Terahertz technology has greatly benefited from the recent development and generalization of prototyping technologies such as 3D printing and laser machining. These techniques can be used to rapidly fabricate optical devices for applications in sensing, imaging and communications. In this paper, we introduce hot stamping, a simple inexpensive and rapid technique to form 2D metallic patterns that are suitable for many terahertz devices. We fabricate several example devices to illustrate the versatility of the technique, including metasurfaces made of arrays of split-ring resonators with resonances up to 550 GHz. We also fabricate a wire-grid polarizer for use as a polarizing beam splitter. The simplicity and low cost of this technique can help in rapid prototyping and realization of future terahertz devices.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Terahertz science and technology is now a mature research area with numerous applications including sensing , imaging  and communications . To enable many of these applications, optical components specially designed for the terahertz spectrum (0.1–10 THz) are required. Recently, there has been an increased interest in using prototyping technologies to fabricate such devices . These techniques vary in nature and can be used to rapidly test a prototype to verify performance characteristics.
On one hand, additive manufacturing technologies have a bottom-up approach [5,6] and can be used to fabricate various THz devices including lenses [7,8], reflectors , diffraction gratings [10,11], waveguides [12–14], corrugated horns , etc. The most commonly used technique is fused deposition modeling (FDM), where a polymer filament (for example polactic acid, PLA) is heated and passes through an extruder nozzle. Mounted on a 3-axis stage, this extruder moves to construct the 3D object layer by layer. The nominal resolution of FDM is limited by the extruder nozzle size, typically ∼0.4 mm. Another popular technique is stereolithography (SLA), in which a photopolymer resin is cured with ultraviolet light. Initially liquid, this resin is poured in receptacle capable of moving in the vertical direction. Beneath the resin, a UV light projects an image corresponding to the axial slice of the 3D object, which in turn chemically solidifies the photopolymer. The receptacle moves vertically, and the object is printed layer by layer. After a careful post-processing step where the unreacted photopolymer is removed, the resolution of SLA can be as low as 50 µm.
In contrast, subtractive manufacturing technologies have a top-down approach and remove material. They have been used in the terahertz range to fabricate lenses [16–18], gratings [19,20], band-pass filters [21,22], polarizers , optical delay lines [24,25], travelling wave tube amplifier [26,27], waveguide [28,29], etc. In computer numerical control (CNC) machining, a spindle is mounted on motorized multi-axis stages and removes material by drilling. The resolution of the fabricated object is typically ∼1 mm, although better resolutions can be achieved with Nano-CNC . This resolution depends mainly on the drill bit size, but also on thermal properties of the material, the vertical and horizontal cutting speeds, etc. Laser machining use a high intensity laser to cut and/or engrave a stock material. Depending on the laser, it can be used on a variety of materials from polymers to metals and glasses. Here, the resolution is mainly limited by the beam spot size to ∼50 µm, and is also greatly affected by the thermal properties, laser power, wavelength, repetition rate, cutting speed, etc. .
The previous techniques can be used to fabricate 3D objects. When 2D patterned surfaces are needed, microfabrication techniques borrowed from the semiconductor industry can be used. These techniques include, among others, photolithography , chemical vapor deposition , and reactive ion etching [33–35]. They can achieve ultrahigh resolution (∼100 nm) that is deeply subwavelength for the THz range. As such, they are mostly suited for higher frequencies, whereas they might be excessive in resolution when used in the lower portion of the spectrum – especially considering their complexity and cost. Other technologies used in the THz range for 2D metallic surfaces include printable electronics (resolutions ∼100 µm [36,37]), super-fine ink-jet printing (resolution ∼10 µm ) and laser decal transfer (resolutions <1 µm ).
In this paper, we present a very simple and inexpensive technique to pattern 2D metallic objects on a paper (nearly transparent) substrate. This technique is known as hot stamping or foil stamping. In contrast with the previously discussed techniques, this method is nearly free and widely accessible, as it simply requires the use of an office printer, a standard laminator and commercially available inexpensive foil sheets. It is therefore amenable to the rapid fabrication of a nearly limitless number of samples, which can enable high-throughput prototyping of terahertz devices. This technique has been recently used to fabricate amplitude masks in super-resolution THz imaging , Bragg gratings for add-drop multiplexer functionality in two-wire waveguide  and leaky-wave antennas for far-field beamforming . Here, we expand on this technique by first describing how to fabricate metallic patterns. Then, to demonstrate its potential, we fabricate and characterize a metasurface featuring an array of split-ring resonators, and a wire-grid polarizer which can be used as a polarizing beam-splitter. Finally, we discuss the limitations of this technique in terms of resolution and potential paths to overcome them.
