Open Access
Add to BibSonomyAbstract
Terahertz (THz) absorbers with surface relief structures (SRSs) were designed and fabricated on a flexible polydimethylsiloxane(PDMS) substrate by using a stamping method. The silicon mold used for the stamping process was prepared by using a crystallographic wet etching method with 45% KOH solution at 80°C. The flexible THz absorber, consisting of micropyramids with a base width of 240 μm, demonstrated nearly perfect absorbance higher than 99% owing to the dramatically reduced surface reflectance of the SRS. The reflectance of the PDMS with the SRS was less than 1%, which is only 1/100th of that measured from a bare PDMS at frequency higher than 1 THz.
©2012 Optical Society of America
1. Introduction
Terahertz (THz) science and technology has received much attention in relation to various application areas such as biological science, medical imaging, security and space science [1–4]. Intensive studies fueled by emerging applications of THz frequency are now focusing on developing various functional devices such as modulators, sources, detectors, and absorbers in efforts to realize THz systems with better performance and new functions [5–9]. In particular, the absorber has become a critical component of security and imaging systems in the THz region [10–12]. Broadband terahertz absorbers are also beneficial to improve the sensitivity of the THz spectroscopy system by suppressing the spurious noise such as unwanted reflected terahertz waves from the background other than the specimen. To realize an absorber with high absorbance, low reflection and low transmission properties are required. Recently, in the design of THz absorbers, research on planar metameterials to suppress the surface reflection has been actively carried out [6, 12–16]. THz absorbers based on various metamaterial designs have exhibited high absorption properties thanks to reduced reflection at the resonance frequency, but the absorption peaks show narrowband characteristics [6, 12–14]. To overcome the narrowband characteristics of the metamaterials, stacked multilayer metamaterials have been developed [15, 16]. These broadband metamaterial absorbers demonstrated absorbance higher than 97% from 4.4 to 5.5 THz [15] and absorbance exceeding 60% from 4.1 to 5.9 THz [16]. A broadband THz absorber can also be realized by fabricating low reflection surface relief structures (SRSs) on top of highly absorbing materials. Similar approach has been investigated to realize broadband absorbers operating in microwave region [18]. Although previously reported SRSs which were fabricated on top of silicon substrates exhibited good performance in reducing the surface reflection in THz region, their reflectance spectrum exhibited narrowband characteristics, which was due to the large refractive index contrast between air and silicon substrates [19].
In this study, we present the design, fabrication, and characterization of SRSs for a broadband THz absorber on a flexible polydimethylsiloxane (PDMS) substrate prepared by a simple stamping process. The PDMS material was chosen as a host material for the broadband flexible absorber because it has a low refractive index and a high absorption coefficient in the THz range as well as high flexibility. The reflection, transmission, and absorbance of the fabricated THz absorber were characterized by using THz time domain spectroscopy (TDS). To investigate the performance of the flexible absorber, reflection measurements were carried out for the absorber bent with different bending radii.
2. Design and simulation
The surface relief structure (SRS) was designed to minimize the surface reflection of the absorber in the THz region. Figure 1(a) shows a schematic illustration of the SRS consisting of close-packed micropyramids and a summary of its structural parameters. SRSs with a micropyramid shape having various base widths of 60, 120, and 240 μm were designed. The height of the micropyramid, h, is determined from the base width, a, of the micropyramid because the crystallographic wet etching of the (100) silicon substrate results in the silicon mold with the pyramidal shape, which is formed by the {111} crystalline planes. The height of the micropyramid, h, is determined from the base width, a, according to the Eq. (1) [20].
Fig. 1 (a) Schematic view of the SRS consisting of close-packed micropyramids for THz antireflection coating. The height was determined by the crystallographic wet etching process on (100) silicon. (b) Refractive index and absorption coefficient of the PDMS extracted from the measured transmittance [21]. (c) Contour plot of the reflectance of the SRS on the PDMS substrate as a function of the frequency and the base width of the micropyramids. (d) The angle-dependent reflectance of the SRS on the PDMS substrate whose micropyramids have base width of 240 μm.
To determine the other parameter, b, which is the spacing between the pyramids, the rigorous coupled-wave analysis (RCWA) method was utilized [21]. The RCWA calculation was carried out with the material parameters of PDMS in Fig. 1(b), which were extracted from the measured transmittance in a frequency range between 0.2 and 2.0 THz [22]. The average reflectance in the frequency range from 0.2 to 2.0 THz was calculated for the three SRS structures with the three base widths shown in the Fig. 1(a). In this calculation, the normalized spacing, b/a, was scanned from 0 to 0.4 with the step of 0.1. All the three samples exhibited the lowest average reflectance at the normalized spacing of 0.1 so that the spacing, b, was set to 0.1a for the following RCWA simulation.
