A method for generating tunable ellipsometric terahertz waves by exciting upright graphene with a dual-beam setup

Abstract

This application relates to the field of terahertz wave generation and polarization control technology, specifically providing a method for generating tunable ellipsometric terahertz waves by exciting upright graphene with a dual-beam setup. The method includes steps 1, generating a first laser beam and a second laser beam, where the wavelength of the first laser beam is twice the wavelength of the second laser beam; steps 2, irradiating the surface of the upright graphene with the first and second laser beams; and steps 3, generating terahertz waves on the back side of the upright graphene. The ellipticity of the terahertz wave is controlled by changing the relative polarization angle between the first and second laser beams. The dual-color light field of this application generates a quantum interference effect, forming a strong coherent injection current. The abundant edge states of the upright graphene further increase the coherent injection current, improving the stability of the terahertz signal. By changing the relative polarization angle between the two laser beams, the proportion of the coherent injection current components in different directions can be adjusted, achieving continuous control of the ellipticity of the terahertz wave.

Description

Technical Field

[0001] This application relates to the field of terahertz wave generation and polarization control technology, and more specifically, to a method for generating tunable ellipsometric terahertz waves by exciting upright graphene with a dual-beam excitation. Background Technology

[0002] Terahertz waves typically refer to electromagnetic waves with a frequency range of 0.1-10 THz, corresponding to a wavelength range of 30-3000 μm. They lie between microwaves and the infrared, possessing characteristics such as a broadband spectrum, non-ionization, and sensitivity to various molecular vibrational and rotational modes. Besides common terahertz imaging and spectroscopy applications, elliptically polarized terahertz waves have unique value in fields requiring the utilization of electromagnetic wave polarization properties. For example, in chiral molecule recognition, terahertz waves with different rotational directions and ellipsoids can produce differential absorption with chiral molecules, thus distinguishing enantiomers; in magnetic materials and spintronics research, elliptically polarized terahertz waves can selectively excite magnons or spin dynamics processes; in anisotropic material characterization, by changing the terahertz polarization and measuring polarization rotation or ellipsoidal changes, the anisotropic dielectric parameters or optical axis orientation of the material can be obtained. Therefore, the generation of stable, tunable elliptically polarized terahertz waves is of great significance for polarization-sensitive applications such as chiral molecule recognition, magnetic material dynamics research, and anisotropic material characterization.

[0003] In existing technologies, terahertz radiation with elliptic polarization characteristics can be generated by irradiating tungsten disulfide and indium arsenide materials with a circularly polarized monochromatic light field under the action of mechanisms such as optical rectification or surface photocurrent. The polarization state of terahertz waves is usually determined by factors such as the crystal structure and anisotropy of the material, as well as the fixed polarization state of the incident light, and the ellipticity and polarization principal axis direction are difficult to continuously control. Previous research has proposed using two laser beams of different frequencies to generate plasma in air to produce tunable ellipticized terahertz waves (Polarization control of terahertz radiation from two-color femtosecond gas breakdown plasma, Optics Letters, 43, Issue 1). When a strong femtosecond laser is focused in air, air molecules are ionized to produce free electrons. These free electrons are accelerated in an asymmetric light field, forming a directional transient current. By adjusting the relative phase of the two laser beams, the direction and intensity of electron motion are changed, thereby achieving control of the polarization state of the terahertz waves. However, plasma generation is closely related to laser energy density, air density, and environmental conditions. Even slight changes in laser intensity or environmental conditions alter the electron density and spatial distribution of the plasma, leading to fluctuations in the intensity and polarization state of the generated terahertz signal, resulting in a low signal-to-noise ratio and poor stability. In practice, increasing the pump laser power can enhance the terahertz signal intensity, but this is insufficient to fundamentally improve stability and may also introduce limitations related to laser safety and system load.

[0004] In summary, existing methods cannot generate stable, tunable ellipsoidal terahertz waves, severely limiting the development of terahertz polarization-sensitive fields such as chiral molecule recognition, magnetic material dynamics research, and anisotropic material characterization. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of the prior art by providing a method for generating tunable ellipsoidal terahertz waves by exciting upright graphene with a dual-beam excitation, thereby solving the problem that existing methods cannot generate stable and tunable ellipsoidal terahertz waves.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This application provides a method for generating tunable ellipsometric terahertz waves by exciting upright graphene with a dual-beam excitation system, comprising the following steps: Step 1: Generate a first laser beam and a second laser beam, wherein the wavelength of the first laser beam is twice the wavelength of the second laser beam; Step 2: The first laser beam and the second laser beam irradiate the surface of the upright graphene. Step 3: Generating terahertz waves on the back of the upright graphene; The ellipticity of the terahertz wave is controlled by changing the relative polarization angle between the first and second laser beams.

