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We report on a new, to the best of our knowledge, type of optical memory that allows for the amplification of the optical signal carrying the stored information during its reading process. The memory mechanism is demonstrated in an ensemble of cold cesium atoms and is based on the multiple parametric four-wave mixing exploring the external atomic degrees of freedom via recoil-induced resonances. We have particularly demonstrated the storage of light carrying orbital angular momentum with a fourfold amplifying factor for the retrieved signal during the reading process. Memory lifetimes of the order of hundreds of microseconds have been measured, and possible applications for this self-amplifying memory are discussed.

We report the investigation of a collimated blue light generated in rubidium vapor due to the combined action of an ultrashort pulse train and a cw diode laser. Each step of the two-photon transition 5S -5P 3/2 -5D is excited by one of the lasers, and the induced coherence between the 5S and 6P 3/2 states is responsible for generating the blue beam. Measurements of the excitation spectrum reveal the frequency comb structure, indicating that each individual mode is responsible for inducing a nonlinear process. The strong signal dependency on the atomic density is characterized by a sharp growth and rapid saturation.Nonlinear interactions of light and atoms can be enhanced dramatically through the generation of quantum coherence among atomic states. In particular, atomic coherence effects have been explored in four-wave mixing to produce efficiently frequency up-conversion using either low power continuous wave (cw) lasers [1][2][3] or pulsed lasers [4,5]. Previous investigations demonstrated the high temporal coherence of the collimated blue light generated in Rb vapor [2] and the ability to transfer orbital angular momentum between the pump and generated beams [6][7][8]. The interest in these investigations includes quantum information processing and memory [9], photon correlation effects [10,11] and tunable coherent sources [12].In this work, we investigate the effects of the twophoton combined interaction of a cw laser and a modelocked (fs) laser in an Rb vapor for the generation of coherent blue light. The two copropagating beams, at 780 nm (cw) and 776 nm (fs), drive each step of the twophoton transition 5S 1/2 → 5P 3/2 → 5D, respectively (see inset of Fig. 1). The induced coherence among 5S 1/2 → 5P 3/2 → 5D → 6P 3/2 transitions produces, by parametric four-wave mixing (PFWM), a coherent beam at 420 nm [1,3,7]. Under the coherent accumulation condition, where the atomic relaxation times are greater than the fs laser repetition period, we show that each individual mode of the frequency comb contribute to the nonlinear signal. The signature of this behavior is the observation of the frequency comb structure in the excitation spectrum of the coherent light, indicating that each mode is responsible for the generation of a blue beam with a frequency determined by the parametric process.The role of each mode of the frequency comb in the two-photon transition of Rb vapor has been investigated using similar experimental schemes, with diode (cw) and mode-locked (fs) beams in a co-and contrapropaganting configurations, but detecting the fluorescence at 90 0 [13,14]. In both cases, the detected signal has its origin in a spontaneous emission process and * vianna@ufpe.br o C and Icw = 1.9 W/cm 2 . This curve is the average of two scans. The saturated absorption signal (upper curve) is detected simultaneously. The inset shows the relevant energy levels of Rb for the PFWM process..reflects the population of the excited state 5D. In the PFWM process investigated here, the nonlinear signal is determined by two-photon coher...

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