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Abstract
Critical factors for terahertz polarizers include broadband operation, high transmittance, and a good extinction ratio. In this paper, using a 5 nm-wide metallic slit array with a 200 nm periodicity as a wire grid polarizer, we achieved over 95% transmittance with an average extinction ratio of 40 dB, over the entire spectrum as defined by the terahertz time-domain spectroscopy (0.4 ∼ 2 THz). Theoretical calculations revealed that the slit array can show 100% transmission up to 5 THz, and wider bandwidths with a higher cutoff frequency can be achieved by reducing the slit periodicity. These results provide a novel approach for achieving a broadband THz polarizer and open a new path for seamless integration of the polarizers with nanophotonic applications.
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1. Introduction
Since it has an ability to divide electromagnetic waves into two perpendicular components, a polarizer is a very useful tool in optical microscopy and spectroscopy for characterizing materials [1–5]. Especially, for long wavelength regions such as terahertz (THz) waves and microwaves, a metallic wire grid polarizer is more practical than other types of polarizers such as Glan-Taylor prism, Wollaston prism, and Brewster angle polarizer, due to its wide incident angular width, broadband uniform efficiency, and high scalability [6–11]. With recent advances in THz technologies, interest in a THz polarizer is increasing in various fields including THz communications, imaging, sensing, etc [12–16]. The distance between two adjacent wires scales with the wavelength and is in the micrometer scale for a typical THz polarizer: for high performance, extreme subwavelength periodicities are needed to prevent reflection leakage [7]. As of now, THz polarizers with micro-sized grid patterns with periods of ∼λ/30 (λ at 1 THz= 300 µm) show less than 80% transmission [17,18]. Reel-wound carbon nanotubes polarizers are easy to fabricate and they possess excellent degree-of-polarization performances, but the overall transmission is small, <50% [19].
Here, to achieve a high-performance in a THz polarizer, we fabricated an array of one-dimensional metallic slot antennas [20,21] and brought the periodicity down to those used in typical infrared wire-grid polarizers, 200 nm [22–24]. For a higher extinction ratio of a THz polarizer, we further increased a high filling factor by reducing the distance between the adjacent wires in a few nanometers scale [25]. Therefore, a THz polarizer consisting of a slit array with a 200 nm periodicity and 5 nm gap width was prepared. With our THz polarizer, nearly perfect THz transmission of >95% was achieved with an extinction ratio of 40 dB in the spectral range of 0.4–2 THz. Theoretical calculations also revealed that the 200 nm period slit array with the 5 nm gap can operate up to 5 THz in 100% transmission, and the operating frequency can be expanded by reducing the slit periodicity. Our results suggest that a THz polarizer consisting of the slit array with a nanogap structure provides a novel approach to a broadband high performance slot antenna-type wire grid polarizer.
2. Experiment
To prepare a THz polarizer using the slit array with a 200 nm periodicity and a 5 nm gap width, we performed the atomic layer lithography [26,27]. In the fabrication process, a 100 nm-thick gold (Au) film was deposited on a 750 µm-thick silicon (Si) substrate with a size of 10 mm × 10 mm. Photolithography using a krypton fluoride (KrF) excimer laser (λ = 248 nm) was carried out on the Au film to make a stripe pattern of a photoresist layer with a line width of 200 nm, a pitch of 400 nm and a length of 2 mm. The whole size of the pattern is 2 mm × 2 mm. After etching the Au layer exposed between the photoresist patterns by a reactive ion etching (RIE) process, the photoresist was eliminated by oxygen plasma treatment to successfully obtain the Au film with the same stripe pattern. In Fig. 1(a), a cross-section of the first patterned Au film was observed by using a focused ion beam (FIB, Helios 650, FEI). It is found that the Au film was perfectly patterned without deformation but the exposed Si surface was slightly etched out during the RIE. Afterward, a 5 nm-thick aluminum oxide (Al2O3) layer as a dielectric gap material was conformally deposited on the patterned Au film by atomic layer deposition (ALD, Lucida D100, NCD Tech). The 100 nm-thick secondary Au layer was then deposited by e-beam evaporation (KVE-E2000, Korea Vacuum Tech) to form the Au-Al2O3-Au structure, where the nanogap is formed along with the pattern. To open the gaps covered with the second Au layer during the deposition, an ion miller (KVET-IM2000L, Korea Vacuum Tech) was implemented with an argon ion (Ar+) beam incident at an angle of 70°. Finally, the 200 nm period slit array with the 5 nm-thick Al2O3 dielectric gap was fabricated as shown in Fig. 1(b) and 1(c) in top and cross-section, respectively.
