Terahertz Modulation Properties Based on ReS₂/Si Heterojunction Films

Abstract

Low cost, low power consumption and high performance are urgent needs for the application of terahertz modulation devices in the 6G field. Rhenium disulfide (ReS₂) is one of the ideal candidate materials due to its unique direct band gap, but it lacks in-depth research. In this work, a highly stable ReS₂ nanodispersion was prepared by liquid-phase exfoliation, and a uniform, dense and well-crystallized ReS₂ film was prepared on high-resistivity silicon by drop casting. The morphological, optical and structural properties of the ReS₂/Si heterojunction film were characterized by OM, SEM, AFM, XRD, RS and PL. The terahertz performance was tested by using a homemade THz-TDS instrument, and the influence of different laser wavelengths and powers on the terahertz modulation performance of the sample was analyzed. The modulation depth of the sample was calculated based on the transmission curve, and the changes in the refractive index and conductivity of the sample with frequency at the corresponding laser power were calculated. The results show that the fabricated ReS₂/Si heterojunction terahertz modulator can stably achieve 30% broadband modulation in the range of 0.3~1.5 THz under the low-power pumping of 1555 mW/cm², and the maximum conductivity is 3.8 Ω⁻¹m⁻¹.

1. Introduction

Terahertz waves are electromagnetic waves between microwaves and infrared waves, with a frequency band of 0.1 THz to 10 THz; so, they have the characteristics of broadband and low energy. Studies in the literature have confirmed the extraordinary potential of terahertz waves. THz pulse sources usually only contain several cycles of electromagnetic oscillations. The frequency band of a single pulse can cover the range from GHz to tens of THz. The vibration and rotation energy levels of many biomacromolecules and the phonon vibration energy levels of dielectrics, semiconductor materials, superconducting materials and thin film materials fall within the THz band. Therefore, they are widely used in medical diagnostics, security inspections, communications, radar and other fields [1,2,3,4,5]. At present, the research on terahertz waves can be roughly divided into the generation of terahertz sources, the development of terahertz detectors, and the use and application of terahertz functional devices. Terahertz sources and detectors are currently the most in-depth research topics. After decades of development, the technology has become relatively mature, but functional terahertz devices still have obvious shortcomings. Terahertz modulators are one of the most fundamental components of terahertz functional devices. They are mainly used in terahertz sensing and terahertz communication, including the current hotspot 6G communication [6]. Terahertz modulators are mainly used to control parameters such as amplitude, phase and frequency to meet process requirements such as cost reduction, energy consumption reduction, efficiency improvement and system simplification [7]. With the advancement of terahertz technology, other new requirements for terahertz modulation devices are constantly emerging. Higher modulation depth and width and faster response speed are the main research goals of terahertz modulation devices. So far, terahertz waves have been successfully modulated in various ways, and the main modulation methods are optical modulation, electrical modulation and thermal modulation. Compared with the other two methods, optical modulation has a simpler operating environment, lower power consumption and faster response time. In contrast, all-optical modulation uses photons from an external laser to replace conventional electrical excitation while reducing interference from the external environment. As a result, the modulation speed is faster and the modulation depth is greater than other optical modulations. While all-optical switches have achieved ultrafast response times in many applications, they usually consume a large amount of energy to meet the requirements. Although energy consumption can be reduced by reducing the response speed, finding a balance between these two factors remains a major challenge [8].

