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## Article

Curr. Opt. Photon. 2021; 5(4): 370-374

Published online August 25, 2021 https://doi.org/10.3807/COPP.2021.5.4.370

## Terahertz Complex Refractive Index and Guiding of White Staghorn Coral

Chul Kang1, Myunghwan Kim1, Hyeongmun Kim1, Jin Young Park1,2, Bok Hyeon Kim1, Inhee Maeng1, SooBong Choi2, Soeun Kim1, Chul-Sik Kee1

1Advanced Photonics Research Institute, Gwangju Institute Science and Technology, Gwangju 61105, Korea
2Department of Physics, Incheon National University, Incheon 22012, Korea

Corresponding author: *cskee@gist.ac.kr, ORCID 0000-0002-3219-5119
Current affiliation: YUHS-KRIBB, Medical Convergence Research Institute, College of Medicine, Yonsei University, Seoul 03722, Korea

Received: May 18, 2021; Revised: June 7, 2021; Accepted: June 8, 2021

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

Corals are the remains of animals that grow on warm beaches. They have been used as decorative jewels because of their variety of colors, and as medicinal materials for treating cancers, AIDS, and other therapeutic uses because of their chemical elements. Corals are mainly composed of calcium carbonate (CaCO3) and have many air pores, tens to hundreds of micrometers in size. The refractive indices and absorption coefficients of dried sliced staghorn corals are investigated using terahertz time-domain spectroscopy. The measured values are similar to those for CaCO3, as expected. It is observed that a sample with a microstructure formed by air pores can guide terahertz waves. The dispersion, effective index, and loss of the guiding modes of coral core surrounded by five triangular air pores are numerically calculated. The simulated spatial distribution of the electric field of the guide mode at 1.25 THz shows the mode to be tightly confined to the core.

Keywords: Coral, Optical waveguide, Terahertz

OCIS codes: (040.2235) Far infrared or terahertz; (060.4510) Optical communication; (130.5296) Photonic crystal waveguides

### I. INTRODUCTION

Coral is a special animal among the cnidarians that was once mistaken for plants or minerals, because it looks like a plant growing on warm beaches and has a calcareous skeleton. It is also very interesting due to the many unique features of its material properties. Coral is used as a decorative gem due to its various colors, and because of its chemical composition it is also used as a medicinal material for treating cancer, AIDS, and other therapeutic uses [1]. In the past, it was also used as a material for ancient wall paintings [2].

These corals can be divided into stony corals and relatively colorful soft corals. Of these, stony corals are composed mostly of calcium carbonate (CaCO3), which can also be used as a mineral [3]. Corals are generally known to have numerous air pores, ranging from tens to hundreds of micrometers in size [46]. There is a possibility that corals can strongly interact with electromagnetic waves in the terahertz (THz) region. For example, corals with air pores hundreds of microns in size can act as microstructured fibers to guide THz waves. However, it has not been reported that THz waves can propagate through the microstructures of corals. Investigating the guiding of THz radiation through corals would be interesting.

In this study, the characteristics of the refractive indices of staghorn coral, rock corals with cylindrical branches ranging from a few cm to a few meters, are investigated in the frequency range from 0.2 to 1.5 THz using THz time-domain spectroscopy. The measured refractive indices are similar to those of CaCO3, as expected [7]. One sample includes a microstructure formed by air pores, like a microstructured optical fiber, around a central region. The guiding of THz waves through the microstructure is observed in a frequency range above 1 THz.

### II. Experiment

To measure their refractive indices in the THz region, staghorn corals were cut in cross section perpendicular to the cylindrical axis, using a mechanical saw. Figures 1(a) and 1(b) show samples with cross-sectional diameters of 12 mm (sample 1) and 8 mm (sample 2) respectively. The lengths of the samples were about 1.57 mm and 0.86 mm respectively. As seen in Fig. 1(b), sample 2 has a structure surrounded by air pores, like a microstructured optical fiber, around a central region of the sample. It would be interesting to investigate whether that microstructure could guide THz waves.

Figure 1.Cross-sectional images of samples: (a) sample 1, (b) sample 2.

