Collision induced and resonantly enhanced extra resonances in four-wave mixing in inhomogeneously broadened four level systems Suneel Singh * , Geetha Govind School of Physics, University of Hyderabad, Hyderabad 500 046, India Received 31 July 2007; received in revised form 7 January 2008; accepted 7 January 2008 Abstract Role of collisions in inducing and enhancing extra resonances in non-linear generation by four-wave mixing in inhomogeneously broadened four level systems is studied. It is shown that collision induced three and two-photon resonances can be enhanced further by tuning to exact one photon (non-absorbing) transitions between unpopulated upper levels. In addition we also find existence of collision induced interference dip at the three-photon resonance that arises due to complete cancellation of signal at the line center. Ó 2008 Elsevier B.V. All rights reserved. PACS: 42.50.Hz; 42.62.Fi; 42.65.Ky 1. Introduction Wave mixing is one of the common manifestations of the non-linear interaction of light with matter. The non-lin- ear optical process of four-wave mixing in multilevel sys- tems particularly has been of great interest and has been studied experimentally and theoretically over the past many years [1–10]. Four-wave mixing (FWM) process in general, denotes interaction of four light waves with differ- ent frequencies and propagation directions. The interaction is due to the third-order non-linear response (susceptibility) of the material. Therefore, enhancement of non-linear sus- ceptibility is essential in order to enhance the efficiency of the four-wave mixing process [11–13]. It is well known that at exact one-photon resonance the non-linear susceptibility involved in the FWM process is highly enhanced, but this also leads to enhanced absorption of the applied field thereby reducing the efficiency of the process. Hence, a near resonant excitation scheme in which the applied field fre- quencies are tuned far from one-photon resonance (to pre- vent absorption) is commonly used. In this case multi- photon (two- or three-photon) resonance are essentially utilized to achieve enhancement of the non-linear suscepti- bility of the material. The generation mechanism of such multi-photon resonances in a multilevel medium is quite complex as they critically depend upon interference between different quantum mechanical pathways of (near resonant) excitation through intermediate states [14]. It was shown as early as 1981 that, dephasing collisions (with the atoms of a foreign gas) could give rise to additional res- onances in FWM susceptibility by removing destructive interference between quantum mechanical amplitudes cor- responding to different paths of excitation. Such pressure induced extra resonances (PIER4) [15–18], and collisional narrowing of resonances [19,20], focussed attention on the role of relaxation processes in four wave mixing. Since then several schemes for observing extra resonances in four-wave mixing in a multilevel system were predicted and these resonances have been observed [21–24]. These schemes of collision induced extra resonances often dealt with excitation of atomic coherence between either close lying (same parity) levels within the same electronic mani- fold [15–17] or different (parity) electronic levels [24–26]. More recently, effect of incoherent processes such as 0030-4018/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2008.01.005 * Corresponding author. Tel.: +91 040 23134336; fax: +91 040 23010227. E-mail address: suneelsp@uohyd.ernet.in (S. Singh). www.elsevier.com/locate/optcom Available online at www.sciencedirect.com Optics Communications 281 (2008) 2670–2679