Nonlinear Optical Films Research Articles

Processing of poled second-order nonlinear optical polymer systems under microgravity conditions

Organic polymeric thin films for photonic applications has been a rapidly exciting area of research over the last decade. Second-order nonlinear optical polymer film is created by aligning the appropriate chromophores into a polymeric host medium by poling in an electric field followed by permanent fixation. The poling is usually conducted near T g but the maintenance of poled order is often difficult even at room temperature because of the relaxation of the chromophore units which destroys the orientation of poled dipoles. It is expected that the rotational relaxation may be affected by polymer structure (cross-linking) and hence T g, device temperature and possibly gravity which acts like a pressure force capable of modifying rotational transition probabilities. We report here the results of our microgravity experiments which were carried out at the ZARM drop tower facility at Bremen, Germany. Two systems were tested: (a) guest–host system using thermal processing and (b) cross-linked system using photocross-linking processing. Five drops of 4.75 s of microgravity were made and for each drop 15 guest–host samples and nine photo cross-linking samples were processed. The sample thickness was fixed at 2 μm ; however, poling field, UV intensity, chromophore type and concentration, poling temperature and cooling rate were varied. The relaxation behavior was monitored using second harmonic generation as a function of time for both the samples prepared at 1 and 10 −6 g . Improvement of relaxation time was observed for certain samples. Simulations using a modified Robertson–Shimha–Curro theory were also carried out. Qualitatively, experimental results support the simulation hypothesis.

Pulsed laser deposition of conductive metallo-dielectric optical filters

We describe the fabrication by room-temperature pulsed laser deposition of a transparent conductor comprising alternating layers of silver and aluminum oxide, forming a metallo-dielectric filter. Transmittances of 0.7 over specific wavelength bands were achieved with resistivities as low as 6.0×10-6 Ω cm, almost two orders of magnitude lower than that of the best single-substrate thin films, such as indium tin oxide. The resistivity can be predicted without adjustable parameters and designed using a simple parallel-circuit model; the optical properties are well described by standard matrix transmission calculations. This demonstrates that pulsed laser deposition may be used to fabricate prototypes of high-quality transparent conductors with predictable properties for conducting windows where low-temperature deposition is critical, as in organic light-emitting diodes and for non-linear optical films.

Photoinduced multipolar tensorial patterning in polymer films by coherent control of molecular orientation

Based on an early proposition by R.J. Glauber in 1967, the possibility to break centrosymmetry in molecular media by interference between one- and two-photon absorption pathways has been implemented in polymers throughout the last decade. This effect has been more recently advantageously generalized to optically induce adjustable multipolar molecular ordering in polymer thin films for nonlinear optics. A major benefit of this approach lies in the control of the final molecular orientational distribution by pure optical parameters such as phase or polarization of write beams. In this article, we extend a tensorial multipolar engineering experiment to encompass tensorial monitoring of photo-induced molecular orientational orders up to the 4th order. Coherent control of optical write beams, combined with real-time tensorial probing of optical properties in the material, provide access to a complete characterization of each excitation pathway contributing to the process. To cite this article: S. Brasselet et al., C. R. Physique 3 (2002) 479–492.

Numerical analysis for photothermal poling process of nonlinear optical polymer film

Numerical simulations of the manufacturing of waveguides with grating patterns on a nonlinear optical (NLO) polymer film using photothermal poling were carried out. The mathematical model was constructed, in which spatial and temporal variations of molecular dipoles during heating with a focused laser beam and electrical poling were considered. The effects of operational factors, i.e., laser power, beam diameter, moving speed of a sample, and electric field strength, on the grating configuration were investigated. As a result, the following guidelines for producing a waveguide with high performance were obtained. The laser beam wavelength should be selected so as to heat uniformly throughout the thickness of the polymer film, the laser power should be as high as possible without causing large deformation in the film, the laser beam diameter should be smaller than half period of the grating, the moving speed of the stage should be slow, and the electric field strength should be as high as possible, if dielectric breakdown is to be avoided.

