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Materials for optical data storage and optical information processing must exhibit good holographic properties. Many materials for these applications have been already proposed. Here we describe a grating inscription process characterized by short inscription time and long-time stability. A series of ruthenium-acetylide organometallic complexes containing an azobenzene fragment were synthesized. Photo-induced gratings were produced by short pulse (16 ps, 532 nm) laser irradiation. The surface relief gratings formed at the same time were observed by atomic force microscope. In this work, we highlight the short inscription times brought into play as well as the good temporal stability of these gratings stored at room temperature. We study the influence of the polarization states and the light intensity of writing beams on the dynamics of the surface relief gratings formation and we compare these results with those of a known representative of azobenzene derivative (Disperse Red 1). Lastly, we show that it is possible to write two-dimensional surface relief gratings.
©2008 Optical Society of America
1. Introduction
In recent years the organometallic systems in nonlinear optics (NLO) have become a topic of intense investigation. Indeed, the incorporation of transition metals in the organic and inorganic systems often used in NLO gave a new dimension to the study of these systems. Transition metals have a broad diversity of oxidation states and ligands which increase nonlinear properties. The interest in the organometallic complexes comes from their particular electronic capacities in term of charge transfer [1,2]. In particular, the organometallic acetylide complexes having a linear structure of type M-C≡C-R generate a strong coupling between the metal and the π-conjugated organic way and consequently have strong optical nonlinearities. The theoretical and experimental studies of these complexes have shown that the strong second-order hyperpolarisability depends on the length of the π-conjugated way and the strength of donor (metal) and acceptor fragments. Moreover, it was observed that the second-order nonlinear response increases due to the multiple metal-carbon bonds which are presented in this type of complexes [3].
The organometallic metal-acetylide complexes present good second and third-order NLO properties. In particular, the ruthenium complexes form part of the organometallic compounds most studied in NLO [2,4,5] due to their versatility and synthetic accessibility, their stability (chemical and thermal) and the reversibility of the redox coupling RuII/RuIII [6]. In this respect, the ruthenium-acetylide fragment is a strong donor which can compete with the strongest organic donors [7,8]. Indeed, these properties come mainly from the result of the overlapping between the d ruthenium orbital and the system, and can thus be modified systematically through the variation of the π-conjugated system or the electronic richness of the ruthenium-acetylide fragment.
This work highlighted the dynamics of photoinduced gratings of organometallic ruthenium(II)-acetylide complexes previously studied in NLO [9,10]. Due to using picosecond pulses, the grating inscription process is faster than under continuous laser regime. Also, short laser irradiation minimizes the thermal effects. Moreover, relatively few works have been published on the inscription of surface relief gratings (SRGs) in pulsed regime [11–14], although the technological projections relating to the high-speed lasers (in particular in term of miniaturization) continue to increase.
2. Experimental
In this work, we studied organometallic complexes (named A-C, as previous (cf. Ref. [9])) having a common donor fragment of ruthenium-acetylide trans[(dppe)2Ru-C≡C-] (dppe=diphenylphosphinoethane) [9]. The phosphine electrodonor ligands are particularly appreciated in NLO since they enrich metal in electrons while improving the molecule stability. Criteria evoked in the Ref. [8], allowing to tend to a better NLO efficiency, suggest a lengthening of the π-conjugated system connecting the donor fragment to the acceptor fragment and impose a multiple bond at the metal-carbon bond of the organometallic fragment.
The complexes A-C are formed by a N,N-dibutylamine fragment and an azobenzene fragment functionalized to study the dynamics of SRGs formation. These complexes were synthesized in the form of powder (50–100 mg) then optically characterized in the form of thin films in order to carry out photo-induced SRGs. The complex A (trans-[Ru(4-C≡CC6H4N=NC6H4-N(C4H9)2)Cl(dppe)2], M=1265.36 g/mol) possesses chlorine fragment attached to the ruthenium-acetylide donor one. In the case of complexes B (trans-[Ru(4- C≡CC6H4N=NC6H4-N(C4H9)2)(4-C≡CC6H4CHO)(dppe)2], M=1359.42 g/mol) and C (trans- [Ru(4-C≡CC6H4N=NC6H4-N(C4H9)2)(4-C≡CC4H2SCHO)(dppe)2], M=1365.38 g/mol), the benzaldehyde and thiophene carboxaldehyde fragments are attached to the donor one, respectively.
