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Abstract
The capability to design, fabricate, and optimize metamaterials based on various structures and material platforms has been crucial for the rapid development of modern terahertz (THz) technology. While the detailed structures of artificial unit cells within a metamaterial is certainly worth investigating, there has been increasing demand to integrate novel metamaterials with a traditional functional photonic device to form a hybrid device, whose performance is so significantly improved as to be promising for real-world applications. In this study, we proposed, for the first time, a THz parallel-plate resonator based on metallic mesh devices (MMDs) for chemical sensing applications. We studied the influences of various structural parameters through simulations, fabricated MMD-based resonator devices, and fully characterized the device performance through THz spectroscopy experiments. Furthermore, we experimentally demonstrated that our device can detect a doxycycline hydrochloride aqueous solution whose concentrations is as low as 1 mg L−1 through resonance frequency shifts, evidencing the device sensitivity capable of delicate chemical sensing tasks. Our work presents a practical and low cost architecture for chemical sensing using THz radiation, which opens new avenues for numerous useful THz devices based on metamaterials.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Terahertz (THz) radiation, as the last part of electromagnetic radiation to be explored, has shown great potential for applications in various fields, including security screening [1], medical biology [2,3], communication [4,5], agriculture and food science [6,7]. However, due to the lack of naturally formed materials for efficient generation, detection, and manipulation of THz waves, existing THz components still can’t meet demands for diverse applications [8,9].
Metamaterials, artificially designed electromagnetic materials arranged with subwavelength structures, have attracted enormous attention in the field of THz science and technology [10,11]. By designing the structures and materials of their unit cells, metamaterials have shown great potential for THz wave generation [12,13], modulation [14–16], polarization [17], sensing [18–20], and imaging [21,22]. Many types of THz metamaterials are present [23–25]. Among them, the metallic mesh devices (MMDs) are thin metallic films with periodic apertures. A resonantly enhanced optical transmission peak appears in the frequency domain spectrum of an MMD due to the excitation of spoof surface plasmon polaritons [26,27]. Owing to their straightforward design, ease of fabrication, and flexible structure free of substrate, MMDs have been frequently applied in signal modulation and sensing applications [28,29].
Thanks to the development of lithography technology and electroplating technology, great performances of THz metamaterials have been achieved [30,31]. However, stringent requirements for fabrication, as well as the typical high cost, sometimes hinder the application of metamaterials in general fields. In addition, current research on THz metamaterials mainly focus on material properties (metal metamaterials [30], all-dielectric metamaterials [32], graphene metamaterials [33], flexible metamaterials [34], etc.) and detailed structures of their unit cells (split ring resonators [35], metallic nanogaps [36], slot antenna [37], Fano resonators [38], etc.). To make metamaterials suitable for more general applications, it is highly meaningful to integrate existing low cost metamaterials, say, MMDs, with an another traditional functional photonic device. Through proper design and optimization, the resulting hybrid device can have significantly improved performances suitable for real-world applications.
In this work, we demonstrated, for the first time, a THz parallel-plate resonator (THz-PPR) integrated with MMDs as a powerful sensor for chemical substances. To explore the characteristics of this THz-PPR, we experimentally and numerically studied the influences of three key parameters, namely, the plate spacing, plate thickness and plate refractive index. We demonstrated that the frequencies of resonance peaks can be modulated by changing the plate spacing, and a higher plate refractive index is beneficial for enhancing the interaction between THz wave and the two parallel plates. In this case, THz MMDs were introduced into the THz-PPR for their high refractive index and extraordinary transmission at a certain band through excitation of spoof surface plasmon polaritons. By utilizing the THz MMDs, the quality factor of the resonator drastically increases, while the relationship between plate spacing and resonant peak frequency remains unchanged. Finally, to put our MMDs-based THz-PPRs to an ultimate test, we performed chemical sensing experiments using our device. We experimentally proved the feasibility for sensing with a 50-micron-thick film attached inside, which agreed well with the simulated data. Using a doxycycline hydrochloride (DCH) aqueous solution as the sensing target, we found that a solution concentration as low as 1 mg L−1 is enough to induce unambiguous shifts of device resonant frequencies, suggesting the potential applications for material sensing and analysis in the THz region.
