Spectral Singularities of a Complex Spherical Barrier Potential and   Their Optical Realization
  The mathematical notion of a spectral singularity admits a physical interpretation as a zero-width resonance. It finds an optical realization as a certain type of lasing effect that occurs at the threshold gain. We explore spectral singularities of a complex spherical barrier potential and study their realization as transverse spherical electromagnetic waves emitted by a gain medium with a spherical geometry. In particular, for a typical dye laser material, we obtain a lower bound on the size of the gain medium for the occurence of this kind of spectral singularities. 
Ali Mostafazadeh and Mustafa Sarisaman
math-ph math.MP physics.optics quant-ph