Electro-optical properties in relaxor ferroelectric materials and the device applications
Abstract (Summary)iii The electro-optic (E-O) effects of the relaxor ferroelectric materials, Lead Magnesium Niobate-Lead Titanate Pb(Mg1/3Nb2/3)O3 ? PbTiO3 (PMN ? PT) single crystals and poly(vinylidene fluoride-trifluoroethylene) [P(VDF ? TrFE)] based polymers, are investigated here. The tetragonal 0.62PMN-0.38PT single crystals poled along the <001> direction (the spontaneous polarization direction) have a stable single domain and show high transparency from the visible to Near-IR range. Using the Mach-Zender interferometry method, large linear E-O coefficients r33 = 70 pm/V, r31 = 25 pm/V, and r15 = 558 pm/V were characterized. P(VDF-TrFE) based terpolymers shows a large Kerr effect where a refractive index change of -2.6 % can be induced under an electric field of 80 V/µm. When combined with the electrostrictive strain, the terpolymer film exhibits a total -5.6 % optical pathlength change under a field of 80 V/µm. Calculations based on density functional theory suggest that such a large E-O effect was caused mainly by the reorientation of the C-F dipoles in the crystalline regions under external field. With the large strain and E-O effect, the tunable graing and Fabry-Perot interferometers (FPIs) were fabricated. By changing the structure of comb-shape electrode for the bottom electrode and the polymer thickness, it was calculated that we could control the electric field distribution leading the different level of strain for grating. This new tunable grating has the advantage of simple fabrication and easy integration. In our first experimental demonstration, 24 % of the first order diffraction efficiency was observed with 100 V/µm. For the strain tunable FPI, which for the terpolymer films under mechanically clamped condition is 1.3 % under 100 V/µm field, we show that a tunable range of 22.5 nm can be obtained at wavelengths near 1.5 µm. On the other hand, the FPI using a terpolymer film directly as the cavity of the etalon shows a smaller tunability (0.78 % under 100 V/µm)due to the opposite sign of the strain and refractive index change. In addition, since the conventional photolithograph technique damages the P(VDF-TrFE) based polymers, stencil mask method was tested to fabricate a strain tunable frequency selective surface. iv In addition to the materials themselves, high performance tunable devices can be accomplished from the artificial periodic structures, so called Photonic Crystals (PCs). Near bandedge of PCs, optical properties can be very sensitive to the changes of the unit cell dimension and the refractive index of the constituents of the PCs. When calculating the optical properties in PCs, most calculations are based on the infinite size of PCs. However, real deivces should have finite size and their propertiese are different from those of the infinite size of PCs. To predict the optical properties in real PCs device, we compared the light propagation properties between infinitely extended and finite size dielectric multilayer layer in terms of photonic crystals (PCs) along the wave propagation direction. It is shown that the multiple reflections at the two end interfaces of a finite size PC give an influence on both the transmittance and phase velocity (and also the group velocity as well as the effective refractive indexes ne f f ). The results clearly demonstrate the importance of the two end interfaces of a finite size PC on the effective properties of photonic crystals, which could be quite different from those deduced from an infinitely extended photonic crystal. For device applications, finite size effect was applied to design the photonic crystals lenses.
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication: