Metasurface device and ultrafast holographic light field generation system

Abstract

The utility model relates to the technical field of superfast holographic light field regulation and control, specifically relates to a kind of metasurface device and the generation system of superfast holographic light field.The metasurface device includes: with at least two annular holograms metasurface, the phase of at least two annular holograms is different, at least two annular holograms are encoded in different spatial frequency channels, the spatial frequency of outer ring hologram in at least two annular holograms is greater than the spatial frequency of inner ring hologram.The utility model in the present application, by encoding hologram of different complexity to high, low spatial frequency channel in Fourier space, and using optical path difference to introduce time domain difference, the time domain separation and dynamic reconstruction of femtosecond pulse light field are realized.

Description

Technical Field

[0001] This utility model relates to the field of ultrafast holographic light field manipulation technology, specifically to a metasurface device and an ultrafast holographic light field generation system. Background Technology

[0002] Currently, the generation of spatiotemporal holographic light fields mainly relies on spectral modulation schemes based on spatial light modulators (SLMs), typically implemented within the framework of 4-f pulse shaping systems. That is, by utilizing the SLM on the Fourier surface of a 4-f system to jointly control the spatial and temporal frequency domains, such systems can flexibly control multiple pulse parameters, including phase, amplitude, and polarization. However, traditional 4-f systems have inherent limitations, such as large size and complex device structures, posing challenges to their integration into compact optical systems. Therefore, overcoming the limitations of traditional SLM-based 4-f systems and developing simplified, compact ultrafast holographic light field modulation schemes has become a key technological challenge in the practical application of spatiotemporal light fields.

[0003] To overcome the limitations of traditional 4-f systems, compact metasurface devices offer a promising alternative. Thanks to their multifunctional structural design, a single-layer metasurface can efficiently control the amplitude, phase, and polarization characteristics of light, thus finding wide application in fields such as spatial holographic light field reconstruction, vector beam encoding, 3D display, and full-color image imaging. In recent years, preliminary attempts have been made to extend metasurfaces to the field of ultrafast pulse manipulation, including embedding them into traditional 4-f systems to achieve femtosecond pulse shaping and broadening, using frequency-gradient metasurfaces to achieve ultrafast angle scanning, controlling femtosecond pulse propagation on a chip, and generating spatiotemporal optical vortices. However, some of these methods still rely on 4-f systems or can only generate relatively simple ultrafast light fields. Therefore, how to achieve the generation of ultrafast holographic light fields without relying on 4-f systems remains a significant challenge in this field. Utility Model Content

[0004] In view of this, the present invention provides a metasurface device and an ultrafast holographic light field generation system to solve the technical problem of how to generate an ultrafast holographic light field without relying on a 4-f system.

[0005] The technical solution provided by this utility model is as follows:

[0006] In a first aspect, the present invention provides a metasurface device, comprising: a metasurface having at least two annular holograms, wherein the at least two annular holograms have different phases, the at least two annular holograms are encoded in different spatial frequency channels, and the spatial frequency of the outer annular hologram in the at least two annular holograms is greater than the spatial frequency of the inner annular hologram.

[0007] In this invention, by encoding holograms of different complexities to high and low spatial frequency channels in Fourier space and introducing temporal differences using optical path difference, temporal separation and dynamic reconstruction of femtosecond pulse light fields are achieved.

[0008] In one alternative embodiment, the metasurface further includes a focusing phase for propagating beams passing through different annular holograms onto the focal plane.

[0009] In this invention, by setting the focusing phase, light of different spatial frequencies corresponding to different annular positions is propagated to the focal plane.

[0010] In one alternative implementation, the metasurface is used to generate time-varying orbital angular momentum beams or time-varying holographic light fields with different patterns.

[0011] In this invention, two typical time-varying ultrafast holographic light fields are generated by setting a metasurface.

[0012] In one alternative embodiment, when the metasurface is used to generate a time-varying orbital angular momentum beam, the spiral phases of at least two annular holograms are different, and the radius of the inner annular hologram in the at least two annular holograms is 0 to 3.2 mm, while the radius of the outer annular hologram is 3.2 mm to 9 mm.