The materials used for modulation are also diverse, including conventional semiconductors, superconducting materials, two-dimensional materials, metasurface materials and liquid crystal materials [9,10,11,12,13,14,15,16]. Among them, two-dimensional materials have attracted attention and are being studied due to their unique properties, and more and more new materials are being discovered and used for optical modulation, such as graphene [17,18], black phosphorus [19], transition metal sulfides [20,21,22,23], etc. The electronic structure of two-dimensional materials enables them to produce significant photoelectric effects over a wide range of electromagnetic wavelengths [24]; at the same time, they can interact strongly with light in a very small space, which allows them to achieve ultrafast responses in micro/nanoscale optoelectronic devices [25]. Moreover, various layered two-dimensional materials hardly suffer from lattice mismatch due to their high mechanical strength and layered spatial structure, and ideal artificial composite heterostructures can be prepared by simple vertical stacking (spin coating method or chemical vapor deposition, abbreviated as CVD) [26,27,28]. For example, the electrode prepared by transferring WS₂ onto MoS₂ exhibited an electron mobility of 65 cm²/Vs and a photoresponsivity of 1.42 A/W, far exceeding the photovoltaic performance of a single component [29]. Most importantly, 2D materials can be manipulated to change their physical and chemical properties, such as all-optical modulation, which uses an external light source to photoexcite the material and change the charge carrier density. Transition Metal Dichalcogenides (TMDCs), an important representative of 2D layered materials, have attracted the attention of researchers because their energy band structure can be controlled by changing the number of layers. When the number of layers is reduced from multiple layers to a few layers or a single layer due to quantum confinement and the interlayer van der Waals forces, the energy band jump of TMDCs changes from indirect jump to direct jump and the band structure changes, which leads to a significant improvement in the optoelectronic properties of the material [30,31]. At the same time, TMDCs are very diverse. According to the conductivity of the material, TMDCs include insulators, semiconductors, metals, etc. Other properties are also very diverse, so researchers have a wide range of choices. Many TMDCs have achieved excellent performance in terahertz modulation devices; for example, a modulator with a MoS₂/Si heterostructure achieved a modulation depth of 96% at a laser pump power of 4.5 W [32]. When Bi₂Se₃ thin films are combined with metasurfaces, the modulator can achieve ultrafast transmission and group delay [33]. Although excellent modulation depth and speed can be achieved, the required power consumption is often high, and modulation devices derived from TMDCs still have a long way to go.

Rhenium disulfide (ReS₂) is a relatively less studied material among TMDCs; unlike most other TMDC materials, the energy band of ReS₂ is independent of the number of layers, its bulk material behaves optically, electronically and vibrationally like an uncoupled monolayer, and its direct band gap always remains at 1.5 eV [34]. ReS₂ shows anisotropy. All those properties indicate that ReS₂ could offer a novel system to study mesoscopic physics of 2D systems without the limitation of obtaining large-area, monolayer-thick flakes. Due to the weak interlayer coupling and insensitive band gap to the layer number, a 2D ReS₂ sample can be prepared facilely and robustly [35]. ReS₂ is used to make field effect transistors and photodetectors with high optical response due to its excellent optoelectronic performance and special properties, which indicates that it also has great potential in terahertz modulation devices. In this article, the terahertz modulation performance of ReS₂/Si heterojunction films was studied using a terahertz time-domain spectroscopy system (TDS). ReS₂ nanodispersions were prepared using the liquid-phase exfoliation method, and ReS₂/Si heterojunction films were prepared by drop casting. Then, the structure of the prepared ReS₂/Si heterojunction films was characterized by optical microscopy, AFM, SEM, XRD and other techniques. Next, the modulation depth and modulation bandwidth of the ReS₂/Si heterojunction films of terahertz waves were investigated using the terahertz time-domain spectroscopy system. Finally, according to the transmittance of the ReS₂/Si heterojunction films and the Fabry–Pérot (FP) effect, the refractive index and conductivity of the ReS₂/Si heterojunction films were determined. The measurement results show that ReS₂/Si heterojunction films have potential applications in the development of high-performance ultrafast tunable devices.

2. Materials and Methods

The preparation process of the ReS₂/Si heterojunction films is shown in Figure 1. Herein, we used the liquid-phase exfoliation method to prepare ReS₂ nanodispersions (ReS₂ nanoparticles and absolute ethanol were mixed in a 1:10 ratio). We placed the high-resistance silicon wafer substrate on a hot plate at 80 °C, poured the nanodispersion droplets on the 1.5 cm × 1.5 cm substrate and repeated the process 2–3 times. With the complete evaporation of the ethanol, the preparation of the ReS₂/Si heterojunction layer was completed.

The preparation of ReS₂ nanodispersions was completed in three steps. (1) A total of 200 mg of rhenium disulfide powder (particle size is micron level, supplied by Shenzhen six carbon technology Co., Ltd., Shenzhen, China) was dissolved in 10 mL of anhydrous ethanol solvent and stirred for one hour at a speed of 1000 rpm using a magnetic stirrer (manufactured by Changzhou Union Instruments Co., Ltd., Changzhou, China) to fully disperse the powder in the anhydrous ethanol solvent. (2) The large particle dispersion of ReS₂ obtained in the previous step was placed in an ultrasonic cleaner (manufactured by Ningbo Scientz Biotechnology Co., Ltd., Ningbo, China), the ultrasonic power was set to 150 W, and the ultrasonic time was 48 h for liquid-phase exfoliation, finally obtaining an ethanol suspension of ReS₂. (3) The ethanol suspension of ReS₂ obtained in the previous step was placed in a centrifuge(manufactured by Chemat Technology Incorporation, Shanghai, China), the speed was set to 1000 rpm, and the centrifugation time was 5 min for centrifugation to remove the large-particle ReS₂ blocks in the dispersion. Subsequently, 2/3 of the supernatant in the centrifuge tube was collected with a pipette, and the supernatant was diluted with anhydrous ethanol at a ratio of 1:10 to finally obtain a nanodispersion of ReS₂.

