Dual-Color Coherent Perfect Absorber.

Perfect absorption of light critically affects light-matter interaction for various applications. Coherent perfect absorbers (CPAs) gain the unique capability of controlling light with light in a linear fashion. Multicolor CPAs [S. Longhi, Time-reversed optical parametric oscillation, Phys. Rev. Lett. 107, 033901 (2011)PRLTAO0031-900710.1103/PhysRevLett.107.033901] are highly desirable for broadband and nonlinear light-to-light coherent control but the experimental demonstration has still remained elusive. Here, for the first time, we experimentally observe a dual-color version of a 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 also be 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 multifrequency domain and paves the way toward all-optical signal processing and quantum information.