[0013] In this invention, the setting of the sizes of the inner and outer ring holograms makes the spatial frequency differences corresponding to different ring holograms significant, realizing the temporal separation and dynamic control of time-varying orbital angular momentum beams, and providing a structural basis for on-demand switching of ultrafast optical fields.

[0014] In one alternative implementation, when the metasurface is used to generate time-varying holographic light fields with different patterns, the phases of at least two annular holograms are determined based on the phase recovery Götzberg-Sachston iterative algorithm, wherein the radius of the inner annular hologram in the at least two annular holograms is 0–5.7 mm and the radius of the outer annular hologram is 5.7 mm–9 mm.

[0015] In this invention, the phase of different annular holograms is determined by using the phase recovery Gerschberg-Sachston iterative algorithm, which is applicable to the phase design of any complex pattern (such as letters or irregular shapes) without the need for analytical expressions.

[0016] Secondly, this utility model 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 of the first aspect of this utility model and any one of the first aspects to generate and measure a time-varying ultrafast holographic light field.

[0017] In related technologies, spatial light modulators and 4f systems are mostly used for time-varying optical field manipulation, resulting in complex optical paths. In this embodiment, the manipulation optical path is designed based on the Mach-Zehnder interferometer principle. Specifically, a signal light and a reference light are generated in the optical path. The signal light interferes with the reference light after passing through a metasurface device, thereby generating a time-varying ultrafast holographic optical field. Simultaneously, by acquiring the interference fringes and based on the phase and intensity changes of the optical field in the time domain before and after manipulation, dynamic measurement of the time-domain optical field is achieved.

[0018] 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 a metasurface; a second polarization modulation unit for polarizing the beam passing through the metasurface; 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.

[0019] In one optional embodiment, the laser beam generated by the laser unit is linearly polarized light, the first polarization modulation unit is used to modulate the linearly polarized light into circularly polarized light, and the second polarization modulation unit is used to modulate the circularly polarized light into linearly polarized light.

[0020] In this invention, the laser beam generated by the laser unit is converted into circularly polarized light by the first polarization modulation unit, which improves the response efficiency of the metasurface; by setting the second polarization modulation unit, the circularly polarized light is converted into linearly polarized light, which makes the signal light and the reference light have the same polarization state, thereby making the final interference fringe contrast optimal.

[0021] In one alternative implementation, the first polarization modulation unit and / or the second polarization modulation unit is a quarter-wave plate, and the optical path adjustment unit includes a displacement adjustment platform and an optical delay line.

[0022] In one optional embodiment, the system further includes a beam expander disposed between the first polarization modulation unit and the metasurface, for expanding the beam polarized by the first polarization modulation unit.

[0023] In this invention, the beam expander not only increases the beam diameter but also eliminates dispersion. Attached Figure Description

[0024] To more clearly illustrate the specific embodiments of this utility model 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 this utility model. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0025] Figure 1(a) is a schematic diagram of the metasurface device in an embodiment of this utility model;

[0026] Figure 1(b) is a schematic diagram of 3D time-domain spatial frequency multiplexing of the metasurface device in the embodiment of this utility model;

[0027] Figure 1(c) is a graph showing the relationship between the transverse wave vector and the propagation time difference in an embodiment of this utility model;

[0028] Figure 2(a) is a schematic diagram of the metasurface structure when a time-varying orbital angular momentum beam is generated using a metasurface;

[0029] Figure 2(b) is a schematic diagram of the theoretical and experimental results of orbital vortex structured light on the focal plane. In this diagram, (b1) is the total pulse time-domain envelope diagram (black line) on the focal plane, with the inner ring generating pulses (red line) and the outer ring generating pulses (blue line). (b2) to (b5) are intensity diagrams at four time points, (b6) to (b9) are phase diagrams at four time points, (c1) to (c9) are the results of splitting the inner ring (TC=-2) from (b1) to (b9), (c3) and (c7) represent the light intensity and phase of the xy plane at the peak time of (c1), (d1) to (d9) are the results of splitting the outer ring (TC=10) from (b1) to (b9), (d3) and (d7) represent the light intensity and phase of the xy plane at the peak time of (d1), and (e1) to (g9) are the experimental results corresponding to (b1) to (b9). The pulse measurement range is -399fs to 399fs.

