We show how a single linearly polarized control field can produce a sharply tunable group velocity of a weak probe field at resonance in a four-level atomic configuration of alkali vapors. The dispersion can be switched from normal to anomalous along with vanishing absorption, just by changing intensity of the resonant control field. In addition, by allowing different intensities of the different polarization components of the control field, the anomalous dispersion can be switched back to the normal. This thereby creates a valley of anomaly in group index variation and offers two sets of control field intensities, for which the system behaves like a vacuum. The explicit analytical expressions for the probe coherence are provided along with all physical explanations. We demonstrate our results in $J = 1/2 leftrightarrow J = 1/2$ transition for D_1 lines in alkali atoms, in which one can obtain a group index as large as $3.2times10^{8}$ and as negative as $-1.5times10^{5}$ using a control field with power as low as 0.017 mW/cm$^2$ and 9.56 mW/cm$^2$ .
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