Nanoplasmonics has recently revolutionized our ability to control light on the nanoscale. Using metallic nanostructures with tailored shapes, it is possible to efficiently focus light into nanoscale field ‘hot spots'. High field enhancement factors have been achieved in such optical nanoantennas, enabling transformative science in the areas of single molecule interactions, highly enhanced nonlinearities and nanoscale waveguiding. Unfortunately, these large enhancements come at the price of high optical losses due to absorption in the metal, severely limiting real-world applications. Via the realization of a novel nanophotonic platform based on dielectric nanostructures to form efficient nanoantennas with ultra-low light-into-heat conversion, here we demonstrate an approach that overcomes these limitations. We show that dimer-like silicon-based single nanoantennas produce both high surface enhanced fluorescence and surface enhanced Raman scattering, while at the same time generating a negligible temperature increase in their hot spots and surrounding environments.

ABSTRACT. We present an all-dielectric germanium nanosystem exhibiting a strong third order nonlinear response and efficient third harmonic generation in the optical regime. A thin germanium nanodisk shows a pronounced valley in its scattering cross section close to the dark anapole mode, while the electric field energy inside the disk is maximized due to high confinement within the dielectric. We investigate the dependence of the third harmonic signal on disk size and pump wavelength to reveal the nature of the anapole mode. Each germanium nanodisk generates a high effective third order susceptibility of (3) = 4.3 10 −9, corresponding to an associated third harmonic conversion efficiency of 0.0001% at a wavelength of 1650 nm, which is four orders of magnitude greater than the case of an unstructured germanium reference film. Furthermore, the nonlinear conversion via the anapole mode outperforms that via the radiative dipolar resonances by about one order of magnitude, which is consistent with our numerical simulations. These findings open new possibilities for the optimization of upconversion processes on the nanoscale through the appropriate engineering of suitable dielectric materials.

Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of ∼20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe2 with judiciously designed plasmonic nanostructures and paves a way towards the implementation of plasmon-enhanced transition metal dichalcogenide photodetectors, sensors and emitters.

We present all-dielectric gallium phosphide (GaP) nanoantennas as an e cient nanophotonic platform for surface-enhanced second harmonic generation (SHG) and uorescence (SEF), showing negligible losses in the visible range. Employing single GaP nanodisks, we observe an increase of more than three orders of magnitude in the SHG signal in comparison with the bulk. This constitutes an SHG e ciency as large as 0.0002%, which is, to the best of our knowledge, the highest yet achieved value produced by a single nano-object in the optical region. Furthermore, we show that GaP dimers with 35 nm gap can enhance up to 3600 times the uorescence emission of dyes located in the gap of the nanoantenna. This is accomplished by a uorescence lifetime reduction of, at least, 22 times, accompanied by a high intensity eld con nement in the gap region. These results open new avenues for low-loss nanophotonics in the optical regime.

One of the main challenges in photonics is the integration of ultrafast coherent sources in silicon compatible platforms at the nanoscale [1]. Generally, the emission of ultra-short pulses is achieved by synchronizing the cavity modes of the system via external active components, such as, e.g., Q-switch or saturable absorbers. Consequently, the required optical setups are complex and difficult to integrate on-chip. To address these difficulties, we propose a novel type of integrated source based on the spontaneous synchronization of several near-field nanolasers. We design our near-field lasers by considering the nonlinear amplification of non-radiating Anapole modes [2]. Anapoles represent an intriguing non-conventional state of radiation, whose excitation is responsible for the formation of scattering suppression states in dielectric nanostructures [3]. Due to their inherent near-field emission properties, an ensemble of anapole-based nanolasers represent an ideal candidate to investigate and tailor spontaneous synchronization phenomena in a silicon-compatible framework. Additionally, their mutual non-linear interaction can be precisely controlled within standard nanofabrication tolerances. We investigated the nonlinear emission from an ensemble of anapole-based nanolasers with our Finite-Differences in Time-Domain simulator NANOCPP, which solves a dispersive set of 3D Maxwell-Bloch equations without approximation [5-9]. The single nanolasers were composed of three-dimensional III-V semiconductor nanostructures. By inducing the formation of a fundamental anapole state in correspondence of the emission frequency of the amplifying semiconductor, we obtained a lasing emission composed of purely evanescent waves (Fig. 1-a). By coupling several anapole sources the system behaves as an integrated ultrafast pulse generation device (Fig. 1-b), in which the collective emission is collected by a silicon waveguide. Interestingly, the system achieves a stable mode-locked emission with optical pulses of less than 100fs (Fig. 1-c). The pulse duration and the repetition rate of the system can be tuned by varying the relative position of the anapole emitters, as shown in Fig. 1-d where the system is optimized to produce ps optical pulses. Fig. 1 Generation of ultrafast pulses from a chain of anapole-based nanolasers. (a) Near-field emission from a 3D anapole nanolaser. (b) Schematic representation of our ultrafast generation device. (c,d) Steady-state emission from the system. The pulse duration and repetition rate can be controlled by optimizing the geometry of the system.

