Enhanced Photoluminescence of Monolayer WS₂ on Ag Films and Nanowire–WS₂–Film Composites

Monolayer transition metal dichalcogenides (TMDCs), due to their structural similarity to graphene, emerge as a promising alternative material of integrated optoelectronic devices. Recently, intense research efforts have been devoted to the combination of atomically thin TMDCs with metallic nanostructures to enhance the light–matter interaction in TMDCs. One crucial parameter for semiconductor–metallic nanostructure hybrids is the spacer thickness between the gain media and the plasmonic resonator, which needs to be optimized to balance radiation enhancement and radiation quenching. In current investigations of TMDCs–plamonic coupling, one often adopts a spacer thickness of ∼5 nm or larger, a typical value for transitional gain media–plasmonic composites. However, it is unclear whether this typical spacer thickness represents the optimal value for TMDCs–plasmonic hybrids. Here we address this critical issue by studying the spacer thickness dependence of the luminescent efficiency in the monolayer tungsten-disulfide (WS₂)–Ag film hybrids. Surprisingly, we discovered that the optimal thickness occurs at ∼1 nm spacer, much smaller than the typical value used previously. In a WS₂–Ag film system, at this optimal spacer thickness, the photoluminescence (PL) is increased by more than an order of magnitude due to exciton-coupled surface plasmon polaritons (SPPs), as compared to the as-grown WS₂ on sapphire. We further explore a new composite system comprising Ag nanowires on top of a WS₂–Ag film and observe additional enhancement of the PL (by a factor of 3) contributed by SPPs that are reflected from the end of the wires. Interestingly, in such a composite system, the additional improvement of the PL signal is observed only when the underlying Ag film is an epitaxial film instead of a commonly available thermal film. This is attributed to the reduction of propagation loss of the SPPs on atomically smooth, epitaxial films

Low-Threshold Plasmonic Lasers on a Single-Crystalline Epitaxial Silver Platform at Telecom Wavelength

We report on the first demonstration of metal–insulator–semiconductor-type plasmonic lasers at the telecom wavelength (∼1.3 μm) using top-down fabricated semiconductor waveguides on single-crystalline metallic platforms formed using epitaxially grown Ag films. The critical role of the Ag film thickness in sustaining plasmonic lasing at the telecom wavelength is investigated systematically. Low-threshold (0.2 MW/cm²) and continuous-wave operation of plasmonic lasing at cryogenic temperatures can be achieved on a 150 nm Ag platform with minimum radiation leakage into the substrate. Plasmonic lasing occurs preferentially through higher-order surface-plasmon-polariton modes, which exhibit a higher mode confinement factor, lower propagation loss, and better field–gain coupling. We observed plasmonic lasing up to ∼200 K under pulsed excitations. The plasmonic lasers on large-area epitaxial Ag films open up a scalable platform for on-chip integrations of plasmonics and optoelectronics at the telecom wavelength