A chiral metasurface optical device, a method of making the same, and an ultrafast circularly polarized light modulation method

Abstract

The application provides a chiral metasurface optical device, a preparation method thereof and an ultrafast circularly polarized light modulation method, the chiral metasurface optical device comprising a substrate and a chiral oligomer nanosuper surface structure arranged on the substrate, the chiral oligomer nanosuper surface structure being formed by a plurality of short-range ordered distribution of structural identical metal nanometer units; the metal nanometer unit comprises six metal nanometer discs, and the centers of the six metal nanometer discs are located on the circumference of a same circle; the structural chiral characteristic is introduced through the circularly distributed metal nanometer discs, the structure is simple, and the circularly polarized light modulation capacity is strong; through the combination of an excitation light source and a detection light source, the optical characteristic of the chiral metasurface optical device is excited on an ultrafast time scale, and a transient circular dichroism signal is realized; by changing the incident direction of the excitation light source and the detection light source, the sign of the transient circular dichroism signal can be reversed, and the chiral metasurface optical device has an ultrafast response time and a relaxation time.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nano-optics, and particularly relates to a chiral metasurface optical device, a preparation method thereof and an ultrafast circularly polarized light modulation method. BACKGROUND

[0002] A metasurface is a planar design of subwavelength artificial nanostructures that can manipulate and enhance the interaction of light and matter at the nanoscale, exhibiting unique optical phenomena. The excellent optical performance, ultra-thin, ultra-integrated and small-sized structural characteristics of the metasurface make it a strong candidate for replacing large optical elements and equipment. With the increasing demand for the speed of optical information transmission, researchers have begun to explore various superfast optical phenomena based on metasurfaces for ultrafast optical modulation at smaller time scales, such as superharmonic generation and ultrafast pulse light switching of metasurfaces. Among them, metal metasurfaces are widely studied and applied due to their unique plasmonic resonance effect and ultrafast hot electron generation. Based on surface plasmon-assisted optical devices, nanoscale collective oscillation of electrons coupled with photons can be achieved, further reducing the control range of photonic devices. However, most of the reported metal metasurfaces modulate light waves by changing the structure 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, and is widely used in contrast-enhanced polarization micro-imaging, biomolecular detection, optical communication, optical remote sensors and optical displays. Traditional circularly polarized light modulation uses chiral polymers for polarization-resolved response in a steady state, but the modulation effect of circularly polarized light is poor and the speed is slow, which 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 the circularly polarized light modulation method.

[0004] CN104865628A discloses a kind of helical metal chiral metamaterial circular polarizer, including substrate, transparent dielectric spacer layer firmly combined with base and embedded in transparent dielectric spacer layer Array of helical metal chiral microstructure.CN105137520A discloses a kind of helical metal chiral metamaterial, circular polarizer, the helical metal chiral metamaterial circular polarizer, including substrate and gradually deposited on substrate Array of helical metal wire;Helical wire array is formed by periodically arranged helical metal wire unit;Each helical wire unit is connected by the helical wire of same rotation direction upper and lower two parts, and the radius of upper part helical wire gradually increases from top to bottom, and the radius of lower part helical wire is equal.These two kinds of circular polarizer structure is complex, presents three-dimensional multilayer structure design, needs complex processing and preparation process, it is difficult to mass production, and chiral polymer is in steady state, and modulation of circular polarized light is interacted with light more, and there is no time response, cannot meet the application demand of high-speed optical device, and cannot be flipped modulation to circular polarized light.

[0005] The existing chiral metasurface optical device structure and preparation process are complex, the modulation speed of circular polarized light is slow, there is no time response, and the preparation and processing of chiral enantiomers are needed to realize the flip modulation of circular polarized light, therefore, a functional flexible and superfast circular polarized light modulation chiral metasurface optical device is developed, the modulation of circular polarized light is realized based on the metasurface of optical device, the amplitude and phase of circular polarized light are accurately controlled, the spatial and temporal degrees of freedom are increased, so that the chiral metasurface optical device can be applied to the design of future high-speed polarization-sensitive optical switch and quantum storage. SUMMARY

[0006] In view of the deficiencies of the prior art, the purpose of the present application is to provide a chiral metasurface optical device, a preparation method and a superfast circular polarized light modulation method, the chiral metasurface optical device introduces structural chiral characteristics by the circular distribution of 6 metal nanodisks of each metal nanometer unit, the structure is simple, the chiral metasurface optical device is excited and detected by the combination of pump light and probe light of different circular polarization states, the optical characteristics of chiral metasurface are excited on the superfast time scale, the modulation of circular polarized light can be reflected by detecting the transient absorption signal of chiral metasurface optical device under the irradiation of different detection light sources, and the transient absorption response with polarization resolution, i.e., transient circular dichroism signal, can also be realized, the superfast circular polarized light modulation capability of the chiral metasurface can be more directly reflected, the sign of transient circular dichroism signal can be realized by changing the incident direction of circular polarized light, the superfast response time and relaxation time are provided, a simple and valuable way is provided for the functional flexibility and high-speed design of polarization optical element, and the present application can be further applied in the fields of superfast optical switch and quantum communication.

