Haitao Chen

Two-dimensional transition-metal dichalcogenides (TMDCs) with intrinsically broken crystal inversion symmetry and large second-order 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 MoSe2 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 MoSe2 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. Combining two-dimensional materials with a silicon waveguide imparts silicon chips with desirable nonlinear optical effects. Two-dimensional transition-metal dichalcogenides exhibit large second-order responses, making them promising as nonlinear light sources, but their subnanometer thickness limits their nonlinear interaction with light. A European−Australian collaboration found that silicon slab waveguides topped with a layer of the transition-metal dichalcogenide MoSe2, a well-known two-dimensional material, produce enhanced second-harmonic generation. This is due to that the evanescent field of the silicon waveguide modes couple with MoSe2, which enables large second-order nonlinear response. Calculations indicate that a 1-mm-long waveguide could provide a second-harmonic signal that is 500 000 times larger than that obtained by pumping the MoSe2 monolayer from above. Other nonlinear effects such as wavelength conversion, parametric amplification and the generation of entangled photons should also be possible.

We demonstrate that a dielectric anapole resonator on a metallic mirror can enhance the third harmonic emission by two orders of magnitude compared to a typical anapole resonator on an insulator substrate. By employing a gold mirror under a silicon nanodisk, we introduce a novel characteristic of the anapole mode through the spatial overlap of resonantly excited Cartesian electric and toroidal dipole modes. This is a remarkable improvement on the early demonstrations of the anapole mode in which the electric and toroidal modes interfere off-resonantly. Therefore, our system produces a significant near-field enhancement, facilitating the nonlinear process. Moreover, the mirror surface boosts the nonlinear emission via the free-charge oscillations within the interface, equivalent to producing a mirror image of the nonlinear source and the pump beneath the interface. We found that these improvements result in an extremely high experimentally obtained efficiency of 0.01%.

Optical nanoantennas have shown a great capacity for efficient extraction of photons from the near to the far field, enabling directional emission from nanoscale single-photon sources. However, their potential for the generation and extraction of multi-photon quantum states remains unexplored. Here we experimentally demonstrate the nanoscale generation of two-photon quantum states at telecommunication wavelengths based on spontaneous parametric down-conversion in an optical nanoantenna. The antenna is a crystalline AlGaAs nanocylinder, possessing Mie-type resonances at both the pump and the bi-photon wavelengths, and when excited by a pump beam it generates photon pairs with a rate of 35 Hz. Normalized to the pump energy stored by the nanoantenna, this rate corresponds to 1.4 GHz/Wm, being 1 order of magnitude higher than conventional on-chip or bulk photon-pair sources. Our experiments open the way for multiplexing several antennas for coherent generation of multi-photon quantum states with complex spatial-mode entanglement and applications in free-space quantum communications and sensing.

Different sizes of CdS nanobelts were synthesized at 800, 850, and 900 °C by the thermal evaporation of CdS powders on Au-coated silicon substrates and were used to study the size effects of Raman scattering and photoluminescent spectra. The Raman spectra of CdS nanobelts clearly exhibit first- and second-order longitudinal optical (LO) Raman peaks, surface phonon peaks, and multiphonon processes when excited using a wavelength of 532 nm. The mechanism of exciton–phonon coupling was observed to be mainly associated with the Fröhlich interaction, and the coupling strength of the exciton–phonon increases with increasing lateral size. Compared with a larger CdS nanobelt, a narrower nanobelt exhibits a larger tensile strain. Recombination of free excitons (FX), excitons bound to neutral impurities (A0X), and donor–acceptor pairs (DAP) were identified from a low-temperature PL spectrum. At temperatures below ∼123 K, a red shift of the FX energy occurs with decreasing lateral size due to a larger uniaxial tensile strain; at temperatures above ∼123 K, a red shift of the FX energy occurs with increasing lateral size because of the reabsorption of the emitted light inside the thicker belt, indicating that the FX energy is affected by both the tensile strain and the surface-depletion-induced quantum confinement (the reabsorption of the emitted light) in the nanobelt.