Atomic layer deposition (ALD) technology is introduced to fabricate a high sensitivity refractive index sensor based on an adiabatic tapered optical fiber. Different thickness of Al2O3 nanofilm is coated around fiber taper precisely and uniformly under different deposition cycles. Attributed to the high refractive index of the Al2O3 nanofilm, an asymmetry Fabry-Perot like interferometer is constructed along the fiber taper. Based on the ray-optic analysis, total internal reflection happens on the nanofilm-surrounding interface. With the ambient refractive index changing, the phase delay induced by the Goos-Hänchen shift is changed. Correspondingly, the transmission resonant spectrum shifts, which can be utilized for realizing high sensitivity sensor. The high sensitivity sensor with 6008 nm/RIU is demonstrated by depositing 3000 layers Al2O3 nanofilm as the ambient refractive index is close to 1.33. This high sensitivity refractive index sensor is expected to have wide applications in biochemical sensors.
© 2015 Optical Society of America
Optical fiber refractive index (RI) sensor is of significant importance to biochemical sensing fields due to the advantages of high sensitivity, compact size and anti-electromagnetic interference. Tapered fiber is one of the commonly used technologies of refractive index sensor which is based on the optical transmission variation of adiabatic tapered single-mode fiber [1, 2] or tapered multimode fiber , and the interference between the fundamental mode and higher order modes in nonadiabatic tapered single-mode fiber [4–6] modulated by the ambient refractive index. For the adiabatic tapered fiber, the sensitivity is higher for the ambient RI close to the cladding RI, but lower for the ambient RI close to 1.33 which is commonly used for biosensor. It has been demonstrated that good sensitivity for RI sensors can be obtained through nanofilm enhancement technology, such as long period gratings (LPGs) with nanofilm overlayer [7–9], double cladding fiber (DCF) with nanofilm overlayer , antiresonant reflecting optical waveguides (ARROWs) structure , hollow-core photonic-crystal fibers (PCFs) , tapered fiber with metal coat [13,14], and cladding-removed multimode optical fiber (CRMMF) with thin film [15–17]. The nanofilm involves resonant enhancement in the interaction between optical fiber mode and ambient RI, allowing creation of high sensitivity refractometer.
Recently, tapered optical fiber coated by nanofilm with layer-by-layer method has been proposed to realize high sensitivity RI sensor [18,19]. This structure is based on lossy mode resonance (LMR), which is a consequence of the coupling between the light guided in the taper and the lossy mode in the nanofilm. In this paper, we demonstrate a high sensitivity ambient RI sensor based on an adiabatic tapered fiber deposited with high refractive index nanofilm overlayer via atomic layer deposition (ALD) technology. An approximate method based on ray-optic is considered to explain the resonance. Light wave leaks out into the nanofilm and multiple beam interference occurs, which can be regarded as a thin film Fabry-Perot resonator. Its resonant transmission spectrum depends on the ambient RI variation, which allows the detection of ambient RI with high sensitivity. In this work, we propose to coat the fiber taper by using the ALD technology which is a conformal deposition process through sequential, self-terminate surface reactions . Compared with the conventional fiber coating technologies, self-assemble, dip-coating for instance, the ALD technology has special advantages including accurate thickness control, good conformality for complex shapes, excellent step coverage, good uniformity and adhesion. Roussey et al.  and Krogulski et al.  have shown the ALD is very suitable for the rod shape of optical fiber and to realize novel optical fiber components.
As shown in Fig. 1, the tapered fiber coated with high refractive index nanofilm consists of three parts: a waist region with small and uniform diameter coated with nanofilm, and two taper regions with gradually expand diameter beside the waist region. When the taper angle is small enough, the tapered fiber is approximatively adiabatic, that is, the transmission loss is almost negligible because of little energy of the excited high-order modes . Thus, only fundamental mode is considered in the taper waist section. Since the refractive index of nanofilm is higher than that of fiber taper, light wave cannot be restricted by total internal reflection (TIR). As depicted in Fig. 1, when the light strikes on the taper-nanofilm interface with grazing incidence, the light will be partially reflected, and then be total reflected at nanofilm-surrounding interface. As a result, multiple beam interference occurs along the fiber taper, just like a thin film Fabry-Perot interferometer. The transmission will present a periodic interference spectrum.
If the propagation light wave in the fiber taper is not polarized, the combination of the reflected power in s and p polarization should be taken account of. The light path presents zigzag ray trajectory due to multiple reflection on the taper-nanofilm interface. The general expression for the transmission is 16,25]26]
A typical transmission spectrum of the tapered fiber coated with nanofilm ( = 500nm, = 1.6 + 0.01i) was theoretically simulated based on Eqs. (1)-(3), as shown in Fig. 2. Strong interference patterns are observed with mth order resonant pairs at different wavelengths which correspond to the different polarization transmission.
