Solitons and Waveguides based on High Performance photorefractive glasses

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Solitons and Waveguides based on High
Performance photorefractive glasses
Marcus X. Asaro Department of Physics and
Astronomy San Francisco State University
Thesis advisor Zhigang Chen, San Francisco
State University
O. Ostroverkhova, W.E. Moerner, Stanford
University M. He, R.J. Twieg, Kent State
University
hn
E
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Outline

• Select review of linear optics
• Linear polarization
• Birefringence
• Nonlinear optics
• Linear electro-optic effect
• Band transport model
• Index change
• Soliton formation in Photorefractive (PR)
  crystals

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Outline

• New PR material
• DCDHF-based organic glass
• Orientational PR nonlinearity
• Experimental observations
• Focusing and defocusing cases
• Optically induced waveguides
• Disussion of other effects
• Conclusion

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Linear optics

• Optical phenomena commonly observed in nature
  such as reflection, refraction, and birefringence
  result from linear interactions with matter.
• In this conventional (linear) regime, the
  polarization induced in the medium is linearly
  proportional to the electric field E of an
  applied optical wave
• P eoc(1)E .

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Linear optics

• In a linear medium the refractive index n0 is a
  constant, independent of beam intensity for a
  given l.
• Also, different f of light encounter slightly
  different indices of refraction
• Given c, a description of the refractive index
  follows
• D eoE P eo(1c)E eoE
• ? e eo(1c) ? n2
  (1c)

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Linear optics

• Some materials have two values of n depending on
  the polarization of the light. These are called
  no and ne
• This property is called birefringence
• Birefringence (BR) occurs in anisotropic
  materials ? c-axis
• If an unpolarized beam propagates along
  c-axis-light does not split

Optic (c-) axis
e-ray
E
o-ray
Extraordinary ray
Ordinary ray k is (? to phase front)
now ? to D, not E. k is ? to both D and E
(D E) S is not to k
S is to k

o-wave feels isotropic medium
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Nonlinear optics

• Certain materials change their optical
  properties (such as n)
• when subjected to an intense applied
  electric field. This
• can be either an optical field (optical
  Kerr effect) or a DC
• field (electro-optic effect). We will focus
  on the second
• effect for this talk.
• ? The large applied field distorts the positions,
  orientations,
• or shapes of the molecules giving rise to
  polarizations that
• exhibit nonlinear behavior.
• P eo(c(1)E c(2)E2 c(3)E3 )
• PLinear Pnon-linear

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Nonlinear optics

• Electro-optic (EO) effect apply an electric
  field gt
• Result refractive index change-two forms
• (a) c(2) ? ?n ? E linear electro-optic or
  Pockels effect
• (b) c(3) ? ?n ? E2 quadratic
  electro-optic or DC Kerr effect
• c(2) process ?

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EO dielectrics? Photorefractive crystals

• Typical values are beam at mW/cm2, E10 V/?m
  
• ? ?n 10-4 - 10-6
• Noncentrosymmetric (lacking inversion symmetry)
• crystals are used.

c-axis
Input beam
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Photorefractive effect ?

• The photorefractive (PR) effect refers to spatial
  modulation of the index of refraction generated
  by a specific mechanism
• Light-induced charge redistribution in a material
  in which the index depends upon the electric
  field

Pockels effect

• To understand PR effect, its physical process
  must be understood

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PR band transport model for inorganics

• Nonuniform illumination

Diffusion



e-
1. Charge photo- generation
Con
duction band


2. Diffusion and driftmigration

u
h



Donor impurities


N
N

3. Trapping of the charges
D
D




Acceptor impurities



N
A


4. Space-charge field arises

Valence band

The Band transport model for organic PR materials
differs somewhat
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Photorefractive effect Index change

• We have seen physically how a net electric field
  is formed.
• How does this affect the index of refraction?

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The photorefractive effect solitons
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.
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The photorefractive effect solitons
Nonlinear medium Photorefractive (PR)
Effect
The presence of
light modifies the refractive index such that a
non-uniform refractive index change, Dn,
results. 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.
Narrowing of a light beam through a nonlinear
effect.
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Optical spatial solitons

• Soliton geometries and resulting beam profiles

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Optical spatial solitons

• In optics, spatial solitons represent a balance
  between self-focusing and diffraction effects.
• Observed in a variety of nonlinear materials

Inorganic PR crystal Optical Kerr media Liquid
crystals ...
Can optical solitons be created in organic
polymers/glasses?
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Compounds under study
C60 (0.5 wt)
DCDHF-6-C7M chromophore Tg33 C, unstable
DCDHF-6 chromophore Tg19 C, unstable
PR gain G220 cm-1 at 30 V/mm Low absorption
a12 cm-1 at 676 nm
DCDHF-6 DCDHF-6-C7M (11 wt mixture) Tg23
C, stable
From O. Ostroverkhova
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Sample construction
Spacer
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2.00kV
o.ookV
y
I(x)
Polarization of Laser
x
x
M. Shih et al., Opt. Lett. (1999).
E(x)
Side View
In
out
x

Dn(x)
gt 0
-
x
x
lt 0
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Mechanism Orientational photorefractive effect

• PR organic polymers/glasses exhibit an
  orientationally enhanced PR effect
• To analyze, note
• NLO chromophores contribute individual PR effects
  ? calculations at the molecular level ? start
  with p not P
• Each rod-like chromophore will exhibit a dipole
  moment
• Due to the rod shape we have
• and

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Mechanism Orientational photorefractive effect

• Macroscopic model needs to account for all
  orientations in the sample ? take the
  orientational average of all the dipole moments
  per unit vol.
• Find the change in macroscopic polarization for
  E0 and EE0
• lt gt can be calculated using dist. function.
  Finally, from n2 1?

