Terahertz metamaterial sensing on polystyrene ... - OSA Publishing

Terahertz metamaterial sensing on polystyrene microbeads: shape dependence S. J. Park, S. W. Jun, A. R. Kim, and Y. H. Ahn* Department of Physics and Department of Energy Systems Research, Ajou University, Suwon 443-749, South Korea * [email protected]

Abstract: The shape dependence of target materials on the sensitivity of terahertz metamaterial sensors was investigated. Polystyrene microbeads with a known dielectric constant and spherical, ovular, lens-shaped, and star-shaped structures were studied. The resonant frequency showed a clear red-shift after the deposition of low-density microbeads owing to the change in the dielectric environment in the gap area of the metamaterials. Results of simulations based on a finite-difference time-domain (FDTD) method, in which the known dielectric constant of a polystyrene sphere was used, showed excellent agreement with experimental results. The shift in the resonant frequency increased linearly with the surface density, saturating at 60–80 GHz when the gap area was full of microbeads. More importantly, the resonant frequency shift was higher for non-spherical microbeads, such as the star-shaped microbeads. Therefore, the shape of the individual target material was a crucial factor in determining metamaterial sensor sensitivity. ©2015 Optical Society of America OCIS codes: (160.3918) Metamaterials; (300.6495) Spectroscopy, terahertz; (050.6624) Subwavelength structures.

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Received 28 May 2015; revised 3 Sep 2015; accepted 3 Sep 2015; published 8 Sep 2015 1 Oct 2015 | Vol. 5, No. 10 | DOI:10.1364/OME.5.002150 | OPTICAL MATERIALS EXPRESS 2150

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1. Introduction Terahertz (THz) spectroscopy is a powerful technique for sensing biological and chemical substances. Recently, there has been an increasing effort to develop fast and sensitive on-site sensors for various applications, such as analyte classification, explosive detection, and microbial-infection diagnosis [1–3]. In most cases, the interaction between a THz wave and analytes is very weak because the target material is typically one hundred times smaller than the THz wavelength (l), resulting in a low scattering cross section [4]. However, with the help of artificial structures, such as metamaterial and plasmonic structures, THz waves enable us to detect the target material because the resonant absorption is linked to the extremely confined electric field in the gap area [4–6]. The resonant frequency of the metamaterial is described by f res = 1 / (2π LC ) , and can be controlled by varying the capacitance (C) and inductance (L) [7–14]. As a result, a shift in the resonant frequency (Δf) occurs in THz transmission when the target materials are located in the gap area because of the change in the dielectric configuration of this area. Recently, we were able to detect microorganisms, such as penicillia, yeast, and bacteria, by using THz metamaterials with high sensitivity [4]. This was possible because of size compatibility: typically sized microorganisms fit well within the gap of the metamaterial. Metamaterial detection is an universal technique because it is based on dielectric sensing. Target-specific detection is also possible by functionalizing substrates. In order to determine the detection mechanisms, we performed finite-difference time-domain (FDTD) simulations and successfully reproduced some of the essential parts of our findings. However, sensitivity in the simulation results was generally lower than those of the experimental results. The main cause for the discrepancy has been attributed to errors in estimating the dielectric constants of the individual microorganisms, and, more importantly, to the fact that the shape of the target material is modeled as a simple sphere. On the other hand, the shape dependence of individual target materials on metamaterial and plasmonic device sensitivity has not been explicitly addressed. In this paper, we present the effects of target material shape on the sensitivity of THz metamaterial sensors. Polystyrene microbeads with a particular dielectric constant were the target materials. We used microbeads with spherical, ovular, lens-shaped, and star-shaped structures. We observed a noticeable shape-dependent resonance shift. The experimental results were compared with those of FDTD simulations. 2. Experimental results and discussion THz transmission amplitudes of the THz metamaterial with polystyrene microbeads were measured using THz time-domain spectroscopy (THz-TDS). A femtosecond laser with a wavelength of 800 nm was incident on the photoconductive antenna to generate a linearly polarized THz pulse. The pulse was focused onto the metamaterial array with a spot size of ~1 mm2. The amplitude and phase of the transmitted THz electric field were measured by varying the time delay between the probe beam and THz pulse. The THz spectrum was

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obtained using a fast Fourier transform (FFT) of the time trace and normalized with respect to a reference. We used commercial polystyrene microbeads of various shapes, such as spheres, ovals, lenses, and stars, with their diameters fixed at 2 μm. The microbead fit within the typical metamaterial gap. The dielectric constant of polystyrene (εp) is 2.56 in the THz frequency range [15]. THz metamaterials were fabricated using photolithography on a 1-mm-thick quartz substrate, followed by Cr/Au (2 nm / 98 nm) deposition using an e-beam evaporator. The metamaterial arrays consisted of electrical split-ring resonators (eSRR) each with a gap width of 3 μm, side length of 36 μm, and periodicity of 50 μm.

