Circularly Polarized Light-Induced Homeotropic Alignment in Azo Dye Doped PMMA Polymer films

Azobenzene-containing polymers have attracted much attention for the fabrication of submicrometer structures for Optical Data Storage, All-Optical Switching and nanotechnologies. The basic mechanism involves the reversible... [ view full abstract ]

Azobenzene-containing polymers have attracted much attention for the fabrication of submicrometer structures for Optical Data Storage, All-Optical Switching and nanotechnologies. The basic mechanism involves the reversible trans-cis photoisomerization reaction which can be interconvert by light and heat. Various azobenzenes have been used as dopants with transparent polymers to form guest-host films and change polymer's properties. In this communication, we present results concerning the photoinduced anisotropy of refractive index changes into initially random AZD1/PMMA guest-host films excited by linearly and circularly polarized pump beams at normal incidence. The absorption spectrum of this guest host system are depicted in Figure 1. Molecular orientation and subsequent photoinduced birefringence in azobenzene doped polymer films were investigated by pumping beams with various intensities and polarization states. A green Argon ion laser pump beam (λ=514,5 nm) at normal incidence with the light intensity and polarization as the control parameter to exposure and an He-Ne laser probe polarized beam (λ=632,8 nm) was used. The mechanism of the energy absorbed by azo-dye molecules for isomerization is known as angular hole burning. Changes in the parallel and vertical polarization components due to the photoinduced birefringence were measured simultaneously during pumping with a linearly an circularly polarized beam at 514,5 nm. To our knowledge, this is the first study of measuring the dynamic process of the refractive index dependence on circularly polarization pump. Dynamic reversible anisotropy of the exposure relaxation cycle in these azo-dye-doped polymer materials was observed (figure 2). The results recorded demonstrate that molecular orientation and anisotropy of refractive index depends on circular polarization of the probe beam. Furthermore, we showed that the direction of azo-dye molecules can be optically controlled from a plane to a line distribution along the wave vector of the pump beam. This work open a new way to azo-isomerization orientational alignment control and change in anisotropic refractive index of guest-host film of doped polymer matrix. The remaining challenge is to find good explanations to these dynamic changes in proprieties of these azopolymer films in function of polarization of the pump beam and prefereniel directional alignement subsequent to the reorientetional hole burning process.