A vortex beam generation system

Abstract

The present invention relates to the field of optical technology, and discloses a vortex beam generation system. The system comprises: a femtosecond laser light source, for generating a laser beam, wherein after the laser beam is split into two, one beam is used as a pump beam and the other beam is used as a probe beam; a pump light shaping module, for shaping the pump beam into a vector beam; a probe light shaping module, for shaping the probe beam into a circularly polarized beam; and a vortex light generation module, for focusing the vector beam and the circularly polarized beam together into a nitrogen-filled gas chamber, inducing a non-adiabatic arrangement of nitrogen molecules in the gas chamber by the vector beam, so that the nitrogen molecules are arranged according to the polarization direction of the vector beam during the recovery period, forming multiple molecular wave plates. After the circularly polarized beam passes through the multiple molecular wave plates, it is modulated into a vortex beam having a hand-direction opposite to that of the circularly polarized beam, thereby improving the quality of the generated vortex beam.

Description

Technical Field

[0001] The present invention relates to the field of optical technology, and in particular to a vortex beam generating system. Background Art

[0002] A vortex beam is a special type of optical beam with a spiral phase wavefront distribution. Due to the presence of a phase singularity at the center of a vortex beam, its central intensity is zero, resulting in a hollow, ring-shaped optical field distribution. A key property of vortex beams is their ability to carry orbital angular momentum (OAM), which gives them significant application potential in a variety of fields.

[0003] The current methods for generating vortex beams are generally as follows:

[0004] One approach involves using devices such as spiral phase plates, liquid crystal spatial light modulators, and cylindrical lenses. However, these methods all perform poorly when generating wide-bandwidth vortex beams. For example, spiral phase plates can cause topological charge dispersion and azimuth-dependent group delay, spatial light modulators can introduce angular dispersion due to diffraction, and cylindrical lenses can produce significant chromatic aberration. The second approach involves post-compression, which generates broadband or short-period optical vortices by post-compressing narrowband vortex pulses. However, during the compression process, it is difficult to maintain the vortex phase characteristics and there is a significant energy loss problem.

[0005] Therefore, the quality of vortex beams generated by current vortex beam generation methods is not high. Summary of the Invention

[0006] The object of the present invention is to provide a vortex beam generating system to generate a high-quality vortex beam.

[0007] To solve the above technical problems, an embodiment of the present invention provides a vortex beam generation system, comprising: a femtosecond laser light source, a pump light shaping module, a probe light shaping module, and a vortex light generation module;

[0008] The femtosecond laser light source is used to generate a laser beam, and after the laser beam is split into two, one beam is used as a pump beam and the other beam is used as a detection beam;

[0009] The pump light shaping module is used to shape the pump light beam into a vector light beam;

[0010] The detection light shaping module is used to shape the detection light beam into a circularly polarized light beam;

[0011] The vortex light generation module is used to focus the vector light beam and the circularly polarized light beam together into a gas chamber filled with nitrogen. The vector light beam induces non-adiabatic arrangement of the nitrogen molecules in the gas chamber, so that the nitrogen molecules are arranged according to the polarization direction of the vector light beam during the recovery period, forming multiple molecular wave plates. After passing through the multiple molecular wave plates, the circularly polarized light beam is modulated into a vortex light beam with a rotation direction opposite to that of the circularly polarized light beam.

[0012] Optionally, the vortex light generating module is specifically used to control and delay the vector light beam and the circularly polarized light beam before focusing them together into an air chamber filled with nitrogen, so that the circularly polarized light beam contacts the nitrogen molecules in the air chamber later than the vector light beam, so that the vector light beam first induces the nitrogen molecules in the air chamber to form a molecular wave plate, and the circularly polarized light beam then passes through the molecular wave plate to form a vortex light beam.

[0013] Optionally, the vortex light generation module is specifically used to combine the vector light beam and the circularly polarized light beam through a dichroic mirror after regulating and delaying the vector light beam and the circularly polarized light beam, and focus the combined vector light beam and the circularly polarized light beam into a gas chamber filled with nitrogen through a silver-plated concave mirror with a focal length of 500mm.

[0014] Optionally, the vortex light generating module is further used to adjust the conversion efficiency of the circularly polarized light beam modulated into a vortex light beam by adjusting the nitrogen pressure in the gas chamber and focusing the vector light beam to a length that induces non-adiabatic arrangement of nitrogen molecules.

