Open Access
Add to BibSonomy
Abstract
Lack of efficient routes to modulate the propagation properties of the terahertz (THz) wave is a major barrier for the further development of THz technology. In recent years, two dimensional transition metal dichalcogenides (2D TMDCs) were applied to the design of effective THz modulator by forming heterostructure with Si. Here, we experimentally demonstrate an optical controlled THz modulator consisting of liquid-exfoliated WS2 nanosheets and a silicon substrate (WS2-Si). By innovatively depositing liquid-exfoliated WS2 nanosheets on the Si instead of growing by chemical vapor deposition (CVD) method, both of the size and the thickness of WS2 film is controlled. The WS2-Si sample presents a flat modulation depth from 0.2 THz to 1.6 THz. The modulation depth reaches 56.7% under a 50 mW pumping power, which is over 5 times enhanced compared with that of the Si substrate. With the increase of illumination power, the modulation depth continues to increase, finally reaching up to 94.8% under 470 mW. Besides, the WS2-Si sample also achieves ~80% modulation depth under 450 nm illumination, indicating its ability to operate under either of wavelength in visible spectra. Moreover, we compare the sample to the reported modulators including CVD growth TMDCs-Si ones and find our sample has comparable modulation effects while is much easy to be prepared. Therefore, we believe our work is meaningful to provide an alternative route to achieve effective modulation of THz waves by adopting liquid-exfoliated 2D materials.
© 2017 Optical Society of America
1. Introduction
With the rapid development in recent decades, THz has revealed bright application prospects in myriad fields including bio-sensing [1], explosion detection [2], medical diagnoses [3,4], communication [5] and molecular spectroscopy [6–8]. However, to date, most progress have been made among THz sources and detectors, while the progress made in THz functional devices is relatively backward [7]. Excellent-performance functional devices operating in THz frequency are in urgent demand. THz modulators is one of the basic functional devices adopted to manipulate the propagation properties of THz wave [9]. It is a basic element of other complex functional devices such THz communication systems [10]. Different types of materials have been utilised to achieve effective modulation of THz waves such as semiconductors, multiple quantum wells, photonic crystals, matamaterials and so on [8,11–18]. However, although the modulation of THz have been achieved to some extent, these modulation schemes have drawbacks such as large insert loss, sub-zero operating environment and narrowed modulation frequency band, which are still difficult to meet the requirements of practical application.
Two dimensional (2D) material with optical/electric tunable carrier properties is naturally suitable for the design of active device, which is regarded as a promising route for the development of THz modulator [19]. Among them, the transition metal dichalcogenides (TMDCs) could form heterostructure with tradition semiconductor to overcome the limited absorption of light due to their atomic thickness [20,21]. In 2016, Y. Cao et al. and S. Chen et al. reported the optically tuned THz modulator based on CVD growth multi-/monolayer molybdenum disulfide (MoS2)-Si heterostructure respectively, which both achieved enhanced modulation effects. However, different from graphene that have realized large-area growth on metal substrates, the production of millimeter-sized continuous film of monolayer TMDCs, to the best of our knowledge, is still a challenging issue. Those unfavorable factors faced with CVD growth TMDCs hinder the further improvement of THz modulator performance. Liquid exfoliation is an alternative method of few-layer TMDCs preparation that obtains TMDCs nanosheets from exfoliating bulk materials by ultrasonic and centrifugation processes [22]. The advantage of this method is that its products originated from bulk material remains minimal lattice defects and therefore outperform CVD growth materials in properties such as mobility and lattice integrity, which is favouring in THz modulation.
In this work, we report an optically controlled THz modulator made by liquid-exfoliated multilayer WS2 nanosheets with Si substrates. Our sample can achieve an effective broadband modulation of THz wave from 0.1 THz to 1.6 THz. The modulation depths reach over 94.8% and 80.3% under 800 nm and 450 nm pumping of ~0.4 W, respectively, indicating the ability to operate well in the whole visible band. This modulation results outweigh the performance of graphene modulator and reach the same scale of the reported THz modulator based on CVD growth MoS2 [9,10]. The modulation mechanism is attributed to the catalysis function of carrier generation in WS2-Si interface due to forming a heterostructure. As our sample achieved comparable THz modulation, our work provides a practicable route to achieve effective modulation of THz wave by adopting liquid exfoliated 2D materials.
2. Experimental setup
General procedure for WS2 nanosheets production
The WS2 nanosheets was prepared by liquid-exfoliation method that has been reported elsewhere [23–25]. The brief preparation processes were offered as follows: All chemical reagents were obtained commercially and were of analytically pure. WS2 power (2 g, 1–6 mm, Aladdin Reagent Inc.) was putted to a 100 mL flask. The mixed solution of ethanol and water (100 mL) with the volume fractions of 20% was added as dispersion solvent. The oxygen in solution was removed though purging argon flow for ~20 min. After that, the ultrasonic treatment was carried out for about 8 h, and then the dispersion was centrifuged at 5000 rpm for 10 mins to remove precipitates. Finally, the liquid supernatant of WS2 nanosheets were transferred to reagent bottle for future experiments.
