Sukant K. Tripathy

Surface relief structures on azo polymer films

All optical fabrication of Surface Relief Grating (SRG) on azobenzene functionalized polymer films is reviewed in this article. The uniqueness of the single step erasable photofabrication of large amplitude surface relief structures on azo polymer films is mainly due to the photoisomerization and photoanisotropic behavior of the azobenzene group. The design and synthesis of several classes of suitable azobenzene functionalized polymers for the SRG formation is presented. Experimental studies are carried out in a systematic manner to understand the chemical and physical intricacies involved in the SRG formation process. Structural requirements and their limitations for the efficient fabrication of SRGs on azo functionalized polymer films are also discussed. Understanding the fundamental mechanism of SRG formation on the azo functionalized polymer films enables us to make use of these structures in a variety of applications.

Gradient force: The mechanism for surface relief grating formation in azobenzene functionalized polymers

A model for the formation of holographic surface relief gratings in azobenzene functionalized polymers is presented. Forces leading to migration of polymer chains upon exposure to light in the absorption band of the azo chromophore are attributed to dipoles interacting with the gradient of the electric field present in the polymer material. Efficient trans–cis cycling in the azobenzenes allows cooperative movement of the chromophores under the influence of gradient forces.

Photoinduced surface deformations on azobenzene polymer films

Laser-induced surface deformation and surface relief grating formation at different intensity levels on azobenzene-functionalized polymer films were investigated. The surface deformation profile induced by a Gaussian laser beam at low intensities reveals strong polarization dependence and establishes that an optical-field component in the direction of the optical-field gradient is essential for this deformation process. A theoretical analysis based on the optical-field gradient force model is presented to account for the deformation behavior at low intensities. In the higher-intensity regime, however, a different mechanism is found to dominate the surface deformation process. Simultaneous formation of two different types of topographic gratings is observed in a two-beam interference experiment. The phase relationship between these surface relief gratings and the interference intensity pattern was deduced based on the results from the single-beam experiments. The erasing behaviors of the two distinct surfa...

A Detailed Investigation of the Polarization-Dependent Surface-Relief-Grating Formation Process on Azo Polymer Films

We have carried out two-beam interference experiments to form surface-relief gratings (SRGs) on azo polymer films under different polarization combinations. The mechanism of SRG formation on azobenzene chromophore functionalized polymer films has been shown to depend on both the spatial variation of the intensity as well as the polarization state of the incident field pattern. The treatment presented in this paper clearly defines the intensity and polarization distribution across the two-beam interference region for different polarization configurations of the interfering beams. The efficient photoisomerization process due to the spatial variation of the optical field renders the material anisotropic and the surface mobile. The simultaneous presence of a component of electric field gradient in the grating vector direction results in large-modulation-depth SRGs. We have carried out this investigation in one class of prototypical amorphous epoxy-based azo polymer thin films. Our results confirm that efficient SRG formation on azo functionalized polymer film requires photoinduced anisotropy and a component of optical field gradient in the direction of the grating vector.

Intensity‐dependent index of refraction in multilayers of polydiacetylene

We have grown multilayer samples of a polydiacetylene upon a metallized grating forming a planar waveguide structure. By measuring the change with optical intensity in the coupling angle between an input laser beam and a planar waveguide mode in the structure we estimate the intensity‐dependent index of refraction, n2≂10−6 (MW/cm2)−1 (λ>7500 A), for the polydiacetylene. Also, a large resonant enhancement of the optical nonlinearity was observed.

Cross‐linked stable second‐order nonlinear optical polymer by photochemical reaction

A new method for obtaining cross‐linked second‐order nonlinear optical polymer by photochemical reaction has been described. An active dye was processed and poled in a photoreactive polymer in a manner similar to doped polymer system, and photo‐cross‐linked in the poled phase by ultraviolet irradiation. The dye doped (20% by weight) cross‐linked polymer has a nonlinear optical coefficient d33 of 5.1 pm/V at 1.540 μm and shows no relaxation after being subjected to thermal treatment at 60 °C for 0.5 h.

Photoinduced surface relief gratings in high‐Tg main‐chain azoaromatic polymer films

We have synthesized two classes of polyureas with mono- and bisazoaromatic groups in their main chains via reactions between isophorone diisocyanate and the corresponding diamines. Holographic gratings were fabricated on azoaromatic polyurea films prepared by spin-coating from solutions. The effect of high glass transition temperature and dipole moment of azo groups on the formation of gratings was investigated. Although the two polymers have relatively high glass transition temperatures (197 and 236°C), chromophore alignment was induced by laser beam irradiation at modest light intensities. Regularly spaced surface relief gratings on the polymer film were also recorded upon exposure to an interference pattern of two polarized argon laser beams. Erasure could be achieved by heating above Tg or by exposure to one of the beams in a manner similar to low-Tg side-chain azo polymers.

A simple experiment for determining Verdet constants using alternating current magnetic fields

A simple experiment suitable for a senior undergraduate and graduate laboratory on the measurement of Faraday rotation using ac magnetic fields is described. The apparatus was used to measure the wavelength dependence of Verdet constants for several materials. A concentration-dependent study on ferric chloride water solution was also carried out to demonstrate that the diamagnetic and paramagnetic contributions to Faraday rotation have opposite signs.

Surface relief gratings from electrostatically layered azo dye films

Surface relief gratings (SRGs) were fabricated on composite films assembled by alternate electrostatic deposition of a polyelectrolyte, poly(dimethyldiallylammonium chloride), and an organic azo dye, congo red. The modulation of SRGs was found to increase with the thickness of the matrix films. Significant photochemical bleaching of the azo dye in the polymeric microenvironment as well as gradient-force-induced migration of the small azo dye contributes to the formation of the SRG structure. This finding demonstrates a facile method to fabricate SRGs for optical and information storage applications using commercially available azo dyes and polyelectrolytes.

ATOMISTIC MODELING OF INTERPHASES IN SPIDER SILK FIBERS

We present a structural description of the intercrystalline polymer interphase based on atomistic modeling for spider dragline silk fibers. In the model small crystallites are connected by interphases. The interphases are assumed to be thin layers (< 4 nm) of statistically arranged chains. The calculated mechanical moduli of these interphases are significantly greater than the isotropic amorphous material.