Fundamental Radiative Processes in Near-Zero-Index Media of Various Dimensionalities

Spontaneous
emission, stimulated emission and absorption are the
three fundamental radiative processes describing light–matter
interactions. Here, we theoretically study the behavior of these fundamental
processes inside an unbounded medium exhibiting a vanishingly small
refractive index, i.e., a near-zero-index (NZI) host medium. We present
a generalized framework to study these processes and find that the
spatial dimension of the NZI medium has profound effects on the nature
of the fundamental radiative processes. Our formalism highlights the
role of the number of available optical modes as well as the ability
of an emitter to couple to these modes as a function of the dimension
and the class of NZI media. We demonstrate that the fundamental radiative
processes are inhibited in 3D homogeneous lossless zero-index materials
but may be strongly enhanced in a zero-index medium of reduced dimensionality.
Our findings have implications in thermal, nonlinear, and quantum
optics as well as in designing quantum metamaterials at optical or
microwave frequencies.