Metasurface device based on step structure and generation system of ultrafast holographic light field

Abstract

The invention relates to the technical field of light field regulation and control, in particular to a metasurface device based on a step structure and a generation system of an ultrafast holographic light field. According to the metasurface device based on the step structures, pulse delay and phase regulation and control are achieved through the thickness difference of the step structures, and multi-form space-time light fields such as space-time vortex light, space-time double-vortex light and space-time vortex ring light are generated through combination of different step structures and different focusing phase structures. The structure is simple, cost is low, expansion is easy, and experiment complexity is remarkably reduced compared with a traditional scheme.

Description

Technical Field

[0001] This invention relates to the field of light field manipulation technology, specifically to metasurface devices based on stepped structures and systems for generating ultrafast holographic light fields. Background Technology

[0002] In recent years, research on novel spatiotemporal optical vortices (STOVs) and spatiotemporal vortex rings has been continuously advancing. These types of optical fields have shown unique potential in ultrafast optical manipulation, information transmission, and high-dimensional encoding. However, most existing methods rely on the combined use of 4f systems and spatial light modulators. While offering flexibility, these systems are bulky and have limited stability, hindering miniaturization and integrated applications. Therefore, exploring compact schemes based on micro / nano-scale devices has become an important direction for advancing research on spatiotemporal vortices and vortex rings. Summary of the Invention

[0003] In view of this, the present invention provides metasurface devices based on stepped structures and a system for generating ultrafast holographic light fields to solve the technical problem of the lack of novel spatiotemporal beam manipulation based on micro-nano scale devices in the prior art.

[0004] The technical solution provided by this invention is as follows: In a first aspect, the present invention provides a metasurface device based on a stepped structure, comprising: a planar device, the planar device including a first surface and a second surface disposed opposite to each other, a focusing phase structure disposed on the first surface, and a stepped structure having a height difference disposed on the second surface; when a light beam passes through the first surface and the second surface of the planar device, the light beam is modulated by the stepped structure and the focusing phase structure to generate a spatiotemporal vortex light field or a spatiotemporal vortex ring light field.

[0005] In one optional embodiment, the step structure includes a single-step structure, which includes a first sub-surface and a second sub-surface that have a height difference and are parallel, and a third sub-surface that is perpendicular to the direction of the first sub-surface, the third sub-surface connecting the first sub-surface and the second sub-surface.

[0006] In one optional embodiment, the focusing phase structure includes a cylindrical focusing phase structure; when the step structure is a single-step structure and the focusing phase structure is a cylindrical focusing phase structure, the generated light field is a spatiotemporal vortex light field.

[0007] In one alternative embodiment, the stepped structure further includes a double-step structure, which includes a through groove or a through protrusion.

[0008] In one optional embodiment, the focusing phase structure includes a cylindrical focusing phase structure and a circular focusing phase structure; when the step structure is a double-step structure and the focusing phase structure is a cylindrical focusing phase structure, the generated light field is a spatiotemporal double vortex light field; when the step structure is a double-step structure and the focusing phase structure is a circular focusing phase structure, the generated light field is a curved spatiotemporal double vortex light field.

[0009] In one alternative embodiment, the step structure further includes a circular step structure, which includes a circular blind hole or a circular protrusion.

[0010] In one optional embodiment, the focusing phase structure includes a circular focusing phase structure; when the step structure is a circular step structure and the focusing phase structure is a circular focusing phase structure, the generated light field is a spatiotemporal vortex ring light field.

[0011] In one optional implementation, when the height difference of the stepped structure changes by more than a first threshold or less than a second threshold, the resulting spatiotemporal vortex light field or spatiotemporal vortex ring light field will change.

[0012] Secondly, the present invention provides a system for generating an ultrafast holographic light field. The system is based on the Mach-Zehnder interferometer principle and uses the metasurface devices of the first aspect and any one of the present invention to generate and measure a time-varying ultrafast holographic light field.

[0013] In one optional embodiment, the system includes: a laser unit for generating a laser beam with a pulse width less than a threshold; a beam splitting unit for splitting the laser beam into two laser beams, one as a reference beam and the other as a signal beam; an optical path adjustment unit for adjusting the optical path of the reference beam; a first polarization modulation unit for polarizing the signal beam and then illuminating the metasurface device; a second polarization modulation unit for polarizing the beam passing through the metasurface device; a beam combining unit for combining the optical path-adjusted reference beam and the beam modulated by the second polarization modulation unit; and a signal acquisition unit for receiving the combined beam to form interference fringes.

[0014] The technical solution of this invention has the following advantages: The metasurface device based on a stepped structure provided by this invention utilizes the thickness difference of the stepped structure to achieve pulse delay and phase modulation. By combining different stepped structures and different focusing phase structures, it can generate various forms of spatiotemporal optical fields, such as spatiotemporal vortex light, spatiotemporal double vortex light, and spatiotemporal vortex ring light. It features a simple structure, low cost, and easy scalability, significantly reducing experimental complexity compared to traditional methods. Attached Figure Description

[0015] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0016] Figure 1 A schematic diagram of a planar device containing a single-step structure; Figure 2 A schematic diagram illustrating the working principle of a planar device containing a single-step structure; Figure 3 A schematic diagram comparing the optical field distribution of planar devices with and without a single-step structure; Figure 4 A schematic diagram comparing the optical field distribution of a planar device with a single-step structure containing different step thicknesses; Figure 5 A comparative schematic diagram of the optical field distribution of a planar device with a single-step structure and a step thickness difference of half-wave condition; Figure 6 This is a structural block diagram of the ultrafast holographic light field generation system in an embodiment of the present invention; Figure 7 This is a schematic diagram of liquid crystal fabrication for a planar device including a single-step structure and a focusing phase structure, as shown in an embodiment of the present invention. Figure 8 A schematic diagram of theoretical and experimental results for a planar device containing a single-step structure at the focal plane. Figure 9 A schematic diagram showing the theoretical and experimental results of the position of a planar device containing a single-step structure before and after the focal plane; Figure 10 A schematic diagram of a planar device containing a double-step structure and a schematic diagram of liquid crystal processing; Figure 11 A schematic diagram illustrating the working principle of a planar device that includes a double-step structure and a cylindrical focusing phase structure; Figure 12 This is a schematic diagram of the theoretical results of the planar device including the double-step structure and the cylindrical focusing phase structure in the embodiment of the present invention at the focal plane position; Figure 13 A schematic diagram showing the theoretical and experimental results of the position of a planar device containing a double-step structure and a cylindrical focusing phase structure in front of the focal plane; Figure 14 A schematic diagram of theoretical and experimental results for the position of a planar device containing a double-step structure and a cylindrical focusing phase structure behind the focal plane; Figure 15A schematic diagram comparing the theoretical errors of the optical field distribution of a planar device containing a double-step structure and a cylindrical focusing phase structure; Figure 16 A schematic diagram illustrating the working principle of a planar device that includes a double-step structure and a circular focusing phase structure; Figure 17 A schematic diagram showing the theoretical and experimental results of the position of a planar device containing a double-step structure and a circular focusing phase structure in front of the focal plane; Figure 18 A schematic diagram illustrating the working principle of a planar device that includes a circular stepped structure and a circular focusing phase structure; Figure 19 A schematic diagram of a planar device containing a circular stepped structure and a schematic diagram of liquid crystal processing; Figure 20 A schematic diagram showing the theoretical and experimental results of the position of a planar device containing a circular stepped structure and a circular focusing phase structure in front of the focal plane; Figure 21 This is a schematic diagram of the theoretical and experimental results for the position of a planar device containing a circular stepped structure and a circular focusing phase structure behind the focal plane. Detailed Implementation