Sample preparation and characterization
The detailed fabrication procedure is presented in experimental section. The AFM image of exfoliated WS2 nanosheets is presented in Fig. 1(a). These liquid-exfoliated WS2 nanosheets were stacked with each other with random orientation. Their thickness distributed from several to dozens of nanometres. The prepared WS2 nanosheets solution was dropped on the substrate. With the gradual evaporation of solvent, the WS2 nanosheets were deposited into a thin film on the Si substrate. The prepared WS2-Si sample is shown in Fig. 1(b). From the Raman spectra of the WS2-Si sample in Fig. 1(d), both Raman peaks of WS2 and Si substrate were measured together. The frequency differences between E2g1 and A1g of WS2 was measured to ~69.5 cm−1 in our sample, which coarsely presented a bulk-like characteristics. Besides, the thickness distribution map of the WS2 film was measured by white light interferometer (presented in Figs. 1(c) and 1(e)). The average thickness of the WS2 film in the tested sample is obtained ~250 nm.
Fig. 1 (a) The AFM image of liquid-exfoliated WS2 nanosheets. (b) The image of the prepared WS2-Si sample. (c) Thickness distribution map of WS2 film measured by white light interferometer. (d) Raman spectra of the WS2-Si sample. (e) The height curve obtained from the data of red line in Fig. 1(c).
A standard THz time-domain spectroscopy (THz-TDS) system with a coaxial pump light was employed in transmission spectra measurement of the WS2-Si sample. A GaAs photoconductive antenna was adopted as the THz source in this system to provide a broadband THz pulse from 0.1 to 1.6 THz. The antenna was powered by a Ti: sapphire laser that operates at the 800 nm with 35 fs duration at 80 MHz repetition rate. The pump light used to irradiate material was a split beam from the incident laser. The fluence of pump light was as low as ~10 nJ/cm2. A single pulse with such weak fluence could only make slight pump-probe response that cannot be detected by our system. Under this circumstance, this pump light could be regarded as a continuous wave (CW) light and the effects of a single pulse is ignored. The diameter of THz pulse and pump beam were 4 mm and 5 mm at sample’s position, respectively. All the experiments were carried out at room temperature and the samples were placed in a sealed cavity with nitrogen atmosphere (the humidity less than 5%) and the disturbance of humidity was less than 1%.
3. Experimental results and discussions
The normalized temporal waveforms through WS2-Si, Si and nitrogen (reference) under dark condition were obtained in Fig. 2(a). The temporal waveforms of WS2-Si and Si were delayed about 4 ps and reduced by 32.43% compared with that of nitrogen. The reduced amplitude of THz wave is attributed to the Fresnel loss in Si/nitrogen interface and the delay is from the additional optical path of silicon layer. We noted that the amplitude of THz waves through WS2-Si and Si were almost the same, indicating that the intrinsic absorption from WS2 film is insignificant under dark condition. Figures 2(b) and 2(c) show the THz waveforms through WS2-Si under varied light power conditions. With the increase of pumping power from 0 mW to 470 mW, the amplitude of THz waves were reduced continuously. In this work, the modulation depth are defined as,where the T is the transmission ratio of THz wave and the Elight and Edark refer to the complex amplitude spectra of THz wave under illuminated and dark conditions, respectively.
Fig. 2 (a) The normalized THz waveforms through WS2-Si (pink), Si (purple) and nitrogen (blue). (b) The THz waveforms through WS2-Si under pumping powers from 0 mW to 470 mW and (c) the frequency spectra obtained from Fig. 2(b) through FFT method.
It can be seen in Figs. 3(a)-3(d) that the WS2-Si sample presented good modulation effects at broadband frequency regions from 0.2 THz to 1.6 THz. Under a lower pump power of 50 mW, the modulation depth of 56.7% has been achieved in WS2-Si sample at 0.5 THz while that of bare Si are only 10.1% in comparison. The over five times increment of modulation depth indicates that our WS2-Si indeed achieves effective modulation of transmitted THz wave. When tuning the pump power higher, the modulation depth continues to increase, finally reaching up to 94.8% under 470 mW, which means almost all THz waveforms are blocked by our WS2-Si modulator.
Fig. 3 The frequency resolved (a) transmittance and (b) modulation depth spectra of WS2-Si sample under different pumping powers. The modulation depth spectra of (c) Gr-Si, (d) Si and (e) WS2-Sapphire under the pumping power from 20 mW to 470 mW. (f) The modulation depths of Si (pink), WS2-Si (purple), WS2-Sapphire(blue) and Gr-Si (cyan) versus pumping power at 0.5 THz.
