Chiral metasurface optical device, preparation method thereof and ultrafast circularly polarized light modulation method

Abstract

The invention provides a chiral metasurface optical device, a preparation method thereof and an ultrafast circularly polarized light modulation method. The chiral metasurface optical device comprises a substrate and a chiral oligomer nano metasurface structure arranged on the substrate, the chiral oligomer nano metasurface structure is formed by a plurality of metal nano units with the same structure in short-range ordered distribution; the metal nanometer unit comprises six metal nanometer plates, and the centers of the six metal nanometer plates are located on the circumference of the same circle. Structural chirality characteristics are introduced through the circularly distributed metal nanoplates, the structure is simple, and the circularly polarized light modulation capability is high; through combination of an excitation light source and a detection light source, optical characteristics of the chiral metasurface optical device are excited on an ultrafast time scale, and transient circular dichroism signals are realized. By changing the incident directions of the excitation light source and the detection light source, the symbol flipping of the transient circular dichroism signal can be realized, and the response time and the relaxation time are ultrafast.

Description

Technical Field

[0001] The present invention belongs to the technical field of nano-optics, and in particular relates to a chiral metasurface optical device, a preparation method thereof and an ultrafast circularly polarized light modulation method. Background Art

[0002] Metasurface is a planar design of subwavelength artificial nanostructures that can manipulate and enhance the interaction between light and matter at the nanoscale, showing unique photophysical phenomena. The excellent optical properties, ultra-thin, ultra-integrated and miniaturized structural features of metasurfaces make them a strong candidate to replace large optical components and equipment today. As the demand for the speed of optical information transmission continues to increase, researchers have begun to explore ultrafast photophysical phenomena based on various metasurfaces for ultrafast light regulation on a smaller time scale, such as high-order harmonic generation and ultrafast pulse optical switching on metasurfaces. Among them, metal metasurfaces have been widely studied and applied for their unique plasmon resonance effect and ultrafast hot electron generation. Optical devices assisted by surface plasmons can realize nanoscale collective oscillations of electrons that are strongly coupled to photons, further narrowing the control range of photonic devices. However, most of the metal metasurfaces reported so far modulate light waves by changing the structural configuration and size, which requires complex processing and preparation processes.

[0003] Circularly polarized light has a chiral wave vector, uniform polarization distribution, and low glare effect, so it is widely used in contrast-enhanced polarization micro-imaging, biomolecule detection, optical communications, optical remote sensors, and optical displays. Traditional circularly polarized light modulation uses chiral polymers to perform polarization-resolved responses in a steady state. The circularly polarized light modulation effect is poor and slow, and cannot meet the application requirements of high-speed optical devices. Therefore, it is an issue that cannot be ignored to construct a chiral system with ultrafast chiral modulation, improve the preparation process of metal metasurfaces, and circularly polarized light modulation methods.

[0004] CN104865628A discloses a spiral metal chiral metamaterial circular polarizer, including a substrate, a transparent medium spacer firmly bonded to the substrate, and a spiral metal chiral microstructure array embedded in the transparent medium spacer. CN105137520A discloses a gradient spiral metal chiral metamaterial and circular polarizer, the gradient spiral metal chiral metamaterial circular polarizer including a substrate and a gradient spiral metal wire array uniformly deposited on the substrate; the spiral wire array is composed of periodically arranged spiral metal wire units; each spiral wire unit is connected by upper and lower parts of spiral wires with the same hand direction, the radius of the upper part of the spiral wire increases from top to bottom, and the lower part of the spiral wire has the same diameter. These two circular polarizers have complex structures, present a three-dimensional multi-layer structural design, require complex processing and preparation processes, and are difficult to mass produce. In addition, the chiral polymers modulate circularly polarized light in a steady state, interact more with light, and have no time response, which cannot meet the application requirements of high-speed optical devices, and cannot flip and modulate circularly polarized light.

[0005] The existing chiral metasurface optical devices have complex structures and preparation processes, slow modulation speed of circularly polarized light, no time response, and require the preparation and processing of chiral enantiomers to achieve flip modulation of circularly polarized light. Therefore, it is a key issue to develop a chiral metasurface optical device with flexible functionality and ultrafast circularly polarized light modulation, modulate circularly polarized light based on the metasurface of the optical device to achieve precise control of its amplitude and phase, increase the spatial and temporal degrees of freedom, and enable this chiral metasurface optical device to be applied to the design of future high-speed polarization-sensitive optical switches and quantum storage. Summary of the invention

[0006] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a chiral metasurface optical device, a preparation method thereof and an ultrafast circularly polarized light modulation method. The chiral metasurface optical device introduces structural chirality characteristics through the circular distribution of 6 metal nanodisks in each metal nanounit, has a simple structure, and excites and detects the chiral metasurface optical device through a combination of pump light and probe light in different circular polarization states, thereby stimulating the optical characteristics of the chiral metasurface on an ultrafast time scale. By detecting the transient absorption signal of the chiral metasurface optical device under the irradiation of different detection light sources, the modulation of the circularly polarized light can be reflected, and at the same time, a transient absorption response with polarization resolution, i.e., a transient circular dichroism signal, can be realized, which can more intuitively reflect the ultrafast circularly polarized light modulation capability of the chiral metasurface. By changing the incident direction of the circularly polarized light, the sign reversal of the transient circular dichroism signal can be realized, and the device has an ultrafast response time and relaxation time, thereby providing a simple and valuable way for the functional flexibility and high-speed design of polarization optical elements, and can be further applied to the fields of ultrafast optical switches and quantum communications.

