Second-Order Nonlinear Optical Susceptibilities of Nonelectrically

Article pubs.acs.org/JPCB

Second-Order Nonlinear Optical Susceptibilities of Nonelectrically Poled DR1−PMMA Guest−Host Polymers Atsushi Sugita,*,† Yasuaki Sato,† Kazuma Ito,† Kenta Murakami,† Yasuaki Tamaki,† Nobuyuki Mase,† Yoshimasa Kawata,‡,§ and Shigeru Tasaka† †

Department of Materials Science, and ‡Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan § CREST, Japan Science and Technology Agency, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8561, Japan ABSTRACT: Guest−host nonlinear optical polymers have attracted considerable interest due to their applications in fast electro-optical modulators and wavelength converters. In general, the electrical poling procedures, for which high DC external fields are applied, are necessary for aligning guest chromophores in polar order and activating the second-order nonlinearity. We present the nonelectrical poling behaviors for guest−host polymers: DR1 (4-[ethyl (2hydroxyethyl) amino]-4′-nitroazobenzene) is the guest, and PMMA (poly (methyl methacrylate)) is the host. Second-order nonlinear optical susceptibility was induced in the conventional guest−host polymers after annealing at temperatures above the glass transition points of the host polymer even without applying the external fields. This phenomenon did not occur in the side-chain polymers, where the guests were directly bonded to the host chains. The guest polar alignments were most likely generated from the guest hydroxyl groups chemisorbing on the substrates. The polar alignments of the guest formed not only near the surface of the substrate, but also inside the host polymers. The optimized conditions for the SHG conversion were examined in the context of the polymer film thickness and guest concentration. The nonelectrical poling techniques described in this study are useful for enhancing the surface nonlinearity in the several materials, and they will be useful for further developments in nanophotonics and plasmonics.

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INTRODUCTION Optical polymers, or plastic optical materials, have been studied for many decades. These materials are indispensable for constructing modern optics and optronics.1−7 The most important applications of these materials include lenses, optical fibers for telecommunication, polarizing films, and light scattering bodies. These materials have inferior thermal and mechanical strengths as compared to inorganic optical materials, such as glass and metal oxides. However, photonic polymers have several unique properties, such as light weightiness, low production costs, and ease of production. Research and development for nonlinear optical (NLO) polymers is critical to the optical polymer technology field.8,9 Many researchers have attempted to develop NLO polymers, particularly aimed at electro-optic modulators and wavelength converters.10−13 In general, NLO polymers comprise the guest−host hybrid structures. In these structures, the guest chromophores are involved in nonlinear light−matter interactions, while the host polymers hold the guest chromophores in a fixed position. Most of the guest chromophores comprise π-conjugated building blocks that bridge the electron-donors and electron-acceptors.14−22 In contrast, amorphous polymers are used as the host materials; poly (methyl methacrylate) (PMMA) and poly (carbonate) are among the most important such materials.23−32 The symmetry of the materials is closely associated with their NLO susceptibility.33 Odd-order susceptibility does not require © 2013 American Chemical Society

any specific symmetry in the materials, while even-order susceptibility requires noninversion symmetry. Frequently, the NLO polymers do not exhibit even-order nonlinearity as prepared, because the guest chromophores are randomly distributed in the host polymers, and the materials have a symmetric structure. Therefore, so-called poling procedures are performed to break the symmetry.23,24,34−40 During a poling procedure, DC electric fields approaching ∼100 kV/mm are applied to the materials. Dielectric breakdown occurs frequently during poling procedures, preventing the induction of nonlinearity in wide areas and large volumes. Several studies have attempted to align the guests in the polar order in the host polymers without applying external electric fields. Nakayama et al. reported second harmonic generations (SHG) from poly (acrylic acid) polymer films doped with novel nonlinear organic ionic materials, pyrylium salt dyes. The guest−host polymer spin-coated thin films exhibited secondorder nonlinearity at d33 ≈ 1.2 pm/V.41 Huang et al. attempted to induce polar order for the phenyltetraene chromophore AJLZ53 in PMMA using external fields from pyroelectric crystals; these researchers recorded second-order nonlinearity via d33 ≈ 80 pm/V.42 Received: August 7, 2013 Revised: October 29, 2013 Published: November 5, 2013 14857