[0015] Optionally, the system further includes: a 60 / 40 beam splitter, which is used to split the laser beam into two, forming a pump beam with 60% of the energy of the laser beam and a detection beam with 40% of the energy of the laser beam.

[0016] Optionally, the system further includes: a BBO crystal and a bandpass filter, wherein the BBO crystal is used to frequency-double the wavelength of the detection beam to 400 nm, the bandpass filter is used to filter out wavelengths other than 400 nm in the frequency-doubled detection beam, and the detection light shaping module is used to shape the detection beam after wavelength filtering into a circularly polarized beam.

[0017] Optionally, the pump light shaping module is composed of a half-wave plate and a vector polarizer, and the detection light shaping module is a quarter-wave plate.

[0018] Optionally, the vector light beam is a radial vector light beam or an angular vector light beam, and the circularly polarized light beam is a left-handed circularly polarized light beam or a right-handed circularly polarized light beam.

[0019] Optionally, the laser beam is a laser pulse with a wavelength of 800 nm and a pulse width of 35 fs, output by a femtosecond laser light source at a repetition rate of 1 kHz.

[0020] Optionally, the system also includes a vortex light detection module, which is used to extract the vortex light beam generated by the vortex light generation module through a circular polarization analyzer composed of a quarter-wave plate and a wire grid polarizer, and to introduce a linearly polarized Gaussian light beam to interfere with the vortex light beam in a small-angle non-parallel manner to form a fork interference pattern, so as to detect the topological charge of the vortex light beam through the number of forks and the direction of the fork opening in the fork interference pattern.

[0021] The vortex beam generation system provided by the present invention has at least the following beneficial effects:

[0022] By preparing a vector beam and a circularly polarized beam, and focusing them into a gas chamber filled with nitrogen, wherein the vector beam can induce a non-adiabatic arrangement of nitrogen molecules in the gas chamber, so that the nitrogen molecules are arranged according to the polarization direction of the vector beam in the recovery period, forming a plurality of molecular wave plates, and the circularly polarized beam can be modulated into a vortex beam with a rotation direction opposite to the circularly polarized beam after passing through the plurality of molecular wave plates, thereby realizing the generation of a vortex beam. Therefore, the present invention introduces the spatial anisotropy of the molecular polarizability through the spatial distribution of the molecular axis, thereby forming a "molecular wave plate" similar to an optical wave plate. The "molecular wave plate" overcomes the wavelength limitation and effectively avoids various problems caused by the wide bandwidth of the vortex beam, such as the problem of topological charge dispersion. Moreover, since the "molecular wave plate" is composed of gas phase molecules, it has a high laser damage threshold and self-healing properties, and is suitable for the generation of high-power, short-cycle vortex pulses, solving the problem that the post-compression method is difficult to maintain the vortex phase characteristics and has significant energy loss when generating such vortex pulses. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] One or more embodiments are exemplarily described by the figures in the corresponding drawings, and these exemplified descriptions do not constitute limitations on the embodiments.

[0024] Figure 1 A vortex beam generating system according to an embodiment of the present invention is shown in FIG. Figure 1 ;

[0025] Figure 2 (a) is a schematic diagram of a vortex beam generating system provided according to an embodiment of the present invention. Figure 2 ;

[0026] Figure 2 (b) is a schematic diagram of a vortex beam intensity distribution according to an embodiment of the present invention;

[0027] Figure 3 (a) is a schematic diagram of a molecular wave plate provided according to an embodiment of the present invention;

[0028] Figure 3(b) is a schematic diagram of the spatial distribution of a light field provided according to an embodiment of the present invention;

[0029] Figure 4 is a schematic diagram of a beam signal field provided according to an embodiment of the present invention;

[0030] Figure 5 is a schematic diagram of a vortex beam provided according to an embodiment of the present invention;

[0031] Figure 6 is a schematic diagram of a detection beam provided according to an embodiment of the present invention;

[0032] Figure 7 It is a schematic diagram of the conversion efficiency of a vortex beam provided according to an embodiment of the present invention. DETAILED DESCRIPTION

[0033] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, it will be understood by those skilled in the art that in the embodiments of the present invention, many technical details are provided to enable the reader to better understand the present invention. However, even without these technical details and the various changes and modifications based on the following embodiments, the technical solutions claimed in the present invention can be implemented. The division of the following embodiments is for convenience of description and should not constitute any limitation on the specific implementation of the present invention. The various embodiments can be combined with each other and referenced to each other under the premise that there is no contradiction.