[0007] To achieve this object, the present invention adopts the following technical solutions:

[0008] In the first aspect, the present invention provides a chiral metasurface optical device, which includes a substrate and a chiral oligomer nano-metasurface structure arranged on the substrate; the chiral oligomer nano-metasurface structure is formed by a short-range orderly distribution of multiple metal nano-units with the same structure; the metal nano-units include 6 metal nano-disks, and the centers of the 6 metal nano-disks are located on the circumference of the same circle.

[0009] In the chiral metasurface optical device provided by the present invention, the chiral oligomer nano-metasurface structure is a two-dimensional planar single-layer chiral metasurface formed by a plurality of chiral metal nano-units distributed in a short-range orderly manner. The structure is simple, and opposite chiral mirror features can be observed through the front and back sides. The short-range orderly distributed chiral metal nano-units introduce asymmetry of the structure, which can produce different responses to different circularly polarized light, that is, induce different plasmon resonance modes, which are specifically reflected in the waveform, amplitude and band of the absorption spectrum. At the same time, the coupling between the metal nano-disks can enhance the interaction with light. Under the interaction with light, the oligomer composed of 6 metal nano-disks can produce Coulomb dipole interaction, which manifests as dipole coupling with chiral optical characteristics. The left-handed circular distribution configuration of the metal nano-disk can produce a stronger capacitive coupling resonance with left-handed circularly polarized light, inducing a stronger electric field. Since the structure has a planar two-dimensional chiral characteristic, the optical chirality can be flipped through front and back excitation.

[0010] In the chiral metasurface optical device provided by the present invention, the chiral oligomer nano-metasurface structure is an array structure composed of multiple metal nano-units with the same structure in short-range order. The "multiple" conventionally means at least two. The specific number of metal nano-units in the present invention can be determined according to the performance of the chiral metasurface optical device, and generally speaking, their number is in the thousands; short-range order means that the distance between the centers of two adjacent metal nano-units is 500±50nm.

[0011] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the objectives and beneficial effects of the present invention can be better achieved and realized.

[0012] As a preferred technical solution, the material of the substrate includes SiO2 or sapphire.

[0013] Preferably, the thickness of the substrate is 0.2-0.5 mm, for example, it can be 0.2 mm, 0.25 mm, 0.3 mm, 0.35 mm, 0.4 mm, 0.45 mm or 0.5 mm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0014] Preferably, the material of the metal nanodisk is gold.

[0015] The metal nanodisks are all made of gold material, have a surface plasmon effect to enhance the interaction with light, and can generate hot electrons ultrafast, which can realize nanoscale collective oscillation of electrons strongly coupled with photons, further narrowing the control range of photonic devices.

[0016] As a preferred technical solution, the center distances between any two adjacent metal nanodisks are equal.

[0017] Preferably, the radii of the six metal nanodisks decrease sequentially in a counterclockwise direction on the circumference.

[0018] Preferably, in order of radius from large to small, the diameters of the six metal nanodisks are: 60-80nm (for example, 62nm, 65nm, 68nm, 70nm, 72nm, 75nm or 78nm, etc.), 50-70nm (for example, 52nm, 55nm, 58nm, 60nm, 62nm, 65nm or 68nm, etc.), 40-60nm (for example, 42nm, 45nm, 48nm, 50nm, 52nm, 55nm or 58nm, etc.), 30-50nm (for example, 32nm, 35nm, 38nm, 40nm, 42nm, 45nm or 48nm, etc.), 20-40nm (for example, 22nm, 25nm , 28nm, 30nm, 32nm, 35nm or 38nm, etc.) and 10-30nm (for example, it can be 12nm, 15nm, 18nm, 20nm, 22nm, 25nm or 28nm, etc.), for example, the diameters of 6 metal nanodisks can be the following combinations: 60nm, 50nm, 40nm, 30nm, 20nm and 10nm; 65nm, 55nm, 45nm, 35nm, 25nm and 15nm; 75nm, 65nm, 55nm, 45nm, 35nm and 25nm; 80nm, 70nm, 60nm, 50nm, 40nm, 30nm, and other diameter combinations that meet the requirements. Due to space limitations and for the sake of simplicity, the present invention will no longer list them exhaustively.

[0019] Preferably, the diameters of the six metal nanodisks are 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, and 20 nm, respectively.

[0020] Preferably, the six metal nanodisks have the same thickness.

[0021] Preferably, the thickness of the metal nanodisk is 15 to 30 nm, for example, 15 nm, 20 nm, 25 nm or 30 nm, as well as specific values ​​between the above values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific values ​​included in the range.

[0022] Preferably, the thickness of the metal nanodisk is 20 nm.

[0023] Preferably, the radius of the circle is 80 to 100 nm, for example, 80 nm, 85 nm, 90 nm, 95 nm or 100 nm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0024] Preferably, the radius of the circle is 80 nm.

[0025] In a second aspect, the present invention provides a method for preparing the chiral metasurface optical device as described in the first aspect, the preparation method comprising the following steps:

[0026] (1) A sacrificial layer is coated on a substrate to obtain a first substrate.

[0027] (2) Coating the nanosphere colloidal solution on the first substrate obtained in step (1) to obtain a second substrate.

[0028] (3) The second substrate obtained in step (2) is subjected to oxygen plasma etching to peel off the nanospheres on the second substrate, remove the sacrificial layer under the nanosphere holes, and obtain a hole mask.

[0029] (4) Depositing metal on the hole mask obtained in step (3) by an inclined angle deposition coating method to obtain the chiral metasurface optical device.

