Open Access
Add to BibSonomy
Abstract
An anti-resonant reflecting guidance coated with a few layers of graphene has been proposed and experimentally demonstrated for the simultaneous measurement of the refractive index and liquid flow rate. A few layers of graphene were coated on the surface of a hollow core photonic crystal fibre. The refractive index and liquid flow rate can be detected by using the resonant condition of the Fabry–Perot resonator and the effective refractive index of the graphene layers heated by a visible laser beam, which are interrogated through the wavelength shift and visibility of the lossy dip in the transmission spectrum. The experimental results show that the sensitivity of up to 1328 nm/RIU and −2.99 dB/(µL/s) are achieved for the refractive index and flow rate measurement in the refractive index range from 1.345 to 1.363 RIU, respectively. The proposed sensor appears to have potential applications for precise measurement in chemistry, medicine, and biology.
© 2017 Optical Society of America
1. Introduction
Optical refractive index (RI) sensors have attracted a great deal of interest over past decades due to their characteristics, such as light weight, small size, high sensitivity, and immunity to electromagnetic fields [1]. Especially in chemical or biological detection, optical RI sensors provide a simple and real-time method not only to study the kinetics of biomolecular interactions, but also to develop the label-free detection of various biomolecules, such as proteins, bacteria, and DNA, etc [2].
Most conventional optical RI sensors need a microfluidic chip to combine with other optical components, such as the light source, lens, and detectors [3]. However, an easy to handle analytical biosensor with low cost and compact size is desired for real applications. Therefore, optical fibre provides an alternative for on-chip waveguides. Especially for the photonic crystal fibres (PCF), inherent hollow holes in the PCF provide the natural in-line optofluidic, which provides robustness, stability, portability, and remote sensing. Generally, in-line fibre optofluidic is also highly sensitive to small changes in the RI occurring inside the holes in the PCF, which can be approximately classified into three types: gratings, interferometers, and surface-plasmon resonance (SPR). With regard to fibre gratings, such as fibre Bragg gratings (FBGs) [4] or long period fibre gratings [5, 6], the RI changes the effective refractive index of the fibre grating, which results in a shift of the resonant wavelength for the fibre grating. With regard to interferometers, the optical path difference (OPD) of the fibre interferometer is sensitive to the RI change. Various fibre interferometers have been researched, such as the Fabry–Pérot interferometer [7], Mach-Zehnder interferometer [8], or Sagnac interferometer [9]. With regard to SPR, surface bound electromagnetic waves are formed at the interface between a metal and a dielectric [10]. It should be noted that the SPR property is highly dependent on the metal, which means careful design and fabrication of the SPR RI sensors is necessary.
Besides the RI sensing, the flow rate of the liquid sample is also a critical parameter for chemical or biological detection. Precise control of the flow rate has been widely used in nanotechnology and cell biology, such as particle counting/separation or sample mixing [11–13]. On the other hand, the immunoreactions between the antibody and the target in the antigen are highly reliant on the flow rate due to the intrinsic adsorption kinetics [14]. Hence precise flow rate measurement could significantly improve the accuracy of many chemical or biological sensors. Compared with conventional mechanisms of thermal transfer [13], electrical admittance [15], and cantilever deflection [16], fibre-optic flowmeters offer a low cost, high sensitivity, and resistance to chemical erosion. The well-known fibre flowmeter is based on the “hot-wire”. The pump laser can be absorbed by various materials in the optical fibre, which generate a quantity of heat. The principle of the “hot-wire” fibre anemometer is based on the heat exchange between the heated materials and the liquid sample [17]. Gao et al. presented an FBG flow sensor based on the near infrared laser self-heated Co2+-doped high-attenuation fibre [18]. Wang et al. developed a hot-wire anemometer based on a silver-coated FBG assisted by a no-core fibre [19]. Liu et al. fabricated a fast-response fibre-optic anemometer based on the Fabry–Pérot silicon interferometer heated by a visible laser [20], and Li et al. researched a microfluidic Fabry–Pérot interferometer flowmeter based on a micro Co2+-doped optical fibre cavity [21]. The laser-heated fibre-optic “hot-wire” flowmeter possesses high sensitivity, low cost, and a wide dynamic range.