2. Results and discussion
2.1 Hot stamping technique
The hot stamping technique uses a special type of foil commercially sold for do-it-yourself (DIY) craft projects. This inexpensive foil is made of a 40-µm thick layer of aluminum-based metallization powder deposited on a thin film of polyethylene terephthalate (PET) (Fig. 1(a)). First, the pattern to be metallized is printed using a regular office toner-based laser printer (Fig. 1(b)). Then, the foil is placed on top of the pattern with the aluminum layer facing the toner, and the pair of sheets is fed through a standard office laminator (Fig. 1(c)). Due to the heat and pressure, the metallization powder and the toner bond together allowing the metallic layer to be transferred onto the printed pattern by peeling the PET sheet (Fig. 1(d)). It is worth mentioning that the hot stamping technique is extremely rapid. The process from printing the pattern to lamination take only 1-2 minutes, which makes it attractive for fast prototyping. Figure 1(e) shows a microscope image of an array of split-ring resonators (which we characterize below). A regular adhesive tape can be used to remove the excess metallization powder that has not been bonded to the toner. The difference between these two regions, before and after excess powder removal, can be seen in the left and right regions of Fig. 1(e). We use an atomic force microscope to characterize the roughness of the obtained metallic surface. Figure 1(f) shows the topography image measured on a 100 µm2 area with 200 nm lateral resolution. We calculate the average roughness to be 5 ± 1 nm, which is several orders of magnitude smaller than the THz wavelength.
We use a THz time-domain spectroscopy system to characterize the various elements used in this fabrication technique. Figure 1(g) and (h) show the measured time traces and corresponding spectra of the samples. The paper (red curve) and the toner-printed paper (green curve) have little impact on the reference THz pulse obtained without any sample in the THz path (blue curve). However, when covered in foil, using our hot stamping technique, the pulse is blocked (gray curve) similar to a thick metallic object blocking the THz pulse (black curve, indistinguishable from zero signal in the time-domain plot). We observe that the efficiency of the blocking by the foil increases with the frequency, which may be explained by the fact that the metallic foil (measured to be 20 µm) is sufficiently thick relative to the skin depth.
2.2 Split-ring resonator
We now demonstrate the efficacy of this fabrication technique using several optical devices that may be useful in THz applications. First, we fabricate an array of split-ring resonators (schematic shown in Fig. 2(a) with the dimensions defined in Table 1). Microscope images of the obtained samples are shown in Fig. 2(b) (where the scale bars correspond to 1 mm), while their measured power transmission spectra are shown in Fig. 2(c) with an incident electric field polarized in the gap direction (vertical direction in Fig. 2(a)). As expected, the resonant frequency of the metasurface increases as the pattern dimensions decrease and 20 dB dips in the transmission spectra can be achieved in all cases. As it is clear from the spectra, the resonances are well defined for samples A-C (low THz frequencies), while they are somewhat broader for samples D-F (high THz frequencies). This can be understood from the microscope images in Fig. 2(b). Features in samples A-C are more uniform, while the defects in samples D-F causes inhomogeneity of the resonant frequencies and lead to an overall broader resonant response. Nevertheless, it is still quite clear that we observe a well-resolved resonance dip at 550 GHz in sample F despite the somewhat lower quality of the reproduced pattern. We believe that this resonance is caused by the LC response of the split-ring resonator, rather by the lattice itself, since our experiments show that it is dependent on the polarization of the incident electric field.
We now study the possibility of fabricating double-layer surfaces using the hot stamping technique on both sides of the sheet of paper. For that purpose, we demonstrate a double-layer split-ring resonator which has recently been subject to increased interest as they provide an additional degree of freedom in the design of metasurfaces [42–44]. We reuse the geometry of sample B and we print it on both sides, with a 180° rotation between the two patterns (Fig. 3(a)). The power transmission spectra in Fig. 3(b) reveals a transmission dip of ∼40 dB, which is in clear difference compared to the ∼20 dB obtained with a single surface design (curve B in Fig. 2(c)). This difference results from electromagnetic coupling between the two separated metasurface arrays. The details are, of course, sensitive to the relative offset between the two patterns. In practice, with our printer, this offset was somehow unpredictable as it depended on the way the sheet of paper was reintroduced in the printer in duplex mode. To avoid this issue, we printed on one side several arrays of split-ring resonators, while on the other side we printed the same arrays shifted by specified amount in horizontal and vertical directions. Therefore, in a single sheet of paper, we obtained >50 different double-sided split-ring resonator arrays that we then measured independently. The result shown in Fig. 3(b) is the one for which we measured the largest absorption. Visually, it corresponded to the case where both resonator arrays were offset. The key point here is that a large number of high-quality samples, comprising all possible relative offsets and orientations, can be fabricated in only minutes.