[0030] As a preferred technical solution, the substrate is pretreated before applying the sacrificial layer.

[0031] Preferably, the pretreatment comprises placing the substrate in a solvent, ultrasonicating it, and then drying it.

[0032] Preferably, the solvent includes a first solvent, a second solvent and a third solvent.

[0033] Preferably, the first solvent comprises acetone, the second solvent comprises ethanol, and the third solvent comprises water.

[0034] Preferably, the specific steps of the pretreatment are placing the substrate in a first solvent, a second solvent and a third solvent in sequence, performing ultrasound, and then blowing the substrate dry with nitrogen.

[0035] Preferably, the material of the sacrificial layer includes polymethyl methacrylate.

[0036] As a preferred technical solution, step (2) further includes subjecting the first substrate to oxygen plasma etching treatment before coating the nanosphere colloidal solution.

[0037] Preferably, the nanosphere colloid solution comprises a polystyrene nanosphere colloid solution.

[0038] Preferably, with the plane of the hole mask as a reference, the azimuth angle of the tilted angle deposition coating is 25° to 35°, for example, it can be 25°, 26°, 27°, 28°, 29°, 30°, 31°, 32°, 33°, 34° or 35°, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0039] The tilt angle of the tilt angle deposition coating is 0°~300°, and the tilt angle is related to the size of the metal nanodisk. The tilt angles of the tilt angle deposition coatings of different metal nanodisks are different. In a preferred technical solution, in the order of the diameters of the 6 metal nanodisks from large to small, the tilt angles are 0°, 60°, 120°, 180°, 240°, and 300° respectively.

[0040] Preferably, the tilted angle deposition coating controls the diameter of the metal nano-disk by gradually reducing the evaporated thickness of the material by counterclockwise rotation.

[0041] Preferably, the evaporation thickness is 20 to 50 nm, for example, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm or 50 nm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0042] Preferably, the deposition is performed six times, so as to form six metal nanodisks in the metal nanounit.

[0043] As a preferred technical solution, after the oblique angle deposition coating is completed, the step of immersing and peeling the sacrificial layer in a solvent is also included.

[0044] Preferably, the solvent comprises acetone.

[0045] Preferably, the immersion temperature is 60-100°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C or 100°C, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0046] Preferably, the soaking time is 30 to 60 minutes, for example, it can be 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes or 60 minutes, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0047] As a preferred technical solution, the preparation method specifically comprises the following steps:

[0048] (1) First, a SiO2 substrate is cleaned, and then a polymethyl methacrylate sacrificial layer is spin-coated onto the SiO2 substrate to obtain a first substrate.

[0049] (2) The first substrate obtained in step (1) is subjected to oxygen plasma etching treatment, and then a polystyrene nanosphere colloidal solution is coated on the first substrate to obtain a second substrate.

[0050] The etching flow rate of the oxygen plasma etching process is 20-100 sccm.

[0051] (3) The second substrate obtained in step (2) is subjected to oxygen plasma etching treatment to peel off the polystyrene nanospheres on the second substrate and remove the polymethyl methacrylate sacrificial layer under the nanosphere holes to obtain a hole mask.

[0052] The etching flow rate of the oxygen plasma etching process is 20-100 sccm.

[0053] (4) depositing gold on the hole mask obtained in step (5) by an oblique angle deposition coating method to obtain a third substrate, and then immersing the third substrate in acetone to peel off the polymethyl methacrylate sacrificial layer to obtain the chiral metasurface optical device.

[0054] Taking the plane of the hole mask as a reference, the azimuth angle of the tilt angle deposition coating is 25° to 35°, and the tilt angle is 0° to 300°.

[0055] The tilt angle deposition coating controls the diameter of the metal nano disk by gradually reducing the evaporated thickness of the material by counterclockwise rotation.

[0056] The evaporation thickness is 20 to 50 nm.

[0057] The deposition was performed 6 times.

[0058] The soaking temperature is 60-100°C.

[0059] The soaking time is 30 to 60 minutes.

[0060] The preparation method of the chiral metasurface optical device provided by the present invention, in step (2), the first substrate is subjected to oxygen plasma etching treatment to improve the hydrophilicity of the first substrate, and the adhesion between the polystyrene (PS) nanosphere colloidal solution and the surface and the electrostatic repulsion between the PS nanospheres will make the PS nanospheres present a short-range ordered distribution state on the second substrate; the distribution of holes on the hole mask template in step (3) corresponds to the distribution position of the metal nanodisk on the substrate, and the holes are formed by peeling off the PS nanospheres. The chiral metasurface optical device is prepared by tilt angle deposition assisted hole mask colloidal lithography, and the preparation method has the advantages of low manufacturing cost, high processing speed and large-area batch processing.

[0061] In a third aspect, the present invention provides an ultrafast circularly polarized light modulation method, the modulation method comprising the following steps:

[0062] (S1) Splitting the linearly polarized laser pulse emitted by the laser into pump light and probe light.

[0063] (S2) The probe light is converted into probe white light through a nonlinear crystal, and the wavelength of the pump light is adjusted to 380-500 nm, for example, 380 nm, 390 nm, 400 nm, 410 nm, 420 nm, 430 nm, 440 nm, 450 nm, 460 nm, 470 nm, 480 nm, 490 nm or 500 nm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0064] (S3) converting the pump light into left-handed circularly polarized pump light or right-handed circularly polarized pump light through a polarizer and a quarter-wave plate, and converting the probe white light into left-handed circularly polarized probe white light or right-handed circularly polarized probe white light, the left-handed circularly polarized pump light or right-handed circularly polarized pump light being used as an excitation light source, and the left-handed circularly polarized probe white light or right-handed circularly polarized probe white light being used as a detection light source.

[0065] (S4) The excitation light source and the detection light source are incident on or back-incident to the same position of the chiral metasurface optical device as described in the first aspect. When the excitation light source is incident on the chiral metasurface optical device, the excitation light source is the left-handed circularly polarized pump light. When the excitation light source is back-incident on the chiral metasurface optical device, the excitation light source is the right-handed circularly polarized pump light.

[0066] (S5) Controlling the delay time of the detection light source relative to the excitation light source arriving at the same position of the chiral metasurface optical device.

[0067] (S6) Under the set delay time, the transient absorption signal of the chiral metasurface optical device under the irradiation of the detection light source is detected, and the modulation and flip modulation of the circularly polarized light by the chiral metasurface optical device is reflected by the transient absorption signal.

[0068] The front side of the chiral metasurface optical device provided by the present invention is the surface on which the chiral oligomer nano-metasurface structure is arranged on the substrate, and the surface on which the chiral oligomer nano-metasurface structure is not arranged is the back side. The front incidence refers to that the excitation light source and the detection light source are incident on the front side of the chiral metasurface optical device, and the back incidence refers to that the excitation light source and the detection light source are incident on the back side of the chiral metasurface optical device. The chiral oligomer nano-metasurface structure is transformed from a ground state to an excited state by the excitation light source, so that the optical characteristics of the chiral metasurface can be excited on an ultrafast time scale. The combination of the excitation light source and the detection light source can realize a transient circular dichroism signal, which can more intuitively reflect the ultrafast circular polarization modulation capability of the chiral metasurface, and by changing the incident directions of the excitation light source and the detection light source, the sign reversal of the transient circular dichroism signal can be realized, and the reversal modulation of ultrafast circularly polarized light can be flexibly realized without complicated enantiomer preparation, and has an ultrafast response time and relaxation time.

[0069] As a preferred technical solution, the pulse width of the linearly polarized laser pulse is 10 to 50 fs, for example, it can be 10 fs, 15 fs, 20 fs, 25 fs, 30 fs, 35 fs, 40 fs, 45 fs or 50 fs, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0070] The repetition frequency of the linearly polarized laser pulse is 1 to 2 kHz, for example, it can be 1.1 kHz, 1.2 kHz, 1.3 kHz, 1.4 kHz, 1.5 kHz, 1.6 kHz, 1.7 kHz, 1.8 kHz, 1.9 kHz or 2 kHz, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0071] Preferably, the wavelength of the probe white light is 440-675 nm, for example, it can be 440 nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm or 675 nm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0072] Preferably, the delay time is -0.7 to 6ps, for example, it can be -0.7ps, -0.6ps, -0.5ps, -0.4ps, -0.3ps, -0.2ps, -0.1ps, 0ps, 1ps, 2ps, 3ps, 4ps, 5ps or 6ps, as well as specific point values ​​between the above point values. Due to limited space and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0073] A negative delay time indicates that the detection light source reaches the chiral metasurface optical device before the excitation light source, and the chiral metasurface optical device is in a ground state. A positive delay time indicates that the detection light source reaches the chiral metasurface optical device after the excitation light source, and the chiral metasurface optical device is in an excited state.

[0074] Preferably, the delay time is 300 fs.

[0075] Compared with the prior art, the present invention has the following beneficial effects:

[0076] (1) The chiral metasurface optical device provided by the present invention has a simple structure and exhibits planar chiral characteristics. Therefore, the polarization-resolving optical characteristics can be flipped by adjusting the incident direction of circularly polarized light, which provides a simple and valuable design approach for the functional flexibility of polarization optical elements.

[0077] (2) The chiral metasurface optical device provided by the present invention is prepared by a hole mask colloidal lithography method assisted by tilt angle deposition, which has the advantages of low manufacturing cost, high processing speed and large-area batch processing. The operation steps are simple and flexible, which is conducive to the application and production of optical devices.

[0078] (3) The chiral metasurface optical device provided by the present invention uses gold as the main material to achieve the chiral optical effect assisted by plasmon enhancement. The advantages of plasmon enhancement effect, band controllability and auxiliary hot electron dynamics process can be used to tune the transient circular dichroism signal.

[0079] (4) The circularly polarized light modulation method provided by the present invention is to stimulate the optical characteristics of the chiral metasurface on an ultrafast time scale by changing the femtosecond pump light and probe light with different circular polarization states, thereby replacing the detection of traditional circular dichroism signals. This breaks through the limitation of no time response in conventional circularly polarized light modulation methods and is helpful for the design and development of the next generation of high-speed optical devices.

