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Abstract
We designed a new type of gas sensor, an optical tentacle, made of highly integrated polymer micro-ring resonators in three-dimensional space on the tiny end-facet of a multicore optical fiber. Two pairs of three polymer micro-ring resonators were hung symmetrically on both sides of three suspended micro-waveguides as the sensing units. The micro-waveguides interlace to form a three-layer nested configuration, which makes the multicore optical fiber a “tentacle” for vapors of volatile organic compounds. Both experiments and theoretical simulation confirmed that the symmetrical coupling of multiple pairs of rings with the micro-waveguide had better resonance than the single ring setup. This is because the symmetrical light modes in the waveguides couple with the rings separately. All the optical micro-components were fabricated by the two-photon lithography technology on the end facet of multicore optical fiber. The optical tentacle shows good sensitivity and reversibility. This approach can also be adopted for sensor array design on a chip. Furthermore, optical sensors that can sense vapors with multiple constituents may be achieved in the future by adding selective sensitive materials to or on the surface of the rings.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Compactness, easy integration and high sensitivity are necessary to realize artificial olfactory receptors that can be installed in a mobile phone for vapor sensing [1]. In bionics, sensors that aim to detect only one type of physical variable, for example, the Hall sensor, geomagnetic sensor, accelerometer, gyro sensor, microphones, temperature sensor, proximity sensor, touchscreen, barometer, and humidity sensor, and so on, can be considered as receptors. Most “smartphones” consist of such sensors. An olfaction device that detects multiple species of chemicals will have a sensor array as the main component [2]. Usually, a sensor array consists of many units, with each of them responding to one species of gas molecules, and the sensitivity and selectivity of the sensing units are determined by the sensing mechanisms and the materials. Therefore, each of the units should be treated with different materials during the process of design and fabrication. On the other hand, the array may need hundreds or thousands of units, and the data processing of such many sensors is a difficult task. It is challenging to satisfy all the demands simultaneously. Therefore, to achieve a compact device that can detect multi gases compositions with high sensitivity, there needs tremendous efforts of researchers from various fields including biology [3], electronics [4,5], chemistry [6–13], material science [14], and optics [15–23]. In this paper, we show that two-photon lithography (an advanced micro- and nano-fabrication technology) is an effective method to achieve the above mentioned aim. To demonstrate this, we propose an “optical tentacle” of suspended polymer micro-rings on a multicore fiber for the detection of vapors of volatile organic compounds (VOCs). Inspired by the tentacles of invertebrates, a multicore optical fiber is employed as the “nerve,” which couples light into and out of the “olfactory tentacle”. The end facets of the optical fibers are intrinsic platforms for micro- and nano-optical sensing devices [24,25], and provide ample scope for the advancement of micro- and nano-fabrication technology [26–32] (six sensing units can be fabricated on the end facet of a seven-core optical fiber). The ring resonators [15,17,21] that support whispering gallery modes [33–35] and show good sensitivity to various gases after sensitive materials are added on them were employed as the sensing component of the tentacle, i.e. “olfactory receptors,” which are located at the end facet of the muliticore optical fiber. There are six olfactory receptors on a single end facet of the multicore optical fiber, each of which can be treated separately during the fabrication. The size of the sensing part of the optical tentacle is 78 µm × 40 µm × 32 µm, and is connected to the optical fiber with only six micro-pillars, which means that the remained area except for the six connected points is still available for other optical or electrical structures, if they are adopted on a chip. Another characteristic of the optical tentacle is that the ring resonators are not adhered to the substrate (the end facet of the optical fiber), but suspended in air, with only a small segment running tangential to another suspended waveguide and welded to it. The advantage of a suspended ring resonator is that it could expand freely with no additional resistance from the substrate; therefore, it will exhibit high sensitivity to analytes or forces.
2. Design and analysis of “optical tentacle”
Figure 1(a) shows the schematic illustration of the sensing end of the optical tentacle, which is mostly a three-part nested structure. Figure 1(b) illustrates a single part with micro-pillars, micro-prisms, micro-tapers, a micro-waveguide, and micro-rings. The two pillars with square cross-sections on two sides are used to couple light from the fiber core beneath and support the other components above. Two prisms with a bottom angle of 45° are located on the two pillars, which are used to reflect the light from the pillar to the taper and vice versa. The two ends of the waveguide are connected to the apexes of the tapers. Two rings are suspended from the center of the waveguide, which are the olfactory receptors of the optical tentacle. Figure 1(b) demonstrates light propagation in the optical tentacle. First, light coupled out of one of the cores (in the front) passes through the pillar and reaches the reflection surface of the micro-prism. This light is reflected onto the taper and squeezed into the waveguide to excite the whispering galley modes in the micro-ring resonator. Finally, light is propagated through the other taper, micro-prism and pillar, and transmitted to the other core of the seven-core optical fiber.
