HTML
-
The all-fiber-coupled EC-SPR fiber-optic sensing system employed is shown in Fig. 5a and comprises a broadband light source (BBS) with bandwidth from 1250 to 1650 nm, a polarizer, a polarization controller (PC), a circulator, a plasmonic optical fiber sensing probe, and an optical spectrum analyzer (OSA). An electrochemical workstation is used for performing conventional electrochemical measurements and collecting supercapacitor data to be correlated to the optical measurements. A computer was used to collect simultaneous data from both systems as the supercapacitor cycled through charge and discharge. Figure 5b presents the detailed configuration of the measurement system containing a supercapacitor (two MnO2@carbon fabric electrodes in liquid electrolyte, with an area of 3 cm2 soaked in electrolyte) and a plasmonic fiber-optic sensing probe coated with a nanometer-scale gold film. The entire plasmonic fiber-optic sensing probe is very compact, with a size of 30 mm in length and 125 μm in diameter (Fig. 5c). The probe can be tightly attached to any electrode of the supercapacitor.
Fig. 5 Electrochemical surface-plasmon-resonance sensing principle and experimental demonstration with an gold-coated TFBG optical fiber sensor.
a Experimental setup of a plasmonic fiber-optic sensing system for monitoring the SOC of supercapacitors. b Photographs of the configuration for the supercapacitor and (c) gold-coated fiber-optic sensing probe. SEM images of (d) the MnO2 electrode and (e) the corresponding magnified image. f Schematic of the measurement of the charge–discharge process of supercapacitors by a plasmonic gold-coated TFBG fiber-optic sensor -
The scanning electron microscopy (SEM) images in Fig. 5d, e show the morphology of the uniform MnO2 nanosheets stacked over the surface of the carbon fiber fabric. A solution containing 0.1 M MnAc2 and 0.1 M NaAc was used to provide Mn ions. The carbon fabric (area of 1 × 3 cm2) was soaked into the Mn ions solution and a constant current density of 1 mA cm−2 was applied. The MnO2 was synthesized by an electrodeposition method that used a three-electrode system with Ag/AgCl as the reference electrode and Pt as the counter electrode. The whole synthesis process was achieved in 5 min.
Figure 5f shows the configuration of the supercapacitor and plasmonic optical fiber sensor. The supercapacitor used here is a pseudo-capacitor. It stores electrical energy on the basis of an electrical double layer effect over the material surface and fast bi-dimensional redox reactions in a very thin electrode surface layer. Sensing is based on the fact that the plasmon waves excited by the grating in the core of the optical fiber probe have a high percentage of their propagating power localized in a 1-μm-thick layer above the metal surface. Therefore, when the fiber probe is positioned in the layer where the redox reactions occur, there are observable changes in the SPR optical spectrum.
-
TFBG probes were manufactured in commercial photosensitive single-mode fiber (provided by Corning Incorporated) using a well-established technique described in ref. 27. Specifically, the TFBG was manufactured using the phase-mask technique by shining UV light pulses from an excimer laser (at a wavelength of 193 nm and with a power of 30 mJ per pulse) onto the surface of a bare fiber, after passing through the diffractive mask where the desired grating pattern was etched. In this manner, a corresponding periodic modulation of the refractive index is formed in the fiber core. Contrary to the case for standard fiber Bragg gratings, the planes of the refractive index modulation were written with a pre-defined tilt relative to the longitudinal axis of the fiber (see Fig. 6, where the grating in the fiber core is colored in pink). The tilt of the grating is an important parameter that determines which set of cladding modes is excited: here, an 18 degree angle is chosen to maximize coupling to cladding modes that are suitable to transfer energy to surface plasmons in the aqueous solutions used for electrolytes in supercapacitors.
Fig. 6 Electrochemical surface-plasmon-resonance sensing principle and experimental demonstration with an gold-coated TFBG optical fiber sensor.
Sketch of the configuration of a plasmonic optical fiber sensor for in situ monitoring of supercapacitorsA 50-nm-thick gold layer of high surface quality was deposited on the above TFBG by sputtering as follows. First, a 2–3-nm buffer layer of chromium is deposited on the optical fiber surface to promote adhesion between the fiber and the gold. Second, gold is sputtered on top of the chromium while the optical fiber is rotated along its axis. This process ensures that the gold layer is uniform around the fiber, which helps in achieving clear SPR effects. Finally, the coated fiber is annealed for 3 h at a temperature of 300 ℃ so that the gold coating has the desired morphology and is robust enough for sensing applications31.
The responses of plasmonic TFBG devices are normally observed in transmission, which requires access to the sensor from both sides. For applications in small areas and, in particular, for this work, it is desirable to have single-ended sensors located close to the end of a fiber, so that it can be inserted into tight spaces. Therefore, an additional (thicker) gold coating is deposited on the end of the fiber, cut a few mm downstream from the TFBG, to act as a broadband mirror with > 90% reflectivity, which enables interrogation of the sensor in reflection. In the reflection measurement method, the Bragg resonance (reflection of the core mode upon itself, used for temperature compensation) appears as a spectral peak while cladding mode resonances appear as narrow troughs in the broadband reflection from the end mirror. Figure 6 shows a cartoon representation of a typical sensor packaged for this work. This configuration also ensures that the sensor is strain free when a single attachment point is used to fix the sensor to the electrode surface.
