Sensors for harsh environments must exhibit robust sensing response and considerable thermal and chemical stability. We report the exploration of a novel all-alumina nanostructured sapphire optical fiber (NSOF) embedded with Au nanorods (Au NRs) for plasmonics-based sensing at high temperatures. Temperature dependence of the localized surface plasmon resonance (LSPR) of Au NRs was studied in conjunction with numerical calculations using the Drude model. It was found that LSPR of Au NRs changes markedly with temperature, red shifting and increasing in transmission amplitude as the temperature increases. Furthermore, this variation is highly localized through tunneling by overlapping the near-field of thin cladding and sapphire optical fiber. The NSOF embedded with Au NRs has the potential for sensing in advanced energy generation systems.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
There is an increasing demand for harsh-environment sensors capable of continuously monitoring the operation of high-efficiency low-emission energy production systems such as combustion turbine engines, coal gasifiers and boilers [1–3]. These sensors should be cost-effective, reliable, robust, and stable over the long term. Fiber optic sensors are promising candidates owing to their electromagnetic interference immunity, elimination of electrical wiring, and availability at reasonable cost . However, conventional silica-based optical fibers are limited in their ability to operate at high temperatures, particularly in chemically reactive environments . Nanostructured sapphire optical fiber (NSOF), consisting of sapphire fiber as the waveguide core and nanoporous anodic aluminum oxide (AAO) coating with highly ordered pore channels aligned vertically to the fiber surface as the cladding, serves as a promising and novel sensing platform with superior chemical and thermal stability [6,7]. Apart from the obvious advantage of the same material composition as the fiber core, AAO cladding can prevent sapphire fiber from surface contaminations [8,9] and possesses the structural tunability to achieve diverse sensing modalities . We report here the first demonstration of sensing at elevated temperatures using NSOF with embedded plasmonic Au nanorods.
Plasmonic nanostructures have been shown to exhibit strong response to variations in temperature and chemical composition [11–18]. The working principle of plasmonic sensors is based on localized surface plasmon resonance (LSPR) of metal (e.g., Au and Ag) nanostructures, the free electrons of which oscillate in resonance with an external electromagnetic field . This oscillation results in absorption or scattering of the incident field that can be observed as strong resonance peaks in absorption or reflection measurements. The LSPR of the plasmonic nanostructure can be dramatically modified by environmental conditions such as the surrounding medium and temperatures. Changes in the LSPR features can be directly associated with changing environmental conditions .
Au nanorods (Au NRs) that are chemically and morphologically stable at high temperatures  are fabricated into the pore channels of AAO cladding of NSOF. The resultant AAO cladding embedded with an array of Au NRs is also a plasmonic metamaterial with optical properties tunable in the visible and near-infrared spectral range. Such cladding configuration exhibits diverse physical properties such as hyperbolic dispersion , negative refraction , nonlocal effects , and light polarization shaping . The optical properties of Au NRs depend strong on temperatures also. For example, the influence of cryogenic temperature down to 80 K on the low-temperature plasmonics of the Au NR-based metamaterials has been measured . However, there is a dearth of experimental study of the high-temperature plasmonics of Au NRs due to their thermodynamic tendency to become spherical nanoparticles at elevated temperatures . In this study, the thermal and morphological stability of Au NRs at elevated temperatures has been significantly improved by geometrical confinement inside AAO pores . High temperatures can drastically alter the transmission of light through Au NRs embedded within the AAO cladding and this altered transmission was successfully observed without the effect from ambient thermal irradiation using NSOF through near-field evanescent tunneling. Numerical modeling was conducted to explain the temperature-dependent Au permittivity using the plasmon frequency and electronic collision frequency as two key parameters.
