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A broadband terahertz (THz) absorber consisting of multilayer glass spheres and polydimethylsiloxane (PDMS) was realized. The multilayer glass spheres were deposited by repeating a self-assembly method used to form monolayer glass spheres and by the spin-coating of PDMS to fill the gaps between the glass spheres. The average reflection at the surface of the absorber was 0.8% and the absorbance was higher than 98% in the frequency range between 0.7 to 2.0 THz.
©2012 Optical Society of America
1. Introduction
The emerging science and technology related to the terahertz (THz) system has various potential applications in the areas of molecular biological science, medical imaging, security, and future communications systems [1–4]. Components such as sources, detectors, absorbers and modulators are required for the realization of a THz system. In particular, an absorber with high absorption and low reflection capabilities is a crucial component in the area of security and imaging systems [5,6]. In spite of the importance of such an absorber, it is difficult to find a natural material with both a high absorption coefficient and a low reflection coefficient reference to the air in the THz frequency range. To overcome this problem, novel techniques to enable the construction of a virtual material with a negative refractive index have been developed by adjusting the complex electric permittivity and magnetic permeability [7–13]. Artificial metamaterials consisting of periodically replicated unit cells with highly conductive metals such as gold or copper have been designed for THz absorption. With the above technique, a metamaterial absorber was realized for single- or dual-band applications, demonstrating good absorption properties. The experimental absorbance of a single-band metamaterial absorber is 70% at 1.3 THz [7], while that of a dual-band metamaterial absorber is 85% at 1.4 THz and 94% at 3.0 THz [8]. Similar approaches have been taken to realize broadband absorbers by stacking multiple metamaterial planes resonating at different but close frequencies [11,12]. The resulting broadband metamaterial absorbers demonstrated the absorbance higher than 97% from 4.4 to 5.5 THz [11] and absorbance higher than 60% from 4.1 to 5.9 THz [12].
In this paper, a broadband THz absorber with multilayer glass spheres was fabricated on a glass substrate using a self-assembly process followed by a spin-coating process. Perfect absorption properties were achieved by combining the self-assembled glass spheres and spin-coated polydimethylsiloxane (PDMS) on a glass substrate, of which the absorption coefficient is high in the THz frequency range [13–15]. The surface reflection was also minimized by the sub-wavelength surface structures [16–20] of the array of glass spheres. The reflection, transmission, and absorption properties of the fabricated broadband THz absorbers based on self-assembled glass spheres are investigated and discussed in this paper.
2. Experimental details
The broadband THz absorber was fabricated according to the following steps. The deposition of monolayer glass spheres with a period of 140 μm was carried out via a self-assembly process on a 1.1-mm-thick glass substrate. For the self-assembly process, a suspension containing glass spheres was initially prepared. Subsequently, the glass sphere suspension was dropped onto a host glass substrate placed on a hot plate, where the temperature was held constant at 90°C during the self-assembly process. Water evaporation caused by the high temperature of the hot plate led to closely packed monolayer glass spheres. The self-assembled monolayer glass spheres do not strongly adhere to the glass substrate via this process, and the gaps between the glass spheres are empty. Next, the resulting glass substrate with closely packed monolayer glass spheres was covered with PDMS by a spin-coating process. With self-assembled monolayer glass spheres and the spin-coated PDMS, the empty space between the glass spheres and the substrate was filled in completely and the monolayer glass spheres became more stable. After the PDMS spin-coating process, a curing process was carried out for 30 minutes in an oven at a temperature of 100°C to remove any remaining solvent. The self-assembly process to form monolayer glass spheres and the PDMS coating process were repeated twice to achieve multilayer glass spheres. With the process steps described above, multilayer glass spheres were realized on a glass substrate. For comparison, a THz absorber with a monolayer glass spheres with a different PDMS coating condition was also fabricated on a glass substrate. The reflectance and transmittance of the various types of devices were measured using a THz time domain spectroscopy system (TPS spectra 3000, TeraView) and their frequency domain responses were obtained by Fourier transform.
