Elliptical polarization terahertz generating device

Abstract

The invention discloses an elliptical polarization terahertz generation device, which relates to the technical field of optics and comprises a laser source used for generating a laser beam which is split into a pump beam and a detection beam through a beam splitting sheet; the first off-axis parabolic mirror is arranged in the propagation direction of the pump light beam and is used for focusing the pump light beam to a focal point of the first off-axis parabolic mirror; the water column is arranged at the focal plane of the first off-axis parabolic mirror; the water column control device is used for controlling the inclination angle of the water column in the propagation direction of the focused pump beam and the offset between the center of the water column and the propagation axis of the focused pump beam, the focused pump beam enters the water column, plasma is generated in the water column, and elliptically polarized terahertz is radiated. The method can be realized by introducing geometric regulation under a monochromatic laser excitation condition. The problems that in an existing scheme, the offset of two laser beams in time and space needs to be accurately controlled, and the requirement for space-time control is extremely strict are solved.

Description

Technical Field

[0001] This invention relates to the field of optical technology, and in particular to an elliptic polarization terahertz generator. Background Technology

[0002] Circularly and elliptically polarized terahertz fields (THz) have significant applications in molecular chirality detection, polarization-sensitive spectroscopy, anisotropic medium imaging, and the study of chiral responses in quantum and magnetic materials due to their unique polarization characteristics. Therefore, the efficient and controllable generation of high-quality elliptically polarized terahertz fields has become a core requirement for the development of terahertz science and technology.

[0003] Over the past few decades, researchers have proposed various methods to achieve controllable terahertz polarization. Traditional methods rely on optical elements, such as Fresnel prisms and terahertz waveplates, but these methods are often limited by bandwidth due to material absorption and transmission losses. As an alternative approach, nonlinear crystals (such as GaP and NiO) can directly generate ellipticized terahertz waves, but their efficiency is often limited by crystal orientation sensitivity and strong laser damage. In recent years, solid-state metasurfaces have been proposed as an emerging solution to achieve broadband and efficient linear-to-circular polarization conversion in the terahertz band. However, solid-state sources are still inevitably limited by polarization sensitivity, orientation dependence, and material breakdown under strong field excitation. In contrast, plasma-based approaches (such as two-color laser-induced gas plasma) not only avoid material damage problems but also allow for flexible control of the polarization state by adjusting the relative phase of the laser. Furthermore, applying external electric or magnetic fields can further enhance polarization control capabilities, but these typically require complex experimental setups.

[0004] In recent years, liquids have demonstrated unique advantages as strong broadband terahertz sources due to their high density and efficient ionization processes. However, under monochromatic laser excitation, the terahertz waves generated by vertical liquid films or columns are typically linearly polarized and remain largely unchanged regardless of the polarization of the pump light. To achieve elliptically polarized radiation, some studies have used two orthogonally polarized femtosecond pulses with spatiotemporal offsets to generate circularly polarized terahertz fields in liquids. While this method can effectively generate the desired polarization state, it requires precise control of the temporal and spatial offsets of the two laser beams, placing extremely stringent demands on spatiotemporal control and resulting in relatively complex experimental setups, thus limiting its feasibility in practical applications. Summary of the Invention

[0005] In view of the defects of the prior art, the present invention provides an elliptic polarization terahertz generator, which solves the existing problems.

[0006] This invention provides an ellipticized terahertz generator, comprising: A laser source for generating a laser beam, which is split into a pump beam and a probe beam by a beam splitter. The first off-axis parabolic mirror is positioned in the propagation direction of the pump beam and is used to focus the pump beam to its focal point. The water column is positioned at the focal plane of the first off-axis parabolic mirror; The water column control device is used to control the tilt angle of the water column in the propagation direction of the focused pump beam and the offset of the center of the water column from the propagation axis of the focused pump beam. The focused pump beam enters the water column, generates plasma in the water column and radiates elliptic polarized terahertz.

[0007] Preferably, the water column consists of a column with a diameter of The nozzle is generated by a circular nozzle, which is driven by a peristaltic pump.

[0008] Preferably, the tilt angle of the water column in the propagation direction of the focused pump beam is controlled by an electric rotating frame, and the offset between the center of the water column and the propagation axis of the focused pump beam is controlled by an electric displacement stage.

[0009] Preferred options also include: The second off-axis parabolic mirror is used to collect the emitted ellipticized terahertz radiation. A first linear grating polarizer and a second linear grating polarizer are used. The transmission axis of the second linear grating polarizer is fixed along the propagation direction of the collected ellipticized terahertz waves. The first linear grating polarizer rotates relative to WGP2. and Terahertz signals enter the first and second linear grid polarizers, resulting in a terahertz signal projected in the horizontal direction. and Based on terahertz signals and Obtain horizontal components and vertical components , horizontal component and vertical components Vector synthesis is performed to obtain the terahertz electric field; A set of reflectors is used to delay the detection beam; ITO is used to combine the delayed probe beam and the terahertz electric field. The third off-axis parabolic mirror is used to focus the combined terahertz electric field and the probe beam onto the ZnTe crystal. The terahertz electric field induces a change in the crystal's refractive index, which alters the polarization state of the probe beam. A quarter-wave plate is used to convert a probe beam with altered polarization into circularly polarized light. Wollaston prisms are used to spatially separate the orthogonal polarization components of circularly polarized light to obtain the intensity difference. A balanced photodetector is used to detect light intensity differences and obtain terahertz differential signals.

[0010] Preferably, a high-resistivity silicon window is provided between the second off-axis parabolic mirror and the first linear grid polarizer to filter out the remaining pump light.

[0011] Preferably, the reflector assembly is adjusted using an electric translation stage.

[0012] Compared with the prior art, the technical solution adopted in this invention can achieve the following beneficial effects: The ellipticized terahertz generator of this invention controls the tilt angle of the water column in the propagation direction of the focused pump beam and the offset between the center of the water column and the propagation axis of the focused pump beam. The focused pump beam enters the water column, generating plasma and radiating ellipticized terahertz radiation. This invention can be achieved by introducing geometric control under monochromatic laser excitation conditions. It solves the problem of existing solutions requiring precise control of the temporal and spatial offsets of two laser beams, which imposes extremely stringent requirements on spatiotemporal control. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 This is a schematic diagram of an elliptic polarization terahertz generating device according to the present invention; Figure 2 These are the terahertz time-domain waveforms measured under different parameters according to the present invention; in, Figure 2 of (a): The terahertz waveform measured at time Figure 2 (b): The terahertz waveform measured at time; Figure 3 For the present invention in The horizontal component of the terahertz electric field measured at time ( ), vertical component ( and the synthesized terahertz electric field vector; in, Figure 3 of (a): A diagram of the water column at that time. Figure 3 (b): corresponds to the horizontal component of the terahertz electric field. ), vertical component ( and the synthesized terahertz electric field vector; Figure 4 For the present invention in The horizontal component of the terahertz electric field measured at time ( ), vertical component ( and the synthesized terahertz electric field vector; in, Figure 4 of (a): A diagram of the water column at that time. Figure 4 (b): corresponds to the horizontal component of the terahertz electric field. ), vertical component ( and the synthesized terahertz electric field vector; Figure 5 For the present invention in The horizontal component of the terahertz electric field measured at time ( ), vertical component ( and the synthesized terahertz electric field vector; in, Figure 5 of (a): A diagram of the water column at that time. Figure 5 (b): corresponds to the horizontal component of the terahertz electric field. ), vertical component ( and the synthesized terahertz electric field vector; Figure 6 For the present invention at three tilt angles ( Under these conditions, the ellipsoidity of the ellipsoidally polarized terahertz field varies with the horizontal offset. Detailed Implementation

[0015] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] This invention proposes an elliptically polarized terahertz generation device, specifically a method for generating elliptically polarized terahertz radiation by driving a water column with a monochromatic laser, referring to... Figure 1 It includes: a femtosecond laser source, a terahertz generation module, a terahertz detection module, and a liquid column control module.

[0017] The femtosecond laser source is a commercial Ti:sapphire femtosecond laser (center wavelength 800nm, pulse width 35fs, repetition frequency 1kHz), and the output beam is split into a pump beam and a probe beam by a beam splitter.

[0018] Terahertz generation module: In x The pump light is linearly polarized in the horizontal direction. z The light propagates in a directional direction and is focused into the water column by a first off-axis parabolic mirror PM1 with a focal length of 5 cm, where it generates plasma and radiates terahertz waves. The generated terahertz waves are collected by a second off-axis parabolic mirror PM2 with an effective focal length of 10 cm, and then passed through a high-resistivity silicon window Si to filter out the remaining 800 nm pump light.

[0019] In the terahertz detection module, the detection beam is delayed by a group of mirrors. The collected terahertz electric field and the detection beam are combined through an ITO (indium tin oxide) lens and then focused onto a 3mm thick ZnTe crystal by a third off-axis parabolic mirror PM3 with a focal length of 10cm. The received terahertz electric field induces a change in the crystal's refractive index, thereby altering the polarization state of the detection beam. The detection beam is then converted into circularly polarized light (with orthogonal polarization components) by a quarter-wave plate (QWP), and then passes through a Wollaston prism WL. After conversion, the orthogonal polarization components of the detection beam are spatially separated. Finally, the intensity difference between the orthogonal components of the detection beam is read by a balanced photodetector BD. The measured differential signal is proportional to the previously received terahertz electric field. A focusing lens (FL) is placed between the ZnTe crystal and the quarter-wave plate.