2. Experimental section
2.1 NSOF fabrication
The essence of NSOF fabrication is to coat an unclad sapphire fiber with Al followed by a simple and cost-effective growth process of porous AAO cladding from Al coating via anodization in an electrochemical cell . Briefly, unclad sapphire fibers (α-Al2O3, MicroMaterials, Inc., SF200-10, 200 µm in diameter, 10 cm in length) with polished distal ends were first cleaned by sonication in acetone (C3H6O, Pharmco-Aaper) and then in Milli-Q water for 10 min. Al with varying thicknesses, which can be controlled by deposition rate and time, were coated on the fiber through thermal evaporation from a heated Al source in a thermal evaporator (Angstrom Engineering, Ontario, Canada). Coating uniformity was achieved by rotating the fiber placed in the path of the Al vapor flux. The Al-coated fibers were then electrochemically anodized to fully convert the Al coating into AAO cladding in 0.3 M oxalic acid (H2C2O4, Sigma-Aldrich, 98%) with vigorous stirring under 60 V for 10 min at 0 °C in an ice-water bath using our home-built anodization unit . The AAO pores were subsequently widened via chemical etching in 0.3 M phosphoric acid (H3PO4, J. T. Baker Chemical Co, 98%) for 50 min at room temperature, followed by thermal annealing at 800 °C for 1 hour in static air to stabilize the pore structure and remove remnant adsorbates. The resultant AAO cladding of NSOF was 5 cm in length, 100 nm in average pore depth and 50 nm in average pore diameter. Planar AAO structures with the same thickness and pore diameter as NSOF cladding were fabricated on sapphire substrates (α-Al2O3, Advalue Technology, 25 × 25 mm, C plane orientation) following the same procedures above.
2.2 Synthesis of Au NRs
The AAO on sapphire substrate or as part of the NSOF was first infiltrated with an aqueous solution of 0.2 mg/ml poly(allylamine hydrochloride) (PAH, Sigma-Aldrich, Mw = 15,000) polymer at pH 9 for 30 min to functionalize the AAO surface. Upon rinsing three times with Milli-Q water to remove any free or loosely bound PAH molecules, the PAH-coated AAO was then immersed into a 5 nm Au NP solution (Nanocomposix, citrate surface, 0.05 mg/mL) for 13 hours, followed by rinsing three times with Milli-Q water to remove any free or loosely bound Au NPs. The Au NPs were immobilized onto the PAH-functionalized AAO top surface and pore walls through electrostatic interactions with PAH and binding between the Au NPs and amino groups of PAH. Subsequently, the 5 nm Au NPs-coated AAO was exposed to a fluorine-based reactive ion etching (RIE, Oxford PlasmaPro NPG80) process to remove any Au NPs on the top surface. Finally, the AAO, with Au NPs only anchored on the pore channel walls, was immersed into an Au plating solution containing 0.079 M gold sodium sulphite (Na3Au(SO3)2, Alfa chemistry, 100 g/L), 0.127 M sodium sulfite (Na2SO3, Sigma-Aldrich, 98%), 0.025 M sodium bicarbonate (NaHCO3, Sigma-Aldrich, 99.7%, ACS reagent) and 0.0625 M formaldehyde (HCHO, Sigma-Aldrich, 36.5-38% in H2O) for 20 hours at 4 °C until Au NRs formed. The pH of the Au plating solution was adjusted to 8 by dropwise addition of 1 M H2SO4 (H2SO4, Sigma-Aldrich, 95-98%). An additional annealing procedure at 250 °C for 1 hour was conducted to densify the Au NRs. The geometry of the resultant Au NRs in AAO was measured via scanning electron microscopy (SEM, Zeiss Auriga).
2.3 NSOF embedded with Au NRs for high-temperature plasmonics
To characterize the high-temperature plasmonics of Au NRs in NSOF under harsh environment conditions, NSOF was first fusion spliced with silica fiber (Thorlabs FGA105-LCA, 105 µm in diameter) on both ends and placed in a tube furnace (Thermo Scientific Lindberg Blue M) with the cladding section in the hot zone as shown in Fig. 1. Tube end caps were custom-built to enable programmable gas flow at elevated temperatures, and the N2 gas flow was maintained at a fixed flow rate of 100 sccm. Optical characterization was performed by coupling light from a broadband halogen light source (Ocean Optics, DH-2000-BAL) into the silica fiber and measuring the transmission spectrum with a UV-VIS spectrometer (Ocean Optics Jaz Spectrometer, JAZA2166-2048 pixels) connected to the other silica fiber. The transmission spectra were taken as temperature increased from room temperature to 800 °C at a ramping rate of 1 °C per minute.