Fig. 1. Schematic illustration of the sensing end of an optical tentacle. (a) The overall arrangement of the three sets of sensing channels on the end facet of a multicore optical fiber. Each sensing channel consists of a pair of polymer micro-rings symmetrically embedded in a waveguide, two tapers at the ends of the waveguide, two prisms connected to the bottom of the tapers, and two pillars supporting the prisms. Red, blue, and yellow denote the light propagating through the structures. The molecules around the fiber indicate the vapor environment of the tentacle. (b) Clear display of one set of sensing channels. The distance between two cores of the fiber is 35 µm. (c) Geometric details of the rings and the waveguide, and how light propagates through them. The cross-sections of the rings and the waveguide are rectangles of 1 µm × 0.5 µm and 1 µm × 1 µm, respectively.
Figure 1(c) shows the geometrical parameters of the rings and the waveguides, and how the electromagnetic field propagates through them. The cross sections of the waveguides and the rings are rectangles of size 1 µm × 1 µm and 1 µm × 0.5 µm, respectively. The radii of the two rings are denoted by r1 and r2, respectively. The rings are embedded in the waveguide at a depth of 0.5 µm, which makes the connection between the rings and the waveguide strong enough to ensure that the rings are hung stably (see the section on fabrication). However, in such configurations, the coupling between the two rings and the waveguide do not meet the critical coupling criterion. Therefore, the quality factors of the resonances cannot be high. This drawback may be overcome by creating gaps between the rings and the waveguide and supporting the rings with a truncated circular cone, like the one we have achieved [35]. The quality factor of the infrared band (1,530 ∼ 1,570 nm) can reach 105. In this work, we focus on improving the number of integrated rings in limited space by this configuration, and exploring its optical properties and sensing characteristics.
Both simulated and experimental results show that the modulation depth of the resonant dips in the transmission spectral of two symmetric rings with the same geometrical parameters (radius and thickness) is much deeper than that with one ring. The modulations depth can be defined as (Tpeak-Tdip)/Tpeak, where Tpeak is the transmission of the peaks between two resonant dips, and Tdip is the transmission at the resonant dips. If the symmetry of the two rings is broken, for example, due to radii difference or dislocation, the resonant dips will split, and the modulation depth will shorten. Figure 2 shows the comprehensive simulation results of the transmission spectral for the two rings with various locations (d), radii (r1 and r2) and thicknesses (t) in the visible spectrum. The simulations are performed on commercially available software by Lumerical FDTD Solutions. Figure 2(a) shows the three main cases considered in the simulation, i.e. case (1) corresponds to the dislocation of d, which is the difference between the two centers of the contact points of the two rings and waveguide; case (2) corresponds to various radii of the two rings, while keeping d = 0; case (3) corresponds to the varying thickness (t) of the two rings, while keeping d = 0 and r1 = r2 = 5 µm. From Fig. 2(b), the maximum modulation depth of the resonant dips of the two rings with exact symmetry (d = 0 and r1 = r2 = 5 µm) is approximately 0.6, while it reduces to 0.35 as the dislocation increases to 1 µm. The modulation depth decreases gradually when d is increased from 0 to 1 µm in steps of 0.2 µm, and there is a minor red shift (1.3 nm) of the resonant dips simultaneously. For the case of a single ring, the modulation depth is only 0.25, as shown in the inset of Fig. 2(b). The difference between the radii of the two rings also causes the modulation depth to decrease. Figure 2(c) shows the simulation results of the transmission spectrum of the two rings, with a fixed r1 (5 µm) and a varying r2 (5 to 5.08 µm in steps of 0.02 µm). Each resonant dip splits into two weak dips, with one is almost always fixed while the other one red shifts to the longer wavelength. The fixed resonant dips correspond to the modes in the ring with r1, while the red shifted resonant dips correspond to the modes in the ring with r2. Because the difference between the free spectral ranges of the two dips is not significant, they coincide again at r2 = 5.08 µm, and the modulations depth reaches almost the same value at r1 = r2 = 5 µm. The symmetry can also be broken by changing the thickness of either of rings (not shown here). Whereas, if the symmetry is maintained by changing the two rings with the same increase in the thickness, for all the resonant dips shown in Fig. 2(d), red shift becomes the governing phenomenon across the spectrum. The average wavelength shift is approximately 9.1 nm, when the thickness of the rings is increased from 0.5 to 0.6 µm.