For chemical or biological detection, simultaneous measurement of the RI and flow rate could improve the accuracy and sensitivity of the fibre sensors significantly. Although some researchers combine FBGs and Fabry–Perot interferometers to detect the RI and flow rate simultaneously, it is inevitable that the liquid flow will be obstructed by the cantilever [22]. In this paper, a fibre-optic sensor for the simultaneous measurement of the RI and flow rate based on graphene-coated optofluidic anti-resonant reflecting guidance has been proposed and experimentally demonstrated. The hollow hole in the cladding of the hollow core photonic crystal fibre (HCPCF) forms an in-line optofluidic channel. A Fabry–Perot resonator can be achieved between the hollow hole and cladding. The resonant condition of the Fabry–Perot resonator is changed with the refractive index (RI) change of the liquid sample, which can be measured through the wavelength shift at the dip wavelength. At the same time, the surface of the HCPCF was coated with a few layers of graphene (FLG). The reflection of the leaky light was modulated through the temperature change of the graphene layers heated by the visible laser beam of a 532 nm laser. The flow rate of the liquid sample was measured by using the wind-cooling effects on the heated graphene layers. The RI and flow rate can be detected simultaneously by interrogation through the wavelength shift and visibility of the lossy dip in the transmission spectrum.
2. Fabrication of the graphene-coated optofluidic anti-resonant reflecting guidance
In the proposed sensor, a HCPCF was employed as the sensing fibre. The HCPCF used in the present work was bought from YOFC Ltd. The core is an air hollow octagon with a side length of 18µm, and the cladding is an air-ring with a diameter of 90 µm. The air-ring cladding is composed of eight hollow holes with an inner diameter of 35 µm. The diameter of the outer cladding is 190µm, as shown in Fig. 1(a). The schematic diagram of the proposed sensor is shown in Fig. 1(c). First, two single mode fibres (SMFs) were spliced with two ends of the HCPCF. In this way, both the air core and air-ring cladding were blocked. The arc fusion procedure was optimized to avoid the collapsing of the air holes in the HCPCF (gap: 10 µm, arc power: 18 mA, arc duration: 140 ms). In order to form an in-line microfluidic channel for one hollow hole in the cladding, two microchannels were fabricated for the delivery of the liquid samples in the HCPCF by using femtosecond laser micromachining, serving as an inlet and outlet, as shown in Fig. 1(b). It should be noted that the microchannel was adjusted to align to one hollow hole in the cladding of the HCPCF. In this way, the liquid sample can be passed through one hollow hole in the cladding of the HCPCF (in-line optofluidic) through two square microchannels.
Fig. 1 (a) Cross-section of the HCPCF. (b) Square microchannel for the inlet and outlet. (c) The schematic diagram of the proposed sensor. (d) FLG-coated HCPCF.
The commercial graphene film was grown on copper foils using the chemical vapor deposition technique. Then the copper foil was removed using Fe(NO3)3 solution, and suspended in DI water for ~12 h to remove the residual Fe ions. Then the graphene film was rinsed in deionized water several times and floated on the water surface. After that, the FLG was then attached to the surface of the HCPCF and dried at room temperature for about an hour [23]. After water evaporation, the FLG was firmly stuck to the surface of the HCPCF by the van der Waals interaction, as shown in Fig. 1(d).
3. Principle of the proposed sensor
In order to investigate the ARROW in the HCPCF, we numerically simulated the mode field by using commercial Comsol Multiphysics software. The refractive index of the liquid sample and pure silica were set at 1.34 and 1.45, respectively. The fundamental core mode is confined in the air core because of the mismatch of the core mode and cladding modes in the HCPCF. As the liquid sample is flowed into the in-line optofluidic, the fundamental core mode radiates and oscillates through the cladding of the liquid sample and silica because the RI of the core (air ~1.000) is less than that of the cladding, as shown in Fig. 2(a) and 2(b). The in-line optofluidic and the silica cladding are formed as a double-layered Fabry–Pérot etalon [24]. At the anti-resonant condition, the light wavelengths mismatch the resonant condition of the resonator, which is reflected by the double-layered Fabry–Pérot etalon, and the guided light is propagated in the air core of the HCPCF. At the resonant condition, when the light wavelengths match the resonant condition, the guided light is transmitted through the double-layered Fabry–Pérot resonator and leaks out of the cladding of the HCPCF, which is referred to resonant wavelengths and leaky modes. In this way, the principle of the in-line optofluidic in the HCPCF can be described as an ARROW. Thus, in the transmission spectrum, a series of lossy dips at the resonant condition of the double-layered Fabry–Pérot etalon will occur [25]. Figure. 2(c) and 2(d) show the mode field distribution at the wavelengths of 1530.00 and 1535.26nm, which correspond to the anti-resonant and resonant conditions, respectively.