Since it is a special two-dimensional material, it is most important to ensure the growth of high-quality grains to obtain high-quality films. Subsequently, the ReS₂/Si heterojunction film was structurally and optically characterized by various instruments listed in

Table 1. Characterization instruments for ReS₂/Si films.

Instrument	Model	Manufacturer
Raman	INVIA	Renishaw (Gloucestershire, UK)
OM	MTZ-600E	Aoka Industry Technology Co., Ltd (Suzhou, China)
AFM	FW-Nanoview 1000	FSM Instruments Co., Ltd (Suzhou, China)
SEM	SU8020	Hitachi (Tkoyo, Japan)
XRD	Empyrean	PANalytical B.V. (Amsterdam, The Netherlands)

, with the results shown in Figure 2. As shown in Figure 2a, ReS₂ nanosheets are distributed on the surface of the silicon wafer, and since ReS₂ is silvery-gray, it presents a silvery-white luster under an optical microscope. This shows that the drip irrigation method used in this experiment meets the requirements of uniform film coating and can meet the conditions for the formation of heterojunctions. As shown in Figure 2b, the roughness of the ReS₂/Si heterojunction films was examined by atomic force microscopy. Because the films are relatively brittle and easily deformed, some damage was caused to the contact surface of the probe during the AFM scanning process, so the image is blurred. From the obtained data, the roughness Ra is 53.0 nm, which proves that the surface of the films is not smooth (the average diameter of ReS₂ crystallites is 16 nm, according to the calculation using the Scherrer formula), but uniform and relatively rough as a whole, which could be improved in a future study. As shown in Figure 2c, the quality of the ReS₂ films and their layer structure were observed using scanning electron microscopy (SEM). The thickness of the ReS₂ films changes with different drop casting times. It can be seen that the surface voids of the ReS₂ films obtained by multiple drop castings are significantly reduced, and ReS₂ evenly covers the entire surface of the silicon substrate. Figure 2e shows the morphological characterization of a ReS₂/Si heterojunction. The optical characterization of the ReS₂/Si heterojunction is shown in Figure 2a, which shows that ReS₂ is indeed coated on Si. In Figure 2d, three vibration peaks corresponding to ReS₂ can be clearly seen, namely A1g at 150 cm⁻¹, A6g at 160 cm⁻¹ and A7g at 211 cm⁻¹ [36,37,38]. Comparing the ReS₂/Si heterojunction with the ReS₂ crystal, the ReS₂/Si heterojunction exhibits strong surface-enhanced Raman scattering (SERS), which is due to the charge transfer between ReS₂ and Si changing the optoelectronic properties of the heterojunction [39].

3. Results and Discussion

We learned that the ReS₂/Si heterojunction film belongs to a certain semiconductor material, and when the photon energy exceeds the semiconductor band gap, the photon energy will be absorbed to generate electron and hole carriers, thereby changing the conductivity of the ReS₂/Si heterojunction semiconductor film and then affecting the transmittance of the terahertz wave, realizing the function of pump light regulation of the terahertz wave. Modulation performance indicators are usually expressed in terms of modulation depth, modulation bandwidth and modulation speed. The number of photogenerated carriers, response time and recombination velocity are important parameters that affect the modulator. Among them, the number of carriers mainly affects the modulation depth of the material, while the response time mainly affects its band gap and modulation speed. Next, we will investigate the modulation characteristics of the ReS₂/Si heterojunction films.

At present, most terahertz time-domain spectroscopy systems use coherent measurement technology, which is a new type of terahertz spectroscopy technology that uses femtosecond lasers to pump photonic crystals or photoconductive antennas to generate terahertz waves. According to the different test optical paths, it is mainly divided into transmission TDS systems, reflective TDS systems and differential TDS systems. Among them, the integrated transmission–reflection TDS systems can not only obtain the specific data of the terahertz wave after the transmission and reflection of the test sample, but also achieve a good signal-to-noise ratio and measurement width. Therefore, this kind of terahertz time-domain spectroscopy system has become one of the main means to study terahertz characteristics.