Fig. 2. Analysis of the simulated transmission spectra of the two rings coupled waveguide structures. (a) Three cases to be analyzed: (i) two rings of same size with dislocation of d, (ii) two rings of different radii, r1 and r2, without dislocation, (iii) two rings symmetrically coupled to waveguide, with varying thickness (t). (b) Transmission spectra corresponding to case (i), with r1 = r2 = 5 µm, t = 0.5 µm, and d = 0, 0.2 0.4, 0.6, 0.8 and 1 µm. (c) Transmission spectra corresponding to case (ii), with r1 = 5 µm, d = 0 µm, t = 0.5 µm, r2 = 5, 5.02, 5.04, 5.06, and 5.08 µm. (d) Transmission spectra corresponding to case (iii), with r1 = r2 = 5 µm, d = 0 µm, t = 0.5, 0.52, 0.54, 0.56, 0.58 and 0.6 µm.
These simulations and analysis imply that if the parameters of the two rings are changed by the same method and the symmetry is maintained, the resonant dips will not split and the modulation depth will be maintained mostly the same. If the analyte in the environment has the same impact on the two rings, a strong modulation depth can be maintained, while if the two rings are doped with materials of different sensitivities, splits in the resonant dips will observed.
To understand why the modulation depths in the resonant dips of the two rings are much deeper than that of a single ring, electromagnetic (EM) field distributions (the absolute values of electrical field intensities) in the two rings and the waveguide for cases with different parameters are simulated, as shown in Fig. 3. Figures 3(a) and 3(f) illustrate two cases of exact symmetry with different thicknesses, t = 0.5 and 0.6 µm, while the other parameters are taken as r1 = r2 = 5 µm and d = 0 µm. The resonant wavelengths in Figs. 3(a) and 3(f) are 716.0 and 824.89 nm, respectively (for other resonant wavelengths, the light field distributions are similar). The field distributions throughout the structure are symmetrical. An interesting characteristic observed here is that the EM fields in the waveguides display a series of beats along the direction of propagation from bottom to upward before they couple with the rings. After the coupling, the light fields are divided into three strands in Fig. 3(a) and two strands in Fig. 3(f). The EM fields in both rings are in resonance with the same intensity, in Figs. 3(a) and 3(f). Figures 3(b) and 3(c) show two cases with d = 0.6 and 1.0 µm, respectively, while the other parameters are taken as r1 = r2 = 5 µm and t = 0.5 µm. Two new characteristics are observed here: a) EM fields in the waveguide do not retain their symmetry after coupling with the two rings; and b) fields in the two rings keep the same intensity but become weak with increasing d.
Fig. 3. Analysis of EM field distributions in the two ring-coupled waveguide structures. Six cases with different parameters are considered. (a) λ = 716.0 nm, r1 = r2 = 5 µm, d = 0 µm, and t = 0.5 µm, (b) λ = 716.5 nm, r1 = r2 = 5 µm, d = 0.6 µm, and t = 0.5 µm, (c) λ = 717.3 nm, r1 = r2 = 5 µm, d = 1.0 µm, and t = 0.5 µm, (d) and (e) with the same geometrical parameters of r1 = 5 µm, r2 = 5.04 µm, d = 0 µm, and t = 0.5 µm and different wavelengths, λ = 697.75 and 700.77 nm. (f) λ = 824.89 nm, r1 = r2 = 5 µm, d = 0 µm, and t = 0.6 µm.
Figures 3(d) and 3(e) show two cases with fixed geometrical parameters (r1 = 5.0 µm, r2 = 5.04 µm, d = 0 µm, and t = 0.5 µm), while the resonant wavelengths take different values (697.75 and 700.77 nm), as marked by two stars in Fig. 2(c) that correspond to the respective resonances of the two rings. In Fig. 3(d), the resonant EM field is mainly located in the left ring, for a resonant wavelength of 697.75 nm. It is mainly located in the right ring for a resonant wavelength of 700.77 nm. The field in the waveguide after coupling shows a serpentine pattern, which means broken symmetry.
