Hybrid plasmonic nano-emitters with controlled single quantum emitter positioning on the local excitation field

Abstract

Hybrid plasmonic nano-emitters based on the combination of quantum dot emitters (QD) and plasmonic nanoantennas open up new perspectives in the control of light. However, precise positioning of any active medium at the nanoscale constitutes a challenge. Here, we report on the optimal overlap of antenna's near-field and active medium whose spatial distribution is controlled via a plasmon-triggered 2-photon polymerization of a photosensitive formulation containing QDs. Au nanoparticles of various geometries are considered. The response of these hybrid nano-emitters is shown to be highly sensitive to the light polarization. Different light emission states are evidenced by photoluminescence measurements. These states correspond to polarization-sensitive nanoscale overlap between the exciting local field and the active medium distribution. The decrease of the QD concentration within the monomer formulation allows trapping of a single quantum dot in the vicinity of the Au particle. The latter objects show polarization-dependent switching in the single-photon regime.

Fig. 1. Characterization of Au nanocubes.

a SEM image, and b AFM image of a representative single Au nanocube (same object). c Nanocube size (edge length) histogram obtained from a set of 100 nanocubes (SEM analysis). d Dark-field scattering image of Au nanocubes on ITO-coated glass substrate. e Dark-field single-nanocube scattering spectrum averaged from 10 nanocubes in air on ITO-coated glass substrate. f Calculated scattering spectrum of a single Au nanocube in air and in polymer (refractive index n = 1.48) on ITO-coated glass substrate (40 nm thickness of ITO layer with refractive index of 2).

Fig. 2. Au nanocube-based hybrid nanostructures made by plasmonic 2-photon polymerization.

a SEM image of the hybrid structure obtained with incident normal laser beam polarized along the cube diagonal (red arrow), λ = 780 nm. Image of the bare nanocube prior to photopolymerization is superimposed to highlight the polymer shell. b Finite difference time domain (FDTD) map of the field modulus |E| in the vicinity of the Au nanocube in polymer corresponding to case a, mid sectional horizontal plane, λ = 780 nm. c SEM image of the hybrid nanostructure obtained with incident polarization along the cube side edge (red arrow). Image of the bare nanocube prior to photopolymerization is superimposed to highlight the polymer shell. d FDTD map of the field modulus |E| in the vicinity of the Au nanocube in polymer corresponding to case c, mid sectional horizontal plane, λ = 780 nm. e Measured polymer elongation along the nanocube diagonal, case a, as a function of the normalized incident dose p used for plasmonic 2-photon polymerization, as determined by SEM analysis.

Fig. 3. Photoluminescence measurements on hybrid nano-emitters.

Single hybrid nanosource based on a Au nanocube and CdSe/CdS/Zn quantum dots embedded within nanometric polymer lobes obtained by plasmonic 2-photon polymerization under ϕ = 45° linear excitation (red arrow). a SEM image of the hybrid plasmonic nanosource with polar coordinates used for discussion. The initial bare nanocube is superimposed to the raw SEM image. b Far-field PL image at polarization angle ϕ = 45°, λ = 405 nm, linear polarization. c PL spectrum. d PL intensity as a function of the angle of polarization of the exciting blue beam. Blue arrows indicate two specific perpendicular polarizations e, f Calculated near-field intensity |E|², λ = 405 nm, mid horizontal section planes, polarizations indicated by blue arrows. Black lines represent the contours of the polymer lobes as deduced from the SEM image (a).

Fig. 4. Photoluminescence measurements on single hybrid nano-emitters.

Hybrid nanosource based on Au nanocube and CdSe/CdS/Zn quantum dots embedded in polymerized volumes obtained under exciting polarization along the cube edge side (red arrow). a Raw SEM image of the hybrid nanosystem. b SEM raw image with superimposed initial bare nanocube. c Far-field PL image at ϕ = 0° polarization angle, λ = 405 nm, linear polarization. d PL intensity as a function of the angle of polarization ϕ of the blue exciting beam. Blue arrows indicate two specific perpendicular polarizations.

Fig. 5. Photoluminescence measurements on single hybrid nano-emitters.

Hybrid nanosystem based on a Au nanodisk and CdSe/CdS/Zn quantum dots embedded within the polymer nanometric lobes. The hybrid nanosystem is obtained by plasmonic 2-photon polymerization using 45° tilted linear polarization (red arrow). a SEM image of the nanosystem (raw image). b Far-field PL image excitation at polarization angle ϕ = 45°, λ = 405 nm, linear polarization. c PL intensity as function of the angle of polarization ϕ, λ = 405 nm, linear polarization is represented as blue arrows.

Fig. 6. Photoluminescence measurements on single hybrid nano-emitters.

Hybrid nanosource based on Au nanodisk and CdSe/CdS/Zn quantum dots embedded in a nanometric polymer shell obtained by plasmonic 2-photon polymerization using circular polarization. a SEM image of the hybrid nanosource. b PL intensity as a function of the polarization angle ϕ of the blue excitation beam. λ = 405 nm, linear polarization.

Fig. 7. Overlap integral ratio.

Computed spatial overlap integral ${\eta }_{\mathrm{nf}/\mathrm{em}}$, defined in Eq. (4), between the local excitation field and the active medium as a function of the incident polarization angle ϕ for three different hybrid nanosources. The incident field is linearly polarized, λ = 405 nm. a Au nanocube with polymer lobes along the diagonal direction, ϕ = 45°. b Au nanocube with polymer deposits at cube faces, ϕ = 0°. c Hybrid Au nanodisk with polymer lobes along ϕ = 45°.

Fig. 8. Hybrid nano-emitters in the single photon regime.

a AFM image of a nanocube-based hybrid nano-emitter. The polymer lobes contain a single or a few QDs. b, c PL spectrum time trace of t = 50 s excited by linearly polarized laser along ϕ = 0° at 405 nm. In b, at time t = 32 s, the polarization direction is rotated to ϕ = 90°. d g⁽²⁾ measurement showing g⁽²⁾(0) = 0.35. e Typical lifetime measurement on a single-QD hybrid nano-emitter, lifetime ∼0.725 ns. The blue curve represents the instrumentation response function of 0.63 ns. f Lifetime measurements. Comparison between single QD in polymer without gold nanocubes (measure: red curve, fitting: black curve 17.5 ns) and single QD in the vicinity of a gold nanocube (hybrid nano-emitter: yellow curve).