Planar AAO embedded with Au NRs on the sapphire substrate was placed in a high-temperature optical transmission cell with sapphire windows (Specac FT-NIR). The sample was illuminated with collimated light from a halogen lamp, vertical to the substrate surface and parallel to the longitudinal axis of Au NRs such that only the transverse LSPR modes would be excited. This is in contrast to the Au NRs integrated NSOF for which both modes would be excited to some extent dependent upon the details of light propagation and polarization state within the fiber. The transmission spectra in response to temperature were also recorded following the same ramping profile as NSOF for direct comparison. The transmission spectra were measured relative to an empty cell under ambient conditions, with both windows in place.
2.4 FDTD simulation
Lumerical FDTD solution of version 8.19 was used to simulate the transmission spectra of Au NRs in planar AAO at elevated temperatures. Periodic Au NR arrays (50 nm in diameter, 100 nm in interrod separation, 22.7% in porosity) in hexagonal arrangement were built using the Cartesian coordinate system. Periodic boundary conditions were defined in the draw mode. For comparison with temperature dependent optical property measurements reported, the complex refractive index of Au NRs at different temperatures was calculated using the Drude model . AAO was treated as an effective homogeneous medium with the effective refractive index modified by porosity as well as being temperature-insensitive. The mesh grid size is 0.5 nm.
3. Results and discussion
There were considerable changes in the transmission spectra through NSOF embedded with Au NRs in response to changing temperature in the environment according to our measurements. Scanning electron micrograph in Fig. 2 shows that the Au NRs are 50 nm in average diameter after 800 °C treatments, and this size has been widely explored regarding the LSPR activity in the literature . The relative change in the transmission spectral energy density (Δuλ=(uλ,T-uλ, 800)/uλ, 800) at different temperatures with respect to 800 °C in nitrogen is presented in Fig. 3(a). A strong wavelength-dependent relative spectral energy density change is observed. The localized feature in the UV-Vis range can be attributed to be the LSPR of Au NRs, red shifting and decreasing in peak intensity as temperature is incrementally increased from 100 to 800 °C. The maximum relative change is observed to be 17.5% for 100 °C compared to 800 °C at a wavelength of 500 nm and the rapid variations in transmission with wavelength are asserted to results from the complex optical anisotropy and periodicity of the Au NR integrated AAO NSOFs. A parallel study on NSOF without embedded Au NRs was conducted with the transmission results illustrated in Fig. 3(b). Additional measurements were also carried out using unclad sapphire fiber, whose transmission spectra exhibited no temperature dependence (results not shown here). The temperature-insensitivity of both bare NSOF and unclad sapphire fiber, relative to NSOF with Au NRs, serves to confirm that the temperature-induced variations shown in Fig. 3(a) originated principally from the response of the LSPR of Au NRs to changing environment temperature. Our work offers the first observation that NSOF embedded with Au NRs can be utilized to investigate plasmonic characteristics of metal nanostructures at high temperatures through near-field evanescent tunneling. These results demonstrate the significant potential of the novel NSOF/Au NRs platform for sensing in harsh environments not suitable for their silica optic fiber counterpart [5,13].
To better understand the temperature induced LSPR-based activity of NSOF embedded with Au NRs, the optical transmission of planar AAO embedded with Au NRs as a function of temperature was measured and presented in Fig. 4(a). In this case, the collective plasmonic transverse mode associated with electron movement perpendicular to the long axis of nanorods was directly stimulated with incident light vertical to the substrate, along with electromagnetic field vector parallel to the short axis of the rods. The transverse mode is observed as a strong LSPR extinction peak in the transmission spectra and the transmission extinction peak shows appreciable red shifting from 529 nm to 539 nm and LSPR broadening as temperature increases. The response is also fully reversible as the temperature decreases, which further corroborates the improved thermal and morphological stability of Au NRs entrapped in AAO pores. The relatively smooth wavelength dependence for planar AAO embedded with Au NRs in a normal incidence geometry, can be attributed to predominant excitation of the transverse LSPR of Au NRs as compared with the results obtained for the NSOFs.