Fig. 2 (a) Schematic diagram of the cross-section of the liquid-infiltrated HCPCF. (b) Guiding mechanism of the liquid -infiltrated HCPCF. Numerical simulation of the HCPCF filled with liquid(c) at the resonant anti-wavelength (1530.00 nm), (d) at the resonant wavelength (1535.26 nm). (e) Raman spectra of FLG using an excitation wavelength of 532 nm. (f) Transmission spectrum of the FLG.
For the lossy dips corresponding to the resonant condition, the wavelength can be expressed as [26]:
where and are the thickness of the in-line optofluidic and the silica cladding, , and are the RI of the air, liquid sample and the silica cladding, respectively, and is the resonance order. According to Eq. (1), the resonant condition of the Fabry–Pérot resonator is also changed when the RI of the liquid sample is changed, which can be detected by interrogating the wavelength shift of the lossy dip.The principle of the fibre flowmeter can be attributed to the absorption of heating light of the FLG. The Raman spectrum of the FLG (iHR 320, HORIBA, 532nm) is shown in Fig. 2(e). Obviously, the intensity of the G band is greater than that of the 2D band, which indicates the characteristic of the few layers for the FLG samples [27]. The transmission spectrum of the FLG in the visible range is shown in Fig. 2(f) (SP-722, Shanghai Spectrum). The measured transmittance is about 88.3%, indicating that the samples mainly consisted of five layers of graphene due to the general rule of 2.3% of absorbance per graphene layer [28]. Irradiation of the FLG with the 532 nm laser generated a temperature increase due to the absorption of the optical intensity [29]. As a result, the conductancefor an FLG at different temperature is modulated as [30]:
where is the charge of an electron, is Boltzmann’s constant, is photon energy. Obviously, the conductivity for the FLG is very sensitive to the change of the temperature. Then, the real part of the refractive index for the FLG () at different temperatures can be obtained as [31]:where = 8.85 × 10−12 F/m, is the image part of, and is the real part of .During the sensing process, the FLG was heated by using the heating light from a 532 nm laser. Thus, when the liquid sample is passed through the hollow hole, it will carry away part of the heat of the FLG, inducing the change of RI of the FLG. The heat loss depends on the flow rate, which can be given by [18]:
where is the power absorbed by the heating element, is the temperature change of the FLG, is wind speed, and and are empirical coefficients.Besides the wavelength of the lossy dip, the light intensity corresponding to the resonant condition can be expressed as [26]:
where is the transmission power at the resonant wavelengths, and are the reflection coefficients of the incident light at the air core-cladding interface and the silica cladding-surrounding FLG interface, is the input light intensity at the resonant wavelength. According to Eq. (5), the light in the transmission spectrum corresponding to the resonant condition is highly sensitive to small changes in the reflection of the outer FLG surrounding the cladding ring. Since the variation of the temperature induced by the flow rate can influence the effective RI of the FLG, the reflective is changed. The FLG-coated HCPCF is also a type of “hot-wire” flowmeter. Therefore, the flow rate and RI can be detected by measuring the visibility and wavelength shift of the lossy dip modulated through the laser-heated FLG simultaneously.4. Experiment and discussion
4.1 Experimental setup
The experimental setup is shown in Fig. 3(a). The HCPCF was illuminated by a 20 mW broadband source based on the amplified spontaneous emission in spectral range of 1525nm to 1565 nm. The transmission spectrum of the sensor was interrogated by using an optical spectrum analyzer (OSA) (AQ6317B, Yokogawa Co., Ltd.). A metal plate, as shown in Fig. 3(b), was designed in to secure the sensor to avoid bending effects. The metal plate consisted of two holders, of which the top surfaces were fabricated with a V-groove channel to fix the HCPCF. Two regions of the HCPCF fabricated with the microchannel were slightly pre-stretched and put on the holders, and two square solidified polydimethylsiloxane (PDMS) chips with a 1.5-mm-diameter hole covered the two holders, respectively. The PDMS was used to seal the space between the fibre and chips to avoid liquid leakage. A syringe pump could precisely control and deliver the flow rate of the liquid sample into the PDMS chips and in-line optofluidic with polytetrafluoroethylene (PTFE) tubes.