[0007] In this application, the wavelengths of the first and second laser beams satisfy a two-fold relationship. Therefore, when irradiating upright graphene, electrons can reach the same final energy state through two different transition paths: one is the single-photon absorption path corresponding to the second laser beam (corresponding to the 400 nm laser beam in the embodiment), and the other is the two-photon absorption path corresponding to the first laser beam (corresponding to the 800 nm laser beam in the embodiment). Quantum interference occurs between the first and second laser beams, forming an asymmetric carrier distribution in momentum space. This transforms the originally canceling carrier motion into a coherent injection current with a macroscopic direction. The change of the coherent injection current over time radiates terahertz waves. This application directly controls the direction and components of the coherent injection current through a two-color light field. Therefore, the polarization state of the terahertz wave is no longer fixed by the intrinsic crystal symmetry of the material and can be actively controlled by external light field parameters, providing a basis for the generation of tunable ellipsometric terahertz waves.

[0008] In this application, upright graphene not only serves as the stimulated material but also plays a crucial role in enhancing the intensity and stability of terahertz radiation. Specifically, the upright graphene is composed of numerous graphene nanosheets grown perpendicular to the substrate, possessing abundant edge structures and a high edge state density. These edge states enhance the interaction between light and the material, improving the generation efficiency of photogenerated carriers. Simultaneously, the edges of the upright graphene nanosheets readily generate localized electric field enhancement effects, amplifying the incident two-color light field in local regions. This further enhances single-photon and two-photon absorption processes, increasing the intensity of quantum interference effects and generating a coherent injection current with a larger amplitude, thereby boosting the intensity of the terahertz wave. With the increased terahertz signal intensity, the signal-to-noise ratio during detection improves, and the influence of external noise, environmental disturbances, and background fluctuations of the detection system on the polarization characterization results is relatively weakened, leading to enhanced stability.

[0009] This application modulates the ellipticity of terahertz waves by changing the relative polarization angle between the first and second laser beams. After the relative polarization angle of the two laser beams changes, the electric field component distribution of the two-color light field in the upright graphene also changes, thereby altering the component ratios of the coherent injection current in different orthogonal directions. Since the terahertz radiation electric field is directly related to the transient current change, when the amplitude ratio of the coherent injection current in the two orthogonal directions changes, the electric field component ratios of the generated terahertz wave in the two orthogonal directions also change, thus altering the polarization ellipse of the terahertz wave, i.e., changing its ellipticity.

[0010] Furthermore, the first and second laser beams pass through a half-wave plate, and their relative polarization angle is changed by rotating the optical axis of the half-wave plate. Both the first and second laser beams pass through the half-wave plate, and rotating the half-wave plate changes the direction of their optical axes, thus altering the relative polarization angle. The half-wave plate acts as a half-wave plate for the first laser beam and a full-wave plate for the second laser beam. Therefore, the polarization direction of the second laser beam remains essentially unchanged, while the polarization direction of the first laser beam changes with the rotation of the half-wave plate, with the rotation angle being twice the rotation angle of the half-wave plate. This change in polarization angle alters the proportion of the coherent injection current induced by the dual-color light field in the upright graphene in different directions, thereby changing the amplitude of the electric field components of the radiated terahertz wave in the two orthogonal directions, thus achieving the control of the ellipticity of the generated terahertz wave.

[0011] Furthermore, the first and second laser beams are simultaneously and perpendicularly incident on the same region of the upright graphene. This simultaneous and perpendicular illumination of the same region ensures that the two laser beams are fully overlapped in space and time, guaranteeing the stable generation of quantum interference effects within the material by the two-color light field, thus forming a stable coherent injection current. Perpendicular incidence reduces the effects of reflection, refraction, and beam shift caused by changes in the incident angle, making the overlap region of the two laser beams more stable. This facilitates the stable control of the terahertz wave ellipticity by altering the relative polarization angle between the two laser beams.

[0012] Furthermore, both the first and second laser beams are linearly polarized light. The fact that both beams are linearly polarized facilitates precise control of their relative polarization. When the polarization directions of the two linearly polarized beams are determined, the polarization angle between them can be changed by rotating the half-wave plate, thereby adjusting the distribution of the electric field components generated by the two-color light field in the upright graphene. This causes the proportion of the coherent injection current components in different directions to change, thus altering the ellipticity of the terahertz wave.

[0013] Furthermore, the second laser beam is obtained by passing the first laser beam through a frequency-doubling crystal. The frequency-doubling crystal utilizes the second-order nonlinear effect of the material. When the fundamental frequency light enters the crystal, under phase-matching conditions, the two fundamental frequency photons can interact within the crystal and be converted into a single photon with twice the frequency, thus generating second-harmonic light. In this application, the polarization directions of the fundamental and frequency-doubled beams are perpendicular to each other, which facilitates a clear determination of the relative polarization relationship between the two beams. This makes the change in the polarization angle between the two beams more stable and controllable by rotating the half-wave plate, and makes it easier to achieve stable control of the direction of the induced current and the ellipticity of the terahertz wave in the two-color light field. Simultaneously, orthogonal polarization can reduce nonlinear coupling and interference noise. When the two beams are polarized identically, it is easier to generate additional interference effects in optical elements or air, introducing additional intensity fluctuations. Orthogonal polarization can reduce the aforementioned coupling, making the structure of the two-color light field more stable and generating stable terahertz radiation.