[0080] (5) The circularly polarized light modulation method provided by the present invention can obtain different transient circular dichroism signals through the measured transient absorption signal, which corresponds to the transient absorption difference obtained by the chiral metasurface optical device to different circularly polarized pump light excitations and different circularly polarized probe white light detections, and can more intuitively reflect the ultrafast circular polarization modulation capability of the chiral metasurface. In addition, the transient circular dichroism signal not only undergoes a sign flip at 300fs, but also maintains the inverted dynamic process on the ultrafast time scale of -0.7ps to 6ps, and has the same hot electron excitation and relaxation time, and can be used in ultrafast optical switches and quantum communication applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Figure 1 A schematic diagram of the structure of the chiral metasurface optical device provided in Example 1;

[0082] Among them, 1-first metal nanodisk, 2-second metal nanodisk, 3-third metal nanodisk, 4-fourth metal nanodisk, 5-fifth metal nanodisk, 6-sixth metal nanodisk;

[0083] Figure 2 Schematic diagram of modulation of circularly polarized light by the chiral metasurface optical device provided in Example 1;

[0084] Figure 3 A comparison diagram of normal-incident transient absorption signals of the chiral metasurface optical device provided in Example 1;

[0085] Figure 4 A comparison diagram of back-incident transient absorption signals of the chiral metasurface optical device provided in Example 1;

[0086] Figure 5 A comparison diagram of transient circular dichroism signals of the chiral metasurface optical device provided in Example 1;

[0087] Figure 6 This is a graph showing the variation of the transient circular dichroism absorption signal of the chiral metasurface optical device provided in Example 1 with the delay time. DETAILED DESCRIPTION

[0088] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and through specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0089] The sources of some components in the following examples are as follows:

[0090] (1) Polymethyl methacrylate: Product number P821346-500g, purchased from Sigma-Aldrich Company;

[0091] (2) Polystyrene nanosphere colloidal solution: diameter 300-500 nm, purchased from Sigma-Aldrich Company.

[0092] The transient absorption spectrometer used in the following examples: brand: CDP, agent: Pioneer Technology Co., Ltd., model: Excipro CDP2022i.

[0093] Example 1

[0094] A chiral metasurface optical device, the structural schematic diagram of which is shown in Figure 1 As shown, the left side is a top view of the structure of the chiral metasurface optical device, including a substrate and a chiral oligomer nano-metasurface structure arranged on the substrate, the material of the substrate is SiO2, and the thickness of the substrate is 0.2 mm; the chiral oligomer nano-metasurface structure is formed by a plurality of metal nano-units with the same structure being distributed in a short range in an orderly manner; the metal nano-unit includes 6 metal nano-disks, and the specific structure is as follows Figure 1 As shown on the right, it includes a first metal nanodisk 1, a second metal nanodisk 2, a third metal nanodisk 3, a fourth metal nanodisk 4, a fifth metal nanodisk 5, and a sixth metal nanodisk 6, which are arranged counterclockwise; the material of each of the metal nanodisks is gold, the centers of the six metal nanodisks are located on the circumference of the same circle, the radius of the circumference is 80nm, the six metal nanodisks have the same thickness of 20nm, the center distances of any two adjacent metal nanodisks are equal, the radii of the six metal nanodisks decrease in a counterclockwise direction on the circumference, and in order of radius from large to small, the diameters of the first metal nanodisk to the sixth metal nanodisk are 70nm, 60nm, 50nm, 40nm, 30nm, and 20nm, respectively.

[0095] The method for preparing the chiral metasurface optical device comprises the following steps:

[0096] (1) Prepare a SiO2 substrate with a thickness of 0.2 mm and a diameter of 25.0 mm, and ultrasonically clean and blow dry it in acetone, ethanol, and deionized water in turn, and finally blow dry it with nitrogen for use. Spin coat a sacrificial layer of polymethyl methacrylate (PMMA) onto the SiO2 substrate using a spin coater (manufacturer: Youlisheng, model: 12A-80) at a speed of 3000 rpm for 2 min to obtain a first substrate.

[0097] (2) The first substrate obtained in step (1) was subjected to oxygen plasma etching treatment (etching flow rate of 50 sccm) using a plasma etcher (manufacturer: Oxford Instruments, model: PlasmaPro100 Cobra300), and then a PS nanosphere colloidal solution was coated on the first substrate to obtain a second substrate.

[0098] (3) The second substrate obtained in step (2) is subjected to oxygen plasma etching treatment (etching flow rate is 40 sccm) using a plasma etcher (manufacturer: Oxford Instruments, model: PlasmaPro100 Cobra300), the PS balls are peeled off by oxygen plasma etching, and a nanopore mask is left on the PMMA layer, and the PMMA exposed under the nanopores in the mask is removed to form a hole mask.

[0099] (4) The hole mask obtained in step 3 was placed in an electron beam vacuum evaporation coating machine (manufacturer: Vistec Lithography Company, model: Vistec EBPG 5000plus ES), with the plane of the hole mask as the reference, the azimuth angle was 25°, and the tilt angles were 0°, 60°, 120°, 180°, 240°, and 300° in descending order of the diameters of the six metal nanodisks, and gold was deposited on the hole mask to obtain a third substrate, the number of depositions was 6 times, and the evaporation thickness was 20 mm, wherein the vacuum degree of the electron beam vacuum evaporation coating machine was not greater than 1×10 -5 torr, and then immerse the third substrate in acetone and place it on a hot plate heated to 60° C. for 30 minutes to peel off the PMMA film, and then blow dry with nitrogen to obtain the chiral metasurface optical device.

[0100] Example 2

[0101] A chiral metasurface optical device, which differs from Example 1 only in that, in order of radius from large to small, the diameters of the first metal nanodisk to the sixth metal nanodisk are 60nm, 50nm, 40nm, 30nm, 20nm, and 10nm, respectively, and the remaining materials and structural parameters are the same as those in Example 1.

[0102] The preparation method of the chiral metasurface optical device is the same as that of Example 1.

[0103] Example 3

[0104] An ultrafast circularly polarized light modulation method, the modulation method comprising the following steps:

[0105] (S1) The linearly polarized laser pulses emitted by a femtosecond titanium sapphire laser (manufacturer: Coherent, model: Astrella-USP-1k) with a pulse width of 50 fs and a repetition rate of 1 kHz are divided into pump light and probe light.

