  In this paper, we begin with a model of a $\Lambda$-type atom whose both
transitions are chirally coupled to a waveguide and then extend the model to
its giant-atom version. We investigate the single-photon scatterings of the
giant-atom model in both the Markovian and non-Markovian regimes. It is shown
that the chiral atom-waveguide couplings enable nonreciprocal, reflectionless,
and efficient frequency conversion, while the giant-atom structure introduces
intriguing interference effects to the scattering behaviors, such as
ultra-narrow scattering windows. The chiral giant-atom model exhibits quite
different scattering spectra in the two regimes and, in particular,
demonstrates non-Markovicity induced nonreciprocity under specific conditions.
These phenomena can be understood from the effective detuning and decay rate of
the giant-atom model. Our results have potential applications in integrated
photonics and quantum network engineering.
