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Abstract
Terahertz (THz) filters with high transmission coefficient (T) in the passband and frequency selectivity are critical in numerous applications such as astronomical detection and next-generation wireless communication. Freestanding bandpass filters eliminate the Fabry–Pérot effect of substrate, thus providing a promising choice for cascaded THz metasurfaces. However, the freestanding bandpass filters (BPFs) using the traditional fabrication process are costly and fragile. Here, we demonstrate a methodology to fabricate THz BPFs using aluminum (Al) foils. We designed a series of filters with center frequencies below 2 THz and manufacture them on 2-inch Al foils with various thicknesses. By optimizing the geometry, T of the filter at the center frequency is over 92%, and the relative full-width half maxima (FWHM) is as narrow as 9%. The responses of BPFs show that “cross-shaped” structures are insensitive to the polarization direction. The simple and low-cost fabrication process of the freestanding BPFs promise their widespread applications in THz systems.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the applications such as next-generation wireless communication and terahertz (THz) astronomy, narrowband bandpass filters (BPFs) are in great demand to suppress the signal outside the working band. The frequency selective surfaces (FSSs) are often a popular choice of BPFs and have been used in various applications [1–9]. FSS consists of a two-dimensional periodic array of metallic structures, which could selectively pass or reject electromagnetic waves at specific frequencies. THz metal mesh filters are the most widely used THz BPFs. In the early study [10], Ulrich discussed the properties of metallic mesh and its complementary structure at the infrared band. The progress of micro- and nano-manufacturing technology significantly contributes to the maturity of THz BPFs.
Fabrication techniques such as photolithography and metallic deposition, nanoimprint, laser ablation, and 3D printing are utilized to make THz BPFs [9,11–15]. In terms of the existence of the substrates, THz BPFs are divided into freestanding and on-chip types. Photolithography and metallic deposition procedure are widely used to manufacture on-chip THz BPFs. However, the support beneath metallic structures must be thin and low-loss at the THz band. Therefore, the substrates need thinning after the fabrication of metallic structures [16]. Otherwise, Fabry–Pérot (FP) will affect the transmission coefficient (T) in the passband and the bandwidth of the filters. The thin plastic films are selected to support metallic components to reduce FP effects. Materials with low loss at the THz band, such as polyimide (PI) [17], and cyclic olefin copolymer (COC) [18], are the ideal candidates. Besides, the multilayer THz BPFs are also fabricated to achieve broadband BPFs [17,19]. However, the wrinkles in the flexible films bring in additional absorption loss. Furthermore, the flexible film is difficult to be applied in extreme conditions, like cryogenic detectors and high-power light sources.
Freestanding THz BPFs are proposed as an excellent solution to the above problems. Laser ablation is widely used to fabricate metallic films to form BPFs [7,12,13]. Due to the high power of lasers, heating damage is inevitable at the edge of the apertures, and high cost of the high-power laser source leads to prolonged mass production time. With the rise of artificial intelligence technology, reverse design algorithms have also been applied to the design of THz filters, showing powerful capabilities in optimizing multiple parameters of THz filters, including peak T, out-of-band T, bandwidth, and polarization stability [9]. Reconfigurable THz filter is a new research direction of THz filter. By integrating the tunable materials (for example, vanadium dioxide) into the traditional filters, parameters such as the center frequency can be tunable [20,21].
In this work, we developed a method of fabricating THz BPFs using aluminum (Al) foils. Traditional photolithography and wet etching are used to fabricate devices, and we also designed filters with different geometric parameters and characterized their transmission properties. The relationship between the polarization of the THz wave and T of the filter is experimentally studied. Our work offers a general method for fabricating freestanding THz devices such as FSS and metasurfaces.
2. Design and fabrication process
Here we chose the cross-shaped metal mesh structures, widely used as the unit cell of THz BPFs. It can be equivalent to a resonant circuit based on the transmission line model, as shown in Fig. 1. For a unit cell, there are three major geometric parameters of the cross-shaped unit length (A), the slit width (B) and the periodicity (P). According to the transmission line theory, the center frequency of the filter is the resonant frequency (ωr) of the circuit, which depends on the inductance and capacitance [22]. R is the resistor, corresponding to the intrinsic loss of the resonator. According to Ref. 23–24, the thickness of the metallic foil (t) is much smaller than the wavelength (${\lambda _r}$) of the electromagnetic wave at the center frequency (i.e., $t \ll {\lambda _r}$), so the center frequency of the THz BPF mainly depends on A, B, and P.
Fig. 1. (a) Geometry of cross-shaped THz filter. A is the arm length of the cross, B is the slit width of the cross, and P is the period length of the unit. (b) Diagram of the equivalent resonant circuit. Pin is the power of the incident pulse. Pout is the power of the THz pulse transmitted through the sample..
