. From the ‘divided’ Z-scan measurements, the values of the DTpv parameter were obtained and have been plotted as a function of the incident laser energy for the ps (see Figure 4a) and ns (see Figure 4b) laser excitation conditions, respectively. As shown, in all cases a very good linear correlation was observed as suggested by the straight lines corresponding to the linear best fits of the experimental data points. From these plots, the nonlinear refractive parameter c0 was determined and then the Rev(3) values of the samples have been calculated using Eq. (4). The determined nonlinear optical parameters b and c0 , together with the deduced values of the third-order susceptibility v(3) are presented in Table 1. Since the value of v(3) can vary for each sample because e.g. the different thickness, in order to provide an easier way to compare the nonlinearity of different films, the linear absorption coefficient a0 of each film was used to define v(3)/a0, which has been also included in Table 1. As can be easily seen from this table, the systems S1 and S3 exhibited significantly larger nonlinear optical response under both pulse durations compared to the S2 system. This is also supported by the fact that the incident laser energies used for samples S1 and S3 were significantly lower than those used for sample S2. In fact, for the S1 and S3 samples’ measurements incident laser energies up to 50 nJ for ps excitation and up to 20 nJ for ns excitation were employed, while as can be easily seen from the abscissas of the plots of Figure 4, the corresponding incident laser energies for sample S2 were surprisingly higher. In general, different physical mechanisms can contribute to the azobenzene optical nonlinearities. The electronic nonlinearity is related to the modification of the dipole moment of the molecule, when it undergoes a transition to an excited state. Such a situation can in principle give rise to large v(3) values, usually being strongly dependent upon the incident laser wavelength [23], exhibiting important resonance effects. In particular, for wavelengths lower than the main resonance, as well as after the 2PA resonance, the v(3) is expected to be positive, while in between the resonance

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1.2

1.2 1.2 1.5

Normalized Transmittance

1.1 1.0

1.1

1.1

1.0 0.5

-20

-10

0

10

0.9

20

-20

-10

0

10

20

1.0

1.0

0.9

0.9

(a) S1, 31 nJ

(b) S1, 7 nJ

0.8

0.8 1.2

1.2

Normalized Transmittance

1.0

1.1

1.0

1.1

0.8

0.6

0.8

-20

-10

0

10

0.6

20

-20

-10

0

10

20

1.0

1.0

0.9

0.9

(c) S2, 270 nJ

(d) S2, 50 nJ

0.8

0.8 1.4

Normalized Transmittance

1.6

1.2

1.1

1.1

1.2

1.0

0.8

0.8

-20

-10

0

10

20

-20

-10

0

10

20

1.0

1.0

0.9

0.9

(f) S3, 4 nJ

(e) S3, 50 nJ 0.8

0.8 -20

-10

0

10

20

z (mm)

-20

-10

0

10

20

z (mm)

Figure 3. Characteristic ‘divided’ and ‘open-aperture’ (insets) Z-scan recordings of the S1, S2 and S3 azobenzene polymer films obtained under 532 nm, 35 ps (left) and 4 ns (right) laser excitation, respectively.

(as it is in the present case) it is expected to be negative. This is in perfect agreement with the current findings. Another contribution on the nonlinear optical response can arise from the trans–cis photo-isomerization of the azobenzene molecules. In fact, it is known, that during the photoisomerization, the distance between the two carbon atoms from which the acceptor and donor groups extend can be reduced, resulting in reduction of the molecule’s dipole moment and which reduces the material’s polarizability providing a large negative nonlinearity, in agreement with the present results under both pulse durations excitation conditions. A detailed discussion of the dynamics of the trans–cis photoisomerization can be found elsewhere [24]. In a similar study, Rangel-Rojo et al. [16] have investigated the nonlinear optical response of some DR1-functionalized PMMA thin films similar to the ones studied here, using Z-scan technique employing 20 ps laser pulses at different excitation wavelengths ranging from 560 to 610 nm. In this Letter, the characteristic p–p⁄ band of the disperse red based molecule was observed at 478 nm, in very good agreement with the present Letter where it appeared at 467 nm (e.g. see Figure 2). Comparing the results they obtained at 560 nm, which is their closest excitation wavelength compared to our measurements, saturable absorption and self-defocusing

have been reported in full agreement with the present Letter. Moreover, the magnitude of their Rev(3) is in perfect agreement with that determined here, while the difference of magnitude existing between the values of the nonlinear absorption parameter b is most probably due to the different film thickness corresponding to different absorption and also to the different laser excitation wavelength resulting to different degree of enhancement. Sun et al. [25] studying the nonlinear optical properties of some organic–inorganic hybrid waveguide films doped with disperse red 1 using Z-scan employing 7 ns, 532 nm laser pulses have determined a third-order susceptibility of 2.52  108 esu, in good agreement with the value determined here for ns excitation. Recently, in another study by Li et al. [26], the nonlinear optical properties of some similar azobenzene polymers with different electron-accepting and electron-donating moieties have been investigated using 4 ns, 532 nm laser excitation. Among them, the azopolymer PNAzo, exhibited its characteristic p–p⁄ band at about 480 nm (see e.g. Figure 1 of Ref. [26]) similarly to the S1 sample whose corresponding band is at 467 nm (see e.g. UV–Vis spectrum of Figure 2). In both cases, the 532 nm laser excitation was occurring very close to this resonance, resulting in saturable absorption behavior and self-defocusing, while the magnitudes of

