Planar nano-antennas for spectroscopy and sensing

Molecular sensing and detection have been established over the past decades with biochemistry and medical diagnostics applications. Among the various sensing methods, fluorescence detection is considered to be one of the most promising ones, especially in vitro diagnostics. Dye molecules used as a label in fluorescence detection are generally dipolar light sources emitting in a random direction, and a significant fraction of the emitted photons can not reach the detectors. This dramatically reduces the brightness of the dye molecules. A practical approach to overcome this problem relies on enhanced light-matter interaction obtained using optical nano-antennas.
In this work, we investigate a planar antenna configuration, which beams the radiation pattern of the dye into a narrow cone. It mimics the operation of a Yagi–Uda antenna, where reflector and director elements are made of thin metal films. Here, the reflector element of the planar Yagi–Uda antenna is an etched flat gold wire, and the director element is made of 10 nm gold film evaporated on the glass coverslip. Our first goal is to elucidate the antenna effects when the reflector-director gap changes (scanning antenna). By scanning the reflector in the axial direction, the radiation pattern of a single fluorescent bead changes, and the emission narrows down to angles of 45° (full width at half maximum) at the antenna resonance. Afterward, we focus on detecting double-stranded DNA molecules labeled by fluorescent Atto-647N in a buffer to determine the sample concentration.
Moreover, a practical way to guide the fluorescence signal to a detector is through an optical fiber. By coating the director on the tip of an etched fiber and using a gold substrate as a reflector, we can direct the emission into the fiber with high efficiency. A numerical simulation has been developed to evaluate the coupling efficiency of a horizontally oriented dipole into a fiber. The result determines more than 70% coupling efficiency, which would only scale by a factor of 2/3 for emitters with a random orientation. This configuration can not only collect the emitted light but also excite the molecule through the same fiber. However, in practice, the fiber shows a high autofluorescence in excitation due to doping or defects in the core material, which needs further investigation. This method is a compact solution to enhance the sensitivity and dynamic range of molecules detection, and it can replace the bulk optics conventionally employed for fluorescence measurements.