Vortex light beam generation system

Abstract

The invention relates to the technical field of optics, and discloses a vortex beam generation system. The system comprises a femtosecond laser light source used for generating a laser beam, and after the laser beam is divided into two beams, one beam serves as a pumping beam, and the other beam serves as a detection beam; the pump light shaping module is used for shaping the pump light beam into a vector light beam; the detection light shaping module is used for shaping the detection light beam into a circularly polarized light beam; the vortex light generation module is used for focusing the vector light beam and the circularly polarized light beam into a gas chamber filled with nitrogen together, and inducing nitrogen molecules in the gas chamber to be arranged in a non-adiabatic manner through the vector light beam, so that the nitrogen molecules are arranged in a recovery period according to the polarization direction of the vector light beam to form a plurality of 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 after passing through the plurality of molecular wave plates, so that the quality of the generated vortex light beam is improved.

Description

Technical Field

[0001] The 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 beam that has a spiral phase wavefront distribution. Since there is a phase singularity at the center of the vortex beam, its central intensity is zero, thus forming a hollow ring-shaped light field distribution. An important feature of a vortex beam is that it can carry orbital angular momentum (OAM), which makes vortex beams have important application potential in many fields.

[0003] The current vortex beam generation methods generally include the following: One is to use devices such as spiral phase plates, liquid crystal spatial light modulators and cylindrical lenses, but these methods all perform poorly when generating wide-bandwidth vortex beams. For example, spiral phase plates can cause topological charge dispersion and azimuth-related group delay, spatial light modulators can introduce angular dispersion caused by diffraction, and cylindrical lenses can produce significant chromatic aberration. The second is to use a post-compression method, which generates broadband or low-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.

[0004] Therefore, the quality of vortex beams generated by current vortex beam generation methods is not high. Summary of the invention

[0005] The object of the present invention is to provide a vortex beam generating system to generate a high-quality vortex beam.

[0006] In order 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 detection light shaping module and a 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 generating module is used to focus the vector light beam and the circularly polarized light beam together into an air chamber filled with nitrogen, and induce the non-adiabatic arrangement of nitrogen molecules in the air chamber by the vector light beam, so that the nitrogen molecules are arranged according to the polarization direction of the vector light beam in the recovery period, forming a plurality of molecular wave plates. After passing through the plurality of molecular wave plates, the circularly polarized light beam is modulated into a vortex light beam having a rotation direction opposite to that of the circularly polarized light beam.

[0007] Optionally, the vortex light generating module is specifically used to perform control and delay processing on the vector light beam and the circularly polarized light beam before focusing the vector light beam and the circularly polarized light beam 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 then the circularly polarized light beam passes through the molecular wave plate to form a vortex light beam.

[0008] Optionally, the vortex light generating 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 together into a gas chamber filled with nitrogen through a silver-coated concave mirror with a focal length of 500 mm.

[0009] 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 a non-adiabatic arrangement of nitrogen molecules.

[0010] Optionally, the system further comprises: a 60 / 40 beam splitter, which is used to split the laser beam into two, forming a pump beam having 60% of the energy of the laser beam and a detection beam having 40% of the energy of the laser beam.

[0011] Optionally, 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 detection beam after doubled frequency, and the detection light shaping module is used to shape the detection beam after wavelength filtering into a circularly polarized beam.

[0012] 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.

[0013] 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.

[0014] Optionally, 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.

[0015] 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 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 fork opening direction in the fork interference pattern.

[0016] The vortex beam generating system provided by the present invention has at least the following beneficial effects: By preparing a vector beam and a circularly polarized beam, 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 in 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 having a rotation direction opposite to the circularly polarized beam after passing through a 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, and 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 characteristics, and is suitable for the generation of high-power, low-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

[0017] One or more embodiments are exemplarily described by the pictures in the corresponding drawings, and these exemplary descriptions do not constitute limitations on the embodiments.