[0020] The time-domain waveform of terahertz radiation was detected and recorded using a standard electro-optic sampling method. The experiment employed standard electro-optic sampling technology, utilizing a motorized translation stage to manipulate a delay line in the probe optical path to precisely adjust the time delay of the probe pulse relative to the pump pulse. The terahertz time-domain waveform was recorded by continuously scanning this time delay. To achieve measurements of terahertz radiation in different polarization states, two linear grid polarizers (WGP1 and WGP2) were placed in the terahertz optical path after the high-resistivity silicon window. The transmission axis of WGP2 was fixed in the horizontal direction. x (direction), while WGP1 rotates relative to WGP2 during the measurement process. and The corresponding time-domain terahertz signal was recorded using the electro-optic sampling method described above. and The horizontal component of the terahertz electric field can then be obtained. and vertical components The calculation relationship is as follows: .

[0021] The liquid column control module uses a water column with a diameter of A circular nozzle is formed, driven by a peristaltic pump with a flow rate set to 30 ml / min. The water column is placed near the pump's focal plane, and its direction of propagation in the laser propagation direction is controlled by an electrically operated rotating frame.z The water column is tilted (in direction), and the electric displacement stage control ensures that the water column moves precisely along the direction. x Directional translation (equivalent to adjusting the horizontal offset of the laser propagation axis relative to the center of the water column).

[0022] The elliptically polarized terahertz generation scheme provided by this invention can be achieved by introducing geometric manipulation under monochromatic laser excitation conditions. Specifically, tilting the water column in the direction of laser propagation yields a vertically polarized terahertz component, while a horizontally polarized terahertz component with a non-zero relative phase is generated when the laser incident axis is laterally offset relative to the center of the water column. The coherent superposition of these two components forms the elliptically polarized terahertz. In the experiment, when the tilt angle is... At this time, the ellipticity can reach 0.75. Moreover, by adjusting the tilt angle of the liquid column and the horizontal offset of the laser incident position relative to the center of the liquid column, not only can the ellipticity of the terahertz wave be flexibly controlled, but also the rotational switching can be achieved.

[0023] The proposed solution avoids the drawbacks of solid materials being easily damaged under strong laser irradiation and the complexity of gas plasma solutions, providing a simple and effective liquid-based method for generating ellipticized terahertz sources. This method is not only significant for the development of controllable polarization terahertz sources, but also has broad application prospects in terahertz spectroscopy, imaging, and information technology.

[0024] Example 1 In the first embodiment of the invention, the terahertz radiation generated by a laser-pumped water column placed vertically perpendicular to the laser propagation direction was first measured. This was achieved by varying the horizontal offset of the laser propagation axis relative to the center of the water column. x The experiment investigated terahertz radiation when a laser was incident at different locations. The results showed that terahertz radiation could only be effectively generated when the laser propagation axis was deviated from the center of the water column. Figure 2 As shown in (a), when the laser axis is shifted to symmetrical positions on both sides of the center of the water column ( When the two terahertz waveforms were measured, they exhibited obvious antisymmetric characteristics. Previous studies explained this phenomenon as the presence of a laser-induced electromotive force current near the liquid column interface due to symmetry breaking. Due to the asymmetry of the cylindrical surface, the horizontal direction ( x Positive and negative currents in the direction of the vertical water column will not completely cancel each other out, while the positive and negative currents in the vertical direction of the vertical water column will not completely cancel each other out. y The direction has interface symmetry, so it is impossible to generate a net current in that direction, and therefore no vertically polarized terahertz will be generated. In the end, the radiated terahertz always remains horizontally polarized.

[0025] Based on the above mechanism, it is known that if the symmetry in the vertical direction is broken, a net current can theoretically be generated in that direction, thereby obtaining vertically polarized terahertz. To achieve this objective, this invention proposes to achieve this by breaking the symmetry in the laser propagation direction (…). z The water column was tilted (in the direction of rotation) to disrupt its vertical symmetry. Experimental results show that tilting the water column can indeed produce vertically polarized terahertz waves. The water column was tilted when the laser axis was aligned with the center of the column. The measured vertically polarized terahertz waveform also exhibits antisymmetry, see... Figure 2 (b). Wherein, the tilt angle... Defined as water column and y The angle between axes, where the negative angle corresponds to the angle along the axis. z The tilt angle corresponds to the opposite tilt angle. Furthermore, the vertically polarized terahertz field intensity generated by the tilted water column is comparable to the horizontally polarized terahertz field intensity, and may even introduce a certain phase difference relative to the horizontal component. Therefore, by simultaneously adjusting the tilt angle of the water column... Horizontal offset of the laser axis relative to the center of the water column x It can generate ellipticized terahertz.