Graphene is one of the 2D materials that have been intensively studied and applied in THz functional device [26–31]. As a comparison, we also made a monolayer graphene sample (Gr-Si) for the same measurement. As shown in Figs. 3(c) and 3(f), the modulation effects of the Gr-Si sample shows a similar increase trend but with a lower extent. The modulation depth of Gr-Si sample reached up to 72.1% under 470 mW laser pumping, which is below the WS2-Si sample by 22.7 percentage points.
The advantage of WS2-Si was embodied especially under lower pumping powers. The modulation depth of WS2-Si under 20 mW pumping power is over 10 times and 2 times higher than that of Si and Gr-Si respectively while these ratios are reduced to around 3 and 1.4 when the pumping power increases to 200 mW due to saturation effects.
After that, the modulation effect of our WS2-Si device at different wavelengths are studied. This is different from the previous study that usually studied with one particular wavelength. As the visible band is one of the most widely used bands in daily application, we conducted measurement under a 450 nm (close to the lower limit of visible band) CW laser illumination. Figures 4(a) and 4(b) present the modulation depths of Si, Gr-Si and WS2-Si under different pumping powers. The WS2-Si sample keeps highest modulation depth among three samples, reaching 80.3% at 0.5 THz under a pumping power of 410 mW.
Fig. 4 (a) The modulation depth spectra of Si (pink), Gr-Si (olive) and WS2-Si (blue) under 450 nm CW light. The pump power is 410 mW. The modulation depths of three samples are declined with the increase of frequency, which accord with the conductivity in Drude model. (b) The modulation depths (at 0.5 THz) of Si (pink), Gr-Si (olive) and WS2-Si (blue) as functions of pump power, respectively. The pump wavelength is adopted as 450 nm.
However, different from the 800 nm pumping condition that had a flat modulation depth curve at different frequencies, the modulation effects were presented to be frequency-dependent that the modulation depths reached the maximum at around 0.28 THz and showed a downward trend with the increase of frequency. The modulation depth of WS2-Si reached over 80% at 0.3 THz while gradually dropped to more than a half (~41%) at 1.6 THz. According to the Drude model, the real conductivity is expressed as follow:
where is the relaxation rate, is the plasma frequency, is the permittivity of free space and is the angular frequency of THz wave. The value of Drude conductivity is declined with the increase of frequency especially when the frequency is close to the plasma frequency, which describes our frequency-dependent modulation depths. This result implies a difference in carrier density between 450 nm and 800 nm illumination.To achieve optimal modulation effects, we possess a comparison to the modulation depths of WS2-Si under 800 nm and 450 nm pumping conditions. The modulation depths adopted are that at 0.4 THz where the spectral component is the optimal. As presented in Fig. 5(a), the modulation depths reach up to 88.6% (800 nm) and 80.3%. (450 nm) under around 0.4 W pumping powers, respectively. The result indicates that the 800 nm light is more efficient in THz modulation of our sample than the 450 nm light, which agrees the aforementioned frequency dependent modulation depth.
Fig. 5 The comparison of modulation depths (at 0.4 THz) of WS2-Si versus (a) pump power and (b) normalized photon density: 800 nm (blue) and 450 nm (green). It is clear that when the same quantity of photon is illuminated in our WS2-Si sample the difference of modulation depth between 800 nm and 450 nm illumination is much reduced.