[0030] Figure 3(a) shows a schematic diagram of the metasurface structure when different patterns of time-varying holographic light fields are generated using metasurfaces;

[0031] Figure 3(b) is a schematic diagram of the theoretical and experimental results of letter holography on the focal plane. (b1) is the temporal envelope diagram (black line) of the strongest point from (b2) to (b5), the letter 'S' is shown in red, and the letter 'Z' is shown in blue. (b2) to (b5) are intensity maps at four time points, (b6) to (b9) are spectrum maps at four time points, and (c1) to (d9) are the temporal decomposition and decoding results from (b1) to (b9). (c1) is the inner ring (letter 'S'). The temporal optical field envelope diagrams are shown. (c1) to (c5) are intensity diagrams at four times, (c6) to (c9) are spectrum diagrams at four times, (c3) represents the light intensity on the xy plane at the peak time of (c1), (d1) to (d9) are the results corresponding to the outer ring (letter 'Z'), (d4) represents the light intensity on the xy plane at the peak time of (d1), and (e1) to (g9) are the experimental results corresponding to (b1) to (b9). The pulse measurement range is -599fs to 599fs.

[0032] Figure 4 This is a schematic diagram of the structure of the ultrafast holographic light field generation system in an embodiment of this utility model. Detailed Implementation

[0033] The technical solution of this utility model 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 this utility model. Based on the embodiments of this utility model, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this utility model.

[0034] In the description of this utility model, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings and are only for the convenience of describing this utility model and 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, and therefore should not be construed as a limitation of this utility model. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0035] In the description of this utility model, 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 utility model based on the specific circumstances.

[0036] Furthermore, the technical features involved in the different embodiments of this utility model described below can be combined with each other as long as they do not conflict with each other.

[0037] This utility model provides a metasurface device, comprising: a metasurface having at least two annular holograms, wherein the at least two annular holograms have different phases, the at least two annular holograms are encoded in different spatial frequency channels, and the spatial frequency of the outer annular hologram in the at least two annular holograms is greater than the spatial frequency of the inner annular hologram.

[0038] In one alternative implementation, besides stacking different annular holograms to form a metasurface, a focusing phase can also be formed in the metasurface. This focusing phase is used to propagate the light beams passing through the different annular holograms to the focal plane. Specifically, the function of the focusing phase is to ensure that light of different spatial frequencies corresponding to different annular positions propagates to the focal plane. This focusing phase can be understood as adding a circular focusing lens with a preset focal length to the different annular holograms. When the metasurface includes multiple annular holograms and a focusing phase, its structural design is shown in Figure 1(a), where (a1) represents the focusing phase, and (a2), (a3), and (a4) represent three annular holograms, with the phases of the three annular holograms being respectively... (a5) represents the superimposed metasurface hologram.

[0039] It should be noted that the number of annular holograms can be set according to the actual situation. In practical applications, multiple annular holograms can be superimposed to form a single-layer metasurface holographic device with multiple annular phase holograms. Furthermore, when a holographic light field is generated through the holographic device, different spatial frequency components of light have different diffraction angles, thus the time taken for the light to propagate from the device plane to the observation plane also differs. Therefore, based on holographic devices encoded in different spatial frequency channels, different target holographic light fields are encoded into annular holograms with different spatial frequencies, thus appearing sequentially on the focal plane with a controllable time delay, forming a time-varying ultrafast holographic light field. Simultaneously, because the phases of different annular holograms are different, the wavefront distribution of the light field is different, resulting in different spatial structures appearing sequentially on the focal plane.

