Open Access
Add to BibSonomy
Abstract
In this article, highly sensitive voltage, thermal and magnetic field fiber sensors were obtained in magnetic nanoparticles-doped E7 liquid crystals filled into photonic crystal fibers (PLCF). The voltage and temperature sensitivity reached at 12.598 nm/V and -3.874 nm/°C, respectively. The minimum voltage response time is 48.2 ms. The phase transition temperature Tc of liquid crystal with magnetic dopant was reduced from 60 °C to 46 °C. The magnetic field sensor based on magnetic nanoparticles-doped PLCF were obtained with sensitivity of 118.2 pm/mT from 400 to 460 mT.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, photonic crystal fibers (PCF) filled with optical materials have been constant interest because of their excellent optical properties [1]. They have excellent responsiveness to external fields such as electric fields, temperature, which have been used to produce as numerous optical devices, such as optical switches [2,3] polarizers [4], filters [5], and sensors [6,7].
Liquid crystals (LC) as a typical optical material were widely used in the regulation of temperature and electric field. By filling LC into the air holes of a PCF, a new type of LC device called photonic liquid crystal fiber (PLCF) was reported [8,9]. Normally, the refractive index of LC materials is higher than that of SiO2 [10]. When LC materials are filled into the air holes of a PCF, the light conduction mode will change from a refractive index guided total reflection type to a photonic band gap type [11]. In addition, the arrangement of the LC molecules in the air hole will be parallel to the radial direction of the air hole [12]. When an external electric field is applied on the PLCF, the orientation of the LC molecules changes, which changes the transmission spectrum of the PLCF due to the birefringence effect of the LC [13]. A. Lorenz et al. fabricated a linear polarizer with a fully filled PLCF using electrical addressing [14]. Y. J. Huang et al. investigated an electro-optical modulation based on PCF for single hole filled LC with a sensitivity of 5.594 nm/Vrms[15]. Q. Liu et al. investigated a high sensitivity voltage sensor based on inserting the PLCF into a Sagnac ring with a sensitivity of up to 7.37 nm/V in the range of 325 ∼ 425 V [7]. In addition, when the temperature reached the phase transition temperature Tc, the LC materials changed from anisotropic to isotropic state and the refractive index changed drastically [16]. S. Tian et al. fabricated a PCF filled with E7 LC and demonstrated a compact multi-band thermo-optical switch with an extinction ratio of 30 dB, which is also an excellent temperature sensor with a sensitivity of 3.9 nm/°C [17]. For the sensing of magnetic field, the magnetic fluids (MF) filled PCF was common. However, the MF based magnetic field sensors have long response times and can only measure weak magnetic fields because of the limitations of the MF [18,19]. Hence, it is meaningful to investigate PLCF-based magnetic field sensors because of the quick response of nematic liquid crystal and measurement of strong magnetic fields.
In this work, we reported the highly sensitive voltage, temperature and magnetic field fiber sensors based on magnetic nanoparticles-doped PLCF. The voltage sensing sensitivity reaches up to 12.598 nm/V, and temperature sensing sensitivity reaches up to -3.874 nm/°C, and the Tc is reduced from 60 to 46 °C because the dopant has decreased the average strength of intermolecular interactions of LC molecules. The PLCF-based magnetic field sensor has been implemented, with a highest sensitivity of 118.2 pm/mT from 400 to 460 mT and the response range of 250 to 460 mT. The all-fiber fiber sensor exhibits simple construction and high sensitivity to real-time monitor the electric, temperature and magnetic fields.
