Churong Ma

Generating scattering dark states through the Fano interference between excitons and an individual silicon nanogroove

Effective interactions between excitons and resonating nanocavities are important for many emerging applications in nanophotonics. Although plasmonic nanocavities are considered promising substitutes for diffraction-limited dielectric microcavities, their practical applications are hindered by large ohmic loss and Joule heating. Other than plasmonic materials, high-refractive-index dielectric nanocavities is a new way to trap light in subwavelength scales. However, studies on the interaction between dielectric nanocavities and excitons are still scarce. Here, for the first time, we demonstrate that the Fano interference between molecular excitons and an individual silicon nanogroove can generate scattering dark modes. By placing J-aggregate excitons into a silicon nanogroove, the leaky magnetic resonant modes filling in the groove can tailor their scattering directivity and reduce the uncoupled radiation decay in a specific direction. This unidirectional ‘dark state’ brings a new approach to tailor the interaction between excitons and nanocavities without large near-field enhancement. By adjusting the resonant modes, the scattering spectra can change from a Fano asymmetric line shape to a significantly suppressed scattering dip. These findings indicate that silicon nanogrooves can provide a platform for integrated on-chip silicon–exciton hybrid optical systems in the future.

Enhanced second harmonic generation in individual barium titanate nanoparticles driven by Mie resonances

Although previous designs of nonlinear optical (NLO) nanostructures have focused on photonic crystals and metal plasmonic nanostructures, complex structures, large ohmic loss, and Joule heating greatly hinder their practical applications. Beyond photonic crystals and metal plasmonic nanostructures, all-dielectric materials (ADMs) offer new ways to generate NLO behavior at subwavelength scales. Herein, we report enhancement in the tunable second harmonic generation (SHG) reflected from individual mid-refractive ADM nanoparticles, BaTiO3 nanoparticles (BTO NPs). Multipole decomposition, as observed in the linear spectra, demonstrated that the SHG enhancement originated from an overlap between the magnetic dipole or quadrupole resonance and the second harmonic wavelength of the pump source. In the vicinity of magnetic resonances, the localized field inside the nanoparticles could be increased by more than one order of magnitude. Compared with the spectral-separated electric and magnetic resonances in high-refractive all-dielectric nanostructures, an overlap of resonances was observed in the mid-refractive all-dielectric nanostructures and it resulted in electromagnetic (EM) mode coupling. A broad spectral characteristic results in moderate EM field enhancements over a wide wavelength range, which is conducive to the tunability of the SHG responses. Our study revealed the relationship between the linear and nonlinear optics at the nanoscale and helped in the design of efficient nonlinear optical devices based on ADMs.

Second harmonic generation from an individual all-dielectric nanoparticle: resonance enhancement versus particle geometry

Nonlinear optical (NLO) nanostructures have played important roles in frequency conversion, optical switching, information storage and biomedical imaging. Although previous designs have focused on photonic crystals and metal plasmonic nanostructures, complex structure, large ohmic loss and Joule heating greatly hinder their practical applications. Beyond photonic crystals and metal plasmonic nanostructures, all-dielectric materials (ADMs) bring new ways to generate NLO behavior at subwavelength scales. We, for the first time to our knowledge, demonstrate irregular-geometry induced second harmonic generation (SHG) enhancement from an individual all-dielectric SiC nanoparticle. SHG conversion efficiency of single irregular-geometry SiC nanoparticles increases to 10−5 under an average excitation power of 6 mW at 880 nm excitation, thirtyfold higher than that of SiC nanosphere. We also establish that not only electric dipole mode but also magnetic dipole mode can exist in the vicinity of nanoparticles in mid-refractive (2 < n < 3) ADMs, and the stronger enhancement of the magnetic response induced by the irregular-geometry makes a great contribution to the SHG enhancement. A modified formula that includes electric and magnetic contributions is proposed to predict the SHG enhancement from ADMs. This discovery makes geometry-tuning all-dielectric nanoparticles promising in NLO nanostructures of nanophotonics in the future.

Giant nonlinear optical responses of carbyne

Carbyne, the third allotrope of carbon with alternating single and triple bonds, consists of sp-hybridized carbon atoms. Theoretically, with two degenerate π-electron bands in one monomer, carbyne exhibits stronger nonlinear optical responses than graphene and it is acknowledged as a superior nonlinear optical material with just one π-electron band. Very recently, carbyne has been synthesized in the laboratory. Here, we demonstrate the giant nonlinear optical responses of carbyne. By employing the Z-scan technique using a femtosecond laser, we show that carbyne exhibits a large nonlinear absorption coefficient β and a refractive index n2 of 3.53 × 10−13 m W−1 and −1.40 × 10−13 esu at 800 nm excitation, respectively. Excellent broadband optical limiting responses of carbyne to femtosecond laser pulses at 800 and 400 nm were observed, respectively. We also establish that extensive π-electron delocalization and electron resonance enhancement of carbyne have made significant contributions to these nonlinear optical responses. These findings open the door to exploring the optical and technological applications of carbyne.