To get quantitative understandings of this difference, the relationship between light wavelength and photon number should be taken into consideration. Under the same pumping power, longer wavelength of pumping light could provide more quantities of photon and thus excite more photocarriers. The ratio of photon number between 450 nm (2.76 eV) and 800 nm (1.55 eV) wavelengths under the same pumping power is 2.76/1.55≈1.78. Besides, only a part of the photons are absorbed while the others are reflected in the interface. The reflectivity of 450 nm light and 800 nm light are 0.42 and 0.33 for our sample, respectively [32,33]. Considering these factors, the modulation depth of WS2-Si as a function of normalized photon density was presented in Fig. 5(b). It is clear that when the same quantity of photon is illuminated in our WS2-Si sample the difference of modulation depth is much reduced compared with Fig. 5(a). From the results, we can get two conclusions: First, our WS2-Si sample could operate well in the either wavelength of light source in visible band as it achieved over 80% modulation depth under both 800 nm and 450 nm illumination. Second, longer wavelength light contains higher photon density under the same light power and thus has more advantage in realizing effective THz modulation. Besides, this conclusion can also be utilized to describe the aforementioned frequency-dependent modulation effect. The plasma frequency positive correlated with the carrier density is lower when pumping with 450 nm light. According to the previous research [34], the THz transmission ratio through a film sample on substrate can be calculated by formula:
where is the complex refractive index of Si (the imaginary part is insignificant compared with its real component), is the vacuum wave impedance, is the effective thickness of optical excited layer is the complex sheet photoconductivity linked with the intensity of pumping power, is the difference of substrate thickness that is negligible in this work.It is also noted that obvious saturation behaviours have been observed in the modulation depth of WS2-Si and Gr-Si in Fig. 3(f). These behaviors are caused by the decrease of conductivity growth rate in higher illumination condition. Pauli blocking due to Pauli exclusion in doped semiconductor can be used to explain the reduced modulation [35]. With the increase of photogenerated carriers, due to Pauli blocking, the phase space available for carriers to translate is gradually reduced and then photogenerated carrier density near the WS2-Si interface [36,37]. Besides this, the reduced carrier lifetime under high fluence pumping also contributes to this phenomenon [38]. According to the reference, nonequilibrium carrier lifetime can be estimated as follows:
where the is carrier recombination rate; ,andare indicated as the intrinsic electron and hole density and the nonequilibrium electron density. When the sample operates in large injection condition (>>), the carrier recombination rate is mainly determined by the nonequilibrium carrier density. In this case, the photogenerated carrier density is proportional to the square root of pump power and behaves a saturated modulation [38]. As obvious saturation modulation effect is observed in our WS2-Si sample, it is believed that the WS2-Si could achieve maximum value of carrier density among these samples.With the measured information of photoconductivity, we can calculate the carrier density and then plasma frequency. Without losing generality, the condition of 200 mW pumping power is taken into consideration as an instance. The transmission ratios in this situations are 66.8% and 47.2% for 800 nm and 450 nm cases and the corresponding carrier density are 7.34 × 1018 cm−3 and 4.87 × 1018 cm−3, respectively. This result indicates that 33.58% larger number of carriers are excited by 800 nm light than by 450 nm light under the same pumping power condition, which is in line with our previous conclusions. As lower carrier density is accumulated with the illumination of 450 nm light, the plasma frequency () decreases to 9.9 THz that is more close to our measured frequency spectra and therefore the frequency-dependent modulation depth presents more obvious in our measurement.
The speed of modulation is another important criteria of active THz modulators, especially for high speed applications. The modulation speed of our WS2-Si sample, as presented in Fig. 6(a), was recorded by using a digital oscilloscope. The fall time, normally defined as the switching duration from 10% to 90% of the maximum modulation magnitude, was estimated to be less than 1 ms. Besides, by mechanical chopping the CW pump laser, a pump beam with the power changing from 0 to 200 mW was adopted to illuminate the WS2-Si sample. The corresponding modulation magnitudes with varied chopping frequency was presented in Fig. 6(b). Generally, the modulation magnitude decreases with the increase of chopping frequency. The 3dB bandwidth of the WS2-Si sample is estimated as ~3 kHz. This value is slightly higher than recently reported graphene-based THz modulator [39].The carrier lifetime of 0.094 ms was calculated by numerical fitting with formula. Where the and are the modulation magnitude under chopping frequency f and without chopping, respectively; the and are indicated as modulation frequency of chopper and time constant of modulator. It is worth noting that the calculated time constant (0.094 ms) from numerical fitting is far faster than the fall time (~1ms) observed in Fig. 6(a). That’s because that the accuracy of fall time is severely restricted by the method of mechanical chopping when the fall time closes to the chopping frequency. For an optically controlled THz modulator, longer carrier lifetime benefits to significant modulation as it helps reach higher non-equilibrium carrier concentration. However, this improvement also slow down the speed of modulation. The modulation speed of this WS2-Si structure is mainly determined by the carrier lifetime of the indirect bandgap silicon. The modulation speed also related to the external condition. According to the previous works, ultrafast carrier recombination in silicon can be achieved within hundreds of ps after pumping with amplified fs pulse [40]. Therefore, by choosing suitable optical sources, this WS2-Si sample has the potentials to achieve much higher modulation rate.
Fig. 6 (a) A complete switching cycle under chopped CW light illumination. The fall time was estimated to be less than 1 ms. (b) Normalized modulation magnitude, showing a 3 dB bandwidth of ~3 KHz. From the fitting result, the time constant of device is calculated as 0.094 ms.
Besides this, for THz modulator fabricated by stacked nanosheets on substrate, the stability of performance among different batch of sample is another factor that matters the application prospect in practical situation. For the stability test, another three of WS2-Si samples prepared with the same procedure are measured under 800 nm illumination. The frequency averaged modulation depth of three sample under different pumping powers are all presented in in Fig. 7(a). The standard error of modulation depth under 200 mW among three samples is only 3.5%, indicating good repeatability in device performance even under such rough control. This little difference mainly arising from manual preparation could be further eliminated in standardized production with specialized instrument.
