Lasing Interactions Disclose Hidden Modes of Necklace States in the Anderson Localized Regime

Anderson light localization is known to be minimally affected by nonlinearity (e.g., gain), as a lasing mode typically arises from an isolated localized mode in a passive system. However, in the Anderson localized regime of low-dimensional systems, local resonances can occasionally experience strong level overlap and form an asymmetric single line, which is also known as necklace states. This case is not a hypothetically rare event and indeed is not uncommon. For such passive transport, we experimentally and theoretically investigate how nonlinearity affects necklace states in multilayered systems. When gain or amplifying media are introduced in a system, an asymmetric line splits into multiple lasing peaks, interacting with one another. By decomposing field spectra into a sum of Lorentzian lines, we find that diverging lasing peaks arise from quasimodes; in turn the lifetime and degree of level overlap of the underlying quasimodes are strongly attributed to the threshold and spectral behavior of the emer...