LAUSR

Band edges at the high symmetry points in reciprocal space of periodic structures hold special interest in materials engineering for their high density of states. In optical metamaterials, standing waves found at these points have facilitated lasing, bound‐states‐in‐the‐continuum, and Bose–Einstein condensation. However, because high symmetry points by definition are localized, properties associated with them are limited to specific energies and wavevectors. Conversely, quasi‐propagating modes along the high symmetry directions are predicted to enable similar phenomena over a continuum of energies and wavevectors. Here, quasi‐propagating modes in 2D nanoparticle lattices are shown to support lasing action over a continuous range of wavelengths and symmetry‐determined directions from a single device. Using lead halide perovskite nanocrystal films as gain materials, lasing is achieved from waveguide‐surface lattice resonance (W‐SLR) modes that can be decomposed into propagating waves along high symmetry directions, and standing waves in the orthogonal direction that provide optical feedback. The characteristics of the lasing beams are analyzed using an analytical 3D model that describes diffracted light in 2D lattices. Demonstrations of lasing across different wavelengths and lattice designs highlight how quasi‐propagating modes offer possibilities to engineer chromatic multibeam emission important in hyperspectral 3D sensing, high‐bandwidth Li‐Fi communication, and laser projection displays.