Research Index / Ultrafast Technology / Electro-Optic Polymers
A two-pronged research thrust has been undertaken to improve the sensitivity and invasiveness properties of electro-optic probes using novel polymers from the group of Prof. David Martin within the Materials Science and Engineering Department at the University of Michigan. Successful progress in this effort would lead to improvements in time-domain electro-optic sampling and support the spatial electro-optic field-mapping program referred to here.
The first of these projects involved the characterization of the second-order nonlinear optical properties of polymer thin films. More specifically, the electro-optic properties of poly(gamma-benzyl-L-glutamate) and poly(hexylisocyanate) were investigated. Both of these polymers, PBLG and PHIC respectively, were attractive due to their large dipole moments. With a few exceptions, most polymers are inherently centrosymmetric and thus are not electro-optic. To remove this inherent centrosymmetry, a process known as electric-field poling is used. In this case, a recently developed technique was applied in which each polymer was first dissolved in an organic solvent, such as chloroform. The poling field was then applied as the solvent evaporated and the polymer solidified, and thus a non-centrosymmetry was induced as the polymer was aligned. The electro-optic coefficients of the polymer films, determined by a crossed polarization experiment, were found to range between 0.6 to 1.6 pm/V. These figures of merit are comparable to those of the GaAs probes currently utilized, but on account of a lower dielectric permittivity, more electric field should enter the polymer probe. It is also anticipated that these electro-optic coefficients can be increased to a much higher value, thus improving the sensitivity of electro-optic probing even further.
In addition to this work, the group has also investigated the properties of conductive polymers, which may take on important roles in structurally-flexible electronic circuitry that could be attached to the outside of curved surfaces or surfaces that move. This research involves the use of Time-Domain Terahertz Spectroscopy (THz-TDS). Since the typical scattering time of conductive polymers is on the order of 100s of femtoseconds, our THz-TDS system, with an operational bandwidth of ~1 THz, is a well-suited spectroscopic technique for this type of material. From the spectroscopic data, the material's refractive indices can be determined, and this information can then be used to determine the conductivity of the material and its mobility. We have collected THz spectroscopic data on the conductive polymer, poly(3,4-ethylenedioxythiophene), also known as PEDOT. We are currently finishing the development of a computer code to extract the material's properties from the experimental data. In the future, we plan to investigate the effect of doping on a conductive polymer's conductivity and to examine the relaxation dynamics of the material.
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