Electrically Controlled Scattering in a Hybrid Dielectric-Plasmonic Nanoantenna

Electrically tunable devices in nanophotonics offer an exciting opportunity to combine electrical and optical functions, opening up their applications in active photonic devices. Silicon as a kind of high refractive index dielectric material has shown comparable performances with plasmonic nanostructures in tailoring and modulating the electromagnetic waves. However, there are few studies on electrically tunable silicon nanoantennas. Here, for the first time we realize the spectral tailoring of an individual silicon nanoparticle in the visible range through changing the applied voltage. We observe that the plasmon-dielectric hybrid resonant peaks experience blue shift and obvious intensity attenuation with increasing the bias voltages from 0 to 1.5 V. A physical model has been established to explain how the applied voltage influences the carrier concentration and how carrier concentration modifies the permittivity of silicon and then the final scattering spectra. Our findings pave a new approach to build excellent tunable nanoantennas or other nanophotonics devices where the optical responses can be purposely controlled by electrical signals.

The optical duality of tellurium nanoparticles for broadband solar energy harvesting and efficient photothermal conversion

Tellurium nanoparticles are used for broadband solar energy harvesting and efficient photothermal conversion. Nanophotonic materials for solar energy harvesting and photothermal conversion are urgently needed to alleviate the global energy crisis. We demonstrate that a broadband absorber made of tellurium (Te) nanoparticles with a wide size distribution can absorb more than 85% solar radiation in the entire spectrum. Temperature of the absorber irradiated by sunlight can increase from 29° to 85°C within 100 s. By dispersing Te nanoparticles into water, the water evaporation rate is improved by three times under solar radiation of 78.9 mW/cm2. This photothermal conversion surpasses that of plasmonic or all-dielectric nanoparticles reported before. We also establish that the unique permittivity of Te is responsible for the high performance. The real part of permittivity experiences a transition from negative to positive in the ultraviolet-visible–near-infrared region, which endows Te nanoparticles with the plasmonic-like and all-dielectric duality. The total absorption covers the entire spectrum of solar radiation due to the enhancement by both plasmonic-like and Mie-type resonances. It is the first reported material that simultaneously has plasmonic-like and all-dielectric properties in the solar radiation region. These findings suggest that the Te nanoparticle can be expected to be an advanced photothermal conversion material for solar-enabled water evaporation.

Active tuning of the Fano resonance from a Si nanosphere dimer by the substrate effect

All-dielectric materials have aroused great interest for their unique light scattering and lower losses compared with plasmonics. Generally, optical properties made by all-dielectric materials can be passively controlled by varying the geometry, size and refractive index at the design stage. Therefore, the realization of active tuning in the field of nanophotonics is important to improve the practicality and achieve light-on-chip technology in the future. Herein, we combine the high refractive index of Si and the phase transition of VO2 to form an active tuning hybrid nanostructure with higher quality factor by depositing Si nanospheres on the VO2 layer with an Al2O3 substrate. As the temperature goes up, the refractive index of the VO2 layer switches from high to low. The scattering intensity of the magnetic dipole resonance of Si nanospheres decreases differently depending on their size, while the intensity of the electric dipole resonance remains almost unchanged. Meanwhile, Fano resonances are observed in the Si nanosphere dimers with a continuous variable Fano lineshape when adjusting the temperature. Mie theory and substrate-induced resonant magneto-electric effects are used to analyze and explain these phenomena. Tuning of the Fano resonance is attributed to the substrate effect from the interaction between Si nanospheres and phase transition of the VO2 layer with temperature. These light scattering properties of such a hybrid nanostructure make it promising for temperature sensing or as a light source at the nanometer scale.

Photoluminescence manipulation of WS2 flakes by an individual Si nanoparticle

Optical manipulation of photoluminescence (PL) emission in 2D materials through nanophotonic structures has attracted a lot of attention. However, it has not been achieved through individual all-dielectric nanoparticles (NPs) so far. Here, we put forward a new hybrid system to manipulate the PL emission, which is composed of an individual Si NP deposited on WS2 flakes of different thicknesses. For monolayer WS2 (1L-WS2), PL quenching accompanied by broadening and redshift is observed when integrated with Si NPs. In contrast, the PL of multilayer WS2 (ML-WS2) is significantly enhanced with the help of Si NPs. The PL manipulation of 1L- and ML-WS2 is attributed to the heating and strain effects due to the presence of Si NPs as well as the interaction between the localized field induced by Si NPs and the exciton dipoles in the WS2 flakes. Based on Mie resonances, Si NPs can be effectively heated up by laser pulses. The localized high temperature and strain enable the 1L-WS2 to transform from a direct to an indirect bandgap, resulting in PL quenching and redshift. On the other hand, the out-of-plane oriented exciton dipoles in ML-WS2 are easier to couple with the resonant optical field in Si NPs than the in-plane oriented exciton dipoles in 1L-WS2, which brings about strong field enhancement in favor of PL emission. The new hybrid system is promising for photodetection and on-chip circuit integration.