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Cite this: DOI: 10.1039/c4nr06642a Received 10th November 2014, Accepted 24th November 2014 DOI: 10.1039/c4nr06642a

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Hybrid photon–plasmon Mach–Zehnder interferometers for highly sensitive hydrogen sensing Fuxing Gu,a Guoqing Wua and Heping Zeng*a,b,c

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By using PdAu nanowires as plasmonic waveguides, hybrid photon–plasmon Mach–Zehnder interferometers by integrating single-crystal PdAu alloy nanowires with silica optical microfibers are demonstrated. Based on an evanescent wave coupling technique using optical fiber tapers, surface plasmon polaritons are efficiently excited and propagated in suspended PdAu nanowires. The interference spectra show attractive properties such as broad and flexible in situ tunability with wavelength spacings ranging from ∼1 to tens of nanometers, and high extinction ratios of over 20 dB. The hybrid Mach–Zehnder interferometers show a higher sensitivity to hydrogen gas than a single-nanowire sensing approach, and the lengths of PdAu nanowires used are less than 20 μm, which are 2 or 3 orders of magnitude shorter than the lengths of Pd coatings in existing fiber-optic hydrogen sensors. Other advantages including good reversibility and low-power operation are also obtained.

1.

Introduction

Hydrogen has attracted significant attention due to its promising applications as a clean energy carrier for use in fuel cells and car engine technology. It is also widely used in scientific research such as that on gas diffusion in metals and biomedicine and industrial manufacturing. Since hydrogen is highly explosive and flammable, it is extremely necessary to develop hydrogen sensors with high reliability, high sensitivity, and fast response.1–3 Palladium (Pd) and its alloys can absorb a large quantity of hydrogen within its crystal lattice and form a palladium hydride phase in a reversible manner, resulting in detectable changes of its electrical and dielectric properties. a

Shanghai Key Laboratory of Modern Optical System, Engineering Research Center of Optical Instrument and System (Ministry of Education), School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China. E-mail: [email protected] b State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China c Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

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These properties have been well developed for hydrogen sensing applications.2–5 In contrast to electrical schemes,6,7 optical sensing offers the advantages of safe operation in an explosive or combustive atmosphere, immunity to electromagnetic interference, and also more options for signal retrieval such as intensity, spectrum, phase, and polarization.8,9 Pd coating-based fiber-optic sensors3,10–13 have been one of the most promising configurations for optical hydrogen detection, including interferometer-based, intensity-based, and fiber grating-based sensors, with the advantages of being riskfree in potentially explosive environments, remote sensing and multipoint measurement. However, because the Pd coatings are very thin (typically less than 100 nm) and most of guided energy is confined inside the optical fibers, the interactions of evanescent waves outside the optical fibers with Pd coatings are insufficient. Thus the signal changes are usually weak, and long lengths of Pd coatings are usually needed (typically longer than 1 mm),3 making it difficult to realize nanophotonic devices with miniaturized sizes and high integration. Due to the short diffusion lengths and large surface-tovolume ratios,6,14 Pd nanostructures such as nanoparticles and nanodisks have been attracting considerable attention in hydrogen plasmonic sensors based on a localized surface plasmon resonance (LSPR) method.15–20 However, because of the nanoscale dimensions of nanostructures, only a small portion of the irradiated light can be intercepted, and the efficiency of photon-to-plasmon conversion is low. Thus the signals are usually weak and relatively high excitation power is needed. In addition, complex and expensive fabrication methods such as using a e-beam lithography technique, and critical sensing requirements such as bulky objectives and low atmospheric-pressure operation are usually needed,18–20 which limit their practical sensing application. Most recently, metal nanowire waveguides have been demonstrated for hydrogen sensing with the attractive advantages of enhanced sensitivity, faster response, and low power operation.21,22 These nanowire sensors are based on measuring changes in light intensity guided along metal single nanowires due to the optical absorption of palladium hydride. The

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Mach–Zehnder interferometer (MZI) is one of the most widely used structures, which is phase sensitive to the refractive index change of the waveguide and/or its surrounding medium.23–26 However, to the best of our knowledge, Pd and its alloy nanowire-based MZIs have not yet been reported. In this work we combine the advantages of optical fibers and surface plasmons and demonstrate hybrid MZIs by integrating singlecrystal PdAu alloy nanowire waveguides with silica optical microfibers. At the optical communication band, MZIs of extinction up to 20 dB are obtained with high sensitivity, short nanowire lengths, and good reversibility to hydrogen sensing.