[0106] (S2) The probe light is converted into probe white light through a nonlinear crystal, and the wavelength of the pump light is adjusted to 390 nm through an OPA optical parametric amplifier (manufacturer: Coherent, model: Coherent OperA Solo).

[0107] (S3) converting the pump light into left-handed circularly polarized pump light and converting the probe white light into left-handed circularly polarized probe white light through a polarizer and a quarter-wave plate, wherein the left-handed circularly polarized pump light serves as an excitation light source and the left-handed circularly polarized probe white light serves as a detection light source.

[0108] (S4) The excitation light source and the detection light source are incident on the same position of the chiral metasurface optical device provided in Example 1.

[0109] (S5) The delay time of the detection light source relative to the excitation light source arriving at the same position of the chiral metasurface optical device is controlled to be 300 fs by a delay line control device of the transient absorption spectrometer.

[0110] (S6) When the delay time is 300 fs, a transient absorption spectrometer is used to detect a transient absorption signal of the chiral metasurface optical device under the irradiation of the detection light source, and the modulation of the circularly polarized light by the chiral metasurface optical device is reflected by the transient absorption signal.

[0111] Example 4

[0112] An ultrafast circularly polarized light modulation method, which differs from Example 3 only in that, in step (S3), the pump light is converted into left-handed circularly polarized pump light by a polarizer and a quarter-wave plate, and the probe white light is converted into right-handed circularly polarized probe white light, the left-handed circularly polarized pump light is used as an excitation light source, and the right-handed circularly polarized probe white light is used as a detection light source; other steps and process parameters are the same as those in Example 3.

[0113] Example 5

[0114] An ultrafast circularly polarized light modulation method, which differs from Example 3 in that, in step (S3), the pump light is converted into right-handed circularly polarized pump light by a polarizer and a quarter-wave plate, and the probe white light is converted into left-handed circularly polarized probe white light, the right-handed circularly polarized pump light is used as an excitation light source, and the left-handed circularly polarized probe white light is used as a detection light source; in step (S4), the excitation light source and the detection light source are back-incident to the same position of the chiral metasurface optical device provided in Example 1; in step (S6), when the delay time is 300fs, a transient absorption spectrometer is used to detect the transient absorption signal of the chiral metasurface optical device under the irradiation of the detection light source, and the transient absorption signal reflects the flip modulation of the circularly polarized light by the chiral metasurface optical device; other steps and process parameters are the same as those in Example 3.

[0115] Example 6

[0116] An ultrafast circularly polarized light modulation method, which differs from Example 5 only in that, in step (S3), the pump light is converted into right-handed circularly polarized pump light by a polarizer and a quarter-wave plate, and the probe white light is converted into right-handed circularly polarized probe white light, the right-handed circularly polarized pump light is used as an excitation light source, and the right-handed circularly polarized probe white light is used as a detection light source; other steps and process parameters are the same as those in Example 5.

[0117] Example 7

[0118] A method for ultrafast circularly polarized light modulation, which differs from Example 3 in that the delay time of step (S5) is 2 ps, and step (S6) is to use a transient absorption spectrometer to detect the transient absorption signal of the chiral metasurface optical device under the irradiation of the detection light source when the delay time is 2 ps, and the modulation of the circularly polarized light by the chiral metasurface optical device is reflected by the transient absorption signal; the other steps and process parameters are the same as those in Example 3.

[0119] Example 8

[0120] An ultrafast circularly polarized light modulation method, which differs from Example 7 only in that, in step (S3), the pump light is converted into left-handed circularly polarized pump light by a polarizer and a quarter-wave plate, and the probe white light is converted into right-handed circularly polarized probe white light, the left-handed circularly polarized pump light is used as an excitation light source, and the right-handed circularly polarized probe white light is used as a detection light source; other steps and process parameters are the same as those in Example 7.

[0121] Example 9

[0122] A method for ultrafast circularly polarized light modulation, which differs from Example 7 in that, in step (S3), the pump light is converted into right-handed circularly polarized pump light by a polarizer and a quarter-wave plate, and the probe white light is converted into left-handed circularly polarized probe white light, the right-handed circularly polarized pump light is used as an excitation light source, and the left-handed circularly polarized probe white light is used as a detection light source; in step (S4), the excitation light source and the detection light source are back-incident to the same position of the chiral metasurface optical device provided in Example 1; in step (S6), the chiral metasurface optical device reflects the flip modulation of circularly polarized light through the transient absorption signal; other steps and process parameters are the same as those in Example 7.

[0123] Example 10

[0124] A method for ultrafast circularly polarized light modulation, which differs from Example 9 in that, in step (S3), the pump light is converted into right-handed circularly polarized pump light by a polarizer and a quarter-wave plate, and the probe white light is converted into right-handed circularly polarized probe white light, the right-handed circularly polarized pump light is used as an excitation light source, and the right-handed circularly polarized probe white light is used as a detection light source; other steps and process parameters are the same as those in Example 9.

