Antireflection self-reference method based on ultrathin metallic nanofilms for improving terahertz reflection spectroscopy

We present the potential of an antireflection self-reference method based on ultrathin tantalum nitride (TaN) nanofilms for improving terahertz (THz) reflection spectroscopy. The antireflection self-reference method is proposed to eliminate mutual interference caused by unwanted reflections, which significantly interferes with the important reflection from the actual sample in THz reflection measurement. The antireflection self-reference model was investigated using a wave-impedance matching approach, and the theoretical model was verified in experimental studies. We experimentally demonstrated this antireflection selfreference method can completely eliminate the effect of mutual interference, accurately recover the actual sample’s reflection and improve THz reflection spectroscopy. Our method paves the way to implement a straightforward, accurate and efficient approach to investigate THz properties of the liquids and biological samples. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement OCIS codes: (310.1210) Antireflection coatings; (300.6495) Spectroscopy, terahertz. References and links 1. B. Ferguson and X. C. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1(1), 26–33 (2002). 2. M. Tonouchi, “Cutting-edge terahertz technology,” Nat. Photonics 1(2), 97–105 (2007). 3. D. G. Cooke, F. A. Hegmann, E. C. Young, and T. Tiedje, “Electron mobility in dilute GaAs bismide and nitride alloys measured by time-resolved terahertz spectroscopy,” Appl. Phys. Lett. 89(12), 122103 (2006). 4. 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). 5. E. Pickwell, B. E. Cole, A. J. Fitzgerald, M. Pepper, and V. P. Wallace, “In vivo study of human skin using pulsed terahertz radiation,” Phys. Med. Biol. 49(9), 1595–1607 (2004). 6. U. Møller, D. G. Cooke, K. Tanaka, and P. U. Jepsen, “Terahertz reflection spectroscopy of Debye relaxation in polar liquids,” J. Opt. Soc. Am. B 26(9), A113–A125 (2009). 7. J. Shi, Y. Wang, T. Chen, D. Xu, H. Zhao, L. Chen, C. Yan, L. Tang, Y. He, H. Feng, and J. Yao, “Automatic evaluation of traumatic brain injury based on terahertz imaging with machine learning,” Opt. Express 26(5), 6371–6381 (2018). 8. 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). 9. W. Withayachumnankul, J. F. O’Hara, W. Cao, I. Al-Naib, and W. Zhang, “Limitation in thin-film sensing with transmission-mode terahertz time-domain spectroscopy,” Opt. Express 22(1), 972–986 (2014). 10. P. U. Jepsen, U. Møller, and H. Merbold, “Investigation of aqueous alcohol and sugar solutions with reflection terahertz time-domain spectroscopy,” Opt. Express 15(22), 14717–14737 (2007). Vol. 26, No. 15 | 23 Jul 2018 | OPTICS EXPRESS 19470 #330282 https://doi.org/10.1364/OE.26.019470 Journal © 2018 Received 7 May 2018; revised 10 Jul 2018; accepted 11 Jul 2018; published 18 Jul 2018 11. P. C. Ashworth, E. Pickwell-MacPherson, E. Provenzano, S. E. Pinder, A. D. Purushotham, M. Pepper, and V. P. Wallace, “Terahertz pulsed spectroscopy of freshly excised human breast cancer,” Opt. Express 17(15), 12444– 12454 (2009). 12. 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). 13. C. Rønne and S. R. Keiding, “Low frequency spectroscopy of liquid water using THz-time domain spectroscopy,” J. Mol. Liq. 101(1-3), 199–218 (2002). 14. S. Y. Huang, Y. X. Wang, D. K. Yeung, A. T. Ahuja, Y. T. Zhang, and E. Pickwell-Macpherson, “Tissue characterization using terahertz pulsed imaging in reflection geometry,” Phys. Med. Biol. 54(1), 149–160 (2009). 15. M. Nagai, H. Yada, T. Arikawa, and K. Tanaka, “Terahertz time-domain attenuated total reflection spectroscopy in water and biological solution,” Int. J. Infrared Millim. Waves 27(4), 505–515 (2006). 16. S. Huang, P. C. Ashworth, K. W. Kan, Y. Chen, V. P. Wallace, Y. T. Zhang, and E. Pickwell-MacPherson, “Improved sample characterization in terahertz reflection imaging and spectroscopy,” Opt. Express 17(5), 3848– 3854 (2009). 17. M. Dressel and G. Grüner, “Electrodynamics of solids: optical properties of electrons in matter,” (AAPT, 2002). 18. A. Thoman, A. Kern, H. Helm, and M. Walther, “Nanostructured gold films as broadband terahertz antireflection coatings,” Phys. Rev. B 77(19), 195405 (2008). 19. 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). 20. W. Lai, N. Born, L. M. Schneider, A. Rahimi-Iman, J. C. Balzer, and M. Koch, “Broadband antireflection coating for optimized terahertz beam splitters,” Opt. Mater. Express 5(12), 2812–2819 (2015). 21. J. Kröll, J. Darmo, and K. Unterrainer, “Metallic wave-impedance matching layers for broadband terahertz optical systems,” Opt. Express 15(11), 6552–6560 (2007). 22. M. Born and E. Wolf, Principles of Optics (Pergamon Press 1980). 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). 24. S. McKnight, K. Stewart, H. Drew, and K. Moorjani, “Wavelength-independent anti-interference coating for the far-infrared,” Infrared Phys. 27(5), 327–333 (1987). 25. E. Pickwell, B. Cole, A. Fitzgerald, V. Wallace, and M. Pepper, “Simulation of terahertz pulse propagation in biological systems,” Appl. Phys. Lett. 84(12), 2190–2192 (2004). 26. J. Kindt and C. Schmuttenmaer, “Far-infrared dielectric properties of polar liquids probed by femtosecond terahertz pulse spectroscopy,” J. Phys. Chem. 100(24), 10373–10379 (1996). 27. J. Barthel, K. Bachhuber, R. Buchner, and H. Hetzenauer, “Dielectric spectra of some common solvents in the microwave region. Water and lower alcohols,” Chem. Phys. Lett. 165(4), 369–373 (1990).