In order to study the terahertz properties of layered ReS₂/Si heterostructure films, a transmission THz TDS system was built for this paper. The optical path diagram of the terahertz time-domain spectroscopy system is shown in Figure 3a, which is mainly composed of femtosecond lasers, beam splitters, terahertz source generators, focusing lenses, terahertz detectors and optical delay systems. The major devices for terahertz source generation are shown in

Table 2. Major devices for terahertz source generation.

Device	Model	Manufacturer
Femtosecond Laser	MaiTai	Newport (Wuxi, China)
Photoconductive Antenna	Tera-SED3	Laser Quantum (Manchester, UK)

. The experimental setup uses photoexcited active materials to regulate the terahertz characteristics, and the TDS test system must be in a dry environment during the test to eliminate the influence of environmental factors such as water vapor on the test results. Since the band gap of the ReS₂/Si sample in the experimental study described above is 1.46 eV, a CW laser with a central wavelength of 520 nm is used as the pump light. During the test, the terahertz wave is vertically irradiated on the surface of the ReS₂/Si film sample; the diameter of the terahertz spot is about 1 mm, the pump laser is obliquely irradiated on theReS₂/Si film, and the spot area is 3 mm × 1.5 mm. The pump spot must coincide with the terahertz radiation source spot and must be larger than the terahertz radiation source spot to ensure that the heterojunction film is completely pumped by the laser at the terahertz wave transmission position, as shown in Figure 3b. At this time, the ReS₂/Si heterojunction (with Si as the substrate) is irradiated by the pump laser, generating a large number of photogenerated carriers in the pump area, changing the conductivity of the sample in this area. When the terahertz wave passes through the sample, it is scattered or absorbed by the photogenerated carriers, reducing the transmission power of the terahertz wave, and ultimately achieving the effect of modulating the transmission characteristics of the terahertz wave.

Then, a laser pumping sample is introduced, the voltage of the laser is gradually increased, and the terahertz time-domain data at different laser power densities are obtained by conversion, as shown in Figure 4a. It can be seen from the figure that as the laser intensity gradually increases, the amplitude of the received terahertz wave also decreases, and the two are inversely proportional, which is also consistent with the above theory. The time-domain spectra obtained in Figure 4a are converted into the corresponding frequency-domain spectra through Fourier transform, as shown in Figure 4b. It can also be seen that as the laser intensity increases, the amplitude of the terahertz wave passing through the sample gradually weakens.

To quantify the terahertz regulation ability of the ReS₂/Si heterojunction film, the transmittance of the terahertz wave passing through the sample can be further calculated and analyzed based on the frequency-domain spectrum extracted above, and the corresponding transmittance calculation formula is shown in Equations (1) and (2) as follows.

$$T(\omega )={\displaystyle \frac{Fop\left(\omega \right)}{Fnop\left(\omega \right)}}={\displaystyle \frac{4\tilde{n}}{(\tilde{n}+1{)}^2}}·\exp (-i(\tilde{n}-1){\displaystyle \frac{\omega d}{c}})·FP(\omega )$$

(1)

$$FP(\omega )={\displaystyle \frac{1}{1-(\frac{\tilde{n}-1}{\tilde{n}+1}{)}^2·\exp (-i\tilde{n}\frac{\omega d}{c})}}$$

(2)

In Equation (1), Fop(ω) is the introduction of external excitation to obtain the terahertz frequency-domain data, and Fnop(ω) is the terahertz frequency-domain data measured without external excitation; $\tilde{n}$ is the complex refractive index of the sample; ω is the angular frequency; d is the thickness of the film; and c is the speed of light. Using the frequency-domain curves of different powers under 520 nm pump light excitation extracted in the previous section, the above formula can be used to calculate the terahertz transmission curves of samples at different laser power densities, as shown in Figure 4c. As can be seen from the figure, as the optical pump power increases, the transmission curve of the sample gradually decreases. For example, when not pumped, the transmittance of terahertz waves through the sample is 100%; when pump light is introduced and the pump power is gradually increased, the terahertz transmittance begins to drop significantly until the laser power reaches 1555 mW/cm² and the transmittance drops to 70%. The above results show that under the action of pump light, the film can effectively modulate terahertz waves. Moreover, within the frequency range of 0.3–1.5 THz, under light excitation with different pump powers, the terahertz transmission curves gradually become smaller and are stored in parallel with each other, indicating that the film can achieve broadband modulation.