[0125] A transient absorption spectrometer is used to detect the transient absorption signals of the chiral metasurface optical device provided in Example 1 under different excitation light sources and detection light sources, and a transient circular dichroism signal diagram is obtained by the difference of the transient absorption signals under different detection light sources. The results are shown in FIG. Figure 3 to Figure 6 As shown:

[0126] (1) Figure 3 and Figure 4 This is a comparison diagram of transient absorption signals of the chiral metasurface optical device provided in Example 1 under normal incidence and back incidence conditions, respectively, when irradiated with white light from probes with different circular polarization states. Figure 3It can be seen that under the excitation of left-handed circularly polarized pump light, the transient absorption spectrum of the chiral metasurface optical device shows signals of two different bands and different signs, namely the positive induced excited state absorption peak corresponding to the short wavelength and the plasmon bleaching peak corresponding to the long wavelength. This is because after being excited by left-handed circularly polarized pump light, electrons transition from the 5d band of gold to the 6sp band or transition to a higher excited state within the 6sp band. When the circular polarization state of the left-handed circularly polarized pump light remains unchanged, and the detection light source is changed from left-handed circularly polarized probe white light to right-handed circularly polarized probe white light, due to the corresponding different electron transition resonance processes, it shows obvious differences in spectral characteristics, that is, it has ultrafast polarization sensitivity; the transient absorption amplitude of the left-handed circularly polarized probe white light is significantly stronger than that of the right-handed circularly polarized probe white light, that is, there is a difference in the number of hot electron couplings between the two, and the former is 0.006 higher than the latter in the positive induced excited state absorption peak and 0.012 higher at the plasmon bleaching peak, indicating that more hot electrons participate in the dipole resonance process of the left-handed circularly polarized probe white light, among which the plasmon resonance will enhance the ultrafast optical response. Figure 4 It can be seen that the transient absorption amplitude of the right-handed circularly polarized probe white light is significantly stronger than that of the left-handed circularly polarized probe white light at back incidence, and more hot electrons participate in the dipole resonance process of the right-handed circularly polarized probe white light.

[0127] (2) Figure 5 To pass Figure 3 and Figure 4 The transient circular dichroism signal comparison diagram obtained by the transient absorption signal difference of the transient absorption spectrum can reflect the polarization sensitivity of the pump light and probe white light with different circular polarization states. Under the excitation of different excitation light sources, the chiral metasurface optical device has different responses to different detection light sources. Therefore, the difference of the transient absorption signal of different detection light sources, that is, the transient circular dichroism signal, can be used to intuitively reflect the ultrafast modulation ability of the chiral metasurface optical device to circularly polarized light. The different number of hot electrons involved is determined by the strength of the plasmon coupling resonance; when the incident direction of the excitation light source and the detection light source is back-incident, from Figure 5 It can be seen that the transient circular dichroism signals corresponding to different detection light sources show spectral characteristics of signal sign reversal at a delay time of 300fs, and the transient absorption amplitude of the right-handed circularly polarized probe white light is significantly stronger than that of the left-handed circularly polarized probe white light. At this time, the plasmon coupling resonance induced by the right-handed circularly polarized probe white light is stronger, that is, the sensitivity of the number of hot electrons participating to light of different polarization states is reversed, proving the ability to modulate ultrafast polarized light by changing the excitation incident direction. In addition, the results show the optical characteristics of the mirror image enantiomer of the chiral structure, so the flip modulation of circularly polarized light can be completed without the preparation and processing of the chiral enantiomer, providing a simple and valuable design approach for the functional flexibility of polarization optical elements.

[0128] (3) Figure 6 The transient circular dichroism signal diagrams of different detection light sources under different delay times for the chiral metasurface optical device provided in Example 1 can be used to observe the evolution of the transient circular dichroism signal of the chiral metasurface optical device over time. Figure 6 It can be seen that the transient circular dichroism signal changes with the delay time, reflecting the response time of ultrafast circular polarization light modulation. When the incident direction of the excitation light source and the detection light source is changed, the transient circular dichroism signal not only flips its sign at 300fs, but also presents an inverted dynamic process on the ultrafast time scale (-0.7 to 6ps), with the amplitude rising rapidly within 300fs and then recovering through exponential decay relaxation. The corresponding hot electron relaxation time can be obtained through exponential fitting, and the same time constant (2ps) is maintained for both forward and back incidence.

[0129] The applicant declares that the present invention illustrates the detailed process flow of the present invention through the above-mentioned embodiments, but the present invention is not limited to the above-mentioned detailed process flow, that is, it does not mean that the present invention must rely on the above-mentioned detailed process flow to be implemented. Those skilled in the art should understand that any improvement of the present invention, equivalent replacement of various raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A chiral metasurface optical device, characterized in that: The chiral metasurface optical device comprises a substrate and a chiral oligomer nano-metasurface structure disposed on the substrate; The chiral oligomer nano-supersurface structure is formed by a plurality of metal nano-units with the same structure being distributed in a short-range orderly manner; the metal nano-units include 6 metal nano-disks, and the centers of the 6 metal nano-disks are located on the circumference of the same circle.

2. The chiral metasurface optical device according to claim 1, characterized in that: The material of the substrate includes SiO2 or sapphire; Preferably, the thickness of the substrate is 0.2 to 0.5 mm; Preferably, the material of the metal nanodisk is gold.

3. The chiral metasurface optical device according to claim 1 or 2, characterized in that: The center distances between any two adjacent metal nanodisks are equal; Preferably, the radii of the six metal nanodisks decrease in sequence in a counterclockwise direction on the circumference; Preferably, in order of radius from large to small, the diameters of the six metal nanodisks are: 60-80 nm, 50-70 nm, 40-60 nm, 30-50 nm, 20-40 nm and 10-30 nm; Preferably, the diameters of the six metal nanodisks are 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, and 20 nm, respectively; Preferably, the 6 metal nanodisks have the same thickness; Preferably, the thickness of the metal nanodisk is 15 to 30 nm; Preferably, the thickness of the metal nanodisk is 20 nm; Preferably, the radius of the circle is 80 to 100 nm; Preferably, the radius of the circle is 80 nm.