[0017] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0019] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can also refer to the internal connection of two components; and they can refer to a wireless connection or a wired connection. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0020] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0021] This invention provides a metasurface device based on a stepped structure, comprising: a planar device including a first surface and a second surface disposed opposite to each other; a focusing phase structure is disposed on the first surface, and a stepped structure with a height difference is disposed on the second surface; when a light beam passes through the first surface and the second surface of the planar device, the light beam is modulated by the stepped structure and the focusing phase structure to generate a spatiotemporal vortex light field or a spatiotemporal vortex ring light field. In this embodiment, a Gaussian femtosecond pulse is used as the incident light source, and the femtosecond pulse light generated by this incident light source is incident on the planar device.

[0022] Specifically, the planar device can be a transparent flat glass structure, and the focusing phase structure is a microstructure disposed on the surface of the planar device to achieve optical wave phase modulation. This microstructure includes multiple nanoscale units arranged in a preset manner. This focusing phase structure effectively achieves a focusing function similar to a lens. Because light travels at different speeds in media with different refractive indices, even if the optical path length is the same, the propagation time will differ. When the same femtosecond pulse light enters regions with different refractive indices simultaneously, the time it takes for the pulse to reach the receiving surface will also differ. The stepped structure set in this embodiment actually introduces a stepped thickness distribution in the planar device. When the light beam passes through the stepped structure with different thickness distributions, the thickness difference of the stepped structure can generate an equivalent phase difference. This phase difference... This can be expressed using the following formula:

[0023] In the formula, , λ represents the working wavelength of the light beam, and n represents the refractive index of the planar device. This represents the thickness difference of the stepped structure. According to the formula for this phase difference, it is slightly different for different wavelengths in the pulse. This phase dispersion effect is also one of the conditions that can generate spacetime vortices.

[0024] Through this phase difference Phase generated by combining focused phase structure The phase of this planar structure can then be expressed by the following formula:

[0025] In the formula, (x0, y0) represents the coordinates of any point on the focusing phase structure of the planar device.

[0026] According to the Fourier transform principle, the time-domain expression of the incident laser pulse... Through frequency domain electric field Represented as:

[0027] In the formula, t represents the pulse duration, and t0 represents the pulse width. Indicates the center angular frequency of the pulse. ω represents angular frequency, and IFT represents Fourier transform.

[0028] According to the Fresnel diffraction formula, the frequency domain light field distribution on the observation plane after the incident light passes through the focusing phase structure and the step structure can be expressed as:

[0029] In the formula, (x, y) represents the coordinates of the observation plane. Based on the Fourier transform relationship between the frequency domain and the time domain, the time-domain light field distribution on the observation plane can be expressed by the following formula:

[0030] Based on this temporal optical field distribution, holographic temporal optical field results for different Z-planes can be obtained.

[0031] In one alternative implementation, such as Figure 1 As shown in (a) to (d), the stepped structure includes a single-step structure, which includes a first sub-surface 11 and a second sub-surface 12 that are parallel and have a height difference, and a third sub-surface 13 that is perpendicular to the direction of the first sub-surface 11. The third sub-surface 13 connects the first sub-surface 11 and the second sub-surface 12. The focusing phase structure includes a cylindrical focusing phase structure; when the stepped structure is a single-step structure and the focusing phase structure is a cylindrical focusing phase structure, the generated light field is a spatiotemporal vortex light field.

[0032] in, Figure 1 Image (a) is a front view of a planar device containing a single-step structure. Figure 1 Image (b) is a side view of a planar device containing a single-step structure. Figure 1 Image (c) is a top view of a planar device containing a single-step structure. Figure 1Image (d) is a rear view of a planar device containing a single-step structure. Figure 1 In (d), Lx represents the diameter of the hologram constructed based on the cylindrical focusing phase structure. The phase of the cylindrical focusing phase structure is expressed by the following formula:

[0033] In the formula, f This indicates the focal length of the hologram.

[0034] For a single-step structure, in order to determine the phase at different locations, a step function is used to calculate the step position. The specific process is shown in the following formula:

[0035] In the formula, The coordinates of the location of the difference in elevation between the steps are: The scale parameter is represented by the theoretically calculated coordinates (x0, y0) of the step's transmittance range, which are the same as the coordinates of the holographic device. To achieve a phase delay of half a wavelength (integer wavelengths), the height difference of the step structure is set to... N represents the thickness coefficient. By selecting an appropriate N, the desired phase delay can be obtained. Then, this phase delay is combined with the function determined by the step function mentioned above. Multiply them to get the final result. .

[0036] Specifically, when the diameter Lx = 2.7 mm, the focal length of the hologram... The incident light source has a center wavelength of 800 nm and a speed of light of _____. A schematic diagram of a femtosecond pulse generated by incident light being incident on a planar device containing a single-step structure and a cylindrical focusing phase structure is shown below. Figure 2 As shown in (a) and (b). Among them, Figure 2 (a) is the main view. Figure 2 (b) is a side view. To further analyze the working principle of this planar device, the focal plane light field distribution of the planar device was theoretically calculated.

[0037] Figure 3 Figures (a1) to (d1) show the light field distribution on the planar device without a stepped structure. Figure 3 Figures (e1) to (h1) show the optical field distribution when a single-step structure is set on a planar device. Figure 3 In the middle (a1), the light intensity distribution on the y-ω plane is... Figure 3 In the diagram (a2), the phase distribution on the y-ω plane is shown. Figure 3 In the middle (b1), the light intensity distribution on the ky-ω surface is shown. Figure 3 In the middle (b2), the phase distribution of the ky-ω surface is shown. Figure 3In the middle (c1), the light intensity distribution on the yt plane is shown. Figure 3 In the middle (c2), the phase distribution of the yt plane is shown. Figure 3 (e1) to (h1) are the corresponding values ​​when a single-step structure is set. Figure 3 The corresponding schematic diagrams from (a1) to (d1) show the intensity and phase distributions of light on different surfaces. Additionally, the time range t in the diagram is 510 fs. The range is 2.299 rad / fs to 2.4133 rad / fs, the y range is 0.39 mm, the ky range is 16.11 rad / mm, the time unit is fs, and the distance unit is mm.

