Strong Enhancement of Two-Photon Absorption and Emergence of Unusual Extinction Saturation in Silver Sulfide Quantum Dots Integrated with Gold and Silica Nanostructures

Abstract

Hybrid nanosystems, such as those combining plasmonic, dielectric, and quantum-confined nanostructures, have long been of interest for enhancing and tailoring diverse light-matter interactions. Here, we present a series of hybrid nanomaterials exhibiting strongly enhanced nonlinear optical (NLO) properties, fabricated by combining silver sulfide quantum dots (Ag₂S QDs) with silica and gold nanostructures. We studied their NLO properties (two-photon absorption and saturable absorption) in colloidal solutions over a wide spectral range (500-1600 nm) using the femtosecond Z-scan technique. Embedding Ag₂S QDs into silica nanospheres gives rise to remarkable enhancement of two-photon absorption (up to a factor of 16 increase in the merit factor σ₂/M compared to bare QDs), whereas covering such QD-doped silica nanospheres with gold nanoparticles or attaching the QDs to the surface of gold nanoshells (NSs) leads to even further enhancement (up to 73-fold increase in σ₂/M), accompanied by a competing effect of saturable absorption. Furthermore, in the case of QD-doped silica spheres covered with a continuous gold layer, we observe a previously unreported saturation of extinction in the near-infrared region that follows an unusual intensity dependence, suggesting the involvement of two-photon absorption as the pumping mechanism. In addition to the experimental studies, we have performed numerical simulations, revealing the plasmonic origin of the observed spectral dependences of the NLO properties, with the underlying enhancement mechanisms involving local field enhancement and, possibly, also coupling between plasmon modes and QD excitons, giving rise to a double peak in the σ₂ spectrum. Our findings demonstrate the unique potential of hybrid NLO nanomaterials combining quantum-confined, plasmonic, and dielectric components.

Keywords: Ag2S; Z-scan; gold nanoparticles; nanomaterials; nanoshells; plasmon resonance; quantum dots; saturable absorption; silver sulfide; two-photon absorption.

Figure 1

Schematic illustration of the six types of hybrid nanomaterials studied in this work: (a) Au nanoshells with silica spacer layer, decorated with Ag₂S QDs (NS@SiO₂@Ag₂S-QDs); (b) Ag₂S QDs with 2-MPA ligand, embedded in silica nanospheres covered by a continuous gold layer (Ag₂S_2MPA@SiO₂_Au-layer); (c, e) Ag₂S QDs embedded in silica nanospheres (Ag₂S@SiO₂). Two different ligands, 2-MPA and 3-MPA, were used for the QDs; thus, the nanostructures are also referred to as Ag₂S_2MPA@SiO₂ and Ag₂S_3MPA@SiO₂; (d, f) Ag₂S QDs embedded in silica (Ag₂S_2MPA@SiO₂ or Ag₂S_3MPA@SiO₂), covered by gold islands (Ag₂S_2MPA@SiO₂_Au-islands or Ag₂S_3MPA@SiO₂_Au-islands). Note: Structures on the left (inside green frame) are discussed in the main text, while the other structures are presented in the SI. The NLO properties of all the structures were investigated using Z-scan, except the one inside the red frame, which was not measured due to its rapid sedimentation.

Figure 2

(a) TEM images of the nanostructures labeled as Ag₂S_2MPA@SiO₂_Au-layer. (b) Spectral dependence of 1/I_sat corresponding to extinction saturation (green squares, the green solid line is used to guide the eye) for Ag₂S_2MPA@SiO₂_Au-layer. The gray-shaded area shows the measured extinction spectrum plotted vs 1λ, while the pink-shaded area corresponds to the same spectrum plotted vs 2λ. The coincidence of the two bands observed in the 1/I_sat spectrum with the two versions of the extinction spectrum may indicate the involvement of both one-photon (1P) and two-photon (2P) driving of the saturation effects. Representative open-aperture Z-scan traces obtained for Ag₂S_2MPA@SiO₂_Au-layer at (c) 725 nm, (d) 825 nm, and (e) 1200 nm. The blue curves correspond to the conventional 1PA saturation model (1P SAT), while the red curves correspond to the proposed 2PA-driven saturation (2P SAT). The excellent fit obtained with the latter model suggests that 2PA-driven extinction saturation is the likely mechanism underlying the observed transmittance variation.

Figure 3

TEM images of the NSs covered with a silica layer. (a, b) Silica layer is functionalized with Ag₂S QDs, which are visible as darker dots on the silica surface (NS@SiO₂@Ag₂S-QDs). For comparison, the QDs are absent in (c) (NS@SiO₂). (d) Two halves of a NS are compared. Left: NS coated with a silica layer and Ag₂S QDs (NS@SiO₂@Ag₂S-QDs). Right: NS with the silica layer only (NS@SiO₂).

Figure 4

(a) Spectral dependence of 2PA cross section (σ₂^eff) measured using the Z-scan technique for the colloidal solution NS@SiO₂@Ag₂S-QDs. The values of σ₂^eff are plotted as green filled squares (the green solid line is used to guide the eye). The gray area represents the measured 1PA spectrum of NSs, the purple area represents the measured 1PA spectrum of Ag₂S QDs vs 1λ, and the beige area represents the same 1PA spectrum plotted vs 2λ. (b) Numerical simulations of the local electric field intensity enhancement around the QD as a function of λ and angle θ. (c) Schematic illustration of the numerically modeled NS and the illumination geometry, with the red arrow indicating the propagation direction of the incident light, and the magenta double-arrow showing the incident light polarization. The model includes only one QD attached to the NS surface. The QD radius is 1.38 nm, and its refractive index is 2.2. The location of the QD with respect to the incident light is defined by the polar angle θ. (d) Representative plots of the local field intensity distribution at the quadrupole (left) and dipole resonance (right). In both cases, the plots are obtained for the QD located in one of the plasmonic hot spots.