[0007] To achieve this purpose, the following technical solutions are adopted in the present application:

[0008] In a first aspect, the present application provides a chiral metasurface optical device, comprising a substrate and a chiral oligomer nanometasurface structure disposed on the substrate; the chiral oligomer nanometasurface structure is formed by a plurality of short-range ordered distribution of structural identical metal nanometer units; the metal nanometer unit comprises six metal nanodisks, and the centers of the six metal nanodisks are located on the circumference of the same circle.

[0009] In the chiral metasurface optical device provided by the present application, the chiral oligomer nanometasurface structure is a two-dimensional planar monolayer chiral metasurface formed by a plurality of short-range ordered distribution of chiral metal nanometer units, which is simple in structure and can be observed on the front and back surfaces to observe opposite chiral mirror image characteristics. The short-range ordered distribution of chiral metal nanometer units introduces structural asymmetry, which can produce different responses to different circularly polarized light, i.e., induce different plasmon resonance modes, which are specifically reflected in the waveform, amplitude and waveband of the absorption spectrum. At the same time, the coupling between the metal nanodisks can enhance the interaction with light. Under the interaction with light, the oligomer composed of six metal nanodisks can produce a Coulomb dipole interaction, which exhibits a dipole coupling with chiral optical characteristics. In addition, the left-handed circular distribution configuration of the metal nanodisks can produce stronger capacitive coupling resonance with left-handed circularly polarized light, inducing a stronger electric field. Since the structure has a planar two-dimensional chiral feature, the optical chirality can be flipped by exciting the front and back surfaces.

[0010] In the chiral metasurface optical device provided by the present application, the chiral oligomer nanometasurface structure is an array structure composed of a plurality of short-range ordered structural identical metal nanometer units. The "plurality" generally means at least two. In the present application, the specific number of metal nanometer units can be determined according to the performance of the chiral metasurface optical device. Generally, the number is in the thousands. The short-range order refers to the distance between the centers of two adjacent metal nanometer units being 500±50 nm.

[0011] The following is a preferred technical solution of the present application, but is not a limitation on the technical solutions provided by the present application. Through the following preferred technical solution, the purpose and beneficial effects of the present application can be better achieved and realized.

[0012] As a preferred technical solution, the material of the substrate comprises 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, and specific point values between the above point values. Due to the limitation of the length and the consideration of simplicity, the present application will not exhaustively list the specific point values included in the range.

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

[0015] The metal nanodisks all adopt gold material, have surface plasmon effect to enhance the interaction with light, and can generate hot electrons at superfast speed, realize nanoscale collective oscillation of electrons coupled with photons, and further reduce the control range of photonic devices.

[0016] As a preferred technical solution, the center-to-center distance of any two adjacent metal nanodisks is equal.

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

[0018] Preferably, in the order of radius from large to small, the diameters of the six metal nanodisks are 60-80 nm (for example, 62 nm, 65 nm, 68 nm, 70 nm, 72 nm, 75 nm, or 78 nm, etc.), 50-70 nm (for example, 52 nm, 55 nm, 58 nm, 60 nm, 62 nm, 65 nm, or 68 nm, etc.), 40-60 nm (for example, 42 nm, 45 nm, 48 nm, 50 nm, 52 nm, 55 nm, or 58 nm, etc.), 30-50 nm (for example, 32 nm, 35 nm, 38 nm, 40 nm, 42 nm, 45 nm, or 48 nm, etc.), 20-40 nm (for example, 22 nm, 25 nm, 28 nm, 30 nm, 32 nm, 35 nm, or 38 nm, etc.), and 10-30 nm (for example, 12 nm, 15 nm, 18 nm, 20 nm, 22 nm, 25 nm, or 28 nm, etc.). For example, the diameters of the six metal nanodisks can be as follows: 60 nm, 50 nm, 40 nm, 30 nm, 20 nm, and 10 nm; 65 nm, 55 nm, 45 nm, 35 nm, 25 nm, and 15 nm; 75 nm, 65 nm, 55 nm, 45 nm, 35 nm, and 25 nm; 80 nm, 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, and other diameter combinations that meet the conditions. Due to the length and the consideration of simplicity, the present application will not be listed 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 thicknesses of the six metal nanodisks are the same.