2. Experimental and principle
In this experiment, Fe3O4@SiO2 nanoparticle-doped modified nematic LC materials were filled inside the air holes of PCFs to investigate the effect of Fe3O4@SiO2 nanoparticles on the electric field and temperature response of LC materials, and the effect of Fe3O4 nanoparticles on the magnetic sensitivity. We fabricated the Fe3O4@SiO2 nanoparticles by chemical co-precipitation. A micro emulsion was prepared by dissolving 1.6 g of sodium dodecyl benzenesulfonate in 15 mL of xylene by sonication. The salt solution consisted of 0.149 g of FeCl2·4H2O, 0.602 g of Fe(NO3)3·9H2O and 0.7 mL of DI water. The salt solution and the microemulsion were mixed thoroughly under vigorous stirring and the solution was kept at room temperature for at least 12 h to stabilize it. Then, the mixture was slowly heated to 90 °C under the protection of continuous flow of nitrogen and the temperature was maintained for 1 h. After that, 1 mL of hydrazine hydrate (34 wt.% aqueous solution) was added slowly dropwise to the solution. The solution was aged at 90 °C for 3 h, and then cooled to 40 °C. To enhance the hydrolysis reaction of tetraethyl orthosilicate (TEOS), 0.3 mL of NaOH solution with a concentration of 7.5 wt.% was added into the magnetite solution. Then, 2 mL TEOS was injected into the mixture under vigorous stirring [20]. The finished Fe3O4@SiO2 nanoparticles obtained are shown in Fig. 1(a), with a total diameter Do = 12.4 nm and core diameter do = 6.1 nm. The Fig. 1(b) show the Fe3O4 nanoparticles image with a diameter of 5 to 15 nm purchased from Hangzhou jikang new materials co. LTD, China. The Fe3O4 nanoparticles without SiO2 shell has better magnetic property than that of the Fe3O4@SiO2 nanoparticles. The E7 LC was obtained from Jiangsu He Cheng Display Technology Co. China. The refractive indices of no = 1.522 and ne = 1.746 at 20 °C at 632.8 nm. The LC materials with different nanoparticles dopant after heated to 65 °C were filled into all the air holes of the PCF by capillary forces to obtain four different PLCFs. The four samples were as follows: PLCF without nanoparticles (pure-PLCF), PLCF doped with 1 wt.% Fe3O4@SiO2 nanoparticles (1 wt.%-Fe3O4@SiO2-PLCF), PLCF doped with 5 wt.% Fe3O4@SiO2 nanoparticles (5 wt.%-Fe3O4@SiO2-PLCF) and PLCF doped with 7 wt.% Fe3O4 nanoparticles (7 wt.%-Fe3O4-PLCF). Figure 1(c) shows the structure of PCF we used from the University of New South Wales, Australia. The PCF has a diameter of D = 130 µm, an air hole diameter of d = 7.9 µm and a lattice size of l = 9 µm. The cladding material for this PCF is SiO2 with the refractive index n = 1.457 at 589 nm.
Fig. 1. (a) TEM of Fe3O4@SiO2 nanoparticles. (b) TEM of Fe3O4 nanoparticles. (c) Structure of the PCF. (d) Schematic diagram of the designed PLCF.
The ends of the PCFs filled with LC materials were spliced to the single mode fibers (SMF) by means of a fiber fusion splicer (Fujikura 60S). As shown in Fig. 1(d), two pieces of ITO glass as electrodes were placed in parallel. The PLCFs were placed between the ITO layers, and two quartz rods of 130 µm diameter were placed to ensure the stability of the structure and the parallelism of two ITO glass plates. Then, the device was fixed using an insulating UV-curable adhesive (H5010, UV Adhesives Lab in Anhui University).
In the structure of a fusion spliced SMF-PLCF-SMF, the PCF was filled with a high refractive index nematic LC. The light-guiding mechanism of PCF changes from refractive index-guided mode to band gap mode. The light beam not only propagates inside the PCF core, but also part of the beam propagates inside the air hole filled with LC materials [21]. There is a slight collapse of the PCF at the end section in fused SMF-PCF-SMF [22]. As a consequence, when the beam passes through the collapsed region, interference occurs between the modes in the LC cladding and PCF core, leading to interference peaks in the transmission spectrum [23]. For the magnetic nanoparticles-doped PLCF, the uniform distribution of nanoparticles in the LC material causes scattering of the light [24]. The interference between the main LC cladding and the PCF fiber core can be described by the following equation [25]:
When Δφ = (2m + 1)π (m is a non-negative integer), the dip can be obtained due to interference, and the dip wavelength can be represented as [25]:
As shown in Fig. 2, when there is no electric field, the orientation of the LC molecules is parallel to the direction of air holes. When the AC signal is applied to the upper and lower electrodes, and the electric field strength is greater than the Freedericksz transform threshold, the orientation of the LC molecules deflects with increasing electric field strength. When the electric field strength exceeds Freedericksz transform threshold, the orientation of LC molecules can be expressed as [26]:
Fig. 2. (a) Orientation of LC molecules without electric field. (b) Orientation of LC molecules have deflected by electric field.