In addition to the free electron density dependence, LSPR frequency of metal is also highly dependent on changes in the dielectric permittivity due to temperature. The complex dielectric permittivity ε(T) of metal nanostructures due to temperature effects can be approximated by the Drude model [13,30]:
It should be noted that the reported thermal expansion coefficient of Au (14.2 × 10−6 K−1) is similar to that of the AAO (16.71 × 10−6 K−1) matrix and hence corrections were not made for the constraints imposed by the surrounding AAO matrix . The size effects of materials due to thermal expansion were not taken into consideration in our simulation. The refractive index was estimated for different temperatures using the Drude model. The key experimental high-temperature plasmonics features in Fig. 4(a) can be reproduced using Finite Element Method in FDTD with temperature dependent optical constants as presented in Fig. 4(b). The simulated extinction peak at around 529 nm also increases in transmission and broadens in width as the temperature increases. The change in extinction is also accompanied by the red shift of the resonance peak. The high-energy band in Fig. 4(a) results from the interband electron transitions in Au, [30,35] not explicitly taken into account in the Drude model calculations. However, while the inclusion of additional interband details in a more sophisticated model would improve agreement between the experimental findings and simulation results at short wavelengths, they would not significantly alter the basic conclusions in the wavelength range of primary interest for the Au NR LSPR since ε∞ = 11.5 used in the Drude model is a reasonable assumption for wavelengths above 516 nm .
The relative changes of Au refractive index at high temperatures with respect to room temperature based on the Drude model are shown in Fig. 5 for comparison. While the imaginary part of the refractive index decreases less than 6% in the UV-Vis range as temperature reaches 700 °C, the change of the real part is much more significant, approximately a 2X increment as compared to room temperature. The absorption cross section (σabs) for Au nanorods is determined by the following equation :
In summary, we have explored a specialty all-alumina NSOF embedded with Au NRs for high-temperature plasmonics study in harsh environments. We have shown that the AAO cladding serves to improve the thermal and morphological stability of Au NRs at elevated temperatures. The optical properties of Au NRs exhibit a strong dependence on temperature. Specifically, the LSPR of Au NRs underwent red shift as well as reduced peak intensity as temperature was raised from 100 to 800 °C. Such dependence can be monitored in the near field via the evanescent tunneling mechanism without interference from other environment variables such as black-body thermal irradiation and floating particulates in a thermal chamber. The NSOF embedded with Au NRs sensing platform offers a potential solution to the decades-old challenges associated with utilization of sapphire optical fiber sensors by virtue of the high thermal and chemical stability of the unique cladding structures in harsh environments.
National Science Foundation (DMR-1506179).
This work was supported in part by an appointment to the National Energy Technology Laboratory Research Participation Program, sponsored by the U.S. Department of Energy and administered by the Oak Ridge Institute for Science and Education. The authors thank Dr. Vishal Narang for supporting the RIE etching process at the City University of New York Advanced Science Research Center (ASRC) nanofabrication facility.
The authors declare no competing financial interest.
1. Y. Jee, Y. Yu, H. W. Abernathy, S. Lee, T. L. Kalapos, G. A. Hackett, and P. R. Ohodnicki, “Plasmonic Conducting Metal Oxide-Based Optical Fiber Sensors for Chemical and Intermediate Temperature-Sensing Applications,” ACS Appl. Mater. Interfaces 10(49), 42552–42563 (2018). [CrossRef]
2. D. G. Senesky, B. Jamshidi, K. B. Cheng, and A. P. Pisano, “Harsh Environment Silicon Carbide Sensors for Health and Performance Monitoring of Aerospace Systems: A Review,” IEEE Sens. J. 9(11), 1472–1478 (2009). [CrossRef]
3. G. Albrecht, S. Kaiser, H. Giessen, and M. Hentschel, “Refractory Plasmonics without Refractory Materials,” Nano Lett. 17(10), 6402–6408 (2017). [CrossRef]
4. T. Wei, X. Lan, H. Xiao, Y. Han, and H.L. Tsai, Optical fiber sensors for high temperature harsh environment sensing, in 2011 IEEE International Instrumentation and Measurement Technology Conference. 2011.
5. H. Chen, M. Buric, P. R. Ohodnicki, J. Nakano, B. Liu, and B. T. Chorpening, “Review and perspective: Sapphire optical fiber cladding development for harsh environment sensing,” Appl. Phys. Rev. 5(1), 011102 (2018). [CrossRef]
6. H. Chen, F. Tian, J. Kanka, and H. Du, “A scalable pathway to nanostructured sapphire optical fiber for evanescent-field sensing and beyond,” Appl. Phys. Lett. 106(11), 111102 (2015). [CrossRef]
7. H. Chen, K. Liu, Y. Ma, F. Tian, and H. Du, “Nanostructured sapphire optical fiber for sensing in harsh environments,” Proc. SPIE 10194, 101941P (2017).
