Aleksandra Apostoluk

Quasi-phase-matched gratings printed by all-optical poling in polymer films.

The all-optical poling technique permits polar orientation of molecules. For efficient poling of thin films the relative phases, amplitudes, and polarizations of the two interfering beams must be controlled. We present an original stable one-arm interferometer that is specific to the recording of two-color interference. It relies on the index dispersion of optical glasses. This interference technique permits true real-time nonperturbative monitoring of the polar orientation process and easy all-optical poling of thin-film materials without the need for phase control. This new configuration opens the door to the realization of customized phase-matched wave-guided frequency-conversion devices for the near infrared.

Photoinduction of surface-relief gratings during all-optical poling of polymer films

Using the all-optical poling method, we observed the formation of a surface-relief grating (SRG) in an amorphous polymer film containing azo dye molecules in side chain positions. The experiment consists of a seeding-type process. We used a recently described experimental setup that permits a periodic nonlinear pattern to be produced by the index dispersion of glass. The particular configuration permits photoinduced translation diffusion of the azo chromophores to be observed as the origin of the SRG formation. Analyses of the gratings recorded by use of s (TE) and p (TM) polarization of the writing beams are conducted by atomic-force microscopy. The effect is attributed to mass transport from regions of high isomerization activity to regions of lower activity.

All-optical poling of DMABI molecules in a polymer matrix

Many organic compounds in solid state have nonlinear optical properties due to the orientation of the molecules in a polymer matrix. In this work, all-optical poling and second harmonic generation in a composition consisting of 1 mass% of N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) compound in poly(methyl methacrylate) (PMMA) matrix were studied. Thin films were prepared by solvent casting. The 1.064-μm fundamental and 532-nm second harmonic wavelengths of a Nd:YAG laser were used. It is shown that DMABI molecules can be oriented by the method of all- optical poling, and that the process is related to the photoinduced switching between two equally stable states of the molecule.

A half-period surface relief grating formed by bicolor interferences in thin azopolymer films

All optical poling technique allows orientation of dye molecules in a polymer matrix by purely optical means. The coherent superposition of two beams at fundamental (FUND)and second harmonic (SH)frequencies results in the presence of the polar field E(t) inside the material, able to break the centrosymmetry of the medium. The temporal average cube 3>t of this field E(t) = Eω(t)+E2ω(t), is non-zero, which leads to an orientation-selective excitation of molecules and the second order χ(2)-susceptibility grating is encoded, with a period satisfying the phase matching condition for SH generation. The physical origin of the effect lies in the orientational hole-burning in the initially isotropic distribution of dye molecules (via trans-cis trans photoisomerisation). We investigate other phenomena responsible for the SHG in polymer films like the electron transfer between a polymer matrix (donor of electrons) and a dye molecule (electron acceptor) and processes which accompany polar orientation mechanism like the one-direction photoinduced molecular migration. The stability of the induced polar order after orientation has been also studied and the crosslinkable polymer system has been developed D side chain molecules with ends can thermally react with epoxy groups were randomly inserted in the side groups of the polymer backbone, which leads to the reticulation and enhancement of the rotational stability. After poling the second order nonlinear response exhibits a much longer relaxation time. In the set-up configuration used we monitor non-perturbatively the all optical poling and there is no necessity of taking into account the phase difference between writing beams (fundamental and SH). The periodical modulation of the relative phase provides the desired control on the polar order at the molecular level as well as the possibility of encoding surface relief gratings with a period equal to the half of the period of the χ(2) grating.

All-optical induction of enantiomer resolution in a racemic mixture

We demonstrate the possibility of optically inducing the resolution of a racemic mixture of molecules. We thus measured an optical induction of optical rotation. For this, we used an all optical method based on a pump (Ar-488 nm) - probe (He-Ne-632 nm) experiment exciting a thin layer of a new chiral photoisomerizable chromophore in a PMMA matrix.