Fig. 3 (a) The experimental setup of the proposed sensor. (b) The metal plate. (c) Thermal image obtained with the thermographic camera. The close-up view of the (d) HCPCF, (e) fluorescent image of the HCPCF filled with anti-mouse IgG -FITC in all hollow holes, (f) and only one hollow hole.
A 532 nm laser (MSL-FN-532, CNIOT Co., Ltd.) connected with a beam expander (GBE05-A-5X, Thorlabs) was used to illuminate the FLG of the HCPCF through the bottom hollow slot of the metal plate. Through the thermographic camera, it can be observed that the FLG-coated HCPCF was heated using the 532 nm laser, as shown in Fig. 3(c).
One hollow hole in the cladding of the HCPCF was injected with the home-made piranha solution (H2O2: H2SO4in a 1:3 ratio by volume) and washed with ultrapure water sequentially 3 times through the syringe pump. The piranha solution could clean the organic matter on the surface of the optofluidic. In order to confirm the effectiveness of the PDMS chip and inlet and outlet microchannels, the goat anti-mouse IgG conjugated with fluorescein isothiocyanate (FITC) was injected into the in-line optofluidic. Meanwhile, the goat anti-mouse IgG was also injected into all the hollow holes in the other HCPCF by using capillary force, respectively. Figure. 3(d) - 3(f) show the close-up view of the HCPCF, the fluorescent image of the HCPCF filled with anti-mouse IgG-FITC in all the hollow holes, and in only one hollow hole. The fluorescence microscopy in Fig. 3(f) clearly confirmed that the two square microchannels can serve as the inlet and outlet, and only one hollow hole of the HCPCF cladding was filled with the liquid sample, which formed an in-line fibre optofluidic.
The transmission spectrum of the in-line optofluidic filled with the ethanol solution was investigated first, as shown in Fig. 4. Clearly, four narrow lossy dips exist in the transmission spectrum of the liquid-infiltrated HCPCF, which indicates the leaky effect in the HCPCF. The wavelengths of the lossy dips are 1530.73, 1541.26, 1551.84 and 1562.32 nm, respectively. There are many ripples in the transmission spectrum, which can be attributed to two reasons: (i) the thickness of the cladding is not very uniform, which may generate many resonant conditions. (ii) the guide light may be reflected between the liquid and silica. Although the reflectivity is weaker than that of two interfaces of the double-layered Fabry–Perot etalon according to the Fresnel reflection, it also may generate many ripples in the transmission spectrum. It should be noted that the modes interference in HCPCF should also be considered, which would generate a sinusoidal interferogram in the transmission spectrum. However, in Fig. 4, there are only lossy dips corresponding to the leaky modes in the transmission spectrum without the sinusoidal interferogram. Therefore, in the proposed sensor the modes interference is too weak to influence the transmission spectrum.
4.2 The RI response of the sensor
The in-line optofluidic was experimentally measured. A series of ethanol solutions with different concentrations were prepared as samples ranging from 1.3465 to 1.3632 RIU (measured with an ATAGO refractometer in the visible wavelength range). The length of the HCPCF is 10 cm, and the temperature is kept at room temperature (25 °C). During each measurement, the liquid was passed through the entire length of the HCPCF, and then pulled out from the outlet. After one measurement, we pumped in a new ethanol solution and recorded a new measurement spectrum. The optical spectra are shown in Fig. 5(a). Due to the multi-dips existed in a same spectrum, one lossy dip was chosen to be recorded at different RI, which is marked with a blue star in Fig. 5(a). It is shown that the initial lossy dip was located at the wavelength of 1530.73 nm. When the RI of the liquid sample was increased, the lossy dip began gradually to red-shift. The wavelength of the lossy dip shows a slight nonlinear relationship against the increase of the liquid RI, which is mainly attributed to the light dispersion in the liquid. The linear fitting curve for the mean wavelength for five times measurement is , as shown in Fig. 5(b). Therefore, the sensitivity of the in-line optofluidic is 1328.4 nm/RIU by interrogating the wavelength shift. Compared with the conventional methods, the sensitivity of the HCPCF interrogated by using the wavelength shift is higher than 1145nm/RIU which is based on the method of a side-channel PCF [5] or 618 pm/RIU which is based on the method of phase-shifted FBG on a side-hole fibre [4]. Each RI measurement was performed five times in order to investigate the reproducibility of the sensor. After each measurement, the in-line optofluidic was washed out. The error bar is shown in Fig. 5(b). The maximum variation is ± 1.3 nm, indicating the in-line optofluidic is able to achieve a good repetition performance.