When the deflection of LC molecules, due to the birefringence of nematic LC materials, the effective refractive index of the LC materials has been changed. The refractive index in both axial directions can be expressed as [27]:
When the temperature changes, the refractive index of the LC material also changes because of the thermo-optical effect of the LC material, and the Δneff will change too. It will cause shift of λdip. The thermo-optical effect of the LC material can be expressed as [17]:
with the change of temperature ΔT, the shift value of λdip can be expressed as [17]:The band gap distribution spectrum of pure PLCF at 25 °C is shown in Fig. 3.
3. Results and discussion
The response of Fe3O4@SiO2 nanoparticles-doped PLCF sensor to electric field signals was investigated. An Ultra-Wideband light source (UWB, Golight, 1250-1650 nm), and an optical spectrum analyzer (OSA, Yokogawa, AQ6370-D) were used to measure the transmission spectra of PLCFs, as shown in Fig. 4(a). An AC signal was used as the electrical signal. And the sensor was placed in the thermal insulation device with 25°C to ensure that it is not affected by the temperature. As shown in Fig. 4(b), there are two band gaps near 1350 nm and 1550 nm respectively, when an electric field of 68 v is applied. Meanwhile, for the Fe3O4@SiO2 nanoparticles-doped PLCF, the light scattering from the nanoparticles causes more transmission loss, and the dips closest to the non-bandgap region was designated as the left and right edges of the bandgap that were used as a reference for the shift of the transmission spectral. When the applied electric field strength is less than the electric field threshold, the orientation of the LC molecules is fixed, and the transmission spectrum does not shift. When the applied electric field strength is close to the Freedericksz transform threshold voltage, the orientation of the LC molecules is incompletely deflected, and the transmission spectrum changes irregularly. When the applied electric field strength exceeds the Freedericksz threshold electric field, the orientation of the LC molecules is deflected as the electric field voltage increases [26]. The effective refractive index also changes due to the birefringence effect, and the wavelength position of the band gap in the transmission spectra.
Fig. 4. (a) Schematic diagram of the device for the measurement of electric field, (b) The band gap of the transmission spectrum of the pure-PLCF with electric field of 68 V.
As shown in Fig. 5(a), the left edge of the photonic band gap of pure-PLCF shifts from 1474.82 nm to 1593.11 nm when the modulated AC voltage increases from 58 to 68 V. The electric field sensitivity is 10.603 nm/V with R2 = 0.9959. In the Fig. 5(b), the left edge of band gap in the transmission spectrum for the 1 wt.%-Fe3O4@SiO2-PLCF shifts from 1517.07 nm to 1642.68 nm when the modulated AC voltage increases from 50 V to 60 V, and the electric field sensitivity is 12.598 nm/V with R2 = 0.9937. In Fig. 5(c), the left band gap edge in the transmission spectrum of the 5 wt.%-Fe3O4@SiO2-PLCF shifts from 1508.17 nm to 1623.57 nm. The electric field sensitivity of this sensor is 11.879 nm/V with R2 = 0.9903. It is noted that the electric field sensitivity of the PLCF doped with Fe3O4@SiO2 nanoparticles is slightly higher than that of the undoped sample. And the range of the operating voltage decreases with the increase of the nanoparticles doping concentration. At the same time, R2 decreases with increasing concentration of dopant, because deflection of LC molecules is inevitably affected by Fe3O4@SiO2 nanoparticles. The stability of deflection of LC moleculars will be slightly broken by the high concentration of dopant. Therefore, we conclude that Fe3O4@SiO2 nanoparticles can improve the sensitivity of the E7 LC material in response to the electric field and can control the measurements range by varying the nanoparticles doping concentration. The sensing range of these sensors encompasses the C-band (1530 nm ∼ 1565 nm) with extinction ratio of ∼30 dB. These sensors are also as a suitable choice for C-band electro-optical switches.
Fig. 5. The spectra and electric sensitivity of the three concentrations of the samples with the applied electric field, (a) pure-PLCF, (b) 1 wt.%-Fe3O4@SiO2-PLCF, (c) 5 wt.%-Fe3O4@SiO2-PLCF.
