Fig. 3. A 5-element Hatano–Nelson phase locked microring laser array.

a Microscope and scanning electron microscope (SEM) images of the 5-element systems. Inset on the top right, Normalized eigenvalues when $\beta L\cong m\pi$. The mode with highest imaginary eigenvalues (dark red circle) will be the only lasing mode in the system. b–j Experimental conditions and results for the 5-element Hatano–Nelson lattice. Under three different pumping profiles, upper (b), middle (e), and lower (h), the 5-element system can be controlled to demonstrate the non-Hermitian skin effect towards two different directions (b, h) or return to its trivial form where the right and left couplings are equal (e). The intensity at various sites are reported (the error bars show the variation over 15 measurements) (c, f, i). The link length is designed to promote single-mode lasing behavior and the measured spectra confirm that this condition is satisfied (d, g, j). The number of the data points in the spectra is 288