2. Methods
All the THz time-domain waveforms were measured by a Z3-XL THz time-domain spectroscopy (THz-TDS) system (Zomega Corporation, East Greenbush, NY, USA) in normal transmission detection mode with a spectral range from 0.1 to 3.5 THz. A large-aperture photoconductive antenna was used for THz pulse generation and an electro-optic ZnTe crystal was used for THz pulse detection, excited by a laser beam with pulse durations less than 100 fs at a repetition rate of 80 MHz. To avoid the possible disturbance from water vapor, all the measurements were carried out at 23 °C ( ± 0.5 °C) with the relative humidity less than 1% after nitrogen purging. Before spectrum collection, the THz-TDS system was warmed up for at least half an hour.
For the bare THz-PPR without MMDs, silica wafers with a measured permittivity ε = 3.83 + 0.0196 i (at 1 THz) were used as resonator plates. The two silica plates, with a thickness of 3 mm, were spaced by hollow spacers with some different thicknesses (50 μm, 100 μm, and 200 μm). The MMD used in our study was manufactured from nickel by the electroforming method. The grid interval of the MMD was 254 μm, with an opening length of 180 μm and a thickness of 60 μm. For the MMDs-based THz-PPR, two MMDs were respectively attached to inner surfaces of the two silica plates. The same hollow spacers were used for spacing generation. A polyester film with a thickness of 50 μm was attached to an MMD to prove the feasibility of the MMDs-THz-PPR for material sensing. And the same polyester film was used as a stage for DCH solution deposited in the same way.
Doxycycline hydrochloride (DCH, >88% ~94%) was purchased from Sangon Biotech (Shanghai, China). Deionized water was obtained from the Milli-Q SP reagent water system (18 MΩ•cm−1, Millipore, Billerica, MA, USA). All of these chemical reagents were used without further purification. In the MMDs-based THz-PPR sensing tests, a series of DCH aqueous solutions with concentrations ranging from 0 to 10000 mg L−1 (deionized water, 1 mg L−1, 10 mg L−1, 100 mg L−1, 1000 mg L−1, and 10000 mg L−1) were prepared by dissolving appropriate amount of DCH powder in deionized water. A volume of 40 μL desired DCH aqueous solution was dropped on the polyester film attached to the MMD and dried at 40 °C for one hour before measurement. Each collected spectrum was an average of four scans and three measurements were conducted for each sample. To ensure that each spectrum was collected from a fixed spot located at the central of the target, an electrically controlled stage was used with an aligned laser diode.
All the simulations in our study were carried out using FDTD Solutions 8.18.1262 (Lumerical Solutions, Inc. Vancouver, Canada). In the part of investigating the influence of plate thickness, the permittivity of the plate was set the same as that of the silica wafer. In the part of investigating the influence of plate refractive index, we just changed the refractive index of the plate and remained the extinction coefficient the same as that of the silica wafer. The nickel was modeled as perfect electric conductor and the polyester film was modeled as a lossless dielectric with a permittivity ε = 3.27 in this case.
3. Results and discussion
3.1 Characterization of the bare THz-PPR without MMDs
To explore the characteristics of a bare THz-PPR without MMDs, we first investigated the influence of plate spacing on the transmission spectrum. The resonator in our study is composed of two identical parallel silica plates, between which some spacers are inserted togenerate desired plate spacing (Fig. 1(a)). Transmittance is defined as T = (Esample/Ereference)2, where Esample (Ereference) is the THz electric field intensity after transmitting through the sample (reference). Figure 1(b) shows the experimental transmission spectra of THz-PPRs with different plate spacing (ranging from 100 to 500 μm) plotted together with corresponding simulated results; an agreement is achieved between them. The slight differences are probably due to the deviations in parameter fitting of the simulated material. In the frequency range of 0.3 to 1.6 THz, the transmission spectra of all resonators have the trend of decreasing with increasing frequency. The resonance peak appearing at lower frequency shows higher transmittance, in consistent with frequency-dependent transmission of a plate. Moreover, more resonance peaks appear as the plate spacing increases, suggesting the possibility of THz signal modulation by tuning the plate spacing. If we consider the lowest frequency peak as the baseband frequency, higher order resonant peak frequencies appear as integer multiples of the baseband frequency. No resonance peak appears when the transmission of the resonator plates was measured individually (not shown), indicating that the resonances exclusively come from the cavity spacing region. To further demonstrate the spacing-dependent transmission of the THz-PPR, the transmission spectra as a function of frequency (from 0.3 to 1.6 THz) and spacing (from 0 to 700 μm) were simulated by the FDTD method; the results are plotted in Fig. 1(c). The simulated data illustrates that larger spacing induces more resonance peaks, and these peaks show a redshift as the spacing increases. No obvious resonance peak appears when the plate spacing was narrowed below 130 μm.