[0040] Specifically, when light propagates from the Fourier frequency domain surface to the image plane used for observation, light of different spatial frequencies exhibits optical path differences, thus giving the information of different spatial frequency channels a clear "sequential order" in the time dimension. This time-domain difference caused by spatial frequency is the physical basis for the time-varying dynamic light field control achieved in this embodiment. As shown in Figure 1(b), when the distance between the holographic device and the observation plane is L, the optical path and time of the light beam reaching the observation plane after passing through the holographic device are represented by S and t, respectively. Simultaneously, because different annular holograms are encoded in different spatial frequency channels within the holographic device, the optical path S and time t required for light to reach the observation plane after passing through different annular holograms are different, and the propagation angle θ is also different. Specifically, the optical path S is determined by the distance L and the propagation angle θ. In the standard coordinate system, θ is the angle between the light ray and the Z-axis, and the corresponding propagation time t can be expressed as:

[0041]

[0042] In the formula, c represents the speed of light in a vacuum. According to the wave vector component relationship, the wave vector k in the y-direction... y The expression is:

[0043]

[0044] Where λ0 is the wavelength, substituting equation (2) into equation (1) yields the light propagation time t and k. y The relationship is shown in the following formula (3):

[0045]

[0046] If the radius of the hologram (the hologram formed by superimposing multiple ring holograms) is 9 mm, the measurement distance is L = 0.65 m, the wavelength of the light source is λ0 = 800 nm, and the speed of light is c = 0.3 μm / fs, the transverse wave vector k shown in Figure 1(c) can be calculated using formula (3). y The graph shows the relationship between k and propagation time difference Δt, where the vertical axis represents the propagation time difference Δt, i.e., different k values. y Compared to horizontally propagating light, spatial frequency light (k y The propagation time difference when k = 0, Δt(k y )=t(k y )-t(k y =0). It can be seen that different wave vector distributions lead to different propagation times, resulting in significant temporal optical field evolution on the observation surface.

[0047] In this invention, by encoding holograms of different complexities to high and low spatial frequency channels in Fourier space and introducing temporal differences using optical path difference, temporal separation and dynamic reconstruction of femtosecond pulse light fields are achieved.

[0048] Furthermore, the optical field of this metasurface is analyzed using a Gaussian femtosecond pulse as the light source. Specifically, according to the Fourier transform principle, the time-domain expression E0(t) of the incident laser pulse can be described by the frequency-domain electric field E0(ω):

[0049]

[0050] Where t is the pulse duration, t0 is the pulse width, ω0 is the pulse center angular frequency, ω is the angular frequency, and FT represents the Fourier transform. Based on Figure 1(a), the initial holographic phase of the optical field is designed by superimposing a circular focusing phase with multiple holograms as follows:

[0051]

[0052] Where f is the focal length and λ0 is the wavelength. For the phase of the hologram on different rings. This indicates the focusing phase. According to the Fresnel diffraction formula, the frequency domain light field distribution on the observation plane after the incident light is transmitted through the hologram can be expressed as:

[0053]

[0054] Where (x0, y0) are the plane coordinates of the holographic device, (x, y) are the coordinates of the observation plane, and k = 2π / λ. Based on the Fourier transform relationship between the frequency and time domains, the time-domain light field distribution on the observation plane can be written as:

[0055]

[0056] According to Formula 7, the time-domain results of the holographic light field in different Z planes can be calculated.

[0057] Based on the above light field analysis, the light fields generated when designing different holograms can be theoretically analyzed. Specifically, in this embodiment, the light fields generated by the design of the hologram include time-varying orbital angular momentum light fields and time-varying holographic light fields with different patterns.

[0058] In an alternative embodiment, when the metasurface is used to generate a time-varying orbital angular momentum (OAM) beam, as shown in Figure 2(a), the metasurface hologram (Hologram 1) shown in (a4) is formed using two annular holograms with different helical phases (vortex phase 1 and vortex phase 2) shown in (a2) and (a3) ​​and the focusing phase shown in (a1). (a5) shows the microscopic local imaging result of the structure corresponding to (a4).

[0059] The device has an overall circular structure with a radius of 9 mm, and the focal length of the focusing phase is 650 mm. In the two annular holograms, the radius of the inner ring hologram ranges from 0 to 3.2 mm, and the radius of the outer ring hologram ranges from 3.2 mm to 9 mm. The corresponding spiral phase expression is for the inner ring. Outer Ring Wherein, the topological charge L1 = -2; L2 = 10; φ represents the azimuth angle of the polar coordinate system in the (x0,y0) plane.