Figure 2 shows a schematic view of the THz time-domain spectroscopy system. To generate the THz waves we employ the p-type InAs substrate that is generally used for THz wave sources with 80-MHz repetition-rate Ti:Sapphire femtosecond laser pulses. The input power of femtosecond laser pulses is 350 mW after the mechanical chopping blade, with a center wavelength of 800 nm. The incidence angle of the femtosecond laser pulses is 45° and they are 5 mm in diameter on the p-type InAs substrate. The four parabolic mirrors focus and collimate the radiated THz waves; the focused beam waist is around 1.5 mm. The detector for THz waves is used with a 5-μm dipole-gap photoconductive antenna on the low-temperature-grown GaAs substrate with 10 mW femtosecond laser power. To eliminate absorption by rotational and vibrational modes of water vapor, we measure under dry-air conditions at 1% humidity in an airtight box.

Figure 2.Schematic view of the THz time-domain spectroscopy system employed in the experiments.

### III. Results and Analysis

To investigate the refractive indices and absorption coefficients of the samples, THz transmission through the samples was investigated by THz time-domain spectroscopy. The refractive indices and absorption coefficient are related to phase delay and transmission in the frequency domain. The refractive indices are obtained using the followings relations [8]:

Oω=Iωexpdαω2expi2πλn1ωd,
n1ω=1+ϕIϕO2πdλ0,αω=2dInOωIω=4πn2λ0,

where O(ω) is the THz transmission through the sample, I(ω) is the electric field of an incident THz wave, n1 is the real refractive index, α(ω) is the power absorption, and n2 is the imaginary refractive index. Also, d is the thickness of the sample, ϕO and ϕI are the phases of the THz transmission signals with and without samples, respectively, and l is the wavelength in the medium. To investigate the guiding property of the microstructure of sample 2, THz transmission through the central region containing the microstructure is compared to that through regions without it.

Figures 3(a) and 3(b) respectively show time- and frequency-domain transmission waveforms through samples with a reference metal hole 2 mm in diameter. The coral samples reduce THz transmission by about 20%. In Fig. 3(a), the slight difference of time delay between transmitted THz waves of the samples is due to the different lengths of samples. Figure 3(b) shows that, for sample1, the spectral characteristics of transmitted THz waves through a central region (S1C) are similar to those through an off-center region (S1O). However, for sample 2 the spectral characteristics of transmitted THz waves through a central region with the microstructure (S2C) are different from those through an off-center region (S2O), in a frequency range above 1 THz.

Figure 3.Transmission waveforms in (a) the time domain and (b) the frequency domain, with a reference metal hole 2 mm in diameter.

Figures 4(a) and 4(b) respectively show the real refractive indices and absorption coefficients of the samples. As expected, the values of both refractive indices and absorption coefficients of the samples are similar to those of CaCO3 [7]. The real refractive index of sample 1 is greater than that of sample 2, implying that the number of air pores per unit volume for sample 1 is smaller than for sample 2. The real refractive indices and absorption coefficients of S1O, S1C, and S2O increase over the frequency range from 0.2 to 1.5 THz. The absorption coefficients of the samples increase as the frequency increases. However, it should be noticed that the refractive index and absorption coefficient of S2C decrease above 1 THz. This implies that the microstructure guides THz waves [9]. These results mean that the guiding mode spreads out more to the air-cladding region as frequency increases, so that the effective refractive index and absorption of the guiding mode decrease as frequency increases.

Figure 4.Optical properties of coral samples: (a) indices of refraction (real) and (b) absorption coefficients.

Figure 5(a) shows the transmitted THz waves through S2O and S2C between 7 and 12 ps in the time domain, which are marked in a circle in the inset. Figure 5(b) presents the spectral amplitudes of the transmitted THz waves. These figures clearly confirm that S2C can guide THz waves in the frequency range between 1.1 and 1.6 THz. However, it is not clear whether the air-pore microstructure guides THz waves around 0.2 THz, because our measurement system contains remarkable noise signals in that frequency range.

Figure 5.THz guiding signals: (a) Transmitted THz waves through S2O and S2C between 7 and 12 ps in the time domain, which are marked in a circle in the inset. (b) Spectral amplitudes of the transmitted THz waves.

It is necessary to study the properties of guiding modes through S2C by using numerical simulations, but it is difficult to reflect exactly the shapes of the air pores in the numerical calculations. Thus the air pore was shaped as an isosceles triangle with apex angle of 53°. The length of the perpendicular bisector is 5 mm. The distance between the center of the coral and the triangle’s vertex facing the center is 1 mm. The length of the sides of triangles is assumed to be 0.85 mm. The commercial simulation program Lumerical MODE is employed in the calculation. Perfectly matched layer (PML) boundary conditions are applied to eliminate reflections at the boundaries, and a dense mesh size of 10 μm is used. The real and imaginary refractive indices of the sample (n and k, 2.55 and 0.07 respectively) are employed in the simulations. The values are the measured indices of the sample at 1.25 THz. Figures 6(a)6(c) present respectively the dispersion, effective index, and loss of the guide modes of the microstructured coral fiber surrounded by five triangular air pores. The values of dispersion are between 1.0 and 1.1 [ps/(nm·km)]. The effective index and loss increase as the frequency increases; this implies that the electric field of the guide mode is strongly confined in the core as the frequency increases. The spatial profile of the squared electric field of the guiding mode at 1.25 THz is presented in Fig. 6(d). The guiding mode was clearly confined to the core surrounded by the triangular air pores.

