A unified approach to Λ-, Ξ- and V-type systems with one continuum

We present a systematic approach to classify the three-level-like models with two bound states coupled to a continuum. It is shown that, when one of the discrete levels of usual three-level Lambda- (Λ), cascade (Ξ) or Vee (V)-type systems is replaced by a continuum of states, the resulting each model can be classified into three distinct categories with nine possible configurations. We show that all these models are exactly solvable. We obtain and compare the asymmetric Fano line shapes of the spectra for all the models. Our results are important for exploring new coherent effects in a variety of physical systems involving continuum-bound coupling such as photoassociation of cold atoms, plasmonics, quantum dots, photonic crystals, electromagnetic metamaterials and so on.