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Mechanism Orientational photorefractive effect
gt 0
? ?
lt 0
D
n(x) gt 0
W. E. Moerner et al., J. Opt. Soc. Am. B (1994).
D
n(x) lt 0
x
M. Shih et al., Opt. Lett. (1999).
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Experimental setup 1-D solitons
Sample
x-polarization
CCD
Cylindrical lens
Imaging lens
Collimation lenses
x z
y
Typical image of diffraction at the
output face
?/2 wave- plate
Diode laser
Samples with different thickness and different
Wt of C60 were tested.
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Can PR glasses support solitons?
Diffracting
55 mm
Conducting polymer
No voltage applied
l780nm at 24mW
2.5mm
Self-focusing
120 m m
ITO-coated glass
12 mm
2.0 kV applied across sample
x
x
z
y
y
M. Shih, F. Sheu, Opt. Lett., 24 1853 (1999)
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Experimental results 1D soliton formation
Input to sample
Output from sample
x
Y-polarized (Self -focusing)
12 mm
X-polarized (Self-defocusing)
y
V0 V2 kV
Poling field along x-direction Insensitive to
polarity of field
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Experimental results Soliton data
Time lapse 160 s
Click to play
Self-defocusing
55 mm
Conducting polymer
80 mm
Vertical polarization
Click to play
Self-focusing
Conducting polymer
12 mm
x
Horizontal polarization
z
x
y
www.physics.sfsu.edu/laser/movies.html
y
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From left to right, the voltage was increased
independently. It appears that there is a
critical value of applied field that favors
soliton formation for a given laser power.
Experimental results Variable bias field
Nonlinearity increases as voltage increaese
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.

Y. S. Kivshar and D. E. Pelinovsky, Phys Report
331, 117 (2000).
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Experimental results Soliton stability
At 0 seconds voltage was applied
150 seconds
500 seconds (decay)

• 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 similar behavior.

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Experimental setup waveguide
Sample
?/2 wave- plate
y-polarization
To CCD
Soliton beam
Cylindrical lens
Collimation lenses
Moveable mirror
Probe beam
x z
y
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Experimental results planar waveguide
Input output (0V) output
(2.7kV) output (V off)
y
1. Stripe soliton created first

• Soliton
• (780nm)
• Probe
• (980nm)

x
2. Probe beam switched on

3. Guidance observed
4. Branching observed when turning off V
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Experimental results planar waveguide
Click to play

• Soliton beam on first
• Probe beam on later
• Probe beam does not
• form soliton itself !

y
x
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Experimental results circular waveguide
Input output (v0) output (v2
kV)
y
Soliton (780nm) Probe (980nm)
19 mm
x
65 s
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2D soliton formation

• The applied field is 16 V/?m
• Beam power at 36 mW
• Self-trapping of the circular beam occurred in
  65 s
• 19 ?m beam diameter

Click to play
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Soliton formation time
780 nm
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Soliton/Waveguide formation speed

• Goal Fast material response for applications
• Preliminary findings faster at 1 dopant
  concentrations
• Future investigation synthetic modifications
  of the DCDHF chromophores mixing
  DCDHF derivatives in various concentrations

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Stability issues
crystallization of chromophores ? scattering,
opaque ? re-heating sample at 130 ?C and cool
down very fast ? optimize sample fabrication
photostability ? slow degradation of
performance ? move to new spot on the
sample ? novel organic compounds electrical
breakdowns ? no HV possible anymore ? purified
materials, cleaner sample preparation ? operation
only in safe region E 0-60 V/mm
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Stability issues
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Conclusions

• A brief discussion of birefringence illustrated
  behavior important to orientationally enhanced
  birefringence.
• The band transport model showed the process of
  photo-charge generation migration, and trapping
  as part of the PR effect.
• An intuitive explanation for soliton formation
  was given
• Index change equations were presented that govern
  the NL response.
• Cont

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Conclusions

• The DCDHF glasses are high performance PR organic
  materials
• Solitons/waveguides were realized in such
  glasses for the first time.
• Optically-induced self-focusing-to-defocusing
  switching
• Both 1D and 2D solitons have been verified.
• Planar and circular soliton waveguides have been
  demonstrated.
• The speed for soliton/waveguide formation can be
  greatly improved.

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APPENDIX 1
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APPENDIX 2 Applications

• PASSIVE APPS
• Polarization induced switching
• Coupling with fiber and recon?gurable directional
  couplers
• based on two bright solitons formed in close
  proximity
• ACTIVE DEVICES
• Logic operations might be carried out by having
  two solitons
• interact
• Using an asymmetric transverse intensity profile,
  direction of
• propagation can be changed by changing the bias
  voltage, as a
• consequence of self-bending

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APPENDIX 3 Sample preparation
all ingredients are dissolved and mixed together
freeze-dried and solvent removed with vacuum
pump
remaining solvent removed in oven
spacer
dripped onto ITO coated glass slides
melted on substrates
sandwiched at 120oC