Single-walled carbon nanotubes (SWCNTs) are applied to realize an enhanced frequency modulation for a suspended THz silicon grating, which is fabricated by a nanosecond laser direct writing and coated with the synthetic SWCNTs/polyacrylic emulsion composite. With terahertz time domain spectroscopy system, the transmission spectra of the bare and SWCNTs coated silicon grating are measured and compared. The SWCNTs coated silicon grating can realize an improved extinction ratio and quality factor, which is due to the SWCNTs caused local field enhancement and can be explained by the theoretical simulation with finite element method. Besides the effective modulation of the grating transmittance, SWCNTs can also be integrated with other platforms and applied in future THz imaging and communication systems.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Terahertz (THz) radiation has been widely applied for biological imaging, medical diagnostics, security inspection, optical communications, etc [1–4]. THz frequency modulation has aroused much attention and can be realized with some material such as semiconductor or graphene [5, 6], or artificial microstructures such as gratings [7–11], while the latter can provide more design freedom. Metal wire gratings can realize THz filtering and sometimes support over 99% transmission at specific frequencies [7–9], but suffering from high insertion loss and complicated fabrication process. A laser-induced filamentary plasma grating can be used to modulate THz wave [10, 11]. It is somewhat problematic due to a poor modulation efficiency (around 3-dB extinction ratio) and unstable device performance. Recently, silicon-graphene was applied to modulate THz wave [12, 13]. A suspended silicon grating can realize high-resolution resonant phenomena, but also limited with a less than 10-dB extinction ratio and low quality factor . Semiconductor nanowires or single-walled carbon nanotubes (SWCNTs) can be applied for THz modulation, and usually used as a polarizing beam splitter, modulator, resonator, and so on [15–21]. SWCNTs usually need to be highly aligned in order to introduce an anisotropic transmission change, whose fabrication is complicated. The corresponding extinction ratio is not so good while the ratio between the transmission amplitude of 90° and 0° is calculated as only about 6.2 . For nonaligned SWCNTs thin film, its electrical/optical properties have been investigated , but the detailed application such as integration with grating structure has not been investigated to the best of our knowledge, where an improved THz frequency modulation could be expected.
In this paper, the modulation performance of SWCNTs coated suspended silicon grating is investigated. Unlike the conventional fabrication technique such as photolithography, deep reactive ion etching, and photo electrochemical etching [24–26], a nanosecond laser direct-writing technique is applied for the suspended grating fabrication. A synthetic SWCNTs/polyacrylic emulsion composite is used to cover the surface of silicon grating. The performance of suspended silicon grating with SWCNTs is measured and discussed, while the grating transmittance can be effectively modulated by SWCNTs.
2. Device fabrication
Suspended silicon grating was fabricated by a nanosecond laser based direct-laser-writing (DLW) technique, which can also be used for high quality laser micromachining of a variety of semiconductors and metals [27, 28]. Figure 1(a) shows a schematic illustration of fabrication process. A double-side polished 200-μm-thick low-doped p-type silicon wafer was used for grating fabrication by the DLW facility [Fig. 1(b)], which consists of a solid-state nanosecond laser and two-dimensional micro-displacement stage. The laser has a wavelength, pulse duration, and output power of 1064 nm, 20 ns, and 20 W, respectively. A 20 × 20 mm2 wafer was used for grating fabrication while the fabricated grating area is 10 × 10 mm2 with a period of 300 μm, which can be easily fabricated by the DLW technique. Then a synthetic SWCNTs/polyacrylic emulsion composite was dropped on the surface of silicon grating and planarized by spin-coater. The sample was transferred into a heating oven and baking at 40 °C for 120 min, in order to get a desired composite structure for the SWCNTs integrated suspended silicon grating.
