An apparatus and method for generating a broadband terahertz vortex beam with adjustable orbital angular momentum

Abstract

The application discloses a device for generating a broadband terahertz vortex beam with adjustable orbital angular momentum, comprising: a vacuum target chamber for providing a vacuum environment for laser and matter interaction; a laser system for providing a driving light pulse; a gas target generating device arranged in the vacuum target chamber, the gas target generating device providing a plasma channel along the propagation direction of the driving light pulse, guiding the driving light pulse to transmit in the plasma channel and generating a broadband terahertz vortex beam; and an adjusting optical path element for adjusting the position and angle of the driving light pulse when it is incident on the plasma channel, so that the driving light pulse transmits along a spiral trajectory in the plasma channel after being incident on the plasma channel, thereby driving a transverse net current at each cross section of the plasma channel, and generating broadband terahertz radiation at different positions of the spiral trajectory in the plasma channel to form a broadband terahertz vortex beam with adjustable orbital angular momentum. The device obtains a broadband terahertz vortex beam with adjustable orbital angular momentum.

Description

Technical Field

[0001] This invention relates to the field of broadband terahertz radiation technology, and in particular to an apparatus and method for generating a broadband terahertz vortex beam with adjustable orbital angular momentum. Background Technology

[0002] Vortex light refers to electromagnetic waves whose wavefronts exhibit a vortex shape during propagation, with their phase continuously varying along the angular direction. Due to its orbital angular momentum, it has wide applications in optical tweezers, optical communication, and optical manipulation. Since the terahertz band covers the rotational energy levels of biological macromolecules, the characteristic energy levels of physical processes such as Bose-Einstein condensation, and the application bands of next-generation communication technologies, the radiation technology of terahertz vortex beams is crucial for the development of related fields.

[0003] Currently, there are two main methods for generating terahertz vortex beams. The first involves directly modulating an already generated terahertz wave using wavefront modulation devices or diffraction elements, imbuing it with an angularly distributed phase difference. These devices are typically only suitable for terahertz waves of a specific frequency and are not applicable to broadband terahertz radiation. Furthermore, a single device can only achieve terahertz vortex wave output with a specific topological charge, lacking tunability. The second method uses a two-color vortex femtosecond laser to excite air plasma, enabling the output of terahertz vortex waves within an air plasma of a certain longitudinal length. The topological charge of the terahertz vortex wave generated by this method depends on the topological charge of the two-color laser field. Therefore, flexible control of the topological charge of terahertz radiation based on this method remains difficult to achieve.

[0004] In general, current methods for generating terahertz vortex beams cannot achieve orbital angular momentum control of broadband terahertz vortex beams, thus failing to meet the needs of related technological applications.

[0005] Therefore, there is an urgent need in the field to develop an apparatus and method for generating broadband terahertz vortex beams with adjustable orbital angular momentum, through which broadband terahertz vortex beams with adjustable orbital angular momentum can be obtained. Summary of the Invention

[0006] The purpose of this application is to provide an apparatus and method for generating a broadband terahertz vortex beam with adjustable orbital angular momentum, which yields a broadband terahertz vortex beam with adjustable orbital angular momentum.

[0007] This application provides an apparatus for generating a broadband terahertz vortex beam with adjustable orbital angular momentum, comprising:

[0008] A vacuum target chamber, which provides a vacuum environment for the interaction of laser and matter;

[0009] A laser system for providing driving optical pulses;

[0010] A gas target generating device is disposed in the vacuum target chamber. The gas target generating device is used to provide a plasma channel along the propagation direction of the driving light pulse, guide the driving light pulse to propagate in the plasma channel, and generate a broadband terahertz vortex beam. The plasma channel has a uniform density distribution in the axial direction and a parabolic density distribution in the radial direction, with a low density in the radial center and a high density on the outer side.

[0011] An adjustable optical path element is located in the vacuum target chamber. The adjustable optical path element is used to adjust the position and angle of the driving light pulse when it is incident on the plasma channel, so that when the adjusted driving light pulse is incident on the plasma channel, due to the plasma lensing effect, the driving light pulse is transmitted along a spiral trajectory in the plasma channel, thereby generating a current in each cross section of the plasma channel and generating broadband terahertz radiation at different positions of the spiral trajectory in the plasma channel, forming a broadband terahertz vortex beam with adjustable orbital angular momentum.

[0012] In another preferred embodiment, the spiral trajectory is a cylindrical spiral trajectory.

[0013] In another preferred embodiment, the plasma channel has a radial density gradient distribution with increasing density in the radial direction of the channel, and a substantially uniform axial density distribution in the axial direction of the channel.