Fig. 1. (a) Schematic of terahertz (THz) metamaterial sensor. (b) Scanning electron microscopy (SEM) image of spherical polystyrene beads deposited on a THz metamaterial device.

Figure 1(a) shows a schematic of a THz metamaterial sensor applied to polystyrene microbeads of various shapes. We measured the change in the THz spectra of the THz metamaterial sensors before and after the deposition of the polystyrene microbeads. Microbeads located in the gap area induced a resonant frequency shift (Δf), which was mainly determined by the effective dielectric constant in the gap area. As a result, the shift occurred during THz transmission when the dielectric materials were located in the gap area [4]. Figure 1(b) is a scanning electron microscopy (SEM) image, which shows spherical polystyrene microbeads randomly distributed on the quartz substrate with the metamaterial sensor. Microbeads were deposited on the THz metamaterial sensor by drying the solution in an oven at 60 °C. The surface density was controlled by manipulating the number of coats using a solution with a density of 1 μg/μl. Because there is a large fluctuation in the local number density of microbeads as shown in the inset of Fig. 2(a), we observe the ensemble average of Δf from more than ~300 eSRR elements located in the focused THz spot. We used spherical polystyrene to study the shape dependence of the THz metamaterial sensor. Figure 2(a) shows a series of transmission spectra for the metamaterial sensors after the deposition of the polystyrene beads. Each time we deposited beads, the surface number density increased by 0.035 μm−2. The resonant frequency shift of the THz metamaterial increased gradually with deposition time. This shift can be clarified using the following relationship [4, 16]:

Δf αN s (ε p − ε air ) = f0 ε eff

(1)

Here, α is the coefficient associated with the sensitivity, Ns is the surface number density of the target material in the gap area, εp is the dielectric constant of the polystyrene, εair is the dielectric constant of the air, εeff ( = neff 2) is the effective dielectric constant without target materials in the metamaterial gap area. In this relation, neff can be characterized with a linear

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combination of the substrate (nsub) and air refractive indices. This yielded neff = 1.72 for a quartz substrate (nsub = 1.93) [14, 17, 18].

Fig. 2. Normalized THz transmission amplitudes for the electrical split-ring resonator (eSRR) with varying surface number density of polystyrene spheres from (a) experiments and (b) simulations. (inset of Fig. 2a) Microscopic images of polystyrene spheres on eSRR arrays with the densities of 0.035/µm2 (left) and 0.140/µm2 (right). (c) Δf/f0 as a function of surface number density extracted from (a) and (b).

It is clear that Δf increased linearly as the surface density increased, as seen in Fig. 2(a). In general, Δf depends critically on the dielectric constant of the target materials and substrates. We used a quartz substrate throughout the experiments. This was because Δf/f0 is inversely proportional to εeff, and therefore, substrates with lower permittivity were preferred [16]. Since εp and εeff were fixed throughout the experiment, the only parameter that determined the sensitivity was α, which was associated with the size and shape of individual target materials. From the results in Fig. 2(a), we obtained α = 3.7 × 10−4 μm2. To confirm our experimental results, we performed FDTD simulations, shown in Fig. 2(b). We tried to mimic the experimental schematic by using linearly polarized plane waves, periodic boundary conditions, and the known dielectric constant of a polystyrene sphere. We used metamaterial patterns with the same geometric parameters utilized in experiments with metal films considered to be perfect electric conductors. The resonant frequency from the simulation (f0 = 1.20 THz) is slightly lower than that of experiments (f0 = 1.26 THz) in Fig. 2(a), which is most likely due to the fabrication errors. To replicate the polystyrene microbeads, we introduced a series of spheres with 2 μm in diameter. THz transmission amplitudes were taken as we increased the number of dielectric spheres in the gap area. The number of dielectric spheres was converted to the surface density (Ns) and normalized by the gap area (3 × 10 µm2). Δf showed a clear red-shift that increased in magnitude with Ns. The frequency shift in the simulation very accurately reproduced our experimental findings. We obtained α = 3.8 × 10−4 μm2, which is very close to our experimental result. For a better comparison, we show a plot of Δf/f0 as a function of Ns in Fig. 2(c) for both the experimental and simulation results. There was only a 5% discrepancy between the experimental results and the simulations regarding the slope. This was because we used target materials with well-defined dielectric constants (εp = 2.56), and, more importantly, a fine spherical shape. This is in contrast to previous studies involving detection of microbes, such as yeasts, molds, and bacteria, with non-spherical shapes.