Fig. 5 (a) The transmission spectra of the in-line optofluidic filled with different RI of ethanol solutions. (b) Relationship between the RI and wavelength of the lossy dip.
4.3 The flow rate response of the sensor
The proposed FLG-coated ARROW was tested for liquid flow rate over the range from 0 µL/s to 0.61µL/s with the step of 0.1µL/s. The corresponding transmission spectra are shown in Fig. 6(a). It is observed that the visibility of the lossy dip decreased rapidly with the increased flow rate. The response of the sensor keeps a good quadratic relationship between the visibility and the flow rate, and the maximum sensitivity reaches −2.99 dB/(µL/s) with the flow rate of 0.61µL/s, as shown in Fig. 6(b). Moreover, Figure. 6(c) shows the visibility of the lossy dip from 0s to 501s corresponding to 0 µL/s to 0.61µL/s of the flow rate. The response time is measured to be 9.6 s for the fibre-optic flowmeter when the flow rate is changed from 0 to 0.3µL/s (shown in Fig. 6(d)), which is attributed to the fast modulation of the FLG. Therefore, the proposed fibre-optic flowmeter can be used for the real-time measurement of the flow rate.
Fig. 6 (a) The transmission spectra with different flow rates. (b) Relationship between the flow rate and visibility of the lossy dip. (c) Visibility of the lossy dip with different flow rates. (d) Time response of the fibre sensor. (e) The relationship between the temperature, visibility and 532 nm heating laser intensity. (f) The visibility at different heating light intensities.
Figure 6(e) shows the experimental results of flow rate measurement at different heating laser intensities. The temperature was measured by using the thermographic camera once the temperature was stable. Both the temperature and the visibility of the lossy dip were increased as a function of the heating laser intensity, indicating the absorption of the heating light and modulation of the RI of the FLG. The linear fitting curves of the temperature and visibility areand, respectively. Figure. 6(f) shows the sensitivity of the FLG-coated ARROW with different heating laser intensities of 20 mW, 40 mW, and 60 mW. It is clearly seen that the sensitivity is enhanced by increasing the heating intensity, and the maximum sensitivity can be estimated as −2.99 dB/(µL/s), −3.50 dB/(µL/s), and −7.56 dB/(µL/s) for the three measurements.
4.4 The temperature response of the sensor
Temperature is a key factor that influences the sensing performance. The temperature dependence of the sensor was also investigated by fixing the sensor into an environmental chamber, of which the temperature range was set from 20°C to 70°C. Figure 7(a) shows the temperature response of the in-line optofluidic. The wavelength shift with the temperature change is 1.8 pm/°C. Compared with the RI sensitivity of 1328nm/RIU, the temperature cross-sensitivity can be neglected. Figure 7(b) presents the visibility response of the proposed fibre-optic flowmeter to the flow rate under different temperatures. The sensitivity of the sensor reduced at high temperature due to the lower heating efficiencies of the FLG. Hence, in practical applications, the heating laser intensity should be adjusted in order to compensate the differing ambient temperature.
5. Conclusion
In conclusion, we have demonstrated a FLG-coated ARROW for the simultaneous measurement of the RI and liquid flow rate. The hollow hole in the cladding of the HCPCF forms an in-line optofluidic channel. A Fabry–Perot resonator can be achieved between the hollow hole and cladding. The resonant condition of the Fabry–Perot resonator is changed with the change of RI of the liquid sample, which can be measured through the wavelength shift of the dip wavelength. At the same time, the surface of the HCPCF was coated with the FLG. The reflection of the leaky light was modulated through the temperature change of the FLG heated by the visible beam of a 532 nm laser. The flow rate of the liquid sample was measured by using the heat exchange effects on the heated FLG. The RI and flow rate can be detected by interrogation through the wavelength shift and visibility of the lossy dip in the transmission spectrum simultaneously. The experimental results show that the sensitivity of up to 1328 nm/RIU and −2.99 dB/(µL/s) are achieved for the RI and flow rate, respectively. Thus the technique appears to have potential applications in research in the fields of chemistry, medicine, and biology.
Funding
National Natural Science Foundation of China (No. 61601436, 61675203, 61377064, 61401432, 61501425); Beijing Natural Science Foundation (4174108), National Basic Research Program of China (No. 2015CB352100); Research Equipment Development Project of Chinese Academy of Sciences (No. YZ201508).
