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.

It is shown that both the spatial and the temporal behavior of a laser pulse can be stored in and recalled from the spatial and spectral distribution of population in the ground state of an inhomogeneously broadened sample. Implications with regard to high-speed frequency-selective optical memories and ultra-high-speed holographic motion pictures are discussed.

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

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Working on the 7 Fo-5 Z>o transition of Eu 3+ in Y2O3, we provide the first detailed characterization of excitation-induced shifts in optical transition frequencies and demonstrate their exaggeration of nominally homogeneous dephasing rates. The shifts (found to vary in rough proportion to the number of Eu 3+ ions excited and to be as small as one part in 10 12 ) were identified through their unique signature in specially designed photon-echo experiments. The effects reported may constitute a rather general complication in optical measurements of homogeneous relaxation rates in solids.