To quantitatively describe the modulation effect of the film, the modulation depth Md is proposed, which is defined as the difference in transmittance before and after the pumping of the sample divided by the transmittance of the unpumped sample and is expressed in Equation (3) as follows:

$$Md={\displaystyle \frac{T_0-T_P}{T_0}}·100\%$$

(3)

T₀ is the terahertz transmittance of the unpumped sample and Tp is the terahertz transmittance of the sample when laser pumped. Figure 5a shows the amplitude modulation depth curve of the sample in the range of 0.3–1.5 THz under different laser pump power conditions. It can be seen that the amplitude modulation depth of the sample in the terahertz region continues to increase with the increase in pump laser intensity, and when the laser power density reaches 1555 mW/cm², the amplitude modulation depth of the sample in the range of 0.3–1.0 THz can be stabilized at about 30%. Figure 5b shows the effect of power density on the amplitude modulation depth at different frequencies (0.5 THz, 0.9 THz, 1.2 THz and 1.6 THz). It can be seen from Figure 5b that the amplitude modulation depth at different frequency points increases with the increase in laser power density, and the maximum modulation depth can reach 31% at the 0.5 THz frequency point, which proves that the ReS₂/Si heterojunction film can achieve efficient and broadband terahertz modulation in this experiment.

The refractive index can reflect the optical dispersion properties of the ReS₂/Si heterojunction film, and according to Equations (4) and (5), the refractive index of the material can be calculated using the transmittance of the sample obtained in the previous section.

$$T_{total}=\{{\displaystyle \frac{({\tilde{n}}_f-{\tilde{n}}_s)({\tilde{n}}_f-1)}{({\tilde{n}}_f+1)({\tilde{n}}_f+{\tilde{n}}_s)}}\cdot \exp (2i{\tilde{n}}_f{\displaystyle \frac{\omega d}{c}}){\}}^n\cdot T_{total}^1(\omega )$$

(4)

$$T_{total}^1={\displaystyle \frac{2}{({\tilde{n}}_s+1)}}{\displaystyle \frac{2{\tilde{n}}_s}{({\tilde{n}}_f+{\tilde{n}}_s)}}{\displaystyle \frac{2{\tilde{n}}_f}{({\tilde{n}}_f+1)}}\cdot \exp \{i({\tilde{n}}_s-1){\displaystyle \frac{\omega d}{c}}\}\cdot \exp \{i({\tilde{n}}_f-1){\displaystyle \frac{\omega t}{c}}\}$$

(5)

${\tilde{n}}_f$ is the refractive index of the film; ${\tilde{n}}_s$ is the refractive index of the substrate; d is the substrate thickness; c is the propagation speed of the electromagnetic wave; t is the film thickness; ω is the frequency; and T¹_total is the transmittance corresponding to the first intercepted pulse signal.

When the THz wave signal passes through the ReS₂/Si heterojunction, the Fabry–Pérot effect (FP) occurs inside, and the FP effect caused by multiple reflections is extracted by a formula, as shown in Equation (6):

$$\begin{array}{ll}FP\left(\omega \right)& ={{\displaystyle \sum _{n=0}^{\infty }}\{{\displaystyle \frac{({\tilde{n}}_s-{\tilde{n}}_f)(1-{\tilde{n}}_f)}{({\tilde{n}}_f+1)({\tilde{n}}_s+{\tilde{n}}_f)}}\cdot \exp {\displaystyle \frac{2i{\tilde{n}}_f\omega t}{c}}\}}^n\\ & =\{1-{\displaystyle \frac{({\tilde{n}}_s-{\tilde{n}}_f)(1-{\tilde{n}}_f)}{({\tilde{n}}_f+1)({\tilde{n}}_s+{\tilde{n}}_f)}}\cdot \exp {\displaystyle \frac{2i{\tilde{n}}_f\omega t}{c}}{\}}^{-1}\end{array}$$

(6)

Normalizing T_total(ω), the complex transmittance function of the film is shown in Equation (7):

$$\begin{array}{ll}{T}_{film}& =T_{total}/T_{ref}\\ & ={\displaystyle \frac{2{\tilde{n}}_f({\tilde{n}}_s+1)}{({\tilde{n}}_s+{\tilde{n}}_f)({\tilde{n}}_f+1)}}\cdot \exp \{i({\tilde{n}}_s-1){\displaystyle \frac{\omega d}{c}})\}\cdot FP(\omega )\\ & ={\displaystyle \frac{2{\tilde{n}}_f({\tilde{n}}_s+1)\exp (-i\frac{\omega d}{c})}{({\tilde{n}}_s{\tilde{n}}_f+{\tilde{n}}_f)\cos ({\tilde{n}}_f\frac{\omega d}{c})-i({{\tilde{n}}_f}^2+{\tilde{n}}_s)\sin ({\tilde{n}}_f\frac{\omega d}{c})}}\end{array}$$