dx.doi.org/10.1021/jp407892b | J. Phys. Chem. B 2013, 117, 14857−14864

The Journal of Physical Chemistry B

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second step, the precursor polymers were postfunctionalized with DR1. The reaction mixture was poured into methanol, and the precipitated polymer was filtered. The DR1 concentration in side-chain polymer was 5 wt %, and the glass transition temperature of the material was Tg ≈ 125 °C. The side-chain polymer thin films were also prepared on the fused silica glass substrates for nonlinear optical spectroscopic experiments. The thin film samples were annealed to break the inversion symmetry of the orientation order of the guests in the NLO polymers. The annealing procedure included a heating step and a subsequent cooling step. First, the samples were heated to 150 °C; the temperature was 40 °C higher than the Tg of PMMA. The temperature was changed at 3 °C/min. Next, the samples were cooled to room temperatures at the same rate. The electrically poled polymers were prepared as a reference. In this step, the corona poling procedure was used to orient the guests. The thin film samples used for corona poling were deposited on glass substrates coated with thin indium tin oxide (ITO) layers. The corona discharge was produced using +4.7 kV connected to a sharp stainless needle 2 cm away from the film surface. The corona poling procedure occurred at 130 °C for 10 min. The second-order nonlinear optical susceptibilities of the samples were characterized using a second harmonic (SH) method. The excitation light sources were femtosecond optical pulses (wavelength, 800 nm; pulse width, 150 fs; and pulse energy, 1 mJ at a 1 kHz repetition rate). The pulse energies were attenuated using a variable neutral density filter. The pump beam diameters and pulse energies for the samples were ∼ϕ1 mm and 20 μJ, respectively. The samples were mounted on a rotational stage to tune the pump beam incidence angles. The excitation light polarization angle was controlled using a half wave plate, while the second harmonic generation (SHG) polarization was resolved using a Glan-Laser prism. The absolute value of the nonlinear optical susceptibilities was determined by comparing with SHG signals emitted from a 1 mm-z-cut quartz plate (d11 = χxxx/2 = 0.3 pm/V) while using the same optical geometry.51

In our previous studies, we reported second-order nonlinear optical susceptibility for the guest−host type polymers utilizing poly (cyano phenylene sulfide) (PCPS) as the host materials.43 The PCPS comprised the one-dimensional alternative of cyanophenylene and sulfur atoms. The polymers exhibited displacement-field hysteresis behaviors even in amorphous phases.44 The pyroelectricity appeared after annealing at temperatures above the glass transition point (Tg). The cyano dipoles were spontaneously arranged over a long-range, even without the applied external fields. We attempted to apply PCPS as a host material for the NLO polymers. Using polarization self-organization behaviors in PCPS, second-order nonlinearity was successfully generated from the PCPS dispersed with guest chromophores by annealing the materials without applying a DC electric field. In the present study, we demonstrate that a nonelectrical poling procedure can be useful for obtaining second-order nonlinearities in conventional guest−host NLO polymers containing the guest DR1 and the host PMMA (Figure 1).

Figure 1. Structures of (a) DR1, (b) PMMA, and (c) the side-chain polymers.

The DR1−PMMA guest−host polymer is one of the most popular NLO polymers, and many researchers have attempted to study the second- and third-order nonlinearities of its poled materials.24,34,45−50 The optimal conditions for activating the second-order nonlinearity were examined in the context of polymer film thicknesses and guest concentrations.