The response times of the three PLCFs were measured, a photodetector and an oscilloscope were used to convert optoelectronic signals and receive electrical signals respectively. As shown in Fig. 6, when the frequency of the drive signal is 50 Hz, significant fluctuations in response time can be observed. This signal fluctuation decreases as the frequency of the drive signal increases. This fluctuation does not disappear completely when the drive signal frequency increases to 200 Hz. When the frequency of the drive signal reaches 500 Hz, this fluctuation disappears, indicating that the drive signal at this point meets the measurement requirements.
The response and recovery times with three kinds of PLCFs are shown in Fig. 7, when the AC signal of 500 Hz is applied, the output of the devices changes from a high level to low and recovers after the AC signal stops. As shown in Fig. 7, the response time of three sensors are 57 ms (pure-PLCF), 56.2 ms (1 wt.%-Fe3O4@SiO2-PLCF) and 48.2 ms (5 wt.%-Fe3O4@SiO2-PLCF), and the recovery times are 114.4 ms, 146 ms and 199.4 ms, respectively. According to the experimental results it can be concluded that as the concentration of Fe3O4@SiO2 nanoparticles increases, the response time of the sensor decreases, while the recovery time increases. This occurs because the Fe3O4@SiO2 nanoparticles brought two different effects, the localized polarization brought by the Fe3O4@SiO2 nanoparticles and the soft anchoring between liquid crystal director and nanoparticles magnetization vector [28]. In our opinion, as the LC molecule rotates, the Fe3O4@SiO2 nanoparticles are driven by the LC molecule as the nanoparticles are coupled to the LC molecule surface due to anchoring forces. When the LC molecule is driven by the electric field, the loss of additional driven Fe3O4@SiO2 nanoparticle motion is outweighed by the gain in enhanced dielectric anisotropy and the response time of the PLCF decreases. In the recovery phase, in the absence of an electric field, the rotation of the LC molecules is driven by elastic forces and the additional driving of the Fe3O4@SiO2 nanoparticles inevitably takes more time, so the recovery time increases with increasing dopant concentration.
Fig. 7. The response and recovery times for the samples with three kinds of concentrations. (a) pure-PLCF, (b) 1 wt.%-Fe3O4@SiO2-PLCF, (c) 5 wt.%-Fe3O4@SiO2-PLCF.
The phase transition temperature Tc is an important parameter for nematic LC materials. When the temperature exceeds the phase transition temperature Tc, the nematic LC materials changes from anisotropic to isotropic, and the birefringence effect disappears. In this paper, the response of PLCFs doped with three concentrations of Fe3O4@SiO2 nanoparticles to temperature were investigated. At the same time, since the band gap is a wide band, it usually consists of multiple dips, so that the band gaps of different spectra usually overlap during the measurement. This situation makes it difficult to recognize the reference dip, and it is more obvious in the samples doped with higher concentration of Fe3O4@SiO2 nanoparticles because of scattering from magnetic nanoparticles. Therefore, the dip wavelengths within the range of 1400 ∼ 1500 nm were used to test the sensitivity as the temperature approaches the phase transition temperature, and the dip wavelengths within the range of 1500 ∼ 1650 nm were used to measure temperature sensitivity above the phase transition temperature. As shown in Fig. 8, the three samples were placed in the heater. As shown in Fig. 9(a), for the pure-PLCF, the band gap red-shifts when the temperature rises from 59 °C to 60 °C, and blue-shifts when temperature rises from 60 °C to 61 °C. And the transmission capacity of PLCF decreases significantly at 60 °C. This turning point corresponding of phase transition temperature Tc of LC materials is 60 °C. When the temperature increases from 65 °C to 85 °C, the dip shifts from 1589.43 to 1514.61 nm, and the sensitivity to temperature response is -3.745 nm/°C with R2 = 0.9998. As shown in Fig. 9(b), for the 1 wt.%-Fe3O4@SiO2-PLCF, the phase transition temperature Tc decreases to 56 °C. When the temperature increases from 60 °C to 80 °C, the dip shifts from 1647.84 to 1570.40 nm, and the temperature sensitivity is -3.874 nm/°C with R2 = 0.9998. As shown in Fig. 9(c), for the 5 wt.%-Fe3O4@SiO2-PLCF, the phase transition temperature Tc decreases to 46 °C. When the temperature increased from 50 °C to 80 °C, the dip shifts from 1622.84 to 1510.52 nm, the temperature sensitivity is -3.748 nm/°C with R2 = 0.9999. As a result, the Fe3O4@SiO2 nanoparticles has a significant effect on the phase transition temperature Tc of the E7 LC. The reason for the reduction in phase transition temperature Tc is explained as the dilution of liquid crystal by spherical Fe3O4@SiO2 nanoparticles. It has been described by a molecular mean-field theory. [29–31]. So that the phase transition temperature Tc decreases significantly with the increase of the doping concentration. Therefore, the temperature sensing range of the PLCF sensors can be extended without reducing the sensing sensitivity, by doping LC materials with different concentrations of Fe3O4@SiO2 nanoparticles.