SWCNTs were prepared by arc discharge method . Typically, pure carbon rod and carbon rod mixed with catalyst were used as cathode and anode, respectively. Arc discharge was generated at a current of 80 A in inert gas. SWCNTs are very difficult to disperse in solvents. A high power ultrasonication process is effective for preparation of SWCNTs dispersion. Firstly, 500-mg SWCNTs, 1-g water-soluble polymer dispersant, and a small amount of distilled water were mixed uniformly in a mortar. After diluted to 50 mL with distilled water, the mixture was sonicated at room temperature for 30 min by means of Ultrasonic Cell Crusher. 50-mL SWCNTs dispersion was mixed with the same volume of polyacrylic emulsion and stirred for 60 min using a magnetic stirrer. SWCNTs/polyacrylic emulsion composite can then be obtained and applied for the following film coating on the grating surface.
Figure 2(a) shows a surface scanning electron microscope (SEM) image for the suspended silicon grating with coated SWCNTs/polyacrylic emulsion composite, with Fig. 2(b) for a cross-sectional view. It can be clearly seen that the suspended silicon grating has a desired period and thickness of 300 and 200 μm, respectively. The grating ridge has a trapezoidal shape with a top and bottom width of 80 and 200 μm, respectively. SWCNTs/polyacrylic emulsion composites can be confirmed to cover the ridge surface of suspended silicon grating successfully.
3. Device characterization and discussion
THz transmission properties of the suspended silicon grating with and without coated SWCNTs were measured by terahertz time-domain spectroscopy (THz-TDS) system, as shown in Fig. 3. A laser with 800-nm central wavelength, 35-fs pulse width, and 1-kHz repetition rate was delivered from a Ti: sapphire amplifier system. A single-crystal InAs was used as the THz emitter, and a GaAs photoconductive dipole antenna was employed as the detector. A silicon lens was used to collimate the generated terahertz pulses into parallel beams. Since the THz beam waist (15-mm diameter at 1 THz) was larger than the sample surface of 10 × 10 cm2, a diaphragm was needed to resize the THz beam to sample size and an aluminum plate with an aperture of 8 × 8 mm2 was used. The sample mounted on it was placed midway between the photoconductive transmitter and receiver at the waist of THz beam, and the THz beam was focused on the sample center. Due to that the THz wave can be easily absorbed by water, all measurements were carried out in a dry nitrogen environment. The transmission characteristics for THz pulses parallel (TE mode) or perpendicular (TM mode) to the ridge direction of suspended grating were measured respectively. The waveforms were recorded over a 40-ps time-window, leading to a spectral resolution of 25 GHz.
Figures 4(a) and 4(b) show the time-domain THz pulses after passing through the suspended silicon grating with and without coated SWCNTs respectively, while a silicon wafer was also measured as a reference. Different temporal waveforms for the grating at TE or TM mode can be observed. For the TE mode in Fig. 4(a), the peak-to-peak amplitude of the suspended silicon grating with and without coated SWCNTs are almost the same compared with the silicon wafer. For TM mode in Fig. 4(b), THz wave travels about 1.2-ps faster in the suspended silicon grating with and without coated SWCNTs compared with the reference silicon wafer, due to the smaller average refractive index of the grating. Moreover, their peak-to-peak amplitude increased slightly compared with the silicon wafer, which may be caused by the enhanced transmittance caused by the grating.
THz Frequency spectrum response for the suspended silicon grating at TE and TM mode were obtained by using Fourier transform to the temporal signal, as shown in Figs. 5(a) and 5(b). Compared with the reference silicon wafer, obvious Mie resonance peaks can be observed for the suspended gratings at either polarization, while the grating with coated SWCNTs can realize an enhanced peak extinction ratio compared with the bare grating. For TE mode, SWCNTs can help to realize resonance extinction ratios of about 30, 40, and 56 dB, which are increased by 14, 24, and 42 dB at the frequencies of 1.32, 1.45, and 1.62 THz, respectively, compared with the bare grating. Corresponding quality factors are about 43, 95, and 155 for the SWCNTs coated grating, compared with 17, 19, and 63 for the bare grating. In the case of TM mode, the resonance extinction ratios for the SWCNTs coated grating are about 24, 33, and 30 dB, which is improved by 6, 16, and 22 dB at the frequency of 0.48, 1.08, and 1.80 THz, respectively. The suspended silicon grating with coated SWCNTs can realize a high quality factor of about 74 at a 1.08-THz frequency compared with 21 at a similar frequency for the bare grating. Compared with the previous results with only silicon grating  or SWCNTs , the integration of SWCNTs and the suspended silicon gratings can help to realize an improved resonance modulation performance.
