Herein, we fabricated and investigated the carbon nanotube (CNT) integrated metamaterial for orthogonal polarization control in the THz regime, which is composed of a sandwiched CNT layer with the adjacent metal gratings in the sub-wavelength integration. Under the mechanism of multilayer polarization selection and multiple reflections in CNT constructed micro-cavity, the perfect orthogonal polarization conversion is achieved and the transmittance spectrum presents multi-band peaks and valleys, which coincide with the theoretical Fabry-Perot resonance. Besides, by controlling the layer number and orientations of the middle CNT, the active modulation of the amplitude and phase in compound metamaterials are realized. Based on the simulation of CNT in the grating model, it obtains a good agreement with the experimental results, and the simulated electric field distribution also confirmed the inner polarization conversion mechanism. This work combines nanomaterials with optical microstructures and successfully applies them to the THz polarization control, which will bring new ideas for design novel THz devices.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
With the promotion of terahertz (THz) source and detection technology, THz science has made unprecedented progress in the past two decades , which shows great application prospects in security check , spectral detection , and the next-generation wireless communication (6G) . However, the key functional devices required in THz applications need to be further developed, especially the THz polarization devices [5,6], which play an important role in polarization analysis, imaging, and polarization communication system [7–9]. However, the existing THz polarization control devices are mainly relying on the natural crystal material with small birefringence, and it is severely limited by the large volume, great loss, and unable to regulate [10,11]. Therefore, it is necessary to develop new materials and structures for THz polarization control.
Nowadays, artificial metamaterials have brought new vitality to electromagnetic wave manipulation owing to their powerful capabilities in amplitude, phase, and polarization control, just by the flexible design of the structural parameters or the layout in the sub-wavelength scale [12–20]. Compare with the dielectric material, metal metamaterials with strong local resonance effects are mostly adopted to realize the polarization transformation, such as the asymmetric metal unit like “C-shaped”, “V-shaped”, “L-shaped”, “H-shaped” and wire-grating structures [21–25]. In particular, as the mature THz polarizer, the sub-wavelength metal grating (MG) plays an irreplaceable role in linear polarization generation and detection. Currently, a variety of MG-based multilayer metamaterials have been demonstrated to achieve the THz polarization conversion [26–28]. For example, N. K. Grady et al. realized an efficient linear polarization conversion based on tri-layer MG with >80% transmission and 1.2THz bandwidth , but the polarization efficiency or transmittance cannot be modulated due to the uncontrollable nature of metal materials. Zhang et al. presented a dynamically tunable THz polarization rotator based on graphene-MG metamaterial by adjusting the Fermi energy of graphene . Although such functional materials integrated metamaterials can realize the active control, they are limited by the need of outfield driving, complex processing, and difficult to implement experimentally.
Carbon nanotube (CNT), made by rolled-up graphene sheet, has drawn broad interests due to the great potential in energy , electronic transistors , and THz science (i.e., THz radiation, detection, and polarization control) [33–36]. In the field of THz polarization control, CNT can be employed as a THz polarizer, which can realize the transmission or absorption of the linear polarized THz waves with different polarization orientations. More importantly, the extinction ratio and the polarization degree of CNT can be actively tuned simply by changing the layer number. For example, Kyoung et al. proposed a freestanding THz polarizer based on highly-oriented multiwalled CNT and realized the increased extinction ratios by changing the CNT layer from 10 to 75 . However, the independent CNT cannot realize the polarization rotation (for example, cross-polarization conversion) and it is also limited by the low polarization degree. In this case, the integration of MG with CNT may bring new opportunities for developing the high-performance THz polarization conversion device.
In this study, we obtain the THz orthogonal polarization control based on the compound metamaterial composed of the CNT layer sandwiched by two MG layers (CSMG). In the integration of CSMG, the Fabry-Perot resonance cavity is constructed by the CNT and MG layers. The experimental results show that the transmission spectrum possesses multiple peaks and valleys with a constant frequency interval. Moreover, the influence of CNT layer number and CNT orientation on polarization conversion of CSMG was studied. By the simulation of CNT in the grating model, the simulation results have a good agreement with the experiments. This work provides a good way towards practical applications for THz polarization manipulation by employ the nanomaterial with optical microstructures.