The surface reflectance of the SRS was calculated as a function of the frequency and the base width of the micropyramids as shown in Fig. 1(c). The surface reflectance monotonically decreases as the frequency and the base width increase when the frequency is lower than the cut-off frequency which is determined by the structural parameters of the SRSs [23]. At frequency lower than the cut-off frequency, only the zeroth-order modes can propagate and the higher order diffracted modes are evanescent. For a given structure, the surface reflectance becomes higher when the frequency of the incident wave exceeds the cut-off frequency because high order backward diffracted modes begin to be generated. However, the increase of the surface reflectance near the cut-off frequency is at most 1% for the PDMS materials, because the refractive index contrast between the air and PDMS is very small. Considering that the height of the micropyramid is proportional to its base width, the SRS consisting of micropyramids with the larger base width exhibits the more gradual change in the effective refractive index from the top to the bottom of the micropyramids. The reflectance at a single fixed frequency thereby tends to decrease as the base width of the micropyramid increases. While micropyramid structures fabricated by crystallographic wet etching process result in the fixed aspect ratio (h/a) of 0.707, the micropyramids with the higher aspect ratio can be fabricated by using deep reactive ion etching (DRIE) process or other advanced fabrication techniques. The SRSs with micropyramids with the higher aspect ratio can achieve the lower reflection properties due to the more gradual change of effective refractive index [23].
The reflectance of the SRS structures is dependent on the incident angle of the incoming THz radiation. The angle-dependent reflectance of the SRSs in the THz region was calculated for the SRSs with micropyramids having base width of 240 μm as shown in the Fig. 1(d). Although the reflectance tends to increase with the larger incident angle, the reflectance can be kept lower than 2% for the THz radiation with incident angle smaller than 50° in the frequency range from 0.8 to 2.0 THz.
3. Experimental details
A THz antireflection coating SRS was fabricated by the following steps, as illustrated in Fig. 2 . A (100) silicon wafer was used to fabricate the silicon mold. Plasma enhanced chemical vapor deposition method was utilized to deposit 500-nm-thick silicon nitride (SiNx) layers on the top and the bottom of the silicon substrate. The square patterns were delineated by conventional ultraviolet (UV) photolithography and they were transferred to the SiNx layer by using reactive ion etching. The silicon substrate with the patterned SiNx layer was dipped in a 45% KOH solution at 80°C to carry out anisotropic wet etching. The etched sample was subsequently dipped in the diluted hydrofluoric acid (HF:H2O = 1:1) at room temperature to remove the SiNx etch mask layer. To realize a flexible THz SRS, diluted polydimethylsiloxane (PDMS, Sylgard 184) was poured onto the fabricated silicon mold and kept in a vacuum chamber to remove the air bubbles at the interface between the PDMS and the silicon mold. Silicon molds coated with PDMS were cured in a N2 ambient oven at 100°C for 30 min. The cured PDMS layer was then carefully detached from the silicon mold. The micropyramids replicated on top of the detached PDMS exhibited essentially the same structural dimensions with the silicon mold. The thickness of the PDMS except the height of the micropyramids was measured to be 3.5 mm. The backside of the detached PDMS sample was left untreated and its average roughness height was measured to be 50 nm.
Fig. 2 Fabrication process of SRS on PDMS substrates by using conventional UV photolithography, anisotropic wet etching, and a stamping technique.
Figure 3 shows the fabricated silicon mold and the detached PDMS samples. Scanning electron microscopy (SEM) images were taken at electron energy of 15 keV for the silicon mold and 5 keV for the PDMS sample, respectively.
Fig. 3 SEM images of (a) the fabricated silicon mold with a base width of 60 μm, (b) detached PDMS sample from the silicon mold by using the stamping method, and (c) photograph of a fabricated flexible PDMS with SRS with a base width of 240 μm.
The reflectance and transmittance of the fabricated flexible PDMS-based SRS samples were measured with a terahertz time-domain spectroscopy (THz-TDS) system (TPS spectra 3000, TeraView). The THz-TDS system was operated at room temperature in a N2 ambient to avoid unwanted THz absorption by the water vapor in the air.