Fig. 9. The spectra of the three concentrations of the samples with the temperature closing and exceeding the Tc, and the sensitivity to temperature response (a) pure-PLCF, (b) 1 wt.%-Fe3O4@SiO2-PLCF, (c) 5 wt.%-Fe3O4@SiO2-PLCF.
We have researched a PLCF doped with Fe3O4 nanoparticles magnetic field sensor. The Fe3O4 nanoparticles were doped into the E7 LC material with concentrations of 7 wt.% and dispersed with ultrasound for 3 hours. As shown in Fig. 10(a), the PLCF was placed inside a uniform magnetic field generator and the radial direction of PLCF is parallel to magnetic field direction. As shown in Fig. 10(b), the transmission spectra of pure-PLCF are essentially unchanged with an applied uniform magnetic field of 460 mT. As shown in Fig. 11(a), the 7 wt.%-Fe3O4-PLCF was placed in a uniform magnetic field, when the magnetic field strength reaches 100 mT and 200 mT, there is no shift in the transmission spectrum. Then, the transmission spectra appear red-shift when the magnetic field strength reaches at 250 mT. When the magnetic field intensity is between 250 and 400 mT, the dip shifts from 1286.17 to 1287.93 nm, when the magnetic field strength is gradually increased from 200 mT. As shown in Fig. 11(c), the transmission spectrum of the sensor shows a nonlinear shift, and the response of the sensor to the magnetic field becomes more and more sensitive as the magnetic field increases from 200 ∼ 400 mT. The Fig. 11(b) presents the spectral variation including a linear magnetic field increase in the range of 400 to 460 mT. The dip red shifts from 1287.925 to 1295.15 nm. As shown in Fig. 11(d), the magnetic field sensitivity reaches 118.2 pm/mT with R2 = 0.9883.
Fig. 10. (a) Schematic diagram of the device for the measurement of magnetic field. (b) The spectra of the pure-PLCF before and after magnetic field addition.
Fig. 11. (a), (b) The spectra of the 7wt%-Fe3O4-PLCF with the magnetic field. (c) Shifts of transmission spectrum from 0 ∼ 400 mT. (d) Magnetic sensitivity of 7wt%-Fe3O4-PLCF from 400 ∼ 460 mT.
Next, we will discuss the reasons for the shifts of transmission spectrum. First, the inner surface of the PCF air hole provides a rigid anchoring to the liquid crystal molecules nearby, so that the main orientation of the liquid crystal molecules in the PCF is parallel to the axial direction of the air hole. At the same time, the spherical nanoparticles are uniformly dispersed in the E7 liquid crystal. Since the size of the nanoparticles is much larger than that of the E7 liquid crystal molecules, some of the liquid crystal molecules are softly anchored on the surface of the nanoparticles [32].
Second, when the sensor is influenced by small magnetic field, the particles can hardly move, and the transmission spectrum of the sensor is almost unchanged because the magnetic force is smaller than the interaction force between the liquid crystal molecules and the magnetic nanoparticles. And this is the reason why the transmission spectrum hardly shifts when the magnetic field is less than 200 mT.
Then, when the magnetic field strength is further increased, the particles are gradually converged by the magnetic field influence. At this point, the magnetic moments of the particles rotate in the same direction and a chain-like structure is gradually formed with magnetic nanoparticles. In this case, the magnetic nanoparticles dopant in the liquid crystal material has both spherical and chain-like structures, and the higher the magnetic field strength, the more chain-like structures are formed. However, the chain-like particles are not softly anchored to the liquid crystal molecules, but rigidly anchored [33]. In this case, the anchoring energy between the E7 liquid crystal molecules and the magnetic nanoparticles increases significantly, which means that the magnetic nanoparticles can move with liquid crystal molecules more easily. Therefore, when the magnetic field strength gradually increased from 200 mT, the transmission spectrum of the sensor shows a nonlinear shift, and the response of the sensor with the magnetic field becomes more and more sensitive as the magnetic field increases.