2. Experimental methods
There are three components in this CNT integrated metamaterial, of which the middle is a CNT layer, the upper and lower is the metal grating layer on a quartz substrate, as shown in Fig. 1(a). Here, the orientations of two MG layers are orthogonal arranged with each other and they are oriented with an angle of 45° or -45° to the middle CNT layer. The MG layers are made of 200 nm-thick gold with the dimensions of p=20 um and w=14 um on a 500-nm-thick quartz substrate, and it has been fabricated through the micro-nano process of masking, lithography, evaporation, stripping, and lastly, the samples with the size of 1 cm × 1 cm are obtained by the laser scribing . CNT here is drawn from long-ranged and highly aligned multiwalled CNT forests (≈6 walls, ≈9 nm of outer diameter), which is grown by chemical vapor deposition (CVD) . Figure 1(b) and Fig. 1(c) show the SEM images of the CNT layer with different magnifications of 2um and 200 nm, and we can find that the CNTs are generally arranged in order, except for very small branches. The thickness of the CNT layer (tc) depends on the CNT layer number, which in the range of a few to tens of nanometers. The integration process of CNT with MG is as follows: firstly, the CNT was transferred from the CNT forests to the quartz side of the MG layer by using glass rods and rectangular frames, and make sure it has a 45° orientation to the grating; secondly, to make the CNT well attach to the quartz surface, the ethanol was spray-coated onto the sample, and the multi-layer CNT can adhere on the back of MG layer after repeat the above steps; lastly, align the grating orientations of the second MG perpendicular with the CNT-attached MG, and complete the packaging with UV glue.
The experiments were performed by using the self-built THz time-domain spectroscopy system (THz-TDS) , as schematically illustrated in Fig. 1(d). Here, the THz pulse was generated by a low-temperature-grown GaAs photoconductive antenna (PCA), and a (110) ZnTe crystal was used for detection. The excitation source was a Ti: sapphire laser with 75 fs duration of 80 MHz repetition rate with a central wavelength of 800 nm. The incident laser is divided into two paths after the BS, to generate and detect the THz waves. The emitted THz wave from PCA is focused on the sample through the gilded parabolic mirror P1 and P2, and then the modulated THz wave that passed through the sample was focused on the ZnTe crystal by the parabolic mirror P3 and P4. Meanwhile, the probe fs-light incident at the same position of the ZnTe crystal through a certain delay optical path. When there is a THz signal, the intensity of the separated fs-light that passes through λ/4, WP, and balance detector will change, and it is proportional to the THz electric field. Based on this, we can extract the THz temporal signal by using this electro-optic sampling approach . The measuring time step was 0.04 ps in this experiment. All the experiments were carried out at room temperature with humidity of less than 5%.
3. Results and discussions
3.1 Polarization characteristics of CSMG with different CNT layers
In our previous work [25,39], we have studied the polarization characteristics of the discrete sub-wavelength MG and CNT in the THz band and concluded that MG can be employed as a perfect THz polarizer, and only the polarization components that perpendicular to the grating directions can be output from the metal gratings. The polarization performance of CNT can be actively tuned by changing the layer number and orientations. In the following, we will mainly discuss the 90° linear polarization conversion of the composite device. As illustrated in Fig. 1(a), the lower metal grating (MG2) is oriented along the x-axis, therefore, the final polarization that output from CSMG must be perpendicular to the direction of MG2. Here the THz wave incident into the CSMG is x-linear polarized, and then the y-linear polarized signal that transmitted from the device is detected. Figure 2(a) shows the experimental results of the measured THz-TDS signals of CSMG with different CNT layers. The whole spectrum can be divided into three parts, the main pulse from 2.5∼10 ps, the small fluctuations from 10∼17.5 ps, and the deputy pulse from 17.5∼25 ps. It should be pointed out that the deputy pulse here is not common to see in the transmitted THz signal, and it may be originated from the internal interaction in CSMG. Besides, by contrast with the different CNT layers, we can find the peak amplitude drops linearly with the decreases of the CNT layers from 50 to 0 layers, and the amplitude is approximately zero when no CNT exists (i.e. 0 layers). By the Fourier transform of time-domain signals, the amplitude transmission was obtained as shown in Fig. 2(b). Here, the CSMG with 50-layer CNT has the highest transmission and it presents multi-band peaks and valleys, and the frequency bandwidths are nearly the same to ∼75 GHz. With the decrease of the CNT layers, the peak transmittance linearly decreased, which is consistent with the variation trend of the time-domain signals in Fig. 2(a). Also, we can find the position of the peak frequency has a slight red-shift with the decrease of the CNT layer, and this is due to the phase lag of the deputy pulse in the time domain signal. If the number of CNT layers is further increased (>50), the corresponding polarization degree of CNT will also be higher, but it will bring more loss to the composite device. Therefore, the high polarization conversion rate and low loss cannot be achieved simultaneously for composite devices, and the CNT achieves a good balance in the case of 50 layers.
