Exciton resonance tuning of an atomically thin lens
Exciton resonances in monolayer 2D semiconductor can be used to realise large-area atomically-thin lenses with a tuneable intensity in the focus.
Excitons are quantum-mechanical quasi-particles composed of an electron and a hole bound by electrostatic forces. In conventional 3D semiconductors, these excitons are dissociated into free charge carriers at room temperature due to the small binding energy. In monolayer 2D semiconductors such as WS2 however, the quantum confinement of the electron wave function and the lack of dielectric screening strongly enhances the binding energy to hundreds of meV. As a result, excitons are stable at room temperature and dominate the optical response of atomically-thin semiconductor layers. Moreover, the exciton resonance is highly sensitive to external fields and carrier density.
Here, we leverage the highly-tuneable properties of such exciton resonances in monolayer WS2 to realise an atomically-thin optical lens for visible light with a tuneable focal intensity. The left image shows a schematic of the device, which is mounted in a transparent electro-chemical cell. Concentric rings composed of nanopatterned monolayer WS2 (brown) are contacted by radial gold electrodes (yellow). The optical response of the lens is dominated by excitons in the semiconductor material (red/blue disks on WS2). A small external bias voltage injects high electron concentrations into the WS2, thereby quenching the exciton resonance and reducing the excitonic light scattering efficiency. As a result, the intensity in the focus is suppressed (see the measured intensity time trace at top of the image). The electrostatic interaction is screened by an ionic liquid in the cell. The right image shows an optical microscope image of the center of the lens, which shows the concentric rings of monolayer WS2. The inset shows the focus formed 2 mm above the surface of the lens, measured using scanning confocal microscopy.