Gold nanowires enable plasmonics in optical fibers

Cylindrical nanophotonic devices in fiber form represent a new class of integrated component that could overcome several limitations faced by their state-of-the-art planar counterparts. Limitations inherent to planar devices include their significant insertion losses and small aspect ratio. The geometry of cylindrical devices, which can be straightforwardly integrated into fiber-optic circuits, may also render the fiber-planar device junction—one of the most severe bottlenecks of all network systems—obsolete. Our approach to resolving such bottlenecks and implementing novel functionalized fibers is based on pressure-assisted melt-filling (PAMF) of holey silica photonic crystal fibers (PCFs) and capillaries. This technique gives PCFs new functionalities and may enable the application of optical fibers in previously inaccessible areas, such as plasmonics.

Our fabrication approach relies on pressing molten materials into silica matrices at very large external pressures.¹ The only prerequisite with PAMF is that the melting temperature of the material to be filled must be lower than the softening temperature of silica (1400°C). The technique therefore intrinsically allows the combination of a greater variety of materials with silica compared with fiber drawing or extrusion, representing a unique approach to altering the properties of optical fibers. The mismatch of thermal expansion coefficients when hole diameters fall below 10μm—a severe problem in fiber drawing—is not relevant within the framework of this technique.

In addition to infiltrating metals, the process has been employed for filling semiconductors² and amorphous materials.^{3, 4} For example, highly nonlinear chalcogenide-silica waveguides have been fabricated by filling arsenic trisulfide glasses into a silica capillary with a bore diameter below 1μm. When exposed to ultrashort optical pulses (center wavelength 2μm), a coherent supercontinuum within the mid-infrared domain is generated which extends up to 4μm.⁵ This technique also allows the fabrication of encapsulated inverse taper structures, called photonic nanospikes, which can boost incoupling efficiencies of subwavelength waveguides by a factor of about 60.⁵

Our fabrication approach enables the combination of plasmonic nanowires (NWs) made from gold with optical fibers, leading to a new class of plasmonic devices in fiber form: see Figure 1(a).¹ Gold does not interact with silica during filling, which enables the fabrication of long NWs with aspect ratios of >10⁴. PCFs which include gold NWs (plasmonic PCF: pPCF) represent an important type of nanophotonic fiber. The collective response of the electron ensemble of the metals leads to localized plasmonic resonances, or propagating plasmonic modes. Plasmonic excitations in metallic NWs can be described as propagating planar plasmons spiraling around the wire circumference: see Figure 1(a).⁶ In contrast to planar film-type structures, our NWs show propagating higher-order plasmonic modes with refractive index dispersions partly falling below the index of the cladding: see Figure 1(b). This feature is essential for the phase-matching of these plasmonic modes to the PCF core mode.

Figure 1. Schematic of a spiraling planar surface plasmon on a single isolated metal wire. Red lines represent the plasmon trajectory on the wire surface and arrows indicate the wave vector diagram of the propagating plasmon. k_SPP: Planar surface plasmon mode wave vector. k_t: Quantized tangential wave vector. β: Longitudinal propagation constant. (b) Complex plane representation of the effective mode index of the different plasmon modes as function of silver wire radius. Each curve starts at a radius of 800nm (small circles) and follows the complex effective mode index for a reducing wire radius (radius reduction indicated by the red and green arrows; wavelength for all calculation: 600nm). The numbers stand for the order of the indicated plasmon mode (order of Bessel function). In the left-hand (light green) area, the real part of the plasmon modes falls below the index of silica. The inset depicts the smallest wire (120nm diameter) fabricated so far with the pressure-assisted melt-filling technique. LR-SPP: Long-range surface plasmon polariton. SR-SPP: Short-range SPP.

By investigating arrays of more than 100 metallic NWs inside of a pPCF, we were able to observe plasmonic mode hybridization⁷ (an effect similar to molecular hybridization in atomic physics). This hybridization, visible as a superplasmonic mode formation, is especially pronounced in the first ring of the NW-array, in which a coupling of quadrupole plasmons can be seen: see Figure 2(a). Using a novel tip calibration technique, we were able to measure the orientation of the local electric field vector (i.e., the nanoscale polarization) with a lateral resolution of more than 60nm.⁸ This represents the first example of a measurement of the transverse near-field pattern and the corresponding near-field polarization of a propagating plasmon mode.

Due to the ductility of gold, cleaving fibers with incorporated NWs leads to the formation of small gold tips at the location of the cleave.⁹ These can be as narrow as 10nm at the tip, defining a small plasmonic resonator. We integrated such a tip at the end of a multimode fiber, with the goal of realizing fiber-integrated near-field probes: see Figure 2(b). In these probes, incoming light excites particular resonances within the plasmonic resonator. Light from these resonances is scattered into the surrounding tapered silica that supports the gold NW. This light is then coupled into the multi-mode fiber and guided to an analyzing instrument. Using our probe, we successfully measured the evanescent near field of a standing wave in the vicinity of the surface of a prism.

Figure 2. (a) Near-field scan of the end face of an array of nanowires. A superplasmonic mode formation occurs when light that is close to the resonance wavelength of coupled systems is launched into the array's defect core. (b) Fiber-integrated near-field probe with nanoscale plasmonic resonator at the tip.

In summary, our fabrication approach using PAMF represents a new way to integrate sophisticated materials into optical fibers. We incorporated noble metallic nanowires into optical fibers and have, for the first time, shown the excitation of spiraling planar plasmons and plasmonic supermodes consisting of more than one hundred individual plasmons. Moreover, we implemented a novel kind of near-field probe based on a nanoscale plasmonic resonator at the end of a functionalized multimode fiber. We are now targeting the integration of a nanotip and waveguide into a single fiber, with the ultimate goal of developing a fully integrated near-field probe that can be used to both excite and collect light.