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Abstract
Aligned Ni nanowire (NW) arrays were investigated for terahertz (THz) wave modulation. By adjusting the NW density and order of the NW arrays, the resonant frequency and intensity of the THz waves can be effectively tuned. The tuning range of the resonant frequency is about 0.29 THz, and a transmittance of less than 40% in the frequency region from 0.5 to 2 THz is achieved by changing the NW density. Although the order of the NW arrays has no influence on the resonant frequency, the transmittance can be tuned about 21%. The ability to tune the intensity and resonant frequency effectively and the ease of fabrication of the Ni-NW arrays make them the potential candidates for THz tunable filters, intensity modulators, and spatial light modulators.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The terahertz (THz) regime ranging from 0.1 to 10 THz bridges the gap between the microwave and infrared regions. Recently, THz wave has received much attention in many applications including noninvasive medical inspection, nondestructive fault analysis, and security surveillance because vast information on the optical properties of samples can be conveyed over a broad spectrum [1–3]. However, THz waves interact minimally with naturally occurring materials and this has hindered the development of THz wave manipulation devices. Therefore, many researchers have focused on improving the performance of THz devices in terms of transmission/reflection and frequency tuning. Numerous tuning methods have been implemented by means of the tunable metamaterials [4–13], metasurface absorption materials [14], multi-resonator graphene structures and periodically sinusoidally-patterned graphene materials [15,16] for the active control of varied THz properties such as magnetic resonance, electrical resonance, absorption, multiband and broadband modulation, and polarization.
Due to the multiple scattering effect, the interaction between the light and the nanostructures in an extremely thin layer is enhanced. Wang et al. found that the microwave absorption property of the urchinlike Ni chains is larger than that of the Ni smooth chains and rings [17]. Moreover, the geometrical parameters, such as the nanowire (NW) lengths and diameters, of NW arrays can effectively tune the intensity and wavelength of the transmission light [18]. Particularly, Ni NWs and its composite structures are widely used to attenuate electromagnetic waves because of magnetic loss [19,20]. In addition, Ni NWs can be facilely synthesized by template synthesis and chemical reduction method [21,22], and aligned to fabricate hierarchical Ni nanostructures using a magnetic alignment technique [23]. In this study, we report new THz tunable structures using the aligned Ni-NW arrays with different NW densities and alignment orders. The THz transmission properties of these structures are studied.
2. Device fabrication and measurement
Firstly, Ni NWs with a 260-nm diameter and a length of 25 μm were synthesized by a template-directed electrode-position method [21]. After template-dissolving and a series of washing processes, the Ni NWs were collected. In order to investigate the influences of the NW density and the alignment order of the aligned Ni-NW arrays on the transmission of the THz waves, two series of samples were fabricated on the polyethylene substrates by using a magnetic alignment technique in alcohol at 50 °C. The first series were single-layered aligned-Ni-NW-array (SA) structures with a NW density in the range of 0.5 × 109 cm−2 to 3.0 × 109 cm−2. The details of the preparation processes were described in our previous report [24]. The bilayered aligned-Ni-NW-array (BA) junctions with junction angles of 0°, 30°, 60°, and 90° were the second series and were fabricated by a two-step magnetic alignment technique reported by Hangarter [23]. In the BA junctions, each aligned-Ni-NW-array layer has the same Ni NW density about 1.5 × 109 cm−2. In addition, a disordered sample with the same density of Ni NWs prepared by using an ultrasonic technique in alcohol without magnetic alignment was used as the control sample. All samples were characterized for six times by using THz time-domain spectroscopy (THz-TDS) in transmission mode. For comparison, the empty polyethylene substrate was used as the reference sample.
3. Results and analysis
The scanning electron microscope (SEM) images of the BA junctions with the junction angles of 30° and 90° are shown in Figs. 1(a) and 1(b) respectively. It can be observed that the BA junctions are successfully prepared with the desired junction angle, although the Ni NWs have a slight lateral aggregation.