[0060] Based on formulas (4) to (7) in the above theoretical analysis, the spatiotemporal light field at the focal plane (Z = 0 mm) is calculated, as shown in (b1) to (b9) of Figure 2(b). Figure 2(b) (b1) shows that the total pulse (black line) is composed of two sub-pulses (blue and red lines) with different topological charges, and there is a clear time interval between the two sub-pulses. As shown in (b2) to (b5) of Figure 2(b), during the temporal intensity change, the light intensity at times 2 and 3 is distributed in a petal-like pattern because the vortex light fields of L = -2 and L = 10 interfere, producing 12 petal-shaped interference fields. The corresponding phase diagram also shows that the center is a spiral phase of L1 = -2, and the outer ring is a spiral phase of L = 10, consistent with the designed hologram. By time step 4, the light intensity is mainly in a ring shape. At this point, the L=-2 vortex has decayed over time, leaving mainly an L=10 OV (Orbital Vortex) light field ring distribution, and its intensity weakens as the pulse moves further away. Then, using an algorithm that splits the light field at different spatial frequencies, the overall light field results in Figure 2(b) from (b1) to (b9) are split into the time-domain results of the L1=-2 and L2=10 OAM pulse light field distributions in Figure 2(b) from (c1) to (c9) and Figure 2(b) from (d1) to (d9). The light intensity results in Figure 2(b) from (c2) to (c4) after splitting show the process of the L1=-2 OAM pulse from rising to falling. It can be seen that the light intensity is in a ring shape, and the phase distribution is a -2 order spiral phase. Similarly, in Figure 2(b), (d2) to (d5) represent the rising and falling process of the split L2=10 OAM pulse. Its intensity distribution is a larger ring, and the phase distribution is a 10th-order spiral phase. Comparing the intensity distribution at four moments in Figure 2(b), (c2) to (c5) and (d2) to (d5), it is clear that the L1=-2 and L2=10 OAM pulses have a time interval. In Figure 2(b), (c3) is the xy plane intensity at the peak of (c1), and (d4) is the xy plane intensity at the peak of (d1). Through the pulse curves of both (the time-domain envelope curves of the strongest intensity points after splitting), the interval between the two pulse peaks is theoretically calculated to be approximately 128 fs.

[0061] Figure 2(b) shows the experimental results corresponding to (b1) to (d9) in Figure 2(b) above. Figure 2(b) shows (f1) to (g9) the individual OAM pulse results for L1 = -2 and L2 = 10 after splitting, also separated from (e1) to (e9) in Figure 2(b) using the same processing method as the theoretical results. It can be seen that the experimentally measured light intensity and phase distribution are consistent with the theory, and the interval between the two pulse peaks is also 128 fs, which also matches the theoretical result. The results in Figure 2(b) demonstrate the effectiveness of the device, namely, that by loading different spiral phases onto light of different spatial frequencies, time-varying OAM light fields can indeed be generated. Although only two time-varying OV light fields with topological charge changes are shown here, theoretically, more time-varying OV light fields with topological charge changes can be generated by splitting more spatial frequency channels (loading more ring holograms).

[0062] In one optional implementation, when the metasurface is used to generate time-varying holographic light fields with different patterns, the phases of at least two annular holograms are determined based on the phase-recovery Gerchberg-Saxton (GS) iterative algorithm. The inner ring hologram has a radius of 0–5.7 mm, and the outer ring hologram has a radius of 5.7 mm–9 mm. The GS algorithm, based on the invertibility of the Fourier transform, infers the phase distribution from known light field amplitude information by alternately applying constraints between the spatial and frequency domains. Specifically, when it is necessary to convert a target pattern (such as a letter) into a phase distribution that can be loaded onto the metasurface, the GS algorithm is used for iterative optimization to infer the corresponding holographic phase from the intensity distribution of the pattern, thereby achieving dynamic temporal light field manipulation.