Fig. 1. FE-SEM images of the slit array with the 200 nm periodicity and the 5 nm-thick Al2O3 dielectric gap. (a) The first 100 nm-thick Au pattern with a 200 nm linewidth and a 400 nm pitch after the RIE process. (b-c) The slit array with the 200 nm periodicity and the 5 nm gap fabricated by the atomic layer lithography in (b) top and (c) cross-section. The 5 nm gap between the first and second Au was determined by the thickness of Al2O3 layer in the ALD process.
To explore the optical properties of the slit array with the 5 nm gap in the THz regime, THz-time domain spectroscopy (THz-TDS) was implemented in the frequency range of 0.4–2 THz. Figure 2(a) schematically represents the THz-TDS setup consisting of a gallium arsenide (GaAs) photoconductive antenna as a THz emitter and a zinc telluride (ZnTe) crystal as an electro-optic detector. In the THz-TDS, a biased GaAs THz emitter was illuminated by a mode-locked femtosecond Ti:sapphire laser operating at a 80 MHz repetition rate and a pulse duration of 130 fs at a center wavelength of 800 nm to generate a THz pulse. The THz waves generated from the emitter were collected by parabolic mirrors and were focused on the sample in normal incidence. The transmitted THz waves were collected again by using parabolic mirrors and detected via electro-optic sampling using ZnTe crystal with (110) orientation. Time traces of the THz amplitude were recorded by moving the delay stage of the pump pulses. It is well known that THz waves irradiated from a photoconductive antenna are a linear polarization. We confirmed that THz waves in the THz-TDS were horizontally polarized, and rotated the sample in order to change polarization directions. In order to evaluate the performances of the slit array as a THz polarizer, the THz transmission in the transverse electric (TE) and the transverse magnetic (TM) modes were measured by focusing the THz waves polarized parallel and perpendicular to the slit direction, respectively. During the measurement, humidity in the THz-TDS was below 5% by nitrogen purge to minimize the THz absorption of ambient moisture.
Fig. 2. THz-TDS measurement of the slit array with the 200 nm periodicity and the 5 nm gap. (a) Schematic illustration of the THz-TDS experiment for the slit array. The THz waves were incident with the polarization of the TM (red arrow) and TE (blue arrow) modes. (b) THz amplitude spectra through the bare Si substrate and the slit array in the frequency domain as a reference (black) and signals in the TM (red) and TE (blue) modes, respectively. the inset is the time traces of the THz pulses through the bare Si and the slit array as the reference and the TM and TE modes, respectively. (c) The normalized THz amplitude through the slit array in the frequency range of 0.4–2.0 THz.
3. Results and discussion
The slit array with the 200 nm periodicity and the 5 nm gap was prepared on a 750 µm-thick Si substrate. However, the thick Si substrate greatly attenuates the intensity of THz waves passing through. In addition, a nanogap structure can be manufactured on any substrate, such as quartz, sapphire, PET, or even free-standing [28–31]. Therefore, to only investigate the slit array, the THz field transmitted through the Si substrate was considered as a reference field. The inset in Fig. 2(b) shows the time traces of the THz amplitude transmitted through the slit array with the 200 nm periodicity and the 5 nm gap width in the TM mode (red curve) and TE mode (blue curve) and the bare Si substrate as a reference (black dashed line). The signal in the TE mode is close to a zero pulse, while the time signal in the TM mode perfectly matches the reference signal. It implies that nearly perfect THz transmission through the 200 nm period slit array with the 5 nm gap occurs in the TM mode. Conversely, in the case of the TE mode, the slit array can block the incident THz waves. By comparing the time integrated THz electric field intensities, it is found that the THz transmission through the slit array in the TM mode was obtained as 96.5%. The THz amplitude in the frequency domain was obtained by applying a fast Fourier transform (FFT) to the time traces, as shown in Fig. 2(b). Some reductions around 0.6, 1.1, and 1.7 THz are observed in the reference and the TM mode due to a slight amount of ambient moisture [32]. In the TM mode, the THz amplitude transmitted through the slit array is slightly lower than that of the reference. Also, the THz amplitude in the TE mode appears close to zero as described in the time pulse. The THz-TDS typically has a bandwidth range of 0.15–3.5 THz. However, for normalization, the reliable amplitude is in the range of 0.4–2 THz. Thus, the normalized THz amplitudes of the slit array were obtained from the FFT results in the spectral range defined as above. Interestingly, as plotted in Fig. 2(c), a constant normalized THz amplitude over 95% for TM mode was observed with the exception of the small reductions due to the THz absorption of the ambient moisture.