Optical characterization of alkyl-thiophenic monomers functionalized with second-order nonlinear chromophores

A series of thiophenic monomers functionalized with different chromophoric groups have been optically characterized in view of preparation of polymeric films for second-order nonlinear optics and electrooptics. In order to investigate the optical behaviour inside a solid host matrix, the molecules have been physically dispersed in poly(methylmethacrylate) (PMMA) films and poled through high-temperature corona-poling for characterization through in situ second harmonic generation and ex situ angle-resolved spectroscopy. The values of dipole moment μ and optical hyperpolarizability β have been estimated inside the host matrix and related to the molecular structure. For one of the chromophores the long-term reorientation dynamics has been monitored in order to investigate the stability and the interaction with the polymeric environment.

The Sense of Chromophore Orientation in Films Made by Alternating Polyelectrolyte Deposition

Abstract : Alternating polyelectrolyte deposition (APD) is a technique for fabricating multilayer thin films at room temperature. The substrate is alternately dipped in two different aqueous solutions - one containing a polycation and the other a polyanion. By incorporating chromophores in the polycation and/or polyanion, it has been demonstrated that the APD process can be used to make multilayer second order nonlinear optical polymer films. In addition, a recent encouraging discovery is that these films exhibit a high degree of thermal robustness.

Study of the single-beam reflection technique for measuring the linear electro-optic coefficient of the nonlinear optical polymer films

The single-beam reflection technique is usually used to measure the linear electro-optic (EO) coefficient (Pockels coefficient) in development of the EO polymer thin films. But there are some problems in the determination of the phase-shift difference between the s and p waves of a laser beam waveguided into the polymer film while a modulating voltage is applied across the electrodes in some articles, and different expressions of the linear EO coefficient have been deduced in these articles. In our research, more accurate expression of the linear EO coefficient was deduced by reasonably considering the phase shift between the s and p waves. The linear EO coefficients of several polymer thin films were measured by the reflection technique, and the results of the linear EO coefficient calculated by different expressions were compared. The limit of the single-beam reflection technique for measuring the linear EO coefficient of the polymer thin films was discussed.

Numerical Analysis for Manufacturing Process of Waveguides with Grating Patterns on NLO Polymer Film by Photoassisted Electrical Poling

Numerical simulations of the manufacturing process of a waveguide with grating patterns on a nonlinear optical (NLO) polymer film by photoassisted electrical poling (PAEP) were carried out. The mathematical model for the process was constructed, which included the theoretical model developed by Dumont and Osman [13, 14] in order to simulate photoinduced orientation of dye molecules in amorphous polymers, and spatial and temporal variations of molecular dipoles in the moving polymer film during photoisomerization by irradiation of a focused laser beam and electrical poling were considered. The effects of operational factors, i.e. , laser beam intensity, beam diameter, moving speed of the sample, and electric field strength, on the grating configuration were investigated. As a result, the following guidelines for producing a waveguide with high performance were created: The laser beam wavelength should be selected so as to make the distributions of beam intensity uniform across the thickness of the polymer film, and the laser beam diameter should be smaller than the half-period of the grating. In addition, the magnitude of the dipole orientation in the polymer film was found to be controlled by the ratio of the beam intensity to the moving speed of the sample.

SNPP and SMAP as Candidate Materials for Langmuir-Blodgett Films for Non-Linear Optics

(S)-N-(4-nitrophenyl)-prolinyl stearate (SNPP) and 2,4-dinitrophenyl-1-alanine stearyl ester (SMAP), stearyl derivatives of the molecules NPP and MAP (which form non-centrosymmetric crystals), have been prepared and studied as candidate materials for Langmuir-Blodgett films for non-linear optics. Pressure-area isotherms were determined under various conditions of sub-phase pH and temperature for both materials, separately and in mixtures with other film-forming species. Surface layers of the pure materials were at best metastable, and Langmuir-Blodgett films deposited from mixtures did not incorporate any SNPP or SMAP. Molecular modelling showed that in SNPP the hydrophobic tail appears to prevent stable film formation by preferring to lie over the weakly hydrophilic nitro group, while in SMAP the orientation of the tail is more favourable but the two nitro groups appear to destabilize the film.

Novel Hybrid Covalent / Ionic Self-Assembly Technique for Improved Second-Order Nonlinear Optical Films

ABSTRACTIonically self-assembled monolayer (ISAM) films have been shown to spontaneously produce noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO) response. Typically, the ISAM films for NLO response are an assemblage of bilayers of oppositely charged polymers whose thickness can be controlled through variation of pH and ionic strength of the immersion solutions. Here, we investigate the effects of replacing the NLO-active polymer layers with layers of monomeric chromophores containing ionic and covalent bonding sites. Films fabricated exclusively using polyelectrolytes contain some fraction of both randomly oriented and anti-parallel oriented chromophores. We have examined the incorporation of monomeric chromophores into ISAM films in order to increase the net polar orientation of the chromophores and reduce bilayer thickness.

