Plasmon-Induced Trap State Emission from Single Quantum Dots

Charge carriers trapped at localized surface defects play a crucial role in quantum dot (QD) photophysics. Surface traps offer longer lifetimes than band-edge emission, expanding the potential of QDs as nanoscale light-emitting excitons and qubits. Here, we demonstrate that a nonradiative plasmon mode drives the transfer from two-photon-excited excitons to trap states. In plasmonic cavities, trap emission dominates while the band-edge recombination is completely suppressed. The induced pathways for excitonic recombination not only shed light on the fundamental interactions of excitonic spins, but also open new avenues in manipulating QD emission, for optoelectronics and nanophotonics applications.

Figure 1

(a) Schematic of QDs in NPoM nanogap. Au NP sits on monolayer of quantum dots self-assembled onto gold film, inset shows core-shell $\mathrm{InP}/\mathrm{ZnS}$ QDs. (b) Photoluminescence spectrum of $\mathrm{InP}/\mathrm{ZnS}$ ${\mathrm{QD}}_{540}$ (${\lambda }_{\max }=540\mathrm{nm}$) in solution (solid line), along with extinction spectrum (dashed line). (c) AFM image of QDs on gold substrate. (d) PL spectra of QDs coupled to the plasmonic cavity (dashed), dark-field spectrum of the NPoM cavity (gray), and QD emission spectrum normalized by the cavity resonance (solid blue).

Figure 2

Two-photon excited photoluminescence (2PPL). (a) ${\mathrm{QD}}_{540}$ emission spectrum excited by 120 fs pulses at 920 nm, for QDs in aqueous phase or air (dashed line) and dried on glass (solid line, as in inset). (b) Power dependence of 2PPL signals from ${\mathrm{QD}}_{540}$ solution, confirming emission is from two-photon absorption. (c) Emission spectra of QDs in a NPoM cavity (as in inset) for increasing excitation power (solid line). Also shown (dashed line) is two-photon-absorption trap-state emission from QDs dried on glass. (d) Quadratic power dependence of QD PL in NPoM.

Figure 3

Selectively pumped trap state emission. (a) Top panel: Normalized PL from one-photon-absorption of QDs in NPoM cavity ($\widetilde{PL}$, blue line) and on glass (dashed orange line) using CW excitation at 447 nm, along with extinction of QDs in solution (solid orange line) and plasmonic cavity resonance (gray line). Middle and bottom panels: 2PPL spectrum for QDs in NPoM (solid line) with (middle) 860 and (bottom) 920 nm pulsed excitation, and from QDs dried on glass (red dashed, while blue dashed line shows from one-photon excitation). Excitation wavelengths and equivalent one-photon doubled wavelengths also marked. (b) 2PPL spectra for a single QD NPoM vs pump excitation tuned from 860 to 960 nm (20 nm steps), with fit curves (gray line) composed of three Gaussians at 764, 775, and 789 nm (red arrows). (c) Integrated 2PPL intensity of QDs in NPoM vs excitation wavelength showing absorption resonance $\sim 920\mathrm{nm}$ (gray line) and one-photon $\alpha$ spectrum of QD solution plotted vs $\lambda /2$ axis (yellow line). (d) Integrated intensity for the three 2PPL modes vs excitation wavelength, showing selective pumping of individual QD trap states.

Figure 4

NPoM-modified radiative recombination processes. (a) Ensemble PL from one-photon-absorption of ${\mathrm{QD}}_{460,540,580}$ on glass. (b) Trap state PL from TPA of ${\mathrm{QDs}}_{1-3}$ on glass (dashed) and inside NPoM cavities (solid line), using 920 nm pulses. (c) Bare QDs on glass: two-photon excitation of dark $J=2$ exciton (double red arrows). Excitons either spin-flip to the ground state $E_g$ (gray arrow) to emit PL (blue arrow), or couple to $J=2$ surface traps to emit red-shifted photons. (d) Dark-field spectra of NPoM cavities formed with ${\mathrm{QDs}}_{1-3}$. (e) Energy alignment of $J=2$ excitons ($E_{2\mathrm{PA}}$, black points show 2PA), emissive surface traps ($E_{\mathrm{TS}}$), and calculated (11) plasmon mode of NPoM. (f) Model of QDs in NPoM: two-photon-excited dark exciton relaxed by (11) gap plasmon into surface trap states, bypassing formation of band-edge excitons.