Light propagation in photorefractive polymers
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Light Propagation in Photorefractive Polymers. h n. 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

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Light Propagation in Photorefractive Polymers

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Light propagation in photorefractive polymers

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


Light propagation in photorefractive polymers

Talk Outline

  • The Photorefractive Effect and solitons

  • Polymeric solitons are possible

  • Characterization of soliton formation

  • Preliminary results!

    • Wave guidance

    • Beam bursting and the


Light propagation in photorefractive polymers

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


Light propagation in photorefractive polymers

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


Light propagation in photorefractive polymers

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.


Light propagation in photorefractive polymers

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.


Can pr polymers support solitons

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!


Light propagation in photorefractive polymers

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.


Light propagation in photorefractive polymers

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


Experimental results soliton data

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!


Experimental results soliton stability

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


Light propagation in photorefractive polymers

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.


Experimental results soliton formation time

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.


Conclusion

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).


Light propagation in photorefractive polymers

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 


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