[0014] In another preferred embodiment, the plasma channel has a density gradient distribution with increasing density in the radial direction of the channel (i.e., in the direction axially outward from the center of the channel) and a substantially uniform axial density distribution in the axial direction of the channel.

[0015] In another preferred embodiment, the plasma channel has a parabolic density gradient in the transverse direction (perpendicular to the laser propagation direction) (i.e., low density in the central region and high density on the outer side), which can effectively guide the long-distance transmission of laser pulses.

[0016] In another preferred embodiment, the plasma channel is a parabolic plasma channel with a low density at the center and a high density at the outer edge.

[0017] In another preferred embodiment, the center wavelength of the laser system is 800 nanometers. Preferably, the center wavelength of the laser system is 1064 nanometers. Preferably, the center wavelength of the laser system is 808 nanometers. Preferably, the center wavelength of the laser system is 980 nanometers. Preferably, the center wavelength of the laser system is 1550 nanometers.

[0018] In another preferred embodiment, the plasma channel is cylindrical. Assuming the longitudinal central axis of the plasma channel is the x-axis, the lines containing the mutually perpendicular diameters of the transverse cross-section of the plasma channel are the y-axis and z-axis respectively, and the center of the transverse cross-section of the plasma channel is the origin O, the position and angle of the driving light pulse incident on the plasma channel include the distance z0 between the incident point on the z-axis and the origin O, and the transmission axis of the driving light pulse being in the xOy plane and forming an angle θ with the y-axis. y .

[0019] In another preferred embodiment, the adjusting optical path element is used to adjust the size of z0 and θ. y The size of z0 is adjusted to continuously regulate the orbital angular momentum of the terahertz vortex beam. The size of z0 should be smaller than the spot diameter of the driving light pulse.

[0020] In another preferred embodiment, the incident point of the driving light pulse is on the z-axis, and the transmission axis of the driving light pulse at the time of incident has an angle θ with the y-axis in the xOy plane. y In another preferred embodiment, z0 is the off-axis amount z0 of the spot of the driving light pulse deviating from the origin O in the z-axis direction, and the included angle θ y The angle θ between the transmission axis and the y-axis when the driving light pulse is incident is... y .

[0021] In another preferred embodiment, the position and angle of the plasma channel include the off-axis direction of the light spot of the driving light pulse relative to the central axis of the parabolic plasma channel (i.e., the distance between the incident point of the driving light pulse and the origin O on the z-axis when the driving light pulse is incident) and the tilt direction of the incident axis (i.e., the transmission axis of the driving light pulse when it is incident has a certain angle θ with the y-axis of the xOy plane). y That is, the tilt angle θ y This allows the off-axis direction and the tilt direction to be in two perpendicular directions, so that the driving light pulse can be transmitted along a cylindrical helical trajectory in the plasma channel through the plasma lens effect, and excite a terahertz vortex beam during the transmission process.

[0022] In another preferred embodiment, the off-axis direction refers to the distance between the incident point of the driving light pulse on the z-axis and the origin O when the driving light pulse is incident, and the incident axis tilt direction refers to the angle between the transmission axis and the y-axis when the driving light pulse is incident (i.e., the tilt angle θ). y ).

[0023] In another preferred embodiment, by controlling and adjusting the optical path element, the sign of z0 is adjusted, thereby changing the chirality of the cylindrical helical trajectory and thus achieving the switching of the sign of the orbital angular momentum of the terahertz vortex beam.

[0024] In another preferred embodiment, the period of the helical trajectory is calculated using the following formula:

[0025] Λ0=2π|z0tanθ y |;

[0026] The refractive index of the gas plasma in the plasma channel for the driving light pulse is approximately 1. The optical path length of the driving light pulse traveling one cycle along a cylindrical helical trajectory in the plasma channel is calculated using the following formula:

[0027] D laser =Λ0(1+1 / 2tan 2 θ y ).

[0028] In another preferred embodiment, during the laser transmission process, a net transverse current is driven in each cross-section of the plasma channel, thereby exciting broadband terahertz radiation. Since the parabolic density distribution plasma channel has a non-uniform plasma density distribution, the terahertz radiation exits from one side of the plasma channel at a certain angle α deviating from the x-axis. The optical path difference between the terahertz radiation generated at the same position in two transmission cycles of the driving optical pulse is calculated using the following formula:

[0029] in, denoted as α, where α is the refractive index of the plasma to terahertz radiation, d is the distance between the centroid of the driving light pulse spot and the lateral edge of the plasma channel, and α is the angle of the terahertz radiation.

[0030] In another preferred embodiment, the ejection from one side of the plasma channel at a certain angle α away from the x-axis means ejecting from one side of the plasma channel at a certain angle α away from the x-axis.