References and links
1. R. Gao, Y. Jiang, W. H. Ding, Z. Wang, and D. Liu, “Filmed extrinsic Fabry–Perot interferometric sensors for the measurement ofarbitrary refractive index of liquid,” Sens. Actuators B Chem. 177, 924–928 (2013).
2. Y.-F. Chen, L. Jiang, M. Mancuso, A. Jain, V. Oncescu, and D. Erickson, “Optofluidic opportunities in global health, food, water and energy,” Nanoscale 4(16), 4839–4857 (2012). [PubMed]
3. D. Psaltis, S. R. Quake, and C. Yang, “Developing optofluidic technology through the fusion of microfluidics and optics,” Nature 442(7101), 381–386 (2006). [PubMed]
4. Q. Zhang, L. Hu, Y. Qi, G. Liu, N. Ianno, and M. Han, “Fiber-optic refractometer based on a phase-shifted fiber Bragg grating on a side-hole fiber,” Opt. Express 23(13), 16750–16759 (2015). [PubMed]
5. N. Zhang, G. Humbert, Z. Wu, K. Li, P. P. Shum, N. M. Y. Zhang, Y. Cui, J. L. Auguste, X. Q. Dinh, and L. Wei, “In-line optofluidic refractive index sensing in a side-channel photonic crystal fiber,” Opt. Express 24(24), 27674–27682 (2016). [PubMed]
6. Z. Wu, Z. Wang, Y. G. Liu, T. Han, S. Li, and H. Wei, “Mechanism and characteristics of long period fiber gratings in simplified hollow-core photonic crystal fibers,” Opt. Express 19(18), 17344–17349 (2011). [PubMed]
7. J. Tian, Z. Lu, M. Quan, Y. Jiao, and Y. Yao, “Fast response Fabry-Perot interferometer microfluidic refractive index fiber sensor based on concave-core photonic crystal fiber,” Opt. Express 24(18), 20132–20142 (2016). [PubMed]
8. N. L. P. Andrews, R. Ross, D. Munzke, C. van Hoorn, A. Brzezinski, J. A. Barnes, O. Reich, and H. P. Loock, “In-fiber Mach-Zehnder interferometer for gas refractive index measurements based on a hollow-core photonic crystal fiber,” Opt. Express 24(13), 14086–14099 (2016). [PubMed]
9. T. Han, Y. G. Liu, Z. Wang, J. Guo, Z. Wu, S. Wang, Z. Li, and W. Zhou, “Unique characteristics of a selective-filling photonic crystal fiber Sagnac interferometer and its application as high sensitivity sensor,” Opt. Express 21(1), 122–128 (2013). [PubMed]
10. R. Otupiri, E. K. Akowuah, and S. Haxha, “Multi-channel SPR biosensor based on PCF for multi-analyte sensing applications,” Opt. Express 23(12), 15716–15727 (2015). [PubMed]
11. J. Collins and A. P. Lee, “Microfluidic flow transducer based on the measurement of electrical admittance,” Lab Chip 4(1), 7–10 (2004). [PubMed]
12. G. J. M. Krijnen, M. Dijkstra, J. J. van Baar, S. S. Shankar, W. J. Kuipers, R. J. de Boer, D. Altpeter, T. S. J. Lammerink, and R. Wiegerink, “MEMS based hair flow-sensors as model systems for acoustic perception studies,” Nanotechnology 17(4), S84–S89 (2006). [PubMed]
13. A. Quist, A. Chand, S. Ramachandran, D. Cohen, and R. Lal, “Piezoresistive cantilever based nanoflow and viscosity sensor for microchannels,” Lab Chip 6(11), 1450–1454 (2006). [PubMed]
14. S. L. Stott, C. H. Hsu, D. I. Tsukrov, M. Yu, D. T. Miyamoto, B. A. Waltman, S. M. Rothenberg, A. M. Shah, M. E. Smas, G. K. Korir, F. P. Floyd Jr, A. J. Gilman, J. B. Lord, D. Winokur, S. Springer, D. Irimia, S. Nagrath, L. V. Sequist, R. J. Lee, K. J. Isselbacher, S. Maheswaran, D. A. Haber, and M. Toner, “Isolation of circulating tumor cells using a microvortex-generating herringbone-chip,” Proc. Natl. Acad. Sci. U.S.A. 107(43), 18392–18397 (2010). [PubMed]
15. M. Ashauer, H. Glosch, F. Hedrich, N. Hey, H. Sandmaier, and W. Lang, “Thermal flow sensor for liquids and gases based on combinations of two principles,” Sens. Actuators A Phys. 73(12), 7–13 (1999).
