Azo Chromophore Monomerically Bonded Mesostructured Silica Films

Aug 13, 2008 - Azo Chromophore Monomerically Bonded Mesostructured Silica Films with Large Third-Order Nonlinearity but Negligible Nonlinear Absorptio...
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J. Phys. Chem. C 2008, 112, 13754–13762

Azo Chromophore Monomerically Bonded Mesostructured Silica Films with Large Third-Order Nonlinearity but Negligible Nonlinear Absorption Jiangtian Li, Jianlin Shi,* Chenyang Wei, Peng Jiang, Weimin Huang, and Dakui Zhuang State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, 1295 Dingxi Road, Shanghai, 200050, China ReceiVed: April 30, 2008; ReVised Manuscript ReceiVed: June 24, 2008

The azobenzene chromophore covalently and monomerically dispersed mesostructured organic-inorganic hybrid films were prepared by a direct-synthesis route thanks to the compatible environment provided by the hydrophobic core of the surfactant micelles. The third-order optical nonlinearity of this mesostructured organic-inorganic hybrid was investigated by using the Z-scan technique in/near the two-photon absorption region (at 1064 nm). It is very interesting that no two-photon absorption could be detected in/near the twophoton absorption region, which is in contrast to those in solution and common nonmesostructured films. Meanwhile, a large nonlinearity with a nonlinear refractive index n2 of 10-9 esu, purely attributed to electronic effect, has been obtained. The special structure, that the hydrophilic alkoxysilane end is covalently anchored on the silica pore surface while the opposite hydrophobic chromophore end is monomerically dispersed in mesopore space, is believed to be responsible for the uncommon nonlinear optical properties. As a result, the special nonlinear optical properties of such mesostructured films can well satisfy the Stegmen figures of merit for the demands of all-optical devices applications. Introduction Nonlinear optics (NLO) has been expected to play an essential role in emerging technologies of photoelectronics and photonics.1,2 In future information technology, all-optical devices, where an incoming beam redirects the other beam through light by light propagating in NLO materials, will have the advantage of ultrafast speed over the electronic or optoelectronic devices in ultrafast communication and signal processing systems.3–6 Unfortunately, most of the components that are currently very much in demand are electro-optical instead of all-optical. What in fact restrains the widespread use of all-optical devices is the lack of breakthroughs needed for commercial applications, i.e., inefficiency of currently available nonlinear optical materials, e.g. low off-resonant third-order nonlinear susceptibility and high optical loss.7 Much research is still focused on searching better and more efficient optical materials. For practical applications, one of the key points to yield good NLO materials is to combine a large off-resonant third-order susceptibility with low optical losses (a low absorption coefficient),6–9 as summarized in the Stegmen figures of merit.10–12 Recently, sol-gel derived organic-inorganic hybrid films doped with active NLO chromophores have attracted considerable interest because of their large nonlinearity as well as low cost and ease of fabrication over inorganic crystals and semiconductors, and especially the possibility to adjust their linear and nonlinear optical parameters with the choice of dopants and their concentrations.13–15 Among various NLO chromophores, azobenzene has good photothermal stability, dissolvability, and easy preparation virtues,16,17 and has been a well-known nonlinear optically active chromophore due to its unique optical properties,18–20 such as photoisomerization and birefringence. The NLO of azobenzene chromophore physically and/or chemically loaded in organic and inorganic polymeric * Corresponding author. E-mail: [email protected]. Phone: 86-2152412712. Fax: 86-21-52413122.

matrices has been widely studied.1,21–28 Nevertheless, as mentioned above, few samples satisfy the figures of merit for alloptical applications even with large nonlinearity. Mesoporous hybrids are an interesting example of multifunctional organic-inorganic systems in which an inorganic or a hybrid organic-inorganic matrix presents tunable pore sizes, extremely high surface areas, and/or functional properties under a wide range of conditions.29 Various organic functional groups or photosensitive chromophores have been covalently incorporated either onto the pore surfaces or in the pore spaces or in the framework of mesoporous materials, such as termini aminoalkyl and thioloalkyl groups, bridged vinyl and phenyl groups and many dye molecules, and so on. These mesoporous hybrids have been well reviewed in the references, and some novel properties due to the effect of mesopores were detected.30–35 Quantum dots or nanoparticles such as PbS and Au incorporated micro- and mesoporous composites have shown very high thirdorder nonlinear optical activities.36–38 In fact, the optical responses of hybrids sensitively depend on the interaction between chromophore and matrix, and a slight change in surrounding environment would lead to a significant change in properties. For example, the aggregation of the chromophore restrained by the supporter may greatly alter the optical performances. A practical device requires the incorporation of a large amount of NLO chromophores without aggregation effects. The main limiting factors of currently available NLO materials for all-optical devices are high linear absorption on resonance and large two-photon absorption (TPA) off resonance. Unfortunately, with some important NLO materials, such as azobenzene chromophores, TPA occurs mainly in the wavelength region of 900-1200 nm, in which the laser signals, such as Nd:YAG lasers of 1064 nm, are widely used. Herein, based on our preliminary work of the synthesis of mesostructured hydrophobic chromophore/silica composites,39 a facile strategy has been developed to covalently and monomerically incorporate azobenzene chromophores into the pore spaces by a direct-