[0034] The existing vortex beam generation schemes are as follows:

[0035] 1. Generate vortex beam based on spiral phase plate:

[0036] The spiral phase plate is a diffractive optical element whose thickness increases with the azimuth angle, thereby achieving wavefront modulation of the incident light beam. The multi-stage spiral phase plate used to generate vortex beams is manufactured on quartz glass using a multi-stage vapor deposition process. It has a high damage threshold and is highly applicable to large-aperture beams. However, the inherent topological charge of the spiral phase plate is This is specific to a specific wavelength. Therefore, when the pulse bandwidth is wide, the spiral topological charge dispersion and group delay reduce the beam quality. This can be compensated by introducing a correction element, but this complicates beam shaping. When processing broadband pulses, an achromatic spiral lens can be used. This lens consists of two elements with the same shape but different refractive indices. The spiral topological charge no longer depends on the wavelength, and the operating bandwidth of the achromatic spiral phase plate is increased to approximately 100nm. However, the material and structure of this element need to be pre-calculated and designed, and materials with matching refractive indices need to be found, making the processing process more complex.

[0037] 2. Generate vortex beam based on holographic grating:

[0038] Holographic gratings can generate vortex beams with arbitrary topological charge. During the generation of vortex beams, due to the presence of angular dispersion, although each spectral component in the broadband light can form a good optical vortex, the diffraction angle of these optical vortices is wavelength-dependent, resulting in spatial chirp. On this basis, the traditional holographic grating method in the field of monochromatic light is improved to generate ultrashort vortex pulses. To address the influence of angular dispersion, a grating pair consisting of a linear grating and a fork grating is used. The linear grating does not have a top fork, but is identical to the fork grating in other respects. The pulse is first pre-chirped by the linear grating, and then an ultrashort vortex pulse with spatial chirp compensated is obtained by the fork grating. However, the vortex light has low contrast, with the central dark spot intensity only about 20% lower than the annular ring intensity, while the ideal vortex light center intensity is zero. In addition, the device is very sensitive to optical path alignment and is not very practical.

[0039] 0. Generate vortex beam based on axisymmetric polarizer and axisymmetric wave plate:

[0040] An ultrashort vortex beam without spatial dispersion and spiral topological charge dispersion can be generated by using an axisymmetric polarizer (ASP). In this scheme, polarization conversion is the key to beam vortexing. After the input Gaussian beam passes through a series of polarization control elements, a high-quality vortex beam can be obtained. Since the elements used are insensitive to wavelength, this scheme can convert ultrashort vortex beams within an ultra-wideband range, and through the design of the ASP, vortex beams with arbitrary topological charges can be generated. However, this scheme has only a 25% conversion efficiency, and due to the low damage threshold of the ASP device, the input beam is limited to Magnitude.

[0041] 4. Generate vortex beam based on diffraction spiral grating:

[0042] By using diffraction spiral grating elements, ultrashort vortex pulses with few cycles and high contrast and topological charge numbers of 1 and 2 can be obtained at a specific transmission distance. For ultrashort pulses with only a few cycles, it is necessary to consider the effects of both angular dispersion and group delay dispersion, so dispersion compensation is very important. The diffraction spiral grating is obtained by multi-stage etching of a 3mm thick fused silica substrate, and the positive group velocity dispersion introduced by the substrate is used to compensate for the negative group velocity dispersion introduced by the grating. However, the structure of the diffraction spiral grating is relatively complex and requires precise calculation and processing in advance, which limits the flexibility of the device.