Fig. 1 THz-PPRs measurement with different plate spacing. (a) Schematic diagram of the THz-PPR for transmission measurements. (b) Experimental and simulated transmission spectra for THz-PPRs with different plate spacing (100 μm, 200 μm, 300 μm, 400 μm, and 500 μm). (c) Simulated transmission spectra as a function of frequency (from 0.3 to 1.6 THz) and spacing (from 0 to 700 μm) for the THz-PPR; the color bar indicates THz transmittance.
Secondly, we investigated the influences of plate thickness and refractive index using FDTD simulations. Figure 2(a) shows the transmission spectra of the THz-PPRs having various plate thicknesses with plate spacing of 500 μm. Due to the fact that the thicker plate causes more absorption of THz waves, the overall transmittance declines as the plate thickness increases. Figure 2(b) shows the transmission spectra of the THz-PPRs with various plate refractive indices with plate spacing of 500 μm. It is obvious that a higher refractive index causes a lower transmittance, which can be explained by Fresnel’s law [39]. More importantly, the resonance peaks become narrow as the refractive index increases, indicating that high refractive index contributes to enhancing the interaction between the THz beam and the two plates. Therefore, a high refractive index helps to enhance the effect of THz-PPRs.
Fig. 2 Simulated results of THz-PPRs with different plate thicknesses and plate refractive indices at plate spacing of 500 μm. (a) Simulated transmission spectra of THz-PPRs with different plate thicknesses (2000 μm, 2500 μm, 3000 μm, 3500 μm, and 4000 μm); the plate refractive index is 1.95. (b) Simulated transmission spectra of THz-PPRs with different plate refractive indices (1.5, 2.0, 2.5, 3.0, and 3.5); the plate thickness is 3000 μm.
To explain these characteristics above, we can consider our THz-PPR as a Fabry-Perot (F-P) cavity, so multiple beam interference theory can be adopted [39]. Because the optical thickness of the plate in our study is about 10 times larger than that of the plate spacing, multiple reflections inside the plate can be easily separated by selecting a proper sampling time window. The interaction between THz waves and the inner surfaces of two plates can be isolated for further analysis.
3.2 Characterization of the MMDs-based THz-PPR
From the results above, we concluded that, in order to have an optimized THz-PPR, it is desirable to replace the plates with a material showing a high refractive index, while still maintaining a reasonable amount of power transmittance. Therefore, MMDs are introduced into the THz-PPR as the desired high index plate material. Figure 3(a) shows the microscopy image of the MMDs with periodic square apertures. The transmission spectrum of the MMD illustrates a pass band around 1 THz with a dip included (Fig. 3(b)); this agrees with other studies [40,41]. Compared to the transmission of a bare MMD, the transmission peak frequency of an MMD-based plate shows a shift to ~0.65 THz with the peak transmittance decreasing to ~0.33, as shown in Fig. 3(b). The redshift is due to the variation of the dielectric environment [42], while the decrease of the transmittance is mainly due to the Fresnel reflection loss as well as the absorption of the plate. The MMDs were carefully attached to each inner surface of the plate to form an MMDs-based THz-PPR, as illustrated in Fig. 4(a).
Fig. 3 The properties of the THz MMD. (a) A microscopy image of the THz MMD. (b) Measured transmission spectra of a bare MMD and an MMD-based plate.
