Solitons and Waveguides based on High Solitons and Waveguides based on High Performance photorefractive glassesPerformance photorefractive glasses
Marcus X. AsaroDepartment of Physics and Astronomy
San Francisco State University
Thesis advisor: Zhigang Chen, San Francisco State University
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N
CN
CN
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O. Ostroverkhova, W.E. Moerner, Stanford UniversityM. He, R.J. Twieg, Kent State University
• 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
OutlineOutline
• New PR material
• DCDHF-based organic glass
• Orientational PR nonlinearity
• Experimental observations
• Focusing and defocusing cases
• Optically induced waveguides
• Disussion of other effects
• Conclusion
OutlineOutline
Linear opticsLinear optics Linear opticsLinear 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 = εo(1)E .
Linear opticsLinear optics Linear opticsLinear optics
In a linear medium the refractive index n0 is a constant, independent of beam intensity for a given .
Also, different f of light encounter slightly different indices of refraction
Given a description of the refractive index follows:
D = εoE + P = εo(1+)E εo
εεo(1+) n2 = (1+)
Linear opticsLinear opticsLinear opticsLinear 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
E
e-ray
o-ray
Optic (c-) axis
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
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 = εo((1)E + (2)E2 + (3)E3 +… ) = PLinear + Pnon-linear
Nonlinear opticsNonlinear optics
Electro-optic (EO) effect: apply an electric field =>
Result: refractive index change−two forms
(a) (2) → n E: linear electro-optic or Pockels effect
(b) (3) → n E2: quadratic electro-optic or DC Kerr effect
(2) process → Ernn eff3
2
1
EO dielectrics→ Photorefractive crystals
• Typical values are: beam at mW/cm2, E=10 V/m n = 10−4 − 10−6
•Noncentrosymmetric (lacking inversion symmetry)crystals are used.
c-axis
z y
x Input beam
Ecos(t)
Ernn e 333
2
1
Photorefractive effect: ?Photorefractive effect: ?Photorefractive effect: ?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
PR band transport model for inorganicsPR band transport model for inorganicsPR band transport model for inorganicsPR band transport model for inorganics Nonuniform illumination
h
e−
ND ND+ NA
Conduction band Donor impurities Acceptor impurities Valence band
Applied electric field E0
Larger density
Smaller density
Diffusion
1. Charge photo- generation
2. Diffusion and drift=migration
3. Trapping of the charges
Esc
4. Space-charge field arises
The Band transport model for organic PR materials differs somewhat
Photorefractive effect: Index change Photorefractive effect: Index change Photorefractive effect: Index change Photorefractive effect: Index change • We have seen physically how a net electric field is formed. • How does this affect the index of refraction?
x
x
x
I(x)
E(x)
n > 0
n=n3reffE/2 < 0
(a)
(b)
(c)
n=0
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.
Diffraction
Broadening due to diffraction.
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, n, 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.
Spatial Soliton
Narrowing of a light beam through a nonlinear effect.
Optical spatial solitonsOptical spatial solitonsOptical spatial solitonsOptical spatial solitons Soliton geometries and resulting beam profiles
y-profile
x-profile
x z y
x-profile
1D soliton 2D soliton
In optics, spatial solitons represent a balance between self-focusing and diffraction effects.
Observed in a variety of nonlinear materials
Inorganic PR crystalOptical Kerr media
Liquid crystals
…...
Optical spatial solitons
Can optical solitons be created in organic polymers/glasses?
O
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N
Wavelength, nm
400 500 600 700
Abs
orpt
ion,
arb
.uni
ts
0
1
O
CN
NCNC
N
O
CN
NCNC
N
Compounds under study*Compounds under study*
DCDHF-6 + DCDHF-6-C7M (1:1 wt mixture)
Tg=23° C, stable
Wavelength, nm
400 600
Abs
orpt
ion,
arb
.uni
ts
0
1
O
CN
NCNC
N
DCDHF-6-C7M chromophore
Tg=33° C, unstable
676 nm 676 nm
C60 (0.5 wt%)DCDHF-6 chromophore
Tg=19° C, unstable
PR gain: ~220 cm-1 at 30 V/mLow absorption ~12 cm-1
at 676 nm*From O. Ostroverkhova
Sample constructionSample constructionSample constructionSample construction
Front view Back view
Organic film ITO
ITO ( )
(+) Electrode
Conducting epoxy
ITO
Soliton beam input
x z y
Vertical glass plate
Spacer
2.00kV
I(x)
E(x)
Polarization of Laser
y
x
o.ookV
-
Side ViewIn out
M. Shih et al., Opt. Lett. (1999).
n(x)
x
x
x
x
22 BRx x dc( )n C E
22 BR1y x dc2( )n C E
> 0
< 0
Mechanism: Orientational photorefractive effectMechanism: Orientational photorefractive effectMechanism: Orientational photorefractive effectMechanism: 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
kjijkjijii EEEp )(E
2211 ||33 33
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 E=0 and E=E0
< > can be calculated using dist. function. Finally, from n2 = 1+
Mechanism: Orientational photorefractive effectMechanism: Orientational photorefractive effectMechanism: Orientational photorefractive effectMechanism: Orientational photorefractive effect
