Aditya Ranade

A widely tunable refractive index in a nanolayered photonic material

A class of composite polymer films is described in which the refractive index can be varied by simple mechanical forces. The films are comprised of 1024 alternating layers of an elastomer and a glassy polymer. When the layer spacing is much less than the wavelength of the probe light, these materials behave as effective medium composites. The layer thickness of the elastomer component and thus the effective index of the composite can be varied by compressing the composite.

Structure-property relationships in coextruded foam/film microlayers

In this work, we have shown for the first time that assemblies of up to 64 alternating foam and film layers can be successfully made via the microlayer coextrusion technology. We have investigated the effect of some important processing variables on the foam-film structure, and have demonstrated how they can be optimized. With the optimum concentration of the right type of chemical blowing agent, the cell size can be significantly reduced for the polypropylene foam-film systems by increasing the number of layers. The foam layer density does not change significantly with increasing number of layers. Measurements of the cell density and cell volume indicate that enhanced nucleation and/or reduced coalescence is responsible for the reduction in cell size with an increasing number of layers. Constrained cell growth comes into play when a single cell/layer morphology is observed. Different types of polymers can be used as the film layer to alter the flexibility of the foam/film composite. The compressive behavior of the hard/hard polypropylene foam/polypropylene film system is similar to that of cork in terms of the compressive modulus, collapse stress, and densification strain. The tensile and compressive moduli increase with the increasing number of layers and this could be explained on the basis of an increase in the fraction of material in the cell walls, using the conventional Gibson-Ashby model. The soft/soft polyethylene foam/ethylene-styrene copolymer film system exhibits enhanced toughness in tension. The effect of the foam/film composition on the tensile and compressive moduli can be explained using the composite series and parallel models.

The refractive index of dye/polymer composite films

Designing photonic devices using organic nonlinear dye/polymer composites requires an accurate determination of the index. A model of the wavelength and concentration dependence of the index that includes the density dependence is given.

Nonlinear Optics in 1D Polymer Structures

Nonlinear dielectric optical polymer structures that behave as 1D photonic crystals with some disorder are readily fabricated. These materials can perform useful optical functions: an intensity dependent nonlinear band gap is demonstrated.

Characterization of dye-doped polymer films for polymer 1D photonic crystals

The incorporation of a nonlinear dye into a nanolayered polymer structure has the ability to greatly enhance the nonlinear transmission of the dye. Several characteristics of the nanolayered structured were studied to optimize the effect.

Variable refractive index polymeric nanolayered films

A new class of polymer films comprised of alternating elastomeric and glassy polymer nanolayers is described. Applying pressure to the composite can reversibly vary the refractive index. The mechanism for the index change was investigated by reflectance measurements, which were also used to assess the modulus of the elastomeric nanolayer.