Open Access
Add to BibSonomy
Abstract
We perform terahertz (THz) transmission spectroscopy to investigate the refractive index of crystalline BiOCl, which appears as a dielectric by-product during a certain process of fabricating superconducting Bi-based THz oscillators. We find that the refractive index is 4.8 or more for THz polarization parallel to the (001) plane; this value is substantially larger than the optical refractive index for parallel light polarization and also than the square root of the static dielectric constant for perpendicular electric field. Our experimental results can be interpreted from a strong dielectric anisotropy of crystalline BiOCl at low frequencies as predicted previously by a few theoretical studies.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Crystalline BiOCl is a unique layer-structured semiconductor with a wide energy gap of ∼3.5 eV and has found promising applications in photocatalysis [1–6] and optoelectronics [6,7]. Furthermore, it appears as an important by-product that surrounds the high-temperature superconductor Bi2Sr2CaCu2O8+x (Bi-2212) when compact terahertz (THz) oscillators based on Bi-2212 intrinsic Josephson junctions [8–10] are fabricated by the acid treatment method [11–14]; crystalline BiOCl has an opportunity of serving as a dielectric material in the THz region. So far, the far-infrared absorption spectrum of BiOCl powder in a frequency range of 1.2–12 THz has been measured by the Nujol mull method [15], and the static dielectric constants of crystalline BiOCl and similar compounds have been theoretically predicted to exhibit strong anisotropy due to their structural uniqueness [16,17]. However, the dielectric response of crystalline BiOCl to THz electromagnetic waves is not well understood.
In this work, we investigated the refractive index and extinction coefficient of crystalline BiOCl by using THz transmission spectroscopy. The THz transmission exhibited a substantial time delay and amplitude reduction, indicating that crystalline BiOCl has a relatively large refractive index of ∼4.8 for THz polarization parallel to the (001) plane (compared with conventional wide-gap semiconductors such as GaN and ZnO). A detailed spectral analysis revealed that the refractive index is indeed 4.8 or more with an anomalous dispersion and that the extinction coefficient is nearly zero at frequencies below 1.7 THz. For comparison, we furthermore evaluated the optical refractive index for light polarization parallel to the (001) plane and the static dielectric constant for electric field perpendicular to the (001) plane. The observed relatively large refractive index of crystalline BiOCl in the THz region can be ascribed to its strong dielectric anisotropy, in semi-quantitative agreement with previous theoretical simulations.
2. Experiment
2.1 Sample preparation
The sample in our experiment was (001)-oriented crystalline BiOCl with a film thickness of 19 μm, prepared in the following way: First, we grew a Bi-2212 single crystal by the traveling solvent floating zone (TSFZ) method [18] and cleaved it into several thin (001)-oriented flakes using a scalpel. Then, we immersed these flakes in dilute hydrochloric acid with a pH of 1.65. In a few hours, they changed to optically transparent BiOCl flakes [12,13]. Being careful to the fragility of the obtained free-standing BiOCl flakes, we selected one as the sample large enough for the measurements of THz transmission and optical transmission/reflection and mounted it on a stainless steel holder with a central hole of 3 mm in diameter. The photograph of the sample is shown in Fig. 1. We also evaluated the crystallinity and static dielectricity of the present BiOCl in similar flakes. All measurements were performed at room temperature.
Fig. 1. Photograph of the (001)-oriented crystalline BiOCl sample mounted on a stainless steel holder with a central hole of 3 mm in diameter.
2.2 Basic characterizations
A typical X-ray diffraction pattern of the present BiOCl is shown in Fig. 2, with seven clear peaks indicating its good crystallinity. We obtained basically the same results for all tested flakes. The diffraction peaks are assigned to (001) and parallel planes of the BiOCl tetragonal structure reported previously [12,13] and thus confirm the crystal orientation of the sample in Fig. 1.
Fig. 2. X-ray diffraction pattern of crystalline BiOCl, with seven peaks assigned to (001) and parallel planes of its tetragonal structure.