Fig. 4 MMDs-based THz-PPRs measurement with different plate spacing. (a) Schematic diagram of the MMDs-based THz-PPR for transmission measurements. (b) Experimental and simulated transmission spectra for MMDs-based THz-PPRs with different plate spacing (120 μm, 220 μm, 320 μm, 420 μm, and 520 μm). (c) Simulated transmission spectra as a function of frequency (from 0.2 to 1.2 THz) and spacing (from 70 to 570 μm) for the MMDs-based THz-PPR; the color bar indicates THz transmittance.
To further explore the effect of the plate spacing on our MMDs-based THz-PPR, devices with various spacing (from 120 to 520 μm) were measured experimentally and simulated numerically (Fig. 4(b)). There is a good consistency between the experimental and simulated results. Three main features of bare THz-PPRs still hold true for the MMDs-based THz-PPR: (1) a larger spacing causes more equally-separated resonance peaks; (2) the peak transmittance varies with the peak frequency, consistent with the trend of the transmittance of a single resonator plate; (3) the resonance peak shows a redshift as the plate spacing increases. Notably, the introduction of MMDs sharpened resonance peaks. The experimental transmission spectra in the frequency range from 0.2 to 1.2 THz with various plate spacing (from 70 to 570 μm) are plotted in Fig. 4(c). The simulated data again illustrates that the resonance peak redshifts as the spacing increases; the peaks at around 0.6 THz show higher transmittance than that at both ends.
The characteristics of this hybrid resonator qualitatively follows those of a bare THz-PPR, only that the quality factor is significantly improved by introducing high-index MMDs. This type of MMDs-based THz-PPRs could realize THz modulation with several narrow pass bands, which may be used for tunable THz filters. We also performed transmission experiments on a THz-PPR with an MMD attached to only one of the inner surfaces of the plate with various plate spacing (from 85 to 485 μm); see Fig. 5.
Fig. 5 Measured transmission spectra for a THz-PPR with only an MMD attached under different plate spacing (85 μm, 185 μm, 285 μm, 385 μm, and 485 μm).
In addition to the enhanced F-P cavity resonance for the high-refractive index of THz MMDs, it can be found that these MMDs-based THz-PPRs can maintain the transmittance at some frequencies to be still larger than 0.45. This is due to the extraordinary transmission of THz MMDs through excitation of spoof surface plasmon polaritons. The detailed process is as follows. As light impinges on a metamaterial, the oscillating electric field of light drives the oscillation of electrons in the unit cells to form plasmons. Upon reasonable engineering of the unit cells, the oscillation paths of the electrons in the confined structure constructively interfere with each other, and thereby, form a resonance. At resonance (THz frequency matches the spoof surface plasmon frequency), the electron oscillation (alternating current in a metallic unit cell) is particularly strong, enhancing the electric field around the structure. There are two consequences induced by this unique effect that we actively utilize for our device. First, the enhanced oscillation significantly increases phase retardation of the THz wave attempting to pass through it; this significantly increases the effective refractive index of the metamaterial layer. Second, the strong alternating currents in the unit cells behave as microscopic antennas, enhancing the transmission of the THz wave (known as extraordinary transmission). These two effects are critical to make our device have a reasonably high quality factor while at the same time maintain the light-cavity coupling efficiency. Moreover, the THz MMDs performance can be enhanced by F-P cavity structure when the unit-cell scale matches the THz wavelength well. Similar enhancement effects have been proved in several papers [43–45].
Based on the analysis of the characteristics of THz-PPRs above, the change of spacing or the refractive index of the medium can cause a frequency shift of the peak, sometimes with a change of intensity. And the attached MMDs promote multiple reflection between plates, allowing for more information about optical path contained. Thus, introducing additional materials in the resonator should change the optical path between plates, and the frequency shifts of the resonance peaks may be used for sensing these materials. Furthermore, the localization of an electromagnetic field around the MMD surface can enhance the interaction between electromagnetic wave and attached materials [46]. Therefore, we demonstrate the sensing performance of our THz-PPRs below.
