Suppressing loss mechanisms in plasmonic structures is critical for the demonstration of high-quality-factor resonances with narrow linewidths. An extensively explored approach for loss reduction in these structures is the implementation of lattice plasmons (LPs) in which the primary loss mechanisms (i.e., Ohmic and radiation losses) can be simultaneously reduced. LPs take advantage of in-plane dipolar coupling of the scattered light from plasmonic arrays to provide narrow resonance linewidths at wavelengths approaching inter-particle distances. Here, we report numerical design and experimental demonstration of ultra-sharp (FWHM ≈ 6 nm) and tunable LP resonance modes in an array of gold (Au) nanopatches separated from a backside metallic film via a thin alumina (Al2O3) spacer layer. We show that oblique excitation of the array induces out-of-plane electric dipoles, which enable diffractive coupling of the incident light to the array, thus, exciting the LP mode. Furthermore, the excitation angle can be controlled to precisely tune important attributes of the LP lineshape including the resonance linewidth and the spectral position. Using spectroscopic ellipsometry measurements and finite-difference time-domain modeling, we show that the LP modes are only achievable through TM-polarized excitations, as a TE-polarized light lacks an out-of-plane electric-field component. The structure reported here holds a great promise for applications seeking strong light-matter interactions.
Alloying has served as a powerful means for tuning the non-vanishing optical bandgap of two-dimensional (2D) transition-metal dichalcogenides (TMDs), a family of 2D materials with optoelectronic properties covering a wide spectral window ranging from visible to near-infrared. In addition to the bandgap engineering, ‘spatial’ modulation of the composition ratio (i.e., x) in a ternary TMD alloy (e.g., MX2xX2(1-x)’; M: transition metal, X, X’: chalcogens) enables formation of lateral heterostructures with complex functionalities within the plane of 2D materials, a new asset that expands the realm of applications in which 2D materials can be incorporated. Despite several demonstrations of alloying in 2D TMDs, the phenomenologically important issue of strain development and its effect on the optical and structural properties of 2D TMD alloys is still missing.
Here, we show that alloying processes induce a biaxial tensile strain that acts on the lattice of 2D TMD alloys and affect their optical properties. In addition, we show that such strain inflicts sever fracture of the alloys via formation of sub-micron-sized cracks. Our experimental characterization combined with detailed theoretical modeling suggest the important role of the Van der Waals interaction between the 2D material and the substrate in formation of the alloying-induced strain. Furthermore, we demonstrate the critical role of crystal defects in cracking of the TMD alloys, which further emphasizes the importance of high quality synthesis of 2D TMD crystals for practical applications.
Nanostructured metals have utilized the strong spatial confinement of surface plasmon polaritons to harness enormous energy densities on their surfaces, and have demonstrated vast potential for the future of nano-optical systems and devices. While the spectral location of the plasmonic resonance can be tailored with relative ease, the control over the spectral linewidth associated with loss represents a more daunting task. In general, plasmonic resonances typically exhibit a spectral linewidth of ~50 nm, limited largely by the combined damping and radiative loss in nanometallic structures. Here, we present one of the sharpest resonance features demonstrated by any plasmonic system reported to date by introducing dark plasmonic modes in diatomic gratings. Each duty cycle of the diatomic grating consists of two nonequivalent metallic stripes, and the asymmetric design leads to the excitation of a dark plasmonic mode under normal incidence. The dark plasmonic mode in our structure, occurring at a prescribed wavelength of ~840 nm, features an ultra-narrow spectral linewidth of about 5 nm, which represents a small fraction of the value commonly seen in typical plasmonic resonances. We leverage the dark plasmonic mode in the metallic nanostructure and demonstrate a resonance enhanced plasmoelectric effect, where the photon-induced electric potential generated in the grating is shown to follow the resonance behavior in the spectral domain. The light concentrating ability of dark plasmonic modes in conjunction with the ultra-sharp resonance feature at a relatively low loss offers a novel route to enhanced light-matter interactions with high spectral sensitivity for diverse applications.