Single-molecule fluorescence detection (SMFD) is able to probe, one molecule at a time, dynamical processes that are crucial for understanding functional mechanisms in biosystems. Fluorescence in the near-infrared (NIR) offers improved signal to noise ratio (SNR) by reducing the scattering, absorption and autofluorescence from biological cellular or tissue samples, and therefore, provides high imaging resolution with increased tissue penetration depths that are important for biomedical applications. However, most NIR-emitters suffer from low quantum yield and the weak NIR fluorescence signal makes the detection extremely difficult.

Plasmonic nanostructures are capable of converting localized electromagnetic energy into free radiation and vice versa. This capability makes them efficient nano-antennas for modulating molecular fluorescence. The plasmonic nano-antenna generally enhances the fluorescence of a nearby molecule by enhancing the excitation rate and the quantum yield of the molecule. In order to optimally enhance the fluorescence, the plasmonic mode of the nano-antenna has to 1) couple strongly to the molecule and 2) radiate strongly to free space. Simultaneously satisfying the two requirements poses a challenge that is impossible to overcome in conventional, symmetric plasmonic nanostructures.