Pronounced Fano Resonance in Single Gold Split Nanodisks with 15 nm Split Gaps for Intensive Second Harmonic Generation

A comparative analysis of surface and bulk contributions to second-harmonic generation in centrosymmetric nanoparticles

Second-harmonic generation (SHG) from nanoparticles made of centrosymmetric materials provides an effective tool to characterize many important properties of photonic structures at the subwavelength scale. Here we study the relative contribution of surface and bulk effects to SHG for plasmonic and dielectric nanostructures made of centrosymmetric materials in both dispersive and non-dispersive regimes. Our calculations of the far-fields generated by the nonlinear surface and bulk currents reveal that the size of the nanoparticle strongly influences the amount and relative contributions of the surface and bulk SHG effects. Importantly, our study reveals that, whereas for plasmonic nanoparticles the surface contribution is always dominant, the bulk and surface SHG effects can become comparable for dielectric nanoparticles, and thus they both should be taken into account when analyzing nonlinear optical properties of all-dielectric nanostructures.

Enhanced second harmonic generation from a plasmonic Fano structure subjected to an azimuthally polarized light beam

We show that an azimuthally polarized beam (APB) excitation of a plasmonic Fano structure made by coupling a split-ring resonator (SRR) to a nanoarc can enhance second harmonic generation (SHG). Strikingly, an almost 30 times enhancement in SHG peak intensity can be achieved when the excitation is switched from a linearly polarized beam (LPB) to an APB. We attribute this significant enhancement of SHG to the corresponding increase in the local field intensity at the fundamental frequency of SHG, resulting from the improved conversion efficiency between the APB excitation and the plasmonic modes of the Fano structure. We also show that unlike LPB, APB excitation creates a symmetric SHG radiation pattern. This effect can be understood by considering an interference model in which the APB can change the total SHG far-field radiation by modifying the amplitudes and phases of two waves originating from the individual SRR and nanoarc of the Fano structure.

Strong magnetic resonances and largely enhanced second-harmonic generation of colloidal MoS2 and ReS2@Au nanoantennas with assembled 2D nanosheets

Colloidal disk-like and sphere-like MoS₂ nanoantennas are synthesized. They consist of curly and interlaced 2D nanosheets. The resonance peak of the MoS₂ nanoantennas can be tuned from 500 to 900 nm by adjusting the size and shape. The strong magnetic and electric resonances of the dielectric antennas are revealed by theoretical calculations with Mie theory. The second harmonic generation (SHG) of the exfoliated nanosheets and the synthesized nanodisks and nanospheres is investigated and compared by scanning the excitation laser wavelength. SHG enhancement of 52 fold is observed for the spherical nanoantennas at 400 nm, which is attributed to the nanoantenna-enhanced two-photon resonance excitation of the D exciton of MoS₂ monolayers. Moreover, ReS₂@Au plasmon-dielectric hybrid nanoantennas are also synthesized. The SHG of Au nanoparticles is enhanced 8.5 times by the coupling of the two types of nanoantennas. This new class of optical nanoantennas consisting of 2D materials and exhibiting unique linear and nonlinear optical responses will bring promising applications ranging from nonlinear photonics to photochemistry.

Broad-Range Electrically Tunable Plasmonic Resonances of a Multilayer Coaxial Nanohole Array with an Electroactive Polymer Wrapper

Plasmonic assemblies featuring high sensitivity that can be readily shifted by external fields are the key for sensitive and versatile sensing devices. In this paper, a novel fast-responsive plasmonic nanocomposite composed of a multilayer nanohole array and a responsive electrochromic polymer is proposed with the plasmonic mode appearance vigorously cycled upon orthogonal electrical stimuli. In this nanocomposite, the coaxially stacked plasmonic nanohole arrays can induce multiple intense Fano resonances, which result from the crosstalk between a broad surface plasmon resonance (SPR) and the designed discrete transmission peaks with ultrahigh sensitivity; the polymer wrapper could provide the sensitive nanohole array with real-time-varied surroundings of refractive indices upon electrical stimuli. Therefore, a pronounced pure electroplasmonic shift up to 72 nm is obtained, which is the largest pure electrotuning SPR range to our knowledge. The stacked nanohole arrays here are also directly used as a working electrode, and they ensure sufficient contact between the working electrode (plasmonic structure) and the electroactive polymer, thus providing considerably improved response speed (within 1 s) for real-time sensing and switching.

High Q-factor with the excitation of anapole modes in dielectric split nanodisk arrays

The simultaneous realization of high Q-factor resonances and strong near-field enhancements around and inside of dielectric nanostructures is important for many applications in nanophotonics. However, the incident fields are often confined within dielectric nanoparticles, which results in poor optical interactions with external environment. Near-field enhancements can be extended outside of dielectric nanostructures with proper design, but the Q-factor is often reduced caused by additional radiation losses. This paper shows that the obstacles to achieve high Q-factor, that is, the radiative losses can be effectively suppressed by using dielectric nanodisk arrays, where the Q-factor is about one order larger than that of the single disks associated with the nonradiating anapole modes and the collective oscillations of the arrays. When the resonance energies of the electric dipole mode and the subradiant mode are degenerate with each other, the destructive interference produces an effect analogous to electromagnetically induced transparency. Furthermore, the Q-factor can be extremely enlarged with dielectric split nanodisk arrays, where the present of the split gap does not induce additional losses. Instead, the coupling between the two interfering modes is modified by adjusting the gap width, which makes it possible to achieve high Q-factor and strong near-field enhancements around and inside of the split disks simultaneously. It is shown that the Q-factor is approaching to 10⁶ when the gap width is about 110 nm, and the near-field enhancements around and inside of the split disks are about two orders stronger than that of the single disk.

Efficient Second-Harmonic Generation in Nanocrystalline Silicon Nanoparticles

Recent trends to employ high-index dielectric particles in nanophotonics are motivated by their reduced dissipative losses and large resonant enhancement of nonlinear effects at the nanoscale. Because silicon is a centrosymmetric material, the studies of nonlinear optical properties of silicon nanoparticles have been targeting primarily the third-harmonic generation effects. Here we demonstrate, both experimentally and theoretically, that resonantly excited nanocrystalline silicon nanoparticles fabricated by an optimized laser printing technique can exhibit strong second-harmonic generation (SHG) effects. We attribute an unexpectedly high yield of the nonlinear conversion to a nanocrystalline structure of nanoparticles supporting the Mie resonances. The demonstrated efficient SHG at green light from a single silicon nanoparticle is 2 orders of magnitude higher than that from unstructured silicon films. This efficiency is significantly higher than that of many plasmonic nanostructures and small silicon nanoparticles in the visible range, and it can be useful for a design of nonlinear nanoantennas and silicon-based integrated light sources.

Constructive-interference-enhanced Fano resonance of silver plasmonic heptamers with a substrate mirror: a numerical study

Plasmonic nanostructures with strong Fano resonance are of fundamental interest. Here, our systematic simulations show that rational positioning of a silver plasmonic heptamer above a highly reflective substrate mirror can significantly enhance its intrinsic Fano-resonance intensity. The silver nanodisk heptamer positioned at an appropriate distance above the reflective substrate enables 2.4 times field enhancement and 3.6 times deeper Fano-dip respectively compared to the heptamer directly placed on silicon oxide substrate. Besides, our results indicate that the Fano-dip position does not shift when the silver nanodisk heptamer gradually shifts away from the reflective substrate mirror (≥60 nm).