Excitonic Beam Steering in an Active van der Waals Metasurface

Abstract

Two-dimensional transition metal dichalcogenides (2D TMDCs) are promising candidates for ultrathin active nanophotonic elements due to the strong tunable excitonic resonances that dominate their optical response. Here, we demonstrate dynamic beam steering by an active van der Waals metasurface that leverages large complex refractive index tunability near excitonic resonances in monolayer molybdenum diselenide (MoSe2). Through varying the radiative and nonradiative rates of the excitons, we can dynamically control both the reflection amplitude and phase profiles, resulting in an excitonic phased array metasurface. Our experiments show reflected light steering to angles between −30° and 30° at different resonant wavelengths corresponding to the A exciton and B exciton. This active van der Waals metasurface relies solely on the excitonic resonances of the monolayer MoSe2 material rather than geometric resonances of patterned nanostructures, suggesting the potential to harness the tunability of excitonic resonances for wavefront shaping in emerging photonic applications.

High-quality-factor space–time metasurface for free-space power isolation at near-infrared regime

“…In particular, electrical gating, optical pumping, thermal annealing, and current-induced heating can be employed for this purpose 14 – 19 Thus far, quantum-confined Stark effects in multiple quantum wells, 20 , 21 molecular phase transitions in vanadium dioxide (VO2) or germanium antimony telluride, 22 reorientation of molecules in liquid crystals, 23 ionic transparent, and carrier-induced field effects in semiconductors, i.e., silicon (Si), gallium arsenide, 24 , 25 transparent conducting oxides (TCOs), 26 – 30 indium arsenide (InAs), 31 , 32 and atomically thin transition metal dichalcogenides, 33 , 34 have been leveraged for index modulation. Among these tuning mechanisms, the free-carrier-induced effects in doped semiconductors have yielded great promise for active tuning thanks to higher modulation frequencies (up to several gigahertz), continuous tunability, and lower power consumption compared to techniques based on the Pockels effect or waveguide modulators based on lithium niobate.…”

Temperature-Dependent Excitonic Light Manipulation with Atomically Thin Optical Elements

“…The intrinsic nature of exciton resonances in 2D-TMDs renders their spectral properties most notably dependent on the band structure of the material, as opposed to the geometry, as with plasmon and Mie-resonators. This allows for their facile integration in more complex architectures. , Furthermore, light scattering by these resonances can be largely and reversibly manipulated via electrostatic free carrier injection, − temperature, , strain, − and external fields, enabling actively tunable nanophotonic devices.…”

“…At the same time, the strong light–matter interaction within a single TMD monolayer poses an intriguing opportunity to realize atomically thin optical elements. In the limit of single layer metasurfaces, the optical function is dictated by the nanopatterned monolayer only, without influences of external (van-der-Waals heterostructure) cavities, , or plasmon resonances in nanopatterned electrodes. , However, to obtain a complex optical functionality such as wavefront shaping by merely nanopatterning the TMD monolayer, the in-plane dimensions of small flakes are typically the limiting factor in achieving the desired functionality. The size obstacle can be resolved by using chemical-vapor deposition (CVD) growth techniques.…”

“…Following the procedure outlined in ref , we then fit this susceptibility (that was retrieved purely numerically) using a physically meaningful model to extract the excitonic decay rates. We use a constant background term and a Lorentzian oscillator − for each of the ground states of the three main exciton resonances of monolayer WS 2 : χ ( E ) = χ ∞ − prefix∑ j = A , B , C ℏ c E j · t · ℏ γ normalr j ( E − E j ) + i ℏ γ nr j 2 …”

Tunable achromatic metalens for generating focused vortex beam in the near-infrared range

“…As for tunable metasurfaces, various functional materials have been proposed to design metasurfaces, including Ge 2 Sb 2 Te 5 (GST), 20) liquid crystals, 21) InSb, 22) graphene, 23) MoSe 2 24) and so on. It is worth noting that GST has gained significant attention in the NIR regime because of its rapid response, minimal optical losses, excellent stability, and substantial variation in refractive index between its crystalline and amorphous states.…”