Coherent population trapping and the effect of laser phase fluctuations

Abstract

I . INTRODUCTION Coherent population trapping occurs in a wide var ie ty of s i tuat ions, some of which are l i s ted in Table I . A more complete survey of the relevent work is given in Reference [5] and within other references l i s ted in Table I . In each case a rate equation approach based only on populations would lead to the expectation that the populations of certain states would decay to zero, due to the presence of i r rever s ib le loss processes. However, the presence of coherence between the states, specif ied via o f f diagonal density matrix elements which appear in a master equation analys is , can in certain circumstances lead to a steady state solut ion where the populations of such po ten t ia l l y decaying states are non zero. Thus coherent population trapping has occured, the f ina l quantum state then being immune from further decay. As the long time spectral features of a system re f lec t the time constants of i t s coupled loss processes,then in coherent trapping si tuat ions we would expect features with narrower widths than the normal decay widths to appear. Coherent trapping has ramif icat ions in spectroscopy, in the search for narrow features on which to base improved time standards [7],and in state select ive photoexci tat ion processes where population trapping would r e s t r i c t the overal l y ie ld of the desired product [5] .