The nearly static dielectric constant for electric field perpendicular to the (001) plane was estimated by measuring the capacitance of BiOCl flakes with areas of more than ∼1.0 mm2 and thicknesses of 20–30 μm sandwiched by pairs of planar gold electrodes. Applying a sinusoidal voltage (with an effective value of 1.0 V) to the electrode pairs through a micro prober, we found the dielectric constant to be approximately 3.5 at low frequencies of 0.1–1.0 MHz. Note that the nearly static measurement is complementary to the THz and optical measurements described below, in terms of electric field direction as well as frequency.
2.3 THz transmission spectroscopy
By using THz time-domain spectroscopy, we measured the THz waveforms transmitted through the sample with the holder and passing through a reference holder of exactly the same type. We carried out this measurement only at normal incidence, considering that the sample was too thin for side incidence and too small for highly oblique incidence. We confirmed that the 90° rotation of the sample around the optical axis of the incident THz waves [19,20] led to no significant change in the THz transmission, as was expected from the BiOCl tetragonal structure. The measurement system was similar to that used recently for nanocellulose composite films [21]. The THz electric fields were emitted from a photoconductive antenna biased at 10 kHz and detected with a 2.0-mm-thick ZnTe electro-optic sensor, triggered by 65-fs-long optical pulses of a mode-locked Ti:sapphire laser. All paths of the THz waves were purged with dry air to reduce the unfavorable absorption induced by water vapor.
After the Fourier transformation of the measured THz waveforms, we obtained the complex transmission coefficient spectrum t(ω) of the sample with respect to frequency ω/2π. The complex refractive index spectrum ñ(ω) = n(ω) + iκ(ω) for THz polarization parallel to the (001) plane is linked to t(ω) [21–23]:
3. Results and discussion
The optical transmission and reflection spectra measured at nearly normal incidence are shown in Fig. 3(a). The transmittance and reflectance have almost constant values of 0.65 and 0.33, respectively, at light wavelengths of 750–850 nm; the sum of them reaches 0.98, indicating that absorption was negligibly small. Figure 3(b) shows the refractive index spectrum derived from Fig. 3(a) for light polarization parallel to the (001) plane. The obtained refractive index has a slightly anomalous dispersion, presumably due to ultraviolet absorption, and is approximately 2.65 in the wavelength range 750–850 nm.
Fig. 3. (a) Optical transmission and reflection spectra of the (001)-oriented crystalline BiOCl sample. (b) Refractive index spectrum for light polarization parallel to the (001) plane.
The transmitted and referenced THz waveforms are shown in Fig. 4 by the red and black curves, respectively. The origin of the time axis [24] is set to an arbitrary position common to these two waveforms. Despite the thinness of the sample, the transmitted signal exhibits a substantial time delay of τ = 0.24 ps together with an amplitude reduction of 62% at its largest negative peak. This indicates that the sample had a relatively large refractive index (compared with conventional wide-gap semiconductors such as GaN with n ∼ 3.1 [25] and ZnO with n ∼ 2.8 [26]), which is roughly estimated to be n = cτ/d + 1 = 4.8 if the dispersion and the internal reflections are ignored. The value n = 4.8 is expected to give a Fresnel reflection loss of 65% in amplitude even without any absorption and to almost account for the observed amplitude reduction. A more accurate analysis of refractive index n and extinction coefficient κ will be made later.
Fig. 4. THz waveforms transmitted through the (001)-oriented crystalline BiOCl sample (red curve) and passing through a reference holder (black curve).
The amplitude spectra obtained from the transmitted and referenced THz waveforms are shown in Fig. 5 by the red and black curves, respectively. The referenced signal peaks at ∼0.8 THz and extends to ∼3.0 THz, having dips at 1.2 and 1.7 THz due to residual water vapor in the measurement system. The transmitted signal clearly loses its strength above ∼2.2 THz, suggesting a kind of stopband, e.g., a possible reststrahlen band induced by optical phonon modes with rather low frequencies [27–29].
Fig. 5. Amplitude spectra obtained from the transmitted and referenced THz waveforms. Dips at 1.2 and 1.7 THz are due to residual water vapor in the measurement system.