[0063] Specifically, as shown in Figure 3(a), two independent holograms are first designed in Fourier space: the inner ring spatial frequency channel corresponds to the phase diagram of the letter "S", and the outer ring spatial frequency channel corresponds to the phase diagram of the letter "Z". As shown in the spherical phase diagram (a1) of Figure 3(a), the overall diameter of this letter hologram is 18mm, and the focal length is 350mm. The holographic phase diagrams of the two different letter structures are obtained using the GS iterative algorithm. and The results are shown in Figure 3(a) (a2) and (a3) ​​respectively. The final hologram is shown in Figure 3(a) (a4), and the processed liquid crystal metasurface sample is shown in Figure 3(a) (a5).

[0064] Based on formulas (4) to (7) in the above theoretical analysis, the following theoretical results are calculated as shown in (b1) to (b9) of Figure 3(b). It can be seen that the information corresponding to the inner ring spatial frequency channel arrives at the focal plane first. Therefore, the light intensity is mainly 'S' shaped in the first two moments, while the light intensity is mainly 'Z' shaped in the third and fourth moments. At this time, the 'S' shape has decayed over time. In addition, the corresponding spatial spectrum on the right shows that as time changes, the spatial frequency gradually moves from the central low-frequency region to the outer high-frequency region, which is in line with the design expectation.

[0065] Next, using an algorithm that splits the light field at different spatial frequencies, the overall light field results in Figure 3(b) from (b1) to (b9) are split into the temporal results of the pulse light field distributions of 'S' and 'Z' in Figure 3(b) from (c1) to (c9) and from (d1) to (d9). Figure 3(b) from (c2) to (c4) shows the light intensity results at the first three moments after the split, illustrating the process of the 'S' pulse rising and falling. Similarly, Figure 3(b) from (d3) to (d5) shows the process of the 'Z' pulse rising and falling after the split. Comparing the light intensity distributions at the four moments in Figure 3(b) from (c2) to (c5) and from (d2) to (d5), it is clear that there is a time interval between the 'S' and 'Z' pulses. Theoretically, the interval between the two pulse peaks is calculated to be 217 fs using the pulse curves (red and blue lines). Similarly, comparing Figure 3 (c6-c9) and Figure 3 (d6-d9), their corresponding spatial spectrograms also show the same temporal evolution: the inner loop spectrum is stronger in the first half of the time, and the outer loop spectrum is stronger in the second half of the time.

[0066] The experimental results are shown in Figures 3(b) (e1) to (g9). The results of the light fields for both letters correspond to the theory, indicating that the superimposed liquid crystal phase plate can simultaneously carry the information of two independent images, "S" and "Z". The interval between the two pulse peaks is 224 fs, which is close to the theoretical result. The reason why the experimental effect is slightly worse than the theoretical result is that the time-domain process in the experiment was measured using interference fringes, which is affected by spatial chirp and the contrast and intensity ratio of the interference fringes. In addition, due to the complex nature of the letter structure, the single-moment average sampling method was used to calculate the inner and outer ring pulse curves (Figure 3(b) (b1)). That is, the intensity value corresponding to each point in the red, black, and blue curves is determined by the average value of the light field intensity at the current moment.

[0067] The experimental results in Figure 3(b) verify the theoretical prediction of this device, namely, that by utilizing the actual optical path differences within different spatial frequency channels, complex patterns can appear sequentially in the temporal domain within a single metasurface structure. In the future, by expanding more spatial frequency channels, more continuous changes in holographic patterns can be generated, forming a dynamic holographic light field similar to an "ultra-fast movie".

[0068] This invention demonstrates two typical light fields through experiments: one is a femtosecond vortex light field with orbital angular momentum varying over time, and the other is a femtosecond holographic pattern switching between different letters over time. Both experimental results are highly consistent with theoretical predictions, effectively verifying the feasibility of the proposed scheme.

[0069] In one alternative embodiment, when fabricating the metasurface device, liquid crystal materials, dielectric materials with low loss and high transparency, metallic materials suitable for plasmonic metasurfaces, phase change materials suitable for tunable metasurfaces, two-dimensional materials and nanofilms, and polymer materials suitable for flexible / stretchable metasurfaces can be used. Other dynamically modulated materials can also be used, such as piezoelectric materials (e.g., PZT) that allow stress-controlled device geometry, ferroelectric materials (e.g., BTO, LiNbO) that allow electric field control of refractive index, and liquid crystal elastomers (LCEs) that combine liquid crystal and mechanical response for multi-physics field manipulation. It should be noted that in the ultrafast optics field of this embodiment, the requirements for materials are more stringent (e.g., response speed, loss, phase depth), and the following can be preferentially considered: high-frequency response liquid crystals (LC), low-loss optical media (e.g., TiO, GaN), fast phase change materials (e.g., GST), graphene, or other 2D materials.

