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

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

Conventional Kerr effect experiment Set-up Photodiode probe pulse Pump pulse Liquid or Crystal l/2 Polariser l/4 Crossed Polariser Dt To lock-in amplifier The pump induces a nonlinear third-order polarization in an isotropic medium The medium becomes birefringent The birefringence induced in medium placed in between two crossed polarisers is sensed by a probe-beam whose polarization is at 45° with respect to the pump beam . Pump and probe pulses can be delayed to perform time-resolved experiment

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

The first THz Kerr effect experiment Kerr effect: Change of refraction induced by an electric field Hence a initially isotropic liquid may become anisotropic when properly excited by the electric field of a THz pulse M.C. Offmann et al., Appl. Phys. Letters 95, 231105 2009

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

THz spectroscopy of GaP

THz-TDS of our GaP Crystal The set-up

THz-TDS of our GaP Crystal FT Absorption band @ 4.5, 5.5 and ~6.5 THz Phonon @ 11 THz

THz-TDS of our GaP Crystal First conclusions: We have a dispersion of the index of refraction and a small absorption in the THz range We are using a 43m cubic crystal: according to Kleinmann’s relation two nonlinear coefficients have to be considered We have a large index mismatch in THz and near I.R. spectral range: n(THz) ~ 3.4 n(l~800nm) ~ 3.66

THz Kerr effect in GaP

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

Theoretical background Third order non linear polarization Rough approximation: R(t) is purely electronic !!! + 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

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

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

Angular dependence We checked the THz Kerr effect was linear with respect to the THz pump intensity Eprobe 45° q X ETHz OXYZ = crystal frame Oxyz = lab frame Fit  u = siiii/siijj  8

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

Conclusions and prospects 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 Nonlinear THz optics: Resonnant Self induced transparency THz photon echos Non-resonnant Self focusing, self phase modulation, THz solitons….

Our laser research in fiber laser Average power 20 W at 74 MHz Pulse duration tunable from 20 ps down to 120 fs Wavelength tunable from 1010 nm to 1080 nm

Our laser research in fiber laser Développement of fiber lasers for ignition of aeronautic engines in partner-ship with a french compagnie (Turbomeca) Demonstration of ignition of combustion chambers Patent on laser ignition of aeronautic engines. Four-wave mixing in birefringent LMA fibers Prospectives: Fondamental research: nanosecondes fiber laser tunable in the visible spectral range Applied reseach: Application to laser ignition to actual aeronautic engines