Electrically Controlled Scattering in a Hybrid Dielectric-Plasmonic Nanoantenna

Ultrafast Sub-100 fs All-Optical Modulation and Efficient Third-Harmonic Generation in Weyl Semimetal Niobium Phosphide Thin Films

Since their experimental discovery in 2015, Weyl semimetals have generated a large amount of attention due their intriguing physical properties that arise from their linear electron dispersion relation and topological surface states. In particular, in the field of nonlinear (NL) optics and light harvesting, Weyl semimetals have shown outstanding performances and achieved record NL conversion coefficients. In this context, the first steps toward Weyl semimetal nanophotonics are performed here by thoroughly characterizing the linear and NL optical behavior of epitaxially grown niobium phosphide (NbP) thin films, covering the visible to the near-infrared regime of the electromagnetic spectrum. Despite the measured high linear absorption, third-harmonic generation studies demonstrate high conversion efficiencies up to 10^-4 % that can be attributed to the topological electron states at the surface of the material. Furthermore, nondegenerate pump-probe measurements with sub-10 fs pulses reveal a maximum modulation depth of ≈1%, completely decaying within 100 fs and therefore suggesting the possibility of developing all-optical switching devices based on NbP. Altogether, this work reveals the promising NL optical properties of Weyl semimetal thin films, which outperform bulk crystals of the same material, laying the grounds for nanoscale applications, enabled by top-down nanostructuring, such as light-harvesting, on-chip frequency conversion, and all-optical processing.

Enhancement of exciton emission in WS2 based on the Kerker effect from the mode engineering of individual Si nanostripes

Coupling between nanostructures and excitons has attracted great attention for potential applications in quantum information technology. Compared with plasmonic platforms, all-dielectric nanostructures with Mie resonances are more practical because of low-loss, low-cost and CMOS compatibility. However, weak field enhancements in single element dielectric nanostructures hinder their applications in both strong and weak coupling regimes. The Kerker effect arising from the far-field electro-magnetic interactions in dielectric nanostructures brings a new mechanism to realize effective coupling with excitons. Until now, it still remains unsolved whether effective Mie-exciton coupling can be realized based on pure far-field Kerker effect. Therefore, we proposed a silicon-on-insulator (SOI) integrated Mie resonator with a 135 nm top oxide layer to exclude the near-field coupling between excitons and silicon (Si) nanostripes. Through tuning the widths of Si nanostripes to obtain highly directional photoluminescence (PL) emission under Kerker conditions, strong PL enhancements can be observed, whose enhancement factors are comparable to the reported best performances of single all-dielectric or even plasmonic nanostructures coupling with 2D excitons. Our findings bring new strategies for strong light-matter interactions with near-zero heating loss and make it possible to construct 2D materials-silicon hybrid integration for future nanophotonic and optoelectronic devices.

Directional radiation and photothermal effect enhanced control of 2D excitonic emission based on germanium nanoparticles

Dielectric nanostructures with Mie resonances have shown promising applications for building nanoantennas and metasurfaces. Coupling between Mie resonators and transition metal dichalcogenide (TMDC) monolayers is of great significance, because the existence of Mie resonances can modulate phases and radiation directions effectively. Recently, monolayer binary and ternary TDMCs have drawn more attention owing to the intriguing and tunable excitonic states from visible to near infrared. However, the coupling mechanism between monolayer TMDCs and Mie resonators has not been well studied. Moreover, it is still a great challenge to realize the control of excitonic emission wavelength and intensity simultaneously. Here, for the first time, we demonstrate that germanium nanoparticles (Ge NPs), a typical high refractive index dielectric Mie resonator, are capable of controlling both the intensity and direction of PL emissions in the near-infrared from monolayer WSe_2(1-x)Te_2x. Through putting Ge NPs below or above monolayers, we observed the obvious emission directivity because of the higher refractive index and higher loss of Ge than silicon. Besides, higher absorption in Ge NPs brings photothermal effects during the interaction with TMDCs. These findings indicate that Ge-based Mie resonators may guide the design of new type nanophotonics devices in the future.

Flexible plasmonic modulators induced by the thermomechanical effect

Reconfigurable plasmon-based flexible devices, composed of artificial plasmonic nanostructures on stretchable substrates, show great promise for dynamic functionalities such as tunability, switching and modulation of electromagnetic waves. Here, we theoretically proposed and experimentally demonstrated a simple and efficient flexible plasmonic modulator based on an array of gold nanostructures on a poly(dimethylsiloxane) (PDMS) substrate. Arising from the current-induced local Joule heat, the local expansion of the PDMS substrate widens the gap distances between the neighboring gold wires, which results in a spectral shift of the plasmon resonance. The experimental results show that the plasmon resonance has a blue-shift of 39 nm under a total power consumption of only 10.5 mW, which results in a high modulation depth of up to 30.5% for the modulator. Such a low power consumption can be ascribed to the small active area and excellent thermal isolation of the PDMS. The optical and thermomechanical responses were confirmed and understood by the electromagnetic and thermomechanical co-simulations based on the finite-difference time-domain and finite-element methods. This novel mechanism to manipulate light provides new opportunities for active optical components and integrated circuits.

Tunable Control of Interlayer Excitons in WS2/MoS2 Heterostructures via Strong Coupling with Enhanced Mie Resonances

Strong Coulomb interactions in monolayer transition metal dichalcogenides (TMDs) produce strongly bound excitons, trions, and biexcitons. The existence of multiexcitonic states has drawn tremendous attention because of its promising applications in quantum information. Combining different monolayer TMDs into van der Waals (vdW) heterostructures opens up opportunities to engineer exciton devices and bring new phenomena. Spatially separated electrons and holes in different layers produce interlayer excitons. Although much progress has been made on excitons in single layers, how interlayer excitons contribute the photoluminescence emission and how to tailor the interlayer exciton emission have not been well understood. Here, room temperature strong coupling between interlayer excitons in the WS2/MoS2 vdW heterostructure and cavity-enhanced Mie resonances in individual silicon nanoparticles (Si NPs) are demonstrated. The heterostructures are inserted into a Si film-Si NP all-dielectric platform to realize effective energy exchanges and Rabi oscillations. Besides mode splitting in scattering, tunable interlayer excitonic emission is also observed. The results make it possible to design TMDs heterostructures with various excitonic states for future photonics devices.

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.

Resonant silicon nanoparticles with controllable crystalline states and nonlinear optical responses

High-throughput laser printing of resonant silicon nanoparticles has emerged as a novel tool for the fabrication of deeply subwavelength objects with various functionalities. The applications of resonant silicon nanoparticles crucially depend on their crystalline state. However, the ways to control the crystalline structure during laser printing remain unstudied. Here we demonstrate, both experimentally and theoretically, how the crystalline structure of silicon nanoparticles fabricated by a laser printing technique can be varied from almost amorphous to a polycrystalline state. In particular, we propose a method of crystalline structure control via changing the distance between the irradiated silicon film and the receiving substrate. This study allows the most optimal conditions for second harmonic generation to be revealed. We believe that the proposed method opens the door to fully controllable laser printing of functional nanoparticles and nanostructures.