[0031] In another preferred embodiment, the terahertz radiation generated at different positions of the helical trajectory in the plasma channel corresponds to different spatial angles and has different optical path differences. The terahertz radiation generated at different positions of the helical trajectory constitutes a terahertz vortex beam, and the topological charge of the terahertz vortex beam is determined by the following formula:

[0032]

[0033] In the formula, Λ0=2π|z0tanθ y| represents the period of the helical trajectory of the driving laser pulse. λ is the refractive index of the plasma to terahertz radiation, d is the distance between the centroid of the driving light pulse spot and the lateral edge of the plasma channel, α is the angle of terahertz radiation, and λ is the refractive index of the plasma to terahertz radiation. THz The wavelength of the terahertz radiation is σ = z₀tanθ. y / |z0tanθ y |

[0034] In another preferred embodiment, the laser system provides a driving optical pulse with a pulse width on the order of femtoseconds, a peak power on the order of terawatts, and a beam waist radius of several micrometers to hundreds of micrometers.

[0035] In another preferred embodiment, the device further includes a synchronization system connected to the laser system, the synchronization system outputting a synchronization signal to the magnetic control valve of the gas target generating device to achieve simultaneous occurrence of the gas generated by the gas target generating device and the driving light pulse emitted by the laser system.

[0036] In another preferred embodiment, the gas target generating device is used to generate gas to form a plasma channel with a parabolic density distribution along the propagation direction of the driving light pulse by laser pre-pulse ionization of the gas or discharge capillary ionization of the gas.

[0037] In another preferred embodiment, the synchronization system is configured to synchronize the time of the gas target generating device with that of the laser system.

[0038] In another preferred embodiment, the plasma in the plasma channel should contain an electron number density of 10. 17 ~10 19 More preferably, it is 10 per cubic centimeter. 18 Each cubic centimeter.

[0039] In another preferred embodiment, the optical path adjustment element is an electrically controlled reflector assembly.

[0040] In another preferred embodiment, an off-axis parabolic mirror is also included for collecting the terahertz radiation.

[0041] In another preferred embodiment, the parabolic density distribution of the plasma channels is obtained by laser pre-pulse ionization of gas or discharge capillary ionization of gas, and / or the density of the parabolic density distribution of the plasma channels is adjusted by laser intensity or delay adjustment, or gas pressure adjustment.

[0042] In another preferred embodiment, the gas may be selected from a single gas or a mixture of multiple gases with a small atomic number, preferably hydrogen, helium, nitrogen, and mixtures thereof.

[0043] Another aspect of this application provides a method for generating a broadband terahertz vortex beam with tunable orbital angular momentum, comprising the following steps:

[0044] (a) Provide driving optical pulses;

[0045] (b) Provides a plasma channel with a parabolic density distribution along the propagation direction of the driving light pulse, the parabolic density distribution plasma channel having a uniform density distribution in the axial direction and a parabolic density distribution with a low density in the radial central region and a high density on the outer side in the radial direction.

[0046] (c) Adjust the position and angle of the driving light pulse when it is incident on the plasma channel, and focus the adjusted driving light pulse onto the plasma channel provided in step (b). Due to the plasma lens effect, the adjusted driving light pulse is transmitted along a cylindrical helical trajectory in the plasma channel, thereby generating a current in each cross section of the plasma channel, and generating broadband terahertz radiation at different positions in the plasma channel, forming a broadband terahertz vortex beam with adjustable orbital angular momentum.

[0047] It should be understood that, within the scope of this invention, the above-described technical features of this invention and the technical features specifically described below (such as in the embodiments) can be combined with each other to form new or preferred technical solutions. Due to space limitations, they will not be described in detail here. Attached Figure Description

[0048] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. It should be understood that the accompanying drawings described below are merely some implementation examples of the present invention, and those skilled in the art can obtain other implementation examples based on these drawings without creative effort.

[0049] Figure 1 A schematic diagram of a broadband terahertz vortex beam realization device with adjustable orbital angular momentum according to an embodiment of this application.

[0050] Figure 2 This is a schematic diagram of the plasma density distribution in a parabolic plasma channel according to an embodiment of this application;

[0051] Figure 3 This is a schematic diagram of the generation process of a terahertz vortex beam in a parabolic plasma channel according to an embodiment of this application.

[0052] In each of the attached figures, the markings are as follows:

[0053] 100-Vacuum Target Chamber

[0054] 1-Laser System

[0055] 2-Electrically controlled reflector assembly

[0056] 3- Parabolic plasma channel generator (gas target generator)

[0057] 4-Synchronization System

[0058] 5-Off-axis parabolic mirror Detailed Implementation

[0059] Through extensive and in-depth research, the inventors have developed for the first time a device and method for generating broadband terahertz vortex beams with adjustable orbital angular momentum. This method achieves the generation of broadband terahertz vortex beams with adjustable orbital angular momentum by injecting femtosecond lasers off-axis obliquely into a parabolic plasma channel.

