Broadband antireflection coating for optimized terahertz beam splitters Weien Lai,1,2,* Norman Born,1 Lorenz Maximilian Schneider,1 Arash Rahimi-Iman,1 Jan C. Balzer,1 and Martin Koch1 1
Faculty of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, Germany 2 State Key Laboratory of Electronic Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China *
[email protected]
Abstract: We investigate the potential of anti-ferromagnetic nanofilms as broadband antireflection coatings in the terahertz frequency range. The antiferromagnetic layer is modeled by an analytic wave-impedance matching approach. The experimental results of the transmission and reflection measurements demonstrate the effectiveness of our antireflection coatings. Furthermore, we use anti-ferromagnetic nanofilms as antireflection coating for a terahertz beam splitter. Compared with conventional terahertz beam splitters consisting of an uncoated thick silicon wafer, the coated silicon beam splitter has two advantages: elimination of multiple reflections and improvement of the signal-to-noise ratio for terahertz time-domain spectroscopy in reflection geometry. ©2015 Optical Society of America OCIS codes: (310.1210) Antireflection coatings; (160.3900) Metals; (300.6495) Spectroscopy, terahertz.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1(2), 97–105 (2007). P. U. Jepsen, D. G. Cooke, and M. Koch, “Terahertz spectroscopy and imaging - Modern techniques and applications,” Laser Photonics Rev. 5(1), 124–166 (2011). P. H. Siegel, “Terahertz technology,” IEEE Trans. Microw. Theory Tech. 50(3), 910–928 (2002). R. Ulbricht, E. Hendry, J. Shan, T. F. Heinz, and M. Bonn, “Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy,” Rev. Mod. Phys. 83(2), 543–586 (2011). B. Ferguson and X.-C. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1(1), 26–33 (2002). B. Scherger, M. Scheller, C. Jansen, M. Koch, and K. Wiesauer, “Terahertz lenses made by compression molding of micropowders,” Appl. Opt. 50(15), 2256–2262 (2011). S. F. Busch, M. Weidenbach, M. Fey, F. Schäfer, M. Koch, and T. Probst, “Optical properties of 3D printable plastics in the THz regime and their application for 3D printed THz optics,” J. Infrared Millim. Terahertz Waves 35(12), 993–997 (2014). K. Nielsen, H. K. Rasmussen, A. J. L. Adam, P. C. Planken, O. Bang, and P. U. Jepsen, “Bendable, low-loss Topas fibers for the terahertz frequency range,” Opt. Express 17(10), 8592–8601 (2009). M. Skorobogatiy and A. Dupuis, “Ferroelectric all-polymer hollow Bragg fibers for terahertz guidance,” Appl. Phys. Lett. 90(11), 113514 (2007). C. Jansen, S. Wietzke, V. Astley, D. M. Mittleman, and M. Koch, “Mechanically flexible polymeric compound one-dimensional photonic crystals for terahertz frequencies,” Appl. Phys. Lett. 96(11), 111108 (2010). B. Scherger, M. Scheller, N. Vieweg, S. T. Cundiff, and M. Koch, “Paper terahertz wave plates,” Opt. Express 19(25), 24884–24889 (2011). C. C. Homes, G. L. Carr, R. P. S. M. Lobo, J. D. Laveigne, and D. B. Tanner, “Silicon beam splitter for farinfrared and terahertz spectroscopy,” Appl. Opt. 46(32), 7884–7888 (2007). C. W. Berry and M. Jarrahi, “Broadband terahertz polarizing beam splitter on a polymer substrate,” J. Infrared Millim. Terahertz Waves 33(2), 127–130 (2012). B. S. Ung, C. Fumeaux, H. Lin, B. M. Fischer, B. W. Ng, and D. Abbott, “Low-cost ultra-thin broadband terahertz beam-splitter,” Opt. Express 20(5), 4968–4978 (2012). J. S. Li, D. G. Xu, and J. Q. Yao, “Compact terahertz wave polarizing beam splitter,” Appl. Opt. 49(24), 4494– 4497 (2010). T. Niu, W. Withayachumnankul, A. Upadhyay, P. Gutruf, D. Abbott, M. Bhaskaran, S. Sriram, and C. Fumeaux, “Terahertz reflectarray as a polarizing beam splitter,” Opt. Express 22(13), 16148–16160 (2014).
