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C entre de Recherche sur les I ons, les MA tériaux et la P hotonique

C entre de Recherche sur les I ons, les MA tériaux et la P hotonique. Diffractive Optical Element for improving the Z-scan technique sensitivity. CIMAP-UMR 6252 ENSICAEN, 6 Bd Maréchal Juin, F14050 Caen, France. Contact : kamel.aitameur@ensicaen.fr. CMDO-Caen-sept 2008. Kerr sample. D 2.

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C entre de Recherche sur les I ons, les MA tériaux et la P hotonique

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  1. Centre de Recherche sur les Ions, les MAtériaux et la Photonique Diffractive Optical Element for improving the Z-scan technique sensitivity CIMAP-UMR 6252 ENSICAEN, 6 Bd Maréchal Juin, F14050 Caen, France Contact : kamel.aitameur@ensicaen.fr CMDO-Caen-sept 2008

  2. Kerr sample D2 -Z +Z D1 Z-scan technique for monitoring n2 the Kerr constant Optical Kerr effect n=n1+n2.I= n1 +n Nonlinear lensing effect causes the beam to focuse or defocuse As the sample is moved, it results a variation in the diaphragm transmittance CMDO-Caen-sept 2008

  3. Peak Useful information Tpv=Tp-Tv determination of n2 the Kerr constant Valley Tpv is small for a thin film Typical Z-scan recording Tpv is proportional to n2 x sample thickness The Z-scan sensitivity improvement consists in increasing Tpv=Tp-Tv for a given nonlinearity. CMDO-Caen-sept 2008

  4. D2 -Z +Z D1 Some methods for improving the Z-scan sensitivity The Z-scan sensitivity improvement consists in increasing Tpv=Tp-Tv for a given nonlinearity : Incident beam shaping : top-hat beam (x 2.5) Bessel-Gauss beam (x 40) EZ-scan : Replacement of the diaphragm Eclipsing Z-scan by a stop (opaque disk) (x 30) CMDO-Caen-sept 2008

  5. D2 -Z +Z D1 The response Tpv is increased Small stop transmission EZ-scan experiment r0 : stop and diaphragm radius Large stop radius r0 Tstop1% CMDO-Caen-sept 2008

  6. Z-scan sensitivity : EZ-scan sensitivity : Ultimate sensitivity commonly admitted Can we obtain better ? CMDO-Caen-sept 2008

  7. + d 2 ( W W ) 0 0 q d q + Z L 0 ê ê - - 10 20 30 40 50 0 20 10 Instead of the Kerr sample, one considers a Gaussian beam with a variable beam waist giving rise to a variable angular divergence  The Z-scan technique is basically a divergence diagnostic CMDO-Caen-sept 2008

  8. + d 2 ( W W ) 0 0 q d q + Z L 0 ê ê - - 10 20 30 40 50 0 20 10 Signal detector : On-axis intensity Small aperture CMDO-Caen-sept 2008

  9. out >in T1 : without divergence amplifier T2 : with divergence amplifier in What is expected Initial idea: Divergence multiplier It is assumed that the beam-waist radius variation arbitrary follows a sinus: Tpv Normalized transmission: CMDO-Caen-sept 2008

  10. One possible candidate: Phase aperture in Near field Far field Ratio of input and output divergences out Which component can play the role of divergence amplifier ? CMDO-Caen-sept 2008

  11. Focal plane Far-field super-Gaussian } doughnut TEM10 The key parameter : =ρ0/W CMDO-Caen-sept 2008

  12. For that the Phase Aperture is set in the beam-waist plane. P.A. The objective is to check if the phase aperture is able to improve the Z-scan signal. For that the Phase Aperture is set in the beam-waist plane. CMDO-Caen-sept 2008

  13. The performance of the sensitivity improvement can be described by a multiplying factor (with P.A.) (without P.A.) P.A. The phase aperture is able to improve the contrast T CMDO-Caen-sept 2008

  14. (with P.A.) (without P.A.) Multiplying factor : ratio of T2 and T1 derivatives CMDO-Caen-sept 2008

  15. 0.83 Multiplying factor for = Fresnel-Kirchhoff integral Appl. Phys. B 90, 513 (2008) CMDO-Caen-sept 2008

  16. h  Influence of the error on the relief etching =  h n=1.5 =1064nm h=10nm  =1.7° Relative variation /1% CMDO-Caen-sept 2008

  17. The multiplying factor is dramatically reduced for 180° CMDO-Caen-sept 2008

  18. in out Is there a compatibility between these 2 graphs ? It could have been expected that the value Y=0.83 should correspond to the steeper part of the out/  in versus Yp 0.83 0.83 CMDO-Caen-sept 2008

  19. D2 EZ-scan -Z +Z T with the nonlinearity Tn = T without the nonlinearity D1 Z-scan Tstop1% Why the EZ-scan sensitivity is improved ? The EZ-scan sensitivity increases because Tn results from the division by a quantity that vanishes. CMDO-Caen-sept 2008

  20. 0.83 T1 : Normalized transmittance without divergence amplifier T2 : Normalized transmittance with divergence amplifier The improvement of the Z-scan sensitivity by the use of a phase aperture occurs for the same reason as the EZ-scan : a division by a small quantity CMDO-Caen-sept 2008

  21. Laser beam Reference arm D1 L4 L1 PD1 PA1 L3 D2 L2 sample PD2 Z PA2 Main arm Rearrangement of the Z-scan experiment CMDO-Caen-sept 2008

  22. 1064 nm CW laser 30mW     Direct measurement of  the sensitivity enhancement PD Diaphragm Phase aperture Home made Deformable Mirror This experiment is currently under test CMDO-Caen-sept 2008

  23. Thank you for your attention

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