2. Experimental Due to the hysteresis effect and poor mechanical properties of pure Pd when exposed to hydrogen, Pd usually should be blended with other metals such as Ni and Au to improve the structural stability.27 Here single-crystal PdAu alloy nanowires are chosen in this work,22,28 and are fabricated using a simple thermal evaporation method.22,29 The as-fabricated PdAu nanowires have lengths up to tens of micrometers with diameters selectable from 30 to 500 nm, as shown in Fig. 1(a). The energy-dispersive spectrometry provided in Fig. 1(b) reveals only stoichiometric Pd and Au signals with a determined composition of Pd0.18Au0.82. Fig. 1(c) shows their high-resolution transmission electron microscopy (TEM) image and the corresponding selected area electron diffraction (SAED) pattern, confirming their single-crystal nature and the growth direction

Fig. 1 (a) Typical TEM image of as-grown PdAu nanowires. (b) Energydispersive spectrometry of a Pd0.18Au0.82 nanowire. (c) High-resolution TEM image of the PdAu nanowire. Inset shows its corresponding SAED pattern. (d–f ) Schematic diagram and optical micrographs of coupling light from a suspended silica fiber taper into a nanowire.

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along [110]. In order to study their plasmonic properties, PdAu nanowires are picked up from the grown substrate via micromanipulation and placed parallel at the tip of a suspended silica fiber taper, as illustrated in Fig. 1(d).8,21,22,30 Based on an evanescent wave coupling technique using optical fiber tapers, surface plasmon polaritons (SPPs) are efficiently excited in PdAu nanowires, and the suspension approach enables low-loss and broadband plasmon propagation. Fig. 1(e) and (f ) show the optical micrographs of a PdAu nanowire guiding lasers at wavelengths of 635, 808, and 1064 nm and a supercontinuum (SC) source, respectively. After propagating for a distance of about 10 μm, bright light spots are observed at the distal end, indicating highly efficient photon–plasmon conversion and a promising possibility of integrating PdAu nanowires into standard optical fibers for hybrid photon– plasmon devices. The schematic diagram of the hybrid MZI is shown in Fig. 2(a), which consists of a hybrid sensing arm and a reference arm. Fig. 2(b) shows a fabricated hybrid MZI, in which the lengths of the sensing and the reference arms are 2037 and 1995 μm, respectively. To construct the sensing arm, two silica microfibers (drawn from standard silica optical fiber), are reversely placed on the MgF2 substrates with their tips protruding out of the substrate.21,22,30 The distance between the two tips is around 15 μm. PdAu nanowires are placed across the tips via micromanipulation. Fig. 2(c)–(e) show optical micrographs of placing a nanowire onto a tip of a suspended microfiber and connecting its end to another microfiber by using a fiber taper. To construct the reference arm, another microfiber is placed close to the sensing arm by an evanescent wave coupling technique.23 Two couplers formed between the silica microfibers are maintained on the substrate by van der Waals force and electrostatic force. For a robust sensing operation,

Fig. 2 (a) Schematic diagram and (b) optical micrograph of the hybrid MZI integrated with a nanowire. (c–e) Optical micrographs of connecting one end of a nanowire to another microfiber via micromanipulation by using a fiber taper. (f ) Optical micrograph of an MZI sensing device.

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the microfibers are also bound on the substrate using a UV curable fluoropolymer (NOA1315; Norland Products Inc.). An amplified spontaneous emission (ASE) source is used as the probing light, and launched into the MZI from the sensing arm. The output signal is analyzed by a spectrometer (AQ6370C, Yokogawa). The MZI elements are sealed in a plastic chamber (Fig. 2(f )) with a vapor inlet/outlet. Hydrogen gas diluted with dry argon gas is introduced into the chamber at a flow rate of 100 sccm. All experiments are carried out at room temperature and under atmospheric pressure.

3. Results and discussion Fig. 3(a) gives a typical transmission spectrum of the hybrid MZI provided in Fig. 2(d), in which a Pd0.51Au0.49 nanowire is used with a diameter of 210 nm and a sensing length (LPd) of 17 μm. For reference, the spectrum of the ASE source is also shown by a dashed line. An apparent interference fringe at a wavelength of 1590.2 nm with an extinction ratio of 11 dB is observed. The wavelength spacing (Δλ) can be defined as Δλ = |λmax − λmin|, where λmax and λmin are the spectral positions of two adjacent maximum and minimum, respectively. The values of λmax and λmin measured from Fig. 3(a) are 1590.2 nm and 1572.7 nm, respectively; and thus experimental Δλ is 17.5 nm. Theoretically the path-length difference (ΔL) of the MZI can be expressed by23–25 Δλ ¼ λmax λmin =2ðng;silica ΔLsilica þ ng;PdAu ΔLPdAu Þ;

ð1Þ

where ng,silica and ng,PdAu are the group indices of the silica microfibers and PdxAu1−x nanowires, respectively, ΔLsilica is

Fig. 3 (a) Transmission spectrum of the hybrid MZI in Fig. 2(d). (b) Measured spectrum of the MZI by increasing the path-length difference.