To analyze the effect of CNT layers on polarization conversion of CSMG, we present the transmittance of CSMG and the degree of polarization (DOP) of CNT with the increased CNT layers, as shown in Fig. 2(c). The red lines represent the peak transmittance of CSMG at 0.468 THz, and it increased from 0.02 of 0-layer CNT to 0.60 of 50-layer CNT. The CNT-DOP is shown in blue lines, and it has high values of 87.2%, 93.5%, and 98.7% when the CNT layers in 20, 35, and 50. Most importantly, the variation trend of the two curves keeps the same pace, the front rises faster (0-20) and the back slowly tends to be flat (20-50). Therefore, we can conclude that the transmittance has a positive correlation with the CNT-DOP, and in turn, the high CNT-DOP brings in the good polarization conversion efficiency of the compound device.
To verify the experimental results, we performed the simulation of CSMG by using CST software. The excitation source and signal detection adopt the configuration of a planar wave with a probe, and two pairs of the periodic boundary condition are set at both x- and y-directions. In the simulation model, the quartz is set to be lossless with the permittivity of 3.61, and the gold is set as the loss metal with the electrical conductivity of 4.5e7 S/m. To reflect the structure of CNT, we simplified it into a conductive grating model with a period of 4um and a diameter of 0.1um, which is optimized by parameter scanning. The specific simulation model of CSMG can be seen in Fig. 4(b). The modulation of CNT-DOP is realized by varying the conductivity in the grating model. Based on this, the transmittance spectrum of CSMG with different CNT layers is achieved. Here, the simulation results are agreed with the experiments, especially in the amplitude variations and frequency intervals. As shown in Fig. 2(d), the transmittance spectrum exhibit a spectra envelope as a function of frequency (i.e., drop at a higher frequency), and it is similar to the frequency selection effect of non-monochromatic light incident into the Fabry-Perot cavity. Since we approximate the carbon nanotubes by using the grating with electrical conductivity, the simulation is only close to the experimental results, but not a perfect match.
3.2 Polarization characteristics of CSMG with different CNT orientations
In Fig. 3, we discuss the effect of the CNT orientations on the polarization conversion of the compound device. Here, the orientation of CNT is defined by the relative angle to the direction of grating ridges of the MG1 layer, for example, 0° means CNT is parallel to the grating ridges. Figure 3(b) shows the measured THz-TDS signals of CSMG with three CNT orientations. When the CNT is in the 0° orientation, no effective polarization component is produced and the CNT loses the ability to realize polarization conversion, therefore the signals almost the background noises, as shown in the yellow line of Fig. 3(c). Besides, we compared two symmetric configurations of 45° and -45° in the time-domain signals, of which possess the same amplitude but with the inversed phase, and one of these lines can be reversed by the other. By the Fourier transform of time-domain signals, the phase shift spectrum can be obtained as shown in Fig. 3(d). We can find δ (45°)-δ (45°) always the integer multiple of π at the given frequency, which confirms the reversed-phase in symmetric configurations. This is because the CNT with the orientation of 45° and -45° converts most of the polarization states into left-handed and right-handed polarized light, and the linear polarization generated by their components after passing through MG2 will naturally have the phase shift of π .
In addition, we present the simulation transmittance results of CSMG when CNT orientates at the angles from 0° to 90° with a 15° interval, as shown in Fig. 4. Here, the structure of CSMG is simulated in the CST software with different CNT orientations. The structure is composed of the front MG1, the middle CNT layer, and the back MG2. The middle CNT orientated into the three angles of 0°, 45°, and 90° is shown in Figs. 4(a)–4(c). The incident light is x-polarized and y-polarized light is detected, which is consistent with the configuration of the experiments. In this way, the simulative transmittance spectrum of CSMG when the angle change from 0° to 90° was obtained, as shown in Figs. 4(d) and 4(e). Since the CNT can realize the better polarization transformation at 45° orientations, therefore the average transmission of CSMG is higher than in other directions. Due to the polarization effect of any two angles that added to 90° is equivalent for the CNT, so their transmittance lines are nearly the same, such as the results in 15° and 75° orientations shown in Fig. 4(d) and Fig. 4(e). Therefore, the above experimental and simulation results manifest that the amplitude of CSMG also can be modulated by changing the orientations of the CNT layer.