[0043] 5. Generate vortex beam based on spiral multi-pinhole plate:

[0044] The spiral multi-pinhole plate is made by punching holes in aluminum foil with a focused femtosecond laser. The pinhole radius is about 26 μm and the number of pinholes is , arranged in a spiral curve along the origin of the plate, the azimuthal increments of these pinholes are constant while the radial distances increase. The modulation of the light beam by the spiral multi-pinhole plate utilizes the interference and diffraction effects of multiple light beams and is suitable for ultra-wideband pulses. This scheme does not cause dispersion caused by the medium during the conversion process, which can avoid the time broadening of ultrashort laser pulses. At the same time, the multi-pinhole plate is easy to manufacture, and the ultrashort vortex light generated has a high contrast. However, this element will cause spiral topological charge dispersion, and the conversion efficiency is low, resulting in a large amount of energy loss when passing through the conversion device.

[0045] 6. Generate vortex beam based on post-compression of vortex beam:

[0046] Currently, the pulse width of ultrashort OAM vortex light generated by active means is still limited to a few hundred femtoseconds, so the output vortex beam generally needs to be post-compressed. Common post-compression technologies include using air-filled hollow-core fibers, bulk materials, multi-pass cavities, solid sheets, etc. These methods are mainly based on the nonlinear effect between vortex pulses and Kerr media. However, these methods usually require focusing the beam to increase the intensity of the interaction to obtain a wider spectrum, which often causes ionization near the focus and destroys the spiral phase structure of the vortex beam. For air-filled hollow-core fibers, lenses are needed to couple the vortex beam into the optical fiber, and its mode selection effect will change the mode of the vortex beam and destroy the phase structure. These post-compression schemes are often accompanied by problems of vortex phase characteristic distortion and energy loss.

[0047] Among them, the generation scheme based on spiral phase plates offers a simple optical path, a high damage threshold, low losses, and high conversion efficiency, making it ideal for high-power, large-aperture applications. However, spiral phase plates are not designed for broadband operation. Once the input pulse bandwidth is wide, the beam quality degrades due to factors such as group delay and spiral topological charge dispersion. While this can be compensated using structures such as dispersion compensation elements and achromatic spiral lenses, this significantly increases the complexity of the scheme.

[0048] Holographic grating-based solutions utilize sophisticated optical path design and common optical components to compensate for dispersion. However, the compensation process is complex and requires strict optical path alignment, while also resulting in a low conversion efficiency of only around 10%.

[0049] The ASP-based generation scheme primarily achieves the conversion of ultrashort vortex pulses through polarization control. Because the modulation elements used in the experiment are wavelength-insensitive, the scheme can generate high-quality output pulses over an ultra-broadband range without angular dispersion or helical topological charge dispersion, with ASP achieving a conversion efficiency of 25%.

[0050] The generation scheme based on diffraction spiral grating has a highly compact structure and can compensate for the dispersion introduced by the broadband vortex pulse during the conversion process without complex optical adjustments, thereby generating a vortex beam with high contrast. However, the structure of the conversion element is relatively complex and requires precise calculation and processing in advance.

[0051] The generation scheme based on spiral multi-pinhole plate has a simple experimental setup and a high damage threshold. The multi-pinhole plate of the conversion device is easy to manufacture and is suitable for ultra-wideband pulses. However, this element will produce spiral topological charge dispersion during the conversion process, and the conversion efficiency is low, resulting in a large amount of energy loss.

[0052] The generation of ultrashort vortex pulses by post-compression of vortex beams based on pulse width compression technology is often accompanied by problems of vortex phase characteristic distortion and energy loss.

[0053] One embodiment of the present invention relates to a vortex beam generating system, which can be used as Figure 1 As shown, it includes: a femtosecond laser light source, a pump light shaping module, a detection light shaping module and a vortex light generation module.

[0054] Among them, the femtosecond laser light source is used to generate a laser beam, and after the laser beam is split into two, one beam is used as a pump beam and the other beam is used as a detection beam; the pump light shaping module is used to shape the pump beam into a vector beam; the detection light shaping module is used to shape the detection beam into a circularly polarized beam; the vortex light generation module is used to focus the vector beam and the circularly polarized beam together into a gas chamber filled with nitrogen, and induce the nitrogen molecules in the gas chamber to arrange non-adiabaticly through the vector beam, so that the nitrogen molecules are arranged according to the polarization direction of the vector beam in the recovery period, forming multiple molecular wave plates. After passing through multiple molecular wave plates, the circularly polarized beam is modulated into a vortex beam with a handedness opposite to that of the circularly polarized beam.

[0055] The implementation details of the vortex beam generating system of this embodiment are described in detail below. The following content is only provided for easy understanding of the implementation details and is not necessary for implementing this solution.