[0060] In the following description, many technical details are presented to help the reader better understand this application. However, those skilled in the art will understand that the technical solutions claimed in this application can be implemented even without these technical details and various variations and modifications based on the following embodiments.

[0061] the term

[0062] As used in this article, “plasma channel”, “parabolic plasma channel”, and “plasma channel with parabolic density distribution” can be used interchangeably.

[0063] As used in this article, "off-axis" refers to the incident position of the driving light pulse being offset from the central x-axis of the plasma channel.

[0064] It should be noted that in this patent application, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. In this patent application, if it refers to performing an action according to an element, it means performing the action at least according to that element, including two cases: performing the action only according to that element, and performing the action according to that element and other elements. Expressions such as "multiple," "repeatedly," and "various" include two, two times, two kinds, and more than two, more than two times, and more than two kinds.

[0065] In this invention, all directional indicators (such as up, down, left, right, front, back, etc.) are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indicator will also change accordingly.

[0066] This application possesses at least one of the following advantages.

[0067] (a) The apparatus and method of this application for generating a broadband terahertz vortex beam with adjustable orbital angular momentum achieves the output of a broadband terahertz vortex beam by injecting a femtosecond laser off-axis obliquely into a parabolic plasma channel, and achieves continuous control of the orbital angular momentum of the terahertz vortex beam by adjusting the laser incident conditions.

[0068] (b) The apparatus and method for generating broadband terahertz vortex beams with adjustable orbital angular momentum proposed in this application are the first to discover or utilize the interaction between a femtosecond Gaussian laser and a plasma parabolic channel to generate broadband terahertz vortex beams.

[0069] (c) The apparatus and method of this application for generating a broadband terahertz vortex beam with adjustable orbital angular momentum, wherein the orbital angular momentum of the terahertz vortex beam can be continuously controlled by changing the laser incident conditions.

[0070] (d) The apparatus and method of this application for generating broadband terahertz vortex beams with tunable orbital angular momentum do not require premodulation of the laser pulse and plasma structure.

[0071] A device for generating broadband terahertz vortex beams with adjustable orbital angular momentum

[0072] The device includes: a vacuum target chamber, which provides a vacuum environment for the interaction of laser and matter;

[0073] A laser system for providing driving optical pulses;

[0074] A gas target generating device is disposed in the vacuum target chamber. The gas target generating device is used to provide a plasma channel with a parabolic density distribution along the propagation direction of the driving light pulse, and to guide the driving light pulse to propagate in the plasma channel and generate a broadband terahertz vortex beam. The parabolic density distribution plasma channel has a density gradient with uniform density distribution in the axial direction and low density in the radial center region and high density on the outer side in the radial direction.

[0075] An adjustable optical path element, located within the vacuum target chamber, is used to adjust the position and angle of the driving light pulse incident on the plasma channel. This ensures that when the adjusted driving light pulse is incident on the plasma channel, due to the plasma lensing effect, it propagates along a helical trajectory within the plasma channel (i.e., the driving light pulse propagates along a helical trajectory after being incident on the plasma channel). This, in turn, drives a net transverse current at each cross-section of the plasma channel and generates broadband terahertz radiation at different positions along the helical trajectory within the plasma channel, forming a broadband terahertz vortex beam with adjustable orbital angular momentum.

[0076] Due to the high refractive index distribution on both sides and low refractive index in the middle of the plasma, the laser trajectory is refracted and deviated, and the final trajectory is changed by the incident conditions. However, the adjustment (or setting) of the position and angle of the driving light pulse when it is incident on the plasma channel in this application allows the adjusted driving light pulse to propagate along a spiral trajectory in the plasma channel.

[0077] Preferably, the spiral trajectory is a cylindrical spiral trajectory.

[0078] Preferably, the plasma channel has a radial density gradient distribution with increasing density in the radial direction of the channel, and a substantially uniform axial density distribution in the axial direction of the channel.

[0079] Preferably, the plasma channel has a density gradient distribution with increasing density in the radial direction of the channel (i.e., the direction from the center of the channel outwards), and a substantially uniform axial density distribution in the axial direction of the channel.

[0080] Preferably, the plasma channel has a parabolic density gradient in the transverse direction (perpendicular to the laser propagation direction) (i.e., low density in the central region and high density on the outer side), which can effectively guide the long-distance transmission of laser pulses.

[0081] Preferably, the plasma channel is a parabolic plasma channel with a low density at the center and a high density at the outer edge.