In order to explain the constantly high THz amplitude transmitted through the slit array in the TM mode, the capacitor model was applied [33]. According to the capacitor model, when THz wave is normally incident to a slit array with a nanogap structure, a current is induced on the Au surface and electric charges are accumulated on the sides of the slits like a capacitor. Thus, an electric field is strongly generated inside the nanogap, allowing the THz waves to pass through the slit array. It is noted that the surface charge density on the sidewalls of the slits is inversely proportional to the incident THz frequency ($f$). Therefore, the THz amplitude transmitted through a nanogap array shows a ${1 / f}$ behavior, as described in the following equation.
where $t$ is the normalized amplitude of the transmitted THz wave, $c$ is speed of light, $f$ is the incident THz frequency, $w$ is the nanogap width, ${\varepsilon _{gap}}$ is the relative permittivity of the gap material, and $h$ and $p$ are the slit thickness and the periodicity, respectively. The fact that the THz transmission cannot exceed 100% suggests that a saturation below a cutoff frequency defined at $t = 1$ occurs depending on the structure of the slit array. Considering the $p$= 200 nm and the ${\varepsilon _{gap}}$ = 4.41 for the 5 nm-thick Al2O3 as the gap material [26], it is found that the cutoff frequency is approximately 5 THz. It sufficiently supports the experimental results of the constantly transmitted THz amplitude over 95% in the spectral range.According to the Eq. (1), it is noted that the normalized amplitude is strongly dependent on the structure of the slit array. Accordingly, it is sufficiently considered that the 200 nm slit periodicity and the 5 nm gap width are highly effective to obtain a higher THz normalized amplitude. However, one consideration should be addressed for characterizing the optical properties of a nanogap structure. As mentioned above, an electric field inside the nanogap is hugely induced when THz waves are illuminated to the nanogap structure. Accordingly, the optical properties of the nanogap structure are characterized by the field enhancement that describes the amplitude ratio of the electric field inside the nanogap to the reference electric field incident to the nanogap structure [34]. In addition, the field enhancement is experimentally extracted by the Kirchhoff integral formalism which explains the relation between near and far electric fields [35]. According to the Kirchhoff integral, the field enhancement is given by ${t / \beta }$, where $\beta = $ w $(gap width){/}p$(period) is the areal coverage ratio of a nanogap structure. Since both the slit periodicity and the gap width are fixed ($\beta $= 0.025), The field enhancement of the slit array with the 200 nm period and the 5 nm gap is predominantly determined by the normalized amplitude, t. As denoted on the right axis in Fig. 2(c), It is found that the field enhancement is limited to nearly 40 due to the saturation of the THz normalized amplitude. Compared to the previously reported nanogap structure with the same 5 nm gap width [31], the field enhancement of 40 is quite small, which could be considered as a disadvantage for nanogap applications using high field enhancement in THz regime [29,36–38]. Typically, as the periodicity of a nanogap structure increases, a field enhancement increases at the same normalized amplitude [39]. Nevertheless, increasing THz transmission through a nanogap structure requires reducing its periodicity, which means that a high slit density is better to obtain higher THz transmission. Consequently, the slit array with the 200 nm periodicity and the 5 nm gap is greatly advantageous for obtaining higher THz transmission.
To theoretically investigate the THz transmission through the slit array, calculations were performed based on modal expansion of electromagnetic waves in and near the slit array. This is a simplified version of the widely used calculation algorithm for rectangular slots [40,41] as a slit is essentially a slot with an infinite length. For the calculations, optical properties of Au reported by Ordal et al. [42] were used and gap width $w$= 5 nm, the Au thickness of 100 nm, refractive index of Al2O3 gap material ${n_{gap}}$ = 2.1, and a silicon substrate with refractive index of $n$= 3.42. Transmission spectra were obtained from 0.01 to 100 THz with the periodicity of the slit array varying from 50 nm to 2 µm. Figure 3(a) displays the THz transmission spectra in the spectral range from 0.01 to 100 THz as a function of slit period. In all cases, the THz transmission exhibits 100% saturation below a cutoff frequency. Then, it follows a ${1 / f}$ dependence as the frequency increases. It is noted that the cutoff frequency is inversely proportional to the slit period. For the 200 nm periodicity, the saturation occurs below 5 THz, which is consistent with the capacitor model. Since the cutoff frequency depends on the slit period, THz transmission at given frequencies were investigated in terms of slit period in Fig. 3(b). While in the case of 0.1 THz, the saturation occurs regardless of the slit periods, for higher frequencies, the THz transmission gradually increases and is saturated as the slit period decreases. And it is found that a periodicity of less than 50 nm is required to achieve the saturation for 10 THz and higher frequencies. As a result, a slit array with a submicron periodicity and a nanogap structure is a crucial to achieve saturation at a high frequency.
