Active Exciton Resonance Tuning for Ultrathin Free-Space Optical Modulators

Emergent optoelectronic technologies such as augmented reality and free-space optical communications call for ultracompact devices with dynamic control over their optical functionalities. This presents a problem, since conventional optoelectronics require bulky optical elements and filters to operate, thereby limiting the minimal device footprint. In recent years, optically resonant nanostructures and metasurfaces have enabled highly compact devices and optical coatings with tailored spectral, angular, and polarization responses. Nevertheless, active tuning of the optical response remains challenging, owing to the weak electro-absorptive and electro-refractive effects in traditional semiconductors and noble metals.<br/><br/>Here, we leverage highly tunable exciton resonances in 2D quantum materials to realize hybrid metasurfaces for active wavefront manipulation. We greatly enhance the excitonic light-matter interaction through coupling with guided mode resonances in dielectric metasurfaces. More specifically, we embed atomically thin WS₂ in an asymmetric Van der Waals heterostructure cavity and integrate it with a subwavelength grating. The resulting hybrid-2D metasurface exhibits highly tunable and near-unity excitonic light absorption at room temperature, which we leverage to realize an ultrathin free-space optical modulator.<br/><br/>We fabricate a heterostructure comprised of atomically thin WS₂ encapsulated by hBN on a Au back-reflector using an all-dry deterministic stamping technique. This method enables sequential stacking of the 2D layers onto the back-reflector, which we prepatterned on a SiO₂/Si substrate. As such, we circumvent direct lithography on the WS₂ flake, thereby largely preserving its pristine optical and electronic properties. Encapsulation of the monolayer with hBN also limits degradation caused by oxidation, environmental adsorbates, and nanofabrication of the grating.<br/><br/>Next, we integrate the heterostructure cavity with a nonlocal dielectric metasurface to maximize the excitonic light-matter interaction. We fabricate a subwavelength grating in the top hBN layer using electron-beam lithography and reactive-ion etching. The grating allows free-space radiation to couple efficiently to a guided mode resonance of the structure. The guided mode confines the field in the structure and therefore interacts strongly with the monolayer. By tailoring the modal dispersion to spectrally overlap with the exciton resonance, we greatly enhance the coupling efficiency at the operation wavelength. In fact, we achieve near-unity excitonic absorption at room temperature through critical coupling to the cavity mode.<br/><br/>Lastly, we employ electrostatic gating to actively manipulate the exciton resonance amplitude. In our device, the WS₂ monolayer is connected to ground, while the Au back-reflector is attached to a voltage source. By applying a gate voltage, we actively modulate the carrier density in the WS₂ via the field effect. By inducing strong n- or p-type doping in the monolayer, the Coulombic electron-hole interaction is effectively screened, and we observe a 90% reduction in excitonic emission at room temperature. We can actively and reversibly switch the device out of the critically coupled state by suppressing the exciton state, allowing for a large modulation depth in reflection. Altogether, our results demonstrate how exciton resonance tuning combined with strong light-matter interaction in hybrid-2D metasurfaces enables dynamically tunable and ultracompact optoelectronic devices.