[0082] Preferably, the plasma channel is cylindrical. Assuming the longitudinal central axis of the plasma channel is the x-axis, the lines containing the mutually perpendicular diameters of the transverse cross-section of the plasma channel are the y-axis and z-axis respectively, and the center of the transverse cross-section of the plasma channel is the origin O, the position and angle of the driving light pulse incident on the plasma channel include the distance z0 between the incident point on the z-axis and the origin O, and the angle θ between the transmission axis of the driving light pulse and the y-axis in the xOy plane. y That is, z0 is the off-axis amount z0 of the light spot of the driving light pulse deviating from the origin O in the z-axis direction, and the included angle θ y The angle θ between the transmission axis and the y-axis when the driving light pulse is incident is... y .

[0083] Preferably, the adjusting optical path element is used to adjust the size of z0 and θ. y The size of z0 is adjusted to continuously regulate the orbital angular momentum of the terahertz vortex beam. The size of z0 should be smaller than the spot diameter of the driving light pulse.

[0084] Preferably, the incident point of the driving optical pulse is on the z-axis, and the transmission axis of the driving optical pulse at the time of incident has an angle θ with the y-axis in the xOy plane. y .

[0085] Preferably, the position and angle of the plasma channel include the off-axis direction of the light spot of the driving light pulse relative to the central axis of the parabolic plasma channel (i.e., the distance between the incident point of the driving light pulse and the origin O on the z-axis when the driving light pulse is incident) and the tilt direction of the incident axis (i.e., the transmission axis of the driving light pulse when it is incident makes a certain angle θ with the y-axis of the xOy plane). y That is, the tilt angle θ y This allows the off-axis direction and the tilt direction to be in two perpendicular directions, so that the driving light pulse can be transmitted along a cylindrical helical trajectory in the plasma channel through the plasma lens effect, and excite a terahertz vortex beam during the transmission process.

[0086] Preferably, the off-axis direction refers to the distance between the incident point of the driving light pulse on the z-axis and the origin O when the driving light pulse is incident, and the incident axis tilt direction refers to the angle between the transmission axis and the y-axis when the driving light pulse is incident (i.e., the tilt angle θ). y ).

[0087] Preferably, the device further includes a synchronization system connected to the laser system. The synchronization system outputs a synchronization signal to the magnetic control valve of the gas target generating device to achieve simultaneous occurrence of the gas generated by the gas target generating device and the driving light pulse emitted by the laser system.

[0088] The gas target generating device is used to generate gas, which forms a plasma channel with a parabolic density distribution along the propagation direction of the driving light pulse by laser pre-pulse ionization of the gas or discharge capillary ionization of the gas.

[0089] In one embodiment, the laser is an fs laser, the gas diffusion is on the order of milliseconds, and the gas cannot continuously operate within the vacuum cavity to maintain the gas target; therefore, both require time synchronization so that the driving light pulse and the gas target appear simultaneously. The synchronization system is configured to synchronize the time of the gas target generation device with that of the laser system.

[0090] A method for generating broadband terahertz vortex beams with tunable orbital angular momentum

[0091] The method includes the following steps:

[0092] (a) Provide driving optical pulses;

[0093] (b) Provides a plasma channel with a parabolic density distribution along the propagation direction of the driving light pulse, the parabolic density distribution plasma channel having a density gradient with uniform density distribution in the axial direction and low density in the radial central region and high density on the outer side in the radial direction.

[0094] (c) Adjust the position and angle of the driving light pulse when it is incident on the plasma channel, and focus the adjusted driving light pulse onto the plasma channel provided in step (b). Due to the plasma lens effect, the adjusted driving light pulse is transmitted along a cylindrical helical trajectory in the plasma channel, thereby generating a current in each cross section of the plasma channel, and generating broadband terahertz radiation at different positions in the plasma channel, forming a broadband terahertz vortex beam with adjustable orbital angular momentum.

[0095] To make the objectives, technical solutions, and advantages of the present invention clearer, embodiments of the present invention will be described in further detail below with reference to the accompanying drawings. It should be understood that these are merely examples provided to the reader of possible implementations of the present invention and are not intended to limit the scope of the invention.

[0096] See Figures 1-3 This application discloses an apparatus and method for generating a broadband terahertz vortex beam with adjustable orbital angular momentum.

[0097] in, Figure 1 A schematic diagram of the device for realizing the broadband terahertz vortex beam with adjustable orbital angular momentum of this application is shown. The device is based on a parabolic plasma channel and includes:

[0098] Vacuum target chamber 100: Used to provide a vacuum environment for laser-matter interaction;

[0099] The femtosecond Gaussian laser provided by laser system 1 serves as the driving light;

[0100] Electro-controlled reflector group 2: Used to adjust the optical path of the laser, thereby adjusting the angle and position of the laser incident plasma channel;

[0101] Parabolic plasma channel generating device 3 (i.e., gas target generating device): The parabolic plasma channel generating device is located in the vacuum target chamber and is used for guiding lasers and generating broadband terahertz vortex beams;

[0102] Synchronization System 4: Connects to the trigger of the laser system and outputs a synchronization signal to the magnetic valve to achieve synchronization between the parabolic plasma channel and the laser system;

[0103] Off-axis parabolic mirror 5: used for collecting terahertz radiation, ultimately obtaining a broadband terahertz vortex beam.

