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We have theoretically and experimentally demonstrated a novel approach to excite Bloch surface wave (BSW) on tapered optical fibers, which are coated with one-dimensional photonic crystal (1DPC) consisting of periodic TiO2 and Al2O3 by atomic layer deposition technology. Two resonant dips are found in transmission spectra that are originated from the excitation of BSW for p-polarized light and s-polarized light, respectively. For the first time, we have demonstrated the developed device for refractive index (RI) sensing.
© 2017 Optical Society of America
1. Introduction
The Bloch surface wave (BSW) in one-dimensional photonic crystal (1DPC) has attracted much attention because of its excellent performance in optical sensing applications [1–7]. BSW presents extremely high sensitivity to the variation of surrounding perturbation attributed to its strong evanescent wave localized on the surface of the 1DPC [8–13]. Based on this feature of BSW, the 1DPC has found many applications in detection of protein aggregation [14], fast optical vapor sensing [15], DNA testing [9], refractive-index-based label-free sensing [9,11], etc. However, most of these 1DPC-based BSW sensors utilize complicated bulk optical components to excite BSW [6,8–15]. In order to reduce the complexity caused by the bulk optical components, researchers show great interest in fiber-optic based BSW sensors and explore new designs with the advantages of optical fiber including compactness, flexibility, high sensitivity, mechanical robustness and convenient manipulating for remote and in vivo analyses [16–20].
In recent years, the investigation of BSW on rod fiber has been conducted, in which Al2O3 and TiO2 with 1DPC are alternatively deposited on the surface of the rod fiber [17]. Due to the excitation of BSW, such kind of structure can achieve the detection of fluorescence of Rhodamine 6G. However, the rod fiber only behaviors as a substrate that is not designed for light propagation within the fiber for a long distance. In order to extend the remote detection capability, the theoretical investigations on the excitation of BSW on D-type fiber [16] as well as fiber-nanoprobe based BSW [18] have been proposed. But D-shape fiber suffers from the polarization dependent loss because the fiber optic structures are not ring-shaped symmetric. The fiber nanoprobe excites BSW on the tip of fiber based on grating coupling and retrieves the sensing signal by testing the reflectance spectrum [18]. Nevertheless, in such fiber nanoprobe scheme, a grating needs to be integrated on the tip of a fiber so the fabrication is complicated. Furthermore, because light has to pass through the grating twice, the signal intensity becomes weak that introduces problem to detection in practice. Recently, it has been reported that the Bloch surface waves are confined in one dimension with a single polymeric nanofiber [19]. But the BSW is not exited on the surface of the nanofiber. In order to overcome these limitations, the BSW fiber sensors based on multi-mode optical fiber deposited with multilayer have been theoretically proposed [20]. Its analyzing results predict that the refractive index sensing resolution can reach 10−6 refractive index unit (RIU) which is comparable with typical bulk prism-coupling based BSW sensor. However, all above works about excitation of BSW on D-shaped fiber, fiber-nanoprobe, and multi-mode fiber for sensing are only done in theory but without the experimental observation.
In this work, we have theoretically and experimentally demonstrated an all-fiber BSW excitation structure by coating 1DPC on a tapered single mode fiber (SMF). The 1DPC consists of periodic TiO2 and Al2O3 that is coated on the fiber by atomic layer deposition (ALD) technology. The ALD technology is believed to be an ideal method for fabricating 1DPC showing a series inherent advantages including accurate thickness control, good conformality for complex shape surface, good uniformity and adhesion [17,21,22]. The researchers have realized highly controllable single layer nanofilm deposition on tapered SMF and special double cladding fiber for achieving high-sensitivity refractive index (RI) sensors [23,24]. By using ALD technology to coat 1DPC on tapered fiber, we have experimentally characterized the developed fiber device that exhibits two resonant dips in the transmission spectrum. The experimental observation of such two resonant dips is consistent with the theoretical simulation, confirming that the two dips are originated from the excitation of BSW for p-polarized light (PPL) and s-polarized light (SPL), respectively. By using the developed tapered fiber with 1DPC, we have demonstrated fiber-based BSW excitation for refractive index (RI) sensing for the first time. Benefiting from the high-sensitivity characteristics of BSW as well as compact and compatible of fiber devices, such tapered fiber-based BSW excitation shows potential application in biosensor and chemical sensing.