4. Discussion
Figure 4(a) shows the measured reflectance spectra of the SRSs consisting of micropyramids with base widths of 60, 120, and 240 μm. Only the zero order backward diffracted mode was measured because the current THz TDS system was set up for measurement of reflected waves only in the surface normal of the device under test. The inset figure shows the fabricated SRS on the PDMS substrate. The reflectance of the bare PDMS substrate is less than 8.4% under a frequency range between 0.3 and 2.0 THz. The PDMS samples with SRSs exhibited lower reflectance than the bare PDMS substrate. Among the four samples, the reflectance value of the sample with a base width of 240 μm was the lowest in the broadband THz range from 0.75 to 2 THz. However, the total reflectance that induces high order backward diffracted modes will be slightly higher than the presented measurement results, because the high order backward diffracted waves are not included in the measurement results. The high order modes may be generated at frequency higher than the cutoff frequency, which is 1.1 THz for the sample with the base width of 240 μm, but their contribution to the total reflectance is less than 1% according to the RCWA simulation as shown in Fig. 1(c). By considering that the measured reflectance values were almost zero from 0.75 to 2 THz, the total reflectance is expected to be as low as 1% in this frequency range.
Fig. 4 (a) Measured reflectance spectra for bare PDMS substrate and PDMS samples with various SRSs. The inset shows the fabricated SRSs on the PDMS substrate. (b) The enhancement ratio, calculated by the equation shown in the figure, was plotted.
To compare the reflectance values between the SRS sample and the reference sample, the enhancement ratio was defined as the ratio of the reflectance of the SRS sample and a bare PDMS sample. The calculated enhancement ratios were plotted for the three SRS samples with different base widths in Fig. 4(b). At 1 THz, the reflectance of the PDMS sample with a base width of 240 μm was less than 1%, which was only 1/100th of that measured from the bare PDMS sample.
To investigate the THz response of the SRS, the electric field (e-field) distribution was simulated by the RCWA method. In Fig. 5 , the e-field distributions are shown at different z-positions for 0.4 and 1.5 THz. The electric fields are uniformly distributed at all the z positions at 0.4 THz, which is lower than the cut-off frequency. This shows that the SRS acts as a graded index structure at 0.4 THz. On the other hand, the electric field distribution at 1.5 THz was quite different from that at 0.4 THz. The transmitted electric field at z = 0 is localized at the center of the micropyramid. The high order diffracted waves constructively interfere at the center of the micropyramid at 1.5 THz. Comparing the intensities at the top of the SRS at 0.4 and 1.5 THz, it is found that the intensity at 0.4 THz was higher than that at the 1.5 THz. The measured reflectance value at 0.4 THz was also higher than that at 1.5 THz, as shown in Fig. 4(a).
Fig. 5 Electric field distribution of the SRS with base width of 240 μm simulated by the RCWA method. The simulation was carried out at different z-positions in the SRS for 0.4 and 1.5 THz.
Figure 6 shows the measured transmittance spectra of the PDMS substrate with and without SRSs in a frequency range between 0.3 and 2.0 THz. From the extracted absorption coefficient as shown in the Fig. 1(b), it is expected that a PDMS substrate thicker than 2 mm can completely absorb the incident THz waves at frequency between 1.0 and 1.6 THz. Both the bare PDMS substrate and the SRS on the PDMS substrate exhibit low transmittance due to the inherently high absorption property of the PDMS material in the THz frequency region. When the PDMS substrates with and without the SRS were compared, the transmittance of the SRS samples was found to be slightly higher than that of the bare PDMS substrate. This is attributed to the lower surface reflectance of the SRS samples relative to that of the bare PDMS sample. The transmittance in the inset of Fig. 6, plotted in log scale, shows that the lowest transmittance occurs at around 1.3 THz, where the absorption coefficient of PDMS is the highest.
Fig. 6 Measured transmittance spectra for bare PDMS substrate and SRS samples with various base widths. The inset shows the transmittance in log scale.
The most important parameter for the absorber is the absorbance, A(f), which is defined as
where R(f) and T(f) are the reflectance and transmittance, respectively [6]. Figure 7 shows the absorbance of the bare PDMS substrate and the SRS samples. The bare PDMS substrate exhibited about 94% absorbance between 0.75 and 2.0 THz. Compared with the bare PDMS sample, the SRSs on the PDMS substrate exhibited higher absorbance and the SRS with a base width of 240 μm showed the highest absorbance among the SRS samples. The sample with a base width of 240 μm exhibited absorbance higher than 99% at frequency above 1 THz. The designed absorber on the PDMS substrate demonstrated excellent absorbance compared with previously reported metamaterial-based THz absorbers, which exhibited 97% absorbance at 1.6 THz single frequency [24], absorbance higher than 97% from 4.4 to 5.5 THz [15], and absorbance higher than 60% from 4.1 to 5.9 THz [16].Fig. 7 Calculated absorbance spectra for bare PDMS substrate and SRS samples with various base widths. The inset shows a schematic view of the THz wave propagation in the PDMS samples with and without the SRS.