[0056] Regarding the femtosecond laser light source, this embodiment uses a Ti:sapphire pulse laser, which outputs laser pulses (i.e., laser beams) with a central wavelength of 800 nm, a pulse width of 35 fs, and a single pulse energy of 7 mJ at a repetition rate (i.e., repetition frequency) of 1 kHz.

[0057] After the femtosecond laser light source outputs the laser beam, it is split into two. This can be achieved by using a beam splitter. One beam is used as the pump beam, and the other as the probe beam. The beam splitter is specifically a 60 / 40 beam splitter. The 60 / 40 beam splitter splits the laser beam into two, forming a beam with 60% of the laser beam's energy and a beam with 40% of the laser beam's energy. The 60% energy beam is used as the pump beam, and the 40% energy beam is used as the probe beam.

[0058] Regarding the pump light shaping module, it consists of a half-wave plate and a vector polarizer. The half-wave plate is used to adjust the polarization direction of the light beam (i.e., the pump beam), and the vector polarizer is used to shape the light beam into a vector beam, which is a radial vector beam or an angular vector beam.

[0059] Regarding the detection light shaping module, it is specifically a quarter-wave plate, which can shape the light beam (i.e., the detection beam) into a circularly polarized beam. The circularly polarized beam is a left-handed circularly polarized beam or a right-handed circularly polarized beam, and the left-handed circularly polarized beam or the right-handed circularly polarized beam has different rotation directions.

[0060] Among them, after obtaining the detection beam, the wavelength of the detection beam is first doubled to 400nm using a BBO crystal, and then a bandpass filter is used to filter out wavelengths other than 400nm in the detection beam after the doubled frequency. Finally, a quarter-wave plate is used to perform beam shaping. That is, the quarter-wave plate shapes the detection beam after wavelength filtering into a circularly polarized beam.

[0061] The vortex light generation module focuses the shaped vector beam and circularly polarized beam into a nitrogen-filled chamber. The vector beam induces a non-adiabatic alignment of the nitrogen molecules in the chamber, causing them to align according to the polarization direction of the vector beam during the recovery period, forming multiple molecular wave plates. After passing through these molecular wave plates, the circularly polarized beam is modulated into a vortex beam with a handedness opposite to that of the circularly polarized beam. For example, if the circularly polarized beam generated by the probe light shaping module is left-handed, the vortex beam is right-handed.

[0062] In the specific implementation, the vortex light generation module will first control and delay the vector light beam and the circularly polarized light beam before focusing them together into a gas chamber filled with nitrogen, so that the circularly polarized light beam contacts the nitrogen molecules in the gas chamber later than the vector light beam. The vector light beam will first induce the nitrogen molecules in the gas chamber to form a molecular wave plate, and then the circularly polarized light beam will pass through the molecular wave plate to form a vortex light beam.

[0063] Among them, the vortex light generation module can accurately control the delay of the circularly polarized light beam through a delay line device. After the vector light beam and the circularly polarized light beam are controlled and delayed, the vector light beam and the circularly polarized light beam are combined through a dichroic mirror, and the combined vector light beam and the circularly polarized light beam are focused together into a nitrogen-filled gas chamber through a silver-plated concave mirror with a focal length of 500mm.

[0064] In one example, the vortex light generation module can adjust the conversion efficiency of a circularly polarized light beam modulated into a vortex light beam by adjusting the nitrogen pressure in the gas chamber and focusing the vector light beam to a length that induces the non-adiabatic arrangement of nitrogen molecules (i.e., the molecular interaction distance).

[0065] After the vortex light generation module generates a vortex beam, the performance of the generated vortex beam can be tested through the vortex light detection module. Specifically, the vortex beam generated by the vortex light generation module is extracted through a circular polarization analyzer composed of a quarter-wave plate and a wire-grid polarizer, and a linearly polarized Gaussian beam is introduced to interfere with the vortex beam in a small-angle non-parallel manner to form a fork-shaped interference pattern. The topological charge of the vortex beam is detected by the number of forks and the direction of the fork opening in the fork-shaped interference pattern. The topological charge can reflect the performance of the generated vortex beam.