[0104] The pulsed laser provided by the laser system 1 has a pulse width on the order of femtoseconds, a peak power on the order of terawatts, and a beam waist radius of several micrometers to hundreds of micrometers.

[0105] Preferably, the center wavelength of the laser system is 800 nanometers. Preferably, the center wavelength of the laser system is 1064 nanometers. Preferably, the center wavelength of the laser system is 808 nanometers. Preferably, the center wavelength of the laser system is 980 nanometers. Preferably, the center wavelength of the laser system is 1550 nanometers.

[0106] Specifically, the parabolic plasma channel generator 3 and the laser system 1 are synchronized using the synchronization system 4. The parabolic plasma channel generator interacts with the laser, and a suitable plasma density is obtained by adjusting the parabolic plasma channel generator 3. The electron number density in the plasma should be 10. 17 ~10 19 10 units per cubic centimeter, preferably 10 18 On the order of magnitude per cubic centimeter.

[0107] In the parabolic plasma channel generating device 3, a parabolic plasma channel with transverse density modulation can be provided in the laser propagation direction by ionizing gas through a discharge capillary, and the plasma density can be adjusted by means of gas pressure regulation, delay regulation, etc. Alternatively, a parabolic plasma channel with transverse density modulation can be provided in the laser propagation direction by ionizing gas through a laser pre-pulse, and the plasma density can be adjusted by means of laser intensity or delay regulation, gas pressure regulation, etc.

[0108] Preferably, the gas can be a single gas or a mixture of multiple gases with a small atomic number, and more preferably hydrogen, helium, nitrogen, or mixtures thereof.

[0109] When the laser, adjusted by the electrically controlled reflector group 2, is incident into the parabolic plasma channel, its spot is controlled relative to the off-axis direction and tilt direction of the central axis of the parabolic plasma channel 3. This ensures that the off-axis and tilt directions are perpendicular to each other, allowing the laser to follow a cylindrical helical trajectory within the plasma channel and exciting a terahertz vortex beam during transmission. The laser injection conditions are expressed as the off-axis amount z0 and the tilt angle θ. y .like Figure 2 As shown, the parabolic plasma channel has a certain lateral dimension, and the plasma density exhibits the distribution of equation (1):

[0110]

[0111] In equation (1), r represents the horizontal coordinate, n represents the plasma density, Δn represents the plasma channel depth, n0 represents the plasma channel bottom density, and r0 represents the plasma channel width.

[0112] Furthermore, such as Figure 3 As shown, the laser propagates in the plasma channel following a cylindrical helical trajectory and excites a terahertz vortex beam during propagation. Its topological charge is determined by equation (2):

[0113]

[0114] In equation (2), Λ0=2π|z0tanθ y | represents the period of the laser helical trajectory. Let λ be the refractive index of the plasma for terahertz radiation, d be the distance between the laser centroid and the transverse edge of the plasma channel, α be the angle of the terahertz radiation, and λ be the refractive index of the plasma for terahertz radiation. THz For terahertz radiation, σ = z0tanθ y / |z0tanθ y |

[0115] The derivation process of equation (2) is briefly introduced below:

[0116] The propagation of the laser in the plasma channel follows a cylindrical helical trajectory, the period of which is determined by the laser incident conditions.

[0117] Λ0=2π|z0tanθ y | (3)

[0118] Since the refractive index of the gas plasma used is approximately 1 for the laser, the optical path length traversed by the laser in one cycle along the cylindrical helical trajectory in the plasma channel is:

[0119] D laser =Λ0(1+1 / 2tan 2 θ y (4)

[0120] During laser transmission, a net transverse current is driven in each cross-section of the channel, thereby exciting broadband terahertz radiation. Due to the non-uniform density distribution of the parabolic plasma channel, the terahertz radiation deviates from the x-axis at a certain angle α from the off-axis side of the laser beam in the plasma channel, such as... Figure 3 As shown. The optical path difference between the terahertz radiation generated at the same position in two laser propagation cycles is...

[0121]

[0122] Terahertz radiation generated at different positions in a parabolic plasma channel corresponds to different spatial angles and carries different optical path differences, thus generating a terahertz vortex beam. The topological charge *l* of the terahertz vortex beam is determined by the optical path difference between the laser and the terahertz radiation, and the sign of the topological charge *l* is determined by the chirality of the laser trajectory.

