Quantum coherence and interference effects in laser physics: Lasers without inversion

Present day lasers encounter difficulties in achieving ultraviolet and X-ray operation due to the requirement of population inversion between the upper and the lower lasing states. The possibility of laser operation without inversion and consequently with much lower threshold pumping power is studied. This possibility is based on the creation of an inherently quantum coherent superpositions of atomic states and the resulting interference of different channels of radiative processes. The present work develops the theory of such lasers appropriate for a multilevel atomic system. A numerical model is described which is necessary to design the experiments that led to the first demonstration of lasers without inversion (LWI) in alkali atoms. The extension of LWI principles to semiconductor lasers and free-electron lasers is developed.