[0066] In this embodiment, a vector beam and a circularly polarized beam are prepared and focused into a nitrogen-filled gas chamber, wherein the vector beam can induce a non-adiabatic arrangement of nitrogen molecules in the gas chamber, so that the nitrogen molecules are arranged according to the polarization direction of the vector beam during the recovery period, forming multiple molecular wave plates. After passing through the multiple molecular wave plates, the circularly polarized beam can be modulated into a vortex beam with a rotation direction opposite to that of the circularly polarized beam, thereby achieving the generation of a vortex beam. Therefore, this embodiment introduces the spatial anisotropy of the molecular polarizability through the spatial distribution of the molecular axis, thereby forming a "molecular wave plate" similar to an optical wave plate. The "molecular wave plate" overcomes the wavelength limitation and effectively avoids various problems caused by the wide bandwidth of the vortex beam, such as the problem of topological charge dispersion. Moreover, since the "molecular wave plate" is composed of gas-phase molecules, it has a high laser damage threshold and self-healing properties, making it suitable for the generation of high-power, short-cycle vortex pulses, solving the problem that the post-compression method is difficult to maintain the vortex phase characteristics and has significant energy loss when generating such vortex pulses.

[0067] The vortex beam generating system of the present invention is described below with a specific embodiment. The vortex beam generating system of this embodiment can be as follows: Figure 2 As shown in (a), the generation process of the vortex beam is as follows:

[0068] The pulsed laser outputs a laser beam, which is split by the beam splitter BS1 to form a pump beam Pump ω (fundamental frequency) and a probe beam. The pump beam, after passing through the half-wave plate HWP and the vector polarizer VWP, forms a vector beam. The probe beam is frequency-doubled by the BBO crystal and input into the bandpass filter F1 to filter out useless wavelength components. It is then split by a beam splitter into the probe beam Probe 2ω (frequency doubled) and the reference beam Rreference 2ω (frequency doubled). The probe beam Probe 2ω is shaped into a circularly polarized beam by the quarter-wave plate QWP. It is then combined with the pump beam Pump ω by the dichroic mirror DM, reflected by the silver-coated mirror, and then focused by the silver-coated concave mirror CM into a nitrogen-filled chamber. The pump beam first interacts with the nitrogen molecules, inducing non-adiabatic alignment of the nitrogen molecules. When the alignment is restored, the probe beam enters the medium to generate a vortex beam.

[0069] The pump beam branch has a delay line device consisting of a pair of mirrors at a 90° angle and an electrically controlled translation stage. This stage has extremely high displacement precision, with a minimum step size of 0.1µm. By moving the stage, the delay between the pump and probe beams can be varied. The reference beam (Rreference 2ω) is used to interfere with the generated vortex beam at a small non-parallel angle to detect the topological charge of the vortex beam.

[0070] It can also be understood that a plurality of reflectors (blue devices) are provided in the figure, which can reflect each light beam into the device that needs to enter.

[0071] Figure 2 (b) shows the intensity distribution of the vortex beam generated under different detection beam delays. Figure 3 (a) shows a schematic diagram of a molecular wave plate, Figure 3 (b) shows the moment when the arrangement of nitrogen molecules is restored, the radius is , the peak intensity is Radial vector beam ( ) driven by the spatial distribution of the first-order polarization tensor components in the cross section of the light field.

[0072] In order to verify the vortex beam generated by the above system, this embodiment uses the spatial polarization topological number The experiment is performed with radial vector beams and right-handed circularly polarized beams:

[0073] Measure the signal field under different pump beam-probe beam delays, Figure 3 The total intensity of the signal beam as a function of time delay and the interferogram between the signal field and the interference field are depicted, with the inset showing the doughnut-shaped profile of the vortex beam. To determine and visualize the orbital angular momentum carried by the signal beam, the interferogram between the signal beam and a Gaussian reference beam was measured at each time delay. Figure 4 The signal field and its interference pattern with the reference light under a half-reply period of 4.2ps under different combinations of pump pulses and probe pulses are shown, where (a)-(b) the pump pulse is The results when the probe pulse is right-handed circularly polarized are (c)-(d) the pump pulse is The results when the probe pulse is left-handed circularly polarized are shown in Figures 2 and 3. (e)-(h) are the same as (a)-(d) but the pump pulse is . It can be seen that the generated vortex beam exhibits a classic donut-shaped intensity distribution, while the interference fringes are the typical pattern of the cross-interference of the vortex beam and the Gaussian beam. In the interference fringe pattern, the difference in the number of fringes on the upper and lower sides (marked by the dotted line in the figure) corresponds to the topological charge of the vortex light. When the right-handed circular polarization (RCP) [left-handed circular polarization (LCP)] detection pulse is used, a topological charge of In order to further verify the effect of doubling the topological charge in the process of interacting with the aligned molecules, a spatial polarization topological number The radially polarized vector beam is used as the pump pulse. Figure 4 (e) and Figure 4 (f)[ Figure 4 (g) and Figure 4(h)] shows the intensity distribution of the signal pulse and its interference with the reference beam, corresponding to the RCP (LCP) probe pulse. The results show that in this configuration, the topological charges of the generated vortex signal beam are and .