[0021] Preferably, the thickness of the metal nanodisk is 15-30 nm, for example, 15 nm, 20 nm, 25 nm or 30 nm, and specific point values between the above point values, limited to the length and for the sake of simplicity, the present application does not list the specific point values included in the range.

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

[0023] Preferably, the radius of the circumference is 80-100 nm, for example, 80 nm, 85 nm, 90 nm, 95 nm or 100 nm, and specific point values between the above point values, limited to the length and for the sake of simplicity, the present application does not list the specific point values included in the range.

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

[0025] In a second aspect, the present application provides a preparation method of the chiral metasurface optical device according to the first aspect, the preparation method comprising the following steps:

[0026] (1) coating a sacrificial layer on the substrate to obtain a first substrate.

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

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

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

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

[0031] Preferably, the pretreatment comprises placing the substrate in a solvent for ultrasonic treatment and then drying.

[0032] Preferably, the solvent comprises 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 the first solvent, the second solvent and the third solvent for ultrasonic treatment in sequence, and then blowing the substrate dry with nitrogen.

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

[0036] As a preferred technical solution, the step (2) further comprises an oxygen plasma etching treatment on the first substrate before the coating of the nanosphere colloidal solution.

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

[0038] Preferably, the azimuth angle of the oblique angle deposition film is 25°-35°, for example, 25°, 26°, 27°, 28°, 29°, 30°, 31°, 32°, 33°, 34° or 35°, and specific point values between the above-mentioned point values, limited to the length and for the sake of simplicity, the present application will not exhaustively list the specific point values included in the range.

[0039] The oblique angle of the oblique angle deposition film is 0°-300°, which is related to the size of the metal nanodisk. The oblique angle of the oblique angle deposition film is different for different metal nanodisks. In a preferred technical solution, according to the order of the diameters of the six metal nanodisks from large to small, the oblique angles are 0°, 60°, 120°, 180°, 240° and 300°, respectively.

[0040] Preferably, the oblique angle deposition film gradually reduces the evaporation thickness of the material by counterclockwise rotation to control the diameter of the metal nanodisk.

[0041] Preferably, the evaporation thickness is 20-50 nm, for example, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm or 50 nm, and specific point values between the above-mentioned point values, limited to the length and for the sake of simplicity, the present application will not exhaustively list the specific point values included in the range.

[0042] Preferably, the number of depositions is 6, thereby forming 6 metal nanodisks in the metal nanocell.

[0043] As a preferred technical solution, after the completion of the oblique angle deposition film, the method further comprises a step of soaking and peeling off the sacrificial layer in a solvent.

[0044] Preferably, the solvent comprises acetone.

[0045] Preferably, the soaking temperature is 60-100℃, for example, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃ or 100℃, and specific point values between the above-mentioned point values, limited to the length and for the sake of simplicity, the present application will not exhaustively list the specific point values included in the range.

[0046] Preferably, the soaking time is 30-60 min, for example, it can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min or 60 min, and specific point values between the above-mentioned point values, limited to the length and for the sake of simplicity, the present application does not enumerate 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, clean the SiO2 substrate, then spin-coat a polymethyl methacrylate sacrificial layer 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 treatment 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 plate.

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

[0053] (4) Gold is deposited on the hole mask plate obtained in step (5) by the method of inclined angle deposition coating to obtain a third substrate, and then the third substrate is soaked in acetone to peel off the polymethyl methacrylate sacrificial layer to obtain the chiral metasurface optical device.

[0054] With the plane of the hole mask plate as the reference, the azimuth angle of the inclined angle deposition coating is 25°-35°, and the tilt angle is 0°-300°.

[0055] The inclined angle deposition coating gradually reduces the evaporation thickness of the material by counterclockwise rotation to control the diameter of the metal nanodisk.

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

[0057] The number of depositions is 6 times.

[0058] The soaking temperature is 60-100℃.

[0059] The soaking time is 30-60 min.

[0060] The preparation method of the chiral metasurface optical device provided by the application, the oxygen plasma etching treatment of the first substrate in step (2) can improve the hydrophilicity of the first substrate, the adhesion of the polystyrene (PS) nanosphere colloidal solution to the surface and the electrostatic repulsion between the PS nanospheres, which can make the PS nanospheres present a short-range ordered distribution state on the second substrate; the distribution of the holes on the hole mask in step (3) corresponds to the distribution position of the metal nanodisks on the substrate, and the holes are formed by peeling off the PS nanospheres. The chiral metasurface optical device is prepared by the tilt angle deposition assisted hole mask colloidal lithography method, 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 application provides an ultrafast circularly polarized light modulation method, which comprises the following steps:

[0062] (S1) Dividing linearly polarized laser pulses emitted by a laser into pump light and probe light.