[0038] from Figure 3 As can be seen from (a1) to (d1), after the 120fs pulse light is focused by a cylindrical beam with a focal length of 650 mm (without spatial phase difference Δh), it exhibits a circularly symmetrical distribution on the yt plane. At this point, the light field is essentially a Gaussian distribution in the spatial, frequency, and time domains, and the phase of the light spot is uniform, consistent with conventional cylindrical focusing characteristics. When a single-step structure is set, as shown... Figure 3 As shown in (e1) to (h1), this represents the height difference of a single-step structure. Time field distribution results. From Figure 3 As can be seen from (e1) and (e2), the 0 / π phase transitions generated by the step at different frequencies are different. Figure 3 A tilted phase transition occurs in (e2). This phase transition occurs in Figure 3 The intensity corresponding to (e1) is a sloping line with zero intensity. Taking the spatial Fourier transform of this result with respect to y yields... Figure 3 The ky-ω plane results for (f1) and (f2) are a vortex optical field. Similarly, Figure 3 (e1) and Figure 3 Taking the Fourier transform of (e2) with respect to frequency ω, we get Figure 3 Zhong (g1) and Figure 3 The result of the yt spacetime plane in (g2) is also a vortex light field. Figure 3In the diagram (h1), the intensity of the three-dimensional isosurface is shown. In the time domain, the center of the light forms a cylindrical dark area parallel to the x-axis and perpendicular to the t-axis, demonstrating that a spatiotemporal vortex light field can be generated in the yt plane using this device. Related technologies have also mentioned generating vortex light fields in the time domain. However, these technologies involve loading a phase map onto a spatial light modulator to generate a vortex structure in the ky-ω plane, which is then projected onto the yt plane using a 4f system. To verify that the light field obtained in this embodiment is indeed a standard vortex light, a Fourier transform is performed on the core light field y-ω to obtain the ky-ω plane result. It can be seen that this frequency domain structure is equivalent to the vortex result obtained using a 4f system in related technologies, and the structure of this embodiment is simpler.

[0039] In one optional implementation, when the height difference of the stepped structure changes by more than a first threshold or less than a second threshold, the resulting spatiotemporal vortex light field or spatiotemporal vortex ring light field changes. That is, regardless of whether the step thickness changes significantly or slightly, it has a significant impact on the light field.

[0040] Specifically, theoretical analysis shows that... This thickness can be used as a standard thickness. To analyze the changes in the optical field when the step thickness varies significantly, the optical field of thinner and thicker step thicknesses was analyzed based on the standard thickness. Figure 4 The middle (a1) to (d1) sections represent thinner steps on a planar device. The light field distribution at time, and various figures. Figure 3 The numbers (a1) to (d1) correspond, and no parameters are labeled. Figure 3 The values ​​(a1) to (d1) correspond to the same values. It can be observed that, relative to the standard thickness, the light field at the thinner step thickness on the yt surface is not completely closed, but the central dark region still forms a complete vortex. From the three-dimensional isosurface... Figure 4 As can be seen in (d1), the vortex characteristics are still obvious. However, as... Figure 4 As shown in (a2), due to the thinness of the step, the tilt angle of the 0-π phase boundary line of the y-ω plane increases.

[0041] Figure 4 The middle (e1) to (h1) sections represent thinner steps on a planar device. The light field distribution at time, and various figures. Figure 4 The corresponding values ​​are 4(d1) to 4(a1). In this case, the 0-π phase boundary line of the y-ω plane is almost vertical, while the closing direction of the yt plane is opposite to that of the thin step case (e.g., Figure 4 As shown in (g1), it presents the same as Figure 3The opposite closing trend is observed in (c1). This demonstrates that the step thickness directly affects the closure of the vortex beam in the yt plane and the tilt angle of the phase distribution in the y-ω plane. Thinner steps tend to produce tilted boundary lines, while thicker steps make the boundary lines closer to vertical, while also altering the closure direction. These characteristics provide a theoretical basis for further designing metasurface devices with specific time-domain vortex structures.

[0042] To analyze the changes in the optical field when the step thickness changes slightly, the influence of a slight change in step thickness on the temporal vortex light was analyzed based on the standard thickness. Figure 5 The thickness difference between (a1) and (d1) represents the step thickness on the planar device. The distribution of the light field at that time, Figure 5 The thickness difference between (e1) and (h1) represents the step thickness on the planar device. The light field distribution at time, and various figures. Figure 4 The correspondence between (a1) and (d1) is as follows, with unlabeled parameters. Figure 3 The values ​​(a1) to (d1) correspond to the same value. This is compared to the standard thickness. ,from Figure 5 As can be seen from (c1), in reducing approximately After the optical path difference, the dark area at the center of the yt plane shifts upward. Conversely, when the step thickness increases by approximately to At that time, the dark area of ​​the light spot shifts downwards. This demonstrates that even a small change in step thickness (less than half a wave) significantly affects the position of the dark area at the center of the time-domain vortex beam and the light field distribution. In practical applications, the effect of even a small change in step thickness on the time-domain vortex beam is one of the most difficult factors to control in experimental fabrication; therefore, this is the key reason for precisely controlling the step thickness in experiments.

[0043] This embodiment also provides a system for generating an ultrafast holographic light field. The system is based on the Mach-Zehnder interferometer principle and uses the metasurface device described in the above embodiment to generate and measure a time-varying ultrafast holographic light field. Specifically, this embodiment, based on the Mach-Zehnder interferometer principle and combined with a time-domain scanning measurement method, uses optical interference to achieve precise measurement of the phase and intensity of the light field in the time domain of a circularly polarized single pulse after it has been modulated by a holographic device.

[0044] In one optional embodiment, the system includes: a laser unit for generating a laser beam with a pulse width less than a threshold; a beam splitting unit for splitting the laser beam into two laser beams, one as a reference beam and the other as a signal beam; an optical path adjustment unit for adjusting the optical path of the reference beam; a first polarization modulation unit for polarizing the signal beam and then illuminating the metasurface device; a second polarization modulation unit for polarizing the beam passing through the metasurface device; a beam combining unit for combining the optical path-adjusted reference beam and the beam modulated by the second polarization modulation unit; and a signal acquisition unit for receiving the combined beam to form interference fringes.

[0045] Specifically, such as Figure 6 As shown, this embodiment uses the center wavelength. A femtosecond laser with a pulse width of approximately 120 fs serves as the laser unit. The beam-splitting unit includes a first beam-splitter BS1 and a second beam-splitter BS2; the optical path adjustment unit includes a movable optical delay line; the first polarization modulation unit includes a first quarter-wave plate QWP1; the second polarization modulation unit includes a second tenth-wave plate QWP2; the beam-combining unit includes a third beam-splitter BS3; and the signal acquisition unit includes a CCD. Additionally, the system includes a first reflecting mirror M1 and a second reflecting mirror M2 for changing the optical path direction.

