Tunable characteristics of the SWCNTs thin film modulator in the THz region

Weijun Wang; Wen Xiong; Jie Ji; Yue Tian; Furi Ling; Jianquan Yao

doi:10.1364/OME.9.001776

1. Introduction

Terahertz (THz) electromagnetic wave is widely used in many fields such as communication, material identification, and biological sensing and have attracted numerous attention from researchers, so the devices tuning THz wave by various means substantially increase [1–6]. The properties of various materials are of great significance to the development of THz devices. However, one-dimensional material [7]with unique structure and properties has been extensively explored in the development of terahertz spectroscopy. As a representative of one-dimensional materials, carbon nanotubes (CNTs) have attracted extensive attention due to their stable and unique carbon structures, which have high mechanical strength and unique electromagnetic and emission properties [8–10]. CNTs are divided into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes. SWCNTs consist of single-layer cylindrical graphite layer. SWCNTs have the small diameter distribution range, fewer defects and higher uniformity. Therefore, their properties are more stable. SWCNTs are direct band gap structure, and their caliber and chirality determine the band gap, so they have wide absorption spectrum from visible region to mid infrared region. The lattice vector parameters define the various types of nanotubes, such as armchair (n, n), zig-zag (n, 0) or chiral (m, n). The values of m and n determine the chirality (twist) of the nanotube, which in turn determines nanotube properties. Therefore, SWCNTs can be divided into metal-type SWCNTs and semiconductor-type SWCNTs [11–13].

At present, most researches focused on the conductivity of carbon nanotubes at different temperatures [14,15]. The conductivity of carbon nanotubes in the presence of optical field and/or electric field was seldom reported. In this paper, we studied the transmission and conductivity spectra of the polyimide-based film of SWCNTs at room temperature by THz spectroscopy technology. the polyimide-based film of SWCNTs showed the excellent modulation effect. During the measurement, modulation characteristics such as transmission of single-wall carbon nanotube composite films tuned by the illumination and voltage were obtained. Harmonic oscillator model was used to analyze the conductivity spectra.

2. Experimental details

The schematic diagram of interdigitated electrodes is shown in Fig. 1. Interdigitated electrodes were grown on a (100)-oriented 20 × 20 × 0.5 mm³ Si crystal. The interdigital electrodes were prepared with standard photolithography and Cr film was subsequently deposited as the adhesion layer and Au film by electron beam evaporation. The interdigital electrodes are composed of 12 pairs of 5 μm wide gold stripes. The gap between each couple of stripes was 300 microns wide. The thickness of the interdigital electrode was 200 nm.

Fig. 1 200 nm high interdigitated electrodes grown on silicon substrate.

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The SWCNTs we use are High Purified Large Surface Area Single-walled Carbon. The purity is higher than 95%. The Outer Diameter is 1-2nm. The Inner Diameter is 0.8-1.6nm. The length is 5-30um. The Special Surface Area is greater than 1075m²/g, and the conductivity is greater than 1000 S/cm. Then polyimide-based film of SWCNTs were prepared as follows. Firstly, sodium dodecyl sulfate (SDS) was added into N, N-dimethylformamide (DMF) and mixed to get the mixture solution. Then SWCNTs powder was mixed with DMF mixture solution under sonication. Then, SWCNTs composite solution was prepared with SWCNTs, DMF mixture solution and polyimide solution under sonication. The thin film coating material was spun onto the interdigital electrodes. Finally, the film was finished after it was dried and hardened in a vacuum oven [9,16].

The atomic force microscope (AFM) was used to analyze the surface roughness of the single-wall carbon nanotube composite film (Figs. 2(a) and 2(b)). The root mean square (RMS) roughness of the film was 5.541 nm, indicating sufficiently smooth surfaces for photoelectric characteristics measurements. To characterize the polyimide-based film of SWCNTs, scanning electron microscope (SEM) was applied to observe the uniformity of SWCNTs in thin films. The SWCNTs composite film was measured from the larger scale of 200 μm and the smaller scale of 1 μm and SEM images at different scales were obtained. The uniformity coefficient of the SWCNTs could be used to analyze the photoelectric properties of the film (Figs. 2(c) and 2(d)) because the spot radius on the samples was 4 mm.