[0063] (S2) Converting the probe light into probe white light through a nonlinear crystal, and adjusting the wavelength of the pump light to be 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, and specific point values between the above point values, for the sake of brevity and simplicity, the application does not list 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 and converting the probe white light into left-handed circularly polarized probe white light or right-handed circularly polarized probe white light through a polarizer and a quarter-wave plate, the left-handed circularly polarized pump light or the right-handed circularly polarized pump light as an excitation light source, and the left-handed circularly polarized probe white light or the right-handed circularly polarized probe white light as a detection light source.

[0065] (S4) The excitation light source and the detection light source are incident 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 normally 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 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 to reach the same position of the chiral metasurface optical device.

[0067] (S6) detecting a transient absorption signal of the chiral metasurface optical device under the set delay time and irradiation of the detection light source, the transient absorption signal reflecting modulation and flip modulation of the chiral metasurface optical device to circularly polarized light.

[0068] The front surface of the chiral metasurface optical device provided by the application is a surface provided with a chiral oligomer nanosuper surface structure on a substrate, and the surface not provided with the chiral oligomer nanosuper surface structure is a back surface. The normal incidence refers to that the excitation light source and the detection light source are incident on the front surface 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 surface of the chiral metasurface optical device. The chiral oligomer nanosuper surface structure is converted 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 directly reflect the ultrafast circular polarization modulation capability of the chiral metasurface. By changing the incident direction of the excitation light source and the detection light source, the sign of the transient circular dichroism signal can be flipped. Without complex enantiomer preparation, the flip modulation of the ultrafast circularly polarized light can be flexibly realized, and the application has ultrafast response time and relaxation time.

[0069] As a preferred technical solution, the pulse width of the linearly polarized laser pulse is 10-50 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, and specific point values between the above point values. Due to the limited length and the consideration of simplicity, the application will not exhaustively list the specific point values included in the range.

[0070] The repetition frequency of the linearly polarized laser pulse is 1-2 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, and specific point values between the above point values. Due to the limited length and the consideration of simplicity, the application will not exhaustively list 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, and specific point values between the above point values. Due to the limited length and the consideration of simplicity, the application will not exhaustively list the specific point values included in the range.

[0072] Preferably, the delay time is -0.7~6ps, for example, can be -0.7ps, -0.6ps, -0.5ps, -0.4ps, -0.3ps, -0.2ps, -0.1ps, 0ps, 1ps, 2ps, 3ps, 4ps, 5ps or 6ps, and the specific point values between the above point values, limited to the length and for the sake of simplicity, the present application does not exhaustively list the specific point values included in the range.

[0073] The delay time is negative, which means that the detection light source reaches the chiral metasurface optical device earlier than the excitation light source, at this time the chiral metasurface optical device is in the ground state, and the positive value means that the detection light source reaches the chiral metasurface optical device later than the excitation light source, at this time the chiral metasurface optical device is in the excited state.

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

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

[0076] (1) The chiral metasurface optical device provided by the present application has simple structure and exhibits planar chirality characteristics, so that the polarization resolution optical characteristics of the device can be reversed by the incident direction of circularly polarized light, thereby providing 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 application is prepared by a tilt angle deposition assisted hole mask colloidal lithography method, which has the advantages of low manufacturing cost, high processing speed and large-area batch processing, simple and flexible operation steps, and is conducive to the application and production of optical devices.

[0078] (3) The chiral metasurface optical device provided by the present application selects gold as the main material to realize plasmon-assisted enhanced chiral optical effect, which can utilize the advantages of plasmon such as enhanced effect, wavelength adjustable control and auxiliary hot electron dynamics process to tune the transient circular dichroism signal.

[0079] (4) The circularly polarized light modulation method provided by the present application is to excite the optical characteristics of the chiral metasurface on the ultrafast time scale by changing the femtosecond pump light and probe light with different circular polarization states, instead of detecting the traditional circular dichroism signal, which breaks through the limitation of no time response in the conventional circularly polarized light modulation method, 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 application can obtain different transient circular dichroism signals through the measured transient absorption signals, can more directly reflect the ultrafast circular polarization modulation capability of the chiral metasurface through the transient absorption difference obtained by exciting different circularly polarized pump light and detecting different circularly polarized probe white light, and in addition, the transient circular dichroism signal not only reverses in sign at 300 fs, but also maintains the inverted dynamic process on the ultrafast time scale of-0.7 ps to 6 ps, and has the same hot electron excitation and relaxation time, and can be used in ultrafast optical switching and quantum communication applications. BRIEF DESCRIPTION OF DRAWINGS

[0081] Figure 1 Structure diagram of the chiral metasurface optical device provided for Example 1;