8. B. Liu, M. Buric, J. Wuenschell, S. Bera, B. Chorpening, and P. Ohodnicki, “Optical properties and long-term stability of unclad single crystal sapphire fiber in harsh environments,” Proc. SPIE 10914, 109140Z (2019).
9. B. A. Wilson, C. M. Petrie, and T. E. Blue, “High-temperature effects on the light transmission through sapphire optical fiber,” J. Am. Ceram. Soc. 101(8), 3452–3459 (2018). [CrossRef]
10. H. Chen, P. Ohodnicki, J. P. Baltrus, G. Holcomb, J. Tylczak, and H. Du, “High-temperature stability of silver nanoparticles geometrically confined in the nanoscale pore channels of anodized aluminum oxide for SERS in harsh environments,” RSC Adv. 6(90), 86930–86937 (2016). [CrossRef]
11. G. Dharmalingam and M. A. Carpenter, “Chemical sensing dependence on metal oxide thickness for high temperature plasmonics-based sensors,” Sens. Actuators, B 251, 1104–1111 (2017). [CrossRef]
12. K. Liu, T. Chen, S. He, J. P. Robbins, S. G. Podkolzin, and F. Tian, “Observation and Identification of an Atomic Oxygen Structure on Catalytic Gold Nanoparticles,” Angew. Chem., Int. Ed. 56(42), 12952–12957 (2017). [CrossRef]
13. P. R. Ohodnicki, M. P. Buric, T. D. Brown, C. Matranga, C. Wang, J. Baltrus, and M. Andio, “Plasmonic nanocomposite thin film enabled fiber optic sensors for simultaneous gas and temperature sensing at extreme temperatures,” Nanoscale 5(19), 9030–9039 (2013). [CrossRef]
14. P. R. Ohodnicki, C. Wang, S. Natesakhawat, J. P. Baltrus, and T. D. Brown, “In-situ and ex-situ characterization of TiO2 and Au nanoparticle incorporated TiO2 thin films for optical gas sensing at extreme temperatures,” J. Appl. Phys. 111(6), 064320 (2012). [CrossRef]
15. P. R. Ohodnicki Jr, T. D. Brown, G. R. Holcomb, J. Tylczak, A. M. Schultz, and J. P. Baltrus, “High temperature optical sensing of gas and temperature using Au-nanoparticle incorporated oxides,” Sens. Actuators, B 202, 489–499 (2014). [CrossRef]
16. P.R. Ohodnicki Jr, T.D. Brown, M.P. Buric, J.P. Baltrus, and B. Chorpening, “Plasmon resonance at extreme temperatures in sputtered Au nanoparticle incorporated TiO2 films,” Proc. SPIE 8456, 845608 (2012).
17. C. Sun, P. Lu, R. Wright, and P.R. Ohodnicki, “Low-cost fiber optic sensor array for simultaneous detection of hydrogen and temperature,” Proc. SPIE 10654, 1065405 (2018).
18. P.R. Ohodnicki, T.D. Brown, M.P. Buric, and C. Matranga, Nanocomposite thin films for optical temperature sensing. 2017, ; National Energy Technology Lab. (NETL), Pittsburgh, PA, and Morgantown, WV (United States). p. Medium: ED.