3. Discussion
The realized THz absorber was composed of self-assembled monolayer or multilayer glass spheres, spin-coated PDMS, and a host glass substrate. Figure 1 shows the measured refractive index and absorption coefficient of the glass and PDMS as a function of the frequency. The refractive index of glass is 2.3 at 1.0 THz, which is significantly higher than that in the visible lightwave frequency range. The high refractive index of the glass in the THz frequency range is due to the contribution of the ionic polarizability to the dielectric constant [13]. Glass and PDMS materials can effectively absorb the incident THz wave due to their inherent high absorption coefficients. The penetration depths, δ, of the glass and PDMS were determined to be 134 and 535 μm at 1 THz, respectively. A glass substrate thicker than 5δ will be opaque by absorbing the incident wave in excess of 99%. The high absorption property of the glass and the PDMS in the THz frequency range was caused by the ionic composition of the material and by the high water composition, respectively. High attenuation of the THz wave can be achieved very easily through the use of a thick absorbing material, but reducing the surface reflection is a design issue.
Fig. 1 The measured refractive index and absorption coefficient of glass and PDMS in the THz frequency range.
Figures 2(a) and 2(b) show cross-sectional scanning electron microscopy (SEM) images of the fabricated monolayer glass spheres with thin PDMS and a 130-μm-thick PDMS coating, respectively. As shown in Fig. 2(a), the gaps between the glass spheres and the substrate in the monolayer-A sample are empty due to the thin PDMS coating. On the other hand, the space in the monolayer-B sample is filled with cured PDMS. The thickness of the PDMS was controlled by the spin-coating speed. The PDMS thickness of the monolayer-B sample is 130 μm, which was achieved at a spin-coating speed of 3000 rpm. As the spin-coating speed decreased, the deposited PDMS thickness increased and the exposed area of the glass spheres decreased, making the top surface of the monolayer-B sample flatter. These surface structures have a period of 140 μm, which is determined by the diameter of the glass sphere. The multilayer glass spheres shown in Fig. 2(c) were achieved by repeating the processes of the deposition of the monolayer glass spheres and the PDMS coating.
Fig. 2 Cross-sectional SEM images of the fabricated monolayer glass spheres with (a) thin PDMS, (b) 130-μm-thick PDMS, and (c) the multilayer glass spheres with PDMS gap filling.
The resulting effective refractive index profiles of the three structures are sketched in the inset of Fig. 3(a) . Compared to the monolayer glass spheres, the multilayer sample has more gradually changing the effective refractive index. Figure 3(a) shows the measured reflectance of glass substrates with and without glass spheres throughout the frequency range of 0.2 to 2.0 THz. A Fabry-Perot interference pattern was observed in the frequency range between 0.2 and 0.7 THz, where the absorption coefficient of the glass substrate is relatively low. At this frequency range, the partially transmitted THz wave is reflected back from the bottom of the glass substrate, creating the Fabry-Perot interference pattern in the reflection spectrum. At a frequency higher than 0.8 THz, the incident THz wave from the top surface was completely absorbed in the absorbing materials so that an interference pattern was not observed. The reflectance of a bare glass substrate is higher than 15.5% in the frequency range of 0.7 to 2.0 THz. The glass substrates with monolayer or multilayer glass spheres exhibited much lower reflectance than a bare glass substrate. The reflectance levels of glass substrates with the monolayer and multilayer configurations are lower than 11.1 and 2.1%, respectively. The reduction in the reflectance was caused by the gradually changing effective refractive index of the surface structure [16–18]. By avoiding an abrupt change in the refractive index at the interface between the air and the glass substrate, the surface reflection could be dramatically reduced. The average reflectance of the multilayer sample is 0.9% in the frequency range from 0.7 to 2.0 THz, which shows that no meaningful reflection occurs in this frequency range.
Fig. 3 The measured reflectance and transmittance of various types of devices with different surface structures.
