Optical fiber is the most broadly used optical waveguide for transmitting light because of its low loss transmission even over long distances and its flexibility, which has been used for illumination and collection of light in various applications including optical fiber biosensors and chemical sensors, fiber lasers, and in-fiber near-field imaging. With the increasing demand for nanoscale photonic devices and quantum communication, an efficient way is needed to focus the light transmitted by optical fiber (the photonic mode) down to nanoscale-confined light. Surface plasmon polariton (SPP) waveguides can control and confine light in the nanometer scale as the light is propagating along the metal-dielectric interface. However, the travel distance of SPP is in the micrometer scale due to the high optical loss. Thus, efficient coupling of low-loss photonic waveguide modes to the highly-confined SPP mode and vice versa is necessary for practical nanoscale fiber optics. The development of these optical fiber nanofocusing devices has been challenging as it requires the phase matching of photonic mode and plasmonic mode that exhibit different mode sizes (microscale vs nanoscale) and mode profiles (with the plasmonic mode being naturally radially polarized). Additionally, current attempts to integrate a nanometer-sized plasmonic nano-waveguide, e.g. a metal nanowire, onto a micrometer-sized fiber have required a challenging fabrication process with multiple procedures and precise alignments.
The collaborative research between Prof. Zhenrong Zhang's group at Baylor University and Prof. Howard Lee's group at the University of California, Irvine in the United States focuses on a simple and straightforward method of efficient coupling and focusing of the optical fiber photonic mode to the nanoscale plasmonic mode (Fig. 1a). The device involved is a needle-like plasmonic nano-antenna fabricated on the end facet of a photonic crystal fiber (PCF) by an electron beam induced evaporation technique (Fig. 1b). The fabrication, experimental characterization, and theoretical simulation of the device are published in Photonics Research Vol. 9, No. 2, 2021 (Khant Minn, Blake Birmingham, Brian Ko, Ho Wai Howard Lee, Zhenrong Zhang. Interfacing photonic crystal fiber with a metallic nanoantenna for enhanced light nanofocusing[J]. Photonics Research, 2021, 9(2): 02000252).
In this platform, introduced by Khant Minn from Baylor University, the fundamental core mode of the PCF propagates through the fiber and couples to the SPP mode on the platinum nano-antenna via the end-fire coupling. Such direct coupling relaxes the challenging phase-matching requirement of the photonic mode and the plasmonic mode, enables broadband coupling, and reduces propagation losses. The coupled SPP's then propagate along the antenna toward the tapered end where they converge to produce a highly enhanced and confined field at the nano-scale apex. The precise control of the position and size of the plasmonic antenna is achieved by directly writing a nanoscale metallic tip onto the fiber in a single step inside a focused-ion and scanning electron microscope (FIB-SEM) chamber.
Fig. 1. The photonic crystal fiber (PCF)-nanoantenna hybrid probe. (a) Schematics of the device and simulated intensity profile of plasmonic mode on metallic nanowire waveguide. (b) Schematics of the electron beam induced deposition process and SEM image of PCF and fabricated nanowire
This work presented a single-step fabrication of a fiber-plasmonic probe which allows for any fiber-antenna coupling configuration to be realized with high precision, in addition to the high yield of the designer 3D antenna with nanometer resolution. For example, during fabrication, a gold thin film rectangular aperture was opened at the base of the metal nanoantenna to enable the asymmetrical coupling and focusing of the commonly-accessible lineally-polarized light from the optical fiber to the radially-polarized nano-focusing plasmons at the tip apex. Input polarization selectiveness of the device observed in the side images of the scattered light demonstrated the plasmonic nature of the focused SPP.
Prof. Zhenrong Zhang from Baylor University and Prof. Howard Lee from the University of California, Irvine believe that the fiber-plasmonic probe is a promising platform for the development of novel optical fiber devices and applications such as remote sensing and nearfield spectroscopes, single photon excitation and quantum sensors, nano-scale optical lithography, and lab-on-fiber devices.
Future work will focus on the optimization of the coupling and focusing efficiencies and the integration of these fiber probes in scanning probe microscopes for nanoscale chemical imaging such as fiber-based tip-enhanced Raman and fluorescence spectroscopies.