Figure 6 shows the complex refractive index spectrum of the sample, plotted by filled circles for its real part n and by open circles for its imaginary part κ in a frequency range of 0.3–2.0 THz, for THz polarization parallel to the (001) plane. The refractive index n exhibits an anomalous dispersion throughout this range and increases monotonically from 4.8 to 6.9 with increasing frequency. The extinction coefficient κ remains nearly zero below 1.7 THz and increases rapidly above that. These values of n and κ, particularly at ∼0.8 THz, are indeed close to the crude estimates made in Fig. 4. The obtained spectral shapes of n and κ can be ascribed to the existence of some stopband at higher frequencies discussed in the preceding paragraph, although they are slightly distorted in the vicinity of the water-vapor absorption peaks. Note that the THz refractive index in Fig. 6 is much larger than the optical refractive index in Fig. 3(b) and also than the square root of the static dielectric constant in Section 2.2.
Fig. 6. Complex refractive index spectrum of crystalline BiOCl for THz polarization parallel to the (001) plane.
Now, let us discuss a possible reason for the observed relatively large refractive index of crystalline BiOCl in the THz region. A few theoretical studies have predicted that crystalline BiOCl and similar compounds will have strong dielectric anisotropy due to a huge contribution from optical phonon modes along the (001) plane: the static dielectric constant for electric field parallel to the (001) plane (εst,‖) will be 5–8 times larger than that for electric field perpendicular to it (εst,⊥) and also 7–12 times larger than the optical dielectric constant for light polarization parallel to the (001) plane (ε∞,‖) [16,17]. Here, εst,‖ can be regarded as similar to the square of n = 4.8 in our experiment because major optical phonon modes should be absent at frequencies below ∼0.8 THz [29]. The present value n2 = 23 is indeed 6.6 times larger than εst,⊥ = 3.5 (see Section 2.2) and also 3.3 times larger than ε∞,‖ = 2.652 = 7.0 [see Fig. 3(b)]. Thus, our experimental results are in semi-quantitative agreement with the theoretical simulations, and can be interpreted from a strong dielectric anisotropy that is due to very different natures of optical phonon modes along and perpendicular to the (001) plane in the unique layered structure of crystalline BiOCl.
4. Summary
We prepared a crystalline BiOCl sample by treating a Bi-2212 single crystal with dilute hydrochloric acid in the same manner as reported for the fabrication of superconducting Bi-2212 THz oscillators, and measured the complex refractive index ñ = n + iκ of the sample by using THz transmission spectroscopy. We found that crystalline BiOCl has relatively large refractive indices n of 4.8 and more (compared with conventional wide-gap semiconductors such as GaN and ZnO) with an anomalous dispersion and has nearly zero extinction coefficients κ at frequencies below 1.7 THz for THz polarization parallel to the (001) plane. The square of n = 4.8 turned out to be 6.6 times larger than the static dielectric constant for electric field perpendicular to the (001) plane and also 3.3 times larger than the square of the optical refractive index for light polarization parallel to the (001) plane, in semi-quantitative agreement with previous theoretical simulations on the anisotropy of optical phonon contributions. Thus, we concluded that the observed relatively large refractive index of crystalline BiOCl in the THz region can be ascribed to a strong dielectric anisotropy originating from its structural uniqueness. This work also shows potential usefulness of crystalline BiOCl as a THz dielectric material, especially for more flexible design of Bi-based compact THz oscillators.
Funding
Nagaoka University of Technology Presidential Research Grant.
Acknowledgments
We thank Ms. Iffah F. A. Hamdany for her preliminary experiments on this subject at Nagaoka University of Technology.
Disclosures
The authors declare no conflicts of interest.