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RESULTS AND DISCUSSION Figure 2 shows a linear absorption spectrum for the DR1− PMMA guest−host polymer thin film. The absorption

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EXPERIMENTAL SECTION Both the DR1 and the PMMA were purchased from Aldrich Co. The average molecular weight for the PMMA was Mw = 1.2 × 105, and its glass transition temperature was Tg ≈ 105 °C. The samples for the nonlinear optical spectroscopic measurements were thin polymer film samples deposited on fused silica SiO2 glass substrates. The polymer films were fabricated using a spin-coating method. The film thickness was controlled by adjusting the solute concentration and the rotational speed of the spin coater. The samples with films between 100 nm and 2 μm thick were prepared. Samples with guest concentrations between 5 and 30 wt % were also prepared. The side-chain polymers in which the DR1 molecules are directly grafted onto the PMMA main chains were also prepared. The side-chain polymers were synthesized using twostep reactions.17 First, copolymers containing the 2-isocyanate ethyl methacrylate (Tokyo Chemical Industry Co. Ltd.) and methyl methacrylate (Tokyo Chemical Industry Co. Ltd.) were synthesized using radical polymerization. The reactions were initiated with 2, 2′-azodiisobutyronitrile (AIBN). For the

Figure 2. Linear absorption spectrum for the guest−host (−) and side-chain polymers (- - -).

spectrum for the side-chain polymer thin film is also presented. The absorption shapes for the two materials are similar, although the absorption peak positions differ slightly. The absorption peak position was λmax = 488 nm for the guest−host polymers and λmax =474 nm for the side-chain polymers. The 14858



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In contrast, the side-chain polymers did not emit SHG signals, even after heating to temperatures above Tg ≈ 125 °C. Therefore, the nonelectrical poling behaviors for the secondorder nonlinearities were observed in the guest−host polymers, but not the side-chain polymers. The electron donor in DR1 was secondary amine functionalized with the hydroxyl groups. In the side-chain polymers, the hydroxyl groups were replaced with ester bonds, when the guests were introduced to the PMMA main chains. Therefore, the hydroxyl groups on DR1 were crucial for activating second-order nonlinearity during the nonelectrical poling. The thin film samples were fabricated on fused silica SiO2 glass substrates. The surfaces of the fused silica glass were partially covered by silanol groups (Si−OH), and the silanol groups can form hydrogen bonds with several types of alcohols.57,58 In the guest−host polymer systems, the guests chemisorbed on the substrate surfaces via hydrogen-bond interactions between the guest hydroxyl groups and the silanol groups on the fused silica surface. In contrast, it was impossible for the guests to chemisorb on the substrates in the side-chain polymers because the hydroxyl groups were removed from the guests upon substitution for the host polymer chains. The nonelectrical poling behaviors were more likely triggered by the chemisorption and subsequent surface alignments of the guests on the SiO2 substrates. Otherwise, the structural constrains of the guest chromophores in the side-chain polymers may avoid guests with polar alignments. In a previous study, infraredvisible sum frequency generation vibration spectroscopy revealed that the ester methyl side chains were oriented on interfaces with the several materials, such as air, water, or sapphire.59−62 The surface orientations of the host polymers may also promote polar alignments for the guests. Figure 4a shows the SHG signal intensities ISHG emitted from the annealed guest−host polymers as a function of the pump

absorption spectrum was consistent with the one reported previously;52,53 in these studies, the absorption was attributed to the absorption band from the DR1 π→π* transitions. The pump beam and SHG wavelengths were 800 and 400 nm, respectively. The materials were transparent at the pump beam wavelength, and the SHG wavelength was at the longer wavelength absorption band tails. Figure 3 shows the SHG signal intensities from the guest− host and side-chain polymers during the annealing procedures.

Figure 3. SHG signal intensities for the guest−host and side-chain polymers during the annealing process.