2.3 Wire-grid polarizer
As a second demonstration of the hot stamping technique, we fabricate wire grid THz polarizers by printing parallel metallic 50-µm lines separated by 200 µm on both sides of the paper sheet (inset in Fig. 4(b)). Figure 4(a) shows the power transmission as a function of the polarizer angle. To ensure that the incident beam has a linear polarization, a commercial wire grid polarizer was placed right after the transmitter. The power was normalized to a measurement without the hot stamped polarizer (only the commercial polarizer in place). As can be inferred from the maximum values in Fig. 4(a), the polarizer introduces ∼3-4 dB insertion losses below 500 GHz. In Fig. 4(b), we plot the extinction ratio defined as the ratio between the maximal and minimal power transmission. We can see that the fabricated polarizer (blue line) works best at lower frequencies, for which the THz radiation is essentially blind to the defects in the fabricated lines (since they are deeply subwavelength). We achieve >30 dB extinction ratios below 250 GHz and >20 dB extinction ratios below 450 GHz. These results were better than those of a single-sided polarizer (not shown here). We also compared the measurement with a commercial polarizer with a wire diameter of 10 µm and a wire spacing of 25 µm (red line in Fig. 4(b)). At lower frequencies, the hot stamped behaves better than the commercial one most probably due to the fact that the geometry of the lines (width and spatial period) is better suited for the lower part of the spectrum. In fact, in the lower part of the spectrum, it shows extinction ratios comparable to those obtained at higher frequencies of the commercial polarizer, at essentially zero cost.
2.4 Polarizing beam-splitter
As a final demonstration of a useful THz optical component, we implement a polarizing beam splitter. We hot stamp a wire grid polarizer with parallel metallic lines of 50 µm separated by 225 µm on a single side of the sheet of the paper. The polarizer is then placed at 45 degrees in the path of a horizontally polarized THz beam obtained with a commercial polarizer placed right after the transmitter (Fig. 5(a)). We measure the THz radiation transmitted (Receiver 1, blue) and reflected (Receiver 2, red) for two orthogonal directions of the polarizer. Figure 5(b) and (c) show the transmission and reflection when the polarization is perpendicular and parallel to the wires respectively. Here, the transmitted power is referenced to a measurement obtained using Receiver 1 (straight path) with no polarizer. When the polarization is perpendicular to the wires (Fig. 5(b)) most of the energy is transmitted (blue curve) while a small portion is reflected (red curve). Conversely, when the wires are parallel to the polarization (Fig. 5(c)), a small portion of the energy is transmitted (red curve) while most of the energy is reflected (blue curve). Again, as before, the polarizing beam splitter works best at lower frequencies.
2.5 Limitations of the hot stamping technique
We now discuss some limitations of the hot stamping technique and various ways to overcome them. First, the achievable resolution restricts the use of this technique to the lower part of the THz spectrum (<0.5 THz). In the experiments shown above, we used a printer with a nominal resolution of 1200 dpi. This means that it can print ∼47 dots per mm with dots having size of ∼21 µm. Better printers are commercially available that can print up to 9600 dpi (∼378 dots per mm with dots size of ∼2.6 µm), which could be used to increase the operational range to above 1 THz. Another limitation of this technique is the uniformity of the printed design, especially the repeatability of miniature features. This depends on a combination of factors such as the toner quality, paper type and thickness. While any type of toner and paper can be used, we found that using a paper with a glossy finish and a weight classification of 32 lbs. (120 g/m2) yields better results as the effect of paper roughness is reduced. It is worth noting that one is not restricted to use paper, as any substrate onto which toner can be printed can be used for this technique. Finally, the temperature of the lamination can be optimized to improve the metallization process. Here, we used the predetermined temperature of the laminator graded for paper of 5 mil thicknesses (∼127 µm). We visually found that passing the sample three to four times through the laminator gives better metallization results; indeed, when only was passing was performed, the metal was not entirely transferred to the toner, therefore using multiple passes can help create higher quality devices.
In conclusion, we demonstrated a simple and rapid technique to pattern 2D metallic surfaces for the THz range. Also known as hot stamping, this technique simply requires an inexpensive metallic foil sheet with a conventional office printer and a laminator. To demonstrate its potential for the terahertz range, we fabricated several useful optical devices. We realized metasurfaces made of arrays of split-ring resonators, and we measured resonances up to ∼550 GHz with power transmission dips of ∼20 dB. We also printed a double-sided wire grid polarizer and we achieved extinction ratios of >20 dB below 450 GHz and >30 dB below 250 GHz, which are on par with commercial polarizers. We also demonstrated a polarizing beam splitter using a single-sided wire grid polarizer. We believe the simplicity of this technique, as well as its inexpensive nature and high speed, can help in prototyping and realizing future terahertz devices.
National Science Foundation; Fonds de recherche du Québec – Nature et technologies.
The authors declare no conflicts of interest.
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