The SRGs were inscribed using a degenerate two wave mixing (DTWM) technique (first time described by Shank et al. [15]). The thin ~300 nm films of complexes A-C were illuminated by two 532 nm beams from 16 ps pulsed Nd:YAG laser (Continuum Leopard D-10) working at 10 Hz repetition rate (coherence length 4.8 mm). The sample was mounted on a stage perpendicularly to the bisectrix of the writing beams. The intensity and the polarization of the writing beams (set up to make an angle of 60°) were controlled by polarizers and half-wave plates placed on the pathways of laser beams. Grating formation was monitored by a cw He-Ne laser (632.8 nm, 30 mW) with vertical polarization in the transmission mode by measuring the first-order of diffraction. The experimental setup was presented by Czaplicki et al. (cf. Ref. [16]). After grating inscription the atomic force microscope (AFM) scans of irradiated surfaces were done.
3. Results and discussion
3.1 Light intensity and polarization dependence
The study on the influence of the intensity of the writing beams (polarization s-s) on the dynamics of the SRGs formation was carried out. Figure 1 illustrates the results of measurements (complex C) taken for different intensities of writing beams (0.1–10.0 GW/cm2). Beyond 10 GW/cm2 (threshold damage for these samples) a photo-bleaching or even a hole (total ablation) were observed due to the thermal effects induced by the strong energy of writing beams (up to 1.3 mJ). When the intensity of the writing beams increases (up to 10 GW/cm2), the first-order diffraction efficiency increases more quickly up to 8.5%, 10.2% and 12.7% for the complexes A, B and C, respectively (see Fig. 2 and Table 1).
Fig. 1. Gratings recording in complex C by two 16 ps beams with the s-s polarization and different intensities.
It is well known that the local molecular orientation in the SRGs depends on the polarization of the incident beams [17]. Our study confirms this tendency (cf. Fig. 2(a)). Table 1 and Fig. 2 illustrate the results of measurement carried out for s-s, p-p and s-p polarization states. Also, we can see that the best result (the strongest diffraction efficiency (12.7%) and average height (100 nm) was obtained for the complex C (Fig. 2(b)).
Fig. 2. First-order diffraction efficiency as the function of irradiation time (a) for complex C with different linear polarization states of writing beams (10 GW/cm2), (b) for complexes A-C with s-s polarization.
Surface relief gratings are easy to scan by atomic force microscope. Figure 3 shows a typical example of three dimensional view of the surface relief grating. The grating spacing, measured with the AFM, was about 0.53 µm, and was consistent with the theoretically calculated one (Eq (1). in Ref. [16]). The resulting image (Fig. 3(a).) gives the possibility to determine the principal directions of the grating orientations. One can deduce that the SRGs were written perpendicularly to the polarization direction of writing beams. The difference of the results obtained between the s-s and p-p polarization states is due to the preferential orientation of chromophores in the perpendicular direction to the substrate [9,10]. In addition, in the polarization state s-p, the electric field vectors of the pump waves are directed so as to slow down the molecular movements at the origin of the SRGs formation (no modulation of the light intensity).
Fig. 3. a) Three dimensional atomic force microscope scan of area exposed to the two s-s polarized laser beams for complex B at 10 GW/cm2 and b) cross section of this SRG.
Several AFM images, carried out over a period of 6 months on these SRGs stored at room temperature highlighted the weak reduction of their average height (lower than 5%) and thus illustrate the good temporal stability of these gratings. In addition, the response of the complexes A-C to the picosecond pulses illustrates the memory effect existing and related to the stability of the molecules brought by their ruthenium-acetylide fragment (Fig. 4). Indeed, after each laser pulse, no decrease of the first-order diffraction efficiency is noted as is the case for other materials [16]. After each laser pulse, the molecules change their position from high to low intensity regions. This process is going on until reaching by molecules a completely static position (saturation of the first-order diffraction efficiency and average height of the SRGs). The time of process is equal to nt imp where n is the number of laser pulses and t imp - the time between two pulses.
Fig. 4. Diffraction efficiency as a function of irradiation time for complex C (10 GW/cm2, s-s polarization).
Table 1 presents the results obtained concerning the inscription times of the SRGs for the complexes A-C for various polarization states of writing beams and for t imp=100 ms. The lowest inscription time of these gratings (0.7 s) was observed for the complex C in s-s polarization, while for the complexes A and B the inscription time is 2.5 and 2 times longer, respectively.