The pump beam incidence angle toward the thin film samples was θ1 = 45°. The p-polarized pump beams excited the samples, and the p-polarized SHG signals were recorded. Before the annealing, the guest−host polymers emitted weak SHG signals. During the heating step, the SHG signals gradually increased at temperatures above ∼80 °C. In contrast, the material continued to emit the SHG signals even after cooling to room temperature. Second-order nonlinear susceptibility can be generated even without applied external fields by annealing the guest−host polymers. The increased SHG signals during the heating step are related to the segmental redistribution motions of the host polymer chains in the rubbery phase above the glass transition points Tg.54 The polar arrangements of the guests may be accompanied by the segmental redistribution motions of the host polymers. When the guests were arranged in a long-range polar order, the guest’s polar conformations were stable even after cooling below Tg. The onset temperature for the increase in SHG signal was slightly lower than the PMMA glass transition point, most likely because the glass transition point was lowered by dispersing the guest chromophores, as previously reported.55,56 The polar alignments of the guests might occur with an aid of the optical poling effect or mechanical stress during the spin coating procedure. To verify these assumptions, two experiments were conducted. First, the SHG signal intensities were recorded by changing the pump beam intensities at temperatures above Tg. The signal intensities did not change significantly when the pump beams were irradiated for a few minutes, making them independent of the intensities of the pump beams. Second, the sample films were prepared at different rotation speeds. Films were also prepared using a drop cast method. Depending on the rotational speeds used during the spin-coating procedure and the film fabrication, different magnitudes of the mechanical stresses could be applied to the guest−host polymers. The SHG signal emissions were commonly observed after annealing, making them independent of the film fabrication methods. These observations indicated that the polar alignment of the guests could be induced without optical poling or mechanical effects.

Figure 4. Incidence angle dependence of the SHG signals from (a) annealed and (b) corona-poled guest−host polymers. “●” and “○” denote the p-polarized SHG components excited with p- and spolarized pump beams, respectively.

beam incidence angles θ1. The materials were 800 nm-thickness with 10 wt %-guests and were annealed at 150 °C. The samples were excited using the p- and s-polarized pump beams. In both cases, p-polarized SHG signals were recorded. Irrespective of the pump beam polarizations, the SHG signal intensity was 14859



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almost zero at θ1 = 0°, increasing with wider incidence angles. The same measurements were generated for the corona-poled guest−host polymers with the same film thickness (Figure 4b). The observed θ1−ISHG curves were similar to those of the annealed materials. The curves were also similar to those of the poled polymers reported previously;63 these studies indicated that the external electric field aligned the guest chromophore static dipole moments with the field direction. Therefore, the guests were oriented along the normal in the annealed materials. Notably, the observed θ1−ISHG trace did not exhibit clear Maker-fringe patterns because the sample film thickness (l = 800 nm) was much narrower than the phase-matching coherence length (lc): lc = (λ/(8·|n2(2ω) − n2(ω)|)), where n2(ω) and n2(2ω) were the refractive indices for the NLO polymers at the fundamental and SHG wavelengths, respectively. Using the refractive indices (n2(ω) = 1.475 and n2(2ω) = 1.487) determined with the ellipsometer, the present guest−host polymer coherence length was lc = 8 μm. The film thickness was much thinner (l = 800 nm) than lc, satisfying the condition l ≪ lc. Figure 5a shows the SHG signal intensities emitted from the annealed guest−host polymers as a function of the pump beam

order of the guest chromophore has C∞v symmetry with only three independent macroscopic second-order nonlinear optical (2) (2) susceptibility tensor components χ(2) zzz, χzii , and χizi . The coordinate z denotes the external field direction or the substrate normal line, while i = x, y represents the in-plane coordinates of the substrates. Macroscopic nonlinear susceptibility arises from the molecular hyperpolarizability of the aligned guests. In the rod-shaped guest chromophores, similar to DR1, the macroscopic nonlinear susceptibility is predominantly determined using the molecular hyperpolarizability βξξξ tensor component; ξ is the molecular long axis.64−68 The macroscopic nonlinear (2) (2) (2) susceptibility tensor components, χzzz , χzii , and χizi , are expressed using the molecular hyperpolarizability (βξξξ) as shown in eq 1. χzzz = N ·Lz(2ω) ·Lz2(ω) ·⟨cos3 Θ⟩βξξξ 1 N ·Lz(2ω) ·Li2(ω) ·⟨cos Θ sin 2 Θ⟩βξξξ 2 1 χizi = N ·Li(2ω) ·Lz(ω) ·Li(ω) ·⟨cos Θ sin 2 Θ⟩βξξξ 2 χzii =

