OPTICAL MANIPULATIONS OF 2D TMDC

Nat. Photonics, 2019, doi: 10.1038/s41566-019-0399-1

Monolayer transition-metal dichacolgenides (TMDCs) present a direct optical bandgap at the Brillouin zones, so-called valleys. Those energetically degenerate valleys (K and K’) present different valley pseudospins, emitting the valley photons with opposite spin angular momentums due to nonlinear optical selection rules. Furthermore, although atomically thin, two-dimensional (2D) TMDCs have giant nonlinearity, which can be enhanced by the valley-excitons. However, there are still big challenges for optical approaches to coherently access and manipulate the valley degree of freedoms of 2D TMDCs, especially in nonlinear region.

Now, an international team led by Prof. Cheng-Wei QIU from National University of Singapore has proposed and demonstrated that they can use the optical 2D metamaterials, namely metasurface, to boost, selectively pump and steer the nonlinear chiral photons from different valleys of monolayer tungsten disulfide (WS₂). The optical metasurface can apply the spin-dependent geometric phases to the fundamental-frequency light, which could further pump the 2D materials. Thanks to the coherent light-matter interaction of second-harmonic generation (SHG) and the spin-valley-exciton-locked SHG in 2D TMDCs, the geometric phase can be further inherited by nonlinear photons, showing a valley-dependent geometric phases and thus spatially steering chiral valley photons with nearly perfect figure or merits. Meanwhile, SHG of 2D WS₂ can be also enhanced by localized plasmon resonance in metasurface. This work points out the possibility to synthesize the optical metamaterials and metasurfaces with 2D materials for double-win applications and could stimulate the widespread interest in nonlinear metamaterials, 2D optoelectronics, quantum optics and others.

Chuanbo Li (School of Science, Minzu University of China, Beijing, China); Ming Li (Institute of Semiconductors, CAS, Beijing, China)

doi: 10.1088/1674-4926/40/6/060201