In this work, traditional ultraviolet lithography and wet etching techniques are introduced to fabricate the freestanding THz BPFs. The fabrication process flow is shown in Fig. 2. First, a thin layer of AZ1500 photoresist is spin-coated onto a transparent glass substrate as an adhesive for the Al foil. The Al foil is then attached to the glass substrate. Since the unavoidable appearance of bubbles will make the surface of the Al foil rough, we gently press the surface of the Al foil to remove the air bubbles. Then, the sample is placed in a dry environment for 3 hours to dry for the photoresist slowly. The second layer of AZ1500 is spin-coated for photolithography. The sample is baked at 95°C for 3 minutes. The Al foil is finally fixed onto the glass substrate. Patterns of filters are transferred to the photoresist using a mask aligner or a direct laser writer. After development, the samples are baked at 95°C for 5 minutes. The deionized water and alkaline developer (6:1) solution was used for Al etching. After the Al foil is etched, the entire sample is placed in an N-methylpyrrolidone (NMP) solution and heated in a water bath for several hours until the Al foil is peeled off the glass substrate. Then, the samples are soaked into isopropanol to remove the residual NMP on the surface. The samples were baked on a hot plate at 100°C until the organic residues on the surface evaporated. The microscopic image of the fabricated THz BPF sample is shown in Fig. 2 (i) and (j). We fabricate filters on Al foils of different thicknesses, with the same process but different etching times. In practice, it takes 5 hours, 2 hours and 1 hour to etch 15 µm, 8 µm, and 5 µm thick Al foils, respectively.
Fig. 2. (a) Cleaning the substrates to remove particles; (b) AZ1500 photoresist spin-coating as the adhesive layer; (c) Attaching the Al foil to the substrate; (d) AZ1500 photoresist spin-coating as the imaging layer; (e) UV lithography; (f) Developing; (g) Wet etching; (h) Lift-off. (i) Optical micrograph of a cross-shaped mesh filter; (j) Photos of the backside and frontside of a fabricated BPF sample.
3. Characterization of the freestanding THz BPFs
All mesh filters are measured by the THz time-domain spectroscopy (THz-TDS) system. The experimental setup is shown in Fig. 3 (a). Figure 3 (b) shows both the transmitted time-domain signals and frequency spectra of the reference and sample. Figure 3 (c) shows the measurement results of the 0.35 THz cross-shaped mesh filters. The experimental results show good consistency with the simulation results. Besides, we measured filters made from Al foils of different thicknesses as shown in Fig. 4 (a). Since the skin depth of Al at above 0.35 THz is lower than 200 nm, there is no apparent difference in the peak T for the filters fabricated on Al foils with the three kinds of thickness.
Fig. 3. (a) Schematic diagram of the optical path of the transmission-type THz time-domain spectroscopy system. (b) Measured transmitted time-domain signals of the reference and the sample.
Fig. 4. (a) Simulated and measured transmission spectra of cross-shaped mesh filter at 0.35 THz. (b) Measurement results of filters with a center frequency of 0.35 THz fabricated on Al foils of different thicknesses (5 µm, 8 µm and 15 µm). (c) Optical micrographs of filters fabricated on aluminum foils with a thickness of 5 µm and 15 µm under the same mask.
However, the filter made from thicker Al foil has a lower center frequency and higher T at the center frequency than those made from thinner foils as shown in Fig. 4 (b). Since the etching is isotropic, the lateral etching is more severe for thicker aluminum foils. As a result, the obtained aperture on the thicker Al foil is larger. Figure 4 (c) shows the difference between BPFs made of Al foils with thicknesses of 5 µm and 15 µm. Besides, the width of the slits in the pattern mask is 22 µm. The deviation between the mask and the samples is approximately equal to the thickness difference of Al foils. The larger the unit area, the higher the T is. Therefore, the photolithography pattern needs to be compensated in advance to obtain the same aperture on Al foils with different thicknesses.
As shown in Fig. 5 (a), BPFs with different P are measured. T at the center frequency gradually decreases as P increases. When P is larger than 650 µm, the descending speed is accelerated. Another significant change is the FWHM, and it decreases as P increases. From the above results, we can get that the FWHM reduces with the increasing P under the condition that T of the center frequency is maintained at a certain high level (>90%).
Fig. 5. (a) Filter measurement results with different unit periods (P) from 550 µm to 1000 µm. (b) Measurement results of THz narrowband bandpass filters with different slit widths (B) from 20 µm to 100 µm; (c) Experimental and fitting data of center wavelength as a function of A.
Figure 5 (b) shows the measurement results of the filters with different B. When B surpasses 30 µm, T at the center frequency exceeds 95%. When B surpasses 70 µm, T is close to 100%. When B increases, T at high frequencies is no longer flat and gradually increases significantly.
We select A = 400 µm, B = 40 µm and P = 650 µm as the filter fabrication parameters. At last, the center frequency of the filter is 0.34 THz, slightly lower than the goal of 0.35 THz. T at the center frequency is about 94%, and the FWHM is 13%. For narrower BPFs, we optimized the geometry, FWHM decreased to 9% while the peak T is 90.6% when A = 390 µm, B = 40 µm and P = 700 µm. Figure 5 (c) shows the measurement results of the filters with different A. By fitting the data, we can see that wavelength at the center frequency has a linear relationship with A, and the slope is 1.56679 ± 0.06457, which is a bit lower than that in Ref. [9].