[0014] Furthermore, the intensity of the first laser beam is greater than that of the second laser beam. The two-photon absorption process corresponding to the first laser beam is more sensitive to the light field intensity. Increasing the intensity of the first laser beam can effectively enhance the two-photon transition probability, making the interference effect between the two transition paths more pronounced, thereby generating stronger transient current changes and radiating stronger terahertz waves. At the same time, a stronger first laser beam helps to improve the signal-to-noise ratio of the terahertz signal, thus improving the stability of the output terahertz wave.

[0015] Furthermore, upright graphene is disposed on a substrate, the plane of which is perpendicular to the propagation direction of the first and second laser beams.

[0016] Furthermore, the thickness of upright graphene is 500-1000 nm.

[0017] Furthermore, upright graphene was obtained via microwave plasma-enhanced chemical vapor deposition.

[0018] Furthermore, the pulse width of the first laser beam source is 10-200 fs, the center wavelength is 400-1550 nm, and the repetition frequency is 1 kHz-82 MHz.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: In this application, a first laser beam and a second laser beam, satisfying a 2:1 ratio, co-excite vertical graphene. The first laser beam absorbs two photon transitions, while the second laser beam absorbs one photon transition, resulting in quantum interference. This creates an asymmetric carrier distribution in momentum space and generates a coherent injection current. Simultaneously, the vertical graphene, composed of numerous vertically grown graphene nanosheets, possesses abundant edge structures and a high electronic state density, which enhances the probability of single-photon and two-photon transitions. This strengthens the quantum interference process induced by the two-color light field, increases the amplitude and rate of change of the coherent injection current, increases the terahertz radiation intensity, improves the signal-to-noise ratio, and ultimately enhances the stability of the output terahertz wave. This application does not amplify the signal by increasing the pump light power, but rather improves the stability of terahertz radiation through the coherent photocurrent mechanism in the solid material and the synergistic effect of the unique micro / nano structure of vertical graphene, thus addressing the stability issues in existing technologies.

[0020] By changing the relative polarization angle between the first and second laser beams, the distribution of the electric field components generated by the two-color light field in upright graphene can be altered, causing a change in the component ratio of the coherent injected current in the two orthogonal directions. Since the terahertz radiation electric field originates from the change in transient current, when the current component ratio in the two directions changes, the amplitude ratio of the corresponding terahertz electric field in the two orthogonal directions also changes, thus achieving adjustment of the terahertz wave ellipsoidity. The method of this application can generate stable and tunable ellipsoidized terahertz waves, laying the foundation for the development of fields such as chiral molecule recognition, magnetic material dynamics research, and anisotropic material characterization. Attached Figure Description

[0021] Figure 1 SEM image of the upright graphene of this application; Figure 2 Comparison of terahertz wave time-domain signals generated by upright graphene excited by the first laser beam, the second laser beam, and the first laser beam + the second laser beam in the direction of normal incidence; Figure 3 The dependence of the electric field of the terahertz wave generated by the method of this application on the pump light power; Figure 4 This is a schematic diagram of the terahertz wave generation and control system of the present invention; Figure 5 The dependence of the two orthogonal components of the terahertz wave generated by the method of this application on the relative polarization angle; Figure 6 The time-domain waveforms of the two orthogonal components when the relative polarization angle is fixed at 230°; Figure 7 The time-domain waveforms of the two orthogonal components when the relative polarization angle is fixed at 40°; Figure 8 The polarization ellipse of the terahertz wave generated when the relative polarization angle is fixed at 0°, 40°, and 230°; Figure 9 This represents the dependence of the ellipticity of terahertz wave polarization on the relative polarization angle. Detailed Implementation

[0022] To make the implementation process of this invention clearer, a detailed description will be provided below in conjunction with the accompanying drawings.

[0023] This application provides a method for generating tunable ellipsometric terahertz waves by exciting upright graphene with a dual-beam excitation system, comprising the following steps: Step 1: Generate a first laser beam and a second laser beam, wherein the wavelength of the first laser beam is twice the wavelength of the second laser beam.

