Asymmetric mode-coupling in Antisymmetric non-Hermitian waveguides

We demonstrate all-optical spatial mode-cleaning in non-Hermitian waveguides by modulating refractive index and gain/loss simultaneously along graded index multimodal waveguides. This approach introduces unidirectional coupling among modes, which, depending on the spatial delay between real and imaginary parts of the modulation favors higher or lower order modes, thereby enhancing nearly-monomode propagation in the latter case. Antisymmetric non-Hermitian modulation effectively cleans modes for any initial field distribution, as demonstrated analytically via coupled mode theory in 1D waveguides and numerically by solving wave propagation equations. For 2D waveguides, we consider different transverse modulations, also leading to the lowest Hermite-Gauss modes upon a higher-mode input beam.