Figure 6.Numerical calculation results: (a) dispersion, (b) effective index, and (c) loss of the guided modes of the microstructured coral fiber surrounded by five triangular air pores. (d) Spatial profile of the squared electric field of the guided mode of the microstructure with five triangles, at 1.25 THz.

### IV. Conclusion

The refractive indices and absorption coefficients of dried sliced staghorn corals were investigated using terahertz time-domain spectroscopy. The measured refractive indices and absorption coefficients have similar values to those of CaCO3, which is the main constituent of a coral. The coral sample with a microstructure surrounded by air pores guided waves between 1.1 and 1.6 THz. The dispersion, effective index, and loss of the guiding modes through the micostructured coral were numerically calculated. The spatial distribution of the electric field of the guide mode at 1.25 THz showed the mode to be tightly confined to the core.

### Acknowledgment

This work was supported by the Gwangju Institute of Science and Technology (GIST) Research Institute (GRI) grant, funded by the GIST in 2021, and the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (NRF-2019R1F1A1063156).

### References

1. R. S. Peixoto, M. Sweet and D. G. Bourne, “Customized Medicine for Corals,” Front. Mar. Sci. 9, 686 (2019).
2. R. J. Gettens, E. W. Fitzhugh and R. L. Feller, “Calcium Carbonate Whites,” Stud. Conserv. 19, 157-184 (1974).
3. T. P. Henkel, “Coral reefs,” Nat. Educ. Knowl. 3, 12 (2010).
4. J. Hu, J. J. Russell, B. Ben-Nissan and R. Vago, “Production and analysis of hydroxyapatite from Australian corals via hydrothermal process,” J. Mater. Sci. Lett. 20, 85-87 (2001).
5. J.-H. Kühne, R. Bartl, B. Frisch, C. Hammer, V. Jansson and M. Zimmer, “Bone formation in coralline hydroxyapatite: effects of pore size studied in rabbits,” Acta Orthop. Scand. 65, 246-252 (1994).
6. C. Demers, C.R. Hamdy, K. Corsi, F. Chellat, M. Tabrizian and L. Yahia, “Natural coral exoskeleton as a bone graft substitute: a review,” Bio-Med. Mater. Eng. 12, 15-35 (2002).
7. M. Mizuno, K. Fukunaga, S. Saito and I. Hosako, “Analysis of calcium carbonate for differentiating between pigment using terahertz spectroscopy,” J. Eur. Opt. Soc. 4, 09044 (2009).
8. T.-I. Jeon, K.-J. Kim, C. Kang, S.-J. Oh, J.-H. Son, K. H. An, D. J. Bae and Y. H. Lee, “Terahertz conductivity of anisotropic single walled carbon nanotube films,” Appl. Phys. Lett. 80, 3403 (2002).
9. M. S. Islam, C. M. B. Cordeiro, M. A. R. Franc, J. Sultana, A. L. S. Cruz and D. Abbotti, “Terahertz optical fibers,” Opt. Express. 28, 16089-16117 (2020).

### Article

#### Article

Curr. Opt. Photon. 2021; 5(4): 370-374

Published online August 25, 2021 https://doi.org/10.3807/COPP.2021.5.4.370

## Terahertz Complex Refractive Index and Guiding of White Staghorn Coral

Chul Kang1, Myunghwan Kim1, Hyeongmun Kim1, Jin Young Park1,2, Bok Hyeon Kim1, Inhee Maeng1, SooBong Choi2, Soeun Kim1, Chul-Sik Kee1

1Advanced Photonics Research Institute, Gwangju Institute Science and Technology, Gwangju 61105, Korea
2Department of Physics, Incheon National University, Incheon 22012, Korea

Correspondence to:*cskee@gist.ac.kr, ORCID 0000-0002-3219-5119
Current affiliation: YUHS-KRIBB, Medical Convergence Research Institute, College of Medicine, Yonsei University, Seoul 03722, Korea