3.3 Sensing applications of the MMDs-based THz-PPR
A polyester film with a thickness of 50 μm was attached to the MMD on one of the plates as the sensing target. The transmission was measured with different plate spacing (from 120 to 420 μm). To reflect the changes induced by the film, the frequencies of the first two resonance peaks of every transmission curve were extracted to compare with those without the film; the results are plotted in Fig. 6(a) (for some scenarios with narrow spacing, there was only one peak included). The solid and dashed lines represent the experimental and simulated results, respectively. The results indicate that the film causes clear redshifts of the two resonance peaks for all plate spacing. The amount of shift decreases as the spacing increases. Due to a nearly two-fold relationship between the two peak frequencies, the frequency shift of the second peak (P2) induced by the attached film is larger than that of the first peaks (P1). We therefore infer that a more remarkable frequency shift can be observed for the resonance peak with a higher interference order. In addition, the effects of target film thickness and refractive index were investigated for the MMDs-based THz-PPR with spacing of 120 μm through simulations. Figure 6(b) shows the peak frequency of transmission for dielectric films detection with various film thicknesses (from 0 to 50 μm) and refractive indices (from 1.5 to 3.0). The peak frequency decreases as the thickness and the refractive index increases. A slower frequency shift can be observed as film thickness increases shown in Fig. 6(b), which is mainly due to a decreasing fraction of optical path change as well as a smaller cavity electric field as reported in [47]. Therefore, the MMDs-based THz-PPR can be used as a platform for sensing, especially for those targets with high refractive indices.
Fig. 6 (a) Experimental (solid line) and simulated (dashed line) results of resonant peak frequencies of MMDs-based THz-PPRs with and without a 50-micro-thick polyester film with different spacing ranging from 120 to 420 μm. (b) Simulated results of resonant peak frequencies for resonators where dielectric films with various thicknesses (from 0 to 50 μm) and refractive indices (from 1.5 to 3.0) are introduced.
We used the MMDs-based THz-PPR to detect DCH aqueous solutions with different concentrations. Firstly, the same 50-micro-thick film was attached to an MMD for depositing the solution; the solutions were transferred and dried on the film. Various concentrations of DCH solutions, ranging from 0 (deionized water) to 10000 mg L−1, were used. The results are plotted in Fig. 7. Figure 7(a) shows how the transmission spectra of the resonator changewith the DCH concentration. These spectra can be clearly distinguished from each other from the salient peak shifts. To emphasis the changes of the peaks, peak transmittance and frequencies were extracted and plotted as functions of DCH concentrations; see Fig. 7(b). The peak frequency decreases monotonically as the concentration increases. The peak transmittance increases with increasing concentrations, but an anomaly appears at 10000 mg L−1. To understand the cause of this anomaly, we may consider the changes of peak frequency and peak transmittance together. The target DCH molecules induce both absorption and additional optical path length of THz waves. As mentioned above, the peak transmittance varies with peak frequency which, sometimes, may increase, which is based on the transmission of a single plate. Therefore, when the DCH concentration is low, the peak transmittance could increase as the concentration increases. Overall, the MMDs-based THz-PPR can detect the DCH solution with a concentration as low as 1 mg L−1, revealing the potential for highly sensitive detection in the THz region. To further enhance the detection sensitivity, besides some work to improve the performance of the THz-PPR itself, conjugating the target to high refractive index materials, such as gold nanoparticles [48], could be an effective method.
Fig. 7 Detection of DCH aqueous solution with different concentrations ranging from 0 to 10000 mg L−1 by the MMDs-based THz-PPR with plate spacing of 120 μm. (a) Experimental transmission spectra obtained using different concentrations of DCH aqueous solutions. (b) Peak frequency (black square) and corresponding peak transmittance (red circle) versus DCH concentrations, the error bars were the standard deviations of three replications.
4. Conclusions
In summary, we proposed a THz parallel-plate resonator based on MMDs. We experimentally and theoretically determined the characteristics of this device and focused on its potential applications for chemical sensing in the THz region. The results show that the resonance peaks could be modulated by the optical path between plates. Our device is able to directly detect a DCH aqueous solution whose concentration is as low as 1 mg L−1. We expect that our work can provide a new design architecture for metamaterial-based hybrid photonic devices for sensing applications.
Funding
Natural Science Foundation of Zhejiang Province for Distinguished Young Scholars (No. LR18C130001).
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