S. Kourtev

Highly efficient nonlinear filter for femtosecond pulse contrast enhancement and pulse shortening.

We propose a highly efficient scheme for temporal filters devoted to femtosecond pulse contrast enhancement. The filter is based on cross-polarized wave generation with a spatially suger-Gaussian-shaped beam. In a single nonlinear crystal scheme the energy conversion to the cross-polarized pulse can reach 28%. We demonstrate that the process enables a significant spectral broadening. For an efficiency of 23% the pulse shortening is estimated to 2.2, leading to an intensity transmission of the nonlinear filter of 50%.

A two crystal arrangement to fight efficiency saturation in cross-polarized wave generation.

We describe a method that overcomes the observed saturation effect in cross polarized wave (XPW) generation. The previously reported internal efficiencies for XPW generation are known to be limited to around 15% whatever the length of the nonlinear medium and/or the input intensity values are. At the opposite, the theoretical limit had been estimated to be close to 25%. Here we show that using two thin BaF(2) crystals separated at optimum distance the saturation level of XPW generation efficiency can be drastically increased. An internal efficiency of 30% is demonstrated experimentally using two BaF(2) crystals.

Nonlinear polarization rotation of elliptical light in cubic crystals, with application to cross-polarized wave generation

We have investigated theoretically and experimentally the nonlinear propagation of intense elliptically polarized light pulses along a fourfold axis of the cubic crystal BaF2. Third-order nonlinear optical processes generate a cross-polarized wave, an effect that presents significant possibilities for application in femtosecond pulse contrast enhancement. The experimental setup consists of an input linear polarized light that passes through a cubic crystal sandwiched between two crossed quarter-wave plates. The exit orthogonal polarization-state production amount is measured at the output of an analyzer. When the light impinging on the sample is elliptically polarized with a quarter-wave plate at 22.5 deg, the achieved efficiency reaches 15%. It is more than twice that of a conventional polarization filter based on nonlinear ellipse rotation in an isotropic medium. This device is compared with previously reported polarization filtering [J. Opt. Soc. Am. B21, 1659 (2004)], in which a linearly polarized light produced a perpendicular field component. The theoretical model describes in detail the obtained dependencies and allows the different nonlinear processes that contribute to the generation of a cross-polarized wave to be distinguished. Possible applications are discussed.

Efficient generation of cross-polarized femtosecond pulses in cubic crystals with holographic cut orientation

We report here an alternative and more efficient orientation of cubic crystals for generation of cross-polarized femtosecond laser pulses. We show both theoretically and experimentally that the cross polarized wave generation (XPWG) is more efficient when the fundamental beam propagates along the [011] direction (holographic cut) in the crystal than along the [001] direction previously reported. With the [011]-cut BaF2 crystal we measured the highest XPWG conversion efficiencies. We prove other very important advantages of the [011]-cut approach: weak induced phase mismatch and no need for its compensation.

Improved nonlinear cross-polarized wave generation in cubic crystals by optimization of the crystal orientation

The efficiency of χ(3)-based cross-polarized wave generation in cubic crystals is investigated for different crystal orientations. It is shown that holographic-cut orientation is the optimal one. A 30% increase of the efficiency can be achieved with this orientation compared with the commonly used z-cut orientation. Another advantage of the holographic-cut crystal orientation is the weaker dependence of the optimal angle of input polarization on the input intensity.

Cross-polarized wave generation in the UV region

We demonstrate experimentally the generation of cross-polarized femtosecond pulses in BaF2 crystal in the UV region. We show that unsaturated cross-polarized wave generation in the UV is six times more efficient than in the visible region, and we deduce the corresponding wavelength dispersion of the third-order nonlinearity.

Comment on "Generation of 10(11) contrast 50 TW laser pulses".

We argue for a different physical interpretation of the results given in the recent Letter by Chvykov et al. [Opt. Lett.31, 1456 (2006)] in which a double nonlinear crystal scheme for cross-polarized wave generation is analyzed. We discuss the most important factors that explain the origin of the two-crystal schemes increased efficiency, namely, the Kerr lensing effect and a Gouy phase shift. The position and orientation of the second crystal relative to the first one are unambiguously defined; related effects are illustrated by already published works on the subject.

Nonlinear mirror based on cross-polarized wave generation

We present a new type of nonlinear mirror based on the generation of a cross-polarized wave through a nonresonant electronic third-order process. It is characterized by a reflection coefficient that depends on the input intensity. Its behavior results from the interference between the nonlinearly generated cross-polarized wave and a pi/2 phase-retarded wave. This setup has a lot of advantages: it does not require any phase matching, it is achromatic and suitable for femtosecond pulses, linear losses are easily adjustable, and the overall behavior is predictable. The device has been experimentally tested using BaF2 and YVO4 crystals.

Nonlinear optical transformation of the polarization state of circularly polarized light with holographic-cut cubic crystals.

Nonlinear modification of circularly polarized light propagating in holographic-cut cubic crystals is theoretically predicted and experimentally observed. To the best of our knowledge this is the first demonstration of nonlinear modification of circularly polarized light with cubic crystals.

Highly efficient XPW temporal contrast filter for ultra intense femtosecond pulses

We present a contrast filter reaching 12 orders of magnitude from micro to milliJoule input pulse with 24% efficiency. It solves contrast issue for Petawatt class lasers in microjoule seeded CPA or double CPA scheme.