Figure 1: (a) Schematic of a laser oscillator: a gain medium with complex refractive index $n=n^{\prime }+i{n}^{\prime \prime }$, embedded in an optical cavity, emits outgoing coherent waves with defined phase and spatial pattern at frequency ${\omega }_0$ when the amount of amplification $n^{\prime \prime }$ 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 $n^{*}=n^{\prime }-i{n}^{\prime \prime }$, 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 $\text{Si}$) of thickness $a$ 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 $\mathrm{\Delta }\theta$ ($\mathrm{\Delta }\theta =0$, blue curve; $\mathrm{\Delta }\theta =\pi$, 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.