[0024] The first laser beam is generated by a Ti:sapphire femtosecond laser with a pulse width of 10-200 fs, a center wavelength of 400-1550 nm, and a repetition rate of 1 kHz-82 MHz. Femtosecond pulses are characterized by high peak power and high time resolution, effectively exciting ultrafast carrier dynamics processes in materials, thus facilitating the generation of strong transient currents and the radiation of terahertz waves. The higher repetition rate helps improve the average signal strength and signal-to-noise ratio, thereby enhancing the stability of the terahertz signal. In this embodiment, the center wavelength of the emitted laser is 800 nm. On the one hand, 800 nm is a typical output wavelength of Ti:sapphire lasers, ensuring stable operation, high beam quality, and mature related optical components. On the other hand, the 800 nm laser can generate 400 nm second-harmonic light through a frequency-doubling crystal, forming a bicolor light field with a frequency ratio of 1:2, which is beneficial for generating quantum interference effects and coherent injection currents in materials.

[0025] The second laser beam is generated by passing the first laser beam through a frequency doubling crystal. In this embodiment, the frequency doubling crystal is... -BBO crystal. -BBO crystals possess large second-order nonlinear coefficients and a wide phase-matching range, making them suitable for frequency doubling processes in femtosecond lasers. An 800 nm fundamental frequency laser enters... After the -BBO crystal is installed, under the condition of Type-I phase matching, two fundamental frequency photons interact within the crystal and are converted into a single photon with twice the frequency, thus generating frequency-doubled light with a wavelength of 400 nm. The fundamental frequency light is o-polarized, while the frequency-doubled light is e-polarized. The polarization directions of the emitted 400 nm frequency-doubled light and the 800 nm fundamental frequency light are usually perpendicular to each other, meaning that the first and second laser beams are linearly polarized light with mutually orthogonal polarization directions.

[0026] Because the frequency doubling process has a certain conversion efficiency, from The laser emitted from the BBO crystal simultaneously contains the remaining 800 nm fundamental frequency light and the newly generated 400 nm frequency-doubled light. The 800 nm light serves as the first laser beam, and the 400 nm light as the second laser beam. The frequency doubling efficiency is related to factors such as crystal thickness, phase matching angle, and incident light intensity; in this embodiment, the efficiency is 10%. The intensity of the first laser beam is typically greater than that of the second laser beam; the intensity of the first laser beam is 15 mW, and the intensity of the second laser beam is 5 mW, with an intensity ratio of 3:1. This intensity ratio is beneficial for simultaneously ensuring high transition probabilities in both the two-photon absorption path and the single-photon absorption path within the material, making the quantum interference effect of the two transition paths more pronounced, enhancing the coherent injection current, and increasing the terahertz radiation intensity. The higher intensity of the first laser beam compared to the second laser beam also helps improve the signal-to-noise ratio of the terahertz signal, making the output terahertz wave more stable.

[0027] Step 2: The first laser beam and the second laser beam irradiate the surface of the upright graphene.

[0028] To enhance the spatial overlap of the two-color light field at the sample and improve the stability of the coherent injection current, a first laser beam and a second laser beam are simultaneously and perpendicularly irradiated onto the same region of the upright graphene. The two laser beams achieve spatial and temporal overlap at the sample, ensuring that the two-color light field can stably generate quantum interference effects and form a stable coherent injection current. The spot diameter of the first laser beam is 2.8-3.2 mm, and the spot diameter of the second laser beam is 2.3-2.8 mm. Preferably, the spot diameters of the first and second laser beams are 3 mm and 2.5 mm, respectively. The spot size of the 400 nm beam on the upright graphene is slightly smaller than that of the 800 nm beam. This ensures that the 400 nm laser is completely within the 800 nm laser spot, making the effective area of ​​the two-color light field more concentrated. This increases the effective overlap area of ​​the two beams at the sample, enhances the quantum interference effect, and improves the intensity and stability of terahertz radiation.

[0029] Vertical graphene is disposed on a sapphire substrate, with the plane of the substrate perpendicular to the propagation directions of the first and second laser beams. The sapphire substrate is positioned on the side furthest from the incident light. Sapphire possesses good mechanical stability and high optical transmittance, exhibiting low absorption loss in both the visible and terahertz wavelengths, which is beneficial for terahertz wave transmission. Simultaneously, sapphire has high thermal conductivity and chemical stability, making it a stable substrate for the growth of vertical graphene. Positioning the sapphire substrate on the terahertz emission side allows the generated terahertz waves to directly pass through the substrate, thereby reducing the influence of interface scattering on the incident side on the terahertz signal and improving the stability of the terahertz waves. In this embodiment, the sapphire substrate has a thickness of 0.5-1 mm and an area of ​​1.2 × 1.2 cm². 2 The area of ​​upright graphene is 1×1 cm. 2 The thickness of upright graphene is 500-1000 nm. If the thickness is too thin, the number of graphene nanosheets is small and the edge state density is insufficient, which is not conducive to enhancing the interaction between light and matter. If the thickness is too thick, it will increase carrier scattering and light absorption loss, which is not conducive to the formation of transient current and the radiation of terahertz waves.

