The spectral and temporal response of an optical cavity resonantly coupled to an ensemble of barium atoms has been investigated experimentally. The empty-cavity transmission resonances are found to split in the presence of the atoms and, under these conditions, the cavity's temporal response is found to be oscillatory. These eA'ects may be viewed as a manifestation of a vacuum-field Rabi splitting, or as a simple consequence of the linear absorption and dispersion of the intracavity atoms.

We report the first observation of continuous-wave two-photon iasing in the optical regime, and demonstrate that its initiation requires the injection of a trigger pulse into the laser resonator. Successful operation of the two-photon laser relies on the use of a novel gain medium consisting of laser-driven, two-level atoms and the use of a high-finesse optical cavity to isolate the two-photon gain from competing processes. Threshold conditions for laser action are in good agreement with recent theoretical predictions.PACS numbers: 42.50.Hz, 42.55.Hq, 42.65.Dr, 42.65.Pc Many of the unique properties of lasers derive from those of the stimulated emission (SE) process on which they are based. Virtually all existing lasers are based on the one-photon SE process-a SE event results in the creation of one new photon. Early on in the laser era, it was suggested [l] that lasers based on higher-order SE processes, wherein each SE event results in the creation of two or more photons, may be possible. It was predicted [2,3] that lasers based on higher-order SE processes possess operational characteristics qualitatively different from those found in normal (one-photon-SE-based) lasers. Unfortunately, tests of these predictions have not been possible, since efforts to realize lasers based on higher-order SE have themselves met with limited success [4]. A primary obstacle to the realization of higherorder-SE-based lasers is the tendency of higher-order SE processes to be weak both in absolute terms and in comparison to one-photon SE processes. In the microwave regime, the use of a unique gain medium and an extremely high-(? resonator has allowed researchers to achieve continuous-wave (cw), two-photon masing [5]. While masers are interesting in their own right, many of the intriguing predictions concerning two-photon lasers cannot be tested with them because their low-frequency photons are difficult to detect.Recently, it has been demonstrated [6] that stronglydriven two-level atoms display two-photon gain, and it has been predicted [7] that this gain can, under reasonable experimental conditions, be useful in the realization of a two-photon laser. We report in this Letter on the use of a driven-atom gain medium to provide the first demonstration of cw two-photon Iasing in the optical regime. The two-photon laser operates in the degenerate mode (both photons generated in the SE process have the same frequency) and displays dynamics that are dramatically different from those found in the case of normal onephoton-gain-based lasers.As discussed in detail elsewhere [6,7], the two-photon gain that arises in the driven two-level-atom system can be understood simply using the dressed-atom picture [8]. The dressed-atom energy eigenstates are shown in Fig. 1(a), where it is seen that the dressed-state doublets are separated in energy by ha)j, where coj is the driving-field frequency, and are split by h ft

Standard classical optical design procedures along with the intuitive concept of hour-glass-type optical modes are employed to produce cavities that provide strong atom-cavity coupling for atoms spread over a relatively large spatial region. Such cavities may be employed to provide macroscopic environments in which ordinarily microscopic quantum optical phenomena play an essential role. Concepts are tested through explicit cavity fabrication combined with an experimental study of cavity-mediated perturbations to the frequency and width of a spontaneous emission line in atomic barium. Manifestly

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A feature associated with continuous-wave two-photon optical gain has been observed in the absorption spectrum of an ensemble of barium atoms driven by a strong near-resonant optical field. In the dressed-atom picture, the observed gain is attributable to inverted two-photon transitions with nearly resonant intermediate states. A cw optical two-photon laser utilizing this gain appears feasible. PACS numbers: 42.50.Hz, 42.55.Hq, 42.65.Dr, 42.65.Pc The interaction of two-level atoms (TLA's) with strong resonant or nearly resonant optical radiation has been analyzed from a number of perspectives. 1,2 Of interest here, it has been shown that TLA's driven by a near-resonant driving field (pump) can act to amplify a weak-probe laser appropriately tuned relative to the atomic and pump frequencies. 3 " 8 In fact, single-photon lasers based on driven TLA gain have been constructed. 9 " 12 Driven TLA gain occurs in the absence of inversion between the ground and excited states. The largest weak-probe gain feature, which has been referred to as Raman gain, 4 can be understood in terms of a stimulated hyper-Raman-scattering process or in terms of population inversions on transitions between dressed atom-field states. Raman gain can be viewed as a single-photon gain process. At higher probe intensities, additional gain features, corresponding to multiphoton analogs of the Raman gain process and involving two-or more-photon gain, 13,14 become important. In this paper, we describe an experimental study of the interaction of a strong probe with a driven TLA and provide the first demonstration of cw two-photon gain in the optical regime.Consider an ensemble of stationary TLA's having a transition frequency v a , an upper-state radiative lifetime T\, and a homogeneous dephasing time T2~2T\. The atoms are driven by a monochromatic pump field of frequency vo, atom-pump detuning A=v fl -vo> and resonant Rabi frequency fto-We study the gain and absorption spectrum of a probe field interacting with the pump-driven TLA's. The probe is assumed to have a frequency vi, probe-pump detuning 8 = v\ -v 0 , and resonant Rabi frequency Q\. This spectrum can be evaluated using the expressions for nonlinear susceptibility derived by Agarwal and Nayak 15 using a continued-fractions method. This approach has also recently been employed by Gruneisen et al. 16 in calculations related to the energy transfer between propagating beams using stimulated Rayleigh scattering.Using the method of Agarwal and Nayak, we have calculated the probe gain and absorption as a function of probe-pump detuning for various probe intensities. The results are presented in Fig.

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