Study on photobleaching of PMMA/DR1 film by <italic>z</italic>-scan measurement

By the z-scan method, we investigated the photobleaching processes of a nonlinear optical polymer film made of poly (methyl methacrylate) with a dispersed red (PMMA/DR1) side chain at the 488 nm wavelength of an argon ion laser. It provides a simple method to determine the parameters in a bleaching model that describes the polymer photobleaching processes.

Dynamic probe of pyroelectricity of nonlinear optical polymer and theoretical analysis of its temporal response

The light pulsing technique was used for detecting the pyroelectricity of poled nonlinear optical polymer. We provided a simple experiment setup and an analytical model for theoretical analysis. Based on the results of our experiment, according to a model that was given in this study, we have calculated the temperature field distribution in the sample caused by the flash lamp irradiation in the polymer by means of numerical analysis, and further, the pyroelectric signal response was obtained. The model could be used for precisely investigating the response from a finite duration of light pulse. This numerical analytical method applied to the thermal pulse method yields unique relations between the time-dependent pyroelectric response and time-dependent temperature distribution for a thin nonlinear optical polymer film. The experimental results compared quite favorably with theoretical analysis.

Second-harmonic generation and absorption spectra of platinum organometallic complexes incorporated in PMMA films

Pt-acetylides with different end-side groups have been synthesized and incorporated in poly(methylmethacrylate) (PMMA) films for second-order nonlinear optics through high-temperature corona-poling. By using two complementary optical techniques, i.e. in-situ second-harmonic generation (SHG) and angle-resolved spectroscopy, the order parameter and the molecular hyperpolarizability β of the chromophores have been characterized inside the polymer matrix. The relaxation of orientational after order poling fits a bi-exponential equation in agreement with the Debye model for guest/host systems. A comparison with the azo-dye Disperse Red 1 (DR1) is discussed within the two-level model and the role of the platinum moiety for conjugated molecules.

Analyzing the polarization distribution in poled polymer films by scanning Kelvin microscopy

We present a method for analyzing the homogeneity of the χ(2) distribution in poled nonlinear optical polymer films. The second-order nonlinear coefficient in these polymers is commonly induced by electric-field poling methods which can lead to a χ(2) distribution with poor spatial homogeneity. In this letter, we analyze the χ(2) distribution using scanning Kelvin microscopy. This allows us to detect the height and the direction of the induced polarization through the probing of the countercharges that are present on the polymer surface. We compare the response to that obtained from the scanning second-harmonic microscopy method, in which the direction of the orientation, and thus the phase of χ(2), cannot be seen.

Nonlinear Optical Polymer Films Based on Sol-Gel Derived Polysilsesquioxane with Pendant Chromophoric Subunits Embedded in Size-matched Pores

Two new hybrid nonlinear optical (NLO) polymeric films have been fabricated from the corresponding monomers, α, ω-bis(trimethoxysilyl)-1,4-dipropoxybenzenes bearing one or two moieties of pendant groups of N-(4)-nitrophenol-(L)-prolinol (NPP) on the spacer respectively by molecular design and “in-situ poling and sol-gel” process. These films are of enhanced thermal stability, extended temporal stability and higher second harmonic generation (SHG) susceptibilities. Their d33 values are 2.0 × 10−8 esu and 3.2 × 10−8 esu., respectively. These properties can be ascribed to the molecule-level embedment of NPP moieties in size-matched pore of the polysilsesquioxanes and fixation of the orientation of the poled chromophore given by the pore formed after the alignment of NPP moieties. The structures and morphologies of the two polymeric films investigated by XRD, FT-IR, DSC, TGA, AFM, etc. show a close correlation with the SHG properties.