[0046] The laser output is linearly polarized light, which, after collimation, is incident on the first beam splitter BS1 and split into two paths: a reference path and a signal path. For the reference path, the beam is split by the second beam splitter BS2, passes through a movable optical delay line, is reflected back by the second mirror M2, and finally guided into the CCD by the third beam splitter BS3, serving as the reference light for interferometry. The precise displacement of the delay line allows the reference pulse to progressively scan the measured optical field over time, thus achieving time-domain resolution.

[0047] For the signal optical path (tuning optical path), the beam is guided by the first reflecting mirror M1 and then converted into circularly polarized light by the first quarter-wave plate QWP1. At the sample, the light field is modulated by a pre-designed metasurface holographic structure, generating a time-domain dynamic holographic light field. The modulated beam then passes through the second quarter-wave plate QWP2 to filter out specific polarization components, thereby enhancing the signal-to-noise ratio. Finally, the beam passes through a beam splitter into the CCD, forming interference fringes with the reference light.

[0048] When two beams of light have an appropriate angle in the CCD plane and the optical path difference is matched, a clear and stable interference fringe pattern can be obtained. By gradually moving the optical delay line, a relative scan of the reference pulse and the measured optical field in the time domain is achieved, thereby acquiring a series of time-domain interferograms. Based on frame-by-frame acquisition and phase reconstruction of the time-domain interference fringes, the temporal evolution information of the manipulated ultrafast optical field can be obtained. The construction of this experimental system provides key support for subsequent verification of the temporal manipulation of ultrafast holographic optical fields, enabling direct comparison between theoretical simulations and actual experimental results.

[0049] Furthermore, this embodiment uses ultra-white glass with a refractive index of 1.5341 to fabricate a metasurface device with a stepped structure. This device exhibits high transmittance in the 320nm-2500nm range and excellent light transmission performance in the visible light region. To save materials and improve experimental efficiency, this embodiment fabricates multiple metasurface structures of size Lx at different step height differences in a single process and then develops them simultaneously. The theoretical design parameters are as follows: ,therefore However, there will be issues during glass sheet processing. Processing errors: When using hydrofluoric acid for wet subtractive processing, there are uneven thicknesses at different cross-sectional positions. Therefore, a large number of comparative experiments are conducted to compensate for this error.

[0050] When the focusing surface structure is fabricated using liquid crystal materials, the fabrication process of this metasurface device is as follows: The B270 glass substrate containing the stepped structure was cleaned. A 1% concentration SD1 solution (N,N-dimethylformamide solution) was spin-coated at 3000 rpm for 30 seconds. It was then baked on a hot plate at 100°C for 10 minutes to completely remove residual N,N-dimethylformamide solvent. The liquid crystal cells were patterned and exposed using a digital micromirror device (DMD). By rotating the polarizer in front of the liquid crystal cells, patterns with different polarization azimuth angles were sequentially projected onto the liquid crystal cells to achieve the desired azimuth angle distribution. Liquid crystal mixture filling and alignment: LCP (Liquid Crystal Polymer) was coated onto the DMD-processed surface using spin-coating at 3000 rpm, 3000 rpm, and 2000 rpm. After each spin-coating, the adhesive was cured by irradiation with a 365nm UV LED to form walls and maintain the gaps between the liquid crystal cells. Through the above process, a liquid crystal polymer (LCP) was obtained. Figure 7 The metasurface device shown has a liquid crystal structure corresponding to a cylindrical focusing phase and a circular focusing phase with a focal length of f=650mm, and a diameter of Lx=2.7mm.

[0051] In addition, it should be noted that the generation of the light field is independent of the specific material used and can be achieved based on various metasurface platforms. Although this embodiment uses liquid crystal material for device fabrication, the same design concept is also applicable to other media or nanostructure metasurfaces, which will not be elaborated here.

[0052] Based on this ultrafast holographic light field generation system, the light field generated by the prepared metasurface device sample can be measured. Specifically, Figure 8 A1 to A5 are for single-step structures. Theoretical calculations of the optical field in the yt plane at position z=f for a cylindrical focusing phase structure with a focal length of 650mm are presented. Figure 8 (a1) represents the light intensity distribution. Figure 8 (a2) and Figure 8 In the middle (a3), the phase distribution of the vortex beam's yt surface is shown. Figure 8 (a2) and Figure 8 The results in (a3) ​​show that the light spot exhibits a counterclockwise rotating vortex structure in the yt plane; Figure 8 (a4) and Figure 8 Image (a5) shows the intensity maps of the (x, y, t) three-dimensional isosurface from different angles. Figure 8 In diagram (a5), a "dark rod" can be found in the central dark region of the three-dimensional isosurface, corresponding to the existence of a phase singularity in the optical vortex. Experimental observations corresponding to the theoretical simulation results are as follows: Figure 8 As shown in (c1) to (c5), to facilitate understanding of the structural characteristics of vortex light, the xy-plane interferogram under time-domain measurement is further presented. The phase behavior observed in the experiment is highly consistent with the numerical simulation results, as shown in... Figure 8 (b1) to (b5) and Figure 8 As shown in (d1) to (d5).

[0053] During the time-domain interferometric fringe imaging process, the displacement stage of the interferometric system scans within the range of 0–0.078 mm, with a step size of [missing value]. The total time span was approximately 520 fs, which is equivalent to taking one image every approximately 2.58 fs, resulting in a total of 201 interferograms. Figure 8 Figures (b1) to (b5) show the xy-plane interferograms at five typical moments. Each interferogram contains the interference of a Gaussian pulse and a modulated vortex beam, yielding xy-plane fringe information. To effectively extract dynamic holographic information from the interferograms, a frequency domain separation method based on Fourier optics was employed in the experiment.

[0054] The specific steps are as follows: First, for Figure 8Perform a two-dimensional Fourier transform on a single-frame interference pattern among (d1) to (d5), converting the interference information from the spatial domain to the frequency domain, thereby clearly showing the distribution characteristics of the 0th, positive 1st, and negative 1st levels of the interference fringes in the frequency domain. Secondly, extract the positive 1st level information in the frequency domain and perform reconstruction. Since the positive 1st level data contains the most complete and clearest information of the measured optical field, extracting this frequency domain information and performing an inverse Fourier transform on it can restore the light intensity and phase distributions in the spatial domain. Based on this method, 200 interference patterns were processed in the same way, thus obtaining the dynamic change results of the entire time domain process of x-y-t. Intercept the position of x = 0, and the y-t light spot as shown in Figure 8 from (c1) to (c5) is obtained. This shows that the structure proposed in this embodiment can effectively generate and maintain a stable vortex optical field at the focal plane position. Among them, Figure 8 in (a1) to (d5), the theoretical calculation time range t is 510 fs, and the y range is 0.39 mm; the experimental measurement time range is 520 fs, and the y range is 1 mm. The time unit is fs, and the distance unit is mm.