Fig. 2 (a) and (b) AFM images of the sample. (c) SEM plot with a scale of 200 μm. (d) SEM image with a scale of 1 μm.

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The transmission spectra of the polyimide -based SWCNTs film in THz frequency domain were obtained by a THz time-domain spectrometer (THz-TDS) system produced by Zomega Terahertz Corporation (USA) as shown in Fig. 3. The detectable frequency range was from 0.3 THz to 1 THz (9.9 cm⁻¹ to 33 cm⁻¹) and the frequency resolution was 4.5 GHz. All-solid-state green laser (center wavelength, 532 nm) was used as an external pump source and the incident angle of the green laser to the film was 60°. During the measurement, the temperature of the THz-TDS system was maintained at 18°C (291 K). In addition, the orientation of the electric field vector of the incident terahertz radiation was vertical to the direction of the interdigital electrodes.

Fig. 3 The THz-TDS system schematic. A green laser is obliquely incident upon the surface of the film at 60° with regard to the polar axis.

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3. Experimental results and discussion

Figure 4 shows the time-domain signal waveforms of air, substrate and the polyimide -based SWCNTs film on the substrate at room temperature. Compared with the signal waveforms of the substrate, the signal waveforms of the film had a significantly decreased amplitude, indicating that the film had a significant effect on the terahertz spectrum. The signal waveforms under different power of the external optical pump slightly shifted to the left and the amplitude of the signal waveform was decreased with the increase in the optical pump power.

Fig. 4 (a) Time-domain signal waveforms of the substrate and air at room temperature. (b) Time-domain signal waveforms of the polyimide -based SWCNTs film on the substrate under illumination.

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Figure 5(a) shows the transmission properties of the polyimide -based SWCNTs film in the power range of the external optical pump from 0 mW to 300 mW. The transmission is calculated as the ratio of the waveform $t (ω) = E s (ω, p) / E r (ω, p)$ , where $E r (ω, p)$ and $E s (ω, p)$ are respectively the frequency waveforms of the bulk sample and the underlying substrate at a certain power of the external optical pump [17,18]. The transmission decreases with the increase in the power of optical pump. Modulation depth is defined as [19]:

M = \frac{T_{0} - T_{1}}{T_{0}},

where

T_{0}

is the transmission without illumination;

T_{1}

denotes the transmission in the presence of the illumination [20]. The modulation depth of the polyimide-based SWCNTs film is shown in Fig. 5(b). The modulation depth was nearly 50% at 25 mW, indicating the predominant photosensitivity. At 300 mW, the modulation depth of the film reached 95.6% at 0.8 THz, indicating the excellent modulation effect. The polyimide-based SWCNTs film exhibited a strong modulation effect under the action of external optical pumps. With the increase in illumination, an obvious absorption saturation phenomenon occurred. Pauli blocking can be applied to explain these behaviors caused by the decrease in conductivity growth rate under stronger optical pump conditions [21].

Fig. 5 (a) Transmission of the polyimide -based SWCNTs film under external optical pump power from 0mW to 300mW. (b) The modulation depth of the polyimide -based SWCNTs film under external optical pump power from 0 mW to 300 mW.

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Through the transmission, the dielectric properties of the polymer-based SWCNTs film can be evaluated by numerically inverting the expression for $t (ω)$ as:

t (ω) = \frac{(N_{s} + 1) e x p (i ω Δ L (N_{s} - 1) / c)}{1 + N_{s} + Z σ (ω) d},

where

c

is the speed of light;

Z

= 377 Ω is the vacuum wave impedance;

d

is the effective thickness of optically excited layer;

N_{s}

is the complex refractive index of the substrate obtained with the ratio of the value of illumination passing through the substrate to the reference illumination [22]. Due to the uniform distribution of the SWCNTs and the interaction between the conducting medium and the dielectric medium, the interaction between the polarized particles was consistent with the effective medium. In the meantime, the SWCNTs were several micrometers long and the wavelength of the terahertz electromagnetic pulse was tens to hundreds of micrometers. The effective dielectric theory can be used to explain the experimental results of the THz spectrum [23]. Therefore, the experimental results were fitted by the effective medium approximation. We can apply the Lorentz model to process the measured THz transmission coefficient spectra together with the directly determined the conductivity spectrum. The conductivity equation under harmonic oscillator model is expressed as [24]:

σ (ω) = ε_{0} ω [\frac{ω_{p}^{2}}{(ω_{0}^{2} - ω^{2}) i + ω γ} - i],

where $ε_{0}$ is the absolute dielectric constant; $ω_{0}$ is the resonance frequency; $ω_{p}$ , $γ$ , and $ω$ are the plasma frequency, the electron scattering rate, and the angular frequency of THz wave, respectively.

