All-optical switching and storage in a four-level tripod-type atomic system M.A. Anto ´n a, * , Oscar G. Caldero ´n a , Sonia Melle a , I. Gonzalo b , F. Carren ˜o a a Escuela Universitaria de O ´ ptica, Universidad Complutense de Madrid, C/Arcos de Jalo ´ n s/n, 28037 Madrid, Spain b F. CC. Fı ´sicas, Universidad Complutense de Madrid, C/Ciudad Universitaria s/n, 28040 Madrid, Spain Received 1 June 2006; received in revised form 27 June 2006; accepted 29 June 2006 Abstract The optical behavior of a four-level tripod-type atomic system in a ring resonator driven by a cavity field and two external coherent fields is studied. It is shown that the atomic response exhibits an ultra-sensitive switch from high absorption to nearly transparency by changing the value of one of the control fields. The optical bistable response can be controlled by means of the external fields. The system can flip from the lower to the upper branch of the hysteresis curve without changing the incident probe. Switching and information stor- age of a light signal are predicted under appropriate triggering of the auxiliary external optical signals. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Optical bistability; Optical switching; Multilevel atoms; Atomic coherence 1. Introduction Control of light by light is essential in all-optical com- munication and optical computing. In the past two dec- ades, all-optical switches based on optical bistability (OB) in two-level atomic systems have been extensively studied [1]. In traditional optical bistability systems there are limi- tations for applications because the only controllable opti- cal beam is the input field, which is part of the bistable curve in the input–output plot. The situation changes when we consider multilevel atoms inside a cavity. First of all, there are new control fields between the different atomic levels, and most interestingly, new physical mechanisms such as quantum interference and coherence effects, which can greatly modify the absorption, dispersion and nonlin- earity of the system. It is well known that the optical prop- erties of atomic media may be dramatically altered if the atoms are placed into appropriate superposition of quan- tum states. A lot of phenomena such as electromagnetically induced transparency (EIT) [2–5], lasing without inversion (LWI) [6–9], refractive index enhancement without absorp- tion [10,11], and spontaneous emission cancellation [12,13] have been predicted and experimentally demonstrated. The role of atomic coherences in the context of collective phenomena, such as OB, has been subject to analysis once again. Walls et al. [14,15] and Harshawardhan and Agar- wal [16] proposed a novel scheme for OB using atomic coherence effects in three-level systems. They found that in K-type atoms the resulting OB is due to a population trapping in a coherent superposition of the ground state sublevels. This coherent superposition is not coupled to the excited level which leads to a narrow non-absorption resonance in the absorption profile. As the intensity of the driving field increases, the non-absorption resonance dip broadens and therefore the medium becomes transpar- ent. Recently, there has been a renewed interest in this sub- ject from a theoretical and experimental point of view. Xiao and coworkers have shown the influence of quantum coherence effects in three-level K atoms such as cavity-line- width narrowing [17], dynamic instability in an optical ring cavity [18], OB [19], and dynamical hysteresis [20] in a sys- tem formed by three-level Rubidium (Rb) atoms. 0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.06.079 * Corresponding author. Tel.: +34 913946855; fax: +34 913946885. E-mail address: antonm@fis.ucm.es (M.A. Anto ´n). www.elsevier.com/locate/optcom Optics Communications 268 (2006) 146–154