Based on these simulation results, a physical mechanism for the enhancement of modulation depth of the two rings compared with a single ring can be deduced (Fig. 2(a)). First, for light of wavelength 0.5 to 0.85 µm, a waveguide with a square cross-section of 1 µm × 1 µm and a refractive index of 1.52 in multimode, the modes in the waveguide are up and down, and right and left symmetrical, and can be displayed with a ray model as shown in Fig. 2(a). For the left ring, only the light indicated by the red ray satisfies the coupling conditions, while only the light indicated by the black ray satisfies the conditions for the right ring. Therefore, the coupling of the EM waves in the waveguide is said to be effective if the two rings are symmetrically coupled with the waves. On the contrary, when a single ring is coupled with the waveguide by embedding it, only the parts of the modes corresponding to the waveguide will satisfy the coupling conditions, which is only half of the total light modes. Therefore, half of the light modes in the waveguide do not couple into the ring. Furthermore, if the conditions are not satisfied, or the resonances of the two rings do not coincide with each other, the modulation depth will reduce. The dislocation-induced changes in the rings break the phase conditions (Fig. 2(b)), while the radii difference of the two rings makes their resonant wavelengths deviate (Fig. 2(c)).
3. Fabrication and sensing characterization of the “optical tentacle”
We have been researching three-dimensional (3D) micro- and nano-sensing devices on top of end facets of various optical fibers for several years. In the course of this research, we developed a technology that enabled us to realize many 3D micro- and nano-sensing structures [35–37]. In this study, we fabricated the optical tentacle using the same technology and the same material on seven-core optical fiber (SM-7C1500(6.1/125) which was obtained from Fibercore Ltd., UK. However, there are still some issues that must be addressed. For instance, due to the limited size of the femtosecond laser focal point, the minimum vertical size of the ring and waveguide cannot exceed 1 µm and the minimum thickness of the ring can only be 0.5 µm, which are the reasons why the cross sections of the waveguide and the rings are 1 µm × 1 µm and 1 µm × 0.5 µm, respectively.
The rings suspended from the waveguide cannot be too large either, else they will be crushed by surface tension of the developer solution. In order to test the sustainability of a large ring embedded 0.5 µm into a waveguide, single rings with various radii were fabricated. The preliminary test shows that the radius of the ring should not be more than 7 µm, to keep them from crushing. The single rings were characterized by three types of organic vapors of various concentrations. Based on these results, we achieved the three-part nested structure of the optical tentacle on the end facet of a seven-core optical fiber, as shown in Figs. 4 and 5. The microscopy images in Fig. 4 show the top view of the head of an optical tentacle, with focus on each olfactory receptors at different heights. For each olfactory receptor, green laser has been coupled into the structures from both ends. Figures 4(a) and 4(d) focus on the olfactory receptor that tilts to the left (the upper channel), Fig. 4(b) and 4(e) focus on the one that tilts to the right (the middle channel), and Figs. 4(c) and 4(f) focus on the horizontal one (the lower channel). For all olfactory receptors, green light flows from the prisms, from the apexes of the cone and the rings, which implies that light has been reflected by the prisms and coupled into the tapers, waveguide, and rings. The scanning electron microscopy (SEM) images of the optical tentacle are given in Fig. 5. Figures 5(a), 5(b) and 5(c) are the overall views in inclined angle, from top and side, respectively. Figures 5(d), 5(e) and 5(f) are the enlarged images of the rings, tapers, and prisms. The 5-µm rings closely retain the circular shape, and the reflection surfaces of the prisms are flat and smooth. Despite the stripes on the outer surfaces of the tapers due to the overlapping of different layers during the layer-by-layer direct writing process, their function of squeezing light into the waveguide is maintained well.
Fig. 4. Microscopy images of the optical tentacle with focus on three sensing channels, respectively. Green light is coupled into the channel from both ends. (a) and (d) focus on the upper channel; (b) and (e) on the middle channel; and (c) and (f) on the lower channel.
Fig. 5. Scanning electron microscopy images of the optical tentacle. (a) Overall view with inclined angle. (b) and (c) Top and side views; (d), (e), and (f) Enlarged sections of the rings, tapers, and prisms.