Fig. 7 (a) The frequency averaged modulation depth of sample 1 (black square), sample 2 (red square) and sample 3 (blue square) under pumping power from 20mW to 200 mW. (b) The peak amplitude of THz wave through THz modulator with different polarization angles with the interval of 30 degree. The blue line and red line are indicated for the case of 0 mW and 100 mW, respectively.
The modulation effects for THz with different polarizations are also studied. As the result shown in Fig. 7(b), the modulation depth of our WS2-Si sample is isotropic. This is reasonable that the WS2 nanosheets in our sample are stacked with randomly arranged orientation and thus no anisotropy phenomenon is exhibited which is normally found in metamaterial-based THz modulators. This feature is of beneficial to the practical application because there is no requirement for precise adjustment of the angle of modulator.
To have a deeper understanding of the modulation mechanism of the WS2-Si structure, the sheet conductivity of the WS2-Si sample under 200 mW and 400 mW pumping powers are calculated theoretically. As shown in Fig. 8(a), the conductivity is close to zero in lower frequency region under a 0 mW pumping condition, agreed with the high DC resistance of Si and WS2 nanosheets. However, the conductivity exhibits a significant increase from 1.58 × 105 S/m to 2.02 × 106 S/m when the WS2-Si sample was illuminated by a 200 mW power light, and this value kept increasing when the pumping power rises to higher level. The photoconductivity of WS2-Si defined as under 400 mW pumping is shown in Fig. 8(b), which is higher by an order of magnitude than the intrinsic conductivity under dark conditions.
Fig. 8 (a) The real part of sheet conductivity of WS2-Si under different powers of 800 nm pumping power: 0 mW (black), 200 mW (orange) and 400 mW (red) pumping powers. (b) The sheet photoconductivity of WS2-Si under a 400 mW pumping power.
Besides, Drude model is adopted to fit the modulation depths under different pumping conditions. The effective mass of carrier ( is the mass of electron) is the average value of electron mass and hole mass in WS2 [41]. The plasma frequency is determined by carrier density. The comparison of experiment data and the simulation results under 20 mW and 200 mW pumping powers are shown in Fig. 9, respectively and corresponding values of carrier density are 1.4 × 1017 cm−3 and 1.6 × 1018 cm−3, respectively. This result confirms that the conductivity of our sample is increased under illumination from the increased Drude term photoexcited carriers and thus induces a decrease of transmittance of THz wave.
Fig. 9 The experimental data (dots) and simulation results (solid lines) of modulation depth of WS2-Si under 20 mW (blue) and 200 mW (red) pumping condition and the corresponding carrier density are 1.4 × 1017 cm−3 and 1.6 × 1018 cm−3, respectively.
As we have discussed the effective modulation of THz wave in the WS2-Si structure, another question still troubled us that where are these carriers coming from, WS2 film or Si surface? As the modulation effects shown in Fig. 3(e), the WS2-sapphire sample under the same pumping power have almost no modulation effects. We measured the intensity variation of pumping light through WS2-sapphire and bare sapphire respectively and the result showed no significant difference. Therefore, Si, as a bulk semiconductor, is deduced to generate the majority of carriers in our sample as considerable parts of light are absorbed on the Si surface. This explanation is similar to the reported THz modulator based on CVD growth MoS2 [9,42]. Besides, compared with the modulation depths of Si, WS2 and WS2-Si shown in Fig. 3(f), we find that only the WS2-Si can achieve effective modulation of THz wave while either Si or WS2 nanosheets can only achieve limited (or no) modulation depth. This difference highlights the significance of the interaction between WS2 nanosheets and Si on the modulation effects of the WS2-Si structure. According to previous researches, several types of 2D materials, such as graphene [43], MoS2 [10,42] and organometal halide perovskite [17] could form a heterostructure with semiconductor Si due to their difference in bandgap and Fermi level. As shown in Fig. 10, the energy band is bent in the junction area between WS2 nanosheets and Si substrate. When the WS2-Si is under illumination, the photoexcited electrons and holes near the junction area are separated each other and drifted into different regions under the function of built-in electric field. Large amounts of electrons are trapped into WS2 in this case. This mechanism decrease the electron and hole product and raises the electron-hole recombination time. Higher concentration of carriers can be aggregated under this mechanism and finally realize a deeper THz modulation. Besides this, the diffusion may also have contributions as the WS2 film offers an additional direction for carrier diffusion along the concentration gradient. This analysis also helps to explain the similar modulation effects among three sample in the stability test. As the WS2 film plays a catalyzer-like role in the carrier accumulation, it is the coverage not the thickness of the WS2 film is dominate factor in the THz modulation. Therefore, the difference of WS2 film during the preparation process may not affect much to our modulation effect while the coverage can be well controlled with subsequent machining process according to the diversity of requirements.
Fig. 10 Illustration of photocarriers movement near the WS2-Si interface. Photogenerated electrons and holes near the interface are divided into different region through drift and diffusion movement.