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17. H. T. Chen, J. Zhou, J. F. O’Hara, and A. J. Taylor, “A numerical investigation of metamaterial antireflection coatings,” Tstnetwork. Org. 3(2), 66–73 (2010). 18. W. E. Lai, H. W. Zhang, Y. H. Zhu, Q. Y. Wen, W. W. Du, and X. L. Tang, “Bilayer metallic nanofilms as broadband antireflection coatings in terahertz optical systems,” Opt. Express 22(3), 2174–2184 (2014). 19. W. E. Lai, H. W. Zhang, Y. H. Zhu, and Q. Y. Wen, “A novel method of terahertz spectroscopy and imaging in reflection geometry,” Appl. Spectrosc. 67(1), 36–39 (2013). 20. K. Kawase and N. Hiromoto, “Terahertz-wave antireflection coating on Ge and GaAs with fused quartz,” Appl. Opt. 37(10), 1862–1866 (1998). 21. A. Thoman, A. Kern, H. Helm, and M. Walther, “Nanostructured gold films as broadband terahertz antireflection coatings,” Phys. Rev. B 77(19), 195405 (2008). 22. J. Kröll, J. Darmo, and K. Unterrainer, “Metallic wave-impedance matching layers for broadband terahertz optical systems,” Opt. Express 15(11), 6552–6560 (2007). 23. H. T. Chen, J. Zhou, J. F. O’Hara, F. Chen, A. K. Azad, and A. J. Taylor, “Antireflection coating using metamaterials and identification of its mechanism,” Phys. Rev. Lett. 105(7), 073901 (2010). 24. Y. Zhou, X. Xu, F. Hu, X. Zheng, W. Li, P. Zhao, J. Bai, and Z. Ren, “Graphene as broadband terahertz antireflection coating Graphene as broadband terahertz antireflection coating,” Appl. Phys. Lett. 104(5), 051106 (2014). 25. F. Yan, E. P. J. Parrott, X. D. Liu, and E. Pickwell-MacPherson, “Low-cost and broadband terahertz antireflection coatings based on DMSO-doped PEDOT/PSS,” Opt. Lett. 40(12), 2886–2889 (2015). 26. 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). 27. X. Wang, Y. Li, B. Cai, Y. Zhu, X. Wang, Y. Li, B. Cai, and Y. Zhu, “High refractive index composite for broadband antireflection in terahertz frequency range High refractive index composite for broadband antireflection in terahertz frequency range,” Appl. Phys. Lett. 106(23), 231107 (2015). 28. L. Ding, Q. Yang, S. Wu, and J. H. Teng, “Polarization independent broadband terahertz antireflection by deepsubwavelength thin metallic mesh,” Laser Photonics Rev. 8(6), 941–945 (2014). 29. M. Wichmann, M. Stein, A. Rahimi-Iman, S. W. Koch, and M. Koch, “Interferometric characterization of a semiconductor disk laser driven terahertz source,” J. Infrared Millim. Terahertz Waves 35(6–7), 503–508 (2014). 30. K. M. S. W. McKnight, K. P. Stewart, H. D. Drew, and K. Moorjani, “Wavelength-independent anti-interference coating for the far-infrared,” Infrared Phys. Technol. 27(5), 327–333 (1987). 31. M. Dressel and G. Gruener, “Electrodynamics of Solids,” Electrodynamics of Solids (Cambridge Press, 2002). 32. M. Born and E. Wolf, “Principles of optics,” Principles of Optics (Pergamon Press, 1980). 33. D. Grischkowsky, S. Keiding, M. Van Exter, and C. Fattinger, “Far-infrared time domain-spectroscopy with terahertz beams of dielectrics an semiconductors,” J. Opt. Soc. Am. B 7(10), 2006–2015 (1990).