Introduction
Terahertz radiation lies between microwave and infrared radiation, which is within the frequency range for macromolecule's vibration and rotational frequencies. The terahertz radiation possesses a unique property for noninvasive spectroscopy and imaging of materials [1,2]. Terahertz spectroscopy and imaging are potential tools to nondestructively probe both chemical and structural information of materials [3], such as liquids and biomaterials [4][5][6]. Terahertz time-domain spectroscopy (THz-TDs) has been shown to be an important technique for studying liquids and biomaterials, and probing drugs-of-abuse and explosives [7,8]. THz-TDs is suitable for determining complex refractive index and dielectric function of materials, which is difficult to be obtained by traditional optical spectrometers [9].
To study terahertz properties of liquids and biological samples, terahertz measurements of liquids and biological samples were usually performed by THz-TDs in reflection geometry [10]. Due to strict requirements of sample preparation and the need of a special holder [11], it is difficult to perform the terahertz measurement of liquids and biological samples in transmission geometry. Moreover, most of liquids and biological samples have strong absorption of THz radiation, so that the sample measurement performed in the transmission geometry is limited. For terahertz measurements of liquids and biological samples, the reflection geometry is more feasible than transmission geometry [12]. In the terahertz measurement of reflection geometry, the liquids and biological samples are typically placed on a substrate, which is made of materials with negligible absorption in the terahertz frequency range, such as high-resistivity silicon and z-cut quartz. As a result, THz detector in the THz reflection measurement receives THz superposed reflection signals from the lower and upper surfaces of the substrate, as shown in Fig. 1(b). This phenomenon was called mutual interference. It arises from the ring tail (unwanted reflection) of the reference reflection from the lower surface of the substrate, which significantly interferes with the sample reflection from the upper surface of the substrate. The mutual interference influences the accuracy of sample information in the THz reflection measurement. Thus, in order to accurately acquire sample information, we need to eliminate mutual interference and to recover the reflection from the actual sample in the reflection measurement. However, the conventional method in the THz reflection measurement generally uses a thicker substrate of higher refractive index (such as silicon) to provide a greater optical delay between the reference reflection (from the lower surface of the substrate) and the sample reflection (from the upper surface of the substrate) [12][13][14]. Since the ringing tail of the reference reflection has further to propagate before it interferes with the sample reflection, the ringing tail is likely to be weaker. But, it has still an influence on accurate sample characterization. In addition, various methods for the THz reflection measurement of liquids and biological samples was proposed, including the ATR(attenuated total reflectance) method [15], the sample baseline method [16] and the optical delay method based on a thicker substrate [13]. However, these methods can partly reduce the effect of mutual interference but cannot completely eliminate the effect and have complicated measurement requirements, which restrict their practical applications.
In this paper, an antireflection (AR) self-reference method based on ultra-thin TaN nanofilms is proposed to eliminate mutual interference, to accurately recover the sample reflection and to improve the THz measurement of liquids and biological samples. This method paves the way to implement an accurate, easy and efficient route to investigate THz properties of liquids and biological samples. In addition, the apparent advantages of TaN nanofilms, such as low-cost, present high-stability and environmental compatibility, make this type of coatings a good candidate for industrial applications. In the future, the substrate covered by antireflection (AR) coatings can be designed as a sensitive sensor in THz spectroscopy systems.

