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Abstract
In this paper, we present our recent research results on light propagation in photonic crystal fibers (PCFs) infiltrated with a 6CHBT nematic liquid crystal (LC) doped with 2-nm gold nanoparticles (NPs) with a concentration in the range of 0.01 – 0.5% wt. Electro-optical response times and thermal tuning of the investigated samples have been studied in detail. We have observed up to ~80% decrease of rise times for different concentrations of gold NPs in the LC. Moreover, a significant reduction of the Fréedericksz threshold voltage (up to 60%) has been observed for samples with higher concentrations of 2-nm gold NPs in 6CHBT.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over the last two decades photonic crystal fibers have gained significant research interest [1] since their appearance at the end of the 20th century. PCFs can be designed in various configurations in which periodically arranged microcapillaries (MCs) form a photonic lattice around the core region. The most exciting property of PCFs is the possibility to infiltrate their MCs with different substances such as gases or liquids, and in this way their optical properties can be modified. Amongst many materials, liquid crystals seem to be the most promising for this purpose. Due to their dielectric anisotropy, they are highly sensitive to an external electric field as well as temperature changes that it can lead to LCs birefringence tuning. PCFs infiltrated with LCs referred to as photonic liquid crystal fibers (PLCFs) have become over the last 15 years an object of extensive research activities that led to the development of a new class of in-fiber devices for sensing and telecommunication applications. The majority of the papers have described the properties and potential applications of PLCFs infiltrated with nematic LCs [2–9]. There have been also efforts to infiltrate PCFs with chiral smectic C (SmC*) liquid crystals that possess ferroelectric properties [10,11] or even with a LC blue phase being characterized by a unique three-dimensional chiral structure [12–14]. To the most crucial parameters of the prospective PLCF-based devices belong their electro-optical switching times. Typical values of both rise and fall times for PLCFs with nematic LCs are in a range of 20-30 ms [15,16], while for PLCFs infiltrated with SmC* LCs they can be even below 100 µs [17]. To main drawbacks in application of SmC*, not only in PLCFs but also in planar cells, belong their domain alignment, tendency to form a zig-zag defects and lack of bistability. All of them can be attributed to occurrence of dislocations between smectic layers, and too strong anchoring to the surface. However, these issues can be overcome by applying additional aligning materials on the inner sides of both PCFs and LC cells with proper anchoring energy [10,18]. In the case of the LC blue phase, these materials do not require additional aligning agents, however a high voltage signal is necessary to switch LC molecules. Recently, a new approach leading to the improvement of electro-optical parameters of PLCFs has been considered [19]. It is expected that doping LC materials with nanoparticles (NPs) will significantly enhance the electro-optical properties of prospective PLCF-based devices and systems [20,21].
In this paper we have investigated the electro-optical properties of a PLCF composed of 2-nm (in diameter) metallic gold (Au) NPs dispersed in a nematic 4-(trans-4'-n-hexylcyclohexyl)- isothiocyanatobenzene (6CHBT) LC [22,23] and introduced into a PCF structure. Both 6CHBT and Au NPs have been synthesized at the Military University of Technology (Warsaw, Poland). Selected concentrations of Au NPs in 6CHBT LC material ranging from 0.01% wt. to 0.5% wt. have been taken into consideration. Due to the optical properties of the Au NPs, they have found applications both in biology [24] and photonics [25–27]. In organic LCs, the non-organic Au NPs can have an influence on lowering N-I phase transition temperature [28,29]. However, it should be noted that chemical composition of LCs, as well as shape, dimensions and applied surface coating [30] of doped NPs have a significant influence on N-I phase transition temperature. An interesting behavior of N-I phase transition temperature can be observed for BaTiO3 NPs which can lower this temperature when dispersed in 8CB LC [31] but can increase the temperature when dispersed in 5CB LC [32]. The current state-of-the-art in this topic has been reported by Orlandi [33]. Moreover, it was proven that the presence of Au NPs in a matrix of nematic LCs can change the switching times and lower the threshold voltage [34–38] that is of a particular interest in PLCF applications.
2. Experimental
The idea of infiltrating PCFs with an Au NPs doped 6CHBT has been reported previously by us elsewhere [39]. However, the main goal of research activities presented in this paper was to significantly improve the switching times of NP-doped LCs by using Au NPs of a much smaller size that is comparable to the size of 6CHBT molecules. It has been predicted that similar sizes of these two materials should improve anchoring and allow NPs to more strongly influence the electro-optical properties of LCs [40–42]. Similar behavior has been already reported in induced chiral nematics when a non-mesogenic chiral dopant of the length that matched the length of the 5CB host nematic LC significantly improved its magneto-optical properties [43]. In our experiments we used an isotropic PCF acronym PCF 061221, manufactured in UMCS (Lublin, Poland), with an outer diameter 135 µm, 6 rows of the holes and inner holes diameters 4.1 µm. The cross-section of the PCF is presented in Fig. 1.