16. M. Dijkstra, J. J. Baar, R. J. Wiegerink, T. S. J. Lammerink, J. H. Boer, and G. J. M. Krijnen, “Artificial sensory hairs based on the flow sensitive receptor hairs of crickets,” J. Micromech. Microeng. 15(7), 132–138 (2005).
17. T. Chen, Q. Wang, B. Zhang, R. Chen, and K. P. Chen, “Distributed flow sensing using optical hot -wire grid,” Opt. Express 20(8), 8240–8249 (2012). [PubMed]
18. S. Gao, A. P. Zhang, H. Y. Tam, L. H. Cho, and C. Lu, “All-optical fiber anemometer based on laser heated fiber Bragg gratings,” Opt. Express 19(11), 10124–10130 (2011). [PubMed]
19. X. H. Wang, X. Y. Dong, Y. Zhou, K. Ni, J. Cheng, and Z. M. Chen, “Hot-wire anemometer based on silver coated fiber Bragg grating assisted by no-core fiber,” IEEE Photonics Technol. Lett. 25(24), 2458–2461 (2013).
20. G. Liu, M. Han, and W. Hou, “High-resolution and fast-response fiber-optic temperature sensor using silicon Fabry-Pérot cavity,” Opt. Express 23(6), 7237–7247 (2015). [PubMed]
21. Y. Li, G. Yan, L. Zhang, and S. He, “Microfluidic flowmeter based on micro “hot-wire” sandwiched Fabry-Perot interferometer,” Opt. Express 23(7), 9483–9493 (2015). [PubMed]
22. J. Sadeghi, A. H. Ghasemi, and H. Latifi, “A label-free infrared opto-fluidic method for real-time determination of flow rate and concentration with temperature cross-sensitivity compensation,” Lab Chip 16(20), 3957–3968 (2016). [PubMed]
23. J. Man, W. Junjie, R. Zhaoyu, Q. Mei, B. Jintao, B. Yang, Z. Yuping, and W. Qunshu, “Synthesis of graphene and its application aswide-band saturable absorbers,” in 2012 12th IEEE Conference on Nanotechnology (IEEE-NANO),Birmingham, UK, August 2012,pp. 1–4.
24. N. M. Litchinitser, A. K. Abeeluck, C. Headley, and B. J. Eggleton, “Antiresonant reflecting photonic crystal optical waveguides,” Opt. Lett. 27(18), 1592–1594 (2002). [PubMed]
25. C.-H. Lai, B. You, J. Y. Lu, T. A. Liu, J. L. Peng, C. K. Sun, and H. C. Chang, “Modal characteristics of antiresonant reflecting pipe waveguides for terahertz waveguiding,” Opt. Express 18(1), 309–322 (2010). [PubMed]
26. S. Liu, Y. Wang, M. Hou, J. Guo, Z. Li, and P. Lu, “Anti-resonant reflecting guidance in alcohol-filled hollow core photonic crystal fiber for sensing applications,” Opt. Express 21(25), 31690–31697 (2013). [PubMed]
27. L. M. Malard, M. A. Pimenta, G. Dresselhaus, and M. S. Dresselhaus, “Raman spectroscopy in graphene,” Phys. Rep. 473(5–6), 51–87 (2009).
28. S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y. J. Kim, K. S. Kim, B. Ozyilmaz, J. H. Ahn, B. H. Hong, and S. Iijima, “Roll-to-roll production of 30-inch graphene films for transparent electrodes,” Nat. Nanotechnol. 5(8), 574–578 (2010). [PubMed]
29. J. L. Benítez, J. H. cordero, S. Muhl, and D. Mendoza, “Few layers graphene as thermally activate doptical modulator in the visible-near IR spectral range,” Opt. Lett. 41(1), 167–170 (2016). [PubMed]
30. G. W. Hanson, “Dyadic Green’s functions and guided surface waves for a surface conductivity model of graphene,” J. Appl. Phys. 103(6), 064302 (2008).
31. A. Vakil and N. Engheta, “Transformation optics using graphene,” Science 332(6035), 1291–1294 (2011). [PubMed]