In order to test the performance of the flexible SRS sample, the reflectance of the samples was measured when they were bent with different radii. In the experiment, the reflectance measurement was carried out by using the sample with a base width of 240 μm. As shown in Fig. 8 , the reflectance was slightly increased as the bending radius was decreased. This is partly due to the decreased aspect ratio of the micropyramids in the SRS compared to the original SRS design on the flat surface [25]. When the THz radiation is incident to the bent sample, the incidence angle off the center of the sample is not zero any more. Thereby, the reflectance increased as the bending radius decreased and the reflectance degradation appeared to be severer in the low frequency range as was expected from the inset of the Fig. 1(d). Even though the bending radius was decreased to 0.25 cm, the reflectance was still lower than that of the flat PDMS sample without the SRS. The flexible absorber realized by the SRS on PDMS can be applied on non-planar surfaces with the minimal degradation of the reflectance in the THz frequency range.
Fig. 8 Measured reflectance spectra of the bent SRS sample with a base width of 240 μm. The sample was bent with different bending radii.
5. Conclusion
A stamping method using a silicon mold and PDMS was employed to fabricate a SRS-based broadband THz absorber. The low reflection property of the designed SRS and PDMS material with a high absorption coefficient led to high absorbance in the THz frequency range. The fabricated SRS on the PDMS substrate exhibited nearly perfect absorbance of 99% in the broadband THz range from 0.75 to 2 THz. From the reflectance of the bent SRS samples, it was also shown that the SRS on the PDMS substrate can be utilized as a broadband THz absorber on a non-planar surface.
Acknowledgment
This research was supported by a NRF grant (No. 20110017603), the World-Class University program funded by the MEST through the NRF of Korea (R31-10026), and the Core Technology Development Program for Next-Generation Solar Cells of the Research Institute for Solar and Sustainable Energies (RISE), GIST.
References and links
1. M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1(2), 97–105 (2007). [CrossRef]
2. B. Ferguson and X. C. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1(1), 26–33 (2002). [CrossRef] [PubMed]
3. B. M. Fischer, M. Hoffmann, H. Helm, R. Wilk, F. Rutz, T. Kleine-Ostmann, M. Koch, and P. Jepsen, “Terahertz time-domain spectroscopy and imaging of artificial RNA,” Opt. Express 13(14), 5205–5215 (2005). [CrossRef] [PubMed]
4. R. M. Woodward, B. E. Cole, V. P. Wallace, R. J. Pye, D. D. Arnone, E. H. Linfield, and M. Pepper, “Terahertz pulse imaging in reflection geometry of human skin cancer and skin tissue,” Phys. Med. Biol. 47(21), 3853–3863 (2002). [CrossRef] [PubMed]
5. H.-T. Chen, W. J. Padilla, M. J. Cich, A. K. Azad, R. D. Averitt, and A. J. Taylor, “A metamaterial solid state terahertz phase modulator,” Nat. Photonics 3(3), 148–151 (2009). [CrossRef]
6. H. Tao, N. I. Landy, C. M. Bingham, X. Zhang, R. D. Averitt, and W. J. Padilla, “A metamaterial absorber for the terahertz regime: Design, fabrication and characterization,” Opt. Express 16(10), 7181–7188 (2008). [CrossRef] [PubMed]
7. S. Hayashi, K. Nawata, H. Sakai, T. Taira, H. Minamide, and K. Kawase, “High-power, single-longitudinal-mode terahertz-wave generation pumped by a microchip Nd:YAG laser [Invited],” Opt. Express 20(3), 2881–2886 (2012). [CrossRef] [PubMed]