10.1021/jp803786x CCC: $40.75  2008 American Chemical Society Published on Web 08/13/2008

Azo Chromophore Mesostructured Silica Films SCHEME 1: Schematic Representation of the Preparation of Mesostructured Film and the Monomeric Dispersion of Azobenzene Chromophore in Mesopore Spaces

synthesis route, and a novel third-order nonlinearity with the absence of two-photon absorption potential for all-optical applications, was detected by the Z-scan technique. In this investigation, the widely studied azo dye disperse red 1 (DR1) was chosen as the target NLO material. DR1 reacted with 3-isocyanatopropyl triethoxylsilane to form the compound DR1ASD,19,20 as shown in Scheme 1. Then the amount of DR1ASD hydrolyzed and co-condensed with tetraethylorthosilicate (TEOS) in acidic condition to form mesoporous films under the direction of the template P123 was calculated. The mesostructured hybrid films were finally prepared by dip- or spin-coating technique after the starting sols were aged for 1 day. As DR1ASD contains a hydrophobic azobenzene group and hydrophilic alkoxysilane groups on opposite sides, during the hydrolysis and condensation alkoxysilane groups react with TEOS and are covalently linked onto the pore surface, while the hydrophobic azobenzene group stretches into the hydrophobic core of the surfactant micelles (Scheme 1A), finally leading to a monomeric dispersion of azobenzene chromophores in the pore space after the removal of template (Scheme 1B). Experimental Section Preparation of Mesostructured Films. Typically, calculated amounts of tetraethoxysilane (TEOS) and DR1ASD with a molar ratio of 1 - x:x were prehydrolyzed in a solution containing 10 mL of tetrahydrofuran (THF) and 3.71 g of diluted hydrochloric acid (pH 2) under vigorous stirring at room temperature. After being stirred for 120 min, this prehydrolyzed silica solution was mixed with a solution containing 1.89 g of P123 dissolved in 30 mL of THF. The resultant solution was further stirred for another 30 min and then aged for 24 h. From this mixture with a final molar composition of TEOS/DR1ASD/P123/H2O/HCl/ THF ) 1 - x/x/0.0094/5/0.004/15, thin films were prepared by dip-coating (75 mm/min) or spin-coating (3000 rpm, 30 s) onto

