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Abstract
With the rapid development of terahertz (THz) technology comes the need to further explore the prospects for various applications of THz systems. Due to the strong need, components and equipment involving the exploration are indispensable. In order to find the most suitable material for THz technology, we selected three common materials for different 3D printing techniques—polyamide (PA), polylactic acid (PLA), and light-curable resin. After mixing each material with a quartz powder of a different weight percentage, we observed the change in absorption coefficients and refractive indices of the mixtures by THz time-domain spectroscopy (THz-TDS). The higher the ratio of a quartz powder to a mixture was, the smaller the absorption coefficient of the mixture would be. The optimum rate of change in the absorption coefficient was attained when the weight percentage of a quartz powder in a mixture was 50 wt%. At 1 THz of the measurement of THz-TDS, the average reduction in the absorption coefficients of the three different materials mixed respectively with a 50 wt% quartz powder was 39.17%. Besides reduced absorption coefficients, the mixtures’ refractive indices also changed as the weight percentage of a quartz powder in the mixtures varied. The PLA-based sample mixed with a 50 wt% quartz powder had the highest increase in the refractive index. Mixing quartz powders with materials, therefore, is an effective method to increase refractive indices and decrease absorption coefficients. The method can be applied in 3D printing techniques in the future to enhance the efficiency of THz components manufactured with 3D printing techniques.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Terahertz (THz) waves are electromagnetic radiation in the range of 0.1 THz to 10 THz. THz technologies have been advancing over these years, and the number of feasible applications of the technologies is also growing. In the future, THz waves are very promising to be applied to the fields including sensors [1–3], imaging [4–6], spectroscopy [7–9], communications [10–12], and security. However, THz technologies still face many challenges, including the lack of usable optical components and the very high price of optical equipment. All application systems related to THz technologies need passive components such as lenses [13,14], wave filters [15], waveguides [16,17], and waveplates [18,19] to be guided and operated. 3D printing is an optimal choice to solve the problems in this aspect.
3D printing is a method of additive manufacturing. It is easier to adjust the model size when creating a model for 3D printing. Thus, 3D printing is suitable for the production of customized commodities. Because 3D printing techniques construct an object by accumulating material together layer by layer, cutting tools are not needed. Therefore, no extra material is wasted, and thus the overall production cost is lower. Common 3D printing techniques include fused deposition modeling (FDM), powder bed fusion (PBF), and vat photopolymerization. The FDM process heats and melts a thermoplastic filament material, squeezes out the melted material through a nozzle, and deposits it layer by layer to form a model [20]. The PBF process paves the powder of a material on a construction platform and then fuses the powder with heat sources such as lasers and electron beams. The fusion techniques of the PBF process fall into two categories, Selective Laser Sintering (SLS) and Selective Laser Melting [21]. Vat photopolymerization shines visible light or UV light on the light-curable liquid resin in a trough to cure it. Different kinds of methods used in vat photopolymerization include stereolithography (SLA), digital light processing, light-emitting diode (LED) processing, continuous digital light processing, and continuous liquid interface production [22]. Different 3D printing techniques require different materials; the material and production method suitable for a particular 3D printing technique should be selected according to the application purpose of a model. Using 3D printing to produce THz optical components is a feasible method; however, the characteristics of 3D printing materials will limit the components’ performance. The removal of the materials’ drawback, high absorption coefficients, can significantly improve the optical performance of the 3D printed optical components.