19. K. A. Willets and R. P. Van Duyne, “Localized surface plasmon resonance spectroscopy and sensing,” Annu. Rev. Phys. Chem. 58(1), 267–297 (2007). [CrossRef]
20. K. S. Lee and M. A. El-Sayed, “Dependence of the enhanced optical scattering efficiency relative to that of absorption for gold metal nanorods on aspect ratio, size, end-cap shape, and medium refractive index,” J. Phys. Chem. B 109(43), 20331–20338 (2005). [CrossRef]
21. A. V. Kabashin, P. Evans, S. Pastkovsky, W. Hendren, G. A. Wurtz, R. Atkinson, R. Pollard, V. A. Podolskiy, and A. V. Zayats, “Plasmonic nanorod metamaterials for biosensing,” Nat. Mater. 8(11), 867–871 (2009). [CrossRef]
22. J. Yao, Z. Liu, Y. Liu, Y. Wang, C. Sun, G. Bartal, A.M. Stacy, and X. Zhang, “Optical Negative Refraction in Bulk Metamaterials of Nanowires,” Science 321(5891), 930 (2008). [CrossRef]
23. G. A. Wurtz, R. Pollard, W. Hendren, G. P. Wiederrecht, D. J. Gosztola, V. A. Podolskiy, and A. V. Zayats, “Designed ultrafast optical nonlinearity in a plasmonic nanorod metamaterial enhanced by nonlocality,” Nat. Nanotechnol. 6(2), 107–111 (2011). [CrossRef]
24. L. H. Nicholls, F. J. Rodríguez-Fortuño, M. E. Nasir, R. M. Córdova-Castro, N. Olivier, G. A. Wurtz, and A. V. Zayats, “Ultrafast synthesis and switching of light polarization in nonlinear anisotropic metamaterials,” Nat. Photonics 11(10), 628–633 (2017). [CrossRef]
25. J. S. G. Bouillard, W. Dickson, D. P. O’Connor, G. A. Wurtz, and A. V. Zayats, “Low-Temperature Plasmonics of Metallic Nanostructures,” Nano Lett. 12(3), 1561–1565 (2012). [CrossRef]
26. H. Petrova, J. P. Juste, I. Pastoriza-Santos, G. V. Hartland, L. M. Liz-Marzán, and P. Mulvaney, “On the temperature stability of gold nanorods: comparison between thermal and ultrafast laser-induced heating,” Phys. Chem. Chem. Phys. 8(7), 814–821 (2006). [CrossRef]
27. K. Liu, P. R. Ohodnicki, X. Kong, S. S. Lee, and H. Du, “Plasmonic Au nanorods stabilized within anodic aluminum oxide pore channels against high-temperature treatment,” Nanotechnology 30(40), 405704 (2019). [CrossRef]
28. K. Liu, Y. Ma, and H. Du, “Tailoring the nanostructure of anodic aluminum oxide cladding on optical fiber,” J. Am. Ceram. Soc. 101(12), 5836–5845 (2018). [CrossRef]
29. H. Chen, F. Tian, K. Liu, J. Kanka, and H. Du, “Strategy and method for nanoporous cladding formation on silica optical fiber,” Opt. Lett. 41(12), 2831–2834 (2016). [CrossRef]
30. A. Alabastri, S. Tuccio, A. Giugni, A. Toma, C. Liberale, G. Das, F.D. Angelis, E.D. Fabrizio, and R.P. Zaccaria, “Molding of Plasmonic Resonances in Metallic Nanostructures: Dependence of the Non-Linear Electric Permittivity on System Size and Temperature,” Materials 6(11), 4879–4910 (2013). [CrossRef]
31. H. S. Sehmi, W. Langbein, and E. A. Muljarov, “Optimizing the Drude-Lorentz model for material permittivity: Method, program, and examples for gold, silver, and copper,” Phys. Rev. B 95(11), 115444 (2017). [CrossRef]
32. S. K. Ozdemir and G. Turhan-Sayan, “Temperature effects on surface plasmon resonance: design considerations for an optical temperature sensor,” J. Lightwave Technol. 21(3), 805–814 (2003). [CrossRef]
33. M. E. Barghouti, A. Akjouj, and A. Mir, “Effect of MoS2 layer on the LSPR in periodic nanostructures,” Optik 171, 237–246 (2018). [CrossRef]
34. X. R. Zhang, T. S. Fisher, A. Raman, and T. D. Sands, “Linear coefficient of thermal expansion of porous anodic alumina thin films from atomic force microscopy,” Nanoscale Microscale Thermophys. Eng. 13(4), 243–252 (2009). [CrossRef]
35. O. A. Yeshchenko, I. S. Bondarchuk, V. S. Gurin, I. M. Dmitruk, and A. V. Kotko, “Temperature dependence of the surface plasmon resonance in gold nanoparticles,” Surf. Sci. 608, 275–281 (2013). [CrossRef]
36. N. Karker, G. Dharmalingam, and M. A. Carpenter, “Thermal energy harvesting plasmonic based chemical sensors,” ACS Nano 8(10), 10953–10962 (2014). [CrossRef]