When the nanofilm RI and thickness is defined, the variation of ambient RI causes and of the nanofilm-surrounding interface to change. As a result, the phase delay induced by the Goos-Hänchen shift changes, which leads to the transmission interference spectrum shift. Therefore, by monitoring the shift () of the interference spectrum at the resonant dip (), the ambient RI sensor can be realized.
3. Sensitivity to ambient RI variation
According to Eq. (4), with changing the ambient RI, only the phase delay will be changed, which means the sensitivity of the 0th resonant dip is larger than others. Thus, the 0th resonant dip is the best choice for high sensitive ambient RI sensor. The imaginary part of the nanofilm refractive index is responsible for the depth of the resonant dip, which agrees with the conclusion in , so we mainly focus on the sensitivity to the refractive index real part and the thickness of the nanofilm. The 0th resonant dip sensitivity for the nanofilm with different refractive index real part and thickness was simulated within the ambient RI range of 1.33 to 1.35. As shown in Fig. 3(a), for a defined nanofilm RI, the sensitivity increases with increasing the nanofilm thickness, which results from the phase change induced by TIR. For the same order resonant dip, the incident angle will increase to satisfy the phase interference condition, which will lead to increase in Goos-Hänchen phase delay change rate according to the TIR feature . Hence higher sensitivity can be obtained with increasing the nanofilm thickness. However, it is important to notice that the 0th resonant dip shifts to longer wavelength with increasing the nanofilm thickness, which may lead to the resonant wavelength beyond the commonly used band of 1310 nm or 1550 nm. Therefore, the nanofilm thickness should be optimized in the sensor head design.
From Fig. 3(a), we also find that higher sensitivity is obtained for the sensor head with relatively higher nanofilm RI. To demonstrate the sensitivity of the phase delay variation related to the ambient RI change, we theoretically simulated the relationship between the resonant wavelength and the nanofilm RI, as shown in Fig. 3(b). The effect of nanofilm RI is similar to that of nanofilm thickness, which is consistent with the conclusions in . The enhanced sensitivity results from the increase of Goos-Hänchen phase delay change rate .Thus, by using ALD technology, some other materials are also good choices to realize high sensitivity with proper deposition thickness, like ZrO2, TiO2.
4. Fabrication of tapered fiber coated with nanofilm
The tapered fiber was fabricated by using the conventional heating and pulling technology. The taper waist radius and the total length are approximate 12.5μm and 16mm, respectively. The insertion loss of pulled fiber taper is less than −1dB. The Al2O3 film was deposited by using the ALD equipment (TFS 200, Beneq). In each deposition cycle, the Al2O3 was formed through the chemical reaction of the two precursors of Al(CH3)3 and O3 at 210°C in the reaction chamber, and then monolayer Al2O3 was deposited on the surface of the taper waist. One deposition cycle was completed in 2.2 seconds. Repeating the deposition cycle, the Al2O3 film can be deposited layer by layer. Attributed to the characteristic of self-terminate, the thickness of monolayer is almost same in each deposition cycle, and the thickness of the final Al2O3 film can be precisely controlled through the number of deposition cycle. The end face of the fiber deposited with 3000 layers Al2O3 nanofilm was observed by using a scanning electron microscope (SEM), as shown in Fig. 4. 16 positions are taken on the nanofilm to measure the corresponding film thickness. The average film thickness is about 269.4 nm, so the average thickness per layer is about 0.09 nm with the standard deviation is 4.2 nm, which shows the good uniformity.
With a broadband light source (NKT Photonics SuperK Compact, 500 nm-2400 nm) and an optical spectrum analyzer (AQ-6315A), the transmission spectra of fiber taper with different Al2O3 nanofilm thickness were measured in air. As shown in Fig. 5(a), for the fiber taper with relatively thin nanofilm, there is no resonant spectrum. By increasing the number of deposition layers, two resonant dips appear in success in the wavelength band of our study, showing a red shift and spectral width increase. Therefore, the resonant wavelength position can be controlled through changing the nanofilm thickness. In Fig. 5(b), the theoretical transmission spectra for different nanofilm thickness (90 nm, 180 nm, and 270 nm) were simulated, where the material dispersion of refractive index was cited from , and the imaginary part was set as 0.001. Comparing Fig. 5 (a) with (b), the theoretical transmission spectra for 90 nm, 180 nm and 270 nm in thickness are corresponding to the experimental results for 1000, 2000, and 3000 deposition layers, respectively, which shows a good agreement between theoretical and experimental results.