Received: May 18, 2021; Revised: June 7, 2021; Accepted: June 8, 2021

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

Corals are the remains of animals that grow on warm beaches. They have been used as decorative jewels because of their variety of colors, and as medicinal materials for treating cancers, AIDS, and other therapeutic uses because of their chemical elements. Corals are mainly composed of calcium carbonate (CaCO3) and have many air pores, tens to hundreds of micrometers in size. The refractive indices and absorption coefficients of dried sliced staghorn corals are investigated using terahertz time-domain spectroscopy. The measured values are similar to those for CaCO3, as expected. It is observed that a sample with a microstructure formed by air pores can guide terahertz waves. The dispersion, effective index, and loss of the guiding modes of coral core surrounded by five triangular air pores are numerically calculated. The simulated spatial distribution of the electric field of the guide mode at 1.25 THz shows the mode to be tightly confined to the core.

Keywords: Coral, Optical waveguide, Terahertz

### I. INTRODUCTION

Coral is a special animal among the cnidarians that was once mistaken for plants or minerals, because it looks like a plant growing on warm beaches and has a calcareous skeleton. It is also very interesting due to the many unique features of its material properties. Coral is used as a decorative gem due to its various colors, and because of its chemical composition it is also used as a medicinal material for treating cancer, AIDS, and other therapeutic uses [1]. In the past, it was also used as a material for ancient wall paintings [2].

These corals can be divided into stony corals and relatively colorful soft corals. Of these, stony corals are composed mostly of calcium carbonate (CaCO3), which can also be used as a mineral [3]. Corals are generally known to have numerous air pores, ranging from tens to hundreds of micrometers in size [46]. There is a possibility that corals can strongly interact with electromagnetic waves in the terahertz (THz) region. For example, corals with air pores hundreds of microns in size can act as microstructured fibers to guide THz waves. However, it has not been reported that THz waves can propagate through the microstructures of corals. Investigating the guiding of THz radiation through corals would be interesting.

In this study, the characteristics of the refractive indices of staghorn coral, rock corals with cylindrical branches ranging from a few cm to a few meters, are investigated in the frequency range from 0.2 to 1.5 THz using THz time-domain spectroscopy. The measured refractive indices are similar to those of CaCO3, as expected [7]. One sample includes a microstructure formed by air pores, like a microstructured optical fiber, around a central region. The guiding of THz waves through the microstructure is observed in a frequency range above 1 THz.

### II. Experiment

To measure their refractive indices in the THz region, staghorn corals were cut in cross section perpendicular to the cylindrical axis, using a mechanical saw. Figures 1(a) and 1(b) show samples with cross-sectional diameters of 12 mm (sample 1) and 8 mm (sample 2) respectively. The lengths of the samples were about 1.57 mm and 0.86 mm respectively. As seen in Fig. 1(b), sample 2 has a structure surrounded by air pores, like a microstructured optical fiber, around a central region of the sample. It would be interesting to investigate whether that microstructure could guide THz waves.

Figure 1. Cross-sectional images of samples: (a) sample 1, (b) sample 2.

Figure 2 shows a schematic view of the THz time-domain spectroscopy system. To generate the THz waves we employ the p-type InAs substrate that is generally used for THz wave sources with 80-MHz repetition-rate Ti:Sapphire femtosecond laser pulses. The input power of femtosecond laser pulses is 350 mW after the mechanical chopping blade, with a center wavelength of 800 nm. The incidence angle of the femtosecond laser pulses is 45° and they are 5 mm in diameter on the p-type InAs substrate. The four parabolic mirrors focus and collimate the radiated THz waves; the focused beam waist is around 1.5 mm. The detector for THz waves is used with a 5-μm dipole-gap photoconductive antenna on the low-temperature-grown GaAs substrate with 10 mW femtosecond laser power. To eliminate absorption by rotational and vibrational modes of water vapor, we measure under dry-air conditions at 1% humidity in an airtight box.

Figure 2. Schematic view of the THz time-domain spectroscopy system employed in the experiments.