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

[0083] Figure 2 Modulation diagram of the chiral metasurface optical device provided for Example 1 on circularly polarized light;

[0084] Figure 3 Comparison diagram of the normal incidence transient absorption signal of the chiral metasurface optical device provided for Example 1;

[0085] Figure 4 Comparison diagram of the back incidence transient absorption signal of the chiral metasurface optical device provided for Example 1;

[0086] Figure 5 Comparison diagram of the transient circular dichroism signal of the chiral metasurface optical device provided for Example 1;

[0087] Figure 6 Diagram of the transient circular dichroism absorption signal of the chiral metasurface optical device provided for Example 1 changing with delay time. DETAILED DESCRIPTION

[0088] The technical solutions of the application will be further described below by means of specific embodiments in conjunction with the accompanying drawings. Those skilled in the art should understand that the embodiments are only used to help understand the application and should not be regarded as specific limitations on the application.

[0089] Some components in the following examples are obtained as follows:

[0090] (1) Polymethyl methacrylate: item 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 is a CDP brand, agent company: Pioneer Technology Co., Ltd., model: Excipro CDP2022i.

[0093] Example 1

[0094] A chiral metasurface optical device, a schematic diagram of the structure is shown as Figure 1 The left side is a top view of the structure of the chiral metasurface optical device, which includes a substrate and a chiral oligomer nanosuperstructure arranged on the substrate, the material of the substrate is SiO2, and the thickness of the substrate is 0.2 mm; the chiral oligomer nanosuperstructure is formed by short-range ordered distribution of a plurality of structurally identical metal nanometer units; the metal nanometer unit includes six metal nanometer discs, and the specific structure is as shown on the right side Figure 1 The first metal nanodisc 1, the second metal nanodisc 2, the third metal nanodisc 3, the fourth metal nanodisc 4, the fifth metal nanodisc 5, and the sixth metal nanodisc 6 are sequentially arranged in a counterclockwise direction; each of the metal nanodiscs is made of gold, the centers of the six metal nanodiscs are located on the circumference of the same circle, the radius of the circumference is 80 nm, the thicknesses of the six metal nanodiscs are the same and are 20 nm, the center-to-center distances of any two adjacent metal nanodiscs are equal, the radii of the six metal nanodiscs decrease in the counterclockwise direction on the circumference, and the diameters of the first metal nanodisc to the sixth metal nanodisc decrease in the order of 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, and 20 nm.

[0095] The preparation method of the chiral metasurface optical device includes the following steps:

[0096] (1) A SiO2 substrate with a thickness of 0.2 mm and a diameter of 25.0 mm is prepared, and is sequentially ultrasonically cleaned in acetone, ethanol, and deionized water, and is finally dried with nitrogen and used, a polymethyl methacrylate (PMMA) sacrificial layer is spin-coated onto the SiO2 substrate, the spin coater (manufacturer: Youlisheng, model: 12A-80) is used at a speed of 3000 rpm for 2 min, and a first substrate is obtained.

[0097] (2) The first substrate obtained in step (1) is subjected to oxygen plasma etching treatment using a plasma etching machine (manufacturer: Oxford Instruments, model: PlasmaPro100 Cobra300) (etching flow rate is 50 sccm), and then a PS nanosphere colloidal solution is 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 etching machine (manufacturer: Oxford Instruments, model: Plasma Pro100 Cobra300) to strip the PS balls by oxygen plasma etching and leave a nanohole mask on the PMMA layer and remove the PMMA exposed below the nanoholes in the mask to form a hole mask plate.

[0099] (4) The hole mask plate obtained in step 3 is placed in an electron beam vacuum evaporation film plating machine (manufacturer: Vistec Lithography Company, model: Vistec EBPG 5000plus ES), and the plane of the hole mask plate is taken as the reference, the azimuth angle is 25°, and the inclination angle is 0°, 60°, 120°, 180°, 240°, and 300° in the order of the diameters of the six metal nanodisks from large to small, respectively, and gold is deposited on the hole mask plate to obtain a third substrate, the deposition times are 6, and the evaporation thickness is 20 mm, wherein the vacuum degree of the electron beam vacuum evaporation film plating machine is not more than 1x10 -5 torr, then the third substrate is soaked in acetone and placed on a hot plate heated to 60℃, and soaked for 30 min to strip the PMMA film, and then dried with nitrogen to obtain the chiral metasurface optical device.

[0100] Example 2

[0101] A chiral metasurface optical device, which is only different from example 1 in that the diameters of the first to sixth metal nanodisks are 60 nm, 50 nm, 40 nm, 30 nm, 20 nm, and 10 nm in the order of the radii from large to small, and the rest of the materials and structural parameters are the same as those of 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 frequency of 1 kHz are divided into pump light and probe light.

