Photo-induced non-linear susceptibility patterns in electro-optic polymers G. Martin, E. Toussaere * , L. Soulier, J. Zyss Laboratoire de Photonique Quantique et Mole Âculaire, UMR 8537 ENS-Cachan, 61 Av. Pre Âsident Wilson, 94235 Cachan, France Abstract We present microscopic scale photo-induced non-linear susceptibility patterns in organic materials. This patterns are observed by re¯ection second harmonic generation microscopy and atomic force microscopy. The characteristics of the non-linear susceptibility tensor are engineered during the active ®lm preparation. Several techniques have been used to pattern the non-linear response: re-alignment through a chrome mask of previously electrically poled samples, Lloyd-mirror interferometric con®guration to imprint any desired susceptibility periodicity in the sample and all optical poling using mutually coherent beams from a frequency-doubled picosecond Nd:Yag laser. Images of different scanned patterns are shown, direct evidence of periodic structures, two-dimensional patterns and applications to 2D photonic crystals are discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microscopy; Non-linear optical materials; Periodic structures; SHG-mapping 1. Introduction Non-linear optical polymers are nowadays recognized as materials for processing electro-optic devices such as mod- ulators or switches for optical telecommunications [1,2]. Their potential is, however, not limited to that applications and extends also to wavelength conversion in integrated optics. The ef®ciency of non-linear effects in bulk materials and integrated optics is, however, limited by the phase mismatch between the different waves interacting in the medium. In the particular case of second harmonic genera- tion, matching the phases of the fundamental and of the harmonic ®eld is critical when one only uses the anisotropy of the material. An alternative way to the ideal phase- matching is the introduction in the material of a periodic modulation of the susceptibility tensor, with a periodicity equal to the coherence length of the interacting waves inside the material [3]. To achieve this modulation, electro-optic polymers are attractive materials due to their compatibility with integrated optics and their ¯exibility, which enables to control both the magnitude and the symmetry of their non- linear susceptibility tensor. Several techniques are known to give the desired pattern- ing of the macroscopic susceptibility tensor, such as photo- bleaching or periodic electrode poling [4±6], one photon induced depolarization [7] or all optical poling through a multiphotonic process [8]. In this work, we will focus on the optical photo-reorientation of the molecules, taking advan- tage of the photochemical sensitivity of the chromophore, in order to modulate the w 2) susceptibility periodically. A patterned irradiation, de®ned through a grating or by inter- ference at speci®c wavelengths, can be transferred to the sample, resulting into a variation of the susceptibility and the thickness only in the illuminated areas, while the dark ones will keep their original conformation. This technique can be applied to the periodic modulation of the non-linear optical susceptibility tensor, and resulting patterns can be mapped by second harmonic microscopy [9] or by atomic force microscopy AFM). 2. Centrosymmetric and axial orientation of azo dyes Azo dyes, among which disperse red one DR1) is often used as a bench-mark chromophore in non-linear optics, are well known for their optically induced orientational proper- ties which are often referred to as the Weigert effect. Basically, such dyes exhibit reversible trans±cis isomeriza- tion under light activation. During this cycles, the dyes may reorient themselves before going back to their ground state, thus resulting after statistical averaging in time) on a net orientational order where the transition dipole of the mole- cules minimize their interaction with the polarized or unpolarized) light beams. Such effects have promoted their use in holographic storage media [10], where the informa- tion is stored as a molecular order inside a polymeric sheet. The reorientational properties are also known to promote Synthetic Metals 127 2002) 49±52 * Corresponding author. Tel.: 33-14740-5552; fax: 33-14740-5567. E-mail address: eric.toussaere@lpqm.ens-cachan.fr E. Toussaere). 0379-6779/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0379-677901)00589-6