The colloidal suspension of Au NPs in an organic solvent was prepared by two-phase redox reaction using a modification of the method reported elsewhere [44]. AuCl4— ions were transferred from aqueous solution to chloroform by using tetraoctylammonium bromide as a phase-transfer reagent and reduced with an aqueous solution of sodium borohydride in the presence of dodecanethiol. To remove excess reagents and other impurities, after reaction the organic phase was separated and washed four times with a diluted hydrochloric acid solution and water. Next, the chloroform phase was mixed with methanol (1:10 v/v) and centrifuged. The supernatant was decanted and the obtained precipitate of Au NPs was diluted in pure chloroform (Fig. 2). Concentration of such a prepared suspension equaled 1.2 mg/mL and obtained gold NPs had diameters in the range of 1-3 nm. The NPs-doped LCs with various Au NPs content were fabricated by mixing various volumes of the colloidal suspension of AuNPs with liquid crystal and evaporation of the solvent at 50 °C for 48 h.
Before PCF infiltration, NPs-doped LCs were stirred with ultrasounds in order to obtain a uniform structure to avoid possible aggregation. Afterwards, the LCs were heated above the 6CHBT clearing temperature and then infiltrated to PCF by capillary forces. We used five different concentrations of Au NPs: 0.01% wt., 0.03% wt., 0.05% wt., 0.1% wt., and 0.5% wt. The mixtures with higher concentrations of NPs were also prepared, but they showed too large density to be introduced into micro-holes of the PCF.
Initially, the thermal properties of NP-doped PLCFs were investigated. The experiment was divided into two parts: in the first one we used microcapillaries of 20-µm diameter as a host material and in the second part we repeated the same experiment while using a PCF as a host material. This splitting was necessary to confirm that the obtained results are mostly related to the properties of the NP-doped LC and not to a specific structure of the PCF. Microcapillaries were placed on a computer-controlled heating plate (Linkam THM6000) and observed in transmission under a microscope (Nikon Eclipse TS-2R) with crossed polarizers. The thermal resolution of the hot stage was 0.1°C.
3. Results and discussion
Selected observation results are presented in Fig. 3 and all transmission temperatures are shown in Tab.1. Figure 3 shows the phase transition process for undoped and an NP-doped microcapillary (diameter: 20 μm) with 0.5% wt. concentration of Au NPs. Because in cylindrical structures the phase transition takes more time than in LC cells, instead of one definite temperature we chose two temperatures describing the beginning and the end of phase transition. As the beginning of N-I phase transition temperature we chose the temperature for which isotropic droplets started to appear, and as the end of transition – the temperature when a stable isotropic phase is achieved. It is clearly seen that the phase transition of the microcapillary with an NP-doped LC occurs at lower temperature than for the microcapillary with an undoped LC.
Fig. 3 Pictures of a microcapillary infiltrated with an undoped (a, b, c), 0.01% wt. (d, e, f), 0.1% wt. (g, h, i) and 0.5% wt. Au NPs-doped 6CHBT LC (j, k, l) under a polarizing microscope with crossed polarizers.
Table 1. N-I phase transition temperatures for a 20-μm microcapillary and PCFs filled with different concentrations of Au NPs.
Afterwards, we replace the microcapillary with a PCF as the host material. The experimental setup for realization of this step is presented in Fig. 4.
Due to a complex structure of the PCF all observations were done by using the broadband light from a halogen lamp (Mikropak HL-2000) launched into the investigated PLCF and collecting the output light with an optical probe (Ocean Optics USB4000). To investigate the thermal effects, we implemented the Peltier module placed under the infiltrated part of the PLCF and the output light propagation spectra were recorded up to temperatures above the nematic/isotropic (N-I) clearing temperature. The temperature was measured by handheld thermometer (Testo AG, Model 735) whose resolution is 0.1°C. The sensing probe was placed on the Peltiere module in proximity to the investigated PLCF. In this host material we also observed a certain range of phase transition instead of one specific N-I phase transition temperature. In Fig. 5a are presented the results for an undoped PLCF and in Fig. 5(b), (c), (d) for an NP-doped PLCF with different NPs concentrations. In all these cases an increase of temperature up to about 40°C resulted in decreasing light intensity, the effect related to the N-I phase transition. It was observed that the flattening of the recorded spectra took place at different temperatures. For Au NPs the phase transition temperature is about 40 - 41.5°C and for the undoped LC it is about 43.2°C. Moreover, a similar effect occurs when we compare the spectra above the N-I phase transition. For the undoped 6CHBT the spectrum is stabilized at a temperature about 45°C. For doped samples the N-I phase transition occurs at temperatures 41.2 - 41.8°C and above these temperatures, the spectra are stable. In conclusion, the beginning of the transition to an isotropic phase takes place for a temperature of 40°C or 41°C (depending on a specific concentration of the dopant) and it is confirmed by macroscopic observation of the material. However, the analysis of the spectra suggests that the transition process is more complicated in PCFs and lasts until the temperature of 42°C is reached. A possible explanation of this process it that NPs start to create clusters and that is why they disrupt the nematic LC phase.