8. E. Castro-Camus, J. Lloyd-Hughes, L. Fu, H. H. Tan, C. Jagadish, and M. B. Johnston, “An ion-implanted InP receiver for polarization resolved terahertz spectroscopy,” Opt. Express 15(11), 7047–7057 (2007). [CrossRef] [PubMed]
9. H.-T. Chen, J. F. O’Hara, A. K. Azad, and A. J. Taylor, “Manipulation of terahertz radiation using metamaterials,” Laser Photon. Rev. 5(4), 513–533 (2011). [CrossRef]
10. J. Grant, Y. Ma, S. Saha, A. Khalid, and D. R. S. Cumming, “Polarization insensitive, broadband terahertz metamaterial absorber,” Opt. Lett. 36(17), 3476–3478 (2011). [CrossRef] [PubMed]
11. L. Zhang, H. Zhong, C. Deng, C. Zhang, and Y. Zhao, “Terahertz wave reference-free phase imaging for identification of explosives,” Appl. Phys. Lett. 92(9), 091117 (2008). [CrossRef]
12. N. I. Landy, C. M. Bingham, T. Tyler, N. Jokerst, D. R. Smith, and W. J. Padilla, “Design, theory, and measurement of a polarization-insensitive absorber for terahertz imaging,” Phys. Rev. B 79(12), 125104 (2009). [CrossRef]
13. H. Tao, C. M. Bingham, D. Pilon, K. Fan, A. C. Strikwerda, D. Shrekenhamer, W. J. Padilla, X. Zhang, and R. D. Averitt, “A dual band terahertz metameterial absorber,” J. Phys. D Appl. Phys. 43(22), 225102 (2010). [CrossRef]
14. X.-J. He, Y. Wang, J.-M. Wang, and T.-L. Gui, “Dual-band terahertz metamaterial absorber with polarization insensitivity and wide incident angle,” Prog. Electromag. Res. 115, 381–397 (2011).
15. Y. Q. Ye, Y. Jin, and S. He, “Omnidirectional, polarization-insensitive and broadband thin absorber in the terahertz regime,” J. Opt. Soc. Am. B 27(3), 498–504 (2010). [CrossRef]
16. J. Grant, Y. Ma, S. Saha, A. Khalid, and D. R. S. Cumming, “Polarization insensitive, broadband terahertz metamaterial absorber,” Opt. Lett. 36(17), 3476–3478 (2011). [CrossRef] [PubMed]
17. Y. W. Chen, P. Y. Han, and X.-C. Zhang, “Tunable broadband antireflection structures for silicon at terahertz frequency,” Appl. Phys. Lett. 94(4), 041106 (2009). [CrossRef]
18. B. T. DeWitt and W. D. Burnside, “Electromagneic scattering by pyramidal and wedge absorber,” IEEE Trans. Antenn. Propag. 36(7), 971–984 (1988). [CrossRef]
19. Y. M. Song and Y. T. Lee, “Investigation of geometrical effects of antireflective subwavelength grating structures for optical device applications,” Opt. Quantum Electron. 41(10), 771–777 (2009). [CrossRef]
20. E. Bassous and E. Bassous, “Fabrication of novel three-dimensional microstructures by the anisotropic etching of (100) and (110) silicon,” IEEE Trans. Electron. Dev. 25(10), 1178–1185 (1978). [CrossRef]
21. M. G. Moharam and T. K. Gaylord, “Three-dimensional vector coupled-wave analysis of planar-grating diffraction,” J. Opt. Soc. Am. 73(9), 1105–1112 (1983). [CrossRef]
22. P. A. George, W. Hui, F. Rana, B. G. Hawkins, A. E. Smith, and B. J. Kirby, “Microfluidic devices for terahertz spectroscopy of biomolecules,” Opt. Express 16(3), 1577–1582 (2008). [CrossRef] [PubMed]
23. C. Brückner, T. Käsebier, B. Pradarutti, S. Riehemann, G. Notni, E.-B. Kley, and A. Tünnermann, “Broadband antireflective structures applied to high resistive float zone silicon in the THz spectral range,” Opt. Express 17(5), 3063–3077 (2009). [CrossRef] [PubMed]
24. H. Tao, C. M. Bingham, A. C. Strikwerda, D. Pilon, D. Shrekenhamer, N. I. Landy, K. Fan, X. Zhang, W. J. Padilla, and R. D. Averitt, “Highly flexible wide angle of incidence terahertz metamaterial absorber: Design, fabrication, and characterization,” Phys. Rev. B 78(24), 241103 (2008). [CrossRef]
25. Z. Wu, J. Walish, A. Nolte, L. Zhai, R. E. Cohen, and M. F. Rubner, “Deformable antireflection coatings from polymer and nanoparticle multilayers,” Adv. Mater. (Deerfield Beach Fla.) 18(20), 2699–2702 (2006). [CrossRef]