[0070] Specifically, when using liquid crystal materials, N-BK7 can be used as the liquid crystal structure substrate, the working wavelength of the liquid crystal polymer is 800nm, the single pixel size is 9um, and the total pixel size is 2000*2000 pixels, thereby obtaining a liquid crystal metasurface sample based on geometric phase.

[0071] This invention also provides a system for generating an ultrafast holographic light field. The system is based on the Mach-Zehnder interferometer principle and utilizes the aforementioned metasurface device to generate and measure a time-varying ultrafast holographic light field. Specifically, in related technologies, spatial light modulators and 4f systems are mostly used for time-varying light field control, resulting in complex optical paths. In this embodiment, the control optical path is designed based on the Mach-Zehnder interferometer principle. Signal light and reference light are generated in the optical path. The signal light interferes with the reference light after passing through the metasurface device, thereby generating a time-varying ultrafast holographic light field. Simultaneously, by acquiring the interference fringes, dynamic measurement of the time-domain light field is achieved based on the phase and intensity changes of the light field before and after control in the time domain.

[0072] 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 a metasurface; a second polarization modulation unit for polarizing the beam passing through the metasurface; 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.

[0073] In this system, when the laser beam generated by the laser unit is linearly polarized, the first polarization modulation unit modulates the linearly polarized light into circularly polarized light, and the second polarization modulation unit modulates the circularly polarized light into linearly polarized light. The system further includes a beam expander disposed between the first polarization modulation unit and the metasurface, used to expand the beam polarized by the first polarization modulation unit. Specifically, the first polarization modulation unit and / or the second polarization modulation unit are quarter-wave plates, and the optical path adjustment unit includes a displacement adjustment platform and an optical delay line. Both the beam splitting unit and the beam combining unit can be beam splitters.

[0074] Specifically, to generate an ultrafast holographic light field, the pulse width of the beam generated by the laser unit needs to meet certain requirements. The shorter the pulse width, the greater the propagation time difference between the beams passing through the inner and outer ring holograms. Based on the pulse width requirement of the laser unit, a femtosecond laser or other lasers with a pulse width less than a threshold can be used. In this embodiment, a femtosecond laser with a center wavelength of 800 nm and a pulse width of approximately 120 fs is used. Furthermore, when a femtosecond laser is used, its output laser pulse is horizontally linearly polarized light. Since the metasurface has the best response efficiency to circularly polarized light, a first polarization modulation unit is set to convert the horizontally linearly polarized light into circularly polarized light. When measuring the ultrafast holographic light field, the reference light and signal light need to have the same polarization state. Therefore, a second polarization modulation unit is set to convert the circularly polarized light passing through the metasurface into linearly polarized light. Simultaneously, the requirement for the signal light and reference light to generate a high-quality interference pattern is that the two beams have an appropriate angle and the same optical path difference. Therefore, an optical path adjustment unit is set up, which includes a displacement control platform and an optical delay line. The displacement control platform can move the optical delay line so that the reference light and the signal light passing through the optical delay line have the same optical path difference.

