1. Light Propagation in Photorefractive Polymershn 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. 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

4. 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...

5. 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.

6. 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 non- uniform 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.

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
l=780nm
at 24mW y
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
0.0 kV applied
2.0 kV applied
(Vertical
Polarization) x
Input face
Output: Diffraction
Output: Self-defocusing y

10. Experimental results: Soliton data
Self-defocusing
Conducting polymer
55 mm
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. 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 kV kV 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. Experimental results: Soliton formation time
The response time is how fast the index change occurs . With a very high bias field, soliton formation occurs in seconds.
The response time is both a function of the applied field and the
beam power.

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