(1)

N denotes the number density for the active guest chromophores that interacts with the pump light waves, and Li(Ω) is the Lorentz factor at frequency Ω (=ω or 2ω) along the i direction (i = x, y, or z) for the NLO polymer. Assuming that the anisotropy of the polymer is negligible, the Lorentz factor can be approximated using Li(Ω) ≈ (n2(Ω) + 2)/3, where n2(Ω) is the refractive index of the NLO polymer. Using the nonlinear susceptibility tensor components in eq 1, the p- and s-polarized SHG components ISHG,p(γp) and ISHG,s(γp) are expressed as shown in eq 2. ISH,p(γp) = K · |cos2 γp·(A · χzzz + B · χizi ) + χzii ·(C·cos2 γp + D·sin 2 γp)|2 ISH,s(γp) = K ·|E · χiyi · cos γp· sin γp|2 (2)

Coefficients A, B, C, D, E, and K are determined using the Fresnel refractions at the air/polymer layer and polymer layer/ fused silica SiO2 substrate interfaces. These coefficients are expressed in detail in eq 3.

Figure 5. Pump beam polarization angle dependence of the SHG signals from the (a) annealed and (b) corona-poled guest−host polymers. “●” and “○” represent the data for the p- and s-polarized components of the SHG waves, respectively.

A = F23, z(2ω) ·F12, z(ω)2 ·sin θ3(2ω) ·sin 2 θ1(ω)

polarization angle (γp); the polarization angle (γp) is defined with respect to the incidence plane. The 800-nm-thick samples containing 10 wt % guest were annealed at 150 °C. The incidence angle of the pump beam was θ1 = 45°. Both the pand the s-polarized SHG signal components were recorded. The p-polarized SHG components have two maxima at γp = 0° and 180°, as well as a minimum at γp = 90°. In contrast, the spolarized SHG components have maxima at γp = 45° and 135°, but have almost zero minima at γp = 0°, 90°, and 180°. The same measurements were conducted for the corona-poled materials (Figure 5b). Both the γp−ISHG,p and the γp−ISHG,s traces for the annealed materials had shapes similar to those of the corona-poled materials. In the electrically poled guest−host polymers, the guest chromophores lie in a noncentro-symmetric arrangements along the external field, and they are randomly oriented in the plane normal to the fields.36−38 In the model, the orientation

B = 2·F23, x(2ω)·F12, z(ω) ·F12, x(ω) ·cos θ3(2ω) ·sin θ1(ω) · cos θ1(ω) C = F23, z(2ω) ·F12, x(ω)2 ·sin θ3(2ω) ·cos2 θ1(ω) D = F23, z(2ω) ·F12, y(ω)2 ·sin θ3(2ω) E = 2·F23, y(2ω)·F12, z(ω)·F12, y(ω) ·sin θ1(ω) K=

8·ω3·l 2·sec 2 θ1(ω) ·I02 n12(ω) ·n3(2ω) ·ε0 ·c 3 (3)

Fab,i(Ω) represents the transmission Fresnel factors at frequency Ω (=ω or 2ω) along the i-direction at the layer a interface with the refractive index na to the layer b interface with nb. 14860


The Journal of Physical Chemistry B Fab , x(Ω) =

2·na(Ω) ·cos θb(Ω) na(Ω) ·cos θb(Ω) + nb(Ω) ·cos θa(Ω)

Fab , y(Ω) =

2·na(Ω) ·cos θa(Ω) na(Ω) ·cos θa(Ω) + nb(Ω) ·cos θb(Ω)

Fab , z(Ω) =

2·nb(Ω) ·cos θa(Ω) na(Ω) ·cos θb(Ω) + nb(Ω) ·cos θa(Ω) ⎛ n (Ω) ⎞2 ·⎜ a ⎟ ⎝ nb(Ω) ⎠