The manufacturing of THz optical components has been relying on microelectromechanical manufacturing techniques; the costs of the manufacturing process and equipment applying the techniques are high. However, manufacturing components using 3D printing is fast and inexpensive. In recent years, 3D printing techniques have been applied to fabricate optical components needed for the THz waveband, such as lenses [23–25], hyperlenses [26–28], and waveguides [29–31]. The enhancement of 3D printing techniques and the improvement in 3D printing materials will boost the performance of THz optical components. Nevertheless, the refractive index, absorption coefficient, and printability of a 3D printing material must be taken into account in terms of the optical application of transmission. For example, a high refractive index and a small absorption coefficient are crucial to the realization of high-performance and wide-bandwidth THz lenses. As a common material for the FDM process, Poly Lactic Acid (PLA) has good mechanical characteristics and is thermoplastic and biodegradable which makes it very eco-friendly [32]. Polyamide (PA) is also thermoplastic. The rigidity and tenacity of PA are better than those of other sorts of plastics; the advantages make it another common material for the FDM process. However, warpage tends to occur in PA parts after they take shape; some researchers, therefore, have mixed PA with other materials to rectify this disadvantage [33]. Additionally, the difference between the melting temperature and the crystallization temperature of PA12 is huge; the huge temperature difference, thus, makes PA12 a common material for 3D printing that applies the SLS technique [34]. Types of resins applied in SLA printing for commercial purposes are very limited; they must be in the liquid state and be hardened rapidly after being shone by light. The initially developed resins could form glassy networks caused by the light-induced polymerization and crosslinking of polyacrylate or epoxy macromers, which are the main compositions of resins. In order to enhance the mechanical strength of the components made of light-curable resin, researchers have been developing other sorts of resins in recent years [35]. Quartz is often used in the optical and semiconductor industries; its main constituent is silicon dioxide (SiO2). When we measure the THz time-domain spectrum (THz-TDS) of a glass component made of fused quartz at 1THz, its refractive index is approximately 1.96, and its absorption coefficient, approximately 5 cm-1 [36]. If we use SiO2 glass or quartz to manufacture optical lenses, the costs are higher. If we adopt plastic materials for 3D printing to manufacture optical lenses, the absorption coefficients of the lenses are higher within the THz waveband [37]. Therefore, we evenly mixed quartz powders of different proportions as the dispersed phase into polymeric materials to fabricate optical components. Next, we measured the components’ THz optical characteristics to find out the mixing ratio that gives a component a high refractive index and a low absorbance. With the discovery of the optimal mixing ratio, we aim to enhance the performance and quality of 3D printing materials by rectifying their disadvantage of high absorbance for THz radiation.
In this study, we mixed quartz powders of different weight percentages into PLA powders, PA powders, and photosensitive resin, respectively. By the measurement of an X-ray diffractometer (XRD), we observed the variance in diffraction peaks of mixtures with different mixing ratios to understand the mixtures’ crystallinity. Next, by the measurement of TDS, we analyzed the variance in the refractive indices and absorption coefficients of the mixtures. Our deduction was based on the Effective Medium Theory, an analytical model that describes the macroscopic characteristics of composite materials. By calculating the averages of each constituent’s characteristics, we could obtain the overall characteristics of a composite material. Due to the characteristics of quartz powder, we can expect that, compared with the optical components made of pure polymeric powder or pure resin, the optical components made of the mixtures of the mentioned 3D printing materials and the quartz powders of different weight percentages will have smaller absorption coefficients and higher refractive indices in the THz waveband.
2. Experimental method
Two groups of research samples were tested in this study. Samples in Group 1 were ingots of the compacted PLA and PA powders respectively mixed with quartz powders of different mixing ratios. Group 2 included the photosensitive resin samples respectively mixed with quartz powders of different mixing ratios. Firstly, each quartz powder of a different mixing ratio in Group 2 was evenly mixed into each uncured liquid resin basis. Next, we cured each mixture with UV light. Finally, the light-cured mixtures were used as our Group 2 samples. By conducting TDS measurement on the samples of both groups, we compared the influence cast by different substrate materials, different mixing ratios of quartz powders, and different material-shaping methods upon the characteristics of a THz optical component.
Firstly, we mixed 10 wt%, 20 wt%, 30 wt%, 40 wt%, and 50 wt% commercial quartz powders into PLA and PA powders, respectively. To assure that the mixtures were evenly mixed, we ground and blended all the powders of different weight percentages with stainless steel balls in a ball mill (Retsch MM 400). The ball mill’s high vibration frequency could reduce the particle size of the powders by collision. Because PLA and PA are thermoplastics, the heat generated by the collision in a long period will fuse their powder together. Therefore, the ball milling process was divided into two phases. In Phase 1, we set the ball mill to operate at 15 Hz for 30 seconds; then, we turned it off for a period to cool down the powder. In Phase 2, the ball mill was set to operate at 5 Hz for three minutes. The powders had to be compacted into ingots before we conducted TDS measurement. We put the powders in a stamping die at different times to compact them separately; the mold in the die was a cylinder trough with a diameter of 13 mm. Samples were compacted with a consistent pressure of 9 tons. The thickness of a sample was between 2.58 mm to 3.95 mm.