[0055] After completing the calculation of the time domain characteristics at the focal plane z = f, to further verify the stability of the vortex light during propagation, the observation points are extended to positions before and after the focal plane (i.e., z < f and z > f). The theoretical analysis results show that at different z positions, the optical field still maintains the vortex characteristics, that is, there is always a helical phase structure. However, compared with the focal plane, the intensity distributions of the light spots obtained before and after the focal plane all show a certain degree of distortion. As shown in Figure 9 from (a1) to (a4), for at, the theoretical calculation results of the optical field at z = 550 mm are shown. Among them, Figure 9 in (a1) represents the light intensity distribution on the y-t plane, Figure 9 in (a2) represents the phase distribution on the y-t plane, Figure 9 in (a3) and Figure 9 in (a4) are the x-y-t three-dimensional isosurface intensity maps; Figure 9 [[ID=zo]]in (b1) to (b4) are the corresponding results at z = 750 mm. It can be seen that when z < f, the intensity distribution of the light spot is significantly distorted, and its uniformity is significantly lower than that at the focal plane ( Figure 8 in (a4)). And at the position of z > f, the light spot also shows a distorted phenomenon, but the distortion direction is opposite to that when z < f, which indicates that the focal plane position has a significant impact on the uniformity and symmetry of the vortex light intensity distribution. Figure 9 in (a1) to (b4), the theoretical calculation time range t is 510 fs, and the y range is 0.39 mm; the time unit is fs, and the distance unit is mm.

[0056] From a physics perspective, although the shape of the light spot will be distorted before and after the focal plane, the phase singularity of the vortex light always exists, indicating that its topological characteristics are relatively stable during propagation. The difference in intensity distribution mainly comes from the diffraction effect during propagation, which further shows that even at positions away from the focal plane, the vortex light can still maintain the basic characteristics of orbital angular momentum. For ease of comparison, the light spot size in the figure is shown using the yt section at y=0.39mm.

[0057] In an optional embodiment, the stepped structure further includes a double-stepped structure, which comprises a through groove or a through protrusion. When the double-stepped structure includes a through groove, its structure is as follows: Figure 10 As shown in (a) to (d). Figure 10 Image (a) is a front view of a planar device containing a double-step structure. Figure 10 Image (b) is a side view of a planar device containing a double-step structure. Figure 10 Image (c) is a top view of a planar device containing a double-step structure. Figure 10 Image (d) is a rear view of a planar device containing a double-step structure. This double-step structure includes a width of [missing information - likely a measurement] designed and fabricated at the center of the planar device. A rectangular groove, maintaining a groove thickness of This structure is equivalent to introducing two independent phase steps simultaneously into the light field. Based on this double-step structure, this embodiment fabricated a corresponding planar device sample and processed a liquid crystal metasurface structure on its back side, wherein the groove depth is approximately... The width is approximately Corresponding phase delay The diameter of the liquid crystal structure is Lx = 2.7 mm, corresponding to a cylindrical focusing phase with a focal length f = 650 mm. To improve experimental efficiency, multiple metasurface structures of size Lx are fabricated simultaneously on the same plane and developed concurrently in subsequent processes, such as... Figure 10 As shown in (e). Figure 10 Image (e) shows the developed liquid crystal device, where the focusing phase structure corresponds to the cylindrical and circular focusing phases with a focal length f = 650 mm, the hologram diameter Lx = 2.7 mm, and the stepped structure. , .

[0058] Based on this double-step structure, the spatiotemporal distribution of the light field at the focal plane and in front of and behind the focal plane was numerically simulated and experimentally verified. The results show that the double-step structure can introduce more complex vortex manipulation characteristics into the light field, providing an effective solution for the generation of multi-vortex ultrafast light fields.

[0059] When a double-step structure is used, a step function is employed to calculate the step position in order to determine the phase at different locations. The specific process is shown in the following formula:

[0060] Wherein, the width of the rectangle is x1 = 0 is the axis of symmetry at the location of the rectangle. The phase delay caused by the step function f2 and the height difference is considered. Multiply them to get the final result. Then change the function. Adding this to the cylindrical focusing phase formula, and combining it with the aforementioned time-domain to frequency-domain transformation formula and frequency-domain light field distribution formula, the final time-domain light field can be obtained. Specifically, when the hologram diameter Lx = 2.7 mm, the hologram focal length f = 650 mm, the incident light source center wavelength is 800 nm, and the speed of light is... A schematic diagram of a femtosecond pulse generated by incident light being incident on a planar device containing a double-step structure and a cylindrical focusing phase structure is shown below. Figure 11 As shown in (a) and (b). Among them, Figure 11 (a) is the main view. Figure 11 (b) is a side view.

[0061] In one optional implementation, when the stepped structure is a double-stepped structure and the focusing phase structure is a cylindrical focusing phase structure, the generated light field is a spatiotemporal double vortex light field. Figure 12 Tables (a1) to (a5) present the theoretical calculation results of the optical field in the yt plane at position z=f for a double-step structure and a cylindrical focusing phase structure with a focal length of 650mm. Figure 12 In the middle (a1), the light intensity distribution on the yt plane is shown. Figure 12 (a2) and Figure 12 In the middle (a3), the phase distribution of the yt plane is shown. Figure 12 (a4) and Figure 12 (a5) is the intensity map of the three-dimensional isosurface of xyt. Figure 12 In diagrams (a1) to (a5), the theoretically calculated time range t is 510 fs, and the y range is 0.39 mm; the time unit is fs, and the distance unit is mm. As can be seen from the figures, the double-step structure cannot form a completely closed dark vortex light field on the focal plane. Figure 12 In the phase distribution shown in (a3), 0-π dislocations are clearly observed, and the isosurface plot fails to present a typical dark vortex structure. This indicates that, compared to a single step, the double step is not located at the beam focusing center, and the two step optical fields influence each other. Therefore, the double step device cannot directly generate an ideal vortex optical field at the focal plane.

[0062] Furthermore, the research scope is extended to a position 200 mm before the focal plane. Figure 13 In (a1) to (b5), for the double-step structure and the cylindrical focusing phase structure with a focal length of 650 mm, the theoretical calculation results of the optical field in the y-t plane at the position where z < f (z = 450 mm) are shown. And Figure 13 In (a1) to (d5), the theoretical calculation time range t is 510 fs, and the y range is 0.39 mm; the theoretical results in the figure show that the double-step structure can effectively generate two spatially separated vortex optical fields and maintain good topological stability during propagation. This provides a reliable theoretical basis for subsequent experimental verification. At the position where z < f, two independent phase singularities can be clearly identified, indicating that the double-step device indeed introduces double vortices in the same optical field. Compared with the single-step structure, the optical field generated by the double-step shows significant double-vortex characteristics in the intensity distribution, while the phase diagram exhibits two helical phase distributions with opposite directions and independent of each other. However, a certain degree of distortion can also be found from the intensity distribution: the two dark regions are not completely surrounded by a uniform light intensity and show partial openings. But when the light intensity threshold is reduced, two complete and closed dark regions can still be observed. This shows that the vortex light is affected by the diffraction effect during propagation, yet the positions of the phase singularities remain stable and do not affect the overall vortex structure characteristics.