J. Phys. Chem. C, Vol. 112, No. 35, 2008 13755 cleaned glass slides or silicon substrates, where x ) 0, 0.01, 0.02, 0.03, and 0.05 corresponding to DR1ASD molar ratio of 0%, 1%, 2%, 3%, 5% to silica source, respectively. The films were dried at 80 °C or room temperature for at least 12 h and then extracted by using ethanol with a little concentrated HCl (37%) being added under reflux conditions for 24 h to remove the surfactant. Preparation of Nonmesostrutured Hybrid Films. The preparation of nonmesostructured hybrid films was similar to that of mesostructured films except that no surfactant P123 was added during the synthesis process. Characterization. The small-angle X-ray diffraction (SAXRD) patterns were recorded on a Rigaku Rotaflex diffractometer equipped with a rotating anode and using Cu ΚR radiation (λ ) 1.5418 Å, 40 kV, 60 mA). TEM was performed with a JEOL 200CX electron microscope operating at 200 kV. UV-vis diffusing reflectance spectra were measured on a Shimadzu UV3101. Thermal analyses (TGA and DTA) were recorded on a STA-449C thermogravity analyzer at 10 deg per min in air. Third Harmonic Generation (THG) Measurements. THG measurements were performed in a transmission mode. A Nd: YAG laser (Continuum Inc., λ ) 1064 nm, repetition rate ) 10 Hz) with a pulse width of 40 ps is used as a fundamental light, which was passed through a narrow band filter and focused with a lens (f ) 200 mm), finally was incident to the sample. The THG signals were focused by a lens (f ) 50 mm) and thereafter processed by a spectrometer and detected with an intensified CCD after removing the fundamental light with a narrow band filter at about λ ) 355 nm. Z-Scan Measurement. The third-order nonlinear optical properties were measured by using a Z-Scan technique. The same fundamental irradiation as that in THG measurements was used to eliminate the accumulative thermal effect. The light intensities transmitted across the samples were measured as a function of the sample position in the Z-direction with respect to the focal plane either through a small aperture (closedaperture/CA Z-scan) or without an aperture (open-aperture/OA Z-scan), in order to resolve the nonlinear refraction and absorption coefficient. The laser beam was focused to a beam waist (ω0) of 35 µm and the corresponding Rayleigh length z0 can be found to be 3.6 mm, calculated by formula z0 ) πω02/λ. Before measuring, CS2 was used as a standard reference to calibrate the Z-scan system. Results and Discussion Morphology of Mesostructured Hybrid Films. Figure 1 shows the SAXRD patterns of mesostructured hybrid films with different DR1ASD concentrations. At lower loading levels, the films show strong (100) and weak (200) reflection peaks, characteristic of a two-dimensional hexagonal (p6mm) structure. With more DR1ASD being introduced into the hybrid films, a significant decrease in the intensity of the reflection peaks is observed, indicating that the structure ordering of the mesostructured organic-inorganic films is subject to the disturbance by higher concentration of DR1ASD.40 This structure change was confirmed by the TEM images (Figure 2). From Figure 2A,B, it can be observed that the long-range-ordered mesostructure was preserved at low concentrations, while short-rangeordered mesopores (Figure 2C) and a worm-like mesostructure (Figure 2D) appear at the high DR1ASD loading levels. However, we can see that the pore diameter was kept at around 6-7 nm. Quantitative determination of the organic group in the mesoporous hybrid materials was performed by thermogravi-

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Figure 1. Small-angle XRD patterns of the mesostrutured films with different DR1ASD concentrations of 1% (A), 2% (B), 3% (C), and 5% (D).

metric analysis (TGA) after the chemical extraction of block copolymer surfactant. The TGA analysis was carried out in air from room temperature to 1200 °C. Theoretically, according to the synthesis procedure, a similar weight concentration should be detected for the mesostructured and nonmesostructured hybrid films. Figure 3 shows a weight loss of about 7% below 200 °C, related to the removal of physisorbed water and ethoxy groups.41 Subsequent large weight losses of about 23% and 28% were observed between 200 and 800 °C for nonmesostructured and mesostructured hybrid films, respectively. In the former, the weight loss was purely attributed to the decomposition of the DR1ASD ligands, while in the latter, an additional 5% weight loss may come from the decomposition of incompletely removed templates. However, an obvious shift of exothermic peak to higher temperature was observed in mesostructured films compared to nonmesostructured films, indicating that the mesostructure improves the thermal stability of the azobenzene chromophore. Linear Optical Properties of Hybrid Films. Figure 4 shows the UV-vis spectra of mesostructured films with different DR1ASD concentrations. Symmetric and sharp absorption bands at about 480 nm can be observed in all mesostructured films, assigned to π-π* electronic transition within the azobenzene chromophore, suggesting that the optical properties of the dye molecules are maintained after covalent linking into the silica matrix. An overall increase of absorbance with a slight blue shift can also be observed with an increase in the chromophore concentration.33,42,43 However, from Figure 5A, a strongly asymmetric absorption band with a large blue shift of the main absorption by more than 50 nm can be observed in nonmesostructured hybrid film, which noticeably differs from the usual spectra obtained with DR1 in some environments and can be attributed to the presence of dye aggregates.19 This is more

