Light Propagation in Photorefractive Polymers

M. Asaro and M. Sheldon Department of Physics and Astronomy San Francisco State University

Thesis Advisor: Z. Chen

*Chemical Synthesis: Stanford University

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Talk Outline

•The Photorefractive Effect and solitons

•Polymeric solitons are possible

•Characterization of soliton formation

•Preliminary results!

Wave guidance

Beam bursting and the

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=εo(1)E .

In this linear medium the refractive index n0 is a constant independent of beam intensity for a given .

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((1) + (2)E1 + (3)E2 +…)E .

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=εo(1)E .

In this linear medium the refractive index n0 is a constant independent of beam intensity for a given .

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((1) + (2)E1 + (3)E2 +…)E .

The Study of nonlinear Optics

Materials are “nonlinear” when they exhibit higher Materials are “nonlinear” when they exhibit higher order susceptibilities, such as order susceptibilities, such as (2)(2)……

The study of NLO is concerned with the The study of NLO is concerned with the effects that effects that light itself induces as it propagates through a medium.light itself induces as it propagates through a medium.

The invention of the laser permitted new ways of The invention of the laser permitted new ways of investigating the optical properties of materials.Thus, investigating the optical properties of materials.Thus, many new nonlinear effects were discovered:many new nonlinear effects were discovered:

-second harmonic generation (SHG) -second harmonic generation (SHG) -third harmonic generation (THG) -third harmonic generation (THG)

- -self-focusingself-focusing......

Materials are “nonlinear” when they exhibit higher Materials are “nonlinear” when they exhibit higher order susceptibilities, such as order susceptibilities, such as (2)(2)……

The study of NLO is concerned with the The study of NLO is concerned with the effects that effects that light itself induces as it propagates through a medium.light itself induces as it propagates through a medium.

The invention of the laser permitted new ways of The invention of the laser permitted new ways of investigating the optical properties of materials.Thus, investigating the optical properties of materials.Thus, many new nonlinear effects were discovered:many new nonlinear effects were discovered:

-second harmonic generation (SHG) -second harmonic generation (SHG) -third harmonic generation (THG) -third harmonic generation (THG)

- -self-focusingself-focusing......

The Study of nonlinear Optics

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 non- uniform refractive index change n.

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?Can PR polymers support solitons?

Diffracting

Self-focusing

•It was suggested that solitons might be formed in PR polymers...

•We have shown that soliton formation does occur in PR polymers!

Conducting polymer

ITO-coated glass

ITO-coated glass

2.5mm

m

=780nmat 24mW

No voltage applied

2.0 kV applied across

sample

12 m

mx

y

z

x

y

In our experiment, a 780-nm laser diode at 24-mW was used with a half-wave plate to rotate polarization.

The beam propagates through the sample while a voltage is applied between the ITO electrodes of the sample to induced self-focusing.

Experimental setup

Laser

/2 plate

Collimation Polymer sample

Cylindrical lens

CCDPolarizer

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.

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.

Experimental results: Optical switching

x

yInput face

Input face Output: Diffraction

Output: Diffraction

Output: Self-focusing

12 m (Horizontal Polarization)

(Vertical Polarization)

0.0 kV applied 0.0 kV applied 2.0 kV applied

Output: Self-defocusing

Experimental results: Soliton dataExperimental results: Soliton data

Self-defocusing

Self-focusing

•We have shown that soliton formation does occur in PR polymers!

12 m

m

x

y

Conducting polymer

Vertical polarization

Conducting polymer

Horizontal polarization

x

y

z

55 m

• 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: Soliton stabilityExperimental results: Soliton stability

150 seconds later 500 seconds (decay)

At 0 seconds voltage was applied

Defocus

Focus

Nonlinearity increases as voltage increases

There is a critical value of applied dc bias field that favors soliton formation for a given laser power. There is a critical value of applied dc bias field that favors soliton formation for a given laser power.

Experimental results: Variable bias field

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 timeExperimental 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.

0

200

400

600

800

1000

1200

0 10 20 30 40

Beam power (mW)

Tim

e (

s)

0

50

100

150

200

250

300

350

0 10 20 30 40

Applied field (V/m)

Tim

e (s

)

ConclusionConclusion 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

inorganic crystals

thick samples good optical quality only doping variable expensive

multiple quantum wells

fast response expensive large absorption narrow window of

polymers / organic glasses

cheap variable composition large external E-field stability

liquid crystals

cheap variable composition small external E-field scattering / thin samples