Paul Rochon

Optically induced surface gratings on azoaromatic polymer films

The surface of an azoaromatic polymer film is optically altered to produce local highly efficient diffraction gratings. The gratings obtained are stable but can be erased by heating the polymer above its glass transition temperature and no permanent damage of the film is observed. Multiple gratings can be simultaneously written and gratings can be overwritten. Atomic force microscopy was used to investigate the gratings produced on the surfaces. Possible mechanisms responsible for the surface alteration are discussed.

Optically induced and erased birefringence and dichroism in azoaromatic polymers

Dichroism and birefringence are shown to be optically induced and erased in high‐glass‐transition azoaromatic polymers. The resulting polarization information is easily detected and exhibits long‐term stability. This optically induced reorientation of the azoaromatic molecules will have wide applications in image recording and in electro‐optical devices.

Model of laser-driven mass transport in thin films of dye-functionalized polymers

Based on Newtonian fluid dynamic relations, a model is constructed to describe laser-induced mass transport in thin films of polymers containing isomerizable azobenzene chromophores, in which surface profile diffraction gratings can be inscribed with an interference pattern of coherent light. The Navier–Stokes equations for laminar flow of a viscous fluid are developed to relate velocity components in the film to pressure gradients in the polymer film, by definition of boundary layer conditions. This general laminar flow model is applicable to the formation of surface gratings through a variety of mechanisms. Considering the mechanism of an isomerization-driven free volume expansion to produce internal pressure gradients, a specific model is developed to describe polymer flow resulting from laser-induced isomerization of the bulky chromophores. This yields an expression relating the time evolution of the surface gratings to properties which could be varied experimentally, such as those of the irradiating ...

Light-induced motions in azobenzene-containing polymers

The following article is a tribute to the late Almeria Natansohn and is based on a brief summary of her research in azopolymers. She showed that reversible trans–cis–trans photoisomerization of aromatic azo groups covalently bonded within polymers could trigger a variety of motions in the polymer materials at molecular, nanometer, and micrometer levels. The photoinduced motions could be limited only to the azo rigid chromophore or could involve many polymer chains and ordered domains. Some of the effects of these motions such as reversible photo-orientation of chromophores, amplification effects, photorefractive effects, formation of surface relief gratings (SRGs), and photoinduced chirality and switching in amorphous and liquid-crystalline (LC) polymer films are discussed in relation to the polymer structure and physical parameters. Possible photonic applications originating from these phenomena are also mentioned.

Self-assembly of colloidal spheres on patterned substrates

We report here on the self-assembly of polystyrene colloidal spheres on patterned substrates. The substrate was a grating with a periodic one-dimensional-height profile. The two-dimensional (2D) arrays of colloidal spheres were obtained by the convective self-assembly method. It is shown that these structures strongly depend on the ratio between the diameter of the sphere (d) and the period of the grating (p) (x=d/p). When 1<x<1.15, and 1.15<x<2, the 2D array showed a centered-rectangular symmetry. For x=1.15 and 2, the ordered 2D array had a hexagonal symmetry structure. The angle between the grating groove direction and the crystal lattice vector (θ) is given by θ=sin−1(d/p). It is suggested that the variety of the 2D structures are formed by the capillary forces and the self-shadowing effects during the drying process.

Azo polymers for reversible optical storage: 13. Photoorientation of rigid side groups containing two azo bonds

Abstract Copolymers of methyl methacrylate with a methacrylate containing a rigid group with two azo bonds (3RM) were prepared and their photoinduced birefringence levels and rates studied. Birefringence levels of 0.11 for the copolymer with 11.6 mol% azo structural units and 0.13 for the copolymer with 30.0 mol% azo structural units were found; this is higher than the birefringence inducible in a typical azo homopolymer containing a chromophore with only one azo group, poly{4′-[(2-(acryloyloxy)ethyl)ethylamino]-4-nitroazobenzene} [poly(DRIA)]. The birefringence per azo structural unit for a copolymer containing 11.6 mol% 3RM is about five times that for a DRIA copolymer with similar azo content, because of the intrinsic structural properties of 3RM (high length/diameter ratio). Dichroism in both u.v. and visible regions of the spectrum contribute to the overall photoinduced birefringence. The rate of inducing birefringence in the 3RM copolymers is lower than in poly(DRIA) and the birefringence stability (91–96% of the induced birefringence is maintained after the writing laser is off) is much better than that for poly(DRIA) (about 80%). The good stability and slow birefringence growth rate are due to the lesser mobility of the larger side group.

Polarization analysis of diffracted orders from a birefringence grating recorded on azobenzene containing polymer

Polarization analyses of transmitted and first-diffracted orders have been performed on photoinduced gratings in azopolymers. The polarization states of diffracted orders depend strongly on irradiation time because of the contribution from both a phase grating at short time and a surface relief grating at longer time. These results are nicely explained using a Jones’ matrix approach which predicts the polarization states of transmitted and diffracted orders of polarization holographic gratings.

Optically inscribed surface relief diffraction gratings on azobenzene‐containing polymers for coupling light into slab waveguides

Surface relief diffraction gratings inscribed on azobenzene‐containing polymers are designed to act as light couplers into slab waveguides. The gratings are inscribed both directly in poly{4‐[2‐(methacryloyloxy)ethyl]azobenzene} (pMEA) waveguide material, and in an azopolymer cladding layer applied to polyimide, silicon nitride, and silicon oxynitride films. Input and output coupling of both 633 and 830 nm light is observed in the waveguides.

Guided mode resonance filters using polymer films

Guided-mode resonance optical filters are made from thin-film structures which consist of azopolymer films deposited on top of slab waveguides. Surface relief gratings can be optically induced on the azopolymer films. The transmission and reflection spectra as a function of the angle of incidence of the probe beam exhibit very sharp resonance features which correspond to coupling into the waveguide modes. Coupling into the overlying azopolymer film is also observed.

Photoinduced liquid crystal alignment based on a surface relief grating in an assembled cell

Surface relief gratings have been optically inscribed on azopolymer films on the interior surface of an empty liquid crystal (LC) cell. Uniform LC alignment is observed in the inscribed regions when the cell is filled. Since the microgrooves are on both sides, this may provide a higher degree of order than other techniques. The stability of the relief gratings and the transmittance of the azopolymer films can be further improved by photobleaching. This alignment technique could have useful applications in the in-plane switching LC displays.