3.2 Complexes in matrices
The dynamics of SRGs formation was also compared with material often used for the study of photo-induced SRGs: Disperse Red 1 (DR1 or [4-(N-(2-hydroxyethyl)-n-ethyl)-amino-4’- nitroazobenzene]). In order to make it film forming, this chromophore was used as a doping agent in a polymeric matrix of polymethylmethacrylate (PMMA) which is a transparent polymer in the visible and near UV (to 250 nm). In order to observe the influence of PMMA matrix on the dynamics of SRGs formation, we also inserted the complexes A-C in a PMMA matrix. The different chromophores and the PMMA were dissolved in trichloroethane. The resulting solutions were filtered before being deposited by spin-coating on BK7 glass substrates (thickness about 1 mm). The obtained compounds: PMMA-A, PMMA-B, PMMA-C (thickness about 300 nm) and PMMA-DR1 (thickness about 1400 nm) were functionalized with a mass doping concentration commonly employed of 30% [18–20]. The maximum absorption of these complexes is about 490 nm.
SRGs were carried out on thin films of these compounds under the same experimental conditions as those of the SRGs obtained on the complexes A-C. Table 1 compares the results for complexes with and without PMMA matrices, and with PMMA-DR1. All results are similarly to these obtained for complexes without matrices and are better than for PMMA-DR1. The strongest first-order diffraction efficiency (8.5%) was obtained for compound PMMA-C in s-s polarization (only 2.1% for PMMA-DR1 (used as reference material)). Also, the weakest inscription time of the SRGs (0.8 s) was obtained for compound PMMA-C in s-s polarization (only 3.2 s for PMMA-DR1).
Being given the matrix-doping agent molecular interactions brought into play in the inscription process of the SRGs, it appears relatively difficult to establish a comparison model between the results obtained for the DR1 with those obtained for the complexes A-C. On the other hand, in comparison with the results presented in Table 1, one can establish that the PMMA matrix induces, for PMMA-A\B\C compounds compared to the complexes A-C, a decrease in the first-order diffraction efficiency, in the average height as well as an increase in the inscription time of the SRGs. This observation is even more marked for the s-p polarization state of writing beams and for the compound PMMA-A having only one chlorine atom like acceptor fragment.
Table 1. Experimental parameters of SRGs: the first-order diffraction efficiency (η+1), the average height (h), and the inscription or writing time (tw) for various polarization states of writing beams (at 10 GW/cm2).
3.3 Two-dimensional structures
In this study, it was also possible to write the so-called “two-dimensional” (2D) gratings (Fig. 5) in a reproducible way. To obtain a 2D SRG, each thin film was turned of 90° between two inscription processes carried out under the same experimental conditions as those of a classical SRG [21–26]. For the complexes A-C, the two classical SRGs separately written are combined (constructive and destructive interferences) to give these 2D SRGs whose cross section is sinusoidal with a grating spacing equal to Λ2D=2Λ1D≈1064 nm. However, according to the results obtained and presented in Table 2, these two-dimensional structures have a maximum first-order diffraction efficiency and an average height (measured after inscription of the 2D SRGs) approximately 60% lower than those of the classical SRGs.
Fig. 5. 2D (left) and 3D (right) AFM images of a photo-induced two-dimensional SRG for complex B (10 GW/cm2, s-s polarization).
In the case of the 2D SRGs, the matter migration is also carried out from high to low intensity regions and the formation of this type of structures is related to the photo-induced auto-organization of azo chromophores of the complexes A-C [21,26].
Table 2. Experimental values of 2D SRGs: the maximum first-order diffraction efficiency (η+1) and the average height (h) for various polarization states of writing beams (at 10 GW/cm2).
4. Conclusion
In this study, photo-induced SRGs were written of the organometallic complexes A-C thin films using a dynamic holography technique in picosecond regime. From the obtained results, it could be clearly established that the dynamics of the formation of these SRGs is dependent on the light intensity and on the polarization of writing beams. Moreover, it was possible to write the two-dimensional structures in a reproducible way. It was observed that these 2D SRGs induce a decreasing of about 60% of the maximum first-order diffraction efficiency and of the average height compared to the classical SRG. Lastly, it is important to note that the photo-induced classical and 2D SRGs could all be completely erased subjecting the thin films to a temperature close to 120°C.
The strongest second and third-order optical nonlinearities of the complexes A-C [9,10] and their good holographic properties observed in picosecond regime (diffraction efficiency, average height, inscription time and stability) make them promising candidates for multiple applications. Indeed, we showed that the ruthenium-acetylide complexes are without a doubt promising materials for future optoelectronics applications and the last projections in terms of molecular engineering and miniaturization of continuous and pulsed laser sources let imagine the potential of applications of these types of materials: optoelectronic components, optical memories and optical data storage.
Acknowledgments
The authors would like to thank the Service Commun d’Imageries et Analyses Microscopiques (Angers University) for performing AFM measurements. Also, the authors thank the COST Action MP0702.
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