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

[0107] (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 left-handed circularly polarized probe white light, the left-handed circularly polarized pump light as an excitation light source, and the left-handed circularly polarized probe white light as a detection light source.

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

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

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

[0111] Embodiment 4

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

[0113] Embodiment 5

[0114] An ultrafast circularly polarized light modulation method, which is different from example 3 in that step (S3) converts the pump light into right circularly polarized pump light and the probe white light into left circularly polarized probe white light through a polarizer and a quarter-wave plate, the right circularly polarized pump light as an excitation light source, and the left circularly polarized probe white light as a detection light source; 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; step (S6) when the delay time is 300 fs, the 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 chiral metasurface optical device to circularly polarized light; 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 is different from example 5 only in that step (S3) converts the pump light into right circularly polarized pump light and the probe white light into right circularly polarized probe white light through a polarizer and a quarter-wave plate, the right circularly polarized pump light as an excitation light source, and the right circularly polarized probe white light as a detection light source; other steps and process parameters are the same as those in example 5.

[0117] Example 7

[0118] An ultrafast circularly polarized light modulation method, which is different from example 3 in that step (S5) the delay time 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 transient absorption signal reflects the modulation of the chiral metasurface optical device to circularly polarized light; 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 is different from example 7 only in that step (S3) converts the pump light into left circularly polarized pump light and the probe white light into right circularly polarized probe white light through a polarizer and a quarter-wave plate, the left circularly polarized pump light as an excitation light source, and the right circularly polarized probe white light 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 is different from example 7 in that step (S3) converts the pump light into right circularly polarized pump light and the probe white light into left circularly polarized probe white light through a polarizer and a quarter-wave plate, the right circularly polarized pump light as the excitation light source, and the left circularly polarized probe white light as the detection light source; 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; step (S6) reflects the flip modulation of the chiral metasurface optical device to circularly polarized light through the transient absorption signal; other steps and process parameters are the same as example 7.

[0123] Example 10

[0124] A method for ultrafast circularly polarized light modulation, which is different from example 9 in that step (S3) converts the pump light into right circularly polarized pump light and the probe white light into right circularly polarized probe white light through a polarizer and a quarter-wave plate, the right circularly polarized pump light as the excitation light source, and the right circularly polarized probe white light as the detection light source; other steps and process parameters are the same as example 9.

[0125] The transient absorption signals of the chiral metasurface optical device provided in example 1 under the irradiation of different excitation light sources and detection light sources were detected using a transient absorption spectrometer, and the transient circular dichroism signal diagram obtained by the difference of the transient absorption signals under the irradiation of different detection light sources is shown in Figures 3 to 6

[0126] (1) Figure 3 and Figure 4 The transient absorption signal comparison diagram of the chiral metasurface optical device provided in example 1 under the irradiation of different circularly polarized probe white light under normal incidence and back incidence conditions. From Figure 3 ​As can be seen, under left-hand circularly polarized pump light excitation, the transient absorption spectrum of the chiral metasurface optical device exhibits two signals with different signs in different bands, namely the positive induced excited state absorption peak at the short wavelength and the plasmon bleaching peak at the long wavelength. This is because after being excited by left-hand circularly polarized pump light, electrons transition from the 5d band of gold to the 6sp band or to a higher excited state within the 6sp band. When the circular polarization state of the left-hand circularly polarized pump light remains unchanged, and the detection source changes from left-hand circularly polarized probe white light to right-hand circularly polarized probe white light, significant differences in spectral characteristics are observed due to the different electronic transition resonance processes, indicating ultrafast polarization sensitivity. The transient absorption amplitude of the left-hand circularly polarized probe white light is significantly stronger than that of the right-hand circularly polarized probe white light, indicating a difference in the number of hot electron couplings. Furthermore, the former is 0.006 higher at the positively induced excited state absorption peak and 0.012 higher at the plasmon bleaching peak compared to the latter, suggesting that more hot electrons participate in the dipole resonance process of the left-hand circularly polarized probe white light. Among these, plasmon resonance enhances the ultrafast optical response. Figure 4 As can be seen, the transient absorption amplitude of the right-hand circularly polarized probe white light under back incidence is significantly stronger than that of the left-hand circularly polarized probe white light, indicating that more hot electrons participate in the dipole resonance process of the right-hand circularly polarized probe white light.

