Vortex nonlinear optics in monolayer van der Waals crystals

Abstract

In addition to wavelength and polarization, coherent light possesses a degree of freedom associated with its spatial topology that, when exploited through nonlinear optics, can unlock a plethora of new photonic phenomena. A prime example involves the use of vortex beams, which allow for the tuning of light's orbital angular momentum (OAM) on demand. Such processes can not only reveal emergent physics but also enable high-density classical and quantum communication paradigms by allowing access to an infinitely large set of orthogonal optical states. Nevertheless, structured nonlinear optics have failed to keep pace with the ever-present need to shrink the length-scale of optoelectronic and photonic technologies to the nanoscale regime. Here, we push the boundaries of vortex nonlinear optics to the ultimate limits of material dimensionality. By exploiting second and third-order nonlinear frequency-mixing processes in van der Waals semiconductor monolayers, we show the free manipulation of the wavelength, topological charge, and radial index of vortex light-fields. We demonstrate that such control can be supported over a broad spectral bandwidth, unconstrained by traditional limits associated with bulk nonlinear optical (NLO) materials, due to the atomically-thin nature of the host crystal. Our work breaks through traditional constraints in optics and promises to herald a new avenue for next-generation optoelectronic and photonics technologies empowered by twisted nanoscale nonlinear light-matter interactions.