2. Theoretical investigation of the excitation of BSW on tapered fiber with 1DPC
Figure 1 shows the schematic diagram of the BSW excitation scheme based on 1DPC on tapered single mode fiber (SMF). In order to ensure the coupling of fundamental HE11 mode from the input of SMF to the tapered fiber with negligible loss, the tapered angle requires small enough to be approximately adiabatic [23,25]. Therefore, the waist radius of the tapered fiber is optimized to be ~10 μm and the total length is ~17 mm. Such 1DPC consists of alternate TiO2 and Al2O3 layers. These dielectric mediums are chosen due to the suitable refractive indices that are 1.6166 for Al2O3 and 2.3003 for TiO2 at wavelength of 1550 nm measured by an ellipsometer (VASE Ellipsometer, J.A. WOOLLAM), the low optical absorption, as well as the mature manufacture technique by ALD technology [17,22–24]. To obtain the BSW resonance locating at near infrared region [16,19], the thickness of Al2O3, TiO2 layers and the surface TiO2 layer are designed to be 320 nm, 260 nm and 100 nm, respectively. Taking into account of the effect of BSW excitation and the time consumption of deposition procedure, the structure of 3 pairs is chosen.
Fig. 1 Schematic diagram of the tapered fiber coated with one-dimensional photonic crystal for the excitation of Bloch surface wave; the insets show the structure diagram of an equivalent slab waveguide.
The model of a tapered fiber coated 1DPC can be equivalent to a slab optical waveguide [20,26]. This equivalent slab optical waveguide is schematically shown in the inset of Fig. 1 where a period of slab dielectric mediums are coated on both sides of the slab waveguide. The calculation is conducted by using advanced finite-difference beam propagation method (BPM) integrated in RSoft Photonics Suite [26]. In the simulation, the refractive indices are set the same as the designed values that are 1.6166 for Al2O3 and 2.3003 for TiO2. The thicknesses of Al2O3, TiO2 layers and the surface TiO2 layer are 320 nm, 260 nm and 100 nm, respectively. The refractive index and the thickness of the slab waveguide are given by 1.4618, and 20 μm, respectively. Based on the theory of the band-gaps given in [1], the band-gaps of the above 1DPC structure for PPL and SPL can be calculated as shown in Figs. 2(a) and 2(b). The white region denotes the allowed bands in which the 1DPC structure can supports many modes of light waves propagating inside it. While the blue region represents the forbidden bands in which the light can't propagate in 1DPC. The BSW exists in the forbidden bands, and its dispersion curves for PPL and SPL are plotted in Figs. 2(a) and 2(b). Similar to the prism-based BSW excitation, the fiber-based BSW can be excited provided the phase of propagating modes in fiber is matched with the phase of BSW [27]. Figures 2(a) and 2(b) plot the dispersion curve of the fundamental mode in the slab silica waveguide. We can find that there is point at which the two dispersion curves intersect. It indicates that the effective refractive indices of fundamental mode and BSW are equal, so that the BSW could be excited and the power of the light can be transferred from the silica waveguide to the 1DPC. Such phase matching points differs for the PPL and SPL that are located 1383 nm and 1481 nm, respectively.
Fig. 2 The calculated bandgaps of the 1DPC structure and dispersion curves of the BSWs (solid lines) and the fundamental modes (dashed lines) for (a) p-polarized light (PPL) and (b) s-polarized light (SPL). The white region denotes the allowed bands; the blue region represents for the forbidden bands.
Figures 3(a) and 3(b) show the field intensity distribution near the region of 1DPC for PPL and SPL at the excitation wavelength of 1383 nm and 1481 nm, respectively. A portion field is localized in 1DPC for both polarizations. Such strong intensity and concentration at the surface of 1DPC confirms that the BSW is excited [28]. In addition, the evanescent field of BSW penetrates into the surroundings that could be applied in sensing application.
Fig. 3 The calculated intensity distribution of electrical field near 1DPC for (a) PPL at wavelength of 1383 nm and (b) SPL at 1481 nm, the dashed lines labels the interface of the different dielectric mediums.
Since a portion of the light could be transferred to the 1DPC structure, the transmission spectrum of slab waveguide will appear a dip at the resonant wavelengths. Figure 4 simulate the transmission spectra of the equivalent slab waveguide by using BPM [26] for SPL and PPL when the surrounding RI (SRI) is 1. As shown in Fig. 4, the dips caused by the excitation of BSW can be observed obviously. The dips appear at 1396 nm for PPL and 1483 nm for SPL, which is consistent with the wavelength of the BSW excitation analyzed by the dispersion curves.
Fig. 4 The simulated transmission spectra of the tapered fiber for PPL and SPL. The dips appear at 1396nm and 1483nm for PPL and SPL, respectively.