4. A method for preparing a chiral metasurface optical device according to any one of claims 1 to 3, characterized in that: The preparation method comprises the following steps: (1) coating a sacrificial layer on a substrate to obtain a first substrate; (2) coating the nanosphere colloidal solution on the first substrate obtained in step (1) to obtain a second substrate; (3) performing oxygen plasma etching on the second substrate obtained in step (2), peeling off the nanospheres on the second substrate, removing the sacrificial layer under the nanosphere holes, and obtaining a hole mask; (4) Depositing metal on the hole mask obtained in step (3) by an inclined angle deposition coating method to obtain the chiral metasurface optical device.

5. The preparation method according to claim 4, characterized in that: Pre-treating the substrate before applying the sacrificial layer; Preferably, the pretreatment comprises placing the substrate in a solvent and drying it after ultrasonication; Preferably, the solvent comprises a first solvent, a second solvent and a third solvent; Preferably, the first solvent comprises acetone, the second solvent comprises ethanol, and the third solvent comprises water; Preferably, the specific steps of the pretreatment are placing the substrate in a first solvent, a second solvent and a third solvent in sequence for ultrasonic treatment, and then drying the substrate with nitrogen; Preferably, the material of the sacrificial layer includes polymethyl methacrylate.

6. The preparation method according to claim 4 or 5, characterized in that: Step (2) also includes subjecting the first substrate to oxygen plasma etching treatment before coating the nanosphere colloidal solution; Preferably, the nanosphere colloid solution comprises a polystyrene nanosphere colloid solution; Preferably, with the plane of the hole mask as a reference, the azimuth angle of the tilt angle deposition coating is 25° to 35°, and the tilt angle is 0° to 300°; Preferably, the tilt angle deposition coating controls the diameter of the metal nano disk by gradually reducing the evaporation thickness of the material by rotating counterclockwise; Preferably, the evaporation thickness is 20 to 50 nm; Preferably, the deposition is performed 6 times.

7. The preparation method according to any one of claims 4 to 6, characterized in that: After the oblique angle deposition coating is completed, the step of immersing the sacrificial layer in a solvent and peeling it off is also included; Preferably, the solvent comprises acetone; Preferably, the soaking temperature is 60-100°C; Preferably, the soaking time is 30 to 60 minutes.

8. The preparation method according to any one of claims 4 to 7, characterized in that: The preparation method specifically comprises the following steps: (1) first cleaning a SiO2 substrate, and then spin coating a polymethyl methacrylate sacrificial layer onto the SiO2 substrate to obtain a first substrate; (2) subjecting the first substrate obtained in step (1) to an oxygen plasma etching treatment, and then coating the first substrate with a polystyrene nanosphere colloidal solution to obtain a second substrate; (3) performing oxygen plasma etching on the second substrate obtained in step (2), peeling off the polystyrene nanospheres on the second substrate, removing the polymethyl methacrylate sacrificial layer under the nanosphere holes, and obtaining a hole mask; (4) depositing gold on the hole mask obtained in step (3) by an oblique angle deposition coating method to obtain a third substrate, and then immersing the third substrate in acetone to peel off the polymethyl methacrylate sacrificial layer to obtain the chiral metasurface optical device; Taking the plane of the hole mask as a reference, the azimuth angle of the tilt angle deposition coating is 25° to 35°, and the tilt angle is 0° to 300°; The tilt angle deposition coating controls the diameter of the metal nano disk by gradually reducing the evaporated thickness of the material by counterclockwise rotation; The evaporation thickness is 20 to 50 nm; The number of depositions is 6 times; The soaking temperature is 60-100°C; The soaking time is 30 to 60 minutes.

9. An ultrafast circularly polarized light modulation method, characterized in that: The modulation method comprises the following steps: (S1) dividing the linearly polarized laser pulse emitted by the laser into pump light and probe light; (S2) converting the probe light into probe white light through a nonlinear crystal, and adjusting the wavelength of the pump light to 380-500 nm; (S3) converting the pump light into left-handed circularly polarized pump light or right-handed circularly polarized pump light through a polarizer and a quarter wave plate, converting the probe white light into left-handed circularly polarized probe white light or right-handed circularly polarized probe white light, using the left-handed circularly polarized pump light or right-handed circularly polarized pump light as an excitation light source, and using the left-handed circularly polarized probe white light or right-handed circularly polarized probe white light as a detection light source; (S4) the excitation light source and the detection light source are incident on the same position of the chiral metasurface optical device as described in any one of claims 1 to 3, when the excitation light source is incident on the chiral metasurface optical device, the excitation light source is the left-handed circularly polarized pump light, and when the excitation light source is incident on the chiral metasurface optical device, the excitation light source is the right-handed circularly polarized pump light; (S5) controlling the delay time of the detection light source relative to the excitation light source arriving at the same position of the chiral metasurface optical device; (S6) Under the set delay time, the transient absorption signal of the chiral metasurface optical device under the irradiation of the detection light source is detected, and the modulation and flip modulation of the circularly polarized light by the chiral metasurface optical device is reflected by the transient absorption signal.

10. The ultrafast circularly polarized light modulation method according to claim 9, characterized in that: The linearly polarized laser pulse has a pulse width of 10 to 50 fs and a repetition frequency of 1 to 2 kHz; Preferably, the wavelength of the probe white light is 440 to 675 nm; Preferably, the delay time is -0.7 to 6 ps; Preferably, the delay time is 300 fs.

Images