[0030] Vertical graphene is randomly grown on a sapphire substrate, exhibiting a macroscopically centrosymmetric structure. Generally, centrosymmetric materials do not possess a second-order nonlinear optical response under the electric dipole approximation, making it difficult to generate terahertz radiation through conventional optical rectification mechanisms. However, in this application, a quantum interference effect is introduced through a two-color laser field, causing electrons to form an asymmetric distribution in momentum space, thereby generating a coherent injection current and radiating terahertz waves. When the first and second laser beams jointly irradiate the surface of the vertical graphene, the two-color light field induces a quantum interference effect in the material. Electrons can transition by absorbing a single 400 nm photon or by absorbing two consecutive 800 nm photons; the two transition paths satisfy an energy matching relationship, thus forming an asymmetric carrier distribution in momentum space and generating a coherent injection current. By changing the relative polarization angle between the two laser beams, the component ratio of the coherent injection current in the two orthogonal directions can be altered, thereby changing the electric field components of the generated terahertz wave in the two orthogonal directions, forming an elliptically polarized terahertz wave. The generated ellipsoidal terahertz waves then propagate outward through the sapphire substrate, achieving stable and tunable ellipsoidal terahertz radiation output.

[0031] Vertical graphene was prepared via microwave-enhanced plasma chemical vapor deposition (MECVD). Specifically, a cleaned sapphire substrate was first placed into a cavity, and the pressure inside the cavity was reduced to 0.1 Pa. A mixture of methane and hydrogen in a 1.5:1 ratio was introduced into the cavity at a flow rate of 125 sccm. The cavity pressure was then maintained at 600 Pa for 3 minutes to obtain a stable growth environment. The microwave plasma source was then turned on, and the growth time was 2 minutes. After growth, the plasma source and carrier gas were immediately turned off. Under the low-pressure hydrogen and methane mixed atmosphere and microwave plasma conditions, the high-energy active particles in the plasma continuously etched the horizontally growing portions of the graphene sheets, while simultaneously promoting carbon source deposition at the edges. This resulted in preferential growth of graphene along a direction perpendicular to the substrate, ultimately forming a vertical structure composed of numerous graphene nanosheets. SEM images of the prepared vertical graphene are shown below. Figure 1 As shown, the scanning electron microscope used was a FEI Helios 600i. The graphene exhibits an upright, high-density, flower-like morphology with uniform growth size, forming a three-dimensional structure composed of numerous upright-grown graphene nanosheets stacked on a substrate and randomly growing along the substrate surface. The prepared upright graphene possesses a large number of vertically oriented graphene nanosheets, forming abundant edge structures and a high local electronic state density. The nanosheet ends can generate a local electric field enhancement effect, thereby enhancing the interaction between light and material and improving the generation efficiency of photogenerated carriers.

[0032] Furthermore, by changing the plasma flux and using a shield or tilted substrate, it is possible to prepare vertical graphene with a thickness gradient, gradually increasing from 500 nm to 1000 nm from one side to the other. In use, the direction of the vertical graphene thickness gradient is parallel to the polarization direction of the 800 nm incident light. The continuous variation of the vertical graphene thickness gradient direction results in a spatially gradual distribution of the two-photon absorption efficiency, local optical field intensity, and transport conditions of photogenerated carriers excited at 800 nm. Since the amplitude of the coherent injection current generated by the two-color light field is more closely related to the change in optical field intensity by the two-photon channel, when the polarization angle changes, the projected electric field of the 800 nm light in different gradient regions changes, causing varying degrees of enhancement in the two-photon absorption probability in each region. This leads to a redistribution and superposition of the transient current components generated in each region along the dominant direction, making the component of the overall coherent injection current in this direction more sensitive to changes in polarization angle. This amplifies the variation in the ratio of current components in the two orthogonal directions, ultimately expanding the terahertz wave E... x and E y The adjustable range of components allows for a wider range of ellipticity control. Furthermore, parallel microgrooves are formed on the surface of a sapphire substrate, upon which upright graphene is grown. The groove direction is parallel to the thickness gradient direction. The groove width is 10-50 μm, the depth is 2-10 μm, and the spacing is 50-200 μm, effectively inducing in-plane orientation consistency in the graphene nanosheets without disrupting continuous growth. The grooves act as growth guiding structures, ensuring a more consistent orientation of the upright graphene nanosheets on the groove sidewalls and in the groove region, thus forming a more continuous and stable conductive channel along the gradient direction. When the 800 nm polarization direction is along this direction, the two-photon absorption channel, the local electric field enhancement direction, and the carrier transport direction tend to align, making the coherent injection current more stable in the principal direction, reducing lateral scattering and local backflow, thereby stabilizing the principal axis of the generated terahertz electric field and improving the repeatability and output stability of the terahertz wave polarization ellipse.

