Supercritical Angle Fluorescence Characterization Using Spatially Resolved Fourier Plane Spectroscopy.

Most fluorescent immunoassays require a wash step prior to read-out due to the otherwise overwhelming signal of the large number of unbound (bulk) fluorescent molecules that dominate over the signal from the molecules of interest, usually bound to a substrate. Supercritical angle fluorescence (SAF) sensing is one of the most promising alternatives to total internal reflection fluorescence for fluorescence imaging and sensing. However, detailed experimental investigation of the influence of collection angle on the SAF surface sensitivity, i.e., signal to background ratio (SBR), is still lacking. In this Letter, we present a novel technique that allows to discriminate the emission patterns of free and bound fluorophores simultaneously by collecting both angular and spectral information. The spectrum was probed at multiple positions in the back focal plane using a multimode fiber connected to a spectrometer and the difference in intensity between two fluorophores was used to calculate the SBR. Our study clearly reveals that increasing the angle of SAF collection enhances the surface sensitivity, albeit at the cost of decreased signal intensity. Furthermore, our findings are fully supported by full-field 3D simulations.

Electrically Driven Unidirectional Optical Nanoantennas.

Directional antennas revolutionized modern day telecommunication by enabling precise beaming of radio and microwave signals with minimal loss of energy. Similarly, directional optical nanoantennas are expected to pave the way toward on-chip wireless communication and information processing. Currently, on-chip integration of such antennas is hampered by their multielement design or the requirement of complicated excitation schemes. Here, we experimentally demonstrate electrical driving of in-plane tunneling nanoantennas to achieve broadband unidirectional emission of light. Far-field interference, as a result of the spectral overlap between the dipolar emission of the tunnel junction and the fundamental quadrupole-like resonance of the nanoantenna, gives rise to a directional radiation pattern. By tuning this overlap using the applied voltage, we record directivities as high as 5 dB. In addition to electrical tunability, we also demonstrate passive tunability of the directivity using the antenna geometry. These fully configurable electrically driven nanoantennas provide a simple way to direct optical energy on-chip using an extremely small device footprint.