Theoretical model
When THz pulses propagates through the interfaces between two media covered by a thin metallic film, there is a theoretical description for electromagnetic waves propagating through such interfaces [17]. If the thickness d of the thin conducting film is smaller than the skin depth 0 δ ( 0 d δ << ), dispersion of electromagnetic waves through the film is negligible [18][19][20]. In the terahertz frequency range, the conductivity of most metals has the real part r σ of frequency independence with a negligible imaginary component i σ [21,22].
Figure 1(a) shows the coated substrate was covered by a metallic antireflection coating. The optimal impedance matching at air-film-substrate interface was achieved by changing the thickness of metallic films, so that the reflection air R at the substrate-film-air interface is suppressed, i.e., the reflection coefficients r are equal to zero. Figure 1(b) shows the sample (liquids or biological samples) was placed on the coated substrate. The complex refractive index of the sample cause the impedance mismatch at sample-film-substrate interface, so that the reflection sam R at the interface exists, i.e., the reflection coefficients r are not equal to zero. In the THz-TDs measurement of the sample on the coated substrate, the measured electric field ( ) , as shown in Fig. 1 in the S-polarization mode (the electric field is perpendicular to the plane of incidence). Here, 0 Z is the impedance of free space, 1 θ is incident angle of THz waves in air, 2 θ is incident angle of THz waves in the substrate, air n is the refractive index of air, sub n is the refractive index of the substrate, sam n  is the complex refractive index of the sample, film σ is the conductivity of the metallic films, film d is the thickness of the metallic films. Similarly, the reflection and transmission coefficients for P-polarization mode (the electric field is parallel to the plane of incidence) is described in the reference [19].
In term of the above equations, the sample information was extracted from the ratio of ( )

Experimental setup
High-resistivity (HR) silicon ( 3000 cm > Ω ⋅ ) with 0.5 mm thickness was chosen as substrate. Due to negligible absorption of high-resistivity silicon in the THz frequency range, the imaginary part of refractive index of the high-resistivity silicon is negligible [23]. The HR silicon substrates can reduce the effect of material dispersion on experimental results in THz-TDs measurement [20]. The TaN films used as AR coating in this paper were prepared by RF magnetron sputtering. The sheet resistances of thin films were determined by a four-probe measurement. According to the antireflection theoretical model [20,24], the matching sheet resistance of the thin film in the impedance matching can be obtained by the theoretical calculation. Hereby, in the experiment, we need to prepare the thin films achieving the matching sheet resistance. Thus, the thickness of the thin film in the impedance matching is measured by a surface profiler. The liquid sample on the coated substrate was measured by using a terahertz time-domain spectroscopy (THz-TDs). The THz-TDs was performed in reflection mode. Photoconductor antenna illuminated by ultra-short laser pulses was used for the generation and detection of THz radiation. The pumping laser provided the pulses of 780 nm center wavelength, 100 fs pulse duration and 100 mW output power at a pulse repetition rate of approximately 80 MHz. The parabolic off-axis focusing mirrors were used to focus the terahertz beam on the sample. The frequency bandwidth in the terahertz system was limited to 1.5 THz. Outside this frequency range, the signal was too weak to obtain an acceptable signal-to-noise ratio in the terahertz system. In the THz reflection measurement, the terahertz beam was focus on the sample (incidence angle 1 30 θ =  ) in the S-polarization mode (the electric field is perpendicular to the plane of incidence).

