A Monolayer Semiconductor Free-Space Optical Modulator

Recent years have witnessed a significantly growing need to develop free-space optical modulators for future optical communications, computer interconnections, and phased-array optical elements. Unfortunately, conventional semiconductors typically show very weak optical modulation effects due to its negligible exciton resonances and very thin accumulation/depletion layers. In sharp contrast, transition metal dichalcogenide (TMDC) monolayers support very strong exciton resonances thanks to its weak dielectric screening, and can be efficiently modulated via electrical gating due to its sub-nanometer thickness. Therefore, TMDC monolayers show great potential to build high-efficiency, ultra-compact free-space optical modulators. However, the sub-nanometer thickness of TMDC monolayers also prevent efficient light-exciton interactions. For this reason, only a ~ 0.5% reflection modulation can be observed for suspended monolayer WS₂ at room temperature.<br/><br/>Here, we demonstrate a monolayer semiconductor free-space optical modulator that affords efficient light modulation by placing a piece of monolayer WS₂ on top of a judiciously engineered silver metasurface gating pad spaced by a 15nm-thick Al₂O₃ gating oxide. The metasurface pad supports surface plasmon polaritons with a periodic perturbation to open the coupling channel with the free-space light. We experimentally show that this platform enables 10% reflection modulation and the modulation on/off ratio reaches 3dB with merely a 3V gating voltage. Consequently, it improves the reflection modulation effect by one and half orders of magnitude compared with the suspended monolayer WS₂. An AC modulation experiment is conducted as well, which verifies the good reversibility of the fabricated optical modulator. We further extend the concept from reflection modulation to phase modulation by designing an asymmetric metasurface grating as the gating pad, where the first order diffraction efficiency of the reflected beam is electrically modulated. We emphasize that the entire fabrication process is cleanroom standard and scalable. The fabrication starts from a millimeter-scale monolayer WS₂ flake exfoliated from bulk crystal, and finally 150 optical modulators are successfully patterned on the single chip.<br/><br/>The observed, significantly enhanced modulation results from designing an optimized photonic environment surrounding the monolayer WS₂. On the one hand, the radiative decay rate of excitons is tremendously enhanced by the Purcell effect through the coupling between the excitons and the surface plasmon resonance supported by the metasurface pad. On the other hand, the total radiative and non-radiative decay rate of the system can be finely tuned by gently modifying the perturbation of the surface plasmon polaritons. Therefore, the critical coupling can be achieved at one designed point, and thus the modulation on/off ratio is maximized. A temporal coupled mode theory is developed to predict the reflection spectrum of the coupled system, and it is in excellent consistence with the full-field simulations. We also build an optical rate-equation model to quantitatively link the modulation to the electrical gating voltage by revealing that the total decay rate of the excitons is doping dependent. Our full-field simulations suggest that a 40% reflection modulation as well as a 10dB on/off ratio can be realized for an optimized device.<br/><br/>These results demonstrate an unprecedented free-space optical modulator platform by merging a highly tunable excitonic monolayer semiconductor in an optimized photonic environment to boost the light-exciton interaction and offer the critical coupling. They can lead to many practical applications, ranging from free-space optical communications to Lidar systems for autonomous driving. The successful integration of nanophotonics with monolayer semiconductor optoelectronic devices further paves the way towards multi-functional, ultra-compact two-dimensional meta-devices.