ISSN 1063-7850, Technical Physics Letters, 2007, Vol. 33, No. 2, pp. 176–179. © Pleiades Publishing, Ltd., 2007. Original Russian Text © T.Yu. Bagaeva, V.V. Popov, N.J.M. Horing, 2007, published in Pis’ma v Zhurnal Tekhnicheskoœ Fiziki, 2007, Vol. 33, No. 4, pp. 79–86. 176 Radiative polaritons in two-dimensional (2D) exci- tonic systems have been theoretically studied [1–7] and experimentally observed in structures with quantum wells [8, 9]. The theoretical approach adopted in [1–5] was based on perturbation theory. According to this a quasi-continuous spectrum of photons in the surround- ing medium (barrier) is represented by eigenwaves in a hypothetical Fabry–Perot resonator, as its dimensions increase to infinity. In this case, the total field of a polariton mode in the barrier represents a superposition of the incident and outgoing plane electromagnetic waves. Within the framework of this approach, the dis- persion branches of radiative polaritons terminate on the surface of a light cone corresponding to the medium surrounding the 2D excitonic system. The frequency of radiative polaritons is practically independent of their wavevectors and, hence, the frequency of light emitted as a result of the radiative decay of polaritons is inde- pendent of the emission angle. The systems with rela- tively weak exciton–photon coupling are quite ade- quately described by perturbation theory, but this the- ory can fail to describe the systems with strong exciton–photon coupling. Another theory [6, 7] employed a self-consistent electrodynamic approach that was based on the repre- sentation of a light field in the surrounding medium in the form of an outgoing inhomogeneous plane electro- magnetic wave satisfying the radiation condition at infinity. In this case, the field of each radiative polariton mode at a fixed moment of time increases in space with the distance from the excitonic layer and decays with time at a fixed point [6]. The dispersion branches of radiative polariton modes enter the slow-wave region on the dispersion plane (i.e., go beyond the light cone) and terminate at the points corresponding to a zero rate of the radiative decay of these modes. The dispersion of radiative excitonic polaritons near the light cone is much stronger than that predicted by perturbation the- ory, which must lead to a strong angular dependence of the frequency of light emitted at grazing angles. It should be emphasized that both approaches are mathematically correct and differ only in the condition at infinity. Indeed, perturbation theory in fact employs the condition of finiteness of the light field at infinity, whereas the self-consistent approach [6, 7] employs the radiation condition. However, the particular behavior of the electromagnetic field at infinity in real physical sit- uations is not as important, since this field cannot reach infinity during a finite observation time. Therefore, a more adequate approach to the excitation of exciton– polariton modes in a 2D excitonic system is necessary, which would include an analysis of the initial condi- tions. This Letter describes an analysis of the nonstation- ary excitation of exciton–polariton modes by obliquely incident light pulses in a 2D excitonic system. The problem is solved using the formalism of Green’s ten- sor (dyadic) function. Consider a 2D excitonic system (material layer) sit- uated in the plane z = 0 of an infinite medium possess- ing permittivity ε h . The response of the material is Temporal Dynamics of Radiative Polariton Modes in a Two-Dimensional Excitonic Layer T. Yu. Bagaeva a, *, V. V. Popov a , and N. J. M. Horing b a Institute of Radio Engineering and Electronics (Saratov Branch), Russian Academy of Sciences, Saratov, 410019 Russia b Department of Physics and Engineering Physics, Stevens Institute of Technology, Hoboken, NJ 07030, USA * e-mail: bagaeva@info.sgu.ru Received September 11, 2006 Abstract—Nonstationary polariton modes excited by light pulses in a two-dimensional excitonic system have been theoretically studied using Green’s tensor function. It is shown that the electromagnetic response contains inhomogeneous radiative polariton modes, which correspond to the complex poles of Green’s function. An analysis of the nonstationary polariton response eliminates the discrepancy that previously existed between two different approaches to the description of radiative polariton modes in two-dimensional excitonic systems. PACS numbers: 73.20.Mf, 71.36.+c, 78.67. De, 78.47.+p DOI: 10.1134/S1063785007020253