EEEpP 0chchch : NNN
xxBR
x ENpNP 2)1(),1( cos
xxxEO
x EENP 03333
),2( cos yy
BRy ENP 3
2),1( cos
yxyxEO
y EENP 03333
23
),2( coscos
2333
0
2
2
0
222
5
1
45
2)()()( E
TkNE
TkNnnn
B
Dch
B
Dch
EOx
BRxx
2333
0
2
2
0
222
15
1
45
1)()()( E
TkNE
TkNnnn
B
Dch
B
Dch
EOy
BRyy
BR EO 2 22 2 2 BR EOx x x x x dc x dc( ) ( ) ( ) ( )n n n C C E C E
BR EO 2 22 2 2 BR EOy y y y y dc y dc( ) ( ) ( ) ( )n n n C C E C E
22 BRx x dc( )n C E
22 BR1y x dc2( )n C E
245
2 )/()(0
TkNC BBRx
0)( //
W. E. Moerner et al., J. Opt. Soc. Am. B (1994).
Mechanism: Orientational photorefractive effectMechanism: Orientational photorefractive effect
> 0
< 0
M. Shih et al., Opt. Lett. (1999).
n(x) < 0x
n(x) > 0
2
2
1 EC
nn BR
bx x
2
4
1 EC
nn BR
by x
2/BRxy CC
Experimental setup: 1-D solitonsExperimental setup: 1-D solitonsExperimental setup: 1-D solitonsExperimental setup: 1-D solitons
x z
y
Cylindrical lens
x-polarization
Collimation lenses
/2 wave- plate
Diode laser
Sample
Imaginglens
CCD
Typical image of diffraction at the output face
Samples with different thicknessand different Wt% of C60 were tested.
Can PR glasses support solitons?Can PR glasses support solitons?
Diffracting
Self-focusing
Conducting polymer
2.5mm
m
=780nmat 24mW
No voltage applied
2.0 kV applied across
sample
12 m
m
x
y
zx
y
M. Shih, F. Sheu, Opt. Lett., 24 1853 (1999)
ITO-coated glass
Experimental results: 1D soliton formationExperimental results: 1D soliton formationExperimental results: 1D soliton formationExperimental results: 1D soliton formation
x
y
=780 nm
V
2. 5 mm
y
xz
ITO-coated glass
120 mITO-coated glass
Input to sample
Y-polarized(Self -focusing)
X-polarized(Self-defocusing)
Output from sample
V=0 V=2 kV
Poling field along x-directionInsensitive to polarity of field
12 m
Experimental results: Soliton dataExperimental results: Soliton data
Self-defocusing
Self-focusing
12 m
m
x
y
Conducting polymer
Vertical polarization
Conducting polymer
Horizontal polarizationx
y
z
m
www.physics.sfsu.edu/~laser/movies.html
Time lapse ~160 s
Click to play
Click to play
Nonlinearity increases as voltage increaese
Y. S. Kivshar and D. E. Pelinovsky, Phys Report 331, 117 (2000).
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.
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
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.
• 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.
Experimental results: Soliton stabilityExperimental results: Soliton stability
150 seconds 500 seconds (decay)
At 0 seconds voltage was applied
Experimental setup: waveguideExperimental setup: waveguideExperimental setup: waveguideExperimental setup: waveguide
x z
y
Cylindrical lens
y-polarization
Collimation lenses
/2 wave- plate
Sample
Soliton beam
Probe beam
To CCD
Moveable mirror
ExperimentalExperimental results: planar waveguideresults: planar waveguideExperimentalExperimental results: planar waveguideresults: planar waveguide
Soliton
(780nm)
Probe
(980nm)
2. Probe beamswitched on
Input output (0V) output (2.7kV) output (V off)
1. Stripe solitoncreated first
3. Guidanceobserved
x
y
4. Branchingobserved whenturning off V
ExperimentalExperimental results: planar waveguideresults: planar waveguideExperimentalExperimental results: planar waveguideresults: planar waveguide
Soliton beam on first
Probe beam on later
Probe beam does not
form soliton itself !
xy
Click to play
Experimental results: circular waveguideExperimental results: circular waveguideExperimental results: circular waveguideExperimental results: circular waveguide
Soliton
(780nm)
Probe
(980nm)
Input output (v=0) output (v=2 kV)
y
x
19 m
~65 s
2D soliton formation2D soliton formation2D soliton formation2D 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
Soliton formation timeSoliton formation timeSoliton formation timeSoliton formation time
The response time depends on poling field and the beam power.Soliton forms faster in a “pre-poled” sample.
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
)
a b
780 nm
Soliton/Waveguide formation speedSoliton/Waveguide formation speedSoliton/Waveguide formation speedSoliton/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
0
0.2
0.4
0.6
0.8
1
1.2
0 20 40 60 80 100 120 140 160
Time (s)
No
rmal
ized
In
ten
sity
1% wt C60
2% wt C60
0.5% wt C60
Stability issuesStability issuesStability issuesStability 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/m
Stability issuesStability issuesStability issuesStability issues
ITO Glass ITO Glass
ITO Glass
Thin film No transmission
a b
y
x
ITO Glass
Conclusions 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…
ConclusionsConclusions
• 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.
APPENDIX 1APPENDIX 1APPENDIX 1APPENDIX 1
x x x
I(x) E(x)
n
n < 0
(a) (b) (c) (d) (e)
n=0
Organic thin film
Soliton beam
Esc
+ +
+ + + + + + E0
z x y
APPENDIX 2: ApplicationsAPPENDIX 2: ApplicationsAPPENDIX 2: ApplicationsAPPENDIX 2: Applications
PASSIVE APPS
•Polarization induced switching•Coupling with fiber and reconfigurable 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
APPENDIX 3: APPENDIX 3: Sample preparationSample preparation
all “ingredients” are dissolved and mixed together
spacer
100oCdripped onto ITO coated glass slides
sandwichedat 120oC
pumpremaining solvent removed in oven
freeze-dried and solvent removed with vacuum
melted on substrates