1. Introduction

In the past 40 years, optical fibers have been very successful in handling light linearly or nonlinearly for wide applications including optical communication, sensing and power delivery [1–3]. While the conventional fiber-optic technology have been well-established, employing new structures or mechanisms for manipulating light has been the driving force to push forward the frontier of fiber optics, among which incorporating fiber optics with plasmonics is one of the current trends for exploring new opportunities. Surface plasmon polaritons (SPPs), which are electromagnetic waves coupled to collective oscillations of electrons on the surface of a conductor [4,5], have been extensively studied recently. By coupling light to free electron oscillations in the metal, SPPs can be guided along metal-dielectric interfaces beyond the diffraction limit and confining light to scales less than λ/10 [6,7], as well as offer the possibility of bridging the gap between the nanoscale electronics and microscale photonics [8]. Since the first demonstration of a fiber-optic chemical sensor based on surface plasmon resonance in 1993 [9], fiber-based SPP components or circuits have been attracting increasing attentions for highly compact and flexible all-fiber applications in optical communication and sensors [10–15]. In recent years, a number of strategies or hybrid structures [13,16–19], such as metal-coated fiber tapers [16,17] or endfaces [18], metal-substrate-supported fiber tapers [13], and metal-nanoparticle-modified fiber tapers [19] have been reported. However, the performances of fiber-based SPP circuits are restricted by high insertion losses of the metal structures and/or low plasmon-to-photon conversion efficiency. Benefitted from strong near-field interaction between tightly confined waveguiding modes in directly contacted photonic nanofibers and plasmonic nanowires, effective coupling between fiber tapers and Ag nanowires was recently demonstrated [12,20–22], which opens opportunities for integrating plasmonic nanowires into standard optical fibers for all-fiber photon-plasmon circuits. Here we demonstrate a simple route to integrating nanowire plasmonics with fiber optics. Relying on high-efficiency near-field photon-plasmon coupling from a fiber taper to a plasmonic nanowire, we demonstrate in-fiber hybrid photon-plasmon probes and all-fiber circuits including loop resonators and Mach-Zehnder interferometers (MZIs) with high flexibilities. Particularly, around the optical communication band, we demonstrate all-fiber photon-plasmon loop resonators with Q-factor of 6 × 10⁶, and MZIs with extinction ratios up to 30 dB and wide-range tunable optical paths. Moreover, as an example for practical applications, we operate the hybrid MZI for plasmonic gas sensing that is fully compatible with standard fiber optics.

2. Fiber-based SPP probes

The Ag nanowires used here were synthesized by reducing silver nitrate (AgNO₃) with ethylene glycol (EG) in the presence of polyvinyl pyrrolidone (PVP) in a soft, self-seeding process [23]. In a typical synthesis, 6 mL of AgNO₃ (0.33g) and PVP (0.17g) solution (in EG) were added drop wise to 5 mL of EG heated at 160°C. The reaction mixture was continued with heating at 160°C for 1 hour until all AgNO₃ had been completely reduced. As-synthesized nanowires were purified by centrifugation, diluted in acetone (5 by volume) to remove EG and then in ethanol (10 by volume) to remove PVP. To tailor a standard optical fiber for nanowire integration, we drew one end of a fiber into sharp taper using a flame-heated taper-drawing technique [24]. By controlling the drawing speed of about 5 m/s, relatively sharp fiber taper with distal end size of about 300 nm can be repeatedly fabricated. Optical characterization shows that, this kind of sharp taper does not only maintain excellent mechanical strength, but also offers nearly adiabatic transition of the fundamental guiding modes of a standard optical fiber into tightly confined modes of the nanofiber at its distal end.