[0026] Example 2 In the second embodiment of the present invention, different combinations of parameters are used. Measurements were performed at terahertz frequencies. Figure 3 Shown in The horizontal component of terahertz measured under the conditions Vertical components The synthesized terahertz electric field vector was obtained. As shown in the figure, a right-handed elliptically polarized terahertz with an ellipticity of 0.73 was obtained under these conditions. Furthermore, and The components exhibit significant waveform reversal under both opposite tilt angles and symmetrical horizontal offset conditions. (Refer to...) Figure 4 When the horizontal offset is changed hour, The components are reversed, thus generating left-handed polarized terahertz waves; reference Figure 5 Similarly, when the tilt angle is changed hour, The component inversion results in a corresponding conversion of terahertz curl. This provides a simple and effective method for controlling terahertz curl.

[0027] Example 3 In a third embodiment of the invention, the measurement was performed when... At that time, the resulting ellipticized terahertz ellipticity varies with the horizontal offset. x Relationships that change, such as Figure 6As shown in the figure. The results show that elliptically polarized terahertz can be generated when the tilt angle and horizontal offset are not both zero. Furthermore, by adjusting these two parameters, the ellipticity and curl of the elliptically polarized terahertz can be effectively controlled. In the measurements of this embodiment, the highest ellipticity obtained reached 0.75. It should be noted that the embodiments of the present invention mainly focus on... The experimental results illustrate its basic principle, and the ellipticity can be further improved through more precise tilt angle optimization. Furthermore, changing the liquid column diameter or using other liquid media can provide new ideas for improving the ellipticity of the generated terahertz waves.

[0028] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0029] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. An elliptic polarization terahertz generator, characterized in that, include: A laser source for generating a laser beam, which is split into a pump beam and a probe beam by a beam splitter. The first off-axis parabolic mirror is positioned in the propagation direction of the pump beam and is used to focus the pump beam to its focal point. The water column is positioned at the focal plane of the first off-axis parabolic mirror; The water column control device is used to control the tilt angle of the water column in the propagation direction of the focused pump beam and the offset of the center of the water column from the propagation axis of the focused pump beam. The focused pump beam enters the water column, generates plasma in the water column and radiates elliptic polarized terahertz.

2. The elliptic polarization terahertz generator as described in claim 1, characterized in that, The water column consists of a column with a diameter of The nozzle is generated by a circular nozzle, which is driven by a peristaltic pump.

3. The elliptic polarization terahertz generator as described in claim 1, characterized in that, The tilt angle of the water column in the propagation direction of the focused pump beam is controlled by an electric rotating frame, and the offset between the center of the water column and the propagation axis of the focused pump beam is controlled by an electric displacement stage.

4. The elliptic polarization terahertz generator as described in claim 1, characterized in that, Also includes: The second off-axis parabolic mirror is used to collect the emitted ellipticized terahertz radiation. A first linear grating polarizer and a second linear grating polarizer are used. The transmission axis of the second linear grating polarizer is fixed along the propagation direction of the collected ellipticized terahertz waves. The first linear grating polarizer rotates relative to WGP2. and Terahertz signals enter the first and second linear grid polarizers, resulting in a terahertz signal projected in the horizontal direction. and Based on terahertz signals and Obtain horizontal components and vertical components , horizontal component and vertical components Vector synthesis is performed to obtain the terahertz electric field; A set of reflectors is used to delay the detection beam; ITO is used to combine the delayed probe beam and the terahertz electric field. The third off-axis parabolic mirror is used to focus the combined terahertz electric field and the probe beam onto the ZnTe crystal. The terahertz electric field induces a change in the crystal's refractive index, which alters the polarization state of the probe beam. A quarter-wave plate is used to convert a probe beam with altered polarization into circularly polarized light. Wollaston prisms are used to spatially separate the orthogonal polarization components of circularly polarized light to obtain the intensity difference. A balanced photodetector is used to detect light intensity differences and obtain terahertz differential signals.

5. The elliptic polarization terahertz generator as described in claim 4, characterized in that, A high-resistivity silicon window is provided between the second off-axis parabolic mirror and the first linear grid polarizer to filter out the remaining pump light.

6. The elliptic polarization terahertz generator as described in claim 4, characterized in that, The mirror assembly is adjusted using an electric translation stage.

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