[0074] At the same time, a molecular wave plate is used to generate a few-cycle vortex pulse numerical simulation:

[0075] Unlike traditional phase modulators [such as spiral phase plates (SPPs)], molecular wave plates overcome wavelength limitations and effectively avoid the topological charge dispersion problem usually caused by the wide bandwidth of ultrashort pulses. This approach has significant advantages in dealing with the limitations brought by ultrashort pulses and provides the possibility of generating short-cycle vortex pulses. To verify this, numerical simulations were performed using probe pulses with a very wide spectral range (266nm, 400nm and 800nm ​​respectively). At these central wavelengths, vortex beams with the same topological charge can be generated, such as Figure 5 Figure 2 shows the intensity distributions of the vortex beams generated by the 266 nm (a), 400 nm (b), and 800 nm (c) probe lasers, respectively. (d)-(f) show the spatial phase distributions corresponding to (a)-(c).

[0076] In this simulation, a radially polarized vector beam pump pulse and a few-cycle (three optical cycles) 400 nm RCP probe pulse are used, as shown in Figure 6 As shown, (a) is the electric field of the incident few-cycle RCP detection pulse, (b) is the spectrum of the few-cycle detection pulse of nitrogen molecules in this wavelength range and the frequency-dependent molecular polarizability, (c) is the theoretically calculated isosurface of the electric field of the few-cycle vortex pulse generated by the "molecular wave plate", (d)-(e) are the generated few-cycle vortex pulses in The intensity distribution and spiral phase at . Figure 6 (a) shows the X and Y components of the electric field of the probe pulse, Figure 6 (b) shows the broad spectrum of the probe pulse, covering the range of 300–600 nm, and demonstrates the frequency-dependent molecular polarizability ( and ), by solving Maxwell's equations, we obtain the topological charge as Few-period LCP vortex beams [see Figure 6 (d)–(e)].

[0077] Ultimately, conversion efficiency plays a crucial role in determining whether the current scheme is feasible. In our scheme, the generation efficiency of the vortex beam can be controlled by adjusting the gas pressure and interaction length. Figure 7 The simulated conversion efficiency at different gas pressures and interaction lengths is shown. Figure 7 As shown, the pump pulse used in the simulation has a radius of , the peak intensity is Radially polarized light ( ), increasing the gas pressure and / or the interaction length can improve the conversion efficiency. Under currently available experimental conditions, the conversion efficiency of vortex beams can be close to 1.

[0078] In this embodiment, since the molecules re-arrange the pulse polarization direction when they are arranged without field, the spatial anisotropy of the refractive index of the molecular ensemble is modulated, thereby forming a molecular wave plate similar to an optical wave plate. When the circularly polarized probe light passes through the molecular wave plate, the opposite circular polarization is generated and the topological charge number is (in This method is applicable to a wide spectral range from ultraviolet to mid-infrared.

[0079] Furthermore, by optimizing the gas pressure and interaction distance, a vortex beam conversion efficiency of up to 100% can be achieved. Furthermore, because the molecular wave plate is composed of gas-phase molecules, it has a high laser damage threshold and self-healing properties, making it suitable for the generation of high-power, ultrashort vortex pulses.

[0080] Furthermore, it avoids the problems of bandwidth limitation, topological charge dispersion and angular dispersion existing in traditional vortex light generation methods (such as spiral phase plates, holographic gratings, etc.), and can generate broadband or short-cycle vortex pulses without topological charge dispersion.