The light-curable resin used in this study was Clear Resin produced by Phrozen. This type of resin is a transparent liquid; its main constituents are acrylate, a photoinitiator, and pigment. 10 wt%, 20 wt%, 30 wt%, 40 wt%, and 50 wt% quartz powders were respectively mixed into the resin. A magnetic stirrer was used to mix each quartz powder and the resin evenly. The spinning speed of the magnetic stirrer was set at 800 rpm for one hour. After being stirred for one hour, the evenly mixed liquid mixture was cured with a UV lamp for 30 minutes until it hardened and turned into a cuboid with a length of 20 mm, a width of 20 mm, and a height of 1.66 mm to 2.5 mm. It is worth noticing that as the mixing ratio of the quartz powder increased, the liquid mixture’s color was closer to opaque yellowish pink, and the mixture’s viscosity was higher.
In this study, we used a seed laser (Vitara, Coherent) to introduce a laser amplifier (Legend Elite HE+ USp-5K-III, Coherent) to amplify the energy, which can provide a pulse width of 40 fs, an average power of 8.2 W, and a repetition rate of 5 kHz at 800 nm. In addition, we adopted the system of THz-TDS [38] constructed by four standard off-axis parabolic mirrors to conduct measurement. Once zinc telluride (ZnTe) was used as the excitation source of THz radiation, and another, the receiving source. The mechanism of electro-optic sampling was applied to detect THz signals. We measured the samples’ optical constants in the frequency range of 0.3 THz to 1.0 THz with THz-TDS; the signal-to-noise ratio (SNR) was about 105. The definition of the system’s SNR is the power of a sample’s signal in the frequency domain divided by the power of a reference signal in the frequency domain. Figure 1(a) and Fig. 1.(b) are THz field of air and samples.
Fig. 1. The temporal waveforms through the reference and samples of (a) PLA sample with the different percentages of quartz, and (b) PA sample with the different percentages of quartz.
Because the thickness of our samples in this experiment was thick enough, the waveform revealed by the reflection could be clearly identified from the THz time-domain spectrum, and the samples’ optical constants, thus, could be calculated with an analysis algorithm [38,39]. In general, assume that THz radiation is a plane wave passing through a sample and air, Esample(ω) and Ereference(ω), respectively, we can obtain the transmission coefficient as Eq. (1),
3. Results and discussion
We used an XRD (Bruker, D8 discover) to measure compacted ingot samples of mixtures of PLA powders and quartz powders, compacted ingot samples of mixtures of PA powders and quartz powders, and light-cured resin samples that had been mixed with quartz powders before being cured, and obtained the samples’ X-ray diffractograms as included in Fig. 2. Figure 2(a) presents the diffractograms of the pure PLA powder as well as mixtures of a PLA powder and a quartz powder of a different mixing ratio. The pure PLA powder sample had obvious diffraction peaks at 19.56° and 22.52°. Figure 2(b) displays the diffractograms of the pure PA powder as well as mixtures of a PA powder and a quartz powder of a different mixing ratio. The diffraction peaks of the pure PA powder sample appeared at 20.1° and 23.9°. Figure 2(c) shows the diffractograms of a light-cured resin sample as well as light-cured resin samples that had been mixed with quartz powders before being cured. Pure resin is amorphous; its diffraction peak stretched wide from 12° to 25°. Quartz powder has extraordinary crystallinity; therefore, after it was mixed into the resin, diffraction peaks characteristic of quartz appeared. As the mixing ratio of a quartz powder increased, the strength of diffraction peak signals became stronger. By comparison, the signals of PLA, PA, and resin were much weaker.
Fig. 2. XRD diffractograms of (a) pure PLA and mixtures of PLA powders and quartz powders of different mixing ratios, (b) pure PA and mixtures of PA powders and quartz powders of different mixing ratios, and (c) pure resin and mixtures of resin and quartz powders of different mixing ratios.