When the surrounding medium is changed, the evanescent field of BSW will be affected and thereby the phase matching condition might be altered. In order to study the RI sensing response of the developed structure, the transmission spectra under different SRI for SPL and PPL are calculated. The results are shown in Figs. 5(a) and 5(b). When the SRI is changed from 1.3367 to 1.4337, the resonant wavelength for PPL shifts from 1500 nm to 1616 nm, while the resonant wavelength for SPL shifts from 1668 nm to 1812 nm. Figure 5(c) shows the shift amount of the resonant wavelengths as a function of SRI for different polarization light. The curves are well fitted following an exponential function and the coefficients of determinations (R2) representing the variability of a factor are estimated to be 0.99987 for SPL and 0.99851 for PPL. The sensitivity is defined as S = ∂λ/∂n, where λ is the wavelength of resonant dip, and n is SRI. Near SRI of 1.3367, the sensitivities are estimated to be 603 nm/RIU and 948 nm/RIU for PPL and SPL, respectively. Near SRI of 1.4337, the sensitivities are estimated to be 2040 nm/RIU and 2184 nm/RIU for PPL and SPL, respectively.
Fig. 5 The simulated transmission spectra of tapered fiber subject to the environment with various surrounding refractive index for (a) PPL and (b) SPL. (c) The points represent the wavelength shifts of the resonant dips originated from BSW as a function of surrounding refractive index; the curves represent the fitting results.
3. Fabrication of tapered fiber coated with 1DPC and the characterization of transmission spectrum
The tapered SMF is fabricated by using the conventional heating and pulling technology. The waist radius of the tapered fiber is uniform and controlled to be ~10 μm approximately. The radius of transition region varies linearly with the distance along propagation direction approximately. The length of the taper waist is ~5 mm, the length of the taper transition regions is ~6 mm, and the total length is ~17 mm, approximately. The insertion loss of the pulled fiber taper is around 1 dB, showing that the tapered fiber is approximately adiabatic [23,25]. The Al2O3 and TiO2 layers on the taper are deposited by using the ALD equipment (TFS 200, Beneq), following the similar procedure described in our previous works [23,24,29]. Al2O3 was formed through the chemical reaction of two precursors Al(CH3)3 and O3. The temperature of the reaction chamber and substrate is maintained at 210°C. The process flow is Al(CH3)3-N2-O3-N2, and the pulse durations are: 100 ms-500 ms-600 ms-1 s correspondingly. The TiO2, which is the same as Al2O3, was formed through the chemical reaction of two other precursors TiCl4 and H2O, and the temperature of the reaction chamber and substrate is maintained at 140°C. The process flow is TiCl4-N2-H2O-N2, and the pulse durations are: 100 ms-3 s-80 ms-3 s correspondingly. The thickness and index of the deposited Al2O3 and TiO2 layer is tested by using an ellipsometer. The thickness of the 3400 cycles Al2O3 layer is 315.8 nm and the growth rates is 0.0929 nm/cycle averagely, while the thickness of the 2000 cycles TiO2 layer is 96.3nm and the growth rates is 0.0481 nm/cycle averagely.
Figures 6(a) and 6(b) shows the measured refractive index of the 3400-cycles Al2O3 layer and 2000-cycles TiO2 layer, respectively. At wavelength of 1550 nm, the RI of the Al2O3 nanofilm is 1.6166, and the RI of the TiO2 nanofilm is 2.3003. In this case, the 1DPC structure is constituted by 3 pairs of alternate TiO2 and Al2O3 layers as shown in Fig. 1. The number of deposition cycle is 3400 for Al2O3 layers, 5300 for TiO2 layers and 2100 for the surface TiO2 layer. According to the measurement of the ellipsometer, the average thicknesses are 315.8 nm, 255.1 nm and 101.1 nm for the Al2O3, TiO2 layers and the surface layers, accordingly. The cross section of the multilayer structure deposited on a SMF is observed by using a scanning electron microscope (SEM) (Apollo 300, CamScan), as shown in Fig. 6(c). The thickness of the layers is well defined.
Fig. 6 The dispersion curve of (a) Al2O3 and (b) TiO2 deposited by ALD. (c) SEM picture of the1DPC multilayer structure
With a broadband light source (NKT Photonics SuperK Compact, 500 nm-2400 nm) and an optical spectrum analyzer (AQ6375, YOKOGAWA), the transmission spectrum of the tapered fiber with above 1DPC is recorded in air. As shown in Fig. 7, a dip appears at wavelength of 1510 nm, which is close to the resonant wavelength of BSW for SPL in simulation results, while another dip appears at wavelength of 1380 nm, which is close to the resonant wavelength for PPL. The shapes of the experiment transmission spectrum and the wavelength of the dips show a good agreement with the simulation results. In order to verity the fabrication technique, two samples are prepared and the transmission spectra show a good agreement with each other, indicating a good uniformity.
Fig. 7 The measured transmission spectra of the two samples of tapered fiber coated with 1DPC in air.
