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Light Propagation in Photorefractive Polymers O O O N hn O N E CN CN CN CN charge generation orientation of chromophore transport trapping M. Asaro and M. Sheldon Department of Physics and Astronomy San Francisco State University Thesis Advisor: Z. Chen *Chemical Synthesis: Stanford University Talk Outline •The Photorefractive Effect and solitons •Polymeric solitons are possible •Characterization of soliton formation •Preliminary results! Wave guidance Beam bursting and the The Study of nonlinear Optics In the regime of conventional (linear) optics, the electric polarization induced in the medium, the electric polarization vector, P, is assumed to be linearly proportional to the electric field E of an applied optical wave: P=εoc(1)E . In this linear medium the refractive index n0 is a constant independent of beam intensity for a given l. When an intense laser beam interacts with an optical medium new effects arise that can be explained if the linear term in P can be replaced by a power series P=εo(c(1) + c(2)E1 + c(3)E2 +…)E . The Study of nonlinear Optics Materials are “nonlinear” when they exhibit higher order susceptibilities, such as c(2)… The study of NLO is concerned with the effects that light itself induces as it propagates through a medium. The invention of the laser permitted new ways of investigating the optical properties of materials.Thus, many new nonlinear effects were discovered: -second harmonic generation (SHG) -third harmonic generation (THG) -self-focusing... The photorefractive effect Self-focusing is a result of the photorefractive effect in a nonlinear optical material... Linear medium (no photorefractive effect): Narrow optical beams propagate w/o affecting the properties of the medium. Optical waves tend to broaden with distance and naturally diffract. Diffraction Broadening due to diffraction. The photorefractive effect Nonlinear medium: Photorefractive (PR) Effect In our case, the presence of light modifies the refractive index (via orientationally enhanced birefringence) to give a nonuniform refractive index change Dn. Self-focusing This index change acts like a lens to the light and so the beam focuses. When the self-focusing exactly compensates for the diffraction of the beam we get a soliton. Spatial Soliton Narrowing of a light beam through a nonlinear effect. Can PR polymers support solitons? •It was suggested that solitons might be formed in PR polymers... Diffracting x z y ITO-coated glass Conducting polymer ITO-coated glass 55 mm No voltage applied x l=780nm at 24mW y 2.5mm 120 m m Self-focusing 12 mm 2.0 kV applied across sample •We have shown that soliton formation does occur in PR polymers! Experimental setup In our experiment, a 780-nm laser diode at 24-mW was used with a half-wave plate to rotate polarization. l/2 plate Cylindrical lens Laser Collimation Polarizer Polymer sample CCD The beam propagates through the sample while a voltage is applied between the ITO electrodes of the sample to induced selffocusing. Experimental results: Optical switching Self-focusing occurs when the laser beam is horizontally (yaxis) polarized; a negative index change. Defocusing occurs when the beam is vertically (x-axis) polarized. Input face Output: Diffraction Output: Self-focusing (Horizontal Polarization) 12 mm 0.0 kV applied 0.0 kV applied 2.0 kV applied (Vertical Polarization) x y Input face Output: Diffraction Output: Self-defocusing Experimental results: Soliton data Self-defocusing Conducting polymer 55 mm 80 mm Vertical polarization Self-focusing 12 mm Conducting polymer x z Horizontal polarization y x y •We have shown that soliton formation does occur in PR polymers! Experimental results: Soliton stability At 0 seconds voltage was applied Focus 150 seconds later 500 seconds (decay) Defocus • Soliton formation from self-trapping occurred 160 sec after a 2.0 kV field was applied. The soliton was stable for more than 100 seconds and then decayed. • Self-defocusing exhibited a similar temporal behavior Experimental results: Variable bias field There is a critical value of applied dc bias field that favors soliton formation for a given laser power. Nonlinearity increases as voltage increases 0.0 kV 1.0 kV 2.0 kV 3.0 kV •If the field is too low only partial focusing occurs. •If the field is too strong, the nonlinearity is too high so the beam breaks up. Experimental results: Soliton formation time 350 1200 300 1000 800 Time (s) Time (s) 250 200 150 600 100 400 50 200 0 0 10 20 Applied field (V/mm ) 30 40 0 0 10 20 30 40 Beam power (mW) The response time is both a function of the applied field and the beam power. The response time is how fast the index change occurs . With a very high bias field, soliton formation occurs in seconds. Conclusion First observation of a soliton in an organic PR polymer. Self-focusing to -defocusing switching occurs by just changing polarization from Horizontal To Vertical. It is independent of polarity. Significance of results; PR polymers are cheaper and easier to dope than the popular PR crystals. Thus, important soliton based applications can now be tested on PR polymers because of our first observation of soliton formation. Z. Chen, M. Asaro et al., to appear, Phys. Rev. Lett. (2003). Comparison of different material classes multiple quantum wells inorganic crystals thick samples good optical quality only doping variable expensive polymers / organic glasses cheap variable composition large external E-field stability fast response expensive large absorption narrow window of l liquid crystals cheap variable composition small external E-field scattering / thin samples