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the length difference of silica microfibers between the sensing and reference arms, and LPdAu is the length of the PdxAu1−x nanowire used. Here ng is 1.46 at 1590 nm wavelength, ΔLsilica is 25 μm, and LPdAu is 17 μm. The ng,PdAu of the Pd0.51Au0.49 nanowire can be calculated using a simple linear combination of refractive indices as ng,PdAu = xng,Pd + (1 − x)ng,Au.31,32 By using ng,Pd = 3.001 and ng,Au = 0.583 at 1590 nm wavelength, the ng,PdAu of the Pd0.51Au0.49 nanowire is calculated to be 1.82. Thus, from eqn (1), the calculated Δλ is about 18.5 nm, which agrees well with the experimental result of 17.5 nm. Since the couplers in the MZI are formed by side-by-side coupling and sustained by van der Waals force and electrostatic attractive force, it is convenient to change the pathlength difference by moving the contact points using micromanipulation. Fig. 3(b) shows two typical spectra of the hybrid MZI by increasing the path-length difference between the sensing and reference arms. As the MZI path-length difference is changed, Δλ also changes, and accordingly a different extinction ratio is achieved. In Fig. 3(b), we have Δλ = 5.4 and 1.8 nm for two typical path-length differences, and the corresponding extinction ratios are 18 dB at 1580.1 nm and 22 dB at 1575.7 nm, respectively. The broad and flexible in situ tunability of the hybrid MZI is promising for large-dynamic optical sensing with quite high sensitivity. Fig. 4(a) gives the transmission spectra of the hybrid MZI (Fig. 2(d)) as hydrogen concentrations increase from 0 to 20%. The dip wavelength of the interference fringe shows a monotonously exponential increase from 1590.2 nm to 1594.0 nm, with a variation in range of about 3.8 nm as plotted in Fig. 4(b). The transmission changes are easily distinguished with a hydrogen concentration of less than 0.5% and become obviously saturated when the concentration exceeds approximately 10%. When exposed to hydrogen gas, the real part of

Fig. 4 (a) Transmission spectra and (b) dip wavelength shift of 1590.2 nm of the MZI integrated with a Pd0.51Au0.49 nanowire as hydrogen concentration increases from 0 to 20%. Inset: intensity change at wavelengths of 1596.4 and 1590.2 nm. (c) Reversible response of the MZI by alternately cycling 3.9% hydrogen inside the chamber. (d) Transmission spectra and (inset) temporal response of the Pd0.51Au0.49 nanowire to 3.9% hydrogen.

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the refractive index of the Pd0.51Au0.49 nanowire increases in the probing spectral region,33 resulting in spectral shifts of the interference peaks. The intensity change at a wavelength of 1590.2 nm is plotted in the inset of Fig. 4(b), in which a monotonous increasing dependence on the hydrogen concentration is observed. The absorbance (defined as A1590.2 = lg[I0/I], where I0 and I are the output intensities in the absence and presence of the hydrogen gas, respectively) at 20% hydrogen is −0.41. Meanwhile, the intensity change at a wavelength of 1596.4 nm is also plotted, in which a monotonous decreasing dependence with an A1596.4 of 0.47 on 20% hydrogen is observed. Thus by using the dual-wavelength measurement, the opposite response at the two wavelengths can also be used to further enhance the sensitivity and accuracy of the MZIs.34 In addition, the A obtained here is higher than most of absolute values in fiber-optic hydrogen sensors (less than 0.3),3,10–13 indicating the higher amplitude sensitivity of our MZI sensors. The temporal response of the nanowire at the 1590.2 nm peak wavelength is tested by alternately cycling 3.9% hydrogen inside the chamber. Compared with the intensely fluctuating dynamic process in pure Pd nanowires,22 an excellent reversibility with high signal-to-noise ratio is observed in Fig. 4(c), indicating that the hysteresis effect is well suppressed in PdAu alloy nanowires. We next investigated the sensitivity of the MZI and compared it with that based on the absorption of signal light in a single-nanowire structure,22 The experimental comparison can be easily done by removing the reference arm of the MZI from the sensing arm using micromanipulation. As shown in Fig. 4(d), with the addition of 3.9% hydrogen gas, the transmission spectrum of the Pd0.51Au0.49 nanowire (without MZI) shows no obvious change as compared with the original spectrum. The inset of Fig. 4(d) shows the temporal response at the 1590.2 nm wavelength, in which slight intensity changes of about 0.04 dB are observed. This value is much smaller than the intensity changes (∼1.9 dB) of the MZI shown in Fig. 4(a) and (c), indicating a higher sensitivity of the MZI structure than those of the single-nanowire structure. From the eqn (1), the wavelength shifts of the MZIs are close to the ratios of LPdAu of PdAu nanowires to the ΔLsilica of silica microfibers. It is expected that higher ratios of ΔLsilica to ΔLsilica will bring larger shifts of the dip wavelengths. For reference, Fig. 5(a) provides the response of a MZI with a Δλ of 4.1 nm, in which a Pd0.53Au0.47 nanowire with a diameter of 250 nm and a length of 13 µm is used. It is shown that with the addition of 9.1% hydrogen, only a 0.2 nm shift at the wavelength of 1594.8 nm is observed, which is much smaller than that obtained in Fig. 4(a). To compare the photonic and plasmonic sensing effects, the Pd0.53Au0.47 nanowire is removed from the coupling tips and attached along the surface of a 2.2 μm diameter silica microfiber, as shown in the upper inset of Fig. 5(b). In this structure, the signal light guided along the silica microfiber leaves a fraction of energy outside the wire as evanescent waves and interacts with the nanowire, which is similar to those reported in Pd coating-based fiber-optic sensors.3,10–13 Fig.