evident if we deconvolute the spectra using Gaussian fitting, by taking into account the presence of two components due to monomers (480 nm) and aggregates (425 nm), shown as dotted lines in Figure 5A.20 Proper thermal treatment has been considered as an efficient way to reduce the chromophore aggregates. From the original and deconvoluted absorption spectra (Figure 5B) of the nonmesostructured film aged at 80 °C, an intensity decrease at 425 nm and an increase at 480 nm in absorbance can be observed; however, serious chromophore aggregates are still present. The two-peak deconvolutions of mesostructured film (Figure 5C,D) reveal the well-matched main absorption band and a weak shoulder band at about 400 nm. Taking into account the almost unchanged absorption intensity after the thermal treatment at 80 °C in mesostructured films, it is considered that the shoulder band at about 400 nm was mainly attributed to the strong interaction between the chromophore and matrix rather than the presence of chromophore aggregates, as indicated in ref 33. The frameworks of both mesostructured and nonmesostructured films are composed of the chromophore modified silica blocks which develop during the coating process, and then continue to polymerize during the aging period. In the absence of P123, the hybrid films were directly packed with the silica clusters upon the evaporation of solvent. Consequently, the dye molecules were bonded and caged in highly dense silica networks, resulting in severe dye restriction and aggregates. However, in the presence of P123, the alignment of surfactant micelles forms a hydrophobic core that can provide a compatible environment for the hydrophobic azobenzene group of the DR1ASD ligands;44–47 meanwhile the hydrophilic alkoxysilane groups on the opposite side were covalently linked onto the pore surface of the silica framework, as shown in Scheme 1. Generally, azobenzene molecules are in the trans configuration in the lowest singlet state in the absence of light. For DR1

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Figure 2. Typical TEM images of hybrid films with different DR1ASD concentrations of 1% (A), 2% (B), 3% (C), and 5% (D).

Figure 3. TGA and DTA spectra of nonmesostructured film (dotted line) and mesostructured film (solid line), respectively.

Figure 4. UV-vis spectra of mesostructured films with different DR1ASD concentrations.

monomers, the size along the long axis is about 1.4 nm in the trans fundamental configuration,1 and an azobenzene chromophore similar to DR1ASD ligands without the termini nitro group has a size of about 1.82 nm,34 so even plus the nitro group and a carbon chain including three carbon atoms, the length of DR1ASD ligands is estimated to be close to but less than 3.0 nm. As illustrated in the TEM images in Figure 2, the mesopores of about 7 nm can provide enough space available for a monomeric dispersion of chromophore molecules after the removal of surfactants, which was verified by the absorption properties in the UV-vis spectra. In return, the DR1ASD ligands also have a remarkable effect on the mesostrucutre. When a high amount (e.g., 5 mol %) of azobenzene ligands stretch into the hydrophobic core, the long-range-ordered mesostructure would be partially destroyed because the ligand length is close to the pore radius, leading to the appearance of short-range-ordered or even worm-like mesostructure as evi-

denced from the XRD patterns in Figure 1. However, the pore sizes of these DR1ASD loaded mesostructures remain almost unchanged (Figure 2) and therefore still can provide enough spaces for the monomeric dispersion of the azobenzene chromophore. Nonlinear Optical Properties of Hybrid Films. The third harmonic generation (THG) technique has the virtue of being easy to operate and can provide direct evidence for the presence or absence of third-order optical nonlinearity before the quantitative measurement. Figure 6 shows the THG signals of mesostructured and nonmesostructured hybrid films with a DR1ASD molar concentration of 5%, as well as the THG signal of glass slide substrate. Both hybrid films have clear and sharp THG peaks at about 355 nm after being excited by the fundamental light at 1064 nm, indicating the presence of thirdorder optical nonlinearity. In glass substrate, no THG peak was observed and the intensity of the THG signal was two

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Figure 5. UV-vis spectra of nonmesostructured (A, B) and mesostructured (C, D) films with a DR1ASD concentration of 5% aged at room temperature (A, C) and 80 °C (B, D), respectively. The dotted lines present the corresponding deconvoluted spectra with Gaussian two-peaking fittings.

Figure 6. THG signals of (A) nonmesostructured, (B) mesostructured hybrid films with a DR1ASD concentration of 5%, and (C) glass substrate, respectively.

magnitudes lower than that of hybrid films, so the contribution to third-order nonlinearity from the glass substrate was negligible in the present measurement condition. The THG signals observed could be attributed to the direct THG process, because no phase-matching condition was taken for all measurements. THG signals observed in our experiment show that the ultrafast optical process of DR1 in the silica matrix is effective. The Z-scan technique is based on the principles of spatial beam distortion and offers simplicity as well as very high sensitivity for measuring both the nonlinear refractive index (n2) and nonlinear absorption coefficient (β) by the measurement of the normalized transmittance through the closed and open aperture versus sample position. If there exists nonlinear absorption in the sample, the closed transmittance will be affected by both the nonlinear refraction and absorption. The pure nonlinear refractive property should be corrected by the division of the normalized closed-aperture (CA) curve by the open-aperture (OA) curve.