Nonlinear optical refractive index and absorption of coordination compounds

We measure third-order optical response of two organometallic compounds using the degenerate four wave mixing method. From measurements of DFWM efficiency, we deduce the values of third-order susceptibilities χ<3>. From measurements of χ<3>, we deduce the values of the second-order hyperpolarisabilities γ. The merit factor for each compound is given and the value obtained for the most efficient compound in terms of γ(second order hyperpolarizability) is 104 larger than the value of CS2, which is a reference material. The obtained optical nonlinearities are compared to those of other compounds previously studied. A preliminary correlation between structure and third-order optical properties is proposed.

Structural and dynamic studies of the polar orientation induced by corona poling and all-optical poling in crosslinkable polymer thin films

The fast development of photonics and progress in laser technologies has created the necessity for materials possessing high nonlinear photoactivity. Polymers containing various dye molecules in grafted as well as doped systems have recently been in the limelight of many research groups because of their various potential applications in electro-optic devices and integrated optics, as they possess large second order nonlinear properties, low dielectric constants, the possibility of inducing large nonlinearities, low attenuation and ease of processing [1–3]. Yet, some important issues concerning temporal stability of induced order and optical loss are still a challenge to researchers [4–7]. A polymer matrix containing non-centrosymmetric molecules is a macroscopically centrosymmetric solution in which second order nonlinear processes are forbidden due to symmetry reasons. Yet, the system properties may be altered by a non-centrosymmetric external field. In order to create second order nonlinear properties in polymer materials, they have to be prepared with required noncentrosymmetry, which is usually accomplished by poling techniques. Some years ago it was demonstrated that the all optical poling technique permits purely optical orientation of molecules and thus induction of the large second order susceptibilities χ(2) [1, 8]. The important advantage of the all-optical technique is that it leads to a periodic ordering of the molecules with the spatial period fulfilling automatically the phase matching condition for second harmonic generation (SHG) [8]. The experiment consists of two phases: the writing (so called ”seeding”) period and the readout one.

All Optical Poling in Polymers and Applications

ABSTRACT The self-induced generation of a second harmonic light was first observed by Margulis and Osterberg 1 in an optical fiber illuminated with intense light at 1.06 μm, and later illuminated at the same time with the beam at the fundamental frequency (1.06 μm) and at 0.532 μm (second harmonic, SH), revealing a way of inducing a second-order nonlinear susceptibility χ (2) by a purely optical method in a centrosymmetric material. However, in the case of the optical fiber, the induced nonlinearities remain relatively small. The induction of noncentrosymmetry in azo-dye materials has been an object of extensive studies 2-5 because of their various potential applications in electro-optic devices and integrated optics, as they possess large second-order nonlinear properties. Side-chain polymer matrices containing organic moieties with large second-order polarizabilities s combine the possibility of inducing significant nonlinearities and the ease of processing of polymers. Structuring materials with nonlinear optical properties in a way that the nonlinearity is modulated spatially in them on a wavelength scale permits us to assure phase matching between light waves and opens up the possibility of making wave guides with frequency doubling properties. It may also lead to the development of new technologies for the production of blue coherent light sources. Organic thin films having large χ (2) nonlinearities are now extensively studied to optimize all optical poling and phase-matching conditions (optimization of the interaction length) to obtain cheap, easy-to-process and fabricate materials that will be used in tuneable optical devices providing the light at any desired wavelength. Spontaneous orientation is not a natural tendency for most molecules, and so the main difficulty lies in the realization of noncentrosymmetric structures. After a brief review of the different standard poling techniques, we present a detailed description of the optical poling technique. This technique is based on a purely optical process enabling us to take full advantage of the rich processing capabilities of optical tools. This technique offers broad possibilities for phase-matching conditions, and it enlarges the achievable poled geometries, leading to the possibility of a full control of the induced symmetry of the macroscopic second-order susceptibility χ (2) .