Last but not least, we list some reported optically controlled THz modulator based on 2D material-semiconductor structure for a comparison to our WS2-Si sample in Table 1 [9,10,16,42,44]. The graphene based THz modulators are slightly inferior in THz modulation than TMDCs based sample except the one with Ge substrate that has higher mobility. Among TMDCs samples, our sample was found to have comparable modulation effects to the reported CVD growth multilayer MoS2-Si sample while requires a lower power. And the enhanced ratio (~5) of modulation depth compared to the corresponding substrate is comparable to the reported monolayer MoS2-Si one. On the other hand, our sample is much easier to be made than the CVD growth one and has no limits of device dimension thus more suitable for large-scale industrial production.
Table 1. Some reported modulation depth of optically controlled THz modulator based on 2D material system.
4. Conclusions
In conclusion, we have experimentally demonstrated an optically controlled THz modulator based on the liquid-exfoliated multilayer WS2 nanosheets under the illumination of 800 nm wavelength. The modulation depth of the WS2-Si sample reached up to 56.7% under a pumping power of 50 mW. This value is over 5 and 1.8 times higher than the bare Si and the Gr-Si sample, respectively. Besides, our sample also exhibited over 80% modulation depth under a 450 nm pumping light, which indicates the WS2-Si modulator can operate under either wavelength of the visible illumination. Through theoretical analysis and numerical stimulation, the THz modulation mechanism is confirmed to be a free carrier absorption followed by Drude model. Moreover, we conclude that the catalysis function of WS2 nanosheets is due to forming heterostructure. Compared with most reported THz modulator based on CVD growth 2D materials, our WS2-Si sample achieves comparable modulation effects but is much easier to be prepared and more stable. As such, we believe our work provides a practicable route to achieve effective modulation of THz wave by adopting liquid-exfoliated 2D materials.
Funding
Scientific Research Foundation of National University of Defense Technology (No. zk16-03-59); Opening Foundation of State Key Laboratory of High Performance Computing (20160101, 201601-03)
References and links
1. X. Wu, X. Pan, B. Quan, X. Xu, C. Gu, and L. Wang, “Self-referenced sensing based on terahertz metamaterial for aqueous solutions,” Appl. Phys. Lett. 102(15), 89 (2013). [CrossRef]
2. R. Beigang, S. G. Biedron, S. Dyjak, F. Ellrich, M. W. Haakestad, D. Hubsch, T. Kartaloglu, E. Ozbay, F. Ospald, N. Palka, et al., “Comparison of terahertz technologies for detection and identification of explosives,” Proc. SPIE 9102, 1–10 (2014).
3. E. P. J. Parrott, Y. Sun, and E. Pickwell-Macpherson, “Terahertz spectroscopy: Its future role in medical diagnoses,” J. Mol. Struct. 1006(1), 66–76 (2011). [CrossRef]
4. P. H. Siegel, “Terahertz technology in biology and medicine,” IEEE Trans. Microw. Theory Tech. 52(10), 2438–2447 (2004). [CrossRef]
5. S. Koenig, D. Lopez-Diaz, J. Antes, F. Boes, R. Henneberger, A. Leuther, A. Tessmann, R. Schmogrow, D. Hillerkuss, R. Palmer, T. Zwick, C. Koos, W. Freude, O. Ambacher, J. Leuthold, and I. Kallfass, “Wireless sub-THz communication system with high data rate,” Nat. Photonics 7(12), 977–981 (2013). [CrossRef]
6. H. T. Chen, A. J. Taylor, and N. Yu, “A review of metasurfaces: physics and applications,” Rep. Prog. Phys. 79(7), 076401 (2016). [CrossRef] [PubMed]
7. M. Rahm, J. Li, and W. J. Padilla, “THz wave modulators: A brief review on different modulation techniques,” J. Infrared Millim. Terahertz Waves 34(1), 1–27 (2013). [CrossRef]
8. I. Chatzakis, Z. Li, A. V. Benderskii, and S. B. Cronin, “Broadband terahertz modulation in electrostatically-doped artificial trilayer graphene,” Nanoscale 9(4), 1721–1726 (2017). [CrossRef] [PubMed]
9. S. Chen, F. Fan, Y. Miao, X. He, K. Zhang, and S. Chang, “Ultrasensitive terahertz modulation by silicon-grown MoS2 nanosheets,” Nanoscale 8(8), 4713–4719 (2016). [CrossRef] [PubMed]
10. Y. Cao, S. Gan, Z. Geng, J. Liu, Y. Yang, Q. Bao, and H. Chen, “Optically tuned terahertz modulator based on annealed multilayer MoS2.,” Sci. Rep. 6(1), 22899 (2016). [CrossRef] [PubMed]
11. D. S. Jessop, C. W. O. Sol, L. Xiao, S. J. Kindness, P. Braeuninger-Weimer, H. Lin, J. P. Griffiths, Y. Ren, V. S. Kamboj, S. Hofmann, J. A. Zeitler, H. E. Beere, D. A. Ritchie, and R. Degl’Innocenti, “Fast terahertz optoelectronic amplitude modulator based on plasmonic metamaterial antenna arrays and graphene,” Proc. SPIE 9747, 1–14 (2016).
