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Abstract
Generation of terahertz (THz) radiation has become increasingly accessible over time. The research and development of THz applications is ongoing, often requiring the use of THz compatible optical components. At the same time, rapid prototyping three-dimensional (3D) printing systems are now commercially accessible and are capable of printing resolutions on the order of the THz wavelengths. Thus, 3D printing systems can be leveraged for use in creating novel THz optical components in varied experiments and applications. The filament material used in 3D printing systems dictates the absorptive and refractive properties of the 3D printed THz optical component. The commercial release of filament materials is continuous, year after year, inducing a need for literature to stay current with characterizing these filament materials over the THz gap. We use terahertz time-domain spectroscopy (THz-TDS) to characterize the absorptive and refractive properties of 3D printing materials over the THz gap. We present a consolidative and comprehensive aggregation of THz-TDS measurements of twenty-three 3D printing materials. A comparison of THz-TDS measurements for thick and thin samples provides verification of measurement accuracy. The measured THz bandwidth of these samples is extended by up to 1.2 THz. Furthermore, to the authors’ best knowledge, the proposed work puts forward the first THz-TDS measurements of polyvinyl butyral, polyetherimide, and low temperature polycaprolactone filament materials. This work primarily focuses on fused deposition modeling (FDM) 3D printed materials, rather than stereolithography (SLA) 3D printed materials.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The terahertz (THz) gap represents frequencies of 0.1–10 THz, which corresponds to wavelengths of 3000–30 µm [1]. The generation and detection of THz radiation has been challenging for traditional radiofrequency and photonic technologies, due to the unique wavelength and low photon energies of THz radiation. Fortunately, through the innovative work of Austin et al. [2] and other researchers, the THz gap became increasingly accessible. Terahertz emitters now make use of photoconductive switching [3] and wave-mixing [4], while THz detectors make use of photo-gating [5] and electro-optic effects [6]. At the same time, researchers have introduced a variety of THz optical components, including lenses [7], diffraction gratings [8], and photonic crystals [9].
Terahertz optical components are constructed from materials that are known to be THz compatible, such as quartz [10], silicon [11], and polytetrafluoroethylene (PTFE) [12]. These THz compatible materials are often implemented with predefined/standard geometries, as shaping of these materials for THz research can be challenging. Fortunately, such a challenge can be mitigated by fabrication equipment that is capable of rapid prototyping [13]. Emergence of novel materials for rapid prototyping is ongoing [14], with characterization of these materials in the THz gap being advantageous for assessing potential use in THz applications [15].
In parallel to the above THz advancements, rapid prototyping has been revolutionized by the advent of commercial three-dimensional (3D) printing systems, enabling enhanced component development in mechanical engineering fields [13]. Rapid prototyping 3D printing systems commonly make use of additive processes: fused deposition modeling (FDM) [16] and stereolithography (SLA) via liquid polymerization [17].
The FDM additive process utilizes filament materials that are melted down and continuously extruded into successive layers to print a component. The typical filament materials used for FDM 3D printers are polylactic acid (PLA) filament material [18], acrylonitrile butadiene styrene (ABS) filament material [19], and polyethylene terephthalate glycol (PETG) filament material [20].
The SLA additive process uses high photon energy light (i.e., ultraviolet light) to solidify photosensitive liquid polymer resins (i.e., photopolymers) into successive structural layers to print a component via liquid polymerization. The liquid polymerization is often performed using the laser based stereolithography (L-SLA) technique, or the digital photomask based stereolithography technique (M-SLA), the latter utilizing a programmable liquid crystal display (LCD) photomask coupled with light emitting diode (LED) illumination. Typical 3D printed materials that are used for liquid polymerization based L- and M-SLA 3D printers are acrylate [21] and methacrylate [22] photopolymer resins. The FDM and SLA 3D printing systems are capable of printing resolutions on the micrometer scale, which is being leveraged in radiofrequency and photonics research applications [23,24]
The spatial resolution of modified FDM 3D printers can be as low as 100 µm [25] (with ubiquitous consumer grade systems achieving 250 µm resolution) and the spatial resolution of industrial grade SLA 3D printers can be below 20 µm [26] (with 100 µm resolution being common with ubiquitous consumer grade systems). These lower limits for feature size of 20 to 100 µm are comparable to wavelengths in the THz spectrum. This makes 3D printing systems well suited to the fabrication of THz optical components.
Photonics researchers have begun to leverage 3D printing systems for the development of THz optical components. For example, Hernandez-Serrano et al. presented gradient-refractive-index THz lenses [27], Guerboukha et al. demonstrated planar porous THz phase plates [28], and Chudpooti et al. constructed hemispherical THz antennas [29]. Additionally, 3D printing and additive manufacturing are being used in terahertz applications including in-core sensing [30], topological waveguides [31], and plasmonic circuits [32]. To complement these investigations, there have been studies on absorptive and refractive properties of 3D printed materials [33,34]. However, the literature has not kept pace with the growing number of 3D printed materials. These studies can also benefit from extensions of bandwidth towards higher THz frequencies.