[0063] After the theoretical simulation was completed, the double-step device was further experimentally verified. The same system as in the single-step research part was used in the experiment. Interference fringes at different delay positions were obtained by scanning with a displacement stage, and the intensity and phase distributions of the optical field were extracted using the frequency-domain separation method of Fourier optics. The experimental results are as Figure 13 shown in (c1) to (d5), where , , the experimental measurement time range is 520 fs, the y range is 3 mm, the time unit is fs, and the distance unit is mm. Corresponding to the observation position where z < f. Two independent phase singularities can be clearly seen, which are highly consistent with the theoretical results Figure 13 in (a1) to (a3). Especially in the phase structure shown in (a2) Figure 13 , the upper half of the y-t plane shows a counterclockwise vortex, while the lower half shows a clockwise vortex, clearly verifying that the double-step structure can stably generate a double-vortex optical field in the same spatio-temporal optical field.

[0064] Figure 14 In (a1) to (b5), for the double-step structure ( , The theoretical calculation results of the light field in the yt plane were obtained for a cylindrical focusing phase structure with a focal length of 650mm, at a position where z is greater than f (z=850mm). The theoretical calculation time range is t=510fs, and the y range is 0.39mm. It can be seen that... Figure 14 In the middle (a1), the light intensity distribution still contains two dark areas, corresponding to two approximate vortex structures; while Figure 14 (a2) and Figure 14 The phase distribution in (a3) ​​is the opposite of that in front of the focal plane: the upper vortex is twisted clockwise, and the lower vortex is twisted counterclockwise, forming a symmetrical structure. This reversal phenomenon stems from the opposite direction of light propagation before and after the focal plane. In the interference results... Figure 14 In (b3), the intersecting interference fringes at the vortex inflection point can also be observed, further verifying this conclusion.

[0065] Experimental observation results are as follows Figure 14 As shown in (c1) to (d5), , The experimental measurement time range was 520 fs, and the y-range was 3.4 mm. All time units were fs, and all distance units were mm. It can be seen that the experimental observations and theoretical results are highly consistent in overall trend. (Comparison) Figure 14 (a3) and Figure 14 In Figure (c3), it can be clearly seen that the rotation direction of the vortex on the yt plane is indeed opposite to that before the focal plane, which is directly related to the difference in the propagation direction of light on both sides of the focal plane. These experimental phenomena fully demonstrate the rationality of the double-step structure design and the effectiveness of the liquid crystal device fabrication. It should be noted that there are still some differences between the experiment and the theory in the details of the light intensity distribution, which may mainly stem from factors such as the limited surface processing precision of the device, insufficient uniformity of the liquid crystal layer, and the stability of the experimental system. However, overall, the experimental results have well verified the theoretical expectations, the rationality of the double-step structure design, and the effectiveness of the liquid crystal device fabrication, laying the foundation for the subsequent realization of more complex light field manipulation. It should be noted that there are still some differences between the experiment and the theory in the local details of the light intensity distribution, which may mainly stem from factors such as the limited surface processing precision of the device, insufficient thickness and uniformity of the liquid crystal layer, and the environmental stability of the experimental system.

[0066] In the theoretical calculations above, the design parameters used were: , , In actual device fabrication, the step thickness of the glass substrate is... Meanwhile, the width of the glass groove is , there are also certain deviations in the actual manufacturing process, and at the same time, it is necessary to calculate in combination with the waist radius of the pulsed light source. Therefore, it is necessary to conduct error analysis on factors such as the step thickness, the gap width between the double steps, and the experimental measurement position, etc., to evaluate the impact of these processing and measurement uncertainties on the experimental results.

[0067] Figure 15 Figures (a1) to (d5) in it give the theoretical error comparison results at the position in front of the focal plane (z < f), where Figure 15 Figures (a1) to (a5) in it are the original comparison data under the reference parameters ( , , z = 450 mm). Figure 15 In figures (a1) to (d5) in it, the theoretical calculation time range t is 510 fs, and the y range is 0.39 mm; the time unit is fs, and the distance unit is mm. <L

[0068] First, change the z position. As Figure 15 shown in figures (b1) to (b5) in it, when z = 550 mm (other parameters are the original data), by comparing Figure 15 figure (a1) in it and Figure 15 figure (b1) in it, it can be observed that the overall intensity of the light spot changes significantly, but the phase rotation direction remains the same. At the same time, in the comparison between Figure 15 figure (a4) in it and Figure 15 figure (b4) in it, it can be seen that the vortex structure is slightly distorted and the spacing between the double vortices decreases.

[0069] Secondly, change the step thickness. As Figure 15 shown in figures (c1) to (c5) in it, when (other parameters are the original data), by comparing Figure 15 figure (a1) in it and Figure 15 figure (c1) in it, it is found that the light spot is split into two parts on the left and right. This is mainly because the too thick glass further increases the phase delay difference between the center and the edge, resulting in an enhanced separation degree of the energy distribution during the y - t evolution. However, it should be noted that the topological structure of the phase vortex still remains stable.

[0070] Finally, change the width between the double steps. As Figure 15 shown in figures (d1) to Figure 15 figures (d5) in it, when increases to (other parameters are the original data), by comparing Figure 15 figure (a1) in it and Figure 15 figure (d1) in it, it is found that the light energy transmitted through the gap position at the center of the light spot increases significantly. This is because the thin step part occupies a wider area in the transverse direction. At the same time, by comparing Figure 15 figure (a4) in it and Figure 15 In (d4), a certain degree of shift in the relative position between the two vortices can also be observed.

[0071] In one optional implementation, when the stepped structure is a double-stepped structure and the focusing phase structure is a circular focusing phase structure, the generated light field is a curved spatiotemporal double-vortex light field. Specifically, the evolution law of the vortex light field under the condition of one-to-one correspondence between the stepped shape and the liquid crystal structure was analyzed above, focusing on the influence of thickness, width, and measurement position error on the spot distribution and phase topology. However, these analyses are all based on the cylindrical focusing phase, whose phase distribution changes linearly in the lateral direction. In order to further investigate the influence of the focusing phase type on the spatiotemporal vortex evolution characteristics, a circular focusing phase structure was introduced on this basis, and theoretical and experimental studies were carried out in conjunction with a double-stepped liquid crystal device.