Fig. 3. Theoretical calculations of the THz transmission through the slit array with the 5 nm gap. (a-b) THz transmission spectra as a function of frequency (a) and slit periodicity (b). In the calculations, the nanogap width is fixed at 5 nm.
To evaluate the polarizer performances of the slit array with the 200 nm periodicity and 5 nm gap, the THz transmission in the TM and TE modes was obtained by squaring the normalized amplitude, $T = {t^2}$. In contrast to the THz transmission in the TM mode described in the capacitor model, it is hard to accumulate the charges at the gap region for the TE mode. Therefore, THz transmission in the TE mode was quite similar to the direct transmission of the Au film, which is affected by skin depth in THz regime [43]. Figure 4(a) shows the THz transmission spectra in the TM and TE mode on red and blue curves, respectively. In addition, the direct THz transmission through a 100 nm-thick Au film on the Si substrate is also plotted on a gray curve for comparison. Most of THz waves in the TE mode and the Au film are typically reflected while the THz fields in the TM mode sufficiently funnels into the nanogap. Thus, in the TE mode, the transmitted signal was very low comparable to the noise level in the THz-TDS (signal-to-noise ratio ∼ 3×103). resulting in oscillating the normalized amplitude in the frequency domain. Furthermore, it is also comparable to the amplitude directly transmitted through the Au film. As a result, the slit array THz polarizer with the 5 nm gap and the 200 nm periodicity effectively blocks the THz waves in the TE mode polarization. The extinction ratio, one of the important factors for the performance of a polarizer, was obtained by the following equation [17],
Fig. 4. Evaluation of the slit array with the 200 nm periodicity and the 5 nm gap as a THz wire grid polarizer. (a) THz transmission spectra in the TM (red curve) and TE (blue curve) modes through the slit array. For comparison, the direct transmission (gray curve) was obtained using the 100 nm-thick Au film on the Si substrate. (b) The extinction ratio of the slit array.
With the respect of the filling factor (= wire width/periodicity) which is one of the specifications of wire grid polarizers, most wire grid polarizers have a filling factor of 0.5 or less [17,22,44–46]. Meanwhile, the 200 nm period slit array works as a wire grid polarizer with the extremely high filling factor of 0.975 because of the 5 nm gap structure. Nevertheless, the THz transmission through the slit array is nearly perfect, reaching over 95%. This is because, according to the Babinet's principle, the nanogap array is a complementary screen of a wire grid, providing the equivalent optical response [47–49]. Furthermore, compared to making ultrathin metallic wires, reducing the width of the nanogap is much easier to reach down to 1 nm or less by a ALD process in the fabrication. As a consequence, the slit array with the nanogap structure as slot antennas can be treated as an ideal wire grid polarizer with a near-unity transmission, high extinction ratio, and broadband operation ranging from infrareds to microwaves, where the functionalities can be tuned by adjusting the slit periodicity as well as the nanogap width.
4. Conclusion
The 200 nm periodicity slit array with the 5 nm Al2O3 gap was demonstrated as a THz wire grid polarizer. The constant THz waves transmittance over 95% and average 40 dB extinction ratio were achieved in the spectral range from 0.4 to 2 THz. The capacitor model and theoretical calculation described that the saturation in the THz transmission occurs below a cutoff frequency that is extended up to 10 THz by tuning the periodicity of the slit array. Also, according to the Babinet’s principle, the slit array with a nanogap structure is regarded as a very narrow metallic wire grid showing equivalent optical behaviors. As a result, it is expected that the slit array with a nanogap structure can open the way for a broadband high-performance wire grid polarizer for various applications such as the THz imaging, communications, sensors, and etc.
Funding
National Research Foundation of Korea (NRF-2015R1A3A2031768, NRF-2021R1C1C1010660); Kangwon National University (2022 Research Grant).
Acknowledgements
Part of this study has been performed using facilities at IBS center for Correlated Electron Systems (CCES) and FIB Facility at National Center for Inter-university Research Facilities (NCIRF) at Seoul National University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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