(7)

Since the sample thickness in the experiment is 1.3 μm, the thickness of the silicon substrate is 1 mm, the refractive index of the substrate is 3.42 and the refractive index of ReS₂ is |${\tilde{n}}_f$| < 50, $n_f\omega t/c~0$; so, the Taylor expansion according to Equations (4) and (5) has $\exp \left(-i\omega d/c\right)~0$, $\cos \left(n_f\omega t/c\right)~1$ and $\sin \left(n_f\omega t/c\right)~n_f\omega t/c$, and the final formula is shown in Equation (8) as follows:

$$T_{film}(\omega )=\frac{{\tilde{n}}_s+1}{({\tilde{n}}_s+1)-i({{\tilde{n}}_f}^2+{\tilde{n}}_s)\frac{\omega d}{c}}$$

(8)

T_film (ω) is the transmittance obtained in the previous section. According to Equations (4)–(6), the refractive indices of ReS₂/Si-based heterojunction-based films at different powers are obtained. Figure 6 shows the refractive index curve based on the ReS₂/Si heterojunction film. It can be seen that as the laser intensity increases, the refractive index of the sample also increases. At a power density of 1555 mW/cm², the refractive index gradually decreases from 1.9 to about 0.4 within the range of 0.3–1.5 THz, showing a decreasing trend.

The degree of metallization of semiconductor materials can be observed by their complex conductivity; so, the conductivity of the sample is further extracted according to the refractive index calculated in the previous section, the conductivity formula is brought into the formula, and the complex conductivity formula is obtained in Equation (9) as follows:

$$\Delta \sigma =-{\displaystyle \frac{1+n_s}{Z_{0}t}}{\displaystyle \frac{1}{T_{film}\left(\omega \right)}}$$

(9)

where Z₀ is the free-space impedance. The conductivity curve of the extracted sample is shown in Figure 7. It can be seen that as the laser intensity increases, the conductivity of the ReS₂/Si heterojunction film gradually increases. For example, when the pump power density increases from 155 mW/cm² to 1555 W/cm², it is found that the conductivity of 0.5 Ω⁻¹m⁻¹ at 0.5 THz gradually climbs to about 3.8 Ω⁻¹m⁻¹, an increase of nearly eight times, which also echoes the terahertz modulation effect mentioned above. The reason is that the substrate and the ReS₂ film form a heterojunction, which generates photogenerated carriers under the irradiation of the laser, and as the laser intensity increases, more photogenerated carriers are generated and the conductivity increases. The higher the conductivity, the more terahertz waves the materials can absorb. Therefore, the greater the transmittance of terahertz waves, the better the modulation.

4. Conclusions

ReS₂/Si heterojunction films were prepared by an easy-to-operate, low-cost liquid-phase exfoliation and drop casting method, and terahertz modulation experiments were carried out. The modulation performance of the ReS₂/Si heterojunction films was evaluated. Conclusions could be drawn as follows:

(1)

The ReS₂/Si heterojunction film modulator can achieve stable and efficient broadband modulation. When the illumination power is 1.55 W/cm², the ReS₂/Si heterojunction film can achieve a modulation depth of 31%, and the modulation bandwidth is between 0.3 and 1.5 THz.

(2)

The modulation effect of the ReS₂/Si heterojunction film can be directly improved by increasing the laser pump intensity. When the pump power density increases from 155 mW/cm² to 1555 mW/cm², the conductivity of the ReS₂/Si heterojunction film gradually increases from 0.5 Ω⁻¹m⁻¹ to about 3.8 Ω⁻¹m⁻¹ at 0.5 THz, an increase of nearly eight times. The ReS₂/Si heterojunction film generates photogenerated carriers under the irradiation of the laser. As the laser intensity increases, the more photogenerated carriers are generated, and the greater the conductivity. The higher conductivity, the more terahertz waves can be absorbed. Therefore, the greater the transmittance of terahertz waves, the better the modulation effect.

(3)

The preparation process of ReS₂/Si heterojunction films can be further optimized by reducing the crystallize sizes and increasing the consistence for a more reliable and effective modulation.