3.3 Theoretical analysis and simulation verification
To clarify the inner polarization conversion mechanism, we show the diagram of the polarization evolution in CSMG, as shown in Fig. 5(a). Here, the x- and y-linear polarizations are represented in red or green arrows, separately. In our design, the MG layer from the front and back plays the role of the polarization selection, and the CNT layer is used to produce a 45° polarization component. When the x-polarized light incident into the CSMG, it can be freely transmitted from MG1 and enter the first microcavity formed by MG1 and CNT. The components that parallel to the CNT orientation will reflect back and forth in this microcavity and the components that perpendicular to the CNT orientation can further penetrate the second microcavity formed by CNT and MG2. Under the process of the multi-reflection, polarization selection, and polarization transformation, the y-polarized wave will be greatly accumulated and then output from MG2 eventually.
In addition, the above analysis can be explained by the classical Fabry-Perot interference theory . The frequencies of the positive interference are equally spaced, and the frequency intervals $\Delta v$ can be expressed by:$\Delta v$ equals to 78.9 GHz, which match the experimental results in theory, and the small discrepancies may come from the uncertainty in the thickness of the quartz substrates. Moreover, we study the relation between cavity length d and frequency interval $\Delta v$. As shown in Fig. 5(b), we can find that the smaller d is, the larger $\Delta v$ is. Therefore, the frequency position and the intervals in the transmission spectrum can be theoretically regulated by changing the cavity length.
Furthermore, we demonstrate the electric field distributions of CSMG to confirm the inner mechanism of multiple resonances and polarization conversion in the x-z cutting view. In Fig. 6(a) and Fig. 6(b), we compare the electric field distributions of CSMG at 0.43 THz and 0.468 THz, in which the frequency is located to the transmission valley and peak in Fig. 3(c). For the transmission valley, we can find it has a strong resonance between MG1and MG2, and the incident energy is reflected by MG2, which cannot output from CSMG, as shown in Fig. 6(a). As to the transmission peak shown in Fig. 6(b), there also has multiple resonances in the cavity but the resonance intensity has some reduced, and most of them transformed into the transmitted wave. Therefore, CSMG has high transmission at the position of the transmission peak. Moreover, to reveal the conversion in polarization state, we present the electric vector distribution of CSMG at 0.468THz with different planes in the x-y cutting view, as shown in Figs. 6(c)–6(e). As mentioned above, the THz wave incident into CSMG is x-linear polarized, and its electric vector distribution is shown in Fig. 6(c). Under the polarization selection and local resonance mechanism, the polarization state inside CSMG is transformed into several parts including elliptic polarization, circular polarization, and rotational linear polarization. So the electric vectors adjacent to the CNT layer present many kinds of forms at the x-y projection plane, as shown in Fig. 6(d). After passing through MG2, the electric vectors at the output plane become vertical to the input plane, which indicated the polarization state is perfectly orthogonally transformed, as shown in Fig. 6(e).
In conclusion, we propose a strategy that integrates CNT with artificial microstructures to realize the perfect orthogonal polarization manipulation in the THz regime. By the measurement of the experimental THz-TDS, the transmission spectrum of the integrated structure shows multi-band peaks and valleys with the same frequency space, which good satisfies the law of the theoretical Fabry-Perot interference. Besides, we achieve the active modulation of the amplitude and phase in CSMG by controlling the CNT layer numbers and orientations. To confirm the authenticity of the experiments, we performed the simulation by simplifying the CNT into a conductive grating model. The simulated electric field distributions good reflect the inner mechanism of multiple resonances and polarization conversion. Based on the interference theory, the working band can be extended to other frequencies by adjusting the media thickness and refractive index of the Fabry-Perot cavity. This work will bring new ideas for the research in developing and broaden novel THz polarization devices with nanomaterials and artificial microstructures.
National Natural Science Foundation of China (62005143, 61971242, 61831012); Natural Science Foundation of Tianjin City (19JCYBJC16600); Young Elite Scientists Sponsorship Program by Tianjin (TJSQNTJ-2017-12).
The authors declare no conflicts of interest.
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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