In OA curves of the nonmesostructured film (Figure 7A), the transmissions are symmetric with respect to the focus where they have minimum transmissions (i.e., maximum absorption), indicating the presence of two-photon absorption. The corrected CA curve of the nonmesostructured film exhibits a peak followed by a valley (Figure 7B), representative of a negative nonlinear refractive index, i.e. n2 < 0. Besides, similar to the THG measurement, no valley or peak signals can be detected for the glass substrate and only mesoporous silica (i.e., without dye loading) coated substrates for either OA or CA curves. For mesostructured films, the OA curve (Figure 7C) does not give signals for decreased transmission at the focus point, while the uncorrected Z-scan CA curve (Figure 7D) is already found to be symmetrical with the characteristic shape of the transmission change, which indicates that TPA is almost absent in our experimental conditions. A valley-peak shape indicates a positive nonlinear refractive index and self-focusing effect in mesostructured films. This self-focusing propagating process is of great practical importance because the intensity at the focal spot of the self-focused beam is usually sufficiently large to optical damage of the materials.48 However, no damage spot was detected in the present mesostructured films during the measurement, which on the other side proves that this hybrid material can resist the optical damage under high laser intensity. A significant feature of the mesostructured film is that all recorded OA Z-scan signals with different incident irradiances show no absorption peaks related to two-photon absorption, which is in contrast to those in the present nonmesostructured film, DR1 solution in our previous work,49 and also the available literature reports which can be found so far for DR1 loaded composites. Considering the identical parameters during the preparation and measurement, the effect of mesopores should be responsible for this interesting phenomenon. The presence of the mesopores makes the azobenzene ligands monomerically

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Figure 7. (A, C) Open-aperture and (B, D) corrected closed-aperture Z-scan results for nonmesostructured films (A, B) and mesostructured films (C, D) with a DR1ASD molar concentration of 5%, λ ) 1064 nm, E0 ) 70 µJ.

TABLE 1: Nonlinear Optical Properties of DR1 Solution (10-2M), Nonmesostructured, and Mesostructured Hybrid Films by the Z-Scan Technique n2 sample

R0c/104 m-1

β/10-10m/W

10-16m2/W

10-9 esu

W

T

χ(3)d/10-10esu

1.2 2.53 2.53

0.00466 3.39 0 0

0.00253 -3.43 1.16 3.25

0.000851 -1.25 0.41 1.14

47.1 7.85 7.85

2.1 0 0

0.00129 2.02 0.64 1.78

liquida hybrid filma meso filma meso filmb

a Incident energy E0 ) 70 µJ. b Incident energy E0 ) 25 µJ. c Linear absorption coefficient; d Third-order susceptibility. The nonlinearities of DR1 solution are referred to ref 47.

disperse in pore space, leading to such a monomer absorption at about 480 nm, the same as that of DR1 in much diluted solution, where chromophore molecules can be considered to exist in a monomer form. However, a great difference between mesostructure and liquid is that the chromophore has one end firmly anchored on the pore wall of silica framework, therefore, the DR1ASD molecules cannot move freely as they do in solution, so the interaction among molecules is almost absent in large enough mesopore spaces in the former, leading to a high chromophore dispersion and mostly isolated azobenzene chromophores. Theoretically, if the absorption band stands closer to the TPA resonance, the nonlinear absorption due to TPA will become stronger. The near-zero absorption of the mesostructured film at/near the focus point in the present case, however, may be attributed to an absorption saturation with a very low energy threshold. Irradiance of low enough energy beyond such a threshold can generate an almost saturated population of the excited states and consequently a reduction of further electron transition from the depleted ground state,1,50 resulting in a negligible absorption from the ground state to the upper energy level, i.e., two-photon absorption. In other words, the complete transmission after a saturated absorption gives a β value close to zero in mesostructured film. In early reported rhodamine 6G-

