Phase Control of Spontaneous Emission

The control of spontaneous emission has attracted much attention for many years. For atoms in free space, atomic coherence and quantum interference are the basic phenomena for controlling spontaneous emission [1– 3]; these have potential applications to lasing without inversion [4–10]. Zhu, Scully, and co-workers have studied the quenching of spontaneous emission using an open V-type atom [11], and gave an experimental verification of their predictions [12]. Here, we study the potential for coherent control in a driven quantum system, using the relative phase between two lasers with equal frequencies va ­ vb ­ v which couple the ground state with the two excited states (see Fig. 1). These laser fields may be distinguished by their different transition characteristics [13,14]. In this way we can obtain efficient control, spectral narrowing, and quenching of spontaneous emission even if we have nontrapping conditions that do not allow control when a single laser is used. The use of two lasers makes the system independent of restrictions involving matrix elements to satisfy the trapping condition of Ref. [11]. Phase dependent effects in spontaneous emission spectra were recently studied in a L-type atom [15] and for an atom near the edge of a photonic band gap [16]. The effects of strong bichromatic excitation in the fluorescence spectrum from a two-level atom have also been studied [17,18]. We use here the wave function approach, and assume that the atom is excited to a superposition of states j0l, j1l, j2l. We apply the Weisskopf-Wigner theory [2,19] and obtain the resulting equations for the probability amplitudes (h̄ ­ 1),