Results and discussion
In the optimal impedance matching, the thickness of TaN films was about 14 nm. Figure 2(a) shows the AFM image of the TaN film produced by RF magnetron sputtering onto the silicon substrate. In the reflection measurements of bare substrate and coated substrate, the measured signal reflected from the bare substrate consists of several THz pulses (dark cyan curve), as shown in Fig. 2(b). The main pulse is reflected from the front side of the bare substrate, and the secondary pulse is reflected from the back side of the bare substrate. For the coated substrate, the second pulse (red curve) are completely suppressed compared with that of the bare substrate, as shown in Fig. 2(b). For the experiment of the AR self-reference method, water as the sample was measured. Firstly, air on the coated silicon substrate was used for measuring the reference refection ref R (as reference) from air-substrate interface, and the reflection air R at the substrate-film-air interface is suppressed, as shown in Fig. 1(a). The time-domain waveforms of the reference refection ref R from air-substrate interface are shown in Fig. 3. Then, the water sample on the coated silicon was measured, and the time-domain waveforms of the measured superposed reflections ref from both interfaces are shown in Fig. 3. In the experiment, the coated substrate was placed on the stage by the holder, so the misplacement error from the coated substrate was avoided. These THz measurements were repeatedly performed for three times, to ensure accuracy of the experimental results.   both interfaces. The mutual interference caused by the ring tail of the reference reflection was obvious. The sample reflection sam R from the substrate-film-water interface was accurately recovered using the AR self-reference method, as shown in Fig. 3. In order to compare our results with published data, the dielectric functions of the water were obtained using the aforementioned theory, and the dielectric spectrum of water was shown in Fig. 4 In Table 1, we compare the fitting parameters obtained using the experiment results with the data from the literature [8,13,[20][21][22]. The results of our analysis are in a good agreement with the values of the Debye parameters published in the literatures [10, 13,[25][26][27]. This demonstrates that the AR self-reference method can precisely characterize the dielectric properties of liquid samples. The small fluctuation of the dielectric spectrum was mainly caused by residual water vapor in the THz beam path. The residual water vapor has an effect on the dielectric spectrum of the testing liquid. As a result, using AR self-reference method, the superposed reflection interfering with the sample reflection was well eliminated, and the sample reflection from the substrate-film-sample interface was accurately recovered, as shown in Fig. 3. Figure 5 depicts the potential application of auto-measurement spectroscopy systems using the self-reference method. Therefore, the AR self-reference method can eliminate mutual interference and improve the THz measurements of liquid and biological samples.

Conclusions
In conclusion, we have described a theoretical model for the AR self-reference method based on the ultra-thin metallic films, which was validated with experimental determination of the dielectric function of water. The ultra-thin TaN nanofilms can be used as an efficient broadband AR coating in the terahertz frequency range. Moreover, we validated that the data extracted from water sample were in a good agreement with the values of the Debye parameters reported in the literature. We have shown the AR self-reference method can completely eliminate mutual interference, accurately recover the sample reflection and efficiently improve the THz measurement for liquid and biological samples. The AR selfreference method provides a promising route for THz spectroscopy sensors in the terahertz application, and makes it attractive to commercial applications.

Funding
The