By calculation of an analysis algorithm, we can obtain all the samples’ optical constants, including their transmittances, refractive indices, extinction coefficients, and absorption coefficients. We cross-analyzed two groups of sample signals and two groups of reference signals and then obtained the data graphs provided in this study. In terms of the pure PLA sample and PLA-based samples, we applied TDS to measure each of them. The PLA-based samples were mixed with 10 wt% to 50 wt% quartz powders, respectively. The results are presented in Fig. 3. From Fig. 3(a), Fig. 3(c), and Fig. 3(d), we can see the higher a quartz powder’s mixing ratio, the lower the absorption coefficient, and the higher the transmittance. The sample containing 50 wt% quartz powder had a lower absorption coefficient and a lower extinction coefficient. The calculation result matched a characteristic of quartz, having a smaller absorption coefficient for THz radiation. Figure 3(b) shows that as the mixing ratio of quartz powder increased, the refractive index increased as well. The sample containing 50 wt% quartz powder had a higher refractive index. We extracted the data of refractive indices and absorption coefficients from Fig. 3(b) and Fig. 3(d) and produced Table 1. The PLA-based sample containing 50 wt% quartz powder had the lowest absorption coefficient, which was 4.04 ± 0.16 at 0.5 THz; even when the frequency reached 1 THz, the absorption coefficient was only 12.30 ± 0.16. Compared with the pure PLA sample, the absorption coefficient of the PLA-based sample containing 50 wt% quartz powder dropped 30.54% at 0.5 THz and 36.14% at 1 THz. The sample containing 50 wt% quartz powder had the highest refractive index, which was 1.68 ± 0.003 at 0.5 THz and 1.67 ± 0.001 at 1 THz. Compared with the pure PLA sample, the refractive index of the PLA-based sample containing 50 wt% quartz powder increased respectively 5.36% and 5.60% at 0.5 THz and 1 THz. These experimental results are very beneficial for the manufacturing of THz lenses in the future. The utilization of quartz’s refractive index will increase a composite material’s refractive index and contribute to the production of THz optical components that are much thinner. Figure 3 and Table 1 demonstrate that the optimal absorption coefficient and the optimal refractive index respectively belonged to the PLA-based sample containing 50 wt% quartz powder. When fabricating a THz optical component, it would be advantageous to select a proper mixing ratio according to the application purposes a manufacturer wishes to serve.
Fig. 3. Change in PLA-based samples’ (a) transmittances, (b) refractive indices, (c) extinction coefficients, and (d) absorption coefficients as the percentage of quartz increased.
Table 1. The refractive indices and absorption coefficients of PLA-based samples with different mixing ratios of quartz powders.
Regarding the pure PA sample and PA-based samples, after mixing each quartz powder of a different mixing ratio into each PA powder basis and compacting all samples into ingots, we used TDS to measure each ingot sample. The results are displayed in Fig. 4. From Fig. 4(a), Fig. 4(c), and Fig. 4(d), we can see the higher a quartz powder’s mixing ratio, the lower the absorption coefficient. The PA-based sample containing 50 wt% quartz powder had the lowest absorption coefficient and extinction coefficient; these results were significantly superior to those of the pure PA sample. Figure 4(b) shows the PA-based sample containing 50 wt% quartz powder had the largest refractive index. The data of refractive indices and absorption coefficients extracted from Fig. 4(b) and Fig. 4(d) were organized in Table 2, which shows that the PA-based sample containing 50 wt% quartz powder had both the optimum refractive index and the optimum absorption coefficient. The sample’s refractive index could reach up to 1.65 ± 0.002 at 0.5 THz and 1.65 ± 0.002 at 1 THz, which was respectively 3.46% and 3.28% higher than the pure PA sample’s refractive index at 0.5 THz and 1 THz. Its absorption coefficient dropped to 2.98 ± 0.17 at 0.5 THz and 12.42 ± 0.17 at 1THz, which was respectively 27.24% and 39.13% lower than the pure PA sample’s absorption coefficient at 0.5 THz and 1 THz.
Fig. 4. Change in PA-based samples’ (a) transmittances, (b) refractive indices, (c) extinction coefficients, and (d) absorption coefficients as the percentage of quartz increased
Table 2. The refractive indices and absorption coefficients of PA-based samples with different mixing ratios of quartz powders.