[0127] (2) Figure 5 To pass Figure 3 and Figure 4 A comparison of transient circular dichroism signals obtained from the difference in transient absorption signals is shown. The transient absorption spectrum can reflect the polarization sensitivity to pump light and probe white light with different circular polarization states. Under different excitation sources, chiral metasurface optical devices exhibit different responses to different detection sources. Therefore, the difference in transient absorption signals to different detection sources, i.e., the transient circular dichroism signal, can intuitively reflect the ultrafast modulation capability of chiral metasurface optical devices to circularly polarized light. The different numbers of hot electrons involved are determined by the strength of plasmon coupling resonance. When the incident directions of the excitation and detection sources are back-incidence, from... Figure 5 It can be observed that the transient circular dichroism signals corresponding to different detection light sources exhibit a sign-reversed spectral characteristic at a delay time of 300 fs. Furthermore, the transient absorption amplitude of the right-hand circularly polarized probe white light is significantly stronger than that of the left-hand circularly polarized probe white light. This indicates a stronger plasmon coupling resonance induced by the right-hand circularly polarized probe white light, meaning the sensitivity of the number of hot electrons to different polarization states is reversed. This demonstrates the ability to modulate ultrafast polarized light by changing the excitation incident direction. In addition, this result exhibits the optical characteristics of a chiral enantiomer, thus eliminating the need for chiral enantiomer fabrication to achieve circular polarization reversal modulation. This provides a simple and valuable design approach for the functional flexibility of polarization optical elements.

[0128] (3)Figure 6 The chiral metasurface optical device provided for example 1 can observe the evolution process of the transient circular dichroism signal of the chiral metasurface optical device over time at different delay times for different detection light sources. The transient circular dichroism signal of the chiral metasurface optical device is shown in FIG. 6. Figure 6 It can be seen that the transient circular dichroism signal changes with the delay time, reflecting the response time of the ultrafast circularly polarized 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 inverts the sign at 300 fs, but also presents an inverted dynamic process on the ultrafast time scale (-0.7-6 ps). The amplitude first rapidly rises within 300 fs, and then recovers through exponential decay relaxation. The corresponding hot electron relaxation time can be obtained by exponential fitting. The positive incidence and back incidence both maintain the same time constant (2 ps).

[0129] The applicant declares that the detailed process flow of the present application is illustrated by the above examples, but the present application is not limited to the above detailed process flow, that is, it does not mean that the present application must rely on the above detailed process flow to be implemented. It should be understood by those skilled in the art that any improvement of the present application, equivalent replacement of each raw material of the product of the present application, addition of auxiliary ingredients, selection of specific modes, etc. fall within the protection scope and disclosure scope of the present application.

Claims

1. A chiral metasurface optical device, characterized in that, The chiral metasurface optical device includes a substrate and a chiral oligomeric nanometasurface structure disposed on the substrate; The chiral oligomeric nanosurface structure is formed by a short-range ordered distribution of multiple identical metal nanounits; each metal nanounit includes six metal nanodisks, the centers of which are located on the circumference of the same circle.

2. The chiral metasurface optical device of claim 1, wherein, The substrate material includes SiO2 or sapphire.

3. The chiral metasurface optical device of claim 1, wherein, The thickness of the substrate is 0.2~0.5 mm.

4. The chiral metasurface optical device of claim 1, wherein, The material of the metal nanodisc is gold.

5. The chiral metasurface optical device of claim 1, wherein, The center-to-center distance between any two adjacent metal nanodisks is equal.

6. The chiral metasurface optical device of claim 1, wherein, The radii of the six metal nanodisks decrease sequentially in a counterclockwise direction around the circumference.

7. The chiral metasurface optical device of claim 6, wherein, The diameters of the six metal nanodisks, arranged in descending order of radius, are: 60-80 nm, 50-70 nm, 40-60 nm, 30-50 nm, 20-40 nm, and 10-30 nm, respectively.

8. The chiral metasurface optical device of claim 7, wherein, The diameters of the six metal nanodisks are 70 nm, 60 nm, 50 nm, 40 nm, 30 nm, and 20 nm, respectively.

9. The chiral metasurface optical device of claim 1, wherein, The six metal nanodisks have the same thickness.

10. The chiral metasurface optical device of claim 9, wherein, The thickness of the metal nanodisc is 15~30 nm.

11. The chiral metasurface optical device of claim 10, wherein, The thickness of the metal nanodisc is 20 nm.

12. The chiral metasurface optical device of claim 1, wherein, The radius of the circumference is 80~100 nm.

13. The chiral metasurface optical device of claim 12, wherein, The radius of the circumference is 80 nm.

14. A method of fabricating a chiral metasurface optical device according to any one of claims 1 to 13, characterized in that, The preparation method includes the following steps: (1) A sacrificial layer is coated on the substrate to obtain the first substrate; (2) Coating the first substrate obtained in step (1) with a nanosphere colloidal solution to obtain a second substrate; (3) Perform oxygen plasma etching on the second substrate obtained in step (2) to peel off the nanospheres on the second substrate and remove the sacrificial layer below the nanosphere holes to obtain a hole mask template; (4) The metal is deposited on the hole mask obtained in step (3) by tilt angle deposition to obtain the chiral metasurface optical device.

