Length-scale dependence of surface relief gratings in azobenzene side-chain polymers B.M. Schulz a , M.R. Huber a , T. Bieringer b , G. Krausch c , S.J. Zilker a,* a Physikalisches Institut und Bayreuther Institut fu Èr Makromoleku Èlforschung, University of Bayreuth, D-95440 Bayreuth, Germany b Bayer AG, D-51368 Leverkusen, Germany c Physikalische Chemie II und Bayreuther Zentrum fu Èr Kolloide und Grenz¯a Èchen, University of Bayreuth, D-95440 Bayreuth, Germany Abstract Polarizedirradiationofazobenzeneside-chainpolymersyieldsintriguingsurfacereliefstructures.Uptonow,thephysicalmechanismof this process is unclear. We report on a study aimed at clarifying the intrinsic length scales of the substantial mass transport which evolves duringlaserirradiation.Wevarytheperiodofanincidentintensitygratingbymorethananorderofmagnitude,andinvestigatetheresulting relief amplitudes with atomic force microscopy. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Surface gratings; Azobenzene copolymer; Holography 1. Introduction Azobenzene side-chain polymers are widely investigated due to potential applications in optical data storage and processing [1±3]. Their mechanism is based on a reorienta- tion of the azobenzene side-chains under irradiation with polarizedlight.Theazomoleculesundergoreversiblelight- induced trans±cis±trans isomerization cycles which pro- mote a reorientation of the chromophore's transition dipole moment in a direction perpendicular to the polarization vector of the light. This process can be used to inscribe stable birefringence gratings with refractive index modula- tions of up to 0.2 [1]. Atomic force microscopy AFM) revealed for the ®rst time in 1995 that the volume reorientation is accompanied by a substantial mass transport in the material [4,5]. As a result, surface relief structures are observed. The origin of this effect, which occurs even in polymers with high glass transition temperatures at low irradiation intensities, is so farnotclear.Severalmodelshavebeenputforward,thetwo most promising ones are a mean-®eld ansatz by Pedersen etal.[6]andagradientforcemodelbyKumaretal.[7].The mean-®eldapproachisbasedonanattractiveinteractionof chromophores which are oriented parallel to each other. The gradient force model assumes that a gradient of the electric ®eld of the incident irradiation exerts an optical gradientforceontheazobenzeneunits.Othermodelswere suggested by Barrett et al. [8], and Nunzi and coworkers [9]. The ®rst one assumes that the isomerization reaction creates internal pressure gradients leading to mass trans- port. In the second model, one assumes an anisotropic translational diffusion of the azobenzenes in a direction parallel to the polarization of the writing beams. None of themodelscanexplainalltheeffectswhichwereobserved experimentally. However, the mean-®eld approach works well in liquid crystalline polymers, whereas the gradient force picture describes most observations in amorphous systems[10].Inanefforttoclarifythegenerationofsurface reliefs,wehavesystematicallyinvestigatedthedependence ofthereliefamplitudeontheperiod L oftheincidentlight intensity grating. The resulting surface structures were characterized by AFM. 2. Experimental The holographic experiments were performed in a setup whichisoptimizedforprovidingasimpleadjustmentofthe angle between the two p-polarized writing beams, yielding different grating periods. The p-polarization was used because it generates reliefs very effectively. The details of the optical path have been published previously [11]. The writingbeamsoriginatefromafrequency-doubledNd-YAG laser which emits cw light at a wavelength of 532 nm. The Synthetic Metals 124 2001) 155±157 * Corresponding author. Tel.: 31-40-274222; fax: 31-40-274335. E-mail address: zilker@philips.com S.J. Zilker). 0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0379-677901)00457-X