[0018] Figure 1 A vortex beam generating system according to an embodiment of the present invention is shown in FIG. Figure 1 ; Figure 2 (a) is a schematic diagram of a vortex beam generating system provided according to an embodiment of the present invention Figure 2 ; Figure 2 (b) is a schematic diagram of vortex beam intensity distribution according to an embodiment of the present invention; Figure 3 (a) is a schematic diagram of a molecular wave plate provided according to an embodiment of the present invention; Figure 3 (b) is a schematic diagram of spatial distribution of a light field provided according to an embodiment of the present invention; Figure 4is a schematic diagram of a beam signal field provided according to an embodiment of the present invention; Figure 5 is a schematic diagram of a vortex beam provided according to an embodiment of the present invention; Figure 6 is a schematic diagram of a detection beam provided according to an embodiment of the present invention; Figure 7 It is a schematic diagram of conversion efficiency of a vortex light beam provided according to an embodiment of the present invention. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings. However, it will be appreciated by those skilled in the art that in the embodiments of the present invention, many technical details are proposed in order to enable the reader to better understand the present invention. However, even without these technical details and various changes and modifications based on the following embodiments, the technical scheme claimed in the present invention can be implemented. The division of the following embodiments is for the convenience of description and should not constitute any limitation on the specific implementation of the present invention. The various embodiments can be combined and referenced with each other without contradiction.

[0020] The existing vortex beam generation schemes are as follows: 1. Generate vortex beam based on spiral phase plate: 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 the vortex beam is manufactured on quartz glass using a multi-stage vapor deposition process. It has a high damage threshold and is highly applicable to large-diameter beams. However, the inherent topological charge of the spiral phase plate is It is for a specific wavelength, so when the pulse bandwidth is wide, the spiral topological charge dispersion and group delay reduce the beam quality, which can be compensated by introducing a correction element, but this will complicate the beam shaping. When processing broadband pulses, an achromatic spiral lens can be used. This lens consists of two elements with the same shape and different refractive indices. The spiral topological charge is no longer dependent on the wavelength, and the working bandwidth of the achromatic spiral phase plate is increased to about 100nm. However, the material and structure of the element need to be calculated and designed in advance, and it is necessary to find a material with a matching refractive index, and the processing process is relatively complicated.

[0021] 2. Generate vortex beam based on holographic grating: Holographic gratings can generate vortex beams with arbitrary topological charges. In the process of generating vortex beams, due to the existence of angular dispersion, although each spectral component in broadband light can form a good optical vortex, the diffraction angle of these optical vortices is related to the wavelength, so spatial chirp will be generated. On this basis, in order to generate ultrashort vortex pulses, the traditional holographic grating method in the field of monochromatic light is improved. In order to solve the influence of angular dispersion, a grating pair consisting of a linear grating and a fork grating is used, in which the linear grating has no top fork, and the rest is exactly the same as the fork grating. First, the pulse is pre-chirped by the linear grating, and then the ultrashort vortex pulse with spatial chirp compensated is obtained by the fork grating. However, the contrast of the vortex light is low, and the intensity of the central dark spot is only about 20% lower than the intensity of the annular ring, 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.

[0022] 0. Generate vortex beam based on axisymmetric polarizer and axisymmetric wave plate: 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.

[0023] 4. Generate vortex beam based on diffraction spiral grating: By using diffraction spiral grating elements, low-cycle, high-contrast ultrashort vortex pulses with topological charges 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.

[0024] 5. Generate vortex beam based on spiral multi-pinhole plate: 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 azimuth increment of these pinholes is constant while the radial distance increases. 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, but this element will cause spiral topological charge dispersion, and the conversion efficiency is low, and there will be a lot of energy loss when passing through the conversion device.

[0025] 6. Generate a vortex beam based on post-compression of the vortex beam: At present, the pulse width of ultrashort OAM vortex light generated by active means is still limited to a few hundred femtoseconds, so it is generally necessary to post-compress the output vortex beam. Common post-compression techniques include air-filled hollow-core fibers, bulk materials, multi-pass cavities, solid sheets, etc. These methods are mainly based on the nonlinear effect of 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.