Finally, when the magnetic field strength is over the threshold value of 400 mT in the manuscript, all magnetic nanoparticles became chain-like structures, at which point the sensitivity could not continue to change and the shifts of transmission spectrum become linear.
Due to strength of intermolecular interactions of LC molecules, the PLCF has a fast reaction and response time under the influence of a magnetic field. Hence, the PLCF based magnetic field sensor is suitable for real-time monitoring systems of magnetic fields and magneto-optical switches in all-fiber structures.
4. Conclusion
In conclusion, we fabricated all-fiber voltage, thermal and magnetic field sensors with high sensitivity by filling Fe3O4 nanoparticles-doped E7 LC into air holes of PCFs. The polarization intensity of LC molecules has changed due to the local electric field on the surface of Fe3O4@SiO2 nanoparticles. Under the effect of an AC signal at a frequency of 500 Hz, the voltage sensitivity of voltage sensors has reached up to 12.598 nm/V, and the response time is as low as 48.2 ms. In addition, as a temperature sensor, the temperature sensitivity has reached up to -3.874 nm/°C, and the Tc is reduced from 60 to 46 °C. By controlling the doping concentration of the particles, the sensing range of the temperature sensor can be tuned to meet a wider range of industrial applications. With the dopants of Fe3O4 nanoparticles into E7 LC materials, the PLCF-based magnetic field fiber sensor has been realized with a modulation range of 7.23 nm and a sensitivity of 118.2 pm/mT. This all-fiber magnetic field sensor has a fast reaction and response time and can be used as a real-time monitoring device for magnetic field signals and magneto-optical switches in all-fiber structures.
Funding
National Natural Science Foundation of China (11874012, 11874126, 12174002, 51771186); Excellent Scientific Research and Innovation Team of Anhui Province (2022AH010003); Key Research and Development Plan of Anhui Province (202104a05020059); Innovation project for the Returned Overseas Scholars of Anhui Province (2021LCX011); The University Synergy Innovation Program of Anhui Province (GXXT-2020-052); Anhui Project (Z010118167).
Acknowledgments
The authors would like to thank the financial supports from the National Natural Science Foundation of China (Grant Nos. 12174002; 11874012, 11874126 and 51771186); Excellent Scientific Research and Innovation Team of Anhui Province (Grant No. 2022AH010003); Key Research and Development Plan of Anhui Province (Grant No. 202104a05020059); Innovation project for the Returned Overseas Scholars of Anhui Province (Grant No. 2021LCX011); The University Synergy Innovation Program of Anhui Province (Grant No. GXXT-2020-052); Anhui Project (Grant No. Z010118167).
Disclosures
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. P. Russell, “Photonic crystal fibers,” Science 299(5605), 358–362 (2003). [CrossRef]
2. Y. Hou, F. Fan, X. Wang, G. Zhao, and S. Chang, “Terahertz switch and polarization controller based on photonic crystal fiber,” Sci. China Inf. Sci. 55(1), 106–113 (2012). [CrossRef]
3. L. Wei, T. T. Alkeskjold, and A. Bjarklev, “Electrically tunable bandpass filter using solid-core photonic crystal fibers filled with multiple liquid crystals,” Opt. Lett. 35(10), 1608–1610 (2010). [CrossRef]
