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 h E O. Ostroverkhova, W.E. Moerner, Stanford University M. 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 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 Outline

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 = ε o  (1) E.

Linear optics  In a linear medium the refractive index n 0 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 = ε o E + P = ε o (1+  )E  ε o   ε  ε o (1+  )  n 2 = (1+  )

Linear optics Some materials have two values of n depending on the polarization of the light. These are called n o and n e  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 = ε o (  (1) E +  (2) E 2 +  (3) E 3 +… ) = P Linear + P non-linear

Nonlinear optics a 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  E 2 : quadratic electro-optic or DC Kerr effect   (2) process →

EO dielectrics→ Photorefractive crystals Typical values are: beam at mW/cm 2, E=10 V/  m   n = 10 −4 − 10 −6 Noncentrosymmetric (lacking inversion symmetry) crystals are used. c-axis Input beam

Photorefractive effect: ? a The photorefractive (PR) effect refers to spatial modulation of the index of refraction generated by a specific mechanism: l Light-induced charge redistribution in a material in which the index depends upon the electric field Pockels effect a To understand PR effect, its physical process must be understood

PR band transport model for inorganics a Nonuniform illumination h  e−e− N D N D+ N A Conduction band Donor impurities Acceptor impurities Valence band Applied electric field E 0 Larger density Smaller density Diffusion 1. Charge photo- generation 2. Diffusion and drift=migration 3. Trapping of the charges E sc 4. Space-charge field arises The Band transport model for organic PR materials differs somewhat

Photorefractive effect: Index change We have seen physically how a net electric field is formed. How does this affect the index of refraction? xxxxxx I(x) E(x)  n > 0  n=  n 3 r eff E/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. Narrowing of a light beam through a nonlinear effect.

Optical spatial solitons a Soliton geometries and resulting beam profiles

a In optics, spatial solitons represent a balance between self-focusing and diffraction effects. a Observed in a variety of nonlinear materials Inorganic PR crystal Optical Kerr media Liquid crystals …... Optical spatial solitons Can optical solitons be created in organic polymers/glasses?

Compounds under study* DCDHF-6 + DCDHF-6-C7M (1:1 wt mixture) T g =23° C, stable DCDHF-6-C7M chromophore T g =33° C, unstable 676 nm C 60 (0.5 wt%) DCDHF-6 chromophore T g =19° C, unstable PR gain:  ~220 cm -1 at 30 V/  m Low absorption  ~12 cm -1 at 676 nm *From O. Ostroverkhova

Sample construction Spacer

2.00kV I(x) E(x) Polarization of Laser y x o.ookV - Side View Inout M. Shih et al., Opt. Lett. (1999). n(x)n(x) x x x x > 0 < 0

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

a Macroscopic model needs to account for all orientations in the sample → take the orientational average of all the dipole moments per unit vol. a Find the change in macroscopic polarization for E=0 and E=E 0 a can be calculated using dist. function. Finally, from n 2 = 1+  Mechanism: Orientational photorefractive effect

W. E. Moerner et al., J. Opt. Soc. Am. B (1994). Mechanism: Orientational photorefractive effect > 0 < 0 M. Shih et al., Opt. Lett. (1999).  n(x) < 0 x  n(x) > 0 

Experimental setup: 1-D solitons x z y Cylindrical lens x -polarization Collimation lenses /2 wave- plate Diode laser Sample Imaging lens CCD Typical image of diffraction at the output face Samples with different thickness and different Wt% of C60 were tested.

Can PR glasses support solitons? Diffracting Self-focusing Conducting polymer 2.5mm  m =780nm at 24mW No voltage applied 2.0 kV applied across sample 12  m  m x y z x y M. Shih, F. Sheu, Opt. Lett., (1999) ITO-coated glass

Experimental results: 1D soliton formation x y Input to sample Y-polarized (Self -focusing) X-polarized (Self-defocusing) Output from sample V=0 V=2 kV Poling field along x-direction Insensitive to polarity of field 12  m

Experimental results: Soliton data Self-defocusing Self-focusing 12  m  m x y Conducting polymer Vertical polarization Conducting polymer Horizontal polarization x y z  m Time lapse ~160 s 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. 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 stability 150 seconds 500 seconds (decay) At 0 seconds voltage was applied

Experimental setup: waveguide x z y Cylindrical lens y -polarization Collimation lenses /2 wave- plate Sample Soliton beam Probe beam To CCD Moveable mirror

Experimental results: planar waveguide Soliton (780nm) Probe (980nm) 2. Probe beam switched on Input output (0V) output (2.7kV) output (V off) 1. Stripe soliton created first 3. Guidance observed x y 4. Branching observed when turning off V

Experimental results: planar waveguide Soliton beam on first Probe beam on later Probe beam does not form soliton itself ! x y Click to play

Experimental results: circular waveguide Soliton (780nm) Probe (980nm) Input output (v=0) output (v=2 kV) y x 19  m ~65 s

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

Soliton formation time The response time depends on poling field and the beam power. Soliton forms faster in a “pre-poled” sample. 780 nm

Soliton/Waveguide formation speed a Goal: Fast material response for applications a Preliminary findings : faster at 1% dopant concentrations  Future investigation: synthetic modifications of the DCDHF chromophores mixing DCDHF derivatives in various concentrations

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/  m

Stability issues ITO Glass Thin film No transmission ab y x ITO Glass

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…

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.

APPENDIX 1

APPENDIX 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: Sample preparation all “ingredients” are dissolved and mixed together spacer 100 o C dripped onto ITO coated glass slides sandwiched at 120 o C pump remaining solvent removed in oven freeze-dried and solvent removed with vacuum melted on substrates