[0026] Among them, the generation scheme based on the spiral phase plate has a simple optical path, a high damage threshold, low loss, and high conversion efficiency, and is very suitable for high-power and large-aperture conditions. However, the spiral phase plate is not designed for broadband. Once the input pulse bandwidth is wide, the beam quality will be reduced due to group delay and spiral topology charge dispersion. Although it can be compensated by dispersion compensation elements and achromatic spiral lenses, this greatly increases the complexity of the scheme.

[0027] The holographic grating-based generation scheme uses common optical elements to compensate for dispersion through sophisticated optical path design. However, the compensation process is relatively complicated and requires high optical path alignment. At the same time, the conversion efficiency is low, only about 10%.

[0028] The ASP generation scheme mainly realizes the conversion of ultrashort vortex pulses by controlling polarization. Since the modulation elements in the experiment are not sensitive to wavelength, the scheme can generate high-quality output pulses in an ultra-wideband range without angular dispersion or spiral topological charge dispersion, and the ASP can achieve a conversion efficiency of 25%.

[0029] The generation scheme based on diffraction spiral grating has a highly compact structure. It can compensate for the dispersion introduced by the broadband vortex pulse during the conversion process without complex optical adjustments, thereby producing a vortex beam with higher contrast. However, the structure of the conversion element is relatively complex and requires precise calculation and processing in advance.

[0030] 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, the 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.

[0031] 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.

[0032] 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 generating module.

[0033] Among them, a femtosecond laser light source is used to generate a laser beam, and after the laser beam is divided into two, one beam is used as a pump beam and the other beam is used as a detection beam; a pump light shaping module is used to shape the pump beam into a vector beam; a detection light shaping module is used to shape the detection beam into a circularly polarized beam; a vortex light generation module is used to focus the vector beam and the circularly polarized beam together into an air chamber filled with nitrogen, and induce the non-adiabatic arrangement of nitrogen molecules in the air chamber 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, and the circularly polarized beam is modulated into a vortex beam with a handedness opposite to that of the circularly polarized beam after passing through the multiple molecular wave plates.

[0034] 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.

[0035] 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 800nm, a pulse width of 35fs, and a single pulse energy of 7mJ at a repetition rate (i.e., repetition frequency) of 1kHz.

[0036] Among them, after the femtosecond laser light source outputs the laser beam, the laser beam is split into two, which can be achieved by a beam splitter. One beam is used as a pump beam and the other is used as a detection beam. The beam splitter is specifically a 60 / 40 beam splitter. The 60 / 40 beam splitter splits the laser beam into two beams with 60% of the energy of the laser beam and 40% of the energy of the laser beam. The beam with 60% energy is used as a pump beam, and the beam with 40% energy is used as a detection beam.

[0037] Regarding the pump light shaping module, it is composed 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.

[0038] 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, which 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.

[0039] 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 doubled detection beam, and then a quarter-wave plate is used to shape the beam, that is, the quarter-wave plate shapes the detection beam after wavelength filtering into a circularly polarized beam.

[0040] Regarding the vortex light generation module, it focuses the shaped vector light beam and circularly polarized light beam together into a gas chamber filled with nitrogen, and induces the non-adiabatic arrangement of nitrogen molecules in the gas chamber through the vector light beam, so that the nitrogen molecules are arranged according to the polarization direction of the vector light beam in the recovery period, forming multiple molecular wave plates. After passing through 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. For example, the circularly polarized light beam generated by the detection light shaping module is left-handed, and the vortex light beam at this time is right-handed.

[0041] In the specific implementation, before the vortex light generation module focuses the vector light beam and the circularly polarized light beam together into an air chamber filled with nitrogen, it will first control and delay the vector light beam and the circularly polarized light beam so that the circularly polarized light beam contacts the nitrogen molecules in the air chamber later than the vector light beam. Then, the vector light beam will first induce the nitrogen molecules in the air 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.

[0042] Among them, the vortex light generation module can accurately control the delay of the circularly polarized light beam through a delay line device, and 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-coated concave mirror with a focal length of 500mm.