[0033] During excitation, preferably, the 800 nm light spot is modulated from a circle to an ellipse, with its major axis parallel to the thickness gradient direction of the upright graphene. The 400 nm light spot remains a small, nearly circular shape and is located within the 800 nm spot, thus spatially expanding the coverage area of ​​the 800 nm beam along the gradient direction. Since the height of the nanosheets, edge state density, and the degree of local electric field enhancement in the upright graphene exhibit continuous changes along the thickness gradient direction, the elliptical major axis along this direction allows the two-photon absorption process excited by 800 nm to occur simultaneously within a longer gradient region. This enhances the modulation effect of structural anisotropy on the coherent injection current, making the current component change more pronounced in the main direction. Simultaneously, maintaining the 400 nm light spot as a slightly smaller circle ensures that the single-photon absorption channel is mainly concentrated in a structurally uniform region, guaranteeing single-photon path stability. By setting an elliptical aperture in the optical path, the circular light spot can be transformed into an elliptical spot, while the 400 nm beam, due to its smaller size and coaxial propagation with the 800 nm beam, can essentially maintain its original nearly circular distribution. This configuration is beneficial for enhancing the coherent injection current induced by the two-color light field and expanding the adjustment range of the terahertz wave ellipticity.

[0034] Step 3: Terahertz waves are generated on the back side of the upright graphene.

[0035] Figure 2 The study investigated the generation of terahertz wave time-domain signals from upright graphene excited by a first laser beam, a second laser beam, and a combination of the first and second laser beams at normal incidence. Notably, no terahertz wave signal was generated under monochromatic light field excitation, but it was generated only under bicolor light field excitation. This indicates that under normal incidence excitation, the terahertz radiation induced by the photo-traction effect is undetectable. The primary physical mechanism is that under normal incidence, only the wave vector perpendicular to the plane contributes to the photocurrent induced by the out-of-plane photo-traction effect. According to dipole radiation theory, the oscillating photocurrent generates terahertz wave radiation perpendicular to the oscillation direction. Under normal incidence conditions, the terahertz waves from the out-of-plane photocurrent cannot be detected. Therefore, the terahertz radiation generated under excitation originates from the contribution of the bicolor light field.

[0036] Figure 3 The electric field of the terahertz wave generated under two-color light field excitation depends on the pump light power. Under this superlinear dependence, the THz radiation under two-color light field excitation can be described as follows: , where η (3) It is the imaginary part of the third-order nonlinear conductivity; E(ω) and E(2ω) represent the electric fields of 800 nm and 400 nm light, respectively; The relative phase is given. In a two-beam excitation system, the relationship between pump light intensity and pump photoelectric field is: and Substituting this relationship into the above equation, we can obtain the relationship between the terahertz wave electric field generated by the quantum interference effect and the pump light intensity as follows: The power measured in the experiment is the power before the laser incident on the frequency doubling crystal. Due to the conversion coefficient (m), the pump light loses some power after passing through the frequency doubling crystal. Therefore, the power excited at 400 nm can be expressed as mI, and the power ratio excited at 800 nm can be expressed as I-mI, where I is the total power of the pump light. Therefore, the expressions for the electric field and power of the terahertz wave generated by normal incident excitation are as follows: This formula indicates that the terahertz radiation generated by upright graphene under two-color light field excitation is mainly dominated by a third-order nonlinear optical process. The detected data points can be fitted by the above formula, that is, the data points and the fitted line coincide well, indicating that the mechanism of this application satisfies the quantum interference effect model.

[0037] The terahertz wave radiation mechanism generated by this invention is the quantum interference effect. Specifically, under two-color light field excitation, electrons are excited from positions below the Fermi level to positions above the Fermi level via two-photon absorption (800 nm) and single-photon absorption (400 nm) through two different transition paths. The quantum interference between these two transition paths leads to an asymmetric distribution of charge carriers in momentum space, thereby generating a net coherent injection photocurrent, which radiates terahertz waves. Since the asymmetric distribution of charge carriers in momentum space is caused by quantum interference, it is not limited by the symmetry of the material. Therefore, the dual-beam excitation method of this application can be extended to other centrosymmetric materials.

[0038] In this application, the ellipticity of the terahertz wave is controlled by changing the relative polarization angle between the first and second laser beams. Specifically, the first and second laser beams pass through a half-wave plate, and the relative polarization angle is changed by rotating the optical axis of the half-wave plate. Figure 4 As shown. Figure 4 This is a dual-beam terahertz emission spectroscopy system based on femtosecond lasers, consisting of a frequency-doubling crystal, a dual-wavelength waveplate, and upright graphene material arranged sequentially. An 800 nm pump light is irradiated onto the frequency-doubling crystal to generate a 400 nm light. Both are then passed through a half-wavelength waveplate polarization adjustment device. Rotating the half-wavelength waveplate continuously changes the relative polarization angle between the 800 nm and 400 nm beams. The half-wavelength waveplate acts as a half-waveplate for the 800 nm light and a full-waveplate for the 400 nm light; therefore, adjusting the half-wavelength waveplate only changes the polarization state of the 800 nm beam, thus adjusting the relative polarization state of the two beams. By changing the optical axis direction of the half-waveplate, the relative polarization angle of the two beams is altered, thereby changing the polarization state of the terahertz wave. In other words, the pump source's polarization state is continuously adjusted through the dual-wavelength waveplate device. Under different polarization states of excitation, the upright graphene material can generate different elliptically polarized terahertz radiation.