doped mesostructured waveguides,51–53 the amplified spontaneous emission threshold was also found to be one magnitude lower than that of common rhodamine 6G-doped sol-gel glass due to the high dispersion of rhodamine 6G in surfactant (cetyltrimethylammonium bromide, CTAB). Besides, an increased transmission near the focus spot was generally detected as a peak in the saturated absorption case; however, it is not present here. We think that the saturated absorption in the present experiment occurred in a time period shorter than the laser pulse width we used, leading to a smooth transmission in the OA curve. All the values for the third-order NLO parameters of DR1 solution, mesostructured, and nonmesostructured films, respectively, are calculated according to the literature54 and summarized in Table 1. It is obvious that nonlinear refractive indexes of the hybrid films are nearly three magnitudes higher than that of the DR1 solution, which can be attributed to the interaction between the matrix and chromophores.49 Similar results have been reported in the azo dyes doped sol-gel system1 and PMMA film.55 For all films, the host matrix enhances the nonlinear optical properties developed by dyes. The large third-order optical nonlinearity of the azobenzene chromophore can be attributed to many mechanisms, such as

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Figure 8. (A) Open-aperture and (B) closed-aperture Z-scan results of mesostructured hybrid film with a DR1 molar concentration of 5%; λ ) 1064 nm, E0 ) 25 µJ.

electronic effect, one-photon and two-photon absorption resonance, thermal effect, and so on. Photoisomerization of the azobenzene chromophore from trans to cis configuration, an important origin of the optical nonlinearity of the azobenzene chromophore at the resonant wavelength in linear absorption region, cannot occur at our utilized wavelength of 1064 nm. The thermal effect on the third-order nonlinearity can also be ruled out within the pulse temporal width. The electronic nonlinearity arises very rapidly within the 40 ps pulse duration. Refractive index changes due to thermal nonlinearity propagates with the acoustic wave speed in materials:7,15 if we estimate it to be 3 × 103 m/s, the time to propagate a distance equal to the beam radius at focus (about 35-40 µm) is about 12 ns, about 300 times longer than the pulse width. In addition, it is a common assumption that Z-scan measurements should be made with a repetition rate of a few tens of hertz to extract a nonlinear refractive index influenced only by electronic effects.15,56 The time scale of this cumulative process is given by tc ) ω02/4D, where ω0 is the beam waist and D is the thermal diffusivity. For most liquids and optical glasses,56 the diffusivity ranges from 1 × 10-7 to 6 × 10-7 m2 s-1. The magnitude of the calculated tc is about 10-3 s, which is much smaller than the time interval between consecutive laser pulses of 0.1 s used in our experiment by two orders, indicating a negligible thermal effect on the third-order nonlinearity. The literature, where the nonlinear refractive indexes of DR1 in PMMA25 and poly(maleic anhydride-co-octadecene)22 were measured to be at the magnitude of 10-17 and 10-19 m2/w at 1064 nm, emphasized the contribution of TPA resonance enhancement to the nonlinearity. Similar contributions of TPA are also present in the nonmesostructured films. In such films, the energy gap between the ground state and excited state becomes higher because of the molecular aggregates (band blueshift in Figure 5A), leading to a decreased contribution from the excited state,15 which implies that an interaction between chromophore and the silica matrix exists. The silane grafting and covalent linkage into silica matrix enhance the π conjugation of the chromophore, and thus increase the π-π* transition band gap,57 in agreement with the blue shift in linear absorption spectra (Figure 5A,B). So we contribute the large nonlinearity of nonmesostructured film to the two-photon absorption resonance, and the enlarged delocalization of π-electron occurred between the chromophore and matrix.47 However, in the mesostructured films, the large nonlinearity can be mainly attributed to the electronic effect, which is induced by either population redistribution or distortion of electronic clouds due to the negligible two-photon absorption. This is consistent with the saturated population in the excited state and the depleted

