Multilevel coherence effects in a two-level atom driven by a trichromatic field Z. Ficek a,b, * , J. Seke a , A.V. Soldatov a,c , G. Adam a , N.N. Bogolubov Jr. a,c a Institut f€ ur Theoretische Physik, Technische Universit€ at Wien, Wiedner Hauptstrasse 8-10/136, A-1040 Wien, Austria b Department of Physics, School of Physical Sciences, The University of Queensland, Brisbane QLD 4072, Australia c Department of Mechanics, V.A. Steklov Mathematical Institute, Gubkin Street 8, 117966 Moscow GSP-1, Russia Received 16 September 2002; received in revised form 6 January 2003; accepted 7 January 2003 Abstract We study the absorption and dispersion properties of a weak probe field monitoring a two-level atom driven by a trichromatic field. We calculate the steady-state linear susceptibility and find that the system can produce a number of multilevelcoherenceeffectspredictedforatomscomposedofthreeandmoreenergylevels.Althoughtheatomhasonly one transition channel, the multilevel effects are possible because there are multichannel transitions between dressed states induced by the driving field. In particular, we show that the system can exhibit multiple electromagnetically induced transparency and can also produce a strong amplification at the central frequency which is not attributed to population inversion in both the atomic bare states and in the dressed atomic states. Moreover, we show that the absorption and dispersion of the probe field is sensitive to the initial relative phase of the components of the driving field. In addition, we show that the group velocity of the probe field can be controlled by changing the initial relative phases or frequencies of the driving fields and can also be varied from subluminal to superluminal. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 32.80.)t; 32.80.Qk; 42.50.Gy; 42.50.Ar Keywords: Two-level atom; Absorption; Dispersion; Coherence; Quantum interference; Group velocity 1. Introduction There has recently been a great interest in the theoretical and experimental studies of changes caused by strong driving fields in the absorption and dispersion properties of a weak probe field monitoring a driven atomic system. In particular, the excitation of a multilevel atom with coherent fields has revealed new phenomena of both con- ceptual and practical importance [1,2]. It has been shownthatinterferencebetweenatomictransitions canreduceorevensuppressspontaneousemission, trap population in coherent superposition states that can lead to the phenomenon of electromag- netically induced transparency (EIT) [3]. The Optics Communications 217 (2003) 299–309 www.elsevier.com/locate/optcom * Corresponding author. Tel.: +61-7-3365-2331; fax: +61-7- 3365-1242. E-mail address: ficek@physics.uq.edu.au (Z. Ficek). 0030-4018/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0030-4018(03)01136-2