[0043] In one example, the vortex light generating module can 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 the non-adiabatic arrangement of nitrogen molecules (i.e., the molecular action distance).

[0044] After the vortex light generating module generates a vortex light beam, the performance of the generated vortex light beam can be detected by the vortex light detecting module. Specifically, the vortex light beam generated by the vortex light generating module is extracted by a circular polarization analyzer composed of a quarter wave plate and a wire grid polarizer, and a linearly polarized Gaussian light beam is introduced to interfere with the vortex light beam in a small-angle non-parallel manner to form a fork interference pattern. The topological charge of the vortex light beam is detected by the number of forks in the fork interference pattern and the direction of the fork opening. The topological charge can reflect the performance of the generated vortex light beam.

[0045] In this embodiment, by preparing a vector beam and a circularly polarized beam, they are focused into an air chamber filled with nitrogen, wherein the vector beam can induce a non-adiabatic arrangement of nitrogen molecules in the air chamber, so that the nitrogen molecules are arranged in 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 having a rotation direction opposite to the circularly polarized beam after passing through a plurality of molecular wave plates, thereby realizing the generation of a vortex beam. Therefore, in this embodiment, the spatial anisotropy of the molecular polarizability is introduced through the spatial distribution of the molecular axis, thereby forming a "molecular wave plate" similar to an optical wave plate, and 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 characteristics, and is suitable for the generation of high-power, low-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.

[0046] The vortex beam generating system of the present invention is described below with a specific embodiment. The vortex beam generating system of the present embodiment can be as follows: Figure 2 As shown in (a), the generation process of the vortex beam is as follows: The pulse laser outputs a laser beam, which is split by the beam splitter BS1 to form a pump beam Pump ω (base frequency) and a probe beam. The pump beam forms a vector beam after passing through a half-wave plate HWP and a vector polarizer VWP. The probe beam is frequency-doubled by a BBO crystal and input into a bandpass filter F1 to filter out useless wavelength components. It is then split by a beam splitter into a probe beam Probe 2ω (frequency doubled) and a reference beam Rreference 2ω (frequency doubled). The probe beam Probe 2ω is shaped into a circularly polarized beam by a quarter-wave plate QWP, and then combined with the pump beam Pump ω by a dichroic mirror DM. After the combination, it is reflected by a silver-coated reflector and then focused by a silver-coated concave mirror CM into a gas chamber filled with nitrogen. The pump beam first interacts with the nitrogen molecules to induce non-adiabatic arrangement of the nitrogen molecules. The probe beam enters the medium at the moment of arrangement recovery to generate a vortex beam.

[0047] Among them, the branch of the pump beam has a delay line device, which consists of a pair of reflectors with an angle of 90° and an electrically controlled translation stage. The translation stage has extremely high displacement accuracy and its minimum moving step is 0.1µm. By moving the translation stage, the delay between the pump beam and the detection beam can be changed. The reference beam Rreference 2ω is used to detect the topological charge of the vortex beam by small-angle non-parallel interference with the generated vortex beam.

[0048] 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.

[0049] 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 The radial vector beam ( ) driven by the first-order polarization tensor component in the light field cross section.

[0050] In order to verify the vortex beam generated by the above system, this embodiment uses the spatial polarization topological number The experiment is carried out with radial vector beams and right-handed circularly polarized beams: Measure the signal field under different pump beam-probe beam delays, Figure 3 The total intensity of the signal beam as a function of the time delay and the interference pattern of the signal field with 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 light field, the interference pattern of the signal beam with a Gaussian reference beam is measured at each time delay. Figure 4 The signal field and its interference pattern with the reference light under the half-reply period of 4.2ps under different pump pulse and probe pulse combinations are shown, where (a)-(b) the pump pulse is The results when the probe pulse is right-handed circularly polarized light, (c)-(d) the pump pulse is The results when the probe pulse is left-handed circularly polarized. (e)-(h) are the same as (a)-(d) but the pump pulse is The generated vortex beam has a classic donut-shaped intensity distribution, while the interference fringes are typical patterns of cross-interference between 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 dotted lines 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 To further verify the effect of doubling the topological charge during the interaction with the aligned molecules, a spatially polarized topological A 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 .