The fiber-based SPP probe was fabricated by coupling a Ag nanowire to the tapered tip of an optical fiber via micromanipulation under an optical microscope. When the Ag nanowire was placed on the fiber tip, optical near field in a subwavelength-diameter nanofiber and plasmonic near field in a plasmonic nanowire may strongly overlap, resulting in highly efficient photon-plasmon conversion in the coupling area. As schematically illustrated in Fig. 1(a), light propagated in a standard fiber was adiabatically squeezed into the nanofiber located at the distal end of the fiber taper, and then effectively coupled to waveguiding plasmons in the Ag nanowire within the overlapping area. When the SPPs propagated to the end of the Ag nanowire, a certain fraction of the SPPs re-emitted into free space as light, while the rest reflected back along the nanowire. Figure 1(b) shows a SEM image of a typical as-fabricated probe assembled with a 300-nm-diameter nanotaper and a 280-nm-diameter Ag nanowire. Although less than 1-μm overlap is long enough for efficient photon-plasmon coupling, here longer than 20 μm Ag nanowire is overlapped on the fiber taper for ensuring the robustness of the probe, leaving about 8.1-μm free-standing Ag nanowire as a plasmonic probe. Figures 1(c) and 1(d) show optical micrographs of the above-mentioned SPP probe before (see Fig. 1(c)) and after (see Fig. 1(d)) optical launching, in which the Ag nanowire is seen under an optical microscope. When a 785-nm-wavelength light was launched from the optical fiber, bright light output from the right end of the Ag nanowire was clearly seen. To estimate the coupling efficiency in Fig. 1(d), we first measure the fractional output of the fiber taper before coupling the Ag nanowire, then the fractional output of the Ag nanowire after coupling it to the fiber taper. The fractional output from the Ag nanowire was obtained as the ratio of output intensity of the Ag nanowire to the output of the fiber taper. To measure the outputs in Fig. 1(d), we considered the fractional output and the guiding losses induced by the Ag nanowire. The guiding loss of the 8.1-μm-length Ag nanowire is about 3.3 dB (about 0.41 dB/μm [25,26]). Therefore, the coupling efficiency was calibrated by deducing the guiding loss from the fractional output (about 43% at 785-nm-wavelength), that is, about 92% at 785-nmwavelength, which is higher than those reported in previous works [12,21,22], and is promising for fiber-based applications such as near-field imaging [27], optical sensing [14] and nanoscale endoscopy [28].

 

Fig. 1 Fiber-based SPP probes. (a) Schematic of a SPP probe. (b) SEM image of a probe assembled with a nanotaper and a 280-nm-diameter Ag nanowire. (c-d) Optical micrographs of the SPP probe taken before and after optical launching. (e) Schematic illustration of a typical in-fiber return-signal plasmonic probe. (f) SEM image of the probe assembled using a 210-nm-diameter Ag nanowire, with about 15-μm-length free-standing nanowire for plasmonic waveguiding. (g-h) Optical micrographs of the in-fiber return-signal plasmonic probe before and after optical launching.

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By coupling back the propagating SPPs in Ag nanowire into a collection optical fiber, a return-signal probe can be obtained. As schematically illustrated in Fig. 1(e), two fiber tapers are placed in parallel and bonded on a low-index substrate (MgF₂ wafer) using low-index UV-cured fluoropolymer (EFIRON PC-373; Luvantix Co. Ltd.) for robust operation. The Ag nanowire is bent to bridge the both tapers. SEM image and optical micrograph of a typical in-fiber return-signal plasmonic probe are shown in Figs. 1(f) and 1(g), respectively. The probe was assembled using a 210-nm-diameter Ag nanowire, with about 15-μm-length free-standing nanowire for plasmonic waveguiding. The bending radius of the Ag nanowire is about 5 μm, which is large enough to avoid bending loss of the propagation SPPs [29]. When a 633-nm-wavelength light was launched from the upper taper, light scattered from the bent Ag nanowire (mostly induced by surface contamination) was clearly seen (see Fig. 1(h)), indicating the propagation SPPs guided through the nanowire bend.

3. Closed-loop all-fiber hybrid photon-plasmon resonators

Relying on the in-fiber return-signal plasmonic probe, a closed-loop all-fiber hybrid photon-plasmon resonator can be readily constructed. As illustrated in Fig. 2(a), by tapering two opposite branches of a commercially available 3-dB X-coupler, and fabricating the probe incorporating a 270-nm-diameter bent Ag nanowire, optical resonance can be established inside the closed loop (see red dashed box in Fig. 2(a)). For optical characterization, a tunable laser (Model: New Focus 6528-LN, linewidth<0.1pm) and an optical power meter (Model: dBm Optics 4650) are used to measure the transmission spectrum. When light from the C-band tunable laser was launched from the left branch, evident resonance was obtained from the right output port of the X-coupler, as shown in Fig. 2(b). The measured free space range (FSR) of the resonance is about 0.7 pm, agrees well with the total optical path of about 3.4 m of the closed loop (30-μm-length Ag nanowire and 2.4-m-length optical fiber). The Q-factor of the hybrid cavity, obtained from the full width at half-maximum (fwhm) of the resonant peak, is about 6 × 10⁶, higher than many other hybrid photon-plasmon cavity reported so far [30–32].

 

Fig. 2 All-fiber hybrid photonic-plasmonic loop resonator. (a) Schematic of the hybrid photonic- plasmonic loop resonator. The black dashed box is the in-fiber return-signal plasmonic probe shown in Figs. 1(e-h) and the red dashed box marks the closed loop. (b) A typical transmission spectrum of the hybrid resonator.

