DOI: 10.1007/s00340-007-2685-8 Appl. Phys. B 87, 595–601 (2007) Rapid communication Lasers and Optics Applied Physics B a. jullien 1,4, ∗ l. canova 1 o. albert 1, ✉ d. boschetto 1 l. antonucci 1 y.-h. cha 3 j.p. rousseau 1 p. chaudet 1 g. ch ´ eriaux 1 j. etchepare 1 s. kourtev 2 n. minkovski 2 s.m. saltiel 2 Spectral broadening and pulse duration reduction during cross-polarized wave generation: influence of the quadratic spectral phase 1 Laboratoire d’Optique Appliqu´ ee, Ecole Nationale Sup´ erieure de Techniques Avanc´ ees, Ecole Polytechnique, Centre National de la Recherche Scientifique, 91761 Palaiseau Cedex, France 2 Faculty of Physics, University of Sofia, 5 J. Bourchier Boulevard, 1164 Sofia, Bulgaria 3 Quantum Optics Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea 4 Thales Laser, Domaine de Corbeville, 91400 Orsay, France Received: 1 February 2007/Revised version: 19 March 2007 Published online: 25 May 2007 • © Springer-Verlag 2007 ABSTRACT Cross-polarized wave (XPW) generation is used for the contrast improve- ment of ultra-intense femtosecond laser pulses in a double CPA configuration. We present theoretical and experimental evidence that the XPW output spectrum depends in a predictable way on the input chirp. Therefore, a chirp controlled pulse can experi- ence a pulse duration shortening up to a factor of √ 3, and an initial amount of chirp that leads to the exact preservation of the spectral width of a given pulse can be predicted. PACS 42.65.-k; 42.65.Re 1 Introduction Cross-polarized wave (XPW) generation is a degenerated four-wave mixing process occurring in a nonlin- ear medium whose third order nonlin- ear susceptibility is anisotropic [1–3]. This process allows the conversion of part of an input linearly polarized wave into an orthogonally polarized wave. The relevance of this mechanism on the design of nonlinear filters devoted to contrast enhancement of ultra-intense femtosecond laser pulses has formerly been demonstrated [4–6]. It has been established that filters based on XPW generation allow improvement of pulse contrast by several orders of magnitude without introducing any spatial or spec- tral distortions. Considering the cru- cial importance of the temporal contrast ratio for the development of petawatt class laser chains, we have also shown in previous work the advantages of ✉ Fax: +33 1 6931 9996, E-mail: olivier.albert@ensta.fr ∗ Currently at Departments of Chemistry and Physics, University of California, Berkeley 94720, USA and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley 94720, USA the XPW nonlinear filter in terms of transmission efficiency and contrast im- provement [7, 8]. Here, we theoretically and experi- mentally investigate the temporal and spectral characteristics (intensity and phase) of the generated cross-polarized wave according to the input pulse du- ration and chirp. Indeed, a way to pro- duce pulses with both high contrast and high peak power is to achieve a dou- ble CPA system including a XPW fil- ter [9]. Knowing the temporal and spec- tral properties of the cleaned pulse is consequently of prime importance to the design of the second CPA setup. Furthermore, an interesting point is that a XPW generated signal presents, in a first approximation, a cubic depen- dence on intensity of the input signal. Therefore, if we assume that pulses ex- hibit a usual temporal Gaussian shape, the duration of the XPW signal should be shorter than the input one and its spectrum should be broadened. This process has experimentally been ob- served in [5] and is a very attractive method for producing ultrashort pulses. In that case, the nonlinear filter allows for the compensation of eventual gain spectral narrowing in the first CPA and for the production of shorter pulses with high temporal contrast. This pa- per demonstrates the conditions on the input pulse to verify this hypothesis. We have particularly investigated the in- fluence of the initial residual spectral phase (chirp), which is easily introduced by optical elements preceding the non- linear medium. We will show that this value has a crucial impact on the dura- tion of the XPW signal. We have short- ened down to 14 fs a 23 fs pulse with XPW by suppressing its initial chirp. Moreover, we demonstrate that for such short pulses an excessive amount of chirp leads to spectrum narrowing and pulse duration broadening, preventing us from using XPW generation for con- trast filtering in the uncompressed pi- cosecond regime. 2 Theoretical considerations 2.1 Hypothesis We consider spectral and as- sociated temporal Gaussian shapes for input and XPW pulses. We also consider linear chirp in this paper; the influence of the coefficients of spectral phase with order higher than two is neglected in our investigation. The effective full-width at half max- imum (FWHM) duration of the initial