Exciton-Resonance Tuning for Atomically-Thin Metasurfaces

Since the development of diffractive optical elements in the 1970s research has focused on replacing bulky optical elements such as lenses and grating by thin counterparts. Over the last decade, nanophotonic metasurfaces rapidly advanced the development of flat optical elements based on the realization that resonant optical antenna elements enable local phase control. Present applications of metasurface flat optical elements include lenses, polarization control, and beam steering.<br/><br/>Next-generation applications of flat optics such as light detection and ranging (LIDAR), dynamic holography, and computational imaging require dynamic control over optical functionalities, e.g. the focal position or efficiency of optical elements. However, most nanophotonic structures are static after design and fabrication. Current approaches for dynamic control like electrical gating exhibit limited tunability due to the finite electrorefraction and electroabsorption effects in metals and semiconductors.<br/><br/>Here, we demonstrate how exciton resonances in monolayer transition-metal dichalcogenides (TMDs) like WS₂ can function as a new type of tunable resonant light-matter interaction in nanophotonic metasurfaces. By directly patterning the monolayer material, the 2D material is turned into the antenna or metamaterial and incorporation of active materials into larger antenna structures will no longer be needed. Due to their sub-nm thickness, these materials are highly tunable through external control. In this presentation, I will present three applications of exciton-enhanced light scattering.<br/><br/>First, we demonstrate actively-tunable and atomically-thin optical lenses by carving them directly out of large-area monolayer WS₂. Using an electrochemical cell, we electrostatically control the carrier density in the monolayer WS₂ and thereby gain active control over the excitonic light scattering amplitude. Using confocal scanning microscopy, we characterize the focal shape and analyze the focal efficiency. We demonstrate dynamic electrical tuning of the focusing efficiency with a 33% modulation depth through manipulating of the excitonic material resonance properties as opposed to tuning of antenna resonances.<br/><br/>Second, we employ these atomically-thin lenses to directly study the influence of exciton decay and dephasing on the metasurface functionality and spectral line shape. To do this, we employ a helium cryostat to measure the efficiency spectrum as a function of temperature. At ambient conditions, the spectrum shows a strong asymmetric line shape revealing that the scattered light fields are directly governed by the monolayer susceptibility. For decreasing temperatures on the other hand, the exciton energy shows a blue-shift, non-radiative decay and dephasing are suppressed and the exciton becomes fully radiative. By comparing the resulting line shapes to an analytical model, we show that the efficiency of the metasurface lens directly scales with the excitonic oscillator strength and decay dynamics.<br/><br/>Third, we use reflection measurements of monolayer TMD on a quartz substrate to highlight that the interference of the substrate and TMD reflections can strongly influence the exciton line shape. By systematically controlling the substrate reflection with index-matching oils, we engineer the interference and thereby the line shape. We further show how basic, room-temperature reflection measurements allow investigation of the quantum mechanical exciton dynamics. By removing the substrate contribution with a properly chosen oil, we can extract the excitonic decay rates including the quantum mechanical dephasing rate.<br/><br/>The strong light-matter interaction and highly tunable nature of these exciton resonances opens an entirely new approach for the design of dynamic flat optics and metasurfaces with applications in free-space beam tapping, wavefront manipulation, and augmented/virtual reality.