Improved Second Harmonic Generation from Organic-Dye-Doped Polymer/Silica Hybrid Materials

In this study, we investigated the second-order optical nonlinearity and its thermal stability of DR1 (disperse red 1) doped poly(methyl methacrylate) (PMMA)/silica gel films. We found that in situ polymerization of sol−gel precursors (tetraethoxysilane, TEOS) in a polymer generated a hybrid matrix which is a good tradeoff between a silica gel and a polymer matrix. The nonlinear thermal stability of the hybrid system increased significantly compared to that of a poled DR1/PMMA film. Our result proved that the combination of a poled polymer and the sol−gel technique provides a way to increase the thermal stability of nonlinear optical thin films fabricated from existing materials rather than synthesizing new chemical compounds. Several factors which affect the second-order optical nonlinearity of the hybrid system were also examined.

Third-harmonic generation and growth mechanism of vanadyl-phthalocyanine single crystals prepared on KBr substrate by molecular beam epitaxy

Vanadyl-phthalocyanine (VOPc) single crystals were prepared on KBr (1 0 0) substrate by molecular beam epitaxy (MBE). The second and third-harmonic generations of nonlinear optical thin films are proportional to the square of the thickness. Therefore, it is important to prepare a large nonlinear optical single crystal. However, the conditions to prepare a large nonlinear optical single crystal are not well understood yet. In this paper, we succeeded to prepare a large nonlinear single crystal. It was obtained with the control of substrate temp., evap. time and annealing time. The orientation and nonlinear property of VOPc single crystal prepared by the MBE method were also studied. The size of maximum single crystal prepared on KBr is 4×9×0.16 μm. The absorption spectrum of sample has an absorption peak at 790 nm. It reveals that the growth of single crystal gradually grow to a large crystal merging small crystals and also suggest that the VOPc single crystal is monoclinically grown from the results of RHEED pattern and UV/VIS spectrum. We estimated the third-order nonlinear optical susceptibility χ (3) from the TH intensity for VOPc single crystal monoclinically grown on KBr. Its χ (3) value estimated is 8.5×10 −10 esu. The value is 3 times higher than the value reported recently.

Ordered Films by Alternating Polyelectrolyte Deposition of Cationic Side Chain and Anionic Main Chain Chromophoric Polymers

Using the method of aqueous solution alternating polyelectrolyte deposition (APD), second-order nonlinear optical (NLO) polymer films were prepared for the first time in which both polymers were NLO-active. Films were prepared by alternately coating a solid substrate with an NLO-active side chain polycation and an NLO-active main chain polyanion. This new polyanion was comprised of α-cinnamoyl chromophores in the syndioregic configuration (an “accordion” polymer). The new polycation was derived from polyepichlorohydrin that was completely substituted with new stilbazolium side chain. The films were transparent and had no texture when observed by polarized microscopy. The increase in intensity of the second harmonic (SH) signal generated in the films was quadratic with each molecular layer to 20 layers. Beyond that, the SH signal intensity saturated as more layers were added.

Nonlinear Optical Films by Alternating Polyelectrolyte Deposition on Hydrophobic and Hydrophilic Substrates

ABSTRACTThe formation of acentric films using alternating polyelectrolyte deposition (APD) has been achieved on hydrophobic and hydrophilic glass substrates. APD is a layer-by-layer technique for the formation of polymer films by alternately immersing a substrate in aqueous solutions of a polyanion and a polycation. APD provides precise control of the overall film thickness that through automated processing may exceed a thousand layers. In this study, APD films were made of an NLO-active polycation, stilbazolium-substituted polyepichlorohydrin (SPECH), and NLO-inactive polyanions. The peak maximum UV-Visible absorbance in transmission through the films was linear as a function of the number of bilayers. Second harmonic generation (SHG) was used as a tool to indicate acentric order of polarizable sidechain chromophores within the APD films. The SHG exhibited the expected quadratic intensity increase with film thickness.

Preparing and polarizing stability of Teflon AF nonlinear optical polymer electret double layer film system

Nonlinear optical activity of an nonlinear optical (NLO) polymer electret originates from the preferential orientation of dipoles doped or chemically attached to the polymer matrix. In this paper, the idea of fabricating a Teflon AF/nonlinear optical polymer electret double layer film system is proposed to improve the orientation stability of the dipole in the polymer electret. The polar stability has been investigated by means of measuring the open-circuit thermally stimulated depolarization (TSD) spectrum, the electro-optical coefficient, and the isothermal surface potential decay. The results indicate that the double layer film system may markedly increase the orientation stability of the dipole in the NLO polymer layer. Because the Teflon AF layer has high charge storage stability, the dipole orientation can be bound by the electric field generated by the space charge in Teflon AF layer.