[0123]

[0124] Substituting equations (4) and (5) into equation (6) yields equation (2).

[0125] Furthermore, a broadband terahertz vortex beam is finally obtained by collecting the beam through an off-axis parabolic mirror 5.

[0126] This specific embodiment also discloses a method for adjusting the orbital angular momentum of a broadband terahertz vortex beam using a parabolic plasma channel. By controlling the electrically controlled mirror group 2, the off-axis amount z0 when the laser is incident on the parabolic plasma channel is adjusted, thereby continuously adjusting the orbital angular momentum of the terahertz vortex beam. During the adjustment process, the off-axis amount should not exceed the laser spot diameter. By controlling the electrically controlled mirror group 2, adjusting the sign of the laser incident off-axis amount z0 changes the chirality of the laser cylindrical helical trajectory, thus switching the sign of the orbital angular momentum of the terahertz vortex beam.

[0127] The specification of this application contains numerous technical features distributed across various technical solutions. Listing all possible combinations of these technical features (i.e., technical solutions) would make the specification excessively lengthy. To avoid this problem, the various technical features disclosed in the above-described invention, the various technical features disclosed in the following embodiments and examples, and the various technical features disclosed in the accompanying drawings can be freely combined to form various new technical solutions (all of which are considered to have been described in this specification), unless such a combination of technical features is technically infeasible. For example, one example discloses feature A+B+C, and another example discloses feature A+B+D+E. Features C and D are equivalent technical means that serve the same function, and technically only one needs to be used; they cannot be used simultaneously. Feature E can technically be combined with feature C. Therefore, the solution A+B+C+D should not be considered as described because it is technically infeasible, while the solution A+B+C+E should be considered as described.

[0128] All documents mentioned in this application are considered to be incorporated in their entirety into the disclosure of this application so that they can serve as a basis for modifications if necessary. Furthermore, it should be understood that after reading the foregoing disclosure of this application, those skilled in the art can make various alterations or modifications to this application, and these equivalent forms also fall within the scope of protection claimed in this application.

Claims

1. A device for generating a broadband terahertz vortex beam with adjustable orbital angular momentum, characterized in that, include: A vacuum target chamber, which provides a vacuum environment for the interaction of laser and matter; A laser system for providing driving optical pulses; A gas target generating device is disposed in the vacuum target chamber. The gas target generating device is used to provide a plasma channel along the propagation direction of the driving light pulse, guide the driving light pulse to propagate in the plasma channel, and generate a broadband terahertz vortex beam. The plasma channel has a uniform density distribution in its axial direction and a parabolic distribution in its radial direction, with a low density in the radial center region and a high density on the outer side. An adjustable optical path element is located in the vacuum target chamber. The adjustable optical path element is used to adjust the position and angle of the driving light pulse when it is incident on the plasma channel, so that when the adjusted driving light pulse is incident on the plasma channel, due to the plasma lensing effect, the adjusted driving light pulse is transmitted along a helical trajectory in the plasma channel, thereby driving a transverse net current at each cross section of the plasma channel, and generating broadband terahertz radiation at different positions of the helical trajectory in the plasma channel, forming a broadband terahertz vortex beam with adjustable orbital angular momentum.

2. The apparatus as claimed in claim 1, characterized in that, The spiral trajectory is a cylindrical spiral trajectory.

3. The apparatus as described in claim 1, characterized in that, The plasma channel has a radial density gradient distribution with increasing density in the radial direction of the channel, and a substantially uniform axial density distribution in the axial direction of the channel.

4. The apparatus as claimed in claim 1, characterized in that, The plasma channel is a parabolic plasma channel with a low density at the center and a high density at the outer edge.

5. The apparatus as claimed in claim 1, characterized in that, The plasma channel is cylindrical in shape. Assuming the longitudinal central axis of the plasma channel is the x-axis, the lines containing the mutually perpendicular diameters of the transverse cross-section of the plasma channel are the y-axis and z-axis, and the center of the transverse cross-section of the plasma channel is the origin O, the position and angle of the driving light pulse incident on the plasma channel include the distance z0 between the incident point on the z-axis and the origin O, and the angle between the transmission axis of the driving light pulse and the y-axis in the xOy plane. .

6. The apparatus as claimed in claim 5, characterized in that, The optical path adjustment element is used to adjust the size of z0 and the... The size of z0 is adjusted to continuously regulate the orbital angular momentum of the terahertz vortex beam. The size of z0 should be smaller than the spot diameter of the driving light pulse.

