Enhanced second-harmonic generation from two-dimensional MoSe_(2) on a silicon waveguide

Two-dimensional transition-metal dichalcogenides(TMDCs)with intrinsically broken crystal inversion symmetry and large secondorder nonlinear responses have shown great promise for future nonlinear light sources.However,the sub-nanometer monolayer thickness of such materials limits the length of their nonlinear interaction with light.Here,we experimentally demonstrate the enhancement of the second-harmonic generation from monolayer MoSe_(2) by its integration onto a 220-nm-thick silicon waveguide.Such on-chip integration allows for a marked increase in the interaction length between the MoSe_(2) and the waveguide mode,further enabling phase matching of the nonlinear process.The demonstrated TMDC–silicon photonic hybrid integration opens the door to second-order nonlinear effects within the silicon photonic platform,including efficient frequency conversion,parametric amplification and the generation of entangled photon pairs.

Giant photoinduced reflectivity modulation of nonlocal resonances in silicon metasurfaces

Metasurfaces offer a unique playground to tailor the electromagnetic field at subwavelength scale to control polarization,wavefront,and nonlinear processes.Tunability of the optical response of these structures is challenging due to the nanoscale size of their constitutive elements.A long-sought solution to achieve tunability at the nanoscale is all-optical modulation by exploiting the ultrafast nonlinear response of materials.However,the nonlinear response of materials is inherently very weak,and,therefore,requires optical excitations with large values of fluence.We show that by properly tuning the equilibrium optical response of a nonlocal metasurface,it is possible to achieve sizable variation of the photoinduced out-ofequilibrium optical response on the picosecond timescale employing fluences smaller than 250μJ∕cm^(2),which is 1 order of magnitude lower than previous studies with comparable reflectivity variations in silicon platforms.Our results pave the way to fast devices with large modulation amplitude.

Fundamental limits for transmission modulation in VO_(2) metasurfaces

The interest in dynamic modulation of light by ultra-thin materials exhibiting insulator–metal phase transition,such as VO_(2),has rapidly grown due to the myriad industrial applications,including smart windows and optical limiters.However,for applications in the telecommunication spectral band,the light modulation through a thin VO_(2) film is low due to the presence of strong material loss.Here,we demonstrate tailored nanostructuring of VO_(2) to dramatically enhance its transmission modulation,reaching a value as high as 0.73,which is 2 times larger than the previous modulation achieved.The resulting designs,including free-topology optimization,demonstrate the fundamental limit in acquiring the desired optical performance,including achieving positive or negative transmission contrast.Our results on nanophotonic management of lossy nanostructured films open new opportunities for applications of VO_(2) metasurfaces.