The field of nanophotonics demands advanced optical nanosources. Hybrid plasmonic nanosources, leveraging energy transfer between metal nanoparticles and semiconductor quantum dots/nanocrystals or organic dyes, are a promising solution for efficient nanosources integrable into photonic nanodevices. Weak coupling (Purcell effect) allows controlled light emission by managing the nano-emitter deexcitation rate. The metal nanostructure's electromagnetic local density of states (LDOS) acts as deexcitation channels, increasing the rate and decreasing lifetime. A major challenge is controlling the nanoscale spatial distribution of nano-emitters relative to the LDOS, potentially using incident field polarization for remote optical control. Equation (1) describes emission rate (γem) as a function of excitation rate (γexc), quantum yield (Q), and emitter presence probability (ρ(x,y,z)dV). γexc is linked to the plasmonic near-field, controllable via incident polarization; however, controlling the nano-emitter spatial distribution ρ(x,y,z) is a significant hurdle. Existing methods like spin coating provide isotropic emitter distribution, while others like electron beam lithography or atomic force microscopy offer nanoscale control but lack simplicity and flexibility. This study presents a simple method for on-demand fabrication of integrated hybrid plasmonic nanosources with controlled active medium positioning using plasmon-based photopolymerization, previously shown to integrate polymer nanostructures containing nano-emitters near metal nanoparticles. This study investigates spatial overlap between optical near-field and quantum emitter distribution in anisotropic hybrid sources based on Au nanocubes, introducing three novelties: controlled infrared two-photon free radical polymerization on single Au nanocubes; nanocube-based hybrid systems with anisotropic integrated active medium, enabling selective excitation of a two-state system with incident polarization; and preliminary observation of polarization-sensitive single-QD hybrid nanosources by reducing QD concentration.

Significant research has explored hybrid plasmonic nanosources, focusing on the Purcell effect and its applications in controlling light emission. Studies have demonstrated fluorescence enhancement using various metal nanostructures (nanoshells, nanorods) and explored the polarization dependence of plasmon-enhanced fluorescence. However, precise control over the spatial arrangement of quantum emitters relative to the plasmonic nanostructure remained a significant challenge. Previous approaches, such as spin coating, resulted in isotropic emitter distributions, requiring extensive sample fabrication to achieve optimal emitter positioning. Other methods, including optical lithography, electron beam lithography, and atomic force microscopy, offered nanoscale precision but were complex and lacked flexibility. DNA-based approaches have shown promise but limited functionalization possibilities. The need for a simpler, on-demand method to create integrated hybrid plasmonic nanosources with precise active medium positioning motivated this research.

The study utilized gold nanocubes (single crystal, 127 ± 2 nm side length) deposited on an indium-tin-oxide (ITO)-coated glass substrate. Nanocubes were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), dark-field white light imaging/spectroscopy, and micro-photoluminescence (PL). Plasmon-assisted two-photon photopolymerization was performed on single gold nanocubes using a formulation of 1%wt Irgacure 819 (photoinitiator) and 99%wt QD-grafted pentaerythritol triacrylate (PETA). Red light-emitting CdSe/CdS/Zn core/shell/shell colloidal quantum dots (QDs) with a photoluminescence wavelength centered at 625 nm were used. Two-photon polymerization was selectively induced near the nanocubes using an incident exposure dose below the polymerization threshold, leveraging plasmonic near-field enhancement. The polymer distribution followed the nanoparticle's dipolar near-field distribution, controlled by incident polarization. A parameter study varied the normalized incident dose (p) to quantify plasmon-induced intensity enhancement and near-field decay length. Photoluminescence measurements (λem = 625 nm, λexc = 405 nm) were performed on single nanocube-based emitters. The polarization dependence of PL was analyzed, revealing a two-state system based on near-field/active medium overlap. Polarization contrast (δPL) was calculated to quantify polarization sensitivity. Different gold nanoparticle geometries (nanodisks) were also investigated to assess the impact of geometry on polarization sensitivity. To achieve single-photon emission, QD concentration in the photosensitive formulation was reduced, allowing trapping of single or few QDs within the polymer lobes. AFM, PL spectroscopy, and autocorrelation function (g(2)) measurements were used to characterize single-QD hybrid nanosources. Finite-difference time-domain (FDTD) calculations simulated electromagnetic fields and quantified the spatial overlap between the near-field and QD distribution. The overlap integral ratio (Inf/em) was defined to quantify the spatial overlap, showing proportionality to PL intensity. Purcell factor was determined by comparing lifetime measurements of single QDs in polymer with and without gold nanocubes.