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4. All-fiber hybrid photonic-plasmonic Mach-Zehnder interferometers (MZIs)

To construct a MZI, we connected two commercial Y-couplers, and inserted a fiber-based plasmonic probe in one arm, as schematically illustrated in Fig. 3(a). The Ag nanowire used for the probe is 270 nm in diameter and about 25 μm in effective length (free-standing part), and the entire fiber-based circuit is connected by standard FC/PC connectors. To operate the MZI, broadband light (1520 nm-1620 nm) from a tunable laser was sent into the circuit from the left-side fiber, split by the first coupler, and recombined at the second coupler after traveling through the reference arm (the upper pure-fiber arm) and the hybrid arm (including the bottom plasmonic probe), respectively. The output signal from right-most fiber of the second coupler was sent to an optical power meter. Typical spectral response of the MZI is shown in Fig. 3(b), in which a clear interference with an extinction ratio up to 30 dB is observed, with a FSR of about 2.63nm.

 

Fig. 3 All-fiber hybrid photonic-plasmonic MZI. (a) Schematic of the hybrid photonic-plasmonic MZI. The structure in the dashed box represents the in-fiber return-signal plasmonic probe shown in Figs. 1(e-h). (b) Typical transmission spectrum of the hybrid MZI.

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Since the all-fiber circuits shown here can be connected by either standard rapid interconnection (hot plugging) FC/PC connectors or fusion splicing, it is convenient to change the optical path by adding/reducing a certain length of the optical fiber. Here, for example, we tuned the optical path of the reference arm of the MZI shown in Fig. 3(a) by plugging in single-mode fiber optic patch cables with different lengths, and evaluated the circuits around the C-band for optical communication. Figure 4 gives the typical results with patch cables of different lengths. It shows that, the FSR of the hybrid MZI can be readily changed from nanometer level (e.g., 2.4 nm of spectral line 1 in Fig. 4(a)) to picometer level (e.g., 1.0 pm in Fig. 4(c)), which are promising for large-dynamic optical sensing with high sensitivity.

 

Fig. 4 Transmission spectra of an all-fiber hybrid photonic-plasmonic MZI with different optical paths. (a) Transmission spectra of the MZI with different path differences. The spectral intensities of (1)–(3) are offset for clarity. Details of the dashed and solid boxes are showed in (b) and (c) respectively.

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To verify the wide-range tunability, we estimate the path-length difference of the two arms by [33]

As an example for practical application, we further explored the possibility of using the all-fiber hybrid MZI for optical sensing of ammonia gas (NH₃). Here the path difference of the MZI was about 27 mm, and the Ag nanowire was 190 nm in diameter and 20 μm in effective length. When the plasmonic probe was exposed to NH₃, the group index of the Ag nanowire was changed due to the change of the resistance [35], resulting in spectral shifts of the interference peaks. As shown in Fig. 5(a), the spectral shift of the interference fringes is clearly seen when the probe was exposed to NH₃ (with N₂ as carrier gas) of 80 ppm (blue line) and 160 ppm (red line), respectively. For in situ sensing, we launched a monochromic 1550-nm-wavelength light into the MZI, and measured the temporal response of the output while alternately cycling pure N₂ and 80 ppm NH₃, with typical response given in Fig. 5(b). The output intensity shows reversible response with high signal-to-noise ratio, indicating that the detection limit can go much lower than 80 ppm. The response time estimated from the close-up time-dependent output (see inset of Fig. 5(b)) is about 400 ms (rising time) and 300 ms (falling time), which is on the same order of other types of fast-response gas sensors [36].

 

Fig. 5 Optical sensing of NH₃ with an all-fiber hybrid photonic-plasmonic MZI. (a) Spectral shifts of the interference peak when the probe is exposed to NH₃ of 80 ppm (blue) and 160 ppm (red), respectively. (b) Temporal response of the MZI measured by alternately cycling pure N₂ and 80 ppm NH₃.

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5. Conclusions

The possibility of integrating nanowire plasmonics with standard fiber optics may open a variety of new opportunities for both fiber optics and nanowire photonics. Generally, the fiber-optic technology can provide highly flexible circuitry from long-haul to chip-to-chip level [37,38], and the nanowire plasmonic waveguiding is ideal for ultra-compact interconnection and redirection from wavelength to sub-wavelength scale [39–41], the seamless integration of nanowire plasmonics with fiber optics offers a promising route to bridge light from macroscopic fiber systems to microscopic nanowire plasmonics down to deep-subwavelength level, as well as a convenient platform for exploring novel fiber-compatible plasmonic-based devices ranging from optical sensing [42] to quantum information technology [43].