7. The apparatus as claimed in claim 6, characterized in that, The position and angle at which the driving light pulse is incident on the plasma channel include the off-axis direction of the light spot of the driving light pulse relative to the central axis of the plasma channel and the tilt direction of the incident axis, such that the off-axis direction and the tilt direction are in two perpendicular directions, so that the driving light pulse can be transmitted along the spiral trajectory in the plasma channel through the plasma lens effect, and a terahertz vortex beam is excited during the transmission process.

8. The apparatus as claimed in claim 7, characterized in that, The off-axis direction refers to the distance between the incident point of the driving light pulse and the origin O on the z-axis when the driving light pulse is incident. The incident axis tilt direction refers to the angle between the transmission axis and the y-axis when the driving light pulse is incident, i.e., the tilt angle. .

9. The apparatus as claimed in claim 7, characterized in that, By controlling and adjusting the optical path components, the... The sign of the chirality of the spiral trajectory is changed by the sign of the chirality, thereby achieving the switching of the sign of the orbital angular momentum of the terahertz vortex beam.

10. The apparatus as claimed in claim 6, characterized in that, The period of the spiral trajectory is calculated using the following formula: ； The refractive index of the gas plasma in the plasma channel for the driving light pulse is approximately 1. The optical path length of the driving light pulse traveling one cycle along the helical trajectory in the plasma channel is calculated using the following formula: 。 11. The apparatus as claimed in claim 10, characterized in that, During the propagation of the driving optical pulse, a net transverse current is generated in each cross-section of the plasma channel, thereby exciting broadband terahertz radiation. Due to the non-uniform plasma density distribution of the parabolic density distribution plasma channel, the terahertz radiation deviates from the x-axis at a certain angle. The optical path difference between the terahertz radiation generated at the same position in two transmission cycles of the driving optical pulse, which exits from one side of the plasma channel, is calculated by the following formula: ， in, Let be the refractive index of the plasma for terahertz radiation. The distance between the centroid of the driving light pulse spot and the lateral edge of the plasma channel is [missing information]. The angle of terahertz radiation.

12. The apparatus as claimed in claim 11, characterized in that, The certain angle deviating from the x-axis Ejection from one side of the plasma channel refers to an emission at a certain angle to the x-axis. It is ejected from one side of the plasma channel away from the x-axis.

13. The apparatus as claimed in claim 11, characterized in that, The terahertz radiation generated at different positions of the helical trajectory in the plasma channel corresponds to different spatial angles and has different optical path differences. The terahertz radiation generated at different positions of the helical trajectory forms a terahertz vortex beam, and the topological charge of the terahertz vortex beam is... Determined by the following formula: ， In the formula, The period is the spiral trajectory of the driving optical pulse. Let be the refractive index of the plasma for terahertz radiation. The distance between the centroid of the driving light pulse spot and the lateral edge of the plasma channel is [missing information]. The angle of terahertz radiation, The wavelength of terahertz radiation. .

14. The apparatus as claimed in claim 1, characterized in that, The laser system provides driving optical pulses with pulse widths on the order of femtoseconds, peak power on the order of terawatts, and beam waist radii ranging from several micrometers to hundreds of micrometers.

15. The apparatus as claimed in claim 1, characterized in that, The device also includes a synchronization system connected to the laser system. The synchronization system outputs a synchronization signal to the magnetic control valve of the gas target generating device to achieve simultaneous occurrence of the gas generated by the gas target generating device and the driving light pulse emitted by the laser system.

16. The apparatus as claimed in claim 1, characterized in that, The parabolic plasma channels are obtained by laser pre-pulse ionization of gas or discharge capillary ionization of gas, and / or by adjusting the density distribution of the plasma channels through laser intensity or delay adjustment or gas pressure adjustment.

17. The apparatus as claimed in claim 1, characterized in that, The plasma in the plasma channel should contain an electron number density of 10. 17 ~10 19 Each cubic centimeter.

18. The apparatus as claimed in claim 1, characterized in that, The optical path adjustment element is an electrically controlled reflector assembly.

19. A method for generating a broadband terahertz vortex beam with adjustable orbital angular momentum, characterized in that, Includes the following steps: (a) Provide driving optical pulses; (b) Provides a plasma channel with a parabolic distribution along the propagation direction of the driving optical pulse, wherein the parabolic plasma channel has a uniform density distribution in the axial direction and exhibits a parabolic distribution with a low density in the radial center region and a high density on the outer side in the radial direction. (c) Adjust the position and angle of the driving light pulse when it is incident on the plasma channel, and focus the adjusted driving light pulse onto the plasma channel provided in step (b). Due to the plasma lens effect, the adjusted driving light pulse is transmitted along a cylindrical helical trajectory in the plasma channel, thereby generating a current in each cross section of the plasma channel, and generating broadband terahertz radiation at different positions in the plasma channel, forming a broadband terahertz vortex beam with adjustable orbital angular momentum.

Images