To characterize the sensing performance of the optical tentacle for various organic vapors with different concentrations, three types of organic vapor environments with stable and quantitative concentrations were prepared. The volatile organic compounds (VOCs) of propylene glycol monomethyl ether acetate (PGMEA), isopropanol (ISO) and alcohol (ALC) were mixed with pure water at various concentrations, and then poured into a sealed chamber with a pinhole for contact with the ambient atmosphere. As demonstrated in our previous work [35], the vapor concentrations in the upper space of the chamber can be accurately controlled by changing the ratios of VOCs and water. For diluted solutions, vapor concentrations can be calculated by a simple formula, which enabled us to characterize the sensing performance of the optical tentacle accurately.
Figure 6 shows the experimental setup used for the optical characterization of the device. A Fan Out which was also obtained from Fibercore Ltd. was used to couple light into and out of the cores where the pillars were located. The Fan Out is an optical fiber coupler with one end of seven single-core optical fibers while the other end of single seven-core optical fiber. The seven single-core optical fibers are connected to the seven cores of the seven-core optical fiber one to one. During the experiment, the end with seven-core of the Fan Out is connected to the sample with cores corresponding to each one to one. Then, we can send light through one of the single-core optical fiber of the Fan Out into the optical tentacle, and get the transmitted light back from another single-core optical fiber. White light was obtained with a laser driven light source (EQ-99 LDLS System, Energetiq, USA) and analyzed under a spectrometer (USB 4000, Ocean optics, USA). In the experiment, we only recorded the transmitted intensities of the light, did not calculate the transmission of the device due to that it is difficult to obtain the corresponding background. Therefore, we name the recorded spectra “Relative transmission”.
Fig. 6. The experimental setup used for the optical characterization of the device. White light was obtained with a laser driven light source (EQ-99 LDLS System, Energetiq, USA), and coupled into the device from one channel of a Fan Out. The transmitted light was sent to the spectrum analyzer (USB 4000, Ocean optics, USA) for measurements. The device is located in a chamber with organic solutions in it.
Figure 7 shows the relative transmission spectra of the optical tentacle in the air and three types of VOCs vapor environments of various concentrations. Figure 7(a) illustrates the relative transmission spectra (wavelength from 450 to 700 nm) of the three olfactory receptors (dual rings ①, ② and ③) when they are in the air. Whispering gallery modes in the whole wavelength range were observed for all the three olfactory receptors in the tentacle, which demonstrates the precision by which the device was fabricated. The maximum modulation depths for the three olfactory receptors were 0.358, 0.269 and 0.386, respectively. The light source used in this experiment was not polarized; the modulation depths can be doubled with polarized light. Thus, the simulation results agree with the experiment. It can be seen that the the average full width at the half maximum of the dips is about 2.7 nm at wavelength of 536 nm, and the free spectrum range is about 5.7 nm. The Q-Factor can be calculated as 198.
Fig. 7. Sensitive characteristics of the optical tentacle for three species of VOC vapors. (a) Transmission spectra of the three sensing channels in the air. (b) The relationships between wavelength shift and concentrations of three species of VOC vapors (PGMEA, ISO and ALC). (c1), (c2) and (c3) are the wavelength shifts of the resonant dips in the transmission spectra of one sensing channel, for PGMEA, ISO and ALC vapor produced from its aqueous solutions with volume ratio varying from 0.2% to 2.0% in steps of 0.2%.
Figures 7(c1), 7(c2) and 7(c3) show the transmission spectra of an olfactory receptor in the vapors of PGMEA, ISO and ALC aqueous solutions with various volumetric ratios ranging from 0.2 to 2.0%. The wavelength shifts (Δλ) of resonance dips for the three species of vapors of aqueous solutions with a volumetric ratio of 2.0% are 8.64, 3.22, and 2.92 nm. The quantitative relationship between the experimental values of wavelength shifts and the vapor concentrations are summarized in Fig. 7(b), where solid circles, squares, and triangles correspond to the experimental results extracted from data shown in Figs. 7(c1), 7(c2) and 7(c3), and the trend lines are best fits according to the Langmuir isotherm, with the wavelength shift being expressed as Δλ = Δλmax ×Kc/(1+ Kc), where K is the equilibrium constant and c is the vapor concentration. The values of parameters Δλmax and K are taken as 18.02, 11.82, and 10.15 nm and 635.25, 245.38 and 281.69, respectively. The fitted lines agree with the experimental results for the three species of organic vapors. The sensitivity of the artificial olfactory receptor to the vapors of PGMEA, ISO, and ALC (aqueous solutions) with a concentration of approximately 150 ppm are 9.54, 2.70, and 2.63 pm/ppm, respectively. The other two olfactory receptors show very similar characters.