12. D. S. Jessop, S. J. Kindness, L. Xiao, P. Braeuninger-Weimer, H. Lin, Y. Ren, C. X. Ren, S. Hofmann, J. A. Zeitler, H. E. Beere, D. A. Ritchie, and R. Degl’Innocenti, “Graphene based plasmonic terahertz amplitude modulator operating above 100 MHz,” Appl. Phys. Lett. 108(96), 171101 (2016). [CrossRef]
13. R. Degl’Innocenti, D. S. Jessop, Y. D. Shah, J. Sibik, J. A. Zeitler, P. R. Kidambi, S. Hofmann, H. E. Beere, and D. A. Ritchie, “Low-bias terahertz amplitude modulator based on split-ring resonators and graphene,” ACS Nano 8(3), 2548–2554 (2014). [CrossRef] [PubMed]
14. H. Chen, W. J. Padilla, M. J. Cich, A. K. Azad, R. D. Averitt, and A. J. Taylor, “A metamaterial solid-state terahertz phase modulator,” Nat. Photonics 3(3), 148–151 (2009). [CrossRef]
15. W. L. Chan, H. Chen, A. J. Taylor, I. Brener, M. J. Cich, and D. M. Mittleman, “A spatial light modulator for terahertz beams,” Appl. Phys. Lett. 94(21), 213511 (2009). [CrossRef]
16. Q. Li, Z. Tian, X. Zhang, R. Singh, L. Du, J. Gu, J. Han, and W. Zhang, “Active graphene-silicon hybrid diode for terahertz waves,” Nat. Commun. 6, 7082 (2015). [CrossRef] [PubMed]
17. B. Zhang, L. Lv, T. He, T. Chen, M. Zang, L. Zhong, X. Wang, J. Shen, and Y. Hou, “Active terahertz device based on optically controlled organometal halide perovskite,” Appl. Phys. Lett. 107(9), 093301 (2015). [CrossRef]
18. I. H. Libon, S. Baumgärtner, M. Hempel, N. E. Hecker, J. Feldmann, M. Koch, and P. Dawson, “An optically controllable terahertz filter,” Appl. Phys. Lett. 76(20), 2821–2823 (2000). [CrossRef]
19. L. Wang, J. Jie, Z. Shao, Q. Zhang, X. Zhang, Y. Wang, Z. Sun, and S. Lee, “MoS2/Si heterojunction with vertically standing layered structure for ultrafast, high-detectivity, self-driven visible-near infrared photodetectors,” Adv. Funct. Mater. 25(19), 2910–2919 (2015). [CrossRef]
20. V. I. Klimov, “Optical nonlinearities and ultrafast carrier dynamics in semiconductor nanocrystals,” J. Phys. Chem. B 104(26), 6112–6123 (2000). [CrossRef]
21. J. Li, L. Luo, H. Huang, C. Ma, Z. Ye, J. Zeng, and H. He, “2D behaviors of excitons in cesium lead halide perovskite nanoplatelets,” J. Phys. Chem. Lett. 8(6), 1161–1168 (2017). [CrossRef] [PubMed]
22. M. Lotya, Y. Hernandez, P. J. King, R. J. Smith, V. Nicolosi, L. S. Karlsson, F. M. Blighe, S. De, Z. Wang, I. T. McGovern, G. S. Duesberg, and J. N. Coleman, “Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions,” J. Am. Chem. Soc. 131(10), 3611–3620 (2009). [CrossRef] [PubMed]
23. K. G. Zhou, N. N. Mao, H. X. Wang, Y. Peng, and H. L. Zhang, “A mixed-solvent strategy for efficient exfoliation of inorganic graphene analogues,” Angew. Chem. Int. Ed. Engl. 50(46), 10839–10842 (2011). [CrossRef] [PubMed]
24. L. Guardia, M. J. Fernández-Merino, J. I. Paredes, P. Solís-Fernández, S. Villar-Rodil, A. Martínez-Alonso, and J. M. D. Tascón, “High-throughput production of pristine graphene in an aqueous dispersion assisted by non-ionic surfactants,” Carbon 49(5), 1653–1662 (2011). [CrossRef]
25. Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De, I. T. McGovern, B. Holland, M. Byrne, Y. K. Gun’Ko, J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari, and J. N. Coleman, “High-yield production of graphene by liquid-phase exfoliation of graphite,” Nat. Nanotechnol. 3(9), 563–568 (2008). [CrossRef] [PubMed]
