Spontaneous feedback for the simultaneous narrowing and elevation of fluorescence spectral lines

Narrow linewidth and high intensity of the fluorescence spectra are two important elements to improve the accuracy and efficiency of related practical measurements, but so far they have not been achievable simultaneously. We propose a new approach to sharpen the linewidth and to heighten the intensity simultaneously. Rather than double coherent mechanisms, it uses the spontaneous emission of inverted atoms in a cavity below the threshold as a resource for feedback control. The spontaneous feedback is simpler in principle and easier to realize technologically, and represents a kind of new way of controlling spontaneous emission.