Abstract
The hybrid combination of two-dimensional (2D) transition metal dichalcogenides (TMDs) and plasmonic materials open up novel means of (ultrafast) optoelectronic applications and manipulation of nanoscale light–matter interaction. However, control of the plasmonic excitations by TMDs themselves has not been investigated. Here, we show that the ultrathin 2D WSe2 crystallites permit nanoscale spatially controlled coherent excitation of surface plasmon polaritons (SPPs) on smooth Au films. The resulting complex plasmonic interference patterns are recorded with nanoscale resolution in a photoemission electron microscope. Modeling shows good agreement with experiments and further indicates how SPPs can be tailored with high spatiotemporal precision using the shape of the 2D TMDs with thicknesses down to single molecular layers. We demonstrate the use of WSe2 nanocrystals as 2D optical elements for exploring the ultrafast dynamics of SPPs. Using few-femtosecond laser pulse pairs we excite an SPP at the boundary of a WSe2 crystal and then have a WSe2 monolayer wedge act as a delay line inducing a spatially varying phase difference down to the attosecond time range. The observed effects are a natural yet unexplored consequence of high dielectric functional values of TMDs in the visible range that should be considered when designing metal–TMD hybrid devices. As the 2D TMD crystals are stable in air, can be defect free, can be synthesized in many shapes, and are reliably positioned on metal surfaces, using them to excite and steer SPPs adds an interesting alternative in designing hybrid structures for plasmonic control.
Highlights
Two-dimensional crystals of transition metal dichalcogenides (TMDs) have been extensively studied in recent years due to their many potential applications in optoelectronics.[1−3] TMDs are characterized by a distinct layered structure, making the fabrication of 2D crystals similar to graphene possible
The surface plasmon polaritons (SPPs) can propagate in both directions: away from and through the WSe2 crystal (Figure 1a, backscattered SPPs are not shown for clarity)
While the present study demonstrates the concept of using 2D TMDs for temporal control and dynamics studies of atomic scale confined systems, it indicates that further studies into the detailed temporal dynamics of SPPs in this 2D semiconductor−metal hybrid system would be interesting
Summary
Two-dimensional crystals of transition metal dichalcogenides (TMDs) have been extensively studied in recent years due to their many potential applications in optoelectronics.[1−3] TMDs are characterized by a distinct layered structure, making the fabrication of 2D crystals similar to graphene possible. Quantum confinement and a reduced dielectric screening change the carrier dynamics and the correlated electron behavior, leading to the presence of interesting exciton phenomena on an ultrafast time scale.[4] To amplify and control the excitations in TMDs spectrally, as well as in time and space, surface plasmon polaritons (SPPs) have recently gained significant interest.[5−8] To induce SPPs by light, the spatial symmetry of the surface has to be broken due to momentum conservation This has usually been accomplished by synthesizing nanocrystallites with naturally confined borders like silver rods or gold flakes[7] or by manufacturing tens of nanometers deep/high holes, ridges, protrusions, or other morphological variations in metal films.[5,9] By combining plasmonic materials with single molecular layers, it has been found that SPPs can be incoherently excited by excitons in 2D materials[10] or by prepared tunnel junctions.[11] the ability to form atomic scale precise boundaries and layer thicknesses in TMD thin films has not been used for coherent SPP excitation and manipulation, despite the high values of the dielectric functions,[12] yielding refractive indices between 4 and 5 in the visible range. This would be highly desirable due to the widespread use of propagating SPPs at metal−dielectric interfaces, which with perfect crystalline systems could lead to coherent control with subfemtosecond temporal precision and low losses in realistically defect free structures
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