[0072] Based on the prototype focusing phase structure, this embodiment designs and fabricates a double-step liquid crystal device with an equivalent focal length of f=650mm, such as... Figure 16 (a) and Figure 16 As shown in (b), Lx = 2.7 mm, step structure. , ,in, Figure 16 (a) is the main view. Figure 16 (b) is a side view. It can be seen that, unlike the cylindrical focusing phase, the circular focusing phase exhibits a radially symmetrical phase distribution in the edge region, resulting in a more concentrated energy field during propagation. Therefore, compared to the double-vortex structure under cylindrical focusing, the circular focusing phase is expected to introduce a significantly curved vortex structure and may exhibit new temporal and genomic control patterns before and after the focal plane, suggesting that the double vortex will exhibit a curved distribution near the focal plane, and may even be accompanied by certain trajectory evolution. To verify this prediction, the device was experimentally fabricated and measured, and the results were systematically compared with theoretical calculations.

[0073] Figure 17 (a1) to (b5) are for double-step structures ( , The theoretical calculation results of the light field in the yt plane at a position where z is less than f (z=450mm) are shown, along with a circular focusing phase structure with a focal length of 650mm. Experimental observation results are as follows: Figure 17 As shown in (c1) to (d5), , The theoretical calculation time range is 510 fs, and the y range is 0.49 mm; the experimental measurement time range is 520 fs, and the y range is 3.4 mm. The time unit is fs, and the distance unit is mm.

[0074] As can be seen, several significant features can be observed when comparing the results with those obtained under the aforementioned cylindrical focusing phase condition. First, from the perspective of the two-dimensional yt intensity distribution, the overall energy distribution pattern remains basically consistent (see...). Figure 17 (a1)). However, if we further examine the distribution of three-dimensional isosurfaces ( Figure 17 In Figure (a4), a significant difference can be clearly observed: under the influence of a circular focusing phase, the red high-intensity region evolves from an open state to a closed state, and the vortex structure changes from a horizontal arrangement to a curved arrangement. This phenomenon can be attributed to the difference in edge phase distribution characteristics between the circular and cylindrical focusing phases, the direct consequence of which is a significant bending of the vortex structure during spatial evolution. The experimental observations and theoretical simulations show a high degree of consistency in light intensity distribution and structural morphology, further verifying the rationality of the above physical analysis.

[0075] In summary, by rationally designing and optimizing the phase distribution of liquid crystals, the controllable arrangement transformation of the double-vortex structure in the spatiotemporal domain can be effectively achieved: the evolution from horizontal alignment to curved alignment. This result not only demonstrates a novel method for manipulating vortex light fields but also provides theoretical and experimental basis for the construction of more complex spatiotemporal vortex structures, which is of great significance for expanding ultrafast light field manipulation techniques.

[0076] In an optional embodiment, the stepped structure further includes a circular stepped structure, which comprises a circular blind hole or a circular protrusion. Specifically, to determine the role of the circular stepped structure in spatiotemporal vortex manipulation, this embodiment processes a diameter of [missing information] at the center of the step. A cylindrical recess (i.e., a circular blind aperture) is used to obtain a standard vortex ring optical field. Devices containing a circular focusing phase (focal length 650mm) and a cylindrical recess include... Figure 18 (a) and Figure 18 As shown in (b), Lx = 2.7 mm, step structure. , ,in, Figure 18 (a) is the main view. Figure 18 (b) is a side view.

[0077] Figure 19 Figures (a) to (e) show schematic diagrams of a stepped liquid crystal device. To achieve this structure, circular steps are fabricated on a glass substrate, wherein... Figure 19 Image (a) is a front view of a planar device containing a circular stepped structure. Figure 19 Image (b) is a side view of a planar device containing a circular stepped structure. Figure 19 Image (c) is a top view of a planar device containing a circular stepped structure. Figure 19Image (d) is a rear view of a planar device containing a circular stepped structure. The cylindrical groove in the circular stepped structure has a depth of approximately... Corresponding phase delay The diameter of the groove is approximately ; Figure 19 In the middle (e), a liquid crystal holographic structure is processed on the back of the step, with a diameter Lx=2.7mm and a circular focusing phase corresponding to a focal length f=650mm.

[0078] When a circular stepped structure is used, a step function is employed to calculate the step position in order to determine the phase at different locations. The specific process is shown in the following formula:

[0079] Wherein, the radius of the cylindrical cross-section is The circular location is at coordinates x1 = 0, y1 = 0. The phase delay caused by the height difference is calculated using the step function f3. Multiply them to get the final result. Then change the function. Adding the circular focusing phase formula to the aforementioned time-domain-frequency domain transformation formula and frequency-domain optical field distribution formula, the final time-domain optical field can be obtained. Specifically, the circular focusing phase formula is expressed as follows:

[0080] In one optional embodiment, when the stepped structure is a circular stepped structure and the focusing phase structure is a circular focusing phase structure, the generated light field is a spatiotemporal vortex ring light field. Specifically, after the device is fabricated, the spatiotemporal distribution of the light field of the circular stepped liquid crystal device at the focal plane and its front and rear positions is theoretically simulated and experimentally measured, and the modulation characteristics of its vortex field are analyzed.

[0081] in, Figure 20 (a1) to (b5) are for circular stepped structures ( , The theoretical calculation results of the light field in the yt plane at a position where z is less than f (z=450mm) are shown, along with a circular focusing phase structure with a focal length of 650mm. Experimental observation results are as follows: Figure 20 As shown in (c1) to (d5), , The theoretical calculation time range is 510 fs, and the y range is 0.49 mm; the experimental measurement time range is 780 fs, and the y range is 3.3 mm. The time unit is fs, and the distance unit is mm.

[0082] based on Figure 20From (a1) to (b5), the distribution on the yt surface shows that the light intensity gradually converges towards the center during propagation. This is a result of the circular focusing phase effect, making the energy distribution more concentrated. When the x=0 section is taken, the position of the double vortex is similar to the double-step structure. However, as the x section gradually approaches the edge, as... Figure 20 (a2) and Figure 20 As shown in (a3), the clockwise and counterclockwise inflection point vortices in the phase structure of the light spot are close together, as... Figure 20 (a4) and Figure 20 As shown in (a5), when the distribution is extended to the full xyt view, a complete annular dark region can be clearly observed. The phase distribution of this dark region covers 0-2π, indicating that the device has achieved the expected vortex annular field structure. Figure 20 The lateral view distribution is given in (a4), while Figure 20 The magnified view (a5) shows the frontal structure of the dark area, presenting the phase distribution of the vortex rings more intuitively.

[0083] based on Figure 20 As can be seen from (c1) to (d5), the light spot in the experimental side view exhibits some distortion due to factors such as beam quality inhomogeneity, liquid crystal layer thickness error, and limitations in the machining accuracy of the circular groove. However, the overall evolution trend is consistent with theoretical calculations, especially in... Figure 20 In the front view of (c5), a relatively standard annular dark area and corresponding phase characteristics can still be identified. This indicates that even with processing and experimental errors, the stepped liquid crystal device can still effectively achieve a stable vortex ring light field before the focal point.