[0051] At the same time, the molecular wave plate is used to generate a few-cycle vortex pulse numerical simulation: Unlike conventional phase modulators such as spiral phase plates (SPPs), molecular wave plates overcome wavelength limitations and effectively avoid the topological charge dispersion problem that is usually caused by the wide bandwidth of ultrashort pulses. This approach has significant advantages in dealing with the limitations imposed by ultrashort pulses, providing 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 center wavelengths, vortex beams with the same topological charge can be generated, such as Figure 5 As shown, (a)-(c) are the intensity distributions of the vortex beams generated by the 266nm (a), 400nm (b) and 800nm ​​(c) detection lasers, respectively. (d)-(f) are the spatial phase distributions corresponding to (a)-(c).

[0052] In this simulation, a radially polarized vector beam pump pulse and a few-cycle (three optical cycles) 400 nm RCP probe pulse were used, as Figure 6 As shown, (a) is the electric field of the incident few-cycle RCP detection pulse, (b) is the few-cycle detection pulse spectrum of nitrogen molecules in this wavelength range and the frequency-dependent molecular polarizability, (c) is the theoretically calculated electric field isosurface of the few-cycle vortex pulse generated by the "molecular wave plate", (d)-(e) are the generated few-cycle vortex pulses at 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 The few-cycle LCP vortex beam [see Figure 6 (d)–(e)].

[0053] 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 the 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.

[0054] In this embodiment, since the molecules are arranged without field, the polarization direction of the pulse is re-arranged, and the spatial anisotropy modulation of the refractive index of the molecular ensemble is achieved, 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 and the topological charge number are generated. (in This method is applicable to a wide spectral range from ultraviolet to mid-infrared.

[0055] Moreover, by optimizing the gas pressure and interaction distance, a vortex beam conversion efficiency of up to 100% can be achieved. At the same time, 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, ultrashort vortex pulses.

[0056] Furthermore, it avoids the problems of bandwidth limitation, topological charge dispersion and character 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.

[0057] Those skilled in the art can understand that the above embodiments are specific embodiments of the present invention, and in practical applications, various changes can be made to them in form and detail without departing from the spirit and scope of the embodiments of the present invention. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the embodiments of the present invention, so the protection scope of the embodiments of the present invention shall be based on 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, detection 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 generating module is used to focus the vector light beam and the circularly polarized light beam together into an air chamber filled with nitrogen, and induce the non-adiabatic arrangement of nitrogen molecules in the air chamber by the vector light beam, so that the nitrogen molecules are arranged according to the polarization direction of the vector light beam in the recovery period, forming a plurality of molecular wave plates. After passing through the plurality of molecular wave plates, the circularly polarized light beam is modulated into a vortex light beam having 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 generating module is specifically used for regulating and delaying the vector light beam and the circularly polarized light beam before focusing the vector light beam and the circularly polarized light beam 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.

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 into a gas chamber filled with nitrogen through a silver-plated concave mirror with a focal length of 500 mm.

4. The vortex beam generating system according to claim 1, characterized in that: 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 air chamber and focusing the vector light beam to a length that induces the non-adiabatic arrangement effect of nitrogen molecules.

5. The vortex beam generating system according to claim 1, characterized in that: The system further comprises: a 60 / 40 beam splitter, which is used to split the laser beam into two, forming a pump beam having 60% of the energy of the laser beam and a detection beam having 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 light beam to 400nm, the bandpass filter is used to filter out wavelengths other than 400nm in the detection light beam after the frequency doubling, and the detection light shaping module is used to shape the detection light beam after the wavelength filtering process into a circularly polarized light 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 800nm ​​and a pulse width of 35fs, which is output by a femtosecond laser light source at a repetition rate of 1kHz.

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 in the fork interference pattern and the direction of the fork opening.

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