[0039] Figure 5 This represents the dependence of the two orthogonal components of a terahertz wave generated under two-color light field excitation on the relative polarization angle, where the relative polarization angle is zero, corresponding to a fast axis of 45 degrees on the half-wave plate. The two components exhibit different dependence on the relative polarization angle, both displaying double rotational symmetry and waveform function dependence, but their dependence characteristics differ significantly. x The peak and valley amplitudes oscillate between positive and negative values ​​as the relative polarization angle changes, while E y The amplitude is always negative. This difference in behavior indicates that the two components have different dependence on the polarization angle, which stems from the anisotropic coherent injection current caused by quantum interference effects.

[0040] A terahertz polarization analysis element is placed after an upright graphene sample to obtain the electric field components of the terahertz wave in two orthogonal directions. This polarization analysis system includes two wire-grid polarizers: one vertically aligned to obtain the horizontal component of the terahertz wave, and a second 45° aligned wire-grid polarizer to obtain the vertical component. The wire-grid polarizers are composed of a periodic array of metal wires. When a terahertz wave is incident on the wire-grid polarizer, the component of the electric field parallel to the direction of the metal grid lines is reflected or absorbed by the excited current in the metal wires, while the component perpendicular to the direction of the grid lines can propagate through the polarizer, thus achieving selective transmission of the polarization components of the terahertz electric field.

[0041] Figure 6 When the relative polarization angle is fixed at 230°, the experimentally measured E x With E y Fourier transform is performed on the time-domain signals of the components to obtain their spectral information. Narrowband filtering is then applied at the center frequency of 0.75 THz to focus on the polarization characteristics at this characteristic frequency. The resulting time-domain waveforms of the two orthogonal components show a phase difference of -0.69 π. Similarly, Figure 7 When the relative polarization angle is fixed at 40°, the time-domain waveforms of the two orthogonal components obtained by the above method show that they differ in phase by 0.12 π; E x and E y The different phase differences between the components indicate that the shape of the electric field vector's trajectory in space changes with time, thus forming polarization ellipses of different shapes and directions. Different phase differences correspond to different terahertz elliptic polarization states. The polarization state of a terahertz wave can be preliminarily determined by the phase difference. A phase difference of (2m + 1 / 2)π indicates left-handed circularly polarized light, and a phase difference of φ = (2m - 1 / 2)π indicates right-handed circularly polarized light. A phase difference of 0 indicates linearly polarized light.

[0042] Figure 8E represents the polarization ellipse of the terahertz wave generated with relative polarization angles fixed at 0°, 40°, and 230°. x and E y Let E be the horizontal and vertical axes, respectively. When two orthogonal electric field components change with time, the trajectories of their endpoints form a polarization ellipse. When the relative polarization angle is 0°, E... x and E y The phase difference between them is close to 0 or π, the two components change approximately in phase or out of phase, and their trajectories are close to a straight line, corresponding to linearly polarized terahertz waves; when the relative polarization angle is 40°, E x and E y They exist simultaneously and have a certain phase difference, thus forming an elliptical trajectory with a significant major axis and a smaller minor axis, corresponding to an elliptically polarized terahertz wave; when the relative polarization angle is 230°, E x and E y The amplitude and phase relationship further change, forming an elliptic trajectory with a different principal axis than the previous ellipse, indicating that both the terahertz polarization direction and ellipticity have changed. The arrows on the ellipse indicate the rotation direction of the electric field vector over time, corresponding to the rotation direction of the polarization ellipse. The polarization trajectory in the figure presents a smooth and continuous closed curve. The ellipse shape obtained under different measurement conditions is stable, with no obvious discrete or scattering points, indicating that E x and E y The amplitude and phase relationships between the components remained stable during the experiment. The polarization ellipse maintained a consistent shape and orientation across multiple scans, indicating that adjusting the angle of the double-waveplate provides good stability and repeatability for controlling the terahertz polarization state.

[0043] Figure 9 The ellipticity of terahertz wave polarization is shown as a dependence of the relative polarization angle. The ellipticity of the corresponding terahertz wave elliptic at each relative polarization angle indicates that stable elliptic polarization generation of terahertz waves can be achieved. The method described in this application can achieve a terahertz wave polarization ellipticity control range of 0-0.5, which is superior to other commonly used methods. For example, the ellipticity control range of an applied magnetic field is generally 0-0.09, far lower than the control range of the method described in this application.