Fig. 6 Measured the real and imaginary parts of conductivity of thin film at room temperature. The scatter plot was the actual measured value, and the curves were the result of simulation using the Eq. (2), which the frequency in terahertz domain were from 0.3 THz to 1 THz.

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In this way, the conductivity of the thin film $σ (w)$ with different power of the external optical pump at room temperature in the range from 0.3 THz to 1.0 THz (9.9 cm⁻¹ to 33 cm⁻¹) could be obtained. With the increase in the power of external optical pump, the real part of the conductivity increased and the peaks of the real part of the conductivity appeared due to the resonance in the SWCNTs under the action of the power of optical pump (Fig. 5). As the power of external optical pump increased, the resonance became more and more intense due to the effect of SWCNTs and the increasingly stronger illumination. With the increase in the power of optical pump, we witnessed the blue shift of the peak frequency. The shift arose for bundled tubes due to the optical pump power accretion dependence of the conductivity of SWCNTs with a fermi level close to the top of the valence band [25–28].

The plasma frequency $ω_{p}$ is obtained by Eq. (2), and the carrier density $N$ can be calculated with $ω_{p} = \sqrt{N e^{2} / ε_{0} m^{*}}$ . The effective mass of carrier $m^{*} = 0.2 m_{e}$ ( $m_{e}$ is the mass of electron) is the average value of electron mass and hole mass in the polyimide-based SWCNTs film [23,29](Fig. 6). Obviously, the mobility and mean-free paths of quantities characterizing individual SWCNTs were far higher than those of the polyimide-based SWCNTs film whose conductivity was determined by the inter-tube contacts and intrinsic response of the SWCNTs. Therefore, the obtained values of these parameters were underestimated compared with those of individual SWCNTs [29]. The raise in conductivity was ascribed to the increase in carriers caused by optical pump. The external optical field drove the electrons to hop to the conduction band in SWCNTs. Apart from electrons produced by the film, the carrier density in the substrate promoted the increase in the carrier density in the film due to the concentration difference when the optical field was applied on the samples.

As shown in Fig. 7(a), the plasma frequency $ω_{p}$ is magnified with the increase in the pump power. The values of the carrier density $N$ under the pump power of 0 mW and 300 mW are 1.04 × 10¹⁷ cm⁻³ and 5.44 × 10¹⁸ cm⁻³, respectively. The electron scattering rate $γ$ shows a linear relationship with the power of optical pump, as shown in Fig. 7(b). When lattice is in thermal equilibrium and the temperature is T, the average number of phonons (c) in the lattice can be described as [30,31]:

Fig. 7 (a) Plasma frequency $w_{p}$ under different power of the external pump. (b) Electron scattering rate $γ$ under the action of illumination.

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\bar{n} (q = 0) = \frac{1}{e^{^{h w_{q = 0} / K T}} - 1},

With the temperature rise, the number of phonons increases. Therefore, with the increase in the power of external optical pump, the number of phonons increase and the carrier density also increases. Therefore, the electron scattering rate $γ$ increases as the increase in the power of external optical pump.

In order to explore the electric field modulation characteristics of the polyimide -based SWCNTs film, the transmission of the thin film at 0mW and 300mW was listed as a comparison. The results of the conductivity under different voltages (0 V, 10 V, 20 V, 30 V and 40 V) were shown in Fig. 8.

Fig. 8 Transmission of the polymer-based SWCNTs film under the power of external pump (0 mW and 300 mW) and different voltages.

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The transmission rose with the increase in voltage under the pump power of 0 mW and 300 mW (Fig. 8). Under the pump power of 0 mW, the transmission was little changed. However, under the pump power of 300 mw, the transmission at 40 V was more than twice of that at 0 V, indicating that the modulation effect was obvious. Under a pump power of 0 mW, the maximum modulation depth of the film was 0.88% at 0.8 THz due to the effect of the voltage. The modulation depth of the film reached 41.87% at 0.7 THz due to the effect of voltage at 300 mW. The change in the modulation depth at 300 mW was much larger than that at 0 mW, indicating that the modulation effect of voltage was more obvious under the condition of illumination. However, the effect of electric field on the change in transmission or modulation depth was far less than the effect of illumination. Therefore, we believed that the optical modulator worked better for the polyimide -based SWCNTs film.

