Light Propagation in Photorefractive Polymers

1. Light Propagation in Photorefractive Polymers hn E 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

2. Talk Outline • The Photorefractive Effect and solitons • Polymeric solitons are possible • Characterization of soliton formation • Preliminary results! • Wave guidance • Beam bursting and the

3. 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

4. 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 Study of nonlinear Optics

5. Diffraction 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. Broadening due to diffraction.

6. Spatial Soliton The photorefractive effect Nonlinear medium: Photorefractive (PR) EffectIn our case, the presence of light modifies the refractive index (via orientationally enhanced birefringence) to give a non- uniform refractive index change Dn. Self-focusingThis 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. Narrowing of a light beam through a nonlinear effect.

7. Can PR polymers support solitons? • It was suggested that solitons might be formed in PR polymers... Diffracting ITO-coated glass 55 mm x Conducting polymer z ITO-coated glass No voltage applied y x y l=780nm at 24mW 2.5mm Self-focusing 120 m m 12 mm 2.0 kV applied across sample • We have shown that soliton formation does occur in PR polymers!

8. Experimental setup In our experiment, a 780-nm laser diode at 24-mW was used with a half-wave plate to rotate polarization. Cylindrical lens l/2 plate Laser Polarizer CCD Collimation Polymer sample The beam propagates through the sample while a voltage is applied between the ITO electrodes of the sample to induced self-focusing.

9. Experimental results: Optical switching Self-focusing occurs when the laser beam is horizontally (y-axis) 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 2.0 kV applied 0.0 kV applied (Vertical Polarization) x Input face Output: Diffraction Output: Self-defocusing y

10. Experimental results: Soliton data Self-defocusing 55mm Conducting polymer 80 mm Vertical polarization Self-focusing Conducting polymer 12 mm x z Horizontal polarization y x y • We have shown that soliton formation does occur in PR polymers!

11. 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

12. There is a critical value of applied dc bias field that favors soliton formation for a given laser power. Experimental results: Variable bias field 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.

13. Experimentalresults:Soliton formation time 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.

14. 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).

15. Comparison of different material classes multiple quantum wells fast response  expensive  large absorption  narrow window of l  inorganic crystals thick samples  good optical quality  only doping variable  expensive  polymers / organic glasses cheap  variable composition  large external E-field  stability  liquid crystals cheap  variable composition  small external E-field  scattering / thin samples 