26. X. He, “Tunable terahertz graphene metamaterials,” Carbon 82, 229–237 (2015). [CrossRef]
27. X. He, P. Gao, and W. Shi, “A further comparison of graphene and thin metal layers for plasmonics,” Nanoscale 8(19), 10388–10397 (2016). [CrossRef] [PubMed]
28. F. Lin, W. Shi, X. He, and X. Zhong, “Investigation of graphene assisted tunable terahertz metamaterials absorber,” Opt. Mater. Express 6(2), 331 (2016). [CrossRef]
29. Y. Fan, F. Zhang, Q. Zhao, Z. Wei, and H. Li, “Tunable terahertz coherent perfect absorption in a monolayer graphene,” Opt. Lett. 39(21), 6269–6272 (2014). [CrossRef] [PubMed]
30. Y. Fan, N. H. Shen, T. Koschny, and C. M. Soukoulis, “Tunable terahertz meta-surface with graphene cut-wires,” ACS Photonics 2(1), 151–156 (2015). [CrossRef]
31. Y. Fan, N. Shen, F. Zhang, Z. Wei, H. Li, Q. Zhao, Q. Fu, P. Zhang, T. Koschny, and C. M. Soukoulis, “Electrically tunable Goos–Hänchen effect with graphene in the terahertz regime,” Adv. Opt. Mater. 4(11), 1824–1828 (2016). [CrossRef]
32. M. A. Green and M. J. Keevers, “Optical properties of intrinsic silicon at 300 K,” Prog. Photovolt. Res. Appl. 3(3), 189–192 (1995). [CrossRef]
33. M. A. Green, “Self-consistent optical parameters of intrinsic silicon at 300 K including temperature coefficients,” Sol. Energy Mater. Sol. Cells 92(11), 1305–1310 (2008). [CrossRef]
34. X. Zou, J. Shang, J. Leaw, Z. Luo, L. Luo, C. La-o-Vorakiat, L. Cheng, S. A. Cheong, H. Su, J. X. Zhu, Y. Liu, K. P. Loh, A. H. Castro Neto, T. Yu, and E. E. Chia, “Terahertz conductivity of twisted bilayer graphene,” Phys. Rev. Lett. 110(6), 067401 (2013). [CrossRef] [PubMed]
35. K. F. Mak, K. He, C. Lee, G. H. Lee, J. Hone, T. F. Heinz, and J. Shan, “Tightly bound trions in monolayer MoS2.,” Nat. Mater. 12(3), 207–211 (2012). [CrossRef] [PubMed]
36. H. Wang, C. Zhang, W. Chan, C. Manolatou, S. Tiwari, and F. Rana, “Radiative lifetimes of excitons and trions in monolayers of the metal dichalcogenide MoS2,” Phys. Rev. B 93(4), 045407 (2014). [CrossRef]
37. C. Zhang, H. Wang, W. Chan, C. Manolatou, and F. Rana, “Absorption of light by excitons and trions in monolayers of metal dichalcogenide MoS2 : Experiments and theory,” Phys. Rev. B 89(20), 205436 (2014). [CrossRef]
38. S. M. Sze and K. K. Ng, Physics of Semiconductor Devices (John wiley & sons, 2006).
39. M. Mittendorff, S. Li, and T. E. Murphy, “Graphene-based waveguide-integrated terahertz modulator,” ACS Photonics 4(2), 316–321 (2017). [CrossRef]
40. L. Hou, X. Shao, L. Yang, Z. Wang, L. Zhang, M. Zhao, and W. Shi, “Study of carrier lifetime of silicon by OPTP method,” Proc. SPIE 9795, 97950Q (2015). [CrossRef]
41. D. Ovchinnikov, A. Allain, Y.-S. Huang, D. Dumcenco, and A. Kis, “Electrical transport properties of single-layer WS2.,” ACS Nano 8(8), 8174–8181 (2014). [CrossRef] [PubMed]
42. W. Zheng, F. Fan, M. Chen, S. Chen, and S.-J. Chang, “Optically pumped terahertz wave modulation in MoS2-Si heterostructure metasurface,” AIP Adv. 6(7), 075105 (2016). [CrossRef]
43. P. Weis, J. L. Garcia-Pomar, M. Höh, B. Reinhard, A. Brodyanski, and M. Rahm, “Spectrally wide-band terahertz wave modulator based on optically tuned graphene,” ACS Nano 6(10), 9118–9124 (2012). [CrossRef] [PubMed]
44. Q. Y. Wen, W. Tian, Q. Mao, Z. Chen, W. W. Liu, Q. H. Yang, M. Sanderson, and H. W. Zhang, “Graphene based all-optical spatial terahertz modulator,” Sci. Rep. 4(1), 7409 (2014). [CrossRef] [PubMed]