The reasons for the changes in the film of the carrier density are shown in Fig. 9. When the external electric field was applied on the film, electron diffusion was affected. Positive and negative voltages were applied at two ends of an interdigital electrode to affect the diffusion of electrons [32]. When the external optical pump was applied, the carriers generated by the excitation and the substrate were increased. The voltage affected the spread of carriers. Due to the electrons and holes collected around the positive and negative electrodes of the interdigital electrodes, the carrier density $N$ in other places would decrease, thus affecting the transmittance of the film in the terahertz band. Under the pump power of 0 mW, the carrier density decreased from 1.11 × 10¹⁷ cm⁻³ to 1.09 × 10¹⁷ cm⁻³ with the increasing of the voltage from 0 V to 40 V. Under the pump power of 300 mW, the carrier density decreased from 5.80 × 10¹⁸ cm⁻³ to 3.17 × 10¹⁸ cm⁻³ with the increasing of the voltage from 0 V to 40 V. The variation of the plasma frequency $ω_{p}$ with the electric field under the pump power of 0 mW and 300 mW proved the reduction of carriers (Figs. 10 (a) and 10(b)).

Fig. 9 (a) Formation of space charge field in the film without illumination. E is the voltage by the interdigital electrodes. Movement of less carriers in the film with the voltage. (b) Under illumination, a lot of carriers move in the film with the voltages.

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Fig. 10 (a) and (b) plasma frequency $ω_{p}$ of the film under the pump power of 0 mW and 300 mW and different voltages of 0 V, 10 V, 20 V, 30 V, and 40 V. (c) and (d) electron scattering rate $γ$ of the film under the pump power of 0 mW and 300 mW and different voltages.

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The electron scattering rate $γ$ of the film gradually decreased from 0.525 THz to 0.464 THz under the pump power of 0 mW and 300 mW when the voltages increased from 0 V to 10 V, 20 V, 30 V, and 40 V (Figs. 10(c) and 10(d)). As the voltage increased, the carrier density of the film gradually decreased and the probability of electron collision in the carbon nanotube diminished under the electric field. However, as the voltage increased, the electric field increased the film temperature and the number of phonons. However, the electron scattering rate $γ$ diminished as the voltage was amplified. Therefore, the carrier density probably had a main influence on the electron scattering rate $γ$ .

4. Summary

Through exploring the modulation characteristics of the thin film on the substrate, we found that the transmission spectra of the thin film dropped greatly with the increase in the power of optical pump. The modulation depth of the film reached 95.6% under the pump power of 300 mW, indicating the excellent modulation effect. It could be observed that the peaks of the real part of the conductivity were blue-shifted with the increase in the power of optical pump due to the resonance. The transmission was determined by carrier density, which responded to conductivity. In order to explore the electric field modulation characteristics of the polyimide-based SWCNTs film, the conductivity without illumination or under the optical pump power of 300 mW was explored. The change of modulation depth under the pump power of 300 mW was much larger than that under the pump power of 0 mW, indicating that the modulation effect of voltage was more obvious in the condition of illumination. Therefore, the modulation effect controlled by optics was more obvious than that in electric field. For the polyimide-based SWCNTs film, the optical modulator was a more suitable choice. Due to good photoelectric characteristics and great changes in transmission under the influence of the film voltage, the polyimide-based SWCNTs film had potential application prospects in terahertz band of terahertz modulators and detectors, especially in flexible devices.

Funding

National Natural Science Foundation of China (61735010, 61675147); National Key Research and Development Program of China (2017YFA0700202).

Acknowledgments

Thanks to Guang Huang engineer in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for the support in preparation of the sample.

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Tunable characteristics of the SWCNTs thin film modulator in the THz region

Abstract

1. Introduction

2. Experimental details

3. Experimental results and discussion

4. Summary

Funding

Acknowledgments

References

Cited By

Figures (10)

Equations (4)

Optical Materials Express