[0044] The large adjustment range of the ellipsoidal terahertz wave in this scheme is mainly due to the combined effect of the coherent control mechanism of the two-color light field and the structural characteristics of upright graphene. By changing the relative polarization angle between the two laser beams, the distribution of the electric field components generated by the two-color light field in the upright graphene can be continuously adjusted, thereby changing the component ratio of the coherent injection current in the two orthogonal directions, and thus making the ET of the terahertz electric field... x and E yThe amplitude relationship of the components varies over a wide range, achieving continuous adjustment from near-linear polarization to high ellipticity and even near-circular polarization. Compared to traditional planar graphene structures, upright graphene possesses numerous edge states and nanosheet structures, enhancing light-matter interaction and local electric fields. This results in a stronger coherent injection current induced by the two-color light field, thereby increasing the adjustment amplitude of the two orthogonal electric field components and expanding the adjustable range of ellipticity. Furthermore, compared to excitation with circularly polarized monochromatic light, this scheme directly controls the direction of the coherent injection current through the two-color light field, independent of the intrinsic anisotropy of the material. Therefore, it not only has a wider adjustment range but also generates terahertz waves with higher intensity and better stability.

[0045] Furthermore, an external magnetic field is applied to the upright graphene sample along a direction perpendicular to the substrate plane, i.e., along the normal direction of the upright graphene. Specifically, permanent magnets or electromagnetic coils can be placed on the upper and lower sides of the sample, so that the magnetic field acts on the sample region along the thickness direction of the upright graphene. Since the dual-color light field generates a coherent injection current in the upright graphene, the photogenerated charge carriers mainly move in the in-plane direction. When the external magnetic field is present, the moving charge carriers are subjected to the Lorentz force under the action of the magnetic field, which will generate a transverse velocity component on the basis of the original current direction, causing the current distribution to be redistributed in two orthogonal directions. The transverse deflection causes the transient current, which is mainly along a single direction, to form a two-dimensional component in the in-plane, enhancing the coupling of the components of the terahertz electric field in the two orthogonal directions, making E x and E y The amplitude ratio and phase relationship are easier to control. This further expands the adjustment range of terahertz wave ellipsoidity and improves the stability and repeatability of polarization control.

[0046] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for generating tunable ellipsometric terahertz waves by dual-beam excitation of upright graphene, characterized in that, The method includes the following steps: Step 1: Generate a first laser beam and a second laser beam, wherein the wavelength of the first laser beam is twice the wavelength of the second laser beam; Step 2: The first laser beam and the second laser beam irradiate the surface of the upright graphene. Step 3: Generating terahertz waves on the back side of the upright graphene; Specifically, the ellipticity of the terahertz wave is controlled by changing the relative polarization angle between the first laser beam and the second laser beam.

2. The method for generating tunable ellipsometric terahertz waves by dual-beam excitation of upright graphene according to claim 1, characterized in that, The first laser beam and the second laser beam pass through a half-wave plate. By rotating the half-wave plate, the direction of the optical axis is changed, thereby changing the relative polarization angle.

3. The method for generating tunable ellipsometric terahertz waves by dual-beam excitation of upright graphene according to claim 2, characterized in that, The first laser beam and the second laser beam simultaneously and perpendicularly irradiate the same region of the upright graphene.

4. The method for generating tunable ellipsometric terahertz waves by dual-beam excitation of upright graphene according to claim 3, characterized in that, Both the first laser beam and the second laser beam are linearly polarized light.

5. The method for generating tunable ellipsometric terahertz waves by dual-beam excitation of upright graphene according to claim 4, characterized in that, The second laser beam is obtained by passing the first laser beam through a frequency doubling crystal.

6. The method for generating tunable ellipsometric terahertz waves by dual-beam excitation of upright graphene according to claim 5, characterized in that, The intensity of the first laser beam is greater than the intensity of the second laser beam.

7. The method for generating tunable ellipsometric terahertz waves by dual-beam excitation of upright graphene according to claim 6, characterized in that, The upright graphene is disposed on a substrate, and the plane of the substrate is perpendicular to the propagation direction of the first laser beam and the second laser beam.

8. The method for generating tunable ellipsometric terahertz waves by dual-beam excitation of upright graphene according to claim 1, characterized in that, The thickness of the upright graphene film is 500-1000 nm.

9. The method for generating tunable ellipsometric terahertz waves by dual-beam excitation of upright graphene according to claim 1, characterized in that, The upright graphene was obtained by microwave plasma-enhanced chemical vapor deposition.

10. The method for generating tunable ellipsometric terahertz waves by dual-beam excitation of upright graphene according to claim 1, characterized in that, The pulse width of the first laser beam source is 10-200 fs, the center wavelength is 400-1550 nm, and the repetition frequency is 1 kHz-82 MHz.

Images