Macro-scale transport of the excitation energy along a metal nanotrack: exciton-plasmon energy transfer mechanism

Abstract

Presently we report (i) excited state (exciton) propagation in a metal nanotrack over macroscopic distances, along with (ii) energy transfer from the nanotrack to adsorbed dye molecules. We measured the rates of both of these processes. We concluded that the effective speed of exciton propagation along the nanotrack is about 8 × 10⁷ cm/s, much lower than the surface plasmon propagation speed of 1.4 × 10¹⁰ cm/s. We report that the transmitted energy yield depends on the nanotrack length, with the energy emitted from the surface much lower than the transmitted energy, i.e. the excited nanotrack mainly emits in its end zone. Our model thus assumes that the limiting step in the exciton propagation is the energy transfer between the originally prepared excitons and surface plasmons, with the rate constant of about 5.7 × 10⁷ s^-1. We also conclude that the energy transfer between the nanotrack and the adsorbed dye is limited by the excited-state lifetime in the nanotrack. Indeed, the measured characteristic buildup time of the dye emission is much longer than the characteristic energy transfer time to the dye of 81 ns, and thus must be determined by the excited state lifetime in the nanotrack. Indeed, the latter is very close to the characteristic buildup time of the dye emission. The data obtained are novel and very promising for a broad range of future applications.

Figure 1

(a) Block diagram of the assembly used for measuring energy transfer along Co nanotracks sized (3 ÷ 12) × 2 mm² and 11.421 nm thick, where 1 is a fused silica lens focusing probing radiation onto one connector 2 of a multimode fiber optic cable 7, 3 is the connector of the second fiber optic cable 7, 4 is a fused silica collimator lens providing a parallel beam, the latter passing first through a filter or going directly to the photodetector, 5 – Co nanotrack deposited on the AlN substrate 6; (b) the connectors of the two fiber cables 2 and 3 were connected to each other directly for recording the baseline; (c) the shape of the nanotrack.

Figure 2

(a) Absorption spectrum of a 11.421 nm Co film deposited on CaF₂ substrate; (b) energy transfer (transmission) spectrum of a 11.421 nm Co nanotrack.

Figure 3

Band maxima/minima vs. the quantum number increment.

Figure 4

Efficiency (quantum yield) of the energy transfer along the nanotrack (circles) and the energy emitted by the nanotrack (squares): (a) steady-state excitation at 633 nm; (b) pulsed excitation at 630 nm.

Figure 5

(1) Dynamics of the radiation transmitted along the nanotrack and (2) of the radiation emitted by the nanotrack; pulsed excitation at 630 nm.

Figure 6

Efficiency of the energy transfer by the nanotrack (a) in function of its length, and (b) of the position of the output fiber optic cable on the 12 mm long nanotrack.

Figure 7

(a) Quantum yields of the energy (1) transmitted along the nanotrack and (2) emitted by the dye adsorbed at the nanotrack surface; (b) time evolution (1) of the energy transmitted along nanotrack and (2) radiation emitted by the dye; (c) emission of bulk oxazine-170. The excitation was performed at 633 nm and 5 mW (steady-state mode) or 630 nm and 5 mJ (pulsed mode).

Figure 8

Kinetic scheme of the excited state dynamics in Co nanotrack and Co nanotrack with adsorbed oxazin dye.