4. T. R. Wolinski, S. Ertman, A. Czapla, P. Lesiak, K. Nowecka, A. Wdomanski, E. Nowinowski-Kruszelnicki, R. Dabrowski, and J. Wojcik, “Polarization effects in photonic liquid crystal fibers,” Meas. Sci. Technol. 18(10), 3061–3069 (2007). [CrossRef]
5. Y. Liu, Y. Wang, B. Sun, C. Liao, J. Song, K. Yang, G. Wang, Q. Wang, G. Yin, and J. Zhou, “Compact tunable multibandpass filters based on liquid-filled photonic crystal fibers,” Opt. Lett. 39(7), 2148–2151 (2014). [CrossRef]
6. S. Mathews, G. Farrell, and Y. Semenova, “Liquid crystal infiltrated photonic crystal fibers for electric field intensity measurements,” Appl. Opt. 50(17), 2628–2635 (2011). [CrossRef]
7. Q. Liu, P. S. Xue, and Z. Y. Liu, “Voltage sensor based on liquid-crystal-infiltrated photonic crystal fiber with high index,” Appl. Phys. Express 13(3), 032005 (2020). [CrossRef]
8. F. Du, Y. Q. Lu, and S. T. Wu, “Electrically tunable liquid-crystal photonic crystal fiber,” Appl. Phys. Lett. 85(12), 2181–2183 (2004). [CrossRef]
9. K. Milenko, T. R. Wolinski, P. P. Shum, and D. J. J. Hu, “Temperature-Sensitive Photonic Liquid Crystal Fiber Modal Interferometer,” IEEE Photonics J. 4(5), 1855–1860 (2012). [CrossRef]
10. T. T. Alkeskjold, L. Scolari, D. Noordegraaf, J. Laegsgaard, J. Weirich, L. Wei, G. Tartarini, P. Bassi, S. Gauza, S. T. Wu, and A. Bjarklev, “Integrating liquid crystal based optical devices in photonic crystal fibers,” Opt. Quantum Electron. 39(12-13), 1009–1019 (2007). [CrossRef]
11. B. T. Kuhlmey, B. J. Eggleton, and D. K. C. Wu, “Fluid-Filled Solid-Core Photonic Bandgap Fibers,” J. Lightwave Technol. 27(11), 1617–1630 (2009). [CrossRef]
12. M. W. Haakestad, T. T. Alkeskjold, M. D. Nielsen, L. Scolari, J. Riishede, H. E. Engan, and A. Bjarklev, “Electrically tunable photonic bandgap guidance in a liquid-crystal-filled photonic crystal fiber,” IEEE Photon. Technol. Lett. 17(4), 819–821 (2005). [CrossRef]
13. L. Scolari, T. T. Alkeskjold, J. Riishede, and A. Bjarklev, “Continuously tunable devices based on electrical control of dual-frequency liquid crystal filled photonic bandgap fibers,” Opt. Express 13(19), 7483–7496 (2005). [CrossRef]
14. A. Lorenz and H. S. Kitzerow, “Efficient electro-optic switching in a photonic liquid crystal fiber,” Appl. Phys. Lett. 98(24), 241106 (2011). [CrossRef]
15. Y. Huang, Y. Wang, L. Zhang, Y. Shao, F. Zhang, C. Liao, and Y. Wang, “Tunable Electro-Optical Modulator Based on a Photonic Crystal Fiber Selectively Filled With Liquid Crystal,” J. Lightwave Technol. 37(9), 1903–1908 (2019). [CrossRef]
16. X. Liu, Y. Liu, and Z. Wang, “Double telecom band thermo-optic switch based on dual-line filled photonic liquid crystal fibres,” Liq. Cryst. 44(3), 479–483 (2017). [CrossRef]
17. S. Tian, T. Y. Yang, J. X. Zhang, K. Xie, J. J. Ma, L. Hong, Y. H. Luo, and Z. J. Hu, “Multi-Band Thermal Optical Switch Based on Nematic Liquid Crystal Filled Photonic Crystal Fiber,” J. Lightwave Technol. 39(10), 3297–3302 (2021). [CrossRef]
18. F. Shao, S. Q. Li, L. Lu, Y. Kuai, Z. G. Cao, F. Xu, Y. Liu, F. Xie, K. Xie, B. L. Yu, and Z. J. Hu, “High sensitivity and dual parameters micro-tapered-LPG sensor,” Opt. Lasers Eng. 164, 107498 (2023). [CrossRef]
19. Y. Zhao, R. Q. Lv, Y. Ying, and Q. Wang, “Hollow-core photonic crystal fiber Fabry-Perot sensor for magnetic field measurement based on magnetic fluid,” Opt. Laser Technol. 44(4), 899–902 (2012). [CrossRef]