In this work, we provide an updated material study of THz time-domain spectroscopy (THz-TDS) measurements on many standard and non-standard 3D printed materials. In particular, our results can be used to assess if the presented 3D printing materials lend themselves to use as THz compatible materials. In our study, we present measurements over an extended THz bandwidth with multiple sample thicknesses. We present the methodologies, measurements, and results for various groups of 3D printing materials: PLA filament materials; common alternatives to PLA filament materials; flexible filament materials; dissolvable support filament materials; nylon filaments materials; packaging thermoplastic filament materials (i.e., polypropylene (PP) filament material, high density polyethylene (HDPE) filament material); and non-standard filament materials. Such work yields several contributions. First, it can provide a comprehensive and consolidative set of THz-TDS measurement, where previously THz-TDS measurements were spread across many studies for FDM filament materials [33,34] and for SLA resin materials [35]. Second, it offers a comparison of thick and thin sample THz-TDS measurements, for verification of measurement accuracy of thick and thin samples, particularly when approaching sample thicknesses above and/or below the dopt = 2/α(f) optimal sample thickness for free-space THz-TDS measurements. (In the previous in-line equaiton dopt is the optimal sample thickness and α(f) is the absorption coefficient as a function of frequency [36].) Third, it extends the measured bandwidth by up to 1.20 THz beyond prior studies, e.g., previous reports provide a maximum THz-TDS bandwidth of 1.80 THz for ABS filament material, high impact polystyrene (HIPS) filament material, and HDPE filament material [34]. In contrast, we show extended bandwidth maximum values of 2.56 THz, 2.66 THz, and 3.00 THz for ABS filament material, HIPS filament material, and HDPE filament material, respectively. Fourth, it gives the first (to the authors’ knowledge) THz-TDS measurements of polyvinyl butyral (PVB) filament material, low temperature polycaprolactone (LT PCL) filament material, and polyetherimide (PEI) filament material. This work primarily focuses on FDM 3D printed materials, rather than SLA 3D printed materials.
2. Terahertz time-domain spectroscopy
2.1 Experimental method and THz-TDS system configuration
A collection of twenty-three 3D printing filament materials are assessed for THz absorptive and refractive properties using the THz-TDS method. When possible, the 3D printing filament materials are chosen to have the corresponding natural filament colour, or otherwise with a white filament colour. These material samples are printed from commercially available 3D printer filament into 25.4 mm diameter disks at an approximate target thickness for two of the following dimensions: 300 µm, 700 µm, 1400 µm, or 2100 µm. The FDM 3D printer is configured to operate at an infill ratio of one hundred percent, producing an approximately solid sample disk. The samples are printed with a generic FDM 3D printer (IIIP 15365) capable of a maximum extruder temperature of 250°C, an X-Y axis resolution of 200 µm, and a Z-axis layer height configuration set to 175 µm. The sample and reference THz electric field measurements are taken in a nitrogen enclosure to mitigate water vapor effects in the THz frequency spectrum.
The THz-TDS method is performed in free space, using the THz system configuration displayed in Fig. 1(a). A 3D computer rendered image of the THz-TDS system is shown as an experimental optomechanical setup on an optical table, with two distinct laser beam paths—the pump beam (Pump), required for photoconductive THz emission, and the probe beam (Probe), required for electro-optic THz detection. The exact component configuration of this system is shown by the top view image in Fig. 1(b). Here, two 780 nm wavelength laser beams, being the pump and probe beams, are generated and sent onto the optical table via an ultrafast Ti:Sapphire pulsed laser (Spectra-Physics Mai Tai HP, MKS Instruments Inc., Massachusetts, USA), with a rated pulse duration of <100 fs, and rated repetition rate of 80 MHz.
The pump beam is directed into a microscope objective (MO) and focused onto the electrodes of a gallium arsenide THz photoconductive emitter (PE), biased with a 100 Vpk-pk square wave of known frequency, fbias. The obtained THz signal from the electro-optic detection is from lock-in detection to fbias. The PE, excited by the pump beam, generates THz radiation (shown in yellow) in a point source emission pattern. This point source of THz radiation is projected onto a parabolic mirror (PM) and shapes the THz point source into a collimated THz beam path. The collimated THz beam path is then incident upon and passes through a sample of 3D printing material (Sample) and continues to pass through a pellicle beam splitter (BSpel). At the same time, the probe beam is directed using standard mirrors (M) onto the BSpel and overlapped spatially with the THz beam path—and subsequently is overlapped temporally using an optical delay stage (IMS600CCHA, MKS Instruments Inc., Massachusetts, USA; not shown on figure).
Both the THz beam (THz) and the probe beam are focused by the second PM onto a zinc telluride electro-optic crystal (EO) that is cut to <110 > orientation. Consequently, electro-optic detection is achieved, where the magnitude of the incident THz radiation is inferenced by a proportional change of probe beam polarization state within the EO crystal. The modulated probe beam is now recollimated by a lens (L) projected onto a quarter-wave plate (QW). The now circular or elliptical polarization of the probe beam (in the presence of THz) is induced by the phase delay of a quarter wavelength, with the horizontal or vertical polarization components of the probe beam separated by a polarizing beam splitter (BSpol). The polarization components of the probe beam are measured by a balanced silicon photodiode (PDSi). The measured optical signal is differentiated and transduced by the PDSi for isolation and acquisition from a lock-in amplifier at fbias (SR830, Stanford Research Systems, California, USA; not shown on figure).
Fig. 1. (a) A three-dimensional (3D) computer-aided design (CAD) model of the terahertz time-domain spectroscopy (THz-TDS) setup is shown upon an optical table. (b) A top-view of the 3D CAD model of the THz-TDS setup is shown. The abbreviation labels are as follows: pump laser beam (Pump), probe laser beam (Probe), microscope objective (MO), photoconductive emitter (PE), parabolic mirror (PM), THz beam path (THz), 3D printed sample material (Sample), pellicle beam splitter (BSpel), electro-optic crystal (EO), lens (L), mirror (M), quarter wave plate (QW), polarization beam splitter (BSpol), and balanced silicon photodiode (PDsi).
2.2 THz-TDS method
Terahertz time-domain spectroscopy is a unique measurement technique because it measures absorption and refraction of a sample over the THz gap. In this study, THz-TDS is applied in a transmission configuration. The electric field of the sample THz pulse is denoted in the time- and frequency-domain as Esam(t) and Esam(f), respectively. The electric field of the reference THz pulse is denoted in the time- and frequency-domain as Eref(t) and Eref(f), respectively. The ratio of the magnitude spectra for the reference and sample THz pulses reveals the absorption spectra (i.e., absorption coefficient) of the sample material as
The maximum measurable absorption spectra has a reciprocal relationship to the sample thickness and its intersection with the absorption spectra defines the maximum measurable frequency. Measurements beyond the maximum frequency are inaccurate [37]. Therefore, it is beneficial to perform multiple measurements at two sample thicknesses. The thinner sample thickness will show a higher maximum frequency. The thicker sample thickness will provide a secondary validation of the measurement across the common frequency spectra, i.e., up to the maximum measurable frequency of the thicker sample.
Two measurements on two different thicknesses is also important because it can reveal any variation in the measurement, although variation between measurements is minimal throughout this study. An example measurement is shown of the well-characterized PLA filament material to demonstrate the above concepts. Figure 2 shows the absorption spectra and the maximum measurable absorption spectra for sample thicknesses of approximately 1400 µm (thick) and 700 µm (thin). Note from this figure that the results for the thick and thin PLA filament materials extend up to different maximum measurable frequencies, at 1.4 to 1.7 THz, respectively, due to their overall losses, at high and low levels, respectively. This distinction will be seen throughout the remainder of this work. The results of Fig. 2 are in agreement with other measurements of the well-studied PLA material [33,34]. This comparison to literature reveals that the THz-TDS setup is well calibrated.
The oscillations present in the maximum absorption coefficient curves of Fig. 2, i.e., the αmax blue and red dashed lines, are due to reflections from the electro-optic crystal which is a <110> ZnTe crystal of 500 µm thickness. An estimate of the free-spectral range (FSR) associated with this oscillation shows the FSR to be approximately 0.1 THz, and this is consistent with the 0.1 THz spacing of local maxima/minima in the oscillations of the Fig. 2 curves. This FSR effect is much less in subsequent absorption coefficient curves, as these calculations divide amplitude of sample and reference measurements, thus largely removing the FSR effect.
The refractive index spectra, while not shown in Fig. 2, can be found from the phase spectra of the sample and reference THz pulse, as
Ultimately, to use any 3D printed material as a THz optical component, one must have knowledge of its overall absorptive and refractive properties. Therefore, we chose to perform THz-TDS on a 3D printed sample, rather than on its fundamental material. The printed sample will show the ultimate properties of the fabricated device. Therefore, the results that are presented will be representative of a realistic printed object and characterize the overall effective absorption and refraction of the system. Any variation in absorption measurements is minimal, and we performed the measurement with multiple thicknesses, with similar absorption and refraction characteristics observed. Effects of the gap between each layer of samples of 3D materials has been previously studied for the THz regime [39]. Naftaly et al. states that although there will be both higher loss coefficient and scattering loss for 3D printed materials, these effects will counteract each other.
Fig. 2. The absorption coefficient (solid curves) and alpha max. (dotted curves) are each shown for polylactic acid (PLA) filament material with approximate sample thicknesses of 700 µm (blue) and 1400 µm (red).
3. Results and discussion
3.1 Polylactic acid filament materials
Polylactic acid is a common filament material for consumer grade FDM printers [40]. In Fig. 3 we compare the standard PLA filament material with non-standard PLA filament materials that are manufactured with blended metallic particles ranging from five to eight percent by volume. The samples are fabricated with FDM. The standard PLA of Fig. 3(a) is measured with a bandwidth of B = Δf = fmax – fmin = 1.76 THz – 0.20 THz = 1.56 THz. (The bandwidth for each filament material can be seen in Table 1.)
Table 1. Summary of absorption coefficient, refractive index, maximum permissible frequency, minimum and maximum measured THz frequency, and measured THz bandwidth.
This PLA filament material is compared for filament materials blended with copper (PLA Cu) in Fig. 3(b), blended with iron (PLA Fe) in Fig. 3(c), and blended with tungsten (PLA W) in Fig. 3(d). The corresponding absorption coefficients and refractive indices are displayed for target sample thicknesses being approximately 700 µm and 1400 µm. The plots for the absorption coefficient and refractive index are displayed up to the maximum measurable frequency (as the intersection of absorption coefficient and alpha max.). Similar absorptive properties are observed across these samples in Fig. 3. There is a notable change in refractive index between the PLA filament material and PLA Cu filament material, from 1.54 to 1.79, respectively.
Fig. 3. The absorption coefficient (blue curves) and refractive index (green curves) are shown for the approximate sample thicknesses of 700 µm (lighter respective colour) and 1400 µm (darker respective colour) for the following sample materials: (a) polylactic acid (PLA) filament material, (b) PLA filament material with embedded copper (PLA Cu) particles, (c) PLA filament material with embedded iron (PLA Fe) particles, and (d) PLA filament material with embedded tungsten (PLA W) particles.
The absorption coefficient and refractive index are important parameters for the application of 3D printed materials for the fabrication of THz optical components. Typically, a material with higher refractive index and lower absorption is desirable for optimal performance of 3D printed THz optical components [41]. This is because higher refractive index materials will refract light at more extreme angles than those of lower refractive index materials for the same thickness of material. Thus, the desired THz beam shaping can be accomplished with less material and less absorption of THz radiation, allowing higher efficiency of THz transmission. For our work and its application, we deem that a 3D printer filament material will be effective for use in THz optical components only up to a maximum permissible frequency, wherein the absorption coefficients at or below this frequency are less than 10 cm-1. This 10 cm-1 value comes about through an analysis of a nominal THz optical component. Such a component may have an order of magnitude greater size than a 300 µm wavelength (corresponding to 1 THz frequency), being 3 mm. Under these conditions, full absorption (defined here as being attenuation of intensity below five percent of initial value) will occur when absorption coefficient is 10 cm-1. Thus this 10 cm-1 absorption coefficient represents a threshold between THz-desirable and THz-undesirable materials. For materials exhibiting comparable absorption coefficients, we deem the material with the higher refractive index to be preferable.
The results of the THz-TDS of PLA filament material, PLA Cu filament material, PLA Fe filament material, and PLA W filament material indicate that these materials can be effective for use in THz optical components up to maximum permissible frequencies of 0.54 THz, 0.48 THz, 0.46 THz, and 0.52 THz, respectively.
In Fig. 4, we compare two non-standard PLA filaments: PLA filament material embedded with pine tree particles (PLA Pine) in Fig. 4(a); and PLA filament material embedded with electrically conductive particles (PLA Cond.) in Fig. 4(b). The samples are fabricated with FDM. The corresponding absorption coefficients and refractive indices are displayed for target sample thicknesses of approximately 700 µm and 1400 µm. The plots for the absorption coefficient and refractive index are displayed until the absorption coefficient intersects alpha max. Due to limitations manifesting from high absorption coefficient and/or low alpha max., the PLA Cond. filament material is measured for a single sample with a thickness of approximately 300 µm. This is because thicker samples of PLA Cond. filament material reach alpha max. at the minimal measurable THz frequency. The PLA Cond. filament material has a high refractive index (i.e., 2.38), but its absorption is not ideal for THz optical components needing strong transmission [41]. Nonetheless, such a material may be of use below the THz spectrum, i.e., below 0.3 THz, where its absorption is low [41]. Continuous wave THz sources could be used here at frequencies of 0.1 THz, 0.2 THz, or 0.3 THz (Terasense Group, Inc. San Jose, California, USA). These match the aforementioned lower absorption range of some 3D printed filament materials.
The results of THz-TDS of the PLA Pine filament material and PLA Cond. filament material indicate that these materials can be effective for use in THz optical components up to maximum permissible frequencies of 0.56 THz, and a frequency below the minimum measured frequency, i.e., fTHz < fmin, respectively. That is, PLA Cond. filament material is highly absorptive to THz radiation and is thus not desirable for transmission based 3D printed THz optical components.
Fig. 4. The absorption coefficient and refractive index are shown for the approximate sample thicknesses of 300 µm, 700 µm, and 1400 µm for the following sample filament materials: (a) polylactic acid filament material embedded with pine wood particles (PLA Pine) and (b) PLA filament material embedded with conductive particles (PLA Cond.).
3.2 Common alternatives to PLA filament materials
Common alternatives to the PLA filament materials are compared in Fig. 5. The figure shows PETG filament material in Fig. 5(a), acrylonitrile styrene acrylate (ASA) filament material in Fig. 5(b), ABS filament material in Fig. 5(c), and polycarbonate co-polyester (PC CPE) filament material in Fig. 5(d). The samples are fabricated with FDM. The corresponding absorption coefficients and refractive indices are displayed for two of three target sample thicknesses being approximately 700 µm, 1400 µm, and 2100 µm. Again, the plots for the absorption coefficient and refractive index are displayed until the absorption coefficient has intersected with alpha max. The results are comparable for both absorption coefficient and refractive index across these samples.
The results of THz-TDS of the PETG filament material, ASA filament material, ABS filament material, and PC CPE filament material indicate that these materials can be effective for use in THz optical components up to maximum permissible frequencies of 0.68 THz, 0.69 THz, 0.70 THz, and 0.66 THz, respectively.
Fig. 5. The absorption coefficient (blue curves) and refractive index (green curves) are shown for the approximate sample thicknesses of 700 µm (lightest respective colour), 1400 µm (medium respective colour), and 2100 µm (darkest respective colour) for the following sample materials: (a) polyethylene terephthalate glycol (PETG) filament material, (b) acrylonitrile styrene acrylate (ASA) filament material, (c) acrylonitrile butadiene styrene (ABS) filament material, and (d) polycarbonate co-polyester (PC CPE) filament material.
3.3 Flexible filament materials
In Fig. 6 we investigate the absorptive and refractive properties of commercially available flexible filament materials, being thermoplastic elastomer (TPE) filament material with polyurethane Fig. 6(a), and thermoplastic polyurethane (TPU) filament material in Fig. 6(b). The samples are fabricated with FDM. The corresponding absorption coefficients and refractive indices are displayed for target sample thicknesses approximately of 700 µm and 1400 µm. Again, the plots for the absorption coefficient and refractive index are displayed until the absorption coefficient intersects with alpha max. The results are comparable for both absorption coefficient and refractive index across these samples.
The results the THz-TDS of the TPE filament material and TPU filament material indicate that these materials can be effective for use in THz optical components up to maximum permissible frequencies of 0.52 THz and 0.52 THz, respectively.
Fig. 6. The absorption coefficient (blue curves) and refractive index (green curves) are shown for the approximate sample thicknesses of 700 µm (lighter respective colour) and 1400 µm (darker respective colour) for the following sample materials: (a) thermoplastic elastomer filament material (TPE) and (b) thermoplastic polyurethane filament material (TPU).
3.4 Dissolvable support filament materials
Figure 7 depicts the absorptive and refractive properties of the dissolvable support filament materials: polyvinyl alcohol (PVA) filament material in Fig. 7(a) and HIPS filament material in Fig. 7(b). The samples are fabricated with FDM. These materials are often used to support the structure of complex components printed in other materials using a dual extruder FDM 3D printer. The support material is dissolved within the appropriate solvent. The solvent used for the PVA filament material is water, and the solvent used for the HIPS filament material is limonene. The corresponding absorption coefficients and refractive indices are displayed for target sample thicknesses of approximately 700 µm and 1400 µm. Again, the plots for the absorption coefficient and refractive index are displayed until the absorption coefficient intersects alpha max.
The absorptive and refractive properties of these samples are very different. The difference in refractive index is 0.26 at 1.0 THz. At the same time, the PVA filament material shows high absorption, at 30.6 cm-1 for 1.0 THz, whereas the HIPS filament material shows low absorption, at 3.50 cm-1 for 1.0 THz, as listed in Table 1.
The results of THz-TDS of the PVA filament material and HIPS filament material indicate that these materials can be effective for use in THz optical components up to maximum permissible frequencies of 0.52 THz and 1.43 THz, respectively.
Fig. 7. The absorption coefficient (blue curves) and refractive index (green curves) are shown for the approximate sample thicknesses of 700 µm (lighter respective colour) and 1400 µm (darker respective colour) for the following sample materials: (a) polyvinyl alcohol (PVA) filament material and (b) high impact polystyrene (HIPS) filament material.
3.5 Nylon filament materials
Figure 8 displays the absorptive and refractive properties of the filament materials blended with different grades of nylon, being Nylon 910 filament material in Fig. 8(a) and Nylon 230 filament material in Fig. 8(b). The samples are fabricated with FDM. The corresponding absorption coefficients and refractive indices are displayed for target sample thicknesses of approximately 700 µm and 1400 µm. Again, the plots for the absorption coefficient and refractive index are displayed until the absorption coefficient has intersected with alpha max. The absorptive and refractive properties of these two samples are comparable.
The results of the THz-TDS of Nylon 910 filament material and Nylon 230 filament material indicate that these materials can be effective for use in THz optical components up to maximum permissible frequencies of 0.48 THz and 0.52 THz, respectively.
Fig. 8. The absorption coefficient (blue curves) and refractive index (green curves) are shown for the approximate sample thicknesses of 700 µm (lighter respective colour) and 1400 µm (darker respective colour) for the following sample materials: (a) Nylon 910 filament material and (b) Nylon 230 filament material.
3.6 Packaging thermoplastic filament materials
Figure 9 depicts the absorptive and refractive properties of the filaments blended with materials common within commercial and consumer packaging, such as PP filament material in Fig. 9(a), and HDPE filament material in Fig. 9(b). The samples are fabricated with FDM. The corresponding absorption coefficients and refractive indices are displayed for two target sample thicknesses selected from approximately 700 µm, 1400 µm, and 2100 µm. There is a high degree of overlap between the 1400 µm and 2100 µm plots for the HDPE filament material. The plots for the absorption coefficient and refractive index are displayed until the absorption coefficient intersects alpha max. The absorption coefficient of the PP filament material data displays minor peaks associated with water vapor vibrational absorption, although they do not change the trend of the plotted data.
The absorptive and refractive properties of these samples are similar, with a difference in refractive index of 0.23 at 1.0 THz. The PP filament material has high absorption coefficients for 1.0 THz and 2.0 THz, at 8.70 and 28.31 cm-1, respectively, in comparison to the HDPE filament material, at 1.2 and 2.39 cm-1, respectively. (See Table 1 for absorptive and refractive data for 1.0 THz.) High density polyethylene filament material has low absorption across the THz bandwidth, making it suitable for use in 3D printing of THz optical components. Although HDPE can be temperamental to work with due to warping, Chong et al. suggests several ways to mitigate these issues in Table 2 of their work [42].
Table 2. Summary of filament material information.
The results of THz-TDS of the PP filament material and HDPE filament material indicate that these materials can be effective for use in THz optical components up to maximum permissible frequencies of 1.04 THz, and above the maximum measured frequency. This indicates that HDPE filament material is very low loss and is acceptable for use in transmission based 3D printed optical components across the entire measured THz frequency bandwidth.
Fig. 9. The absorption coefficient (blue curves) and refractive index (green curves) are shown for the approximate sample thicknesses of 700 µm (lightest respective colour), 1400 µm (medium respective colour), and 2100 µm (darkest respective colour) for the following sample filament materials: (a) polypropylene (PP) filament material and (b) high density polyethylene (HDPE) filament material.
3.7 Non-standard filament materials
Figure 10 depicts the absorptive and refractive properties of LT PCL filament material in Fig. 10(a) and PVB filament material in Fig. 10(b). The samples are fabricated with FDM. The corresponding absorption coefficients and refractive indices are displayed for target sample thicknesses of approximately 700 µm, and 1400 µm. The plots for the absorption coefficient and refractive index are displayed until the absorption coefficient intersects alpha max.
To the authors’ best knowledge, this is the first THz absorptive and refractive characterizations of the LT PCL filament material and PVB filament material. The high refractive index (i.e., 1.89 at 1.0 THz) of LT PCL filament material is promising for THz optical elements in the 0.2 to 0.5 THz range, given the low absorption coefficients in this range. The material LT PCL can be highlighted as an interesting material, given its unique properties, being that it has relatively high refractive index (approx. n = 2), rather than the lower refractive indices (approx. n = 1.5) of many of the other 3D printed materials. This could therefore be a useful material for refractive-based applications, such as rapid prototyping of lenses and diffraction gratings.
The absorptive and refractive properties of PVB filament material outperform those of the standard PLA filament material, reaching a lower max. absorption coefficient over a broader THz frequency range, making it useful for THz optical components where higher THz transmission is preferable [37].
The results of the THz-TDS of LT PCL filament material and PVB filament material indicate that these materials can be effective for use in THz optical components up to maximum permissible frequencies of 0.4 THz and 0.82 THz, respectively.
Figure 11 shows the absorptive and refractive properties of polymethylmethacrylate (PMMA) filament material (i.e., acrylic filament material). The samples are fabricated with FDM. Such acrylic is often used in optical applications as a waveguide over the visible spectrum. The corresponding absorption coefficients and refractive indices are displayed for target sample thicknesses of approximately 1400 µm and 2100 µm. The plots for the absorption coefficient and refractive index are displayed until the absorption coefficient intersects alpha max. The results of THz-TDS of the PMMA filament indicate that this material can be effective for use in THz optical components up to a maximum permissible frequency of 0.68 THz.
Fig. 10. The absorption coefficient (blue curves) and refractive index (green curves) are shown for the approximate sample thicknesses of 700 µm (lighter respective colour) and 1400 µm (darker respective colour) for the following sample filament materials: (a) low temperature polycaprolactone (LT PCL) filament material and (b) polyvinyl butyral (PVB) filament material.
Figure 12 depicts the absorptive and refractive properties of photopolymer liquid resin for SLA printers (i.e., SLA), in Fig. 12(a), and PEI filament material, in Fig. 12(b). The Fig. 12(a) results are performed on a SLA material sample that is fabricated with a ELEGOO Saturn MSLA 3D printer. The Fig. 12(b) results are performed on a PEI material sample that is fabricated with a heated hydraulic press. The corresponding absorption coefficients and refractive index are displayed for target sample thicknesses of approximately 700 µm, 1400 µm, and 2100 µm. The plots for the absorption coefficient and refractive index are displayed until the absorption coefficient intersects alpha max. To the authors’ best knowledge, this is the first THz absorptive and refractive characterizations of the PEI filament material.
The PEI filament material is designed for high performance 3D printers with printing temperatures in excess of 290°C. The FDM 3D printer used in this experiment has a maximum extruder temperature of 250°C, which is insufficient to melt the PEI filament material. This challenge is mitigated by melting down and compressing the PEI filament material into a solid cylindrical disk sample (i.e., PEI material sample) with a heated hydraulic press (Model: 4122, CARVER Inc., Indiana, USA). The obtained PEI material samples are fabricated for two different target thicknesses and are characterized via THz-TDS over the THz gap. To facilitate the comprehensive aggregation of filament material absorptive and refractive data for this manuscript, two samples of different thickness of photopolymer liquid resin are produced (i.e., SLA material samples) using the SLA additive process and are characterized with THz-TDS. The SLA material samples are printed with an ELEGOO Saturn MSLA 3D printer. In this experiment, both the SLA and PEI material samples are completely solid.
The results of the THz-TDS of SLA and PEI material samples indicate that these materials can be effective for use in THz optical components up to maximum permissible frequencies of 0.68 THz and 0.66 THz, respectively.
Fig. 11. The absorption coefficient (blue curves) and refractive index (green curves) are shown for the approximate sample thicknesses of 1400 µm (medium respective colour), and 2100 µm (darkest respective colour) for the following sample material: polymethylmethacrylate (PMMA) filament material.
3.8 Sample images of 3D printed materials
The images of the 3D printed materials are shown in Fig. 13. The images are taken with a digital microscope and a scalebar is included to shown the size of the images.
Fig. 12. The absorption coefficient (blue curves) and refractive index (green curves) are shown for the approximate sample thicknesses of 700 µm (lightest respective colour), 1400 µm (medium respective colour), and 2100 µm (darkest respective colour) for the following sample materials: (a) photopolymer liquid resin for SLA printers (i.e., SLA) and (b) polyetherimide (PEI) filament material.
3.9 Summary of absorptive and refractive properties and product details on 3D printed filament materials
In addition to the presented 3D printed materials, there are many other materials of interest for THz applications, including TOPAS, Teflon, and Zeonex. These materials are thoroughly characterized in Saiful Islam et al. [43] with full THz-TDS characterization for absorption coefficient and refractive index.
Table 1 summarizes the absorption coefficient and refractive index measurements of all the filament materials under test at a frequency of 1.0 THz and a target sample thickness of approximately 1400 µm. This summarizes absorption coefficient and refractive index at a frequency of 1 THz, shows the maximum permissible frequency, shows the minimum and maximum measured frequency, and shows the measured THz bandwidth (being the difference between the maximum and minimum measured frequencies).
To note trends and provide comparisons it is helpful to look at this one specific wavelength of 1 THz. The frequency 1 THz is chosen as this is below the maximum measurable frequency for all samples, and thus has a value of each sample for absorption coefficient and refractive index. Additionally, the choice of 1 THz follows the methods of Jin et al. [44], which sets a convention of setting loss tangent analyses at 1 THz frequency. A loss tangent analysis is presented later in this manuscript.
To aid in the application of this work, specific material details are summarized. Table 2 shows the filament material acronym, the corresponding filament material name, the product name with material color (where available on product specification sheet), and the product brand name. The filament material acronyms from Table 2 are used as labels on previous figures of this manuscript.
3.10 Absorption coefficient, refractive index, and loss tangent analysis of 3D printed materials set
The absorption coefficient data versus loss tangent (at 1 THz) is shown in Fig. 14 with Fig. 14(a) showing 23 materials, and Fig. 14(b) showing all but the highest absorption coefficient material. The loss tangent is
The PLA filament material embedded with electrically conductive particles (PLA Cond.) is particularly absorptive (i.e., absorption coefficient beyond 250 cm-1). This is as one would expect given that the free electrons and high conductivity will severely attenuate a THz electromagnetic wave, particular given the relatively close match of the plasma frequency. The PLA material has covalent bonding, lactic acid monomer, and is within the polymer class polyesters.
The low temperature polycaprolactone (LT PCL) filament material represents the next regime of interest with absorption coefficient beyond 50 cm-1. The LT PCL material is within the polymer class esters.
The PLA family of materials is spread from approximately 25-45 cm-1, and consists of PLA, PLA blended with pine tree particles (PLA Pine), PLA blended with iron (PLA Fe), PLA blended with copper (PLA Cu), and PLA blended with tungsten (PLA W). In general, the integration of metal tends to trend up to higher absorption while the integration of wood (e.g., pine) tends to trend down to lower absorption of the THz wave. Given the high absorption of metals [45] and the low absorption of wood for THz [46], this result is consistent with expectations. As stated above, the PLA material has covalent bonding, lactic acid monomer, and is within the polymer class polyesters.
There is a cluster of materials (other than PLA) in the 25-35 cm-1 regime, including Nylon 230 and Nylon 910 and thermoplastic elastomer (TPE), thermoplastic polyurethane (TPU), and polyvinyl alcohol (PVA). The Nylon 230 and Nylon 910 materials are within the polymer class amides. The TPE and TPU materials are within the polymer class polyurethanes. The PVA material is within the polymer class vinyl.
The next cluster of materials is in the 10-25 cm-1 regime, including photopolymer liquid resin for SLA printers (SLA), polymethylmethacrylate (PMMA), polycarbonate co-polyester (PC CPE), acrylonitrile styrene acrylate (ASA), acrylonitrile butadiene styrene (ABS), polyethylene terephthalate glycol (PETG), polyvinyl butyral (PVB), and polyetherimide (PEI). These materials can be classified as photopolymers (SLA), methacrylates (PMMA), copolyesters (PC CPE and PETG), acrylonitrile copolymers (ASA and ABS), vinyl (PVB), and ethers (PEI).
Of particular importance and relevance to application in THz optical components is materials with absorption coefficient below 10 cm-1, and this includes the cluster of 3D printed materials that includes polypropylene (PP), high impact polystyrene (HIPS), and high density polyethylene (HDPE). The material PP has propylene monomer and is within the classification of olefins. The materials HDPE has ethylene monomer and is within the classification of olefins. The material HIPS has styrene monomer.
In Fig. 15, the refractive indices versus loss tangent (at 1 THz) is shown with Fig. 15(a) showing 23 materials, and Fig. 15(b) showing all but the highest refractive index material. Most of the samples fall within the refractive index range of 1.3-1.9.
4. Conclusion
This manuscript presented THz-TDS measurements in a comprehensive study of twenty-three 3D printer materials. The work also presented a comparison of thick and thin samples for THz-TDS measurements—establishing verification of measurement accuracy. Such work extended the measured THz bandwidth beyond prior reported data. At the same time, the work put forward the first THz-TDS measurements, to the authors’ best knowledge, of PVB filament material, LT PCL filament material, and PEI filament material. It is hoped that this work will become a useful resource in the assessment of absorptive and refractive properties for 3D printing filaments and their suitability for custom THz optics. The THz-TDS results shown in this work indicated that the four best 3D printer filament materials, from the collection measured here, are the HDPE filament material, HIPS filament material, PP filament material, and PVB filament material. These 3D printing filament materials exhibit relatively low absorption at higher THz frequencies, in comparison to the other 3D printer filament materials evaluated in this study.
Funding
Canada Foundation for Innovation (16659, 37389); Natural Sciences and Engineering Research Council of Canada (RGPIN-2017-04022, RGPIN-2017-04073).
Acknowledgments
The authors acknowledge that the University of British Columbia's Okanagan campus is on the traditional, ancestral, and unceded territory of the Syilx Okanagan Nation. The authors acknowledge laboratory technician Jiyee Yoon for technical assistance.
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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