[0081] Those skilled in the art will appreciate that the above-described embodiments are specific embodiments for implementing the present invention, and that in actual applications, various changes in form and detail may be made thereto without departing from the spirit and scope of the embodiments of the present invention. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the embodiments of the present invention. Therefore, the scope of protection of the embodiments of the present invention shall be subject to the scope defined in the claims.

Claims

1. A vortex beam generating system, characterized in that: include: Femtosecond laser light source, pump light shaping module, probe light shaping module and vortex light generation module; The femtosecond laser light source is used to generate a laser beam, and after the laser beam is split into two, one beam is used as a pump beam and the other beam is used as a detection beam; The pump light shaping module is used to shape the pump light beam into a vector light beam; The detection light shaping module is used to shape the detection light beam into a circularly polarized light beam; The vortex light generation module is used to focus the vector light beam and the circularly polarized light beam together into a gas chamber filled with nitrogen. The vector light beam induces non-adiabatic arrangement of the nitrogen molecules in the gas chamber, so that the nitrogen molecules are arranged according to the polarization direction of the vector light beam during the recovery period, forming multiple molecular wave plates. After passing through the multiple molecular wave plates, the circularly polarized light beam is modulated into a vortex light beam with a rotation direction opposite to that of the circularly polarized light beam.

2. The vortex beam generating system according to claim 1, characterized in that: The vortex light generation module is specifically used to control and delay the vector light beam and the circularly polarized light beam before focusing them together into a gas chamber filled with nitrogen, so that the circularly polarized light beam contacts the nitrogen molecules in the gas chamber later than the vector light beam, so that the vector light beam first induces the nitrogen molecules in the gas chamber to form a molecular wave plate, and the circularly polarized light beam then passes through the molecular wave plate to form a vortex light beam.

3. The vortex beam generating system according to claim 2, characterized in that: The vortex light generation module is specifically used to combine the vector light beam and the circularly polarized light beam through a dichroic mirror after regulating and delaying the vector light beam and the circularly polarized light beam, and to focus the combined vector light beam and the circularly polarized light beam together into a nitrogen-filled gas chamber through a silver-plated concave mirror with a focal length of 500 mm.

4. The vortex beam generating system according to claim 1, wherein: The vortex light generating module is also used to adjust the conversion efficiency of the circularly polarized light beam modulated into the vortex light beam by adjusting the nitrogen pressure in the gas chamber and focusing the vector light beam to a length that induces the non-adiabatic arrangement of nitrogen molecules.

5. The vortex beam generating system according to claim 1, characterized in that: The system further includes a 60 / 40 beam splitter, which is used to split the laser beam into two, forming a pump beam with 60% of the energy of the laser beam and a detection beam with 40% of the energy of the laser beam.

6. The vortex beam generating system according to claim 5, characterized in that: The system also includes: a BBO crystal and a bandpass filter. The BBO crystal is used to double the wavelength of the detection beam to 400nm. The bandpass filter is used to filter out wavelengths other than 400nm in the doubled detection beam. The detection light shaping module is used to shape the detection beam after wavelength filtering into a circularly polarized beam.

7. The vortex beam generating system according to claim 1, characterized in that: The pump light shaping module is composed of a half-wave plate and a vector polarizer, and the detection light shaping module is a quarter-wave plate.

8. The vortex beam generating system according to any one of claims 1 to 7, characterized in that: The vector light beam is a radial vector light beam or an angular vector light beam, and the circularly polarized light beam is a left-handed circularly polarized light beam or a right-handed circularly polarized light beam.

9. The vortex beam generating system according to any one of claims 1 to 7, characterized in that: The laser beam is a laser pulse with a wavelength of 800 nm and a pulse width of 35 fs, which is output by a femtosecond laser light source at a repetition rate of 1 kHz.

10. The vortex beam generating system according to claim 1, characterized in that: The system also includes a vortex light detection module, which is used to extract the vortex light beam generated by the vortex light generation module through a circular polarization analyzer composed of a quarter-wave plate and a wire grid polarizer, and introduce a linearly polarized Gaussian light beam to interfere with the vortex light beam in a small-angle non-parallel manner to form a fork interference pattern, so as to detect the topological charge of the vortex light beam through the number of forks and the direction of the fork openings in the fork interference pattern.

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