References
1. H. Cheng, B. Huang, and Y. Dai, “Engineering BiOX (X = Cl, Br, I) nanostructures for highly efficient photocatalytic applications,” Nanoscale 6(4), 2009–2026 (2014). [CrossRef]
2. J. Li, Y. Yu, and L. Zhang, “Bismuth oxyhalide nanomaterials: layered structures meet photocatalysis,” Nanoscale 6(15), 8473–8488 (2014). [CrossRef]
3. J. Li, H. Li, G. Zhan, and L. Zhang, “Solar water splitting and nitrogen fixation with layered bismuth oxyhalides,” Acc. Chem. Res. 50(1), 112–121 (2017). [CrossRef]
4. X. Zhang, X. Liu, C. Fan, Y. Wang, Y. Wang, and Z. Liang, “A novel BiOCl thin film prepared by electrochemical method and its application in photocatalysis,” Appl. Catal. B 132-133, 332–341 (2013). [CrossRef]
5. K. Li, Y. Tang, Y. Xu, Y. Wang, Y. Huo, H. Li, and J. Jia, “A BiOCl film synthesis from Bi2O3 film and its UV and visible light photocatalytic activity,” Appl. Catal. B 140-141, 179–188 (2013). [CrossRef]
6. M. Li, J. Zhang, H. Gao, F. Li, S.-E. Lindquist, N. Wu, and R. Wang, “Microsized BiOCl square nanosheets as ultraviolet photodetectors and photocatalysts,” ACS Appl. Mater. Interfaces 8(10), 6662–6668 (2016). [CrossRef]
7. L. Kang, X. Yu, X. Zhao, Q. Ouyang, J. Di, M. Xu, D. Tian, W. Gan, C. C. I. Ang, S. Ning, Q. Fu, J. Zhou, R. G. Kutty, Y. Deng, P. Song, Q. Zeng, S. J. Pennycook, J. Shen, K.-T. Yong, and Z. Liu, “Space-confined microwave synthesis of ternary-layered BiOCl crystals with high-performance ultraviolet photodetection,” InfoMat 2(3), 593–600 (2020). [CrossRef]
8. L. Ozyuzer, A. E. Koshelev, C. Kurter, N. Gopalsami, Q. Li, M. Tachiki, K. Kadowaki, T. Yamamoto, H. Minami, H. Yamaguchi, T. Tachiki, K. E. Gray, W.-K. Kwok, and U. Welp, “Emission of Coherent THz Radiation from Superconductors,” Science 318(5854), 1291–1293 (2007). [CrossRef]
9. T. Kashiwagi, M. Tsujimoto, T. Yamamoto, H. Minami, K. Yamaki, K. Delfanazari, K. Deguchi, N. Orita, T. Koike, R. Nakayama, T. Kitamura, M. Sawamura, S. Hagino, K. Ishida, K. Ivanovic, H. Asai, M. Tachiki, R. A. Klemm, and K. Kadowaki, “High temperature superconductor terahertz emitters: fundamental physics and its applications,” Jpn. J. Appl. Phys. 51(1R), 010113 (2012). [CrossRef]
10. R. Kleiner and H. Wang, “Terahertz emission from Bi2Sr2CaCu2O8+x intrinsic Josephson junction stacks,” J. Appl. Phys. 126(17), 171101 (2019). [CrossRef]
11. T. Kato, J. Chen, S. Sunaga, H. Mizumaru, T. Asano, H. Shimakage, K. Yasui, and K. Hamasaki, “Characterization of Bi2Sr2CaCu2O8+x stacks fabricated by acid treatment process,” IEEE Trans. Appl. Supercond. 21(3), 172–175 (2011). [CrossRef]
12. T. Kato, N. Iso, A. Miwa, H. Suematsu, A. Kawakami, K. Yasui, and K. Hamasaki, “Fabrication of a Bi2Sr2CaCu2O8+x intrinsic DC-SQUID with a shunt resistor,” IEEE Trans. Appl. Supercond. 21(3), 379–382 (2011). [CrossRef]
13. T. Kato, H. Ishida, H. Suematsu, K. Yasui, and K. Hamasaki, “Fabrication process of intrinsic Josephson junction stacks in Bi2Sr2CaCu2O8+x crystals by double-sided patterning process using dilute hydrochloric acid,” Cryogenics 52(7-9), 398–402 (2012). [CrossRef]