Moreover, the transmission spectra of the tapered fiber with 270 nm Al2O3 nanofilm for s and p polarization were simulated, respectively. As shown in Fig. 6, the transmission spectra for s and p polarization coincide with dip A and dip B of the transmission spectrum with 270 nm-thick Al2O3 layer, respectively. Therefore, it is confirmed that the resonant dips at different wavelengths result from different polarizations.
5. Experiment on RI sensor
In the sensing experiment for ambient RI, the fiber taper sensor head was fixed on a microscope glass slide, the liquid sample with different RI was dropped on the sensor head. The mixture of glycerol and deionized water with different ratios was used as the liquid sample. The RI is from 1.33 to 1.35, which was certified by using an Abbe refractometer with the resolution of 0.0001. The temperature around the sensor head was maintained at 20°C in order to avoid the RI/temperature cross sensitivity. In addition, the sensor head was cleaned by deionized water and dried after each sample measurement.
The transmission spectra of tapered fiber with different thickness of Al2O3 nanofilm in deionized water are shown in Fig. 7. By comparing the Fig. 5(a) with Fig. 7, it is found that, for a certain thickness of Al2O3 nanofilm, the resonant wavelengths shift to the red with switching from air to deionized water, which indicates that the change in the ambient RI can be monitored from the shift of the transmission spectrum. In the wavelength band of our study, there are dual dips for both of the sensor heads with 180 nm-thick and 270 nm-thick Al2O3 layer.
The transmission spectra versus ambient RI are shown in Fig. 8(a)-8(d) for the tapered fibers deposited with 180 nm-thick and 270 nm-thick Al2O3 layer, respectively. As the ambient RI increases, all of the transmission spectra shift toward the longer wavelength. According to Eq. (4), this relationship results from the decrease in the phase delay induced by the Goos-Hänchen shift in the total internal reflection on the nanofilm-surrounding interface. In addition, the shift amount for the dual dips of 270 nm-thick Al2O3 layer is larger than that of 180 nm-thick Al2O3 layer. This has been demonstrated in previous simulation results, as shown in Fig. 3(a).
In order to demonstrate the sensitivity of the tapered fiber with nanofilm, linear fitting to the response of transmission spectrum to the ambient RI variation is given, as shown in Fig. 9. The coefficient of determination R2 is above 0.97 indicating that the response has good linear property. It is shown that the sensitivity is enhanced for the sensor head with thicker nanofilm. For 270 nm-thick Al2O3 layer, the sensitivities for dip A and B is 6008 nm/RIU and 4114 nm/RIU, respectively. It is almost two times more than the sensitivity of the corresponding dips which are 2919 nm/RIU and 2178 nm/RIU respectively for 180 nm-thick Al2O3 layer. These results also agree with previous discussion that increasing nanofilm thickness can improve the sensitivity for the same order resonant dip.
In this paper, a high sensitive refractive index sensor is fabricated by coating an adiabatic tapered optical fiber with Al2O3 nanofilm via ALD technology. This sensor demonstrates high sensitivity to the relatively low ambient RI near to 1.33 which is commonly used in biosensor applications. The fiber taper with nanofilm coating structure can be regarded as a thin film Fabry-Perot interferometer. Thus, the ray-optic method is adopted to analyze the transmission spectrum and characterize the sensing properties. Attributed to the thin thickness of the coating, the 0th order resonant dip appears in the near infrared windows of silica optical fiber (1310 nm and 1550 nm). The phase delay induced by the Goos-Hänchen shift has quite largely impact on the resonant phase condition for the 0th order resonant dip. Thus, a very small change in ambient RI near to 1.33 can cause a larger resonant wavelength shift. Based on the advantages of ALD technology, tunable thickness of Al2O3 nanofilm ensures controlling the transmission spectrum precisely. The experimental results demonstrate that the high sensitivity of 6008 nm/RIU at the ambient RI of 1.33 is easily obtained by modifying the nanofilm thickness. In addition, from the theoretical results, the sensitivity can be enhanced further through increasing the nanofilm refractive index. There are a variety of choices to perform this optimization in ALD technology, such as TiO2, ZrO2, and Ta2O5.
This project was funded by the Natural Science Foundation of China (61275090, 61422507, 61227012), Science and Technology Commission of Shanghai Municipality (STCSM) (13510500300, 14511105602, 14DZ1201403), and Innovation Program of Shanghai Municipal Education Commission (14ZZ093).
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