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a

0.0

S1 S2 S3

ΔT P-V

-0.1

-0.2

-0.3

(a) 532 nm, 35 ps -0.4 0.0

0.1

0.2

0.3

0.4

0.5

Energy (μJ)

b

0.0

S1 S2 S3

ΔT P-V

-0.2

smaller than the v(3) values obtained by other techniques, as for example by the Z-scan technique which can include other sources of nonlinear optical response as well. So, although direct comparisons of the absolute v(3) values obtained by THG and Z-scan techniques cannot be made, the relative magnitudes of the third-order susceptibility of the investigated samples can be compared. So, both techniques firmly confirm that azopolymer S1 possesses the strongest optical nonlinearity of all three azopolymers studied, followed by azopolymers S3 and S2, respectively. Another interesting issue concerning the effect of the laser pulse duration on the strength of the nonlinear optical response is that the v(3) values of the studied azopolymers were found to be lower by 2–3 orders of magnitude under ps excitation compared to those found using ns pulses, as it can be easily seen from Table 1. This situation can be attributed to the transient optical response resulting usually under ns laser excitation where population dynamics can enhance importantly the observed third-order susceptibility. Similar observations have been done in the case of studies of the nonlinear optical response of fullerenes and have been discussed elsewhere [28].

4. Conclusion

-0.4

-0.6

(b) 532 nm, 4 ns 0.00

0.02

0.04

0.06

0.08

0.10

Energy (μJ) Figure 4. Variation of the DTpv parameter as a function of the incident laser energy for the azobenzene polymers studied under visible (a) 35 ps, and (b) 4 ns laser excitation.

the nonlinear optical parameters b and c0 were found to be in very good agreement. Similarly, Muto et al. [27] having used ns visible laser pulses and a DFWM setup to study some DR1-doped alumina films, have reported v(3) values of the order of 107 esu in very good agreement with the v(3) values determined here for ns excitation. In a very recent study [14], the third-order optical nonlinearity of the azopolymers investigated here, have been studied by means of Third Harmonic Generation (THG) technique employing 30 ps, 1064 nm laser pulses. THG technique is known to provide the third-order susceptibility of a system corresponding to its pure electronic response, whose absolute magnitude is usually much

In conclusion, in the present Letter the electronic and transient nonlinear optical response of three push–pull azobenzene polymers has been investigated by means of the Z-scan technique using visible laser pulses of different durations (e.g., 35 ps and 4 ns). All azopolymer systems were found to exhibit very large values of third-order nonlinear susceptibility, the v(3) values determined under ns excitation being 3–4 orders of magnitude larger. Furthermore, all azopolymer samples were found to exhibit self-defocusing (i.e., negative nonlinear refraction) under both excitation regimes. Besides, S1 and S3 samples were found to exhibit saturable absorption (i.e., negative absorption) as a result of the neighboring of the laser excitation wavelength to their p–p⁄ resonance. Interestingly, sample S2 did not show any significant nonlinear absorption, exhibiting only important nonlinear refraction. In all cases, the the observed very strong nonlinear optical response can be attributed to the trans– cis photo-isomerization of the azobenzenes when excited with visible laser light.

Acknowledgments S.C. and I.P. acknowledge partial support by the European Union (European Social Fund–ESF) and Greek national funds through the Operational Program ‘Education and Lifelong Learning’ of the National Strategic Reference Framework (NSRF)-Research Funding Program: THALIS. Investing in knowledge society through the European Social Fund.

Table 1 Nonlinear optical parameters of the azobenzene polymers determined using the Z-scan technique. Sample

c0  1014 (m2/W)

35 ps, 532 nm S1 (64.76 ± 2.73) S2 (1.65 ± 0.26) S3 (13.87 ± 0.40) Sample

c0  1011 (m2/W)

4 ns, 532 nm S1 (6.16 ± 1.90) S2 (0.60 ± 0.17) S3 (7.18 ± 1.80)

b  106 (m/W)

Rev(3)  109 (esu)

Imv(3)  109 (esu)

v(3)  109 (esu)

a0  104 (cm1)

v(3)/a0  1013 (esu/cm1)

(6.01 ± 0.28) – (0.70 ± 0.35)

(92.40 ± 3.90) (2.35 ± 0.37) (19.78 ± 0.57)

(35.17 ± 1.65) – (4.11 ± 2.05)

98.86 ± 4.23 2.35 ± 0.37 20.2 ± 3.56

8.35 0.84 3.18

11.89 ± 0.51 2.80 ± 0.44 6.35 ± 1.12

b  104 (m/W)

Rev(3)  106 (esu)

Imv(3)  106 (esu)

v(3)  106 (esu)

a0  104 (cm1)

v(3)/a0  1010 (esu/cm1)

13.3 ± 4.82 0.35 ± 0.1 12.07 ± 3.73

8.35 0.84 3.18

(16.1 ± 7.0) – (11.0 ± 5.0)

(8.78 ± 2.7) 0.35 ± 0.1 10.24 ± 2.57

(10.0 ± 4.0) – (6.4 ± 2.7)

1.59 ± 0.58 0.42 ± 0.12 3.79 ± 1.17

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