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film thickness increased, the SHG signal intensities increased in the range l < 1500 nm, above which they decreased monotonously. In contrast, the molecular order parameter was nearly independent of the film thickness (l). The observed l−ISHG,p (γp = 0°) trace indicates that the nonlinearities of the annealed materials were generated due to the surface-aligned guests and the guests oriented inside the host polymers. As mentioned above, the chemisorption on the substrates was crucial for the nonelectrical poling behavior. The nonzero SHG signals components observed in infinitesimally thin films were likely due to the surface-aligned guests. In contrast, the SHG signals increased, as the film thickness increased, and were likely associated with the guests oriented inside the host polymers. Long-range ordering for the guest was generated from within the host polymers, which were stacked on the surface-aligned guest chromophores. Consequently, the nonlinear susceptibility of the annealed guest−host polymers was decomposed into two components: one component was associated with the guests aligned on the substrate surface (χ(2) ijk,surface), and the other component was associated with the guests oriented inside the host polymers (χ(2) ijk,bulk l). The SHG signal intensities ISHG are related to the function describing the film thickness (l):

(4)

In this equation, n1 = 1.0, n2(Ω), and n3(Ω) are the refractive indices for the air, polymer layer, and fused silica substrate. The refractive index of the polymer was determined using an ellipsometer as discussed above. The fused silica refractive indices were n3(ω) = 1.45332 and n3(2ω) = 1.47012 when using the references.69 θ1(Ω) = 45° is the pump light incidence angles; θ2(Ω) and θ3(Ω) are the refraction angles in the polymer layer and fused silica glass substrate, respectively. The γp−ISHG,p and γp−ISHG,s traces in Figure 5 are consistent with eq 2 when using the fitted parameters χzii/χzzz = 0.26 and χizi/χzzz = 0.29 for the annealed materials as well as χzii/χzzz = 0.37 and χizi/χzzz = 0.40 for the corona-poled materials. The degree of molecular orientation for the guests is frequently evaluated using ⟨cos2 Θ⟩, as defined in eq 5. ⟨cos2 Θ⟩ =

ISHG ∝ |χijk(2) |2 = |χijk(2),surface + χijk(2),bulk ·l|2

χzzz χzzz + 2·χzii

(6)

The l−ISHG,p(γp = 0) trace in Figure 6 is consistent with eq 6. The nonlinear susceptibility tensor component from the molecular orientations inside the host polymer is separated using eq 6 and is χ(2) zzz,bulk = 0.26 pm/V. The nonlinear susceptibility for the corona-poled materials in Figure 5b was χ(2) zzz,corona = 18 pm/V. According to eq 1, the nonlinear susceptibility tensor components are related to the molecular hyperpolarizability (βξξξ), the average molecular tilt angle Θ, and the oriented guest chromophore density (N′). The molecular hyperpolarizability of the annealed materials should be similar to that in the corona-poled materials. Using the average molecular tilt angles (Θ) determined using the fitting procedures, the ratio of the aligned guests in the annealed materials versus the corona-poled guests is Nanneal′/ Ncorona′ ≈ 0.7%. Finally, the nonlinearity and the orientation order of the guests were examined in the annealed guest−host polymers with different guest concentrations (N). The samples were l = 800 ± 50 nm thick and annealed at 150 °C. The SHG signal intensities ISHG,p(γp = 0) and the molecular orientation order (⟨cos2 Θ⟩) were plotted against N in Figure 7. The molecular order parameter was not significantly dependent on the guest concentration. In contrast, the SHG intensity (ISHG,p (γp = 0)) increased monotonously when N < 15 wt %, and the curve was consistent with a square function. The SHG signal intensity was saturated at the higher concentrations. The decreased SHG signal intensities are caused by aggregates formed from adjacent guests. The DR1 aggregates were revealed in a previous report on the anisotropy in a linear absorption spectrum.70,71 In the aggregates, a pair of adjacent DR1 molecules adopts an antiparallel conformation to stabilize the electrostatic potential energies. At the higher concentrations, the guests are more likely to adopt the aggregated conformations, which prevents them from aligning inline and generating long-range polar order.