14. T. Nishikata, T. Kato, Y. Kotaki, H. Suematsu, A. Kawakami, and K. Yasui, “Hydrochloric acid modification process for fabricating Bi2Sr2CaCu2O8+x THz oscillator stack on-chip coupled to THz detector,” Jpn. J. Appl. Phys. 53(4S), 04EJ02 (2014). [CrossRef]
15. J. E. D. Davis, “Solid state vibrational spectroscopy—III [1] the infrared and Raman spectra of the bismuth (III) oxide halides,” J. Inorg. Nucl. Chem. 35(5), 1531–1534 (1973). [CrossRef]
16. H. Shi, W. Ming, and M.-H. Du, “Bismuth chalcohalides and oxyhalides as optoelectronic materials,” Phys. Rev. B 93(10), 104108 (2016). [CrossRef]
17. Z. Ran, X. Wang, Y. Li, D. Yang, X.-G. Zhao, K. Biswas, D. J. Singh, and L. Zhang, “Bismuth and antimony-based oxyhalides and chalcohalides as potential optoelectronic materials,” npj Comput. Mater. 4(1), 14 (2018). [CrossRef]
18. T. Mochiku and K. Kadowaki, “Growth and properties of Bi2Sr2(Ca, Y)Cu2O8+δ single crystals,” Phys. C (Amsterdam, Neth.) 235-240(1), 523–524 (1994). [CrossRef]
19. T. Unuma, A. Umemoto, and H. Kishida, “Anisotropic terahertz complex conductivities in oriented polythiophene films,” Appl. Phys. Lett. 103(21), 213305 (2013). [CrossRef]
20. Y. Wang, Z. Cui, D. Zhu, X. Zhang, and L. Qian, “Tailoring terahertz surface plasmon wave through free-standing multi-walled carbon nanotubes metasurface,” Opt. Express 26(12), 15343–15352 (2018). [CrossRef]
21. T. Unuma, O. Kobayashi, I. F. A. Hamdany, V. Kumar, and J. J. Saarinen, “Terahertz complex conductivity of nanofibrillar cellulose-PEDOT:PSS composite films,” Cellulose 26(5), 3247–3253 (2019). [CrossRef]
22. T. Unuma, K. Fujii, H. Kishida, and A. Nakamura, “Terahertz complex conductivities of carriers with partial localization in doped polythiophenes,” Appl. Phys. Lett. 97(3), 033308 (2010). [CrossRef]
23. T. Unuma, N. Yamada, A. Nakamura, H. Kishida, S.-C. Lee, E.-Y. Hong, S.-H. Lee, and O.-P. Kwon, “Direct observation of carrier delocalization in highly conducting polyaniline,” Appl. Phys. Lett. 103(5), 053303 (2013). [CrossRef]
24. T. Unuma and K. Minami, “Generalized framework for determining time origin in terahertz emission spectroscopy on the basis of causality,” Opt. Express 27(4), 5136–5143 (2019). [CrossRef]
25. M. T. Hibberd, V. Frey, B. F. Spencer, P. W. Mitchell, P. Dawson, M. J. Kappers, R. A. Oliver, C. J. Humphreys, and D. M. Graham, “Dielectric response of wurtzite gallium nitride in the terahertz frequency range,” Solid State Commun. 247, 68–71 (2016). [CrossRef]
26. A. K. Azad, J. Han, and W. Zhang, “Terahertz dielectric properties of high-resistivity single-crystal ZnO,” Appl. Phys. Lett. 88(2), 021103 (2006). [CrossRef]
27. A. Rulmont, “Raman spectra of a single crystal of BiOCl,” Spectrochim. Acta, Part A 30(1), 311–313 (1974). [CrossRef]
28. S. Bunda and V. Bunda, “Raman spectra of bismuth oxyhalide single crystals,” Acta Phys. Pol., A 126(1), 272–273 (2014). [CrossRef]
29. J. Lu, W. Zhou, X. Zhang, and G. Xiang, “Electronic structures and lattice dynamics of layered BiOCl single crystals,” J. Phys. Chem. Lett. 11(3), 1038–1044 (2020). [CrossRef]