### III. Results and Analysis

To investigate the refractive indices and absorption coefficients of the samples, THz transmission through the samples was investigated by THz time-domain spectroscopy. The refractive indices and absorption coefficient are related to phase delay and transmission in the frequency domain. The refractive indices are obtained using the followings relations [8]:

$Oω=Iωexp−dαω2expi2πλn1ωd,$
$n1ω=1+ϕI−ϕO2πdλ0,αω=−2dInOωIω=4πn2λ0,$

where O(ω) is the THz transmission through the sample, I(ω) is the electric field of an incident THz wave, n1 is the real refractive index, α(ω) is the power absorption, and n2 is the imaginary refractive index. Also, d is the thickness of the sample, ϕO and ϕI are the phases of the THz transmission signals with and without samples, respectively, and l is the wavelength in the medium. To investigate the guiding property of the microstructure of sample 2, THz transmission through the central region containing the microstructure is compared to that through regions without it.

Figures 3(a) and 3(b) respectively show time- and frequency-domain transmission waveforms through samples with a reference metal hole 2 mm in diameter. The coral samples reduce THz transmission by about 20%. In Fig. 3(a), the slight difference of time delay between transmitted THz waves of the samples is due to the different lengths of samples. Figure 3(b) shows that, for sample1, the spectral characteristics of transmitted THz waves through a central region (S1C) are similar to those through an off-center region (S1O). However, for sample 2 the spectral characteristics of transmitted THz waves through a central region with the microstructure (S2C) are different from those through an off-center region (S2O), in a frequency range above 1 THz.

Figure 3. Transmission waveforms in (a) the time domain and (b) the frequency domain, with a reference metal hole 2 mm in diameter.

Figures 4(a) and 4(b) respectively show the real refractive indices and absorption coefficients of the samples. As expected, the values of both refractive indices and absorption coefficients of the samples are similar to those of CaCO3 [7]. The real refractive index of sample 1 is greater than that of sample 2, implying that the number of air pores per unit volume for sample 1 is smaller than for sample 2. The real refractive indices and absorption coefficients of S1O, S1C, and S2O increase over the frequency range from 0.2 to 1.5 THz. The absorption coefficients of the samples increase as the frequency increases. However, it should be noticed that the refractive index and absorption coefficient of S2C decrease above 1 THz. This implies that the microstructure guides THz waves [9]. These results mean that the guiding mode spreads out more to the air-cladding region as frequency increases, so that the effective refractive index and absorption of the guiding mode decrease as frequency increases.

Figure 4. Optical properties of coral samples: (a) indices of refraction (real) and (b) absorption coefficients.

Figure 5(a) shows the transmitted THz waves through S2O and S2C between 7 and 12 ps in the time domain, which are marked in a circle in the inset. Figure 5(b) presents the spectral amplitudes of the transmitted THz waves. These figures clearly confirm that S2C can guide THz waves in the frequency range between 1.1 and 1.6 THz. However, it is not clear whether the air-pore microstructure guides THz waves around 0.2 THz, because our measurement system contains remarkable noise signals in that frequency range.

Figure 5. THz guiding signals: (a) Transmitted THz waves through S2O and S2C between 7 and 12 ps in the time domain, which are marked in a circle in the inset. (b) Spectral amplitudes of the transmitted THz waves.

It is necessary to study the properties of guiding modes through S2C by using numerical simulations, but it is difficult to reflect exactly the shapes of the air pores in the numerical calculations. Thus the air pore was shaped as an isosceles triangle with apex angle of 53°. The length of the perpendicular bisector is 5 mm. The distance between the center of the coral and the triangle’s vertex facing the center is 1 mm. The length of the sides of triangles is assumed to be 0.85 mm. The commercial simulation program Lumerical MODE is employed in the calculation. Perfectly matched layer (PML) boundary conditions are applied to eliminate reflections at the boundaries, and a dense mesh size of 10 μm is used. The real and imaginary refractive indices of the sample (n and k, 2.55 and 0.07 respectively) are employed in the simulations. The values are the measured indices of the sample at 1.25 THz. Figures 6(a)6(c) present respectively the dispersion, effective index, and loss of the guide modes of the microstructured coral fiber surrounded by five triangular air pores. The values of dispersion are between 1.0 and 1.1 [ps/(nm·km)]. The effective index and loss increase as the frequency increases; this implies that the electric field of the guide mode is strongly confined in the core as the frequency increases. The spatial profile of the squared electric field of the guiding mode at 1.25 THz is presented in Fig. 6(d). The guiding mode was clearly confined to the core surrounded by the triangular air pores.

Figure 6. Numerical calculation results: (a) dispersion, (b) effective index, and (c) loss of the guided modes of the microstructured coral fiber surrounded by five triangular air pores. (d) Spatial profile of the squared electric field of the guided mode of the microstructure with five triangles, at 1.25 THz.