1. Introduction Recently, terahertz (THz) technology has become increasingly attractive because of its potential applications [1–3]. Important applications of THz technology are spectroscopy and imaging, which are becoming tools for characterizing a variety of materials including semiconductors, high-temperature superconductors, and biomaterial specimens [4, 5]. A mature THz technology does not only require emitters and receivers but also passive components to guide THz waves. This includes lenses [6, 7], waveguides [8, 9], mirrors [10], wave plates [11], and beam splitters [12–16]. Many THz components and devices are fabricated on substrates with a high refractive index [17, 18]. As a consequence, these components and devices cause unwanted multiple reflections arising from the refractive index mismatch at the dielectric interface. These multiple reflections lead to an undesired modulation in the frequency domain due to Fabry-Pérot oscillations. This modulation can significantly influence the performance of THz systems [19]. In order to eliminate the unwanted reflections, various antireflection layers in the THz range have been studied. One approach which is well known from optical frequencies uses quarter-wave dielectric layers as antireflection coating [20]. This approach is quite challenging in the broadband terahertz frequency range since it is difficult to satisfy the requirements for a broad frequency region with only one layer. Furthermore, antireflection layers using non-magnetic films have been demonstrated [21, 22]. However, these antireflection layers are mainly based on a homogeneous single-layer metallic film with a high conductivity and were only characterized in transmission geometry. Further concepts for antireflection coatings, such as those employing metamaterials [23], graphene [24], dimethylsulfoxide compounds [25] and nanostructures [26–28], have been recently demonstrated. Yet, metamaterials only work within a narrow frequency band. Furthermore, each of the aforementioned concepts relies on #251282 Received 2 Oct 2015; revised 5 Nov 2015; accepted 9 Nov 2015; published 16 Nov 2015 © 2015 OSA 1 Dec 2015 | Vol. 5, No. 12 | DOI:10.1364/OME.5.002812 | OPTICAL MATERIALS EXPRESS 2813
complex fabrication techniques. There is still a large demand for new materials with better performances. In this paper, we study a broadband antireflection coating consisting of anti-ferromagnetic Ir25Mn75 thin films for the THz range. Antiferromagnetic materials are versatile materials that are stable, low-cost and easy to fabricate. The performance of the antireflection layer is verified in transmission and reflection geometries. In addition, we propose to use our antireflection coating for a THz beam splitter made of silicon. THz beam splitters are important components in terahertz time-domain spectrometer (THz-TDS) systems working in reflection geometry at normal incidence. Furthermore, they can be found in Michelson interferometers for THz waves [29]. In the past several approaches for beam splitters have been presented. Some of them lead to polarizing beam splitters. Some use very thin layers which could be sensitive to vibrations. Hence, in most cases beam splitters in terahertz systems are made of a thick silicon wafer [12]. Yet, unwanted multiple reflections may cause a problem in this case. However, the method we propose here can eliminate such unwanted reflections and improve the signal-to-noise ratio compared with a bare silicon beam splitter. Additionally, it offers a cost-effective alternative to thick silicon beam splitters as the silicon wafers can be considerably thinner. 2. Theoretical model of antireflection coatings Figure 1 schematically shows a substrate with refractive index n2 which is covered by the antireflection coating. It also shows the transmitted THz beam and its multiple reflections. Let us briefly recall the theoretical description for electromagnetic waves encountering such interfaces. One important parameter is the characteristic impedance [30, 31] which is Z expressed as Z = 0 . Here n = εr is the complex refractive index and Z 0 is the impedance n of free space. If the thickness d of the thin conducting film is smaller than the skin depth δ 0 ( d
