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Theory of coupled waveguides with modal degeneracies and gain

We investigate the novel characteristics of coupled waveguides exhibiting state eigenmode degeneracies and gain. We explore the theory of such coupled-waveguides with exceptional points of degeneracies (EPD) using a generic coupled transmission-line (CTL) approach. We show that giant enhancement in the local density of states (LDOS) occurs in finite length resonators with second and fourth order degeneracies. We also demonstrate the possibility of low threshold generation (lasing) in such cavities. The developed framework provides a promising performance boost for several applications including low-threshold lasers, highly efficient high microwave power sources, and sensors.