(5)

Using the fitted parameters (χzii/χzzz), the order parameter was ⟨cos2 Θ⟩anneal = 0.65 for the annealed materials and ⟨cos2 Θ⟩corona = 0.58 for the corona-poled materials. The same measurements were conducted for the annealed materials with different film thicknesses. Figure 6 shows the ppolarized SHG signals excited with p-polarized pump beams (ISHG,p(γp = 0°)) and molecular order parameter (⟨cos2 Θ⟩) against the film thickness (l). Here, the guest concentration of the materials was 10 wt %. The signal intensity approached a nonzero value as the film thickness approached zero. When the

Figure 6. (a) The film thickness dependence of the p-polarized SHG signal intensities emitted from the annealed materials. The pump beams were p-polarized. The dashed curve is the fit generated by the square function. (b) The molecular order parameter ⟨cos2 Θ⟩ for the same materials is also shown. 14861



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(3) Węgłowski, R.; Kłosowicz, S. J.; Majchrowski, A.; Tkaczyk, S.; Reshak, A. H.; Pisarek, J.; Kityk, I. V. Enhancement of the Kerr response in polymer-dispersed liquid crystal complexes due to incorporation of BiB3O6 nanocrystallites. Mater. Lett. 2010, 64, 1176−1178. (4) Majchrowski, A.; Ebothe, J.; Sanetra, J.; Ozga, K.; Kityk, I. V.; Reshak, A. H.; Lukasiewicz, T. Electrooptics parameters of the BiBO:Tm3+ glass nanoparticles embedded in polymer matrices. J. Mater. Sci.: Mater. Electron 2010, 21, 726−729. (5) Reshak, A. H.; Ebothe, J.; Wojciechowski, A.; Kuznik, W.; Popeda, A. Optically stimulated non-linear optics effects in semiconducting nano-crystallites. Physica E 2010, 42, 1769−1771. (6) Kityk, I. V.; AlZayed, N. S.; El-Naggar, A. M.; Reben, M.; Wasylak, J.; Lakshminarayana, G.; Reshak, A. H.; Brik, M. G. Er−Pr doped tellurite glass nanocomposites for white light emitting diodes. Opt. Commun. 2012, 285, 655−658. (7) Alzayed, N.; Reshak, A. H.; Plucinski, K. J.; Berdowski, J.; FuksJanczarek, I.; Miedzinski, R.; Tylczynski, Z. Optically-operated elastooptical effects in polymer matrices with nanocrystallites. Funct. Mater. Lett. 2011, 4, 357−359. Kityk, I. V.; Majchrowski, A.; Lakshminarayana, G.; Reshak, A. H.; Michalski, E.; Olifierczuk, M.; Ozga, K.; Jaroszewicz, L.; Lukasiewicz, T.; Szota, M.; Nabialek, M. Laser operated elasto-optical features of La2CaB10O19:Pr3+ polymer nanocomposites. J. Lumin. 2012, 132, 2577−2580. (8) Nalwa, H. S.; Watanabe, T.; Miyata, S. In Nonlinear Optics of Organic Molecules and Polymers; Nalwa, H. S., Miyata, S., Eds.; CRC Press: New York, 1996; pp 89−350. (9) Dalton, L. R.; Sullivan, P. A.; Bale, D. H. Electric field poled organic electro-optic materials: State of the art and future prospects. Chem. Rev. 2010, 110, 25−55. (10) Wang, F.; Cao, Z.; Shen, Q. Reflective-type configuration for nearly phase-matching Cerenkov second-harmonic generation in a nonlinear-optical polymer waveguide. Opt. Lett. 2005, 30, 522−524. (11) Poberezhskiy, I. Y.; Bortnik, B. J.; Kim, S.-K.; Fetterman, H. R. Electro-optic polymer frequency shifter activated by input optical pulses. Opt. Lett. 2003, 28, 1570−1572. (12) Reshak, A. H.; Majchrowski, A.; Imiolek, W. Pockels effect in yttrium aluminum borate single crystals. Laser Phys. 2008, 18, 1204− 1206. (13) Majchrowski, A.; Klosowicz, S.; Weglowski, R.; Cieslik, I.; Piasecki, M.; Kityk, I. V.; Reshak, A. H. Electro-optical properties of BixLayScz(BO3)4 (x + y + z = 4) nanocrystallites having huntite structure incorporated into polymer matrices. J. Alloys Compd. 2010, 488, 291−293. (14) Duff, A.-C. L.; Canva, M.; Lévy, Y.; Brun, A.; Ricci, V.; Pliska, T.; Meier, J.; Stegeman, G. I.; Chaput, F.; Boilot, J.-P. Disperse Red 1doped solgel planar waveguides for nonlinear optical devices operating at telecommunications wavelengths. J. Opt. Soc. Am. B 2001, 18, 1827−1831. (15) Sun, S.-S.; Zhang, C.; Dalton, L. R. 1,3-Bis(dicyanomethylidene) indane-based second-order NLO materials. Chem. Mater. 1996, 8, 2539−2541. (16) He, M.; Leslie, T. M.; Sinicropi, J. A. α-Hydroxy ketone precursors leading to a novel class of electro-optic acceptors. Chem. Mater. 2002, 14, 2393−2400. (17) Piao, X.; Zhang, X.; Mori, Y.; Koishi, M.; Nakaya, A.; Inoue, S.; Aoki, I.; Otomo, A.; Yokoyama, S. Nonlinear optical side-chain polymers post-functionalized with high-β chromophores exhibiting large electro-optic property. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 47−54. (18) Reshak, A. H.; Auluck, S.; Stys, D.; Kityk, I. V.; Kamarudin, H.; Berdowski, J.; Tylczynski, Z. Dispersion of linear and non-linear optical susceptibilities for amino acid 2-aminopropanoic CH3CH(NH2)COOH single crystals: experimental and theoretical investigations. J. Mater. Chem. 2011, 21, 17219−17228. (19) Reshak, A. H.; Stys, D.; Auluck, S.; Kityk, I. V.; Kamarudin, H. Structural properties and bonding nature of 3-methyl-4-phenyl-5-(2pyridyl)-1,2,4-triazole single crystal. Mater. Chem. Phys. 2011, 130, 458−465.