To clarify the interaction between the SWCNTs and silicon grating resonance, THz response of SWCNTs/polyacrylic emulsion or polyacrylic emulsion on bare silicon wafer was also measured by the THz-TDS system, with the corresponding frequency spectra shown in Fig. 6. The frequency spectrum for the bare silicon wafer was also presented as a reference. As can be seen from Fig. 6, there is no significant difference between the silicon wafer and that coated with polyacrylic emulsion or SWCNTs. It can be confirmed that the enhanced resonance filter performance is caused by the interaction between SWCNTs and the suspended silicon grating, not by the material itself.
In order to investigate the interaction between SWCNTs and silicon grating, a finite-element simulation method (CST Microwave Studio) was used to calculate the transmission spectrum and corresponding electric field distribution. By applying a periodic boundary condition, simulation cell is composed of a single grating period with the trapezoid ridge as in Fig. 1(b). Effective dielectric parameters from SWCNTs/polyacrylic emulsion composites were first derived to model SWCNTs in simulations . Figures 7(a) and 7(b) show the calculated transmission spectra for the TE and TM modes, respectively. The simulated spectra for the bare silicon wafer are also presented, which are almost the same as the measured results in Fig. 5. For the silicon grating, polarization-dependent transmission property can be observed.
Experimental and theoretical frequency spectra for the SWCNTs coated suspended silicon grating are also compared in Figs. 8(a) and 8(b). Different from the experimental results, more resonance peaks are observed in the simulation. This may be caused by the limited frequency-domain resolution of the THz-TDS system. Nonetheless, the trend of theoretical results and some main resonance peaks are roughly similar to the experimental ones. There is a main theoretical resonance peak of 1.61 THz for TE and 1.05 THz for TM mode. Both are close to the measured 1.62 and 1.08 THz. An improved resonance extinction ratio and quality factor can be obtained from the simulation for the SWCNTs-covered grating, which is consistent with the observed experimental results.
Figures 9(a) and 9(b) shows the distinct electric behaviors at the resonance frequency of 1.61 THz for TE mode of the bare and SWCNTs coated suspended silicon grating. Compared with the bare silicon grating, much stronger electric field distribution in the SWCNTs coated grating ridge is observed, and a thin-layer SWCNTs can help to enhance the local resonance field. It can be confirmed that the physical mechanism of SWCNTs enhanced frequency modulation is caused by its interaction with the suspended silicon grating.
To summarize, a polarization-dependent THz frequency modulator is realized by a suspended silicon grating. Nonaligned SWCNTs can help to enhance the local resonance field and improve the grating frequency modulation performance. The silicon grating is fabricated by a nanosecond laser direct-writing technique. The synthetic SWCNTs/polyacrylic emulsion composite is used for integration with silicon grating. For the frequency modulation, SWCNTs coated grating can realize an enhanced extinction ratio and high quality factor, which are about 56 dB and 155 at a 1.62-THz frequency for TE mode and about 33 dB and 74 at a 1.08-THz frequency for TM mode. Theoretical simulation is also performed to explain the physical mechanism, which is due to the interaction between SWCNTs and grating. SWCNTs are also convenient to integrate with other device platforms, which would have a wide range of applications in the future.
National Natural Science Foundation of China (NSFC) (11774235, 61705130, 61505106); Natural Science Foundation of Shanghai (17ZR1443400); Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
We thank the reviewers for the valuable comments.
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