[0084] To further verify the light field distribution characteristics of the circular stepped liquid crystal device behind the focal plane, theoretical calculations and experimental observations were performed on the field distribution behind the focal length. Figure 21 (a1) to (b5) are for circular stepped structures ( , The theoretical calculations of the optical field in the yt plane at a position z greater than f (z=850mm) using a circular focusing phase structure with a focal length of 650mm are presented. Experimental observations are as follows. Figure 21 As shown in (c1) to (d5), , The theoretical calculation time range is 510 fs, and the y range is 0.49 mm; the experimental measurement time range is 780 fs, and the y range is 2.2 mm. The time unit is fs, and the distance unit is mm.

[0085] from Figure 21 As can be seen from (a1) to (b5), compared with the result before the focal plane, the phase circumference direction of the vortex rings in the light field behind the focal plane is reversed, indicating that the phase structure has a symmetrical propagation characteristic under the focusing effect of the lens. Figure 21As can be seen from (c1) to (d5), the intensity distribution of the light spot in the experiment differs somewhat from the theoretical simulation. This deviation may mainly stem from the measurement error of the position z behind the focal plane. In the side view, there is a small gap in the vortex ring, which indicates an error in the actual fabrication of the liquid crystal step thickness and width. In this experiment, the measured parameters of the liquid crystal step are as follows: , As can be seen, the overall trend is still consistent with the theoretical calculation. The experimental results verify that the circular step liquid crystal device can also form a stable vortex ring light field behind the focal plane, and reflect the symmetrical characteristics of the vortex phase structure before and after the focal plane.

[0086] During the research, it was found that changing the stepped structure from a groove to a convex shape resulted in a reversal of the rotation direction of the spacetime vortex rings, but the rest of the light field evolution patterns remained consistent with the current phenomena. The relevant stepped structure and the resulting light field structure are not shown in this embodiment.

[0087] This invention achieves the generation and evolution of spatiotemporal optical vortices (STOV) and vortex ring optical fields under single-pulse conditions. This metasurface device, by designing step thickness differences of varying heights in space, effectively introduces a controllable phase delay in the time domain, thereby achieving temporal modulation of femtosecond pulses. Experiments successfully generated time-evolving vortex optical fields, double vortex optical fields with bending of the topological ring in the time domain, and vortex ring optical fields of a circularly stepped metalens structure. The experimental results are highly consistent with theoretical calculations. This structure can utilize various planar metalens materials, demonstrating the universality of combining planar focusing lenses with stepped delay structures in ultrafast spatiotemporal optical field manipulation, and breaking through the technical bottleneck of generating and controlling femtosecond spatiotemporal vortex optical fields with monolithic devices.

[0088] While exemplary embodiments and their advantages have been described in detail, those skilled in the art can make various changes, substitutions, and modifications to these embodiments without departing from the spirit of the invention and the scope of protection defined by the appended claims. Such modifications and variations all fall within the scope defined by the appended claims. For other examples, those skilled in the art should readily understand that the order of process steps can be changed while remaining within the scope of the invention.

[0089] Furthermore, the scope of this invention is not limited to the processes, mechanisms, manufacturing methods, material compositions, means, methods, and steps of the specific embodiments described in the specification. From the disclosure of this invention, those skilled in the art will readily understand that any existing or future processes, mechanisms, manufacturing methods, material compositions, means, methods, or steps that perform substantially the same function or obtain substantially the same results as the corresponding embodiments described in this invention can be applied according to this invention. Therefore, the appended claims are intended to include these processes, mechanisms, manufacturing methods, material compositions, means, methods, or steps within their scope of protection.

Claims

1. A metasurface device based on a stepped structure, characterized in that, include: A planar device, the planar device comprising a first surface and a second surface disposed opposite to each other, the first surface being provided with a focusing phase structure, and the second surface being provided with a stepped structure having a height difference; When the light beam passes through the first and second surfaces of the planar device, the light beam is modulated by the stepped structure and the focusing phase structure to generate a spatiotemporal vortex light field or a spatiotemporal vortex ring light field.

2. The metasurface device based on a stepped structure according to claim 1, characterized in that, The stepped structure includes a single-step structure, which includes a first sub-surface and a second sub-surface that have a height difference and are parallel, and a third sub-surface that is perpendicular to the direction of the first sub-surface. The third sub-surface connects the first sub-surface and the second sub-surface.

3. The metasurface device based on a stepped structure according to claim 2, characterized in that, The focusing phase structure includes a cylindrical focusing phase structure; When the stepped structure is a single-step structure and the focusing phase structure is a cylindrical focusing phase structure, the generated light field is a spatiotemporal vortex light field.

4. The metasurface device based on a stepped structure according to claim 1, characterized in that, The stepped structure also includes a double-step structure, which includes a through groove or a through protrusion.

5. The metasurface device based on a stepped structure according to claim 4, characterized in that, The focusing phase structure includes a cylindrical focusing phase structure and a circular focusing phase structure; When the stepped structure is a double-step structure and the focusing phase structure is a cylindrical focusing phase structure, the generated light field is a spatiotemporal double vortex light field. When the stepped structure is a double-step structure and the focusing phase structure is a circular focusing phase structure, the generated light field is a curved spatiotemporal double vortex light field.

6. The metasurface device based on a stepped structure according to claim 1, characterized in that, The stepped structure also includes a circular stepped structure, which includes a circular blind hole or a circular protrusion.

7. The metasurface device based on a stepped structure according to claim 6, characterized in that, The focusing phase structure includes a circular focusing phase structure; When the stepped structure is a circular stepped structure and the focusing phase structure is a circular focusing phase structure, the generated light field is a spatiotemporal vortex ring light field.

8. The metasurface device based on a stepped structure according to claim 1, characterized in that, When the height difference of the stepped structure changes by more than the first threshold or less than the second threshold, the resulting spatiotemporal vortex light field or spatiotemporal vortex ring light field will change.

9. A system for generating an ultrafast holographic light field, characterized in that, The system is based on the Mach-Zehnder interferometer principle and uses the metasurface device of any one of claims 1-8 to generate and measure a time-varying ultrafast holographic light field.

10. The ultrafast holographic light field generation system according to claim 9, characterized in that, The system includes: The laser unit is used to generate a laser beam with a pulse width less than a threshold. The beam splitting unit is used to split the laser beam into two laser beams, one as a reference beam and the other as a signal beam; An optical path adjustment unit is used to adjust the optical path of the reference beam; The first polarization modulation unit is used to polarize and modulate the signal beam before irradiating the metasurface device. The second polarization modulation unit is used to polarize the light beam passing through the metasurface device. The beam combining unit is used to combine the reference beam after optical path adjustment and the beam modulated by the second polarization modulation unit. The signal acquisition unit is used to receive the interference fringes formed by the combined beam.

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