Figure 2 shows the THz-TDS of the BA junctions with the different junction angles. It is evident that the THz pulse has a reflective oscillation in the sample that induces two transmission peaks in the spectrum. In addition, the intensity of the first peak decreases as the junction angle increases from 0° to 90°. Moreover, the random sample has the lowest peak density. For the SA structures, similar THz-TDS were found (not shown here) and the intensity of the first peak increased with the decrease in the NW density; this is in agreement with the results reported in Ref. 24.
In order to investigate the uniformity of the Ni-NW arrays, the optical thickness (dOT) of the NW arrays was determined; it is expressed as dOT = c × (t1-t2)/2, where c is the speed of the THz wave in the air, t1 and t2 are the delays of the two peaks in the THz-TDS for the Ni-NW samples and the reference sample, respectively. Figure 3(a) shows the changes in the optical thickness for the SA structures as a function of the NW density; a linear relationship is observed. The optical thicknesses for the BA junctions with the different junction angles are shown in Fig. 3(b). It is observed that the thicknesses of the BA junctions range from 120 μm to 130 μm, indicating that the order of the aligned Ni-NW arrays has no influence on the uniformity of the Ni-NW arrays.
Figure 4(a) shows the frequency transmission spectra (FTS) of the SA structures that are calculated from the THz-TDS after the application of a fast Fourier transform. The transmission peak frequency exhibits a slight red shift with increasing NW density. When the NW density changes from 0.5 × 109 cm−2 to 3.0 × 109 cm−2, the resonant frequency is tuned from 0.69 THz to 0.4 THz. The normalized transmittance of the SA structures is shown in Fig. 4(b), which shows that the transmittance in the THz frequency range (0.25–2.25 THz) greatly attenuates as the NW density increases. Moreover, the degree of attenuation is greater for the high-frequency wave than the low-frequency wave. It is observed that, at a frequency of 1.5 THz, the transmittance is about 92%, 58%, and 23% for the NW density of the SA structures of 0.5 × 109, 1.5 × 109, and 3 × 109 cm−2, respectively. The transmittance of the SA structure with a NW density of 3 × 109 cm−2 is less than 40% in the frequency range of 0.5 to 2.0 THz. The FTS of the BA junctions with different junction angles are shown in Fig. 4(c). The results showed that no significant spectral shift is observed, indicating that the order of the Ni-NW arrays is not the main factor affecting the change in the resonant frequency. However, the transmittance of the BA junctions in the resonant region gradually decreases with the increasing junction angle. Specifically, the control sample has the lowest resonant peak intensity and is ~1.7 times smaller than that of the sample with junction angle of 0°. The transmittance decreases about 21% as the junction angle is changed from 0° to 90°. The transmission attenuation possibly resulted from the magnetic responses of the Ni-NW arrays [25]. Because the direction of the vibration of the incident magnetic field in the THz wave region is random, the interaction between the Ni-NW arrays and the incident magnetic field is enhanced by the disorder of the Ni NWs, inducing the THz absorption increases.
Fig. 4 the FTS (a) and changes in the normalized transmittance (b) of the SA junctions, (c) the frequency transmission spectra of the BA junctions.
4. Conclusion
In summary, the transmission properties of aligned Ni-NW arrays with different NW densities and alignment orders have been investigated. Several interesting phenomena related to resonance shifting and tunable absorption are observed in the aligned Ni NW arrays using THz-TDS. The results show that the NW density is the major parameter modifying the intensity and the wavelength of the THz waves. With increasing NW density from 0.5 × 109 cm−2 to 3.0 × 109 cm−2, the resonant frequency is tuned from 0.69 THz to 0.4 THz and the terahertz transmittance is effectively reduced from 92% to 23% at 1.5 THz. Moreover, at a NW density of 3.0 × 109 cm−2, the normalized transmittance is smaller than 40% in the bandwidth of 1.5 THz ranging from 0.5 to 2 THz. The order of the Ni-NW arrays is not the main cause of the resonant frequency shift but it affects the change of the transmittance. The transmittance of the BA structures decreases about 21% when the junction angle increases from 0° to 90°. Based on our experimental results, it is evident that the aligned Ni-NW arrays can be used for THz tunable filters, intensity modulators, and spatial light modulators.
Funding
National Natural Science Foundation of Beijing (4142047); Excellent Young Teachers Program of China University of Petroleum (2462015YQ0603).
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