ELSFNIER 4 November 1996 Physics Letters A 222 (1996) 258-262 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH PHYSICS LETTERS A Polariton-atom bound state in a dispersive medium Valery I. Rupasov ‘, M. Singh 2 Centrefor zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Chemical Physics and Department of Physics, University of W estern Ontario, London, Onz., Canada N6A SK7 Received 13 May 1996; accepted for publication 2 August 1996 Communicated by L.J. Sham Abstract The quantum electrodynamics of a single two-level atom placed within a frequency dispersive medium whose polariton spectrum has a gap due to photon coupling to a medium excitation (exciton, optical phonon, etc.) is studied. If the atomic resonance frequency lies near the gap, the spectrum of the system is shown to contain a novel polariton-atom bound state with an eigenfrequency lying within the gap. The radiation and medium polarization of the bound state are localized in the vicinity of the atom. A signi~c~t suppression of spontaneous emission due to the bound state is predicted. The prediction [1,2] and the first experimental observation [3] of beg-dimensional dielectric struc- tures, which exhibit a complete photonic band gap (PBG) for all directions of electromagnetic propaga- tion, have created a great interest in a number of fundamental phenomena of classical and quantum electrodynamics in these materials. It has been shown that the existence of PI3G materials gives rise to such interesting phenomena as the suppression of sponta- neous emission [4], the formation of strongly local- ized states of light [3], photon-atom bound states [S], and the localization of superradiance near a photonic band gap f61. In ~ificial PBG materials, a suppression of the photon density of states over a narrow frequency range results from multiple photon scattering by ’ On leave from the Landau Institute for Theoretical Physics and the Institute of Spectroscopy, Russian Academy of Sciences. E-mail: v~p~ov~juli~.uwo.ca. ’ E-mail: msingh@uwovax.uwo.ca. spatially correlated scatterers. But it is well known that a frequency gap for propagating ele~~oma~etic modes exists also in many natural dielectrics and semiconductors. In contrast to PBG materials, gaps in these media are caused by photon couplings to elementary excitations (excitons, optical phonons, etc.) of the media [‘7,8]. The “normal” electromag- netic modes in frequency dispersive media (DM) are determined by the Maxwell equations with a fre- quency-dependent dielectric permeability and are treated as “photons in a medium” (or “polaritons”, i.e. photons dressed by an interaction with a medium excitation). Their spectrum consists of two branches of allowed states separated by a gap in which propa- gating polariton modes are completely forbidden_ Therefore, the intriguing question arises whether quantum optical phenomena predicted for PBG ma- terials are observed in dispersive media. In this paper we report some first results of qu~~rn elec~~yn~ics of ~l~itons interacting with a single two-level atom whose resonance fre- 03759601/96/$12.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SO375-9601(96)00626-3