Two-dimensional hybrid organic-inorganic Ruddlesden-Popper perovskites (RPPs) have attracted considerable attention due to their rich photonic and optoelectronic properties. The natural multi-quantum-well structure of 2D RPPs has been predicted to exhibit a large third-order nonlinearity. However, nonlinear optical studies on 2D RPPs have previously been conducted only on bulk polycrystalline samples, in which only weak third-harmonic generation (THG) has been observed. Here, we perform parametric nonlinear optical characterization of 2D perovskite nanosheets mechanically exfoliated from four different lead halide RPP single crystals, from which we observe ultrastrong THG with a maximum effective third-order susceptibility (χ) of 1.12 × 10 m V. A maximum conversion efficiency of 0.006% is attained, which is more than 5 orders of magnitude higher than previously reported values for 2D materials. The THG emission is resonantly enhanced at the excitonic band gap energy of the 2D RPP crystals and can be tuned from violet to red by selecting the RPP homologue with the requisite resonance. Due to signal depletion effects and phase-matching conditions, the strongest nonlinear response is achieved for thicknesses less than 100 nm.

In emerging optoelectronic applications, such as water photolysis, exciton fission and novel photovoltaics involving low-dimensional nanomaterials, hot-carrier relaxation and extraction mechanisms play an indispensable and intriguing role in their photo-electron conversion processes. Two-dimensional transition metal dichalcogenides have attracted much attention in above fields recently; however, insight into the relaxation mechanism of hot electron-hole pairs in the band nesting region denoted as C-excitons, remains elusive. Using MoS2 monolayers as a model two-dimensional transition metal dichalcogenide system, here we report a slower hot-carrier cooling for C-excitons, in comparison with band-edge excitons. We deduce that this effect arises from the favourable band alignment and transient excited-state Coulomb environment, rather than solely on quantum confinement in two-dimension systems. We identify the screening-sensitive bandgap renormalization for MoS2 monolayer/graphene heterostructures, and confirm the initial hot-carrier extraction for the C-exciton state with an unprecedented efficiency of 80%, accompanied by a twofold reduction in the exciton binding energy.

High refractive index dielectric nanoantennas are expected to become key elements for nonlinear nano-optics applications due to their large nonlinearities, low energy losses, and ability to produce high electric field enhancements in relatively large nanoscale volumes. In this work, we show that the nonlinear response from a high-index dielectric nanoantenna can be significantly improved by adding a metallic component to build a metal-dielectric hybrid nanostructure. We demonstrate that the plasmonic resonance of a Au nanoring can boost the anapole mode supported by a Si nanodisk, strongly enhancing the electric field inside the large third-order susceptibility dielectric. As a result, a high third harmonic conversion efficiency, which reaches 0.007% at a third harmonic wavelength of 440 nm, is obtained. In addition, by suitably modifying geometrical parameters of the hybrid nanoantenna, we tune the enhanced third harmonic emission throughout the optical regime. Coupling metallic and dielectric nanoantennas to expand the potential of subwavelength structures opens new paths for efficient nonlinear optical effects in the visible range on the nanoscale.