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TeraHertz Kerr effect in GaP crystal

TeraHertz Kerr effect in GaP crystal. J. Degert, M. Geye , E. Abraham, E. Freysz Laboratoire Ondes et Matière d’Aquitaine University of Bordeaux France. Outline. Introduction: 2. THz spectroscopy application to the GaP crystal 3. THz - Kerr effect in GaP

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TeraHertz Kerr effect in GaP crystal

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  1. TeraHertz Kerr effect in GaPcrystal J. Degert, M. Geye, E. Abraham, E. Freysz Laboratoire Ondes et Matière d’Aquitaine University of Bordeaux France

  2. Outline • Introduction: • 2. THzspectroscopy application to the GaPcrystal • 3. THz- Kerr effect in GaP • 4. Conclusions and prospects Dept. Physics, Bangalore, October 24 2011

  3. Conventional Kerr effectexperiment Set-up Polariser l/4 probe pulse To lock-in amplifier Dt Photodiode Liquid or Crystal Crossed Polariser l/2 Pump pulse • The pumpinduces a nonlinearthird-orderpolarizationin an isotropic medium • The medium becomesbirefringent • The birefringenceinduced in medium placed in betweentwocrossed • polarisersissensed by a probe-beamwhosepolarizationisat 45° • with respect to the pumpbeam. • Pump and probe pulses canbedelayed to perform time-resolvedexperiment

  4. Conventional Kerr effect experiment • This technique is usually applied in the optical spectral range, in degenerate • wavelength configuration, in transparent and isotropic medium • Liquids • Glasses • The dispersion and absorption of the medium is usually negligible • No dispersion and absorption of the pump or probe pulses mismatch • No group velocity mismatch • Automatic phase matching • Only one nonlinear third order coefficient as to be known: all the other are • related ! • We will see the situation is quite different, if you perform a THz Kerr effect • experiment in a cubic crystal (GaP) using a visible pulse as a probe

  5. The first THzKerr effectexperiment Kerr effect: Change of refractioninduced by an electricfield Hence a initiallyisotropicliquidmaybecomeanisotropicwhenproperlyexcited by the electricfield of a THz pulse • M.C. Offmann et al., Appl. Phys. Letters 95, 231105 2009

  6. THz Kerr effect in liquid • M.C. Offmann et al., Appl. Phys. Letters 95, 231105 2009

  7. THzspectroscopy of GaP

  8. THz-TDS of ourGaP Crystal The set-up

  9. THz-TDS of ourGaP Crystal FT Absorption band @ 4.5, 5.5 and ~6.5 THz Phonon @ 11 THz

  10. THz-TDS of ourGaP Crystal • First conclusions: • We have a dispersion of the index of refraction and a small absorption in the THz range • We are using a 43mcubiccrystal: according to Kleinmann’s relation twononlinear coefficients have to beconsidered • We have a large index mismatch in THz and near I.R. spectral range: • n(THz) ~ 3.4 • n(l~800nm) ~ 3.66

  11. THz Kerr effect in GaP

  12. Response function THz pulse • Theoretical background IThirdorder non linearpolarization In the reference frame of the crystal(OXYZ), the THz induced third order polarization is: with Geometry of the experiment Eprobe 45° Oxyz = reference frame of the laboratory q X ETHz

  13. Theoretical background Thirdorder non linearpolarization Rough approximation: R(t) ispurelyelectronic !!! + Crystal symmetry: + An other approximation: THz and optical pulse are phase matched in GaP In the frame of the laboratory (Oxyz) with u = a/b

  14. Theoretical background Kerr effect signal Detection set-up GaP <100> L=1 mm t Ss PD1 Wollaston I SKerr(t)=Ss–Sp λ/4 Sp PD2

  15. Kerr effect in a GaPcrystal: Experimentalset-up Eprobe 45° X ETHz Pellicle Wollaston Lock-in Amplifier λ/4 Balanced Detector Type I BBO 100 µm Si-filter GaP <100> 1 mm Computer f = 25 cm 700 µJ THz 120 kV/cm Probe pulse C. P. A. λ0 = 795 nm τp = 35 fs 1 kHz Delay Beamsplitter (R=90%) EMax (THz)= 120 kV.cm-1

  16. Angulardependence Wechecked the THz Kerr effectwaslinearwith respect to the THzpumpintensity Eprobe 45° q X ETHz OXYZ = crystal frame Oxyz = lab frame Fit  u =siiii/siijj  8

  17. Temporal dependence If the vf (THz) =vg (800 nm) then S(t)~I THz(t) Not our case

  18. Conclusions and prospects Conclusions: We have investigate the THz Kerr effect in a <100> GaP crystal The angular dependence results from the symmetry of the crystal The temporal dependence of the Kerr signal is mainly affected by the velocity mismatch and the dispersion of the THz pulse during its propagation within the crystal. The latter is related to c(1) (THz) In near future we would like to investigate the dispersion of c(3) (THz,THz,visible) • Our prospects in NonlinearTHzoptics: • Resonnant • Self inducedtransparency • THz photon echos • Non-resonnant • Self focusing, self phase modulation, THzsolitons….

  19. Our laser research in fiber laser • Average power 20 Wat 74 MHz • Pulse durationtunablefrom 20 ps down to 120 fs • Wavelengthtunablefrom 1010 nm to 1080 nm

  20. Our laser research in fiber laser • Développement of fiber lasers for ignition of aeronauticengines • in partner-shipwith a french compagnie (Turbomeca) • Demonstration of ignition of combustion chambers • Patent on laser ignition of aeronauticengines. • Four-wavemixing in birefringent LMA fibers • Prospectives: • Fondamental research: nanosecondes fiber laser tunable in the visible spectral range • Appliedreseach: Application to laser ignition to actualaeronauticengines

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