We discuss asymmetric reflectance in surface plasmon Bragg gratings incorporating optical gain, referred to as
active asymmetric surface plasmon Bragg gratings. It is shown that balanced modulation of index and gain/loss with
quarter pitch spatial shift causes unidirectional coupling between contra-propagating modes in long-range surface
plasmon polariton Bragg gratings. Such gratings operate at the breaking threshold of parity-time symmetry
(exceptional point). Two active asymmetric surface plasmon Bragg gratings designs are proposed and their
performance is examined through modal and transfer matrix method computations.
In this paper, we review our recent work on active plasmonic structures composed of optically pumped dye molecules infiltrated in a polymer host as the cladding of long-range surface plasmon polariton (LRSPP) structures. In particular, concepts for distributed Bragg and distributed feedback (DBR/DFB) lasers, and a spatially non-reciprocal Bragg grating (NRBG) are reviewed. The LRSPP Bragg grating is a fundamental element in these devices which is created by stepping the width of a metal stripe to produce modulation of refractive index. The gain medium in all of these active devices is assumed to be a thin film (~1μm) of polymer (poly (methyl methacrylate)) doped with organic laser dye molecules IR- 140. The gain medium is assumed pumped optically through the top of the devices via 10 ns laser pulses at 810 nm with 500 kW/cm2 power intensity to enable stimulated emission at 880 nm. The maximum material gain coefficient of this medium was measured independently as 68 cm-1.
Incorporation of a solid-state gain medium in the cladding of a Long Range Surface Plasmon Polariton (LRSPP)
waveguide in order to create a single-mode near-infrared laser source is proposed. LRSPP Bragg gratings based on
stepping the width of the metal strip are used to form the laser’s cavity. Three laser configurations are presented: The
first 2 lasers employ DBRs (Distributed Bragg Reflectors) in ECL (External Cavity Laser) architecture while the third is
based on the DFB (Distributed Feedback) configuration. All 3 configurations are thermally tunable by heating the
gratings directly by injecting current. The lasers are convenient to fabricate leading to inexpensive sources that could be
used in optical integrated circuits or waveguide biosensors.