We introduce a unique technique for generating directional coherent emissions that could be utilized to create coherent sources in a wide range of frequencies from the extreme ultraviolet(XUV)to the deep infrared.This is accomplished without population inversion by pumping a two-level system with a far-detuned strong optical field that induces the splitting of the two-level system.A nonlinear process of four-wave mixing then occurs across the split system,driving coherent emission at sidebands both redand blue-detuned from the pump frequency,and propagates both forward and backward along the pump beam path.We observed this phenomenon in dense rubidium vapor along both the D_(1) and D_(2) transitions.The sideband emission exhibits a short pulse duration(o1 ns)with threshold-like behavior dependent on both the pump intensity and Rb vapor density.This technique offers a new capability for manipulating the emission frequency simply through intensity-induced atomic modulation that can be scaled to most frequency regimes using various atomic/molecular ensembles and pump energies.
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