Appl. Phys. B 35, 7 10 (1984) Applied physics Physics B "" Chemistry 9 Springer-Verlag 1984 Observation of Superfluorescence and Stimulated Emission in Bi I after Nonresonant Two-Photon Pumping C. Cremer and G. Gerber Fakult/it f/Jr Physik, Universit/it, D-7800 Freiburg, Fed. Rep. Germany Received 6 April 1984/Accepted 14 May 1984 Abstract. We report the observation of superfluorescence and of stimulated emission in the near-infrared at 965.69 nm in atomic Bi. The atoms are prepared in the 6pZ(3Po)Tp3/2 upper state by nonresonant two-photon excitation. The cooperative character of the emission is demonstrated by the delay of the light pulses and their N 2 dependence upon the atomic density. PACS: 32 Superfluorescence is the cooperative emission of radi- ation of an ensemble of initially uncorrelated atoms or molecules. Superradiance, a closely related pheno- menon, has been predicted by Dicke [1] in 1954. The cooperative emission is characterised by the radiation damping time TR= 8rc/7,bN22L, where 7,b is the tran- sition probability and 2 the wavelength of the observed emission, N is the initial population inversion density, and L is the length of the sample [2]. Superfluorescence is observable only if T R is shorter than the phase relaxation time T2* of the atomic dipoles [3]. Hence, a high inversion density of the fluorescent transition of wavelength /t is necessary for the occurrence of co- operative emission. Superfluorescence occurs with an average delay time To after the inversion. To is proportional to N-1 and its time duration Tw is also proportional to N 1 There has been a great deal of theoretical and experi- mental work on the phenomenon of cooperative emission of radiation in recent years ([3] and re- ferences therein). Observation of superradiant or su- perfluorescent light pulses have been reported by several groups [4-10]. In all reported experimental work optical pumping by laser pulses was used to achieve the necessary high inversion density. In the course of experiments to multiphoton excitation and ionization of atomic and molecular bismuth we ob- served directional radiation. In this paper we present experimental evidence for superfluorescent pulses at 2 = 965.69 nm in atomic bismuth, where the inversion was achieved by nonresonant two-photon exci- tation. 1. Experiment The relevant levels of the studied system are shown in Fig. 1. Bismuth atoms are prepared in the 6p2(3Po)7p3/2 level (state I1)) by a nonresonant two- photon transition from the ground state 6p 34S3/2 (state 10)) at the laser wavelength 2 = 465.634 nm. The 6p 3 aPj/2 level is energetically close to the virtual state (190.7cm -1 detuning). This level, however, has the same parity as the ground state 10) and the excited state I1). Thus, only magnetic dipole or electric quadrupole transitions are allowed to and from this intermediate state. In Bi I this "forbidden" transition to the ground state is well known [11] and has a transition rate of 61.2s -1. The also forbidden tran- sition from I1) to this intermediate state has not yet been observed. So we estimate this rate to be 60 s- 1 or even less. The next allowed electric dipole transition connects the ground state with 6p2(3Po)7S4Pl/2 (state 12)). This strongest line in the BiI spectrum has a transition rate of 2 x 108 s - 1 [-12]. The transition rate between state 12) and the upper state 11) was mea- sured by us to be 4x 107s -~. Despite the large detuning of 11117.9cm -~ from the virtual state this