20. J. Lee, Y. Lee, J. K. Youn, H. Bin Na, T. Yu, H. Kim, S. M. Lee, Y. M. Koo, J. H. Kwak, H. G. Park, H. N. Chang, M. Hwang, J. G. Park, J. Kim, and T. Hyeon, “Simple synthesis of functionalized superparamagnetic magnetite/silica core/shell nanoparticles and their application as magnetically separable high-performance biocatalysts,” Small 4(1), 143–152 (2008). [CrossRef]
21. J. J. Hu, G. Ren, P. Shum, X. Yu, G. Wang, and C. Lu, “Analytical method for band structure calculation of photonic crystal fibers filled with liquid crystal,” Opt. Express 16(9), 6668–6674 (2008). [CrossRef]
22. B. Sun, Y. J. Huang, D. Luo, C. Wang, J. He, C. R. Liao, G. L. Yin, J. T. Zhou, S. Liu, J. Zhao, and Y. P. Wang, “Broadband Thermo-Optic Switching Effect Based on Liquid Crystal Infiltrated Photonic Crystal Fibers,” IEEE Photonics J. 7(4), 1–7 (2015). [CrossRef]
23. H. Du, X. Sun, Y. Hu, X. Dong, and J. Zhou, “High sensitive refractive index sensor based on cladding etched photonic crystal fiber Mach-Zehnder interferometer,” Photonic Sens. 9(2), 126–134 (2019). [CrossRef]
24. E. S. Iranizad, Z. Dehghani, and M. Nadafan, “Nonlinear optical properties of nematic liquid crystal doped with different compositional percentage of synthesis of Fe3O4 nanoparticles,” J. Mol. Liq. 190, 6–9 (2014). [CrossRef]
25. Y. Geng, X. Li, X. Tan, Y. Deng, and X. Hong, “Compact and ultrasensitive temperature sensor with a fully liquid-filled photonic crystal fiber Mach–Zehnder interferometer,” IEEE Sensors J. 14(1), 167–170 (2014). [CrossRef]
26. Y. B. Liu, W. H. Lin, M. I. Vai, P. P. Shum, L. Y. Shao, W. He, S. Q. Liu, F. Zhao, W. Z. Wang, and Y. H. Liu, “Fiber Optic Electric Field Intensity Sensor Based on Liquid Crystal-Filled Photonic Crystal Fiber Incorporated Ring Laser,” IEEE Photonics J. 14(1), 1–5 (2022). [CrossRef]
27. Y. Liu, C. Zhao, Y.-n. Zhang, G. Ma, X. Li, and Y. Zhao, “Electrically tunable optical fiber device based on hollow-core fiber infiltrated with liquid crystal,” Sens. Actuators, A 318, 112500 (2021). [CrossRef]
28. A. Zakhlevnykh and D. Petrov, “Weak coupling effects and re-entrant transitions in ferronematic liquid crystals,” J. Mol. Liq. 198, 223–233 (2014). [CrossRef]
29. M. V. Gorkunov and M. A. J. S. M. Osipov, “Mean-field theory of a nematic liquid crystal doped with anisotropic nanoparticles,” Soft Matter. 7(9), 4348–4356 (2011). [CrossRef]
30. V. Gdovinová, N. Tomašovičová, N. Éber, T. Tóth-Katona, V. Závišová, M. Timko, and P. J. L. C. Kopčanský, “Influence of the anisometry of magnetic particles on the isotropic–nematic phase transition,” Liq. Cryst. 41(12), 1773–1777 (2014). [CrossRef]
31. K. Zakutanska, V. Lacková, N. Tomašovičová, S. Burylov, N. Burylova, V. Skosar, A. Juríková, M. Vojtko, J. Jadźyn, and P. J. J. o. M. L. Kopčanský, “Nanoparticle's size, surfactant and concentration effects on stability and isotropic-nematic transition in ferronematic liquid crystal,” J. Mol. Liq. 289, 111125 (2019). [CrossRef]
32. P. Kopčanský, I. Potočová, M. Koneracká, M. Timko, J. Jadzyn, G. Czechowski, and A. M. G. Jansen, “The structural instabilities of ferronematic based on liquid crystal with low negative magnetic susceptibility,” Phys. Status Solidi B 236(2), 450–453 (2003). [CrossRef]
33. P. Kopcansky, N. Tomasovicova, M. Koneracka, V. Zavisova, M. Timko, A. Dzarova, A. Sprincova, N. Eber, K. Fodor-Csorba, T. Toth-Katona, A. Vajda, and J. Jadzyn, “Structural changes in the 6CHBT liquid crystal doped with spherical, rodlike, and chainlike magnetic particles,” Phys. Rev. E 78(1), 011702 (2008). [CrossRef]
