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

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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?

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Wavelength, nm

400 500 600 700

Abs

orpt

ion,

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0

1

O

CN

NCNC

N

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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