(a) Schematic of a laser oscillator: a gain medium with complex refractive index , embedded in an optical cavity, emits outgoing coherent waves with defined phase and spatial pattern at frequency when the amount of amplification reaches a threshold value. (b) Schematic of the time-reversed laser oscillator that realizes a coherent perfect absorber: when the gain medium is replaced by an absorbing medium with complex refractive index , incoming waves with definite phase and spatial pattern [the time-reversed waves of the lasing mode in (a)] are fully absorbed in the medium. (c) Typical behavior of total intensity reflection coefficient versus frequency in a simple two-port photonic device, comprising a slab of lossy material (like ) of thickness embedded in a fiber or waveguide (inset), excited at the two input sides. The red and blue lines show the spectral reflection curves for coherent input waves with controlled phase difference (, blue curve; , red curve). The dashed black line depicts the spectral reflection curve for incoherent input waves. The peaks in the coherent reflection curves correspond to the odd- and even-symmetric modes sustained by the slab structure.