Optical Fiber Axicon Enabled Plasmofluidic Trapping: Sensing With Enhanced Figure of Merit

In this letter, trapping of colloidal gold nanoparticles (AuNPs) using an optical fiber axicon, and its use in plasmon-enhanced sensing have been reported. The AuNPs of 40 nm size were trapped in a Bessel beam from an optical fiber negative axicon coupled with a laser of 532 nm wavelength. The fluorescence signal from a mixture of AuNPs and Rhodamine 6G (Rh6G) of varying concentrations was studied on a custom-made optical setup. Control experiments on an optical fiber with Gaussian beam illumination, and on NPs of varying sizes were performed to achieve optimal performance. An overall plasmonic enhancement factor of
$10^{3}$
per molecule in the fluorescence signal was estimated. A performance enhancement of about 480 times was achieved through trapping, as compared to normal optical fiber illumination. A limit of detection (LoD) of 10 ng/ml of Rh6G was achieved. This study is useful in the development of application-specific metasurfaces, and biosensors using structured beam illumination of plasmonic-fluids. This study also shows that structured illumination can lead to manifold increase in plasmon enhanced fluorescence signal.