The reversibility of the new optical tentacle is also demonstrated by the wavelength shift and recovery when it is immersed and taken out from different environments (PGMEA, ISO and ALC). The tentacle is immersed in the three environments for 100 s (Fig. 8(a)), and then taken out immediately (<0.1 s) for another 100 s (Fig. 8(b)). Four cycles of exposed and removal (Fig. 8(c)) were carried out; both the processes follow exponential behaviors which corresponds to the absorption and release of molecules on the surface of the rings. The reversibility of gas sensors is determined by the type of absorption process of the molecules on the sensitive materials. In this case, there is no chemical reaction between the volatile organic compounds and the photoresist; the absorption is only a physical process. Therefore, the optical tentacle shows good reversibility. Another noticeable character of the device is different response time for the three vapor species, which depends on the specific process of absorption by the material and deposition on the photoresist.
Fig. 8. Reversible characteristics of the optical tentacle for three species of VOC vapors. (a) The variation of transmission spectra of one sensing channel when it is immersed in a vapor environment of 2% ISO aqueous solution. (b) When the channel is lifted off the vapor. (c) Four cycle tests for the reversibility of the optical tentacle in three species of vapors (PGMEA, ISO and ALC). The solid curves are exponentially fit with experimental data.
4. Summary
We demonstrated the manufacturing process of six suspended polymer micro-rings and their supporting facilities, such as micro-waveguide, micro-tapers and micro-prisms in a tiny space of 78 µm × 40 µm × 32 µm using two-photon lithography, a 3D manufacturing technology. Analogous to the olfactory receptors in living beings, the optical micro-components interlace with each other in three-dimensional space to form a new method of three-dimensional integration, which is effective to increase the number of sensing units in a sensor array. Two-photon lithography is a suitable method to fabricate the three-dimensional integrated optical micro-components. To demonstrate, we built the optical micro-components on an end facet of a multicore optical fiber. The optical micro-components form three VOC vapor sensing units on the fiber, which transforms the multicore optical fiber into an optical tentacle. The six suspended polymer micro-rings are divided into three pairs, each of which is symmetrically embedded in a waveguide. The enhancement of modulation depth was analyzed theoretically and verified experimentally. We proposed an explanation for the underlying physical mechanism based on the analysis of EM field distributions in the structure.
The sensing performance of the optical tentacle for three species of organic vapors was investigated. The results showed sensitivities of 9.54, 2.70, and 2.63 pm/ppm for PGMEA, isopropanol, and alcohol vapors within the low concentration range (<150 ppm) and good reversibility. We only focus on improving the number of sensing units in a compact space, and there is no selectively of this opitcal tentacle to different types of vapors. In the future, if selective sensitive materials are added into or on the surface of the rings, then the optical tentacle may be used for sensing vapors with multiple constituents.
Funding
National Natural Science Foundation of China (11404224, 11474206, 1174243, 11774246, 61735002); Capital Normal University (008/19530050146); Beijing Nova Program (Z161100004916100); Capacity Building for Sci-Tech Innovation - Fundamental Scientific Research Funds (008/18530500186, 008/19530050170, 008/19530050180, 025185305000/142).
Disclosures
The authors declare no conflicts of interest.
References
1. N. J. Tao, “Adding New Senses to Your Cell Phone: Opportunities for Gas Sensing,” ACS Sens. 2(3), 317 (2017). [CrossRef]