Fig. 5 Propagation spectra for thermal switching of PLCF with (a) undoped LC, (b) 0.01% wt. Au NPs, (c) 0.1% wt. Au NPs and 0.5% wt. Au NPs. Shifts of PBGs are visible in all cases.
The main part of our studies was to investigate whether the presence of Au NPs could improve (i.e. decrease) the switching times under the action of an external electric field. The experimental setup used in our studies is shown in Fig. 6.
In this experiment, as a light source a He-Ne laser operating at a wavelength of 632 nm was used since all the samples under investigation had relatively good propagation properties at this wavelength. The signal was collected by an optical power meter (Newport 2936-R) connected to a digital oscilloscope (LeCroy 104MXi). The infiltrated part of the investigated PLCF was placed between two electrodes connected to a function generator (Rigol DG1022A) through a voltage amplifier (FLC A800DI). The distance between electrodes was estimated to be ~140µm. As a rise (fall) time we assume the time required for a change in the light transmission change through the investigated PLCF sample from 10% to 90% (or vice-versa) of its maximal value when voltage from a signal generator is switched on (or off) in the range from 200V to 1500V. These values correspond to electric field intensities from 1.4 V/µm to 10.4 V/µm respectively. The obtained switching times for doped and undoped samples are compared in Fig. 7, Tab. 2, and Tab. 3.
Fig. 7 Rise times (a) and fall times (b) for undoped, 0.01% wt. Au NPs-doped and 0.5% wt. Au NPs-doped PLCF under an external electric field.
Table 2. Comparison of rise times for undoped and NPs-doped LCs. In brackets there are given percentage changes of the time value (rounded to the full value) compared to the rise time of an undoped sample.
Table 3. Comparison of fall times for undoped and NPs-doped LCs. In brackets there are given percentage changes of the time value (rounded to the full value) compared to the rise time of an undoped sample.
For 0.01% wt. concentration of NPs, the rise times for lower field values are about 24-26% faster than for an undoped PLCF. After rising the external voltage over 7.0 V/µm, the percentage change of time value drops to 10-11%. A similar effect can be observed with a much higher concentration of NPs. Using 0.5% wt. NPs in a PLCF we can achieve even an 82% improvement in the rise time compared to an undoped PLCF. Similarly, as in a lower concentration of NPs, the improvement also drops with a rise of amplitude of an external electric field, however the drop is much lower than for other mixtures. The relaxation (fall) times demonstrate a slightly different behavior. In both NP-doped samples we can observe lowering of the relaxation times (about 8% faster for 0.01% wt. NPs concentration and about 50% for 0.5% wt. concentration). However, contrary to the behavior of rise times, here an increase of electric field amplitude has a little impact on the speed of LC reorientation and relaxation times are still at a similar level. There is only a visible difference in value of switching times for PLCF with 0.01% and 0.5% concentrations of NPs. For example, for an electric field intensity of 10.4 V/µm the rise time is about 0.3 ms (11%) for 0.01% wt. NP-doped PLCF and about 1.88 ms (67%) for an 0.5% wt. NP-doped PLCF, faster than for the undoped PLCF. This effect could be related to electric constants of Au NPs. This material is characterized by relatively high conductivity, which with a small size of NPs used could influence the electric properties of the nematic LC. The influence of a metal NP on the electro-optical properties of a host material has been known for some time, but the real origin of the effect has not been fully revealed. We attribute this behavior to be related to the order parameter of the nematic LC. After doping of NPs to an LC, the order parameter around the NP is affected and, as a result, both the local order parameter and dielectric anisotropy are lowered.
4. Conclusions
To conclude, we have demonstrated that 2-nm gold NPs doped in a 6CHBT LC can influence the electro-optical properties of single silica-glass MCs and PLCFs. In comparison to our previous results reported in [39], a significant improvement (of the rate ~50%) for the rise and fall times has been observed for a PLCF infiltrated with a 6CHBT LC doped with 0.5% Au NPs with the maximal decrease that reached even 82% for the rise time. Moreover, a significant reduction of the Fréedericksz threshold voltage (up to 60%) has been observed for the same PLCF with 0.5% Au NPs in comparison to the undoped PLCF. Another conclusion is a decrease of the N-I phase transition temperature correlated to an increase of Au NPs concentration. This phenomenon can be related to a lower LC order parameter caused by the presence of NPs in a LC matrix, and is responsible for decreasing the threshold voltage. All obtained results hold great potential for prospective PLCF systems and devices in which lower switching voltages and faster response times are required to efficiently control the alignment of LC molecules.
Funding
National Science Center (NCN 2015/19/B/ST7/03650)
Acknowledgment
The authors acknowledge fruitful collaboration with Professor Roman Dąbrowski (Military University of Technology, Warsaw, Poland) and Dr. Paweł Mergo (UMCS, Lublin, Poland).
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