ground state due to the saturation absorption with a very low energy threshold, as discussed above. In such films, the strong interaction between chromophore and matrix also increases the electronic transition gap from the ground state,33 shown as the shoulder band in linear absorption spectra (Figures 4 and 5D), which has the same contribution to the large third-order nonlinearity as in the nonmesostructured film. As a result, the electronic contribution, also with that by the saturation absorption with a very low energy threshold probably, leads to a smaller n2 of the mesostructured film compared to that of the nonmesostructured film with the same incident irradiance. To further prove the absorption saturation, decreased incident irradiance was used to modulate the nonlinearity by keeping other parameters identical, and a negligible two-photon absorption was also observed, as shown in the OA curve (Figure 8A), which indicates a very low threshold for absorption saturation at the focus point. Besides, there was no obvious change in the difference of normalized transmission between the peak and valley in the CA curve (Figure 8B), which shows again that the irradiance threshold for the absorption saturation of the mesostructured film is very low. Beyond the threshold, the difference of transmission would hardly change. This is important as it provides a way to obtain the large nonlinearity with low optical loss due to the TPA. According to a two-level model, the third-order electronic nonlinearity should be positive for laser wavelengths longer than the TPA resonance of the sample.12,48,50 However, if the nonlinearity of these samples is due to TPA, a negative n2 should be obtained when the absorption wavelength maximum is shorter than the two-photon resonance condition of the laser (532 nm in present study, half of the 1064 nm used in the experiment).25 A positive n2 in the mesostructured film is consistent with the two-level model for third-order electronic nonlinearity, while in the nonmesostructured film, a negative n2 indicates that the two-photon resonance is dominant over the contribution of electronic nonlinearity. The different mechanisms for third-order nonlinearity lead to a self-focusing process in mesostructured films and a self-defocusing process in nonmesostructured films, respectively. For practical use in ultrafast all-optical devices, many considerations have to been taken into account to investigate the efficiency of NLO materials. Two Stegman figures of merit have to be satisfied for expected phase shift to evaluate the applicability of optical films in such devices.10–12 The first is that the effect of linear absorption must be weak compared to the effect of nonlinearity, which pertains primarily to spectral regions in the immediate vicinity of resonant nonlinearities. This limit can be expressed as follows: W ) ∆n/R0λ > 1, where ∆n

Azo Chromophore Mesostructured Silica Films ) n2I0 and I0 is the irradiance. The second is that the effect of two-photon absorption must be weak compared to the nonlinear effect, which is important whenever two-photon absorption dominates the loss. This limit can be expressed as follows: T ) 2βλ/n2 < 1. The W and T values were also summarized in Table 1. According to the figures of merit, the region near the two-photon absorption was considered unavailable for photonic switching applications because of the presence of strong twophoton absorption for common azobenzene-containing materials despite a large electronic contribution to the Kerr coefficient in this region. In the present study, however, such a region exists for mesostructured hybrid films. First, as the utilized wavelength 1064 nm lies far from the resonant one-photon transition band of the hybrid films, the linear absorption is negligible and the first limit can be satisfied for all hybrid films, as confirmed by the large calculated W values. So the second limitation, i.e., the effect of two-photon absorption in/near the two-photon absorption region, becomes crucial for photonic switching application. The absence of two-photon absorption in the mesostructured film gives a T value equal to zero (Table 1), so the mesostructured hybrid film can also well satisfy the second limit. This is important for the mesostructured hybrid films to satisfy both limits at the 1064 nm laser input as 1064 nm is a very common laser signal in practical applications. Despite no contribution from two-photon resonance, the measured nonlinearity, n2 ≈ 10-9 esu (χ(3) ≈ 10-10 esu), is still large enough to be applied in all-optical devices.58 Operations at wavelengths in the region near two-photon absorption but where two-photon absorption is absent and nonlinear refraction remains large enough would be particularly interesting for photonic switching applications. Conclusions In summary, this work reports for the first time the covalently bonded mesostructured and nonmesostructured hybrid silica films with active azobenzene chromophore either monomerically dispersed in mesoporous silica films or incorporated in dense silica films, and their novel and interesting NLO properties. The azobenzene chromophores, which were covalently anchored onto the pore surface in mesoporous silica on one hydrophilic end, were monomerically dispersed on the opposite hydrophobic end within the pore spaces through the compatible environment provided by the hydrophobic core of the surfactant micelles. Both large enough third-order nonlinearities were detected for the two kinds of hybrid films, while significantly large thirdorder nonlinearity contributed by electronic nonlinearity but with negligible nonlinear absorption near the two-photon absorption region was achieved in the mesostructured organic-inorganic hybrid films, which satisfies the two limits in the Stegmen figures of merit. Therefore the present mesostructured hybrid films monomerically incorporated with active azobenzene chromophore are very promising to meet the demand for applications of photonic switching in all-optical devices. Since there are many different types of NLO chromophores and many different types of mesoporous films with different pore sizes and pore structures, this work provides a promising new direction to search for highly sensitive and applicable thirdorder nonlinear optical materials. Acknowledgment. The financial support of National Natural Science Foundation of China (Grant Nos. 20633090 and 50672115)andtheNationalBasicResearchProject(2002CB613300) is gratefully acknowledged.

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