Mode conversion and absorption in an optical waveguide under cascaded complex modulations

We investigate the mode conversion in a dielectric waveguide under spatially complex modulation, which refers to modulating both the real and imaginary parts of the dielectric permittivity. The complex modulations are applied separately in two distinct regions in the waveguide. The mode conversion can be controlled by adjusting the initial phase difference of the modulations in the two cascaded regions and by varying the phase difference of real and imaginary modulations in each region. The former is analogous to an Aharonov–Bohm (AB) phase and the latter suggests a symmetric propagation phase. Comparing to one modulated region, the cascaded modulations could benefit to realizing laser, coherent perfect absorber, and bidirectional invisibility. The study may contribute to the applications in optical switches and mode converters.