It is observed that the spectral bandwidth of the dips in the experiment as shown in Fig. 7 is larger than that in simulation as shown in Fig. 4. The reason could be explained by the effect of fiber tapering. Although the tapered fiber is designed to be adiabatic, the BSW still can be excited near the taper transition region where the core is too small to confine the light [29] and affected by 1DPC. In order to investigate the effect of taper transition region on the BSW excitation, the simulation is further conducted. We have calculated the variation range of the effective RI of the fundamental mode in the taper transition regions due to different fiber radius for different polarization light as shown in Figs. 8(a) and 8(b). The thickness of 1DPC is remained for different fiber radius and thus the effective RI of BSW keeps invariable. According to the phase-matching condition [27], the light wave will excite the BSW within a range of wavelength. In such wavelength range, the dispersion curves of the BSW falls in the range of the effective RI of the fundamental mode due to fiber tapering. In Fig. 8, the effective RI of 1.4614 corresponds to the tapered waist. The effective RI of 1.4618 corresponds to the taper transition with a radius of 24 μm. Under this radius, the fundamental mode expands from the core into the cladding adiabatically, and the excitation of BSW starts. As a result, the spectral bandwidth of dips is enlarged both for PPL and SPL, confirming the experimental observation.
Fig. 8 The dispersion curves of the BSWs (solid lines) and variation range of the effective refractive index of the fundamental modes (green area) for (a) p-polarized light (PPL) and (b) s-polarized light (SPL).
4. Excitation of BSW on tapered fiber for refractive index sensing application
In order to explore the application of BSW in the area of fiber sensor, we preliminarily test the RI sensing characteristic of one of the prepared sample. In the experiment, the liquid samples with different RI are dropped on the fiber tapered waist, and the tapered fiber is fixed on a glass slide. The liquid samples are the mixture of glycerol and deionized water with different ratios with RI ranging from 1.3367 to 1.4337, which is measured by using an Abbe refractometer with the resolution of 0.0001. The temperature around the taper is maintained at 23°C so as to avoid the interference of the temperature.
Figure 9(a) shows the transmission spectra versus SRI ranging from 1.3367 to 1.4337. As SRI increases, two dips shift towards the longer wavelength monotonously and observably. Figure 9 (b) shows the wavelength shift of resonant dips which is consistent with the simulated results as shown in Fig. 5(c). The curves are well fitted following an exponential function and the coefficients of determinations (R2) are estimated to be 0.99921 for SPL and 0.99961 for PPL. The sensitivities are calculated using the same formula as that used in the chapter 2. For PPL, the sensitivity is 650 nm/RIU near SRI of 1.3370 and its maximum sensitivity is achieved to be 1900 nm/RIU near SRI of 1.4337. For SPL, the sensitivity is 930 nm/RIU near SRI of 1.3370 and the maximum sensitivity is 2030 nm/RIU near SRI of 1.4337. Importantly, when SRI = 1.4337, the sensitivity of this novel scheme is much higher than that of other fiber sensors based on BSW reported recently, for example, 560 nm/RIU in [18] and 535 nm/RIU in [20]. The figure of merit (FoM) is used to estimate the sensing performance. The FoM is defined as FoM = S/FWHM [20], where S is sensitivity, and FWHM is full width half maximum. From Fig. 9 (a), when the SRI is 1.4337, the FWHMs are ~54.2 nm for SPL, and ~113 nm for PPL. Thus, the FoM are ~37.5 RIU−1 for SPL, and ~16.8 RIU−1 for PPL. It is believed that the FoM of the developed fiber sensors can be further improved if the spectral bandwidth of resonant dip is narrow, which can be achieved by preventing the tapering region from being coating by 1DPC during ALD deposition [29].
Fig. 9 (a) The experimental results of the optical spectra of tapered fiber with BSW excitation subjected to different surrounding refractive index. (b) The measured wavelength shift of resonant dips as a function of surrounding refractive index, and the curves refer to the data fitting.
5. Conclusion
We have theoretically and experimentally demonstrated a novel approach of BSW excitation based on tapered SMF deposited with 1DPC consisting of TiO2 and Al2O3. By using ALD technology, the thickness and the uniformity of the multilayer structure are controlled accurately. The transmission spectrum of the tapered with multilayer is observed experimentally, showing a good agreement with the simulation results. Furthermore, the appearance of the BSW resonance dips and sensing character studies prove the feasibility of this novel fiber sensor. The sensing characters have been studied both in experiment and in simulation, and they show a great agreement with each other. We have demonstrated fiber-based BSW excitation for refractive index (RI) sensing for the first time. Due to the high sensitivity to the surrounding medium, the proposed sensor is potential in the area of biosensor and chemical sensing.
Funding
National Natural Science Foundation of China (NSFC) (61422507, 61605108); National Key Research and Development Program of China (2016YFF0100603); Science and Technology Commission of Shanghai Municipality (STCSM) (15511105402)
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