  Perfect absorption of light critically affects light-matter interaction for
various applications. Coherent perfect absorbers (CPA) gain the unique
capability of controlling light with light in a linear fashion. Multi-color
CPAs [Phys. Rev. Lett. 107, 033901] are highly desirable for broadband and
nonlinear light-to-light coherent control, however, the experimental
demonstration has still remained elusive. Here we experimentally observe a
dual-color version of CPA (DC-CPA) through a second harmonic generation in a
single whispering-gallery-mode microcavity. The DC-CPA enables simultaneous
perfect absorption of both the incoming fundamental wave and its second
harmonic. Similar to its linear counterpart, coherent control in the DC-CPA can
be also realized by tuning the relative phase and intensity between the
two-colored waves through nonlinear interference instead of the linear one.
This scheme breaks the linear boundary of the traditional CPA into a
multi-frequency domain and paves the way toward all-optically signal processing
and quantum information.
