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

2. 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

3. 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

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 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,

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

7. THz spectroscopy of GaP

8. THz-TDS of our GaP Crystal
The set-up

9. THz-TDS of our GaP CrystalFT
Absorption 4.5, 5.5 and ~6.5 THz
11 THz

10. 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

11. THz Kerr effect in GaP

12. 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

13. 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

14. 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

15. 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

16. 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

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

18. 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….

19. 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

20. 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