Surface plasmon distributed feedback lasers and parity-time symmetric gratings

This paper discusses the amplification of surface plasmons (SPs) via propagation through one or two optically-pumped dipolar gain materials incorporated into the claddings adjacent to a thin metal plane or stripe. Then, cavity designs to ensure single-mode SP lasing are discussed, emphasizing distributed feedback (DFB) concepts. Single-mode DFB lasers are then implemented as a step-in-width metal stripe waveguide Bragg grating to produce distributed SP modal feedback, which in conjunction with SP modal gain, enables lasing in one SP mode. By also structuring the gain medium it is possible to interleave a gain grating with a loss grating such that a parity-time (PT) symmetric grating is enabled. Parity-time (PT) symmetric optical materials or devices are synthetic structures where the refractive index distribution is judiciously synthesized such that n(r) = n * (−r) about an axis or plane of symmetry. By altering the refractive index of the medium at a certain critical threshold, PT symmetry breaks down sharply. This breaking threshold is referred to as an exceptional point or a spontaneous phase transition, where many fascinating optical phenomena can be observed. One phenomenon of particular interest is asymmetric reflectance produced in waveguide Bragg gratings where the period of the grating satisfies this relationship. In a carefully-designed structure, the asymmetry can be ideal, i.e., the reflectance from one end is non-zero but the reflectance from the other is zero.