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Fig. 3. Resonant frequency shift as a function of surface number density for various polystyrene types: (a) sphere, (b) oval, (c) lens, and (d) star.

Figure 3 shows the microbead shape dependence on Δf using the microbeads with nonspherical shapes (oval, lens, and star) in addition to the spherical shape. Here, we increased Ns until Δf was saturated. Δf increased gradually with Ns, until it reached saturation, which, in the spherical case, occurred at Δfsat = 65 GHz, as shown in Fig. 3(a). We calculated a saturation surface density Nsat = 0.24 µm−2, by using the simple exponential fitting shown below: Δf = Δfsat (1 − exp(− N s / N sat )) (2) Shown together in Fig. 3(b)–3(d) are a series of experimental results for the non-spherical shapes with diameters very close to those of the spheres (2 µm). Similar saturation behaviors were found after deposition of the microbeads with different shapes. The saturation surface densities of ovular and lens-shaped beads were very close to each other at Nsat = 0.14 µm−2, which was significantly lower than that of the spheres. Star-shaped microbeads had densities as low as Nsat = 0.08 µm−2, which was three times smaller than that of spherical bead.

Fig. 4. Saturation surface number density of spherical, ovular, lens-shaped, and star-shaped polystyrene in THz metamaterial sensor.

The results for Nsat and Δfsat are summarized in Fig. 4. The lower saturation density for sensing the non-spherical shapes simply suggested that they were more sensitive compared to the simple spherical case. We also note that the discrepancy in the sensitivities between the simulation and experimental results, which were previously reported in metamaterial and plasmonic sensing, could be understood by considering the non-spherical shapes of fungi and bacteria [4, 16].

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Thus far, the sensitivity has not been addressed in terms of the shape of the target materials in the dielectric sensor. The enhanced sensitivity cannot simply be understood in terms of the different filling fractions of the dielectric materials because we used beads of similar size. In addition, the values of Δfsat (60–85 GHz) were not far from each other, as shown in Fig. 4. This was because of the formation of a thin film as we increased the deposition time, whereas the slight variations in Δfsat were likely due to different packing fractions for the different bead shapes. In addition, the dielectric constant of the microbeads, which has been measured from very thick film (~2 mm) consisting of closely packed microbeads, was not far from each other. This confirms that possible deviation in the volume density of different polystyerene types is not the main cause for the increased sensitivity in sensing star-shaped microbeads (by 3-fold). We found that all of our experimental results were highly reproducible. Therefore, it was likely that the field enhancement effects that originated from the pointed shape of the target dielectric materials in the gap region were responsible for the enhancement. Though we tried simulations using different shapes, we could not precisely reproduce our experimental results. Reproducibility was limited by errors associated with the ultrafine structures of the nonspherical shapes with size ranges less than λ/10,000, although the overall size of the target materials was already λ/150. We believe that this issue has to be addressed in the near future. In addition, although we recoreded the ensemble average of the frequency shifts from more than hundreds of eSRRs, it will be very interesting if we can control the number and the position of microbeads in the gap areas precisely. This could be achived by explointing the inkjet printing techniques which have been successfully demonstrated for locating the microbeads and single-cells [19]. 3. Conclusion

We found that polystyrene microbeads, which have well-known dielectric properties, were an ideal test bed to study the shape dependence of target materials for metamaterial sensors and find an optimized design for them. We observed a clear red-shift in the resonant frequency due to the change in the dielectric constant in the gap area following the microbead deposition. The sensitivity of THz metamaterial depended on microbead shape. We performed FDTD simulations to reproduce the experimental results for spherical polystyrene and the simulation results were in strong agreement with experimental data. We investigated the effect of microbead shape on the sensitivity of metamaterial sensors by examining various polystyrene structures with the same surface number density. The sensitivity for star-shaped microbeads was three times that of spherical beads in the low-density regime, implying the field enhancement effect was at work. This study provides critical information useful to the development and optimization of metamaterial sensors. Acknowledgments

This work was supported by the Midcareer Researcher Program (2014R1A2A1A11052108) and PRC Program (2009-0094046), and Public Welfare & Safety Research Program (20110020819) through a National Research Foundation grant funded by the Korea Government (MSIP).

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