Concerning the pure light-cured resin sample and light-cured resin-based samples, similarly, the higher the mixing ratio of a quartz powder in a sample, the lower the sample’s absorption coefficient, as shown in Fig. 5(a), Fig. 5(c), and Fig. 5(d). There was no significant difference between the absorption coefficients of the samples that contained respectively 40 wt% and 50 wt% quartz powders, and neither was there between the extinction coefficients of both samples. The absorption coefficients and extinction coefficients of the two samples were also lower than those of the other samples. Nevertheless, as shown in Fig. 5(b), the refractive index of the sample containing 50 wt% quartz powder was slightly higher than that containing 40 wt% quartz powder. The data of refractive indices and absorption coefficients extracted from Fig. 5(b) and Fig. 5(d) were organized in Table 3, which shows that the refractive index of the resin-based sample containing 50 wt% quartz powder could reach up to 1.67 ± 0.002 at 0.5 THz and 1.66 ± 0.003 at 1 THz, and the absorption coefficient of the sample dropped to 5.87 ± 0.19 at 0.5 THz and 12.18 ± 0.19 at 1THz. The refractive index of this sample was about the same as that of the pure resin one; however, the absorption coefficient of this sample was 21.56% lower at 0.5 THz and 42.24% lower at 1 THz than that of the pure resin sample at the same frequencies.
Fig. 5. Change in resin-based samples’ (a) transmittances, (b) refractive indices, (c) extinction coefficients, and (d) absorption coefficients as the percentage of quartz increased
Table 3. The refractive indices and absorption coefficients of resin-based samples with different mixing ratios of quartz powders
To synthesize the measurement results, Fig. 6 shows from the comparison of the refractive indices and absorption coefficients of the PLA-based, PA-based, and light-cured resin-based samples at 1 THz that contained quartz powders of different mixing ratios, we obtained that the mixture of PLA and quartz powder had the highest refractive index among all the mixtures of the three different kinds of bases that contained 50 wt% quartz powders. Because the refractive index of resin is higher than those of the other two polymeric materials, the efficacy of the enhancement was less obvious. Figure 6(a) and Fig. 6(c) show that the refractive index of PA-based and PLA-based samples increased as the mixing ratio of quartz powder. It is noticed that the refractive index of a resin-based sample decreased as the mixing ratio of quartz powder increased. The resin-based sample containing 30 wt% quartz powder had the lowest refractive index. However, as the mixing ratio of quartz powder exceeded 30 wt%, the higher the mixing ratio of quartz powder contained in a sample, the higher the sample’s refractive index, as shown in Fig. 6(f). Concerning exposure time as a light-curing parameter, each light-curable resin-based sample with a different mixing ratio of quartz powder requires a different duration of exposure time to reach its own optimum curing quality. Therefore, when the exposure time of a light-curing process is set to last 30 minutes as it was in our experiment, the sample with 30 wt% quartz powder will have the least compact density after the curing process. From Fig. 6(b), Fig. 6(d), and Fig. 6(f). The absorption coefficient is PA contained 50% quartz powders as the lowest, and the change rate is the most with resin contained 50% quartz powders.
Fig. 6. is the (a)(c)(e) refractive index and (b)(d)(f) absorption coefficient of each PLA-based, PA-based and resin-based sample mixed with a quartz powder of a different weight percentage when measured at 1 THz.
4. Conclusion
The convenience of 3D printing will help fabricate optical components for the THz waveband; therefore, we attempted to enhance several kinds of common materials used for 3D printing by mixing quartz powders of different mixing ratios into them to understand their optical characteristics. Compared with polymer materials, the quartz has lower absorption coefficients and higher refractive indices. By applying the Effective Medium Theory, absorption coefficients decrease and refractive indices increase since the percentage of quartz weight increases. To synthesize the measurement results, the absorption coefficients of PLA-based, PA-based, and light-cured resin-based samples could have an average decrease of 39.17% at 1THz when the mixing ratio of quartz powder mixed into them was 50 wt%. Among the three kinds of materials, the mixture of light-cured resin and 50 wt% quartz powder had the largest decrease 42.24% in the absorption coefficient. The same mixture also had the highest refractive index; however, the mixture of PA powder and 50 wt% quartz powder had the largest increase in the refractive index. Mixing quartz powder into a material, therefore, is an effective way to increase its refractive index and decrease its absorption coefficients, and this method also applies to the light-curable resin used in SLA printing as well.
Funding
Ministry of Science and Technology, Taiwan (107-2112-M-003-014-MY3, 107-2221-E-003-008, 109-2112-M-007-033-, 110-2112-M-003-012-MY3, 110-2224-E-006-006, 110-2923-E-007-006).
Acknowledgments
The authors would like to thank Prof. Tun-Ping Teng for lending us Ball Mill, Prof. Ci-Ling Pan for providing some optical components, and Mr. Yi-Sheng Cheng for optimizing the THz-TDS system.
Disclosures
The authors declare no conflicts of interest.
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.
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