Low-Threshold Lasing and Coherent Perfect Absorption in Generalized PT -Symmetric Optical Structures

A necessary, but not sufficient, condition for parity-time (P00exT) symmetry to hold in an optical system is that spatially separated gain and loss must be exactly balanced. This study introduces generalized P00exT-symmetric optical structures that can have asymmetric and unbalanced gain-loss profiles, yet offer similar scattering properties and phase transitions as traditional P00exT-symmetric ones. The concept of general P00exT symmetry may help to reduce the threshold gain in recently discovered P00exT-enabled applications, such as the coherent-perfect-absorber laser and exceptional-point dynamics, and will facilitate optical and photonic devices by offering greater design freedom.