Investigations of optical and chemical properties are currently in high demand for medical diagnostics and biochemical analysis. Optical-fiber-based sensors, e.g., refractometers, where refractive index (RI) is investigated, gained a lot of attention due to their possible probe-like shape, repeatable and low cost fabrication of the optical fibers and their interrogation systems, which are well-developed for telecommunication [1]. Long-period gratings (LPGs) have been known for over a decade [2]. LPGs are periodic modulations of the refractive index in fiber core along the length of a fiber. Under certain phase-matching conditions, the grating couples the fundamental core mode and discrete cladding modes that are attenuated due to absorption and scattering. The coupling is wavelength-dependent, so one can obtain a spectrally selective loss. A number of sensors based on the LPGs have been proposed for temperature, humidity, strain, hydrostatic pressure, bending and RI sensing, including a number of biosensors [3-6].
It has also been shown that the deposition of some overlays can significantly modify sensitivity of LPG structures to certain external influences. Deposition of the high-refractive-index (high-
Diamond is a wide band gap semiconductor with E
In this paper we analyze several cm long optical fiber sections coated with B-NCD overlay using the Microwave Plasma Assisted Chemical Vapor Deposited (MW PA CVD) method. The work focuses on investigation of overlay properties and application of the coated device for both optical and electrochemical investigations of liquids.
Fabrication procedure of LPGs with computer-assisted precision arc-discharge apparatus has been described previously in [15]. In this stage of the experiment, we used Corning SMF28 fiber samples to write gratings with period of
Fibers were nucleated by means of dip-coating in dispersion consisting of detonation nanodiamond (DND) in dimethyl sulfoxide (DMSO) with polyvinyl alcohol (PVA). Next, the B-NCD films were deposited on the fibers using an MW PA CVD system (SEKI Technotron). The deposition process took 60 minutes with the total flow rate of gases reaching 300 sccm at methane molar ratio of 4% and 5000 ppm [B]/[C] ratio in gas phase. The holder temperature was 475 ℃ and process pressure was kept at 50 Torr.
The morphological studies were performed with a Hitachi S-3400N scanning electron microscope (SEM). The molecular composition of the films was studied by means of Raman spectroscopy using a Horiba LabRAM ARAMIS Raman confocal microscope. Spectroscopic ellipsometry (SE) investigations on reference Si samples were carried out with a Horiba Jobin-Yvon UVISEL phase-modulated ellipsometer [16].
Furthermore, cyclic voltammograms (CV) were recorded to determine the electrochemical window and reaction reversibility at the B-NCD fiber-based electrode. CV measurements were performed in aqueous media consisting of 5 mM K3[Fe(CN)6] in 0.5 M Na2SO4 at scan rate of 0.1 V s−1. The electro-chemical system includes three electrodes, i.e., B-NCD-coated fiber, Ag/AgCl/0.1 M KCl and platinum wire as working, reference and counter electrodes, respectively. The electrochemical experiment was performed using an Autolab PGSTAT30 system.
Next, the transmission of the LPG samples was measured in wavelength range 1150 to 1650 nm using a Yokogawa AQ4305 white light source and Yokogawa AQ6370B optical spectrum analyzer. For the RI measurements, the nano-coated LPG was immersed in various liquids prepared by mixing water and glycerin. The RI of the obtained liquid (
As a result of SE analysis performed on B-NCD-coated Si reference samples, the thickness and optical constants of diamond film, i.e. refractive index
Fiber cross-sections reveal no significant change, i.e., dopant diffusion, in the fiber core after high-temperature B-NCD film deposition (Fig. 2(a)). The obtained diamond overlay is continuous and homogenous along the fiber (Fig. 2(b)), in agreement with previously obtained results on the fused glass slides [23, 24]. Average crystallite size is between 100 and 200 nm (Fig. 3(b)) and thickness of the films is below 500 nm. A deposition rate of 10 nm/min was reached.
Raman spectrum shown in Fig. 1(b) proves the presence of strong bands assigned to the diamond lattice, and confirms a high content of sp3 carbon in the film structure. Its shift from typical for diamond 1330 to about 1324 cm−1 is caused by a stress introduced by boron doping [25]. Moreover, the boron presence in the sample can be confirmed by a band observed at 1226 cm−1 and by a strong signal intensity drop above 1650 cm−1. The D band at 1487 cm−1 can be assigned to the mixture of amorphous carbon phase sp2 and sp3, while the G band at 1535 cm−1, to sp2-bonded amorphous carbon.
The effect of the B-NCD overlay deposition on LPG response to external RI is shown in Fig. 3. As shown for RI close to that of water (nD=1.333) where most of the biosensors operate, the deposition process had significant influence on LPG resonance, i.e., decrease in depth and their spectral shift towards shorter wavelengths (Fig. 3(a)). The change in response induced by the deposition process when compared to a bare LPG sample can be attributed to both high temperature of the process and B-NCD overlay deposition [26]. The decrease in depth results from the temperature-induced decrease in refractive index modulation in fiber core at the LPG region. For the B-NCD deposition, the temperature at the fiber can locally reach 550 ℃. The distribution of temperature along the fiber is mostly driven by density of the microwave power. The fused silica is transparent at microwave frequencies as well as to infrared radiation generated by the induction heater. Thus, local changes in temperatures are induced mainly by conduction heat transfer phenomenon and different parts of the fiber can be heated differently at the deposition stage. What is more, different values of thermal expansion coefficient of polycrystalline diamond film and the amorphous fused silica substrate also have an influence also on deposition-induced strain effect. However, both the processes exhibit nanothermomechanic character and cannot be controlled or measured locally due to high temperature and ionic plasma character. Previously presented micromorphological SEM studies [27]reveal that fibers do not show any changes in the cladding diameter (125 μm) and fiber core diameter (~ 9 μm) due to temperature-induced stress and microwave power during the CVD process.
Moreover, an increase in external RI, which is also obtained as a result of high-
Response of LPG to dipping in several liquids with different external RI is shown in Fig. 3(b) and 3(c). For LPGs with no high-n overlays, the resonance-based spectral response disappears when external RI matches the one of the fiber cladding. When no cladding modes can be formed, no coupling takes place. In these conditions, the RI sensitivity is the highest, but the resonances are difficult to trace. In turn, the resonances are visible when external RI is lower or higher than the one of the fiber cladding. Deposition of the high-n overlay shifts appearance of this effect, i.e., vanishing of the resonances and higher sensitivity, towards lower external RI. Thin film with high-
CV is considered a very useful method for investigating electrochemical properties of the material being in contact with an electrolyte. Figures 4(a) and 4(b) show the electrochemical windows and redox reaction reversibility for potassium ferricyanides solution measured with the reference microcrystalline boron-doped diamond (BDD) electrode (Fig. 4(a)) and B-NCD- coated fiber (Fig. 4(b)). It can also be seen that the electrochemical window is 2.4 times narrower than for the reference BDD film, i.e., 2.4 and 1.03 V for micro-crystalline BDD and B-NCD-coated fiber, respectively. The Fe(CN)63−/4− redox system is sensitive to surface properties such as
The recorded CV curves obtained for microcrystalline BDD and B-NCD-coated fiber electrodes were investigated in 5 mM K3Fe(CN)6/0.5 M NaSO4 solution
In this work we have investigated optical and electro-chemical response of optical fibers coated with B-NCD overlay. We have found that the overlay can tune response of the LPGs to variations on external refractive index and makes possible electrochemical measurements with B-NCD optical fiber. Application of electrically conductive B-NCD films allows for developing both optical and electrochemical sensing devices, and what is more, allows for combining these two capabilities in one sensing structure.