Optical fiber sensor probe: 3D quasi-plasmonic

Picture: (a) A schematic diagram of the semi-three-dimensional structure of the device. (b) Zoom view shows nanocaps and nanoslits. (c) Electric field strength distribution, where the fiber-guided light wave incident from below, mediated by the FP etalon, excites SPPs on the aqueous solution side.
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Credit: By Xiaqing Sun, Zeyu Lei, Hao Zhong, Chenjia He, Sihang Liu, Qingfeng Meng, Qingwei Liu, Shengfu Chen, Xiangyang Kong, Tian Yang

The optical fiber-mounted miniature sensors are being widely recognized as an important future solution for immediate and point-of-care medical diagnosis and on-site agricultural product inspection. Plasmonic devices on the flat sides of single mode fiber make full use of the convenient and fast operation of optical fiber devices. They can be dipped directly into fine specimens or inserted with minimal invasiveness. However, most devices of this type have been restricted by low resonance quality (Q) factors or low coupling efficiency when coupling plasmons and fiber-guided optical waves. Thus, the signal-to-noise ratio (SNR) of refractive index change detection has lagged far behind free-space optics or side-horn waveguide analogs, preventing fiber-tipped SPRs from meeting the requirements of real applications where target concentrations are often low.

A team of scientists, led by Professor Tian Yang from Shanghai Jiao Tong University, has reported remarkable advances in device design, manufacturing technology and SNR for polariton (SPP) sensors on the end aspects of single-mode optical fibers. This work was published in Light: advanced manufacturing Titled, “A quasi-three-dimensional Fano resonance cavity on an optical fiber terminal interface for high signal-to-noise ratio dipping and readout for surface plasmon sensing.”

Based on their previous work on SPP crystal microcavities (Appl. Phys. Lett.108, 231105, 2016; Appl. Phys. Lett.110, 171107, 2017), the researchers used a hybrid structure that produces a Fano resonance between the SPP cavity and a Fabry Perot Italon. . The SPP crystal consists of nano-illuminated periodic arrays in a thin gold film. The SPP cavity consists of an SPP bandgap region located in the center and aligned with the fiber core, and a SPP bandgap region, which is located in the periphery. The SPP-band region converts the incident light normally into second-order SPP-band oriented waves through a grating coupling effect. Finally, the fiber-guided optical wave is converted into a SPP oscillating surface wave on the aqueous solution side of the SPP cavity, using FP etalon as a medium so that high quality and high coupling efficiency can be achieved.

Arrays of Fano-resonance structures were first fabricated on planar glass substrates, then transferred to the end sides of the flat fibers and fixed by UV glue. The interface between devices and glass substrates should be of low adhesion to ensure the quality and success rate of the transfer process. At the same time, this interface should allow efficient tunneling between the light fields in the etalon and the SPPs on the aqueous solution side. For this purpose, the authors invented a nanocap interface in which a few nanometer-thick metallic layer covers prominent dielectric nanocracks.

The fiber probes were installed in standard fiber optic connectors and used to monitor the change of the refractive index and the physical absorption of bovine serum albumin. Test results show that the noise equivalent detection limit of fiber-tip SPR sensors is up to 10-7 RIU level. It is three times less hardware of its kind based on different design approaches and is already comparable to commercial SPR tools based on prism coupling.

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