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Fig. 5 (a) Transmission spectra of a MZI integrated with a Pd0.53Au0.47 nanowire as the hydrogen concentration increases from 0 to 9.1%. (b) Transmission spectra of the MZI by attaching the nanowire along the surface of a silica microfiber as the hydrogen concentration increases from 0 to 9.1%. Top inset: optical micrograph of attaching the nanowire along a fiber taper of the MZI. Bottom inset: power distribution (Poynting vectors) of lower-order guided modes at the transverse cross plane of a 2.2 μm diameter silica microfiber attached with a Pd0.53Au0.47 nanowire.

5(b) shows that with the addition of 9.1% hydrogen, no obvious spectral change is observed. Three-dimensional finite-difference time domain (3D-FDTD) simulation (upper inset of Fig. 5(b)) shows the energy fields in a 2.2 μm diameter silica microfiber attached to a PdAu nanowire, in which most of the guided energy is confined inside the silica microfiber and a very small fraction interacts with the PdAu nanowire. In our work using the PdAu nanowires as plasmonic waveguides, obvious signal changes are observed with nanowire lengths less than 20 μm, which are 2 or 3 orders of magnitude shorter than the lengths of Pd coatings in existing fiber-optic hydrogen sensors based on the photonic effect. The compositions of Pd and Au in alloy nanowires have great influence on the sensing performance of the hybrid MZIs. Fig. 6 gives the transmission spectra of the hybrid MZI integrated with a 180 nm diameter Pd0.45Au0.55, in which the dip wavelength at 1587.9 nm shows a redshift with the addition of hydrogen. For hydrogen concentrations less than 10%, the redshift shows a good linear response with a variation range of about 2.1 nm (inset), which is smaller than that of the Pd0.51Au0.49 nanowire in Fig. 4. For thorough experimental comparisons, many experimental tests are done with PdAu alloy nanowires of various compositions. By increasing the Au composition in PdAu nanowires, it is found that the linear responses of the MZI sensors are improved but the

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Notes and references

Fig. 6 Transmission spectra and (inset) dip wavelength shift of 1587.9 nm of the MZI integrated with a Pd0.45Au0.55 nanowire as the hydrogen concentration increases from 0 to 20%.

dynamic variation range decreases. There is a balance between these two factors.

4.

Conclusion

In conclusion, by using the PdAu nanowires as plasmonic waveguides we have demonstrated simple and robust hybrid MZIs by integrating single-crystal PdAu alloy nanowires with silica optical microfibers. The interference spectra show broad and flexible in situ tunability with Δλ ranging from ∼1 to tens of nanometers, and the extinction ratios can be over 20 dB. The nanowire lengths used are less than 20 μm, which are 2 or 3 orders of magnitude shorter than the lengths of Pd coatings in existing fiber-optic hydrogen sensors. Compared with the single-nanowire sensing scheme, the hybrid MZI structure shows higher sensitivity responses and good reversibility. In addition, the power spectral density used in hybrid MZIs are less than 1 nW, and the critical sensing requirements usually used in LSPR-based methods such as bulky objectives and low atmospheric-pressure operations are not needed here. Benefitting from the compatibility with standard optical fiber systems, the hybrid MZI devices are more practical and may stimulate further exploration of metal nanowire-based plasmonic sensors with the advantages of low power consumption, remote sensing, and multiplexing signals in one optical fiber.

Acknowledgements This work was partly supported by the National Natural Science Foundation of China (11304202 and 91221304), Natural Science Foundation of Shanghai (13ZR1458000), National Key Scientific Instrument Project (2012YQ150092), and National Basic Research Program of China (2011CB808105).

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Hybrid photon-plasmon Mach-Zehnder interferometers for highly sensitive hydrogen sensing.

By using PdAu nanowires as plasmonic waveguides, hybrid photon-plasmon Mach-Zehnder interferometers by integrating single-crystal PdAu alloy nanowires...
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