The study successfully demonstrated controlled two-photon polymerization of a photosensitive formulation containing quantum dots near single gold nanocubes, enabling the creation of hybrid plasmonic nano-emitters with precisely controlled anisotropic active medium distribution. The resulting hybrid nanostructures exhibited strong polarization-dependent photoluminescence (PL), with the PL intensity directly correlated to the spatial overlap between the plasmonic near-field and the distribution of quantum dots. This polarization sensitivity was quantified using polarization contrast (δPL), revealing a significant difference between maximum and minimum PL intensities. Different nanoparticle geometries (nanocubes and nanodisks) were compared, showing that nanocube-based emitters offer stronger polarization sensitivity due to sharper near-field enhancements at their corners. The study further demonstrated polarization-sensitive single-photon emission from hybrid nano-emitters by reducing the concentration of QDs in the photosensitive formulation, trapping single or a few QDs within the polymer lobes. The single-photon nature was confirmed by g(2) measurements, and a significant Purcell effect was observed due to the interaction between the trapped QD and the gold nanocube. The calculated spatial overlap integral (Inf/em) accurately predicted the experimental PL intensity, establishing a direct link between spatial overlap and emission characteristics. The results provided a quantitative understanding of the relationship between the geometry of plasmonic nanoparticles, the spatial arrangement of the quantum dots, and the resulting polarization-dependent PL emission in these hybrid nanosystems. In essence, the research established a new class of anisotropic plasmonic nano-emitters with controllable polarization-dependent emission characteristics, suitable for applications such as polarization-driven tunable nano-emitters, including nanolasers and single-photon emitters.

The findings demonstrate a significant advance in the control of light emission from hybrid plasmonic nanosources. By precisely controlling the spatial distribution of quantum dots using plasmon-triggered two-photon polymerization, the researchers achieved anisotropic nano-emitters with tunable polarization-dependent PL. The strong correlation between the calculated spatial overlap integral and the experimental PL intensity validates the approach and provides a quantitative framework for designing and optimizing these hybrid systems. The demonstration of polarization-sensitive single-photon switching opens new avenues for developing advanced nanoscale light sources. The results are highly relevant to the development of advanced nanophotonic devices, such as tunable nanolasers and single-photon sources, impacting various fields including quantum information processing, biosensing, and advanced optical imaging.

This study successfully fabricated anisotropic hybrid plasmonic nano-emitters with controlled active medium distribution using plasmon-based two-photon polymerization. The polarization-dependent PL was analyzed and quantified using new parameters: active medium angular filling factor, nanoscale spatial overlap integral, and PL polarization contrast. The achieved control over emitter positioning surpasses existing methods and opens avenues for creating polarization-driven tunable nano-emitters. A key result is the demonstration of a polarization-driven single-photon switch, showcasing the potential of these hybrid systems in single-photon applications. Future work should explore the optimization of these systems for higher polarization contrast and efficiency, as well as exploring their use in complex nanophotonic devices.

The study focused on specific nanoparticle geometries (nanocubes and nanodisks), and further investigation with other shapes is warranted to fully explore the design space. The assumption of homogeneous QD distribution within the polymer matrix may not be entirely accurate, potentially affecting the quantitative analysis of the spatial overlap integral. Long-term stability studies of the single-QD hybrid nanosources are necessary to assess their performance over extended periods. The relatively low numerical aperture of the objective lens may have limited the accuracy of polarization control in some aspects of the study.