2. P. Wang, Q. J. Liu, C. S. Wu, and K. Jimmy Hsia, “Bioinspired Smell and Taste Sensors,” Springer, (2015).
3. A. J. M. Barbosa, A. R. Oliveira, and A. C. A. Roque, “Protein- and Peptide-Based Biosensors in Artificial Olfaction,” Trends Biotechnol. 36(12), 1244–1258 (2018). [CrossRef]
4. C. Hagleitner, A. Hierlemann, D. Lange, A. Kummer, N. Kerness, O. Brand, and H. Baltes, “Smart single-chip gas sensor microsystem,” Nature 414(6861), 293–296 (2001). [CrossRef]
5. X. Y. Xu, X. F. Jiang, G. M. Zhao, and L. Yang, “Phone-sized whispering-gallery microresonator sensing system,” Opt. Express 24(23), 25905–25910 (2016). [CrossRef]
6. N. J. Tao, Y. T. Long, and J. J. Gooding, “Sensing Chemicals in Gas Phase-Addressing an Unmet Need,” ACS Sens. 1(5), 460–461 (2016). [CrossRef]
7. B. Wang, J. C. Cancilla, J. S. Torrecilla, and H. Haick, “Artificial Sensing Intelligence with Silicon Nanowires for Ultraselective Detection in the Gas Phase,” Nano Lett. 14(2), 933–938 (2014). [CrossRef]
8. T. Wasilewski, J. Gebicki, and W. Kamysz, “Advances in olfaction-inspired biomaterials applied to bioelectronic noses,” Sens. Actuators, B 257, 511–537 (2018). [CrossRef]
9. R. Blue and D. Uttamchandani, “Chemicapacitors as a versatile platform for miniature gas and vapor sensors,” Meas. Sci. Technol. 28(2), 022001 (2017). [CrossRef]
10. B. Raman, J. L. Hertz, K. D. Benkstein, and S. Semancik, “Bioinspired Methodology for Artificial Olfaction,” Anal. Chem. 80(22), 8364–8371 (2008). [CrossRef]
11. R. A. Potyrailo, C. Surman, N. Nagraj, and A. Burns, “Materials and Transducers Toward Selective Wireless Gas Sensing,” Chem. Rev. 111(11), 7315–7354 (2011). [CrossRef]
12. R. A. Potyrailo, “Toward high value sensing: monolayer-protected metal nanoparticles in multivariable gas and vapor sensors,” Chem. Soc. Rev. 46(17), 5311–5346 (2017). [CrossRef]
13. X. Zhou, S. Y. Lee, Z. C. Xu, and J. Y. Yoon, “Recent Progress on the Development of Chemosensors for Gases,” Chem. Rev. 115(15), 7944–8000 (2015). [CrossRef]
14. J. Zhang, X. H. Liu, G. Neri, and N. Pinna, “Nanostructured Materials for Room-Temperature Gas Sensors,” Adv. Mater. 28(5), 795–831 (2016). [CrossRef]
15. N. A. Yebo, P. Lommens, Z. Hens, and R. Baets, “An integrated optic ethanol vapor sensor based on a silicon-on-insulator microring resonator coated with a porous ZnO film,” Opt. Express 18(11), 11859–11866 (2010). [CrossRef]
16. H. Z. Wang, L. Yuan, C. Kim, X. W. Lan, J. Huang, and Y. F. Ma, “Integrated chemical vapor sensor based on thin wall capillary coupled porous glass microsphere optical resonator,” Sens. Actuators, B 216, 332–336 (2015). [CrossRef]
17. A. Ramachandran, S. Wang, J. Clarke, S. J. Ja, D. Goad, L. Wald, E. M. Flood, E. Knobbe, J. V. Hryniewicz, and S. T. Chu, “A universal biosensing platform based on optical micro-ring resonators,” Biosens. Bioelectron. 23(7), 939–944 (2008). [CrossRef]
18. B. Yao, C. Yu, Y. Wu, S. W. Huang, H. Wu, Y. Gong, Y. Chen, Y. Li, C. W. Wong, X. D. Fan, and Y. Rao, “Graphene-Enhanced Brillouin Optomechanical Microresonator for Ultrasensitive Gas Detection,” Nano Lett. 17(8), 4996–5002 (2017). [CrossRef]
19. M. Eryürek, Z. Tasdemir, Y. Karadag, S. Anand, N. Kilinc, B. E. Alaca, and A. Kiraz, “Integrated humidity sensor based on SU-8 polymer microdisk microresonator,” Sens. Actuators, B 242, 1115–1120 (2017). [CrossRef]
20. K. Scholten, W. R. Collin, X. Fan, and E. T. Zellers, “Nanoparticle-coated micro-optofluidic ring resonator as a detector for microscale gas chromatographic vapor analysis,” Nanoscale 7(20), 9282–9289 (2015). [CrossRef]