The measured transmittance of the sample with multilayer glass spheres was compared with that of a bare glass substrate, as shown in Fig. 3(b). The inset shows a tilted SEM image of the fabricated sample with multilayer glass spheres. Both the bare glass substrate and the glass substrate with the multilayer glass spheres exhibit low transmittance due to the inherently high absorption properties of the glass and the PDMS material in the THz frequency region investigated in this study. Although the surface reflection was reduced for the sample with the multilayer glass spheres, as shown in Fig. 3(a), its transmittance was also reduced because the thickness of the absorbing material was increased after the adding of the multilayer glass spheres and PDMS.
To investigate the difference in the THz responses of the three samples, a three-dimensional electromagnetic simulation was carried out using the rigorous coupled-wave analysis (RCWA) method. The results are summarized in Fig. 4 . In this RCWA simulation study, it was found that surface structures made of glass spheres support only the 0th-order diffracted mode and do not generate any higher-order diffracted modes in the frequency range investigated in this study because the period of the structure is smaller than the wavelength of the incident wave. The higher-order modes begin to appear at frequency higher than 2.1 THz, where the total reflection spectrum and the 0th-order reflection spectrum begin to deviate. This shows that the surface structures made of glass spheres act as subwavelength structures at a frequency lower than 2.1 THz [16, 18]. When the three samples are compared, the field distribution in the multilayer glass sample is more uniform than the other samples, which shows that the multilayer glass spheres result in a more gradual effective refractive index profile, as expected from the inset of Fig. 3(a). When the intensities at the top and the bottom of the three surface structures shown in Fig. 4(b) and 4(d) are compared, the lowest intensities appear at the top and the bottom of the surface structures in the sample with the multilayer glass spheres. It agrees well with the measured results shown in Fig. 3.
Fig. 4 (a) The electric field distribution at the upper part of the absorbers with the monolayer-A, monolayer-B, and multilayer glass spheres at 0.7 THz and (b) the corresponding intensity distribution at the top, middle, and bottom of the glass spheres. (c) The electric field distribution at the upper part of the absorbers with the monolayer-A, monolayer-B, and multilayer glass spheres at 1.5 THz and (d) the corresponding intensity distribution at the top, middle, and bottom of the glass spheres.
From the measured reflectance and transmittance, the corresponding absorbance, A, can be calculated using
where R and T are the measured reflectance and transmittance, respectively. The calculated absorbance is shown in Fig. 5 . The absorbance of the bare glass substrate is 80% over the frequency range of 0.7 to 2.0 THz. Compared to the bare glass substrate, the fabricated broadband absorber with multilayer glass spheres absorbs more than 98% of the incident THz wave in the same frequency range. This broadband absorption property was facilitated by the complete removal of the surface reflection.The characteristics of the metamaterial absorbers and the fabricated absorbers are compared in Table 1 . Single- and dual-band metamaterial absorbers have one and two resonant absorption peaks, respectively [7,8]. Although a metamaterial absorber can achieve high absorption properties, the application of these devices is restricted to narrow-bandwidth applications. The broadband absorbers were also designed and fabricated by stacking multilayer metamaterial planes [11,12]. They achieved high absorption properties together with broadband characteristics. The absorber based on multilayer glass spheres as fabricated in this study demonstrated nearly perfect absorption properties in the broadband frequency range of 0.7 to 2.0 THz without using any metamaterial structures which require fabrication processes such as lithography and metallization techniques.
Table 1. Comparison of the Bandwidth and Absorbance of the Metamaterial Absorber and the Monolayer and Multilayer Glass Spheres Absorber
4. Conclusion
Self-assembly and spin-coating methods were utilized to realize multilayer glass spheres on a glass substrate for a broadband THz absorber. The low reflection from the surface structure and the high absorption properties of the host materials led to high absorbance properties in the THz frequency range. The fabricated device with multilayer glass spheres can be an excellent candidate for broadband THz absorbers.
Acknowledgment
This work was supported by the NRF grant (No. 20110017603), by the World-Class University program funded by the MEST through the NRF of Korea (R31-10026), and by the Core Technology Development Program for Next-Generation Solar Cells of Research Institute for Solar and Sustainable Energies (RISE), GIST.
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