-
Surface plasmon polaritons (SPPs) are near-infrared or visible-frequency electromagnetic waves that travel along a metal–dielectric or metal–air interface and decay exponentially away from the interface. The evanescent field of a fiber cladding mode can tunnel through a thin metal coating and transfer energy to an SPP wave of the outside interface of the metal when two necessary conditions are satisfied: (1) the propagation constant of the cladding mode equals that of the SPP for that particular combination of metal and surrounding medium; and (2) the polarization of the light must be perpendicular to the metal surface, i.e., TM like31, 32. Only a small subset of the cladding modes of any fiber can meet these conditions.
The propagation constant βSPP of SPP is expressed as:
$$ \beta _{{\rm SPP}} = \frac{\omega }{c}\sqrt {\frac{{\varepsilon _{\rm m}\varepsilon _{\rm s}}}{{\varepsilon _{\rm m} + \varepsilon _{\rm s}}}} $$ (1) where c is the speed of light in vacuum, ω is the angular frequency of the light, and εm and εs are the complex relative permittivities of the metal film and the surrounding material adjacent to the metal interface where the SPP is located, respectively.
On the other hand, the propagation constants βclad, i of cladding modes (labeled by the subscript i) in a standard fiber with a cladding diameter on the order of 100 times the wavelength can take a large, closely spaced set of values, and the associated fields have widely different polarization properties. The phase-match condition between propagation constants can be expressed as
$$ \beta _{{\rm SPP}} = \beta _{{\rm clad},i} = 2\pi N_{{\rm clad},i}^{{\rm eff}}{\mathrm{/}}\lambda $$ (2) where the last equality introduces $N_{{\rm clad}, i}^{{\rm eff}}$ which is defined as the effective index of the ith cladding mode at wavelength λ29.
Finally, the phase-matching condition can be observed and measured with great accuracy from the transmission spectrum of a fiber grating because of the one-to-one relationship between effective indices (hence propagation constants) of modes and their resonance wavelengths in the spectrum, expressed by the following additional phase-matching rule:
$$ \lambda _{{\rm clad},i} = \left( {N_{{\rm clad},i}^{{\rm eff}} + N_{{\rm core}}^{{\rm eff}}} \right){\mathrm{\Lambda }} $$ (3) where $N_{{\rm core}}^{{\rm eff}}$ is the effective index of the input core mode and Λ is the period of the grating (measured along the fiber axis, i.e., not equal to the distance between the tilted grating planes). As can be seen in the measured spectra in Fig. 7 (left), there are many such resonances corresponding to the set of modes supported by the relatively large cladding. A first discrimination between modes is provided by using polarized input core mode light. It was demonstrated that with input light polarized parallel to the inclination plane of the grating (P-polarized), the excited cladding modes have electric fields polarized radially at the surface of the cladding and can thus excite surface plasmons, while the orthogonal input polarization (S-polarized) excites tangentially polarized cladding modes that cannot couple to plasmons33. This is clearly demonstrated in Fig. 7 by the fact that only the P-polarized spectrum (red curve) shows a characteristic attenuation of the cladding mode resonance amplitudes for wavelengths near 1550 nm (indicating that power has been "lost" or transferred from the cladding to the surface plasmon) and further by simulations of the electric field profiles for the two cases. The measured S-polarized spectrum (black curve) shows no such attenuation.
Fig. 7 (Left) Reflection spectra of a mirror-ended TFBG optical fiber coated with 50 nm of gold and immersed in water.
P-polarized incident light showing SPR near 1550 nm (red curve) and S-polarized incident light (black curve, no SPR observed); (right) Simulated electric mode field profiles for two neighboring high-order cladding modes: modes excited by P-polarized core mode input light have electric fields that are oriented predominantly radially at the boundary (upper), while modes excited from S-polarized input have predominantly tangential electric fields around the fiber cladding boundary (bottom). The color scale reflects the magnitude of the electric fields and the arrows their orientation. The transfer of energy from cladding mode to a surface plasmon shows up as a bright ring around the fiber cladding for P-polarized inputHaving identified plasmon-coupled resonances in the spectrum at specific wavelengths, Eqs. 1–3 provide a direct link between these wavelengths and the permittivity of the medium just above the metal layer (εs). If that permittivity changes, the corresponding resonance position and amplitude will change as well34. Such changes include the formation of a new material layer on top of the gold, such as in the well-known and widely applied affinity studies for biomolecules and also in the measurements of faradic currents and double layer charging currents in the metal film itself leading to charge density changes in the metal film and modifications of εm. The latter phenomenon is commonly named electrochemical surface plasmon resonance (EC-SPR), and it is a powerful tool to study and identify the electrochemical activity of the "surface" and "localized" charge state of the ions adjacent to the electrode interfaces. Simulations similar to those shown in Fig. 7 reveal that up to 70% of the light power of cladding modes phase matched to plasmons propagating as a bound wave in the external medium, while the "normal" evanescent waves of the cladding modes of bare fiber (with the same TFBG inside) can only carry between 2 and 5% of the mode power33.
Finally, another factor important in the current device is that it operates in near infrared instead of the more commonly used visible wavelengths for SPR applications. This extends the penetration depth of the fields of plasmon waves from the 200–300 nm range to more than 1 μm, a more suitable distance for monitoring the electrochemical activities of ions just over the surface of the electrodes.
-
The data that support the findings of this study are available from the corresponding authors upon request.