[0075] Specifically, such as Figure 4As shown, the system specifically includes a first beam splitter BS1, a second beam splitter BS2, a third beam splitter BS3, a first reflecting mirror M1, a second reflecting mirror M2 on the optical delay line, a first quarter-wave plate QWP1, a beam expander BE, a holographic device sample, a second quarter-wave plate QWP2, and a signal acquisition unit CCD. During operation, the femtosecond laser outputs a horizontally linearly polarized femtosecond laser pulse, which, after collimation, is incident on the first beam splitter BS1, splitting the beam into two. One beam serves as a reference beam, which, after passing through the second beam splitter BS2, is reflected back by the second reflecting mirror M2 on the optical delay line, and finally reflected by the third beam splitter BS3 into the CCD, providing a precise reference optical path for interferometry. The other beam is a signal beam (modulation beam). This signal beam passes through the first reflecting mirror M1, then is converted into circularly polarized light by the first quarter-wave plate QWP1, and then passes through a 10x achromatic beam expander BE to enlarge the beam diameter to cover the holographic device sample. It is then modulated by a pre-designed holographic phase device to generate an ultrafast holographic light field. The modulated light is filtered out again by the second quarter-wave plate QWP2, and finally enters the CCD through the third beam splitter BS3. When the two beams have an appropriate angle and the optical path difference is the same, a high-quality interference fringe pattern is formed on the CCD plane. By moving the optical delay line, the reference light and the beam under test are scanned in real time, and each frame of the interferogram is accurately acquired, realizing dynamic measurement of the time-domain optical field.

[0076] This invention achieves, for the first time, dynamic control of femtosecond time-varying light fields on a single metasurface holographic device through system design. Compared to traditional SLM-based 4-f systems, this system offers significant advantages such as strong versatility, compact device structure, and ease of integration, and has broad application prospects in fields such as spatiotemporal light field manipulation, integrated ultrafast photonics, and ultrafast optical information processing.

[0077] 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 this 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 protection of this invention.

[0078] Furthermore, the scope of application of this utility model 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 utility model, those skilled in the art will readily understand that existing or future-developed processes, mechanisms, manufacturing methods, material compositions, means, methods, or steps that perform substantially the same function or achieve substantially the same results as the corresponding embodiments described in this utility model can be applied according to this utility model. Therefore, the appended claims of this utility model aim to include these processes, mechanisms, manufacturing methods, material compositions, means, methods, or steps within their scope of protection.

Claims

1. A metasurface device, characterized in that, include: A metasurface having at least two annular holograms, wherein the at least two annular holograms have different phases, the at least two annular holograms are encoded in different spatial frequency channels, and the spatial frequency of the outer annular hologram in the at least two annular holograms is greater than the spatial frequency of the inner annular hologram.

2. The metasurface device according to claim 1, characterized in that, The metasurface also includes a focusing phase, which is used to propagate the light beams passing through different annular holograms onto the focal plane.

3. The metasurface device according to claim 1, characterized in that, The metasurface is used to generate time-varying orbital angular momentum beams or time-varying holographic light fields with different patterns.

4. The metasurface device according to claim 3, characterized in that, When the metasurface is used to generate a time-varying orbital angular momentum beam, the spiral phases of at least two annular holograms are different, and the radius of the inner annular hologram in the at least two annular holograms is 0 to 3.2 mm, while the radius of the outer annular hologram is 3.2 mm to 9 mm.

5. The metasurface device according to claim 3, characterized in that, When the metasurface is used to generate time-varying holographic light fields with different patterns, the phases of at least two annular holograms are determined based on the phase recovery Götzberg-Sachston iterative algorithm. The radius of the inner annular hologram in the at least two annular holograms is 0–5.7 mm, and the radius of the outer annular hologram is 5.7 mm–9 mm.

6. 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 1-5 to generate and measure a time-varying ultrafast holographic light field.

7. The ultrafast holographic light field generation system according to claim 6, 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 and then irradiate it onto the metasurface. The second polarization modulation unit is used to polarize the beam passing through the metasurface. 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.

8. The ultrafast holographic light field generation system according to claim 7, characterized in that, The laser beam generated by the laser unit is linearly polarized light. The first polarization modulation unit is used to modulate the linearly polarized light into circularly polarized light, and the second polarization modulation unit is used to modulate the circularly polarized light into linearly polarized light.

9. The ultrafast holographic light field generation system according to claim 7, characterized in that, The first polarization modulation unit and / or the second polarization modulation unit are quarter-wave plates, and the optical path adjustment unit includes a displacement adjustment platform and an optical delay line.

10. The ultrafast holographic light field generation system according to claim 7, characterized in that, The system further includes a beam expander disposed between the first polarization modulation unit and the metasurface, for expanding the beam polarized by the first polarization modulation unit.

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