The measured THz transmission spectra of BPFs with different center frequencies are plotted in Fig. 6(a). The FWHM becomes larger as the center frequency increases. It is pretty challenging to fabricate a high-frequency THz narrowband bandpass filter using the existing methods because the periodicity and the unit size are small, which approach the thickness of Al foil. Another problem at high frequencies is that the center frequency of the device is sensitive to geometric size. It is necessary to precisely control the wet etching time to reduce the deviation between the actual and the target sizes. By analyzing the FWHM and the peak T, the peak T decreases above 1.6 THz as the center frequency increases. Therefore, for fabricating the filters with a higher center frequency, we need to optimize the geometry further to obtain a high T.
Fig. 6. (a) Measurement results of mesh filters with different center frequencies. FWHM is shown on the right. The peak T is also shown in figure on the right. (b) BPF-i (i = 1, 2, 3, 4) is the result of single BPF on the left. Stack and product results are shown on the right. Product-i (i = 1, 2, 3) is the product of BPF-i. Product-1 is the product of BPF-1, BPF-2 and BPF-3. Product-2 is the product of BPF-2, BPF-3 and BPF-4. Product-3 is the product of BPF-1, BPF-2, BPF-3 and BPF-4. BPF Stack-i (i = 1, 2, 3) is the measurement result of BPF configuration corresponding to Product-i.
As Fig. 6(b) shows, FWHM and the peak T decrease as the number of metallic layers increases. For the four-layer configuration, the peak T is 53% and the FWHM is 8.2%. We also compare the product of T of a single filter with the measurement results of multiple-layer stacks. Near the center frequency, the product matches the measurement results of stacks well. However, at frequencies far from the center frequency, the measurement result differs significantly from the product of a single BPF. Through the product, the out-of-band noise is reduced by orders of magnitude.
As shown in Fig. 7(a), the two kinds of BPFs with different geometries are insensitive to the polarization direction. The Y-shaped filters are slightly sensitive to the polarization direction. The average peak T from the multiple measurements is 92.2% with a standard deviation of 4.1%, which is less than one-tenth of the mean. For practical applications, this fluctuation is at an acceptable level. In contrast, the cross-shaped THz narrowband bandpass filter is nearly insensitive to the polarization direction. The average peak T from the multiple measurements is 97.9% with a standard deviation of 1.6%, less than 2% of the average. Therefore, the cross-shaped filter is a more suitable choice for applications. From Fig. 7(b), we can see that the vertical and horizontal lengths of the cross slots are different. Therefore, we attribute the weak sensitivity to the polarization of the BPF in measurement to this difference. Besides that, the zigzag shape in the slot also causes the inhomogeneity of the THz transmission spectra.
Fig. 7. (a) Color contour plots of T and different polarization directions. The angular interval is 15°. Plots on the left are the results of “Y”-shaped BPFs, and on the right are the results of cross-shaped BPFs; (b) Scanning electron microscope photos of a pixel and enlarged slot of the fabricated sample.
To verify the repeatability, we fabricated 30 samples and measured the transmission spectra of these BPFs. Besides, we also measured transmission spectra at 15 different positions on a filter. As Fig. 8(a) shows, the samples show good consistency at the center frequency. However, the peak transmission coefficient shows a difference. Due to the nonuniformity of the thickness, it is difficult to control the area of a unit cell on the filters. Therefore, the recipe of the wet etchant needs optimization, and the etching time also requires standardization. As shown in Fig. 8(b), the 15 different points on the filter maintained good consistency in the center frequency and peak transmission coefficient, with an indistinguishable difference. From the results above, we think our fabrication method can be applied in THz researches.
Fig. 8. (a) Measured THz transmission spectra of 30 samples; (b) THz transmission spectra of a BPF measured at 15 different points on the sample.
4. Conclusion
In conclusion, we fabricated freestanding THz mesh filters using Al foils with different thicknesses. To acquire a THz narrowband bandpass filter with a high T and a narrow bandwidth, we optimized the geometry of the unit cells. Finally, we realized it at the target center frequency. The developed fabrication method does not require a thin film deposition process. Besides, wafer-level manufacturing can also be realized. The obtained peak T of the fabricated filters is as high as 94% and the FWHM is 13% at 0.35 THz. The measured THz transmission spectra match well with the simulation results. The polarization sensitivity measurement shows that the filters are insensitive to the polarization direction. The stacked filters have good performance in peak T, bandwidth, and out-of-band rejection. The excellent performance of the fabricated filters plus the low cost and ease of fabrication process make us believe the freestanding filters may find a variety of applications at THz systems.
Funding
Xiaomi Young Scholar Program; Jiangsu Provincial Key Laboratory of Advanced Manipulating Technique of Electromagnetic Wave; Fundamental Research Funds for the Central Universities; National Natural Science Foundation of China (62004093, 62027807, 62035014, 62227820, 62288101).
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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