21. J. T. Robinson, L. Chen, and M. Lipson, “On-chip gas detection in silicon optical microcavities,” Opt. Express 16(6), 4296–4301 (2008). [CrossRef]
22. S. I. Shopova, I. M. White, Y. Z. Sun, H. Y. Zhu, X. D. Fan, G. Frye-Mason, A. Thompson, and S. Ja, “On-column micro gas chromatography detection with capillary-based optical ring resonators,” Anal. Chem. 80(6), 2232–2238 (2008). [CrossRef]
23. M. Eryürek, Y. Karadag, N. Tasaltin, N. Kilinc, and A. Kiraz, “Optical sensor for hydrogen gas based on a palladium-coated polymer microresonator,” Sens. Actuators, B 212, 78–83 (2015). [CrossRef]
24. G. Kostovski, P. R. Stoddart, and A. Mitchell, “The Optical Fiber Tip: An Inherently Light-Coupled Microscopic Platform for Micro- and Nanotechnologies,” Adv. Mater. 26(23), 3798–3820 (2014). [CrossRef]
25. M. Consales, A. Ricciardi, A. Crescitelli, E. Esposito, A. Cutolo, and A. Cusano, “Lab-on-Fiber Technology: Toward Multifunctional Optical Nanoprobes,” ACS Nano 6(4), 3163–3170 (2012). [CrossRef]
26. H. E. Williams, D. J. Freppon, S. M. Kuebler, R. C. Rumpf, and M. A. Melino, “Fabrication of three-dimensional micro-photonic structures on the tip of optical fibers using SU-8,” Opt. Express 19(23), 22910–22922 (2011). [CrossRef]
27. J. S. Wu, M. J. Yin, K. Seefeldt, A. Dani, R. Guterman, J. Y. Yuan, A. P. Zhang, and H. Y. Tam, “In situ mu-printed optical fiber-tip CO2 sensor using a photocrosslinkable poly(ionic liquid),” Sens. Actuators, B 259, 833–839 (2018). [CrossRef]
28. M. Yao, X. Ouyang, J. S. Wu, A. P. Zhang, H. Y. Tam, and P. K. A. Wai, “Optical Fiber-Tip Sensors Based on In-Situ mu-Printed Polymer Suspended-Microbeams,” Sensors 18(6), 1825 (2018). [CrossRef]
29. K. Markiewicz and P. Wasylczyk, “Photonic-chip-on-tip: compound photonic devices fabricated on optical fibers,” Opt. Express 27(6), 8440–8445 (2019). [CrossRef]
30. Y. Liu, J. Y. Guang, C. Liu, S. Bi, Q. Liu, P. Li, N. Zhang, S. M. Chen, H. Z. Yuan, D. P. Zhou, and W. Peng, “Simple and Low-Cost Plasmonic Fiber-Optic Probe as SERS and Biosensing Platform,” Adv. Opt. Mater. 7(19), 1900337 (2019). [CrossRef]
31. T. Gissibl, S. Thiele, A. Herkommer, and H. Giessen, “Two-photon direct laser writing of ultracompact multi-lens objectives,” Nat. Photonics 10(8), 554–560 (2016). [CrossRef]
32. T. Gissibl, S. Thiele, A. Herkommer, and H. Giessen, “Sub-micrometre accurate free-form optics by three-dimensional printing on single-mode fibres,” Nat. Commun. 7(1), 11763 (2016). [CrossRef]
33. F. Vollmer and L. Yang, “Label-free detection with high-Q microcavities: a review of biosensing mechanisms for integrated devices,” Nanophotonics 1(3-4), 267–291 (2012). [CrossRef]
34. M. R. Foreman, J. D. Swaim, and F. Vollmer, “Whispering gallery mode sensors,” Adv. Opt. Photonics 7(2), 168–240 (2015). [CrossRef]
35. S. Y. Zhang, S. J. Tang, S. F. Feng, Y. F. Xiao, W. Y. Cui, X. K. Wang, W. F. Sun, J. S. Ye, P. Han, X. P. Zhang, and Y. Zhang, “High-Q Polymer Microcavities Integrated on a Multicore Fiber Facet for Vapor Sensing,” Adv. Opt. Mater. 7(20), 1900602 (2019). [CrossRef]
36. Z. W. Xie, S. F. Feng, P. J. Wang, L. S. Zhang, X. Ren, L. Cui, T. R. Zhai, J. Chen, Y. L. Wang, X. K. Wang, W. F. Sun, J. S. Ye, P. Han, P. J. Klar, and Y. Zhang, “Demonstration of a 3D Radar-Like SERS Sensor Micro- and Nanofabricated on an Optical Fiber,” Adv. Opt. Mater. 3(9), 1232–1239 (2015). [CrossRef]
37. H. Wang, Z. W. Xie, M. L. Zhang, H. L. Cui, J. S. He, S. F. Feng, X. K. Wang, W. F. Sun, J. S. Ye, P. Han, and Y. Zhang, “[INVITED] A miniaturized optical fiber microphone with concentric nanorings grating and microsprings structured diaphragm,” Opt. Laser Technol. 78, 110–115 (2016). [CrossRef]