### IV. Conclusion

The refractive indices and absorption coefficients of dried sliced staghorn corals were investigated using terahertz time-domain spectroscopy. The measured refractive indices and absorption coefficients have similar values to those of CaCO3, which is the main constituent of a coral. The coral sample with a microstructure surrounded by air pores guided waves between 1.1 and 1.6 THz. The dispersion, effective index, and loss of the guiding modes through the micostructured coral were numerically calculated. The spatial distribution of the electric field of the guide mode at 1.25 THz showed the mode to be tightly confined to the core.

### Acknowledgment

This work was supported by the Gwangju Institute of Science and Technology (GIST) Research Institute (GRI) grant, funded by the GIST in 2021, and the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (NRF-2019R1F1A1063156).

### Fig 1.

Figure 1.Cross-sectional images of samples: (a) sample 1, (b) sample 2.
Current Optics and Photonics 2021; 5: 370-374https://doi.org/10.3807/COPP.2021.5.4.370

### Fig 2.

Figure 2.Schematic view of the THz time-domain spectroscopy system employed in the experiments.
Current Optics and Photonics 2021; 5: 370-374https://doi.org/10.3807/COPP.2021.5.4.370

### Fig 3.

Figure 3.Transmission waveforms in (a) the time domain and (b) the frequency domain, with a reference metal hole 2 mm in diameter.
Current Optics and Photonics 2021; 5: 370-374https://doi.org/10.3807/COPP.2021.5.4.370

### Fig 4.

Figure 4.Optical properties of coral samples: (a) indices of refraction (real) and (b) absorption coefficients.
Current Optics and Photonics 2021; 5: 370-374https://doi.org/10.3807/COPP.2021.5.4.370

### Fig 5.

Figure 5.THz guiding signals: (a) Transmitted THz waves through S2O and S2C between 7 and 12 ps in the time domain, which are marked in a circle in the inset. (b) Spectral amplitudes of the transmitted THz waves.
Current Optics and Photonics 2021; 5: 370-374https://doi.org/10.3807/COPP.2021.5.4.370

### Fig 6.

Figure 6.Numerical calculation results: (a) dispersion, (b) effective index, and (c) loss of the guided modes of the microstructured coral fiber surrounded by five triangular air pores. (d) Spatial profile of the squared electric field of the guided mode of the microstructure with five triangles, at 1.25 THz.
Current Optics and Photonics 2021; 5: 370-374https://doi.org/10.3807/COPP.2021.5.4.370

### References

1. R. S. Peixoto, M. Sweet and D. G. Bourne, “Customized Medicine for Corals,” Front. Mar. Sci. 9, 686 (2019).
2. R. J. Gettens, E. W. Fitzhugh and R. L. Feller, “Calcium Carbonate Whites,” Stud. Conserv. 19, 157-184 (1974).
3. T. P. Henkel, “Coral reefs,” Nat. Educ. Knowl. 3, 12 (2010).
4. J. Hu, J. J. Russell, B. Ben-Nissan and R. Vago, “Production and analysis of hydroxyapatite from Australian corals via hydrothermal process,” J. Mater. Sci. Lett. 20, 85-87 (2001).
5. J.-H. Kühne, R. Bartl, B. Frisch, C. Hammer, V. Jansson and M. Zimmer, “Bone formation in coralline hydroxyapatite: effects of pore size studied in rabbits,” Acta Orthop. Scand. 65, 246-252 (1994).
6. C. Demers, C.R. Hamdy, K. Corsi, F. Chellat, M. Tabrizian and L. Yahia, “Natural coral exoskeleton as a bone graft substitute: a review,” Bio-Med. Mater. Eng. 12, 15-35 (2002).
7. M. Mizuno, K. Fukunaga, S. Saito and I. Hosako, “Analysis of calcium carbonate for differentiating between pigment using terahertz spectroscopy,” J. Eur. Opt. Soc. 4, 09044 (2009).
8. T.-I. Jeon, K.-J. Kim, C. Kang, S.-J. Oh, J.-H. Son, K. H. An, D. J. Bae and Y. H. Lee, “Terahertz conductivity of anisotropic single walled carbon nanotube films,” Appl. Phys. Lett. 80, 3403 (2002).
9. M. S. Islam, C. M. B. Cordeiro, M. A. R. Franc, J. Sultana, A. L. S. Cruz and D. Abbotti, “Terahertz optical fibers,” Opt. Express. 28, 16089-16117 (2020).

Wonshik Choi,
Editor-in-chief