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Solitons and Waveguides based on High Performance photorefractive glasses

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

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Solitons and Waveguides based on High Performance photorefractive glasses

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  1. 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

  2. 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

  3. 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

  4. 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 = εoc(1)E .

  5. 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 = εoE + P = εo(1+c)E = εoE  ε = εo(1+c)  n2 = (1+c)

  6. 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, notE. k is  to both D and E (D || E) S is not || to k S is || to k o-wave “feels” isotropic medium

  7. 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(c(1)E + c(2)E2 +c(3)E3 +… ) • = PLinear+ Pnon-linear

  8. Nonlinear optics • Electro-optic (EO) effect: apply an electric field => 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→

  9. 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 Input beam

  10. 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

  11. Smaller Larger density density Applied electric field E0 Esc PR band transport model for inorganics Diffusion • Nonuniform illumination 1. Charge photo- generation e− Con duction band 2. Diffusion and drift=migration 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

  12. I(x) • E(x) • n > 0 • n=n3reffE/2 < 0 (a) (b) (c) x x x n=0 Photorefractive effect: Index change • We have seen physically how a net electric field is formed. • How does this affect the index of refraction?

  13. Diffraction 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.

  14. Spatial Soliton The photorefractive effect: solitons Nonlinear medium: Photorefractive (PR) EffectThe presence of light modifies the refractive index such that a non-uniform refractive index change, Dn, results. Self-focusingThis 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.

  15. Optical spatial solitons • Soliton geometries and resulting beam profiles

  16. 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?

  17. Compounds under study* 676 nm 676 nm C60 (0.5 wt%) • DCDHF-6-C7M chromophore • Tg=33° C, unstable DCDHF-6 chromophore Tg=19° C, unstable PR gain: G~220 cm-1 at 30 V/mm Low absorption a~12 cm-1 at 676 nm DCDHF-6 + DCDHF-6-C7M (1:1 wt mixture) Tg=23° C, stable *From O. Ostroverkhova

  18. Sample construction Spacer

  19. 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) > 0 - x x < 0

  20. 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

  21. 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 E=0 and E=E0 • < > can be calculated using dist. function. Finally, from n2 = 1+

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

  23. 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.

  24. Can PR glasses support solitons? Diffracting 55 mm Conducting polymer No voltage applied l=780nm 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)

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

  26. 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

  27. 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).

  28. 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.

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

  30. Experimentalresults: 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

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

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

  33. 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

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

  35. 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

  36. 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

  37. a b ITO Glass ITO Glass No transmission Thin film y x ITO Glass ITO Glass Stability issues

  38. 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…

  39. 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.

  40. APPENDIX 1

  41. 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

  42. 100oC 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

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