Elliptically polarized Terahertz emission through four-wave mixing in a two-color filament in air Yanping Chen a, * , Claude Marceau a , Simon Génier a , Francis Théberge b , Marc Châteauneuf b , Jacques Dubois b , See Leang Chin a a Centre d’Optique, Photonique et Laser (COPL) and Département de physique, de génie physique et d’optique, Université Laval, Québec, Canada G1V 0A6 b Defence Research and Development Canada – Valcartier, 2459 Pie-XI Blvd. North, Québec, Canada G3J 1X5 article info Article history: Received 25 March 2009 Received in revised form 22 July 2009 Accepted 23 July 2009 PACS: 42.25.Ja 42.65.Jx 42.65.Ky Keywords: Terahertz (THz) Filamentation Four-wave rectification Elliptical polarization abstract We diagnosed the polarization characteristics of Terahertz emission from a two-color femtosecond laser filament when the polarizations of x and 2x beams are orthogonal. We discovered that the THz pulse is elliptically polarized. The generation mechanism could be through four-wave optical rectification inside the filament zone where the inversion symmetry of air is broken. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Since the first demonstration by Cook and Hochstrasser [1], Ter- ahertz (THz) generation in air by superposition of an intense fun- damental (x) pulse and its second-harmonic (2x) pulse inside a plasma filament induced by the fundamental pulses has attracted great interest [2–9]. Compared with photo-excitation of semicon- ductors [10] or optical rectification in second-order nonlinear crystal [11], this technique can be performed at higher laser inten- sity. The attainable peak THz electric field is enhanced by more than one order than that emitted from a single filament [4].A THz pulse with high-energy (>5 lJ) and super-broadband spectrum (75 THz) has been reported with this technique [9]. The polarization of the THz pulse from a two-color (x and 2x) filament is still puzzling. Kress et al. [2] claimed that only the tensor component v ð3Þ xxxx of the nonlinear optical susceptibility was responsible for THz radiation, i.e., there is no THz radiation when the polarizations of x and 2x are orthogonal ðv ð3Þ yyxx ¼ 0Þ. However, based on the same experimental configuration and detection technique, Bartel et al. [3] observed strong THz radia- tion polarized perpendicular to x when the polarization of x is perpendicular to 2x, i.e., v ð3Þ yyxx term dominates. There is an obvi- ous discrepancy on THz generation between Refs. [2,3] when the polarizations of x and 2x are orthogonal. By separating the two beams (x and 2x) and independently controlling the polar- ization directions of x and 2x, Xie et al. [4] and Houard et al. [8] proved that THz radiation can be generated in both x and y planes (v ð3Þ xyxx and v ð3Þ yyxx ) when the polarizations of x and 2x are orthogo- nal. Compared with v ð3Þ xxxx , THz conversion efficiency from v ð3Þ xyxx and v ð3Þ yyxx is much lower, but not negligible. However, the coherence of the THz signals polarized at two orthogonal directions has not been proven yet. In this paper, we use the experimental configuration reported in Refs. [2,3], sending a x beam through a BBO crystal and then cre- ating a two-color filament after the BBO crystal. We analyze the polarization of the THz emission from the two-color laser filament when x and 2x beams have orthogonal polarizations. We confirm that THz waves can indeed be obtained either parallel or orthogo- nal to the polarization of x. Furthermore, we prove that the two polarization components of the THz wave are coherent and the combined electric field vector obeys an ellipse; i.e., the THz pulse is elliptically polarized. 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.07.044 * Corresponding author. Tel.: +1 418 656 2131; fax: +1 418 656 2623. E-mail address: yanping.chen.1@ulaval.ca (Y. Chen). Optics Communications 282 (2009) 4283–4287 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom