In this Perspective contribution, we present a brief review of the literature available on optical devices for terahertz frequencies, followed by an analysis of the challenges faced by this technology and its future potential to generate complex photonic systems, and in principle the possibilities of this technique for the production of components for the infrared and visible band.

The development of the fused deposition modeling (FDM) method has allowed the use of affordable and reliable three-dimensional printers over the last decade, to the point that a good number of research laboratories have them within their toolboxes, and even “amateur” developers have access to this technology at home. The capacity of making a computer assisted three-dimensional design and having it fabricated within minutes has revolutionized the way prototypes are conceived and developed. While it might be obvious that 3D printing can be used to produce components that would traditionally be made in a mechanical workshop, optical components might not, a priori, be natural candidates for 3D printer based fabrication. Yet, if we take the term “optical” in a broad sense, i.e., not restricted to the visible band of the electromagnetic spectrum, the resolution of these printers is enough to produce optical and photonic components for wavelengths falling in the terahertz regime.1 Yet, materials which are being used for optical elements need to be largely transparent over the frequency range in which they should be used. Many standard materials that are being used for 3D printing show a considerable absorption and cannot be used to print THz lenses or waveguides. As a rule-of-thumb, one can state that, just like liquids,2–4 polar polymers are absorbing while non-polar polymers show a reasonable transparency in the THz range. Polyethylene and polypropylene show a very good transparency in the lower THz frequency range as shown in Fig. 1, but unfortunately they are inappropriate for 3D printers based on fused material deposition. In fact, any thermoplastic material can be used for printing; however, the printing quality varies considerably depending on the melting point, thermal expansion coefficient, and elasticity of the material. Fortunately, several materials appropriate for 3D printing, such as polystyrene (ps), Cyclic Olefin Copolymer (TOPAS), and Bendley, have low enough absorption in the terahertz band to be considered transparent,5–12 opening the possibility to produce geometrically complex dielectric devices, which would be hard, if not impossible, to produce in the more traditional visible or near-infrared regions.

FIG. 1.

Refractive index and absorption coefficient of various thermoplastics used in FDM 3D-printing.5,13 The acronyms and abbreviations are HDPE = High Density Polyethylene, Polyprop. = Polypropylene, ABS = Acrylonitrile Butadiene Styrene, and PLA = Polyactic acid. Data from Refs. 5 and 13.

FIG. 1.

Refractive index and absorption coefficient of various thermoplastics used in FDM 3D-printing.5,13 The acronyms and abbreviations are HDPE = High Density Polyethylene, Polyprop. = Polypropylene, ABS = Acrylonitrile Butadiene Styrene, and PLA = Polyactic acid. Data from Refs. 5 and 13.

Close modal

With the ideas mentioned in the previous paragraphs in mind, many groups around the world have tested this technique for the production of components that range from very conventional mirrors,6,16 lenses,5,17,18 prisms,19 gratings,13,20–29 and antireflection structures 30 to optical components that are less conventional such as Gradient-Refractive-INdex (GRIN) lenses14,31–33 [see Figs. 2(a), 3, and 4] and diffractive devices.34–36 In addition, some beam modifier components such as Airy37,38 or Bessel39–41 beam generators have been introduced such as the ones shown in the figure. Furthermore, designs of variable or active optical components like an Alvarez lens42 have been published.

FIG. 2.

Two examples of 3D printed optical components. (a) A Gradient-Refractive-INdex (GRIN) lens,14 which benefits from the sub-wavelength resolution, at a few hundred GHz, of the printer, in order to produce a quasi-continuous variation of the effective refractive index as function of the radial coordinate, which induces the lens behavior, although the component has flat faces in the plane of the page. (b) A topological waveplate,15 which also uses the sub-wavelength resolution of the printer in order to generate form-birefringence at 150GHz, yet the direction of the slow and fast axes of the material are a function of the position; this component can be used in order to generate exotic polarization modes such as radial or azimuthal “polarization.”

FIG. 2.

Two examples of 3D printed optical components. (a) A Gradient-Refractive-INdex (GRIN) lens,14 which benefits from the sub-wavelength resolution, at a few hundred GHz, of the printer, in order to produce a quasi-continuous variation of the effective refractive index as function of the radial coordinate, which induces the lens behavior, although the component has flat faces in the plane of the page. (b) A topological waveplate,15 which also uses the sub-wavelength resolution of the printer in order to generate form-birefringence at 150GHz, yet the direction of the slow and fast axes of the material are a function of the position; this component can be used in order to generate exotic polarization modes such as radial or azimuthal “polarization.”

Close modal
FIG. 3.

Height profiles [(a) and (c)] and photos [(b) and (d)] of two 3D-printed elements which, in combination, generate an Airy beam.38 Reproduced with permission from Liu et al., Opt. Express 24, 29342 (2016). Copyright 2016 The Optical Society.

FIG. 3.

Height profiles [(a) and (c)] and photos [(b) and (d)] of two 3D-printed elements which, in combination, generate an Airy beam.38 Reproduced with permission from Liu et al., Opt. Express 24, 29342 (2016). Copyright 2016 The Optical Society.

Close modal
FIG. 4.

A 3D terahertz gradient-refractive index lens designed by transformation optics is achieved by fabricating “woodpile” structures with varying dimensions of subwavelength dielectric unit cells using the projection microstereolithography technique. Both simulation and experimental investigations confirm that the lens delivers an imaging resolution very close to the diffraction limit over a frequency range from 0.4 to 0.6 THz.31 Reproduced with permission from Zhou et al., Adv. Opt. Mater. 4, 1034 (2016). Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

FIG. 4.

A 3D terahertz gradient-refractive index lens designed by transformation optics is achieved by fabricating “woodpile” structures with varying dimensions of subwavelength dielectric unit cells using the projection microstereolithography technique. Both simulation and experimental investigations confirm that the lens delivers an imaging resolution very close to the diffraction limit over a frequency range from 0.4 to 0.6 THz.31 Reproduced with permission from Zhou et al., Adv. Opt. Mater. 4, 1034 (2016). Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

Close modal

Since 3D printing opens the possibility of creating rather complex geometrical structures, a possibility that seems very appealing is to produce rectangular dielectric,43–46 plasmonic,47 metal-dielectric,48 and other forms of waveguides49 and waveguide-based filters,50 as well as Bragg,51–53 photonic crystal,54–57 such as the ones depicted in Fig. 5, and hollow core optical fibers,58–62 as well as various preforms for optical fibers.63–65 Additionally, photonic crystals44,55,66–69 among other metamaterials70–73 appear in recent publications that include some engineered corrugated surfaces to couple electromagnetic radiation with polaritons.74 All these components can open the possibility to interconnect various devices and, in principle, create integrated optical circuitry at terahertz frequencies. In fact, the first experiments to transfer waveguide “wire”-based data streams at a few hundred GHz have recently been carried out.75,76 Yet, very recently, it was also found that the humidity in the ambient air leads to a water film to form on bare polymer wires.77 This water film is highly absorbing and limits the transmission distances for polymer wire-bound transmission. This shows that THz waveguides will require a cladding or other more complex inner structures. Here, 3D printed waveguides or 3D printed preforms for pulled waveguides could be a solution.

FIG. 5.

(a) Photograph and (b) calculated fundamental guided mode structure at 1 THz for a “large” mode area fiber, and (c) photograph and (d) calculated fundamental mode structure at 1 THz of a “small” mode area (SMA) fiber. (e) Photograph of fibers shaped into 90° bends.54 Reprinted with permission from Nielsen et al., Opt. Express 17, 8592 (2009). Copyright 2009 The Optical Society.

FIG. 5.

(a) Photograph and (b) calculated fundamental guided mode structure at 1 THz for a “large” mode area fiber, and (c) photograph and (d) calculated fundamental mode structure at 1 THz of a “small” mode area (SMA) fiber. (e) Photograph of fibers shaped into 90° bends.54 Reprinted with permission from Nielsen et al., Opt. Express 17, 8592 (2009). Copyright 2009 The Optical Society.

Close modal

Polarization handling and modifying components have also been introduced. For instance, polarizers,78–80 polarization splitters,81 waveplates,82 topological plates15 [see Fig. 2(b)], vortex beam generators83–86 and polarized grids79 have been explored and reported. In addition, the use of 3D printing technology has allowed to demonstrate waveguides that act as focusing probes87 for near field microscopy or for endoscopic purposes.88 Various holograms have also been reported.26,27,89–93

A quick inspection of the list of references cited in the previous paragraphs allows us to realize that most of the investigation in this field has happened in the last 5 years; therefore, it can still be considered an emerging research topic with enormous potential in the near future for two reasons. First, the number and complexity of photonic components for the THz band is expected to quickly grow in the next decade driven by the imminent shift of wireless communications that currently use the 3 GHz band, to higher frequencies.94 Second, because with the improvement of resolution of the printers, 3D printing will soon be able to produce components for the mid-infrared, and eventually for the visible band. As a matter of fact, a few 3D printed components for the infrared have already been explored,95–97 such as the one shown in Fig. 6. Therefore, the experience gained at terahertz frequencies, will be crucial to exploit this technology on other bands of the electromagnetic spectrum.

FIG. 6.

Spiral phase plates have been fabricated by 3D printing on the tip of optical fibers, demonstrating the possibility of using additive manufacture of components, in this case for the near-infrared region. (a) shows an scanning-electron-micrograph, (b) an optical-micrograph, and (c) an optical profilometry reconstruction of the device after fabrication. (d) illustrates how this device can be used to modify the angular momentum state of optical beams.97 Reproduced with permission from Weber et al., Opt. Express 25, 19672 (2017). Copyright 2017 The Optical Society.

FIG. 6.

Spiral phase plates have been fabricated by 3D printing on the tip of optical fibers, demonstrating the possibility of using additive manufacture of components, in this case for the near-infrared region. (a) shows an scanning-electron-micrograph, (b) an optical-micrograph, and (c) an optical profilometry reconstruction of the device after fabrication. (d) illustrates how this device can be used to modify the angular momentum state of optical beams.97 Reproduced with permission from Weber et al., Opt. Express 25, 19672 (2017). Copyright 2017 The Optical Society.

Close modal

The main two limitations for this technique are the, still, relatively poor resolution of economical FDM 3D printers, and the limited number of highly transparent printable polymers, all with refractive indices in the vicinity of 1.5.

The consequence of the poor resolution of FDM printers is that devices operate in the few hundred gigahertz region, which is only the low frequency end of the THz band. This can be overcome by using other 3D printing techniques, which unfortunately are currently not as widely available or as accessible in terms of cost. Furthermore, those printers use a completely different set of materials, which are sometimes sold in sealed cartridges by the printer manufacturers only, and little, or no information about their composition is available. Yet, the FMD technology is expected to still evolve and perhaps it will reach the desired resolutions in the future.

The limited number of THz-transparent polymers that can be 3D printed, which all have very similar optical properties (n1.5) prevents the possibility of fabricating several interesting components, such as photonic crystals with large bandgaps, where a significant refractive index contrast is required.

As seen in the context section, many different quasi-optical/photonic devices have been demonstrated in the last few years, some of which were only predicted theoretically until these reports. Some of such devices have not been demonstrated for the visible range or other bands of the spectrum, mainly because of the impossibility of fabricating them with conventional techniques such as polishing of traditional materials like glass in the complex geometries required.

A topic search in the Web of Science using the chain “terahertz 3D print” (on October 18, 2019) produces the results shown in Fig. 7. The figure shows the number of articles published (a) and their citations (b) by year. The dashed line, provided as a guide-to-the-eye, is the result of fitting an exponential function; while we do not expect this exponential to be a very accurate model, it provides a qualitative approximation of the past and short-term-future behavior. We consider that this area is an emerging topic with great potential, of course, for the terahertz community and for the broad optics and photonics community too. We can foresee that in the coming few years, the resolution of 3D printers will increase substantially, opening the possibility to prototype and mass produce components appropriate for wavelengths from the microwave to the visible bands of great complexity, that today cannot be produced even in highly specialized research facilities. One can only imagine, perhaps, photonic crystals with a complex three-dimensionally distributed defect structure, three-dimensional waveguide structures of specifically engineered dispersion, or the frequency selective transmission that interconnect photonic logic gates in order to form complex integrated optical circuits, all of this fabricated in minutes from simple computer assisted designs. The potential applications are still to be seen. In this sense, those of us working in the wavelength range from approximately 100μm to a few millimeters, i.e., the terahertz band, are lucky to be the first ones to explore this prototyping technology for photonic devices.

FIG. 7.

(a) Number of articles per year (circles) as reported by Web of Science on October 21, 2019 for the search chain “terahertz 3D print”; the square is an estimation for the end of 2019. (b) Number of citations to the articles in (a) per year (circles) as reported by Web of Science on October 21, 2019; the square is an estimation for the end of 2019.

FIG. 7.

(a) Number of articles per year (circles) as reported by Web of Science on October 21, 2019 for the search chain “terahertz 3D print”; the square is an estimation for the end of 2019. (b) Number of citations to the articles in (a) per year (circles) as reported by Web of Science on October 21, 2019; the square is an estimation for the end of 2019.

Close modal

It is worth mentioning at this point that other 3D-printing technologies, apart from FMD, have recently been explored for the fabrication of terahertz devices. For instance, photo-polymerization based printing is an interesting option. This technique consists of projecting optical images on a reservoir of liquid polymer, which hardens only on the exposed regions, forming a layer of the 3D model; this layer is then mechanically lifted, the following layer is exposed, and the process is repeated until the 3D shape is completed. Initial characterization of some of the photo-polymer materials used have been carried out98 and some components have been demonstrated using this technique.99 Other examples include material jetting,99 binder jetting, and selective laser melting;45 the last two open the possibility of making metallic structures, such as antennas, with complex geometries.

As for the number of 3D-printable materials available, the selection of commercially available polymers has grown in the last few years. Yet, most of them are either not transparent enough or their refractive index is, for all cases, close to 1.5. This is a challenge to address in the coming years. Finding non-conventional polymers with melt and re-solidifying temperatures and times appropriate for 3D printing that show refractive indices greater than 2 and low absorption are highly desirable. Another approach that we have tried is the incorporation of additives to conventional 3D printable polymers that can increase their refractive index;100 however, these materials are not appropriate for 3D printing yet, but there should be interesting opportunities in that direction. In particular, it is conceivable that new non-polar polymers appropriate for 3D printing and with a lower absorption in the THz range than polyethylene and polypropylene will be developed. One should recall that ordinary glass is not very transparent at optical frequencies. A glass window only a few millimeters thick is transparent; however, a block of ordinary glass with a thickness of 1 m is in fact opaque. Yet the need to produce optical fibers pushed the development of new extremely transparent glasses in the 1970s and 1980s, which enabled the glass fiber based communication network in place around the globe nowadays.

One additional aspect that makes printable optical components desirable is the ability to print not only the optics but also the mounts that hold them in an optical system; this allows the “makers,” low-budget laboratories in developing countries and even to pre-university schools to set up terahertz experiments.101 This effectively can make terahertz technology and research widely accessible to professionals and amateurs worldwide.

The possibilities that three-dimensional printing has opened for both prototyping and production are still hard to appreciate in many fields; optical and photonic components are not the exception. A plethora of conventional and also non-conventional optical components for the terahertz band have been demonstrated recently. The trend shown by the number of publications in this area seems to indicate that the interest in this field is growing. It is to be expected that soon the availability of new materials and the improvement on the printing hardware will allow the fabrication of more complex and interesting devices in the near future. It is even possible to foresee that this technology will soon expand to shorter wavelengths with potential for the near-infrared and visible regions of the spectrum.

We would like to acknowledge support of the Alexander von Humboldt Foundation through an Experienced Research Fellowship awarded to E.C.-C. hosted by M.K.

The data that support the findings of this study are available from the corresponding author upon reasonable request unless it comes from a cited reference.

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