(Invited) Gold Nanobridge Enhanced Fiber Optic Sensor
Authors: Floyd Adam , Scott Brian , Song Yunbin , Fraser Michael , Yu Zhihao , Wang Anbo , Pickrell Gary 
Published: February 2020

A wide variety of optical sensors have been used for gas detection. Hollow core waveguides have shown particular usefulness due to the larger interaction volume of the optical signal with the gas species through direct absorption instead the lower interaction volume of evanescent field spectroscopy. In particular porous capillary tube waveguides (PCTW) have been of particular interest because of their very fast response time. One limitation, in particular, is the increased loss due to the nearly free space transmission through the hollow core. PCTWs have been successfully tested on a variety of gases and across a wide temperature range by our group previously.
In this paper, we present the results for a fiber optic sensor structure which is composed of gold nanobridges. The gold nanobridges are formed by coating the inside of a microcapillary glass tube with gold. The glass tube is phase separated and one of the phases is removed by a leaching process. This leaves a three dimensionally interconnected porous phase and a three dimensionally interconnected glass phase which is coated with gold. The structure was analyzed for topology and chemistry using a scanning electron microscope. SEM micrographs of the microcapillary tubes both before and after phase separation were obtained. In addition, EDAX was used to confirm the presence of the gold coating on the samples after the leaching process. The porous gold film was found to be composed of approximately 100nm ligaments in an interconnected network and the glass had approximately 30nm pores around these ligaments.
The process to produce these gold coated nanoporous elements is abbreviated here due to word count restriction for this abstract. Porous capillary tubes were produced from a phase separable glass composition. This type of glass produces 3-dimensionally interconnected phases through a compositional phase separation process induced by heat treatment. Porosity is created when the secondary phase or soluble phase is removed by immersion in a dilute acid solution that dissolves and leaches out the secondary phase. Capillary tubes to be plated were drawn with an inner diameter of 170-220 µm and an outer diameter range of 400-500 µm. Dimensions were selected to provide an adequate ID for plating and then the insertion of the lead-in and lead-out fibers. Drawn capillary tubes were plated using a commercially available electroless gold plating solution.
Plating was accomplished after fibers were annealed to induce phase separation in order to minimize thermal changes to the gold film that the necessary heat treatment would cause. The inner diameter of the capillary tube was coated by flowing the plating solutions through the tubes. Plating was carried out per the manufacturer’s directions with the exception that the solutions were in a flowing state compared to the static application recommended. Plating occurred in a three step process with the first step being a sensitizing solution, then a brief rinse, followed by a combined mixture of an activated gold salt and a reducing agent. Flow rates of the plating solution ranged from 0.5 mL/min-2mL/min depending on tube ID. After plating, the tube and gold film was air dried followed by acid leaching of the secondary phase.
Optical transmission spectra were characterized over the spectral range from 1520-1570 using a commercially available spectrometer which contained a swept laser interrogator. A Micron Optics Si-720 Component Testing System (CTS) and SMF-28 lead-in and lead-out fibers were used in the transmission testing. The apparatus constructed for the transmission tests consisted of the CTS unit with the swept laser being used as the light source connected to a SMF optical fiber as the lead-in fiber. A MMF was used for the lead out fiber to the oscilloscope which was attached to a photodetector for detection. The transmission spectrums were studied as a function of the microcapillary tubing length. By comparing the transmission spectrums of the microcapillaries before gold coating and after gold coating, it was shown that the gold coated microcapillaries had higher transmission than the uncoated for each length studied. The gold coated nanobridge enhanced fiber optic sensor system was tested in a gas cell using acetylene gas. Test concentrations ranging from 10% Acetylene down to 100ppm were obtained using a Sierra Instruments gas mixing system. From the optical spectrums measured, it was shown that the absorption peaks associated with the acetylene gas are clearly visible in the optical spectrums. The absorption peak strengths were in the range from 5-10dB depending upon which line in the acetylene absorption spectrum one is considering and what the gas concentration was.

Figure 1