Figure 7. (a) The guest-concentration dependence of the p-polarized SHG signal intensities emitted from the annealed materials. The pump beams were p-polarized. The dashed curve is the fit generated by the square function. (b) The molecular order parameter ⟨cos2 Θ⟩ for the same materials is also shown.

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CONCLUSIONS We reported the nonelectrical poling behaviors in the DR1− PMMA guest−host polymers. Second-order nonlinearity was activated by annealing the materials at temperatures above Tg without applying an external field. The nonelectrical poling procedure was unavailable for the side-chain polymers, because the DR1 was directly bonded to the PMMA chains. The guest hydroxyl groups were indispensable for the nonelectrical poling behaviors. The molecular polar order in the guests was expressed using the molecular orientation models proposed for the poled polymers in a previous study. The guests aligned noncentro-symmetrically near the substrate surface and inside the polymer layers. The oriented guest density in the annealed guest−host polymers was approximately 0.7% as compared to that of the corona-poled samples. The optimal nonelectrical poling conditions were also examined in the context of the film thickness and the guest concentrations. The optimal film thickness and the guest concentrations were ∼1500 nm and 15 wt %, respectively. Although the nonlinearity generated using the nonelectrical poling technique was one magnitude smaller than that obtained via the conventional electrical poling techniques, the simple procedure for generating the secondorder nonlinearity facilitates further research in plastic optics and plastic optronics.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +81-53-478-1622. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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