Transcript Self-focusing - SFSU Physics & Astronomy

Light Propagation in Photorefractive
Polymers
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charge
generation
orientation of
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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
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polymers / organic glasses
cheap
variable composition
large external E-field
stability
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fast response
expensive
large absorption
narrow window of l
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liquid crystals
cheap
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variable composition 
small external E-field 
scattering / thin samples 