15. The preparation method according to claim 14, characterized in that, The substrate is pretreated before the sacrificial layer is applied.

16. The method of claim 15, wherein, Its features are, The pretreatment includes immersing the substrate in a solvent, followed by ultrasonication and drying.

17. The method of claim 16, wherein, The solvents include a first solvent, a second solvent, and a third solvent.

18. The method of claim 17, wherein, The first solvent includes acetone, the second solvent includes ethanol, and the third solvent includes water.

19. The method of claim 18, wherein, The specific steps of the pretreatment are to place the substrate in the first solvent, the second solvent and the third solvent in sequence for ultrasonication, and then dry the substrate with nitrogen gas.

20. The method of claim 14, wherein, The material of the sacrificial layer includes polymethyl methacrylate.

21. The method of claim 14, wherein, Step (2) before coating the nanosphere colloidal solution further includes oxygen plasma etching of the first substrate.

22. The method of claim 14, wherein, The nanosphere colloidal solution includes a polystyrene nanosphere colloidal solution.

23. The method of claim 14, wherein, With the plane of the hole mask template as a reference, the azimuth angle of the tilt angle deposition coating is 25°~35°, and the tilt angle is 0°~300°.

24. The method of claim 14, wherein, The diameter of the metal nanodisk is controlled by gradually reducing the evaporation thickness of the material through counterclockwise rotation of the tilted angle deposition coating.

25. The method of claim 24, wherein, The evaporation thickness is 20~50 nm.

26. The method of claim 14, wherein, The deposition was performed 6 times.

27. The method of claim 14, wherein, After the tilt angle deposition is completed, the process also includes a step of immersing and peeling the sacrificial layer in a solvent.

28. The preparation method according to claim 27, characterized in that, The solvent includes acetone.

29. The preparation method according to claim 27, characterized in that, The soaking temperature is 60~100℃.

30. The preparation method according to claim 27, characterized in that, The soaking time is 30-60 minutes.

31. The preparation method according to claim 14, characterized in that, The preparation method specifically includes the following steps: (1) First, the SiO2 substrate is cleaned, and then the polymethyl methacrylate sacrificial layer is spin-coated onto the SiO2 substrate to obtain the first substrate; (2) The first substrate obtained in step (1) is subjected to oxygen plasma etching treatment, and then polystyrene nanosphere colloidal solution is coated on the first substrate to obtain the second substrate; (3) Perform oxygen plasma etching on the second substrate obtained in step (2) to peel off the polystyrene nanospheres on the second substrate and remove the polymethyl methacrylate sacrificial layer below the nanosphere pores to obtain the pore mask template. (4) Gold is deposited on the hole mask obtained in step (3) by tilt angle deposition to obtain a third substrate. Then the third substrate is immersed in acetone to remove the polymethyl methacrylate sacrificial layer to obtain the chiral metasurface optical device. With the plane of the hole mask template as a reference, the azimuth angle of the tilt angle deposition coating is 25°~35°, and the tilt angle is 0°~300°; The diameter of the metal nanodisk is controlled by gradually reducing the evaporation thickness of the material through counterclockwise rotation during the tilted angle deposition coating. The evaporation thickness is 20~50 nm; The deposition was performed 6 times; The soaking temperature is 60~100℃; The soaking time is 30-60 minutes.

32. A method for ultrafast circularly polarized light modulation, characterized in that, The modulation method includes the following steps: (S1) The linearly polarized laser pulse emitted from the laser is divided into pump light and probe light; (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. (S3) The pump light is converted into left-hand circularly polarized pump light or right-hand circularly polarized pump light by a polarizer and a quarter-wave plate, and the probe white light is converted into left-hand circularly polarized probe white light or right-hand circularly polarized probe white light. The left-hand circularly polarized pump light or right-hand circularly polarized pump light is used as an excitation source, and the left-hand circularly polarized probe white light or right-hand circularly polarized probe white light is used as a detection 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 13. 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 incident on the chiral metasurface optical device, the excitation light source is the right-handed circularly polarized pump light. (S5) Control the delay time of the detection light source relative to the excitation light source when it reaches the same position of the chiral metasurface optical device; (S6) At the set delay time, the transient absorption signal of the chiral metasurface optical device under the illumination of the detection light source is detected, and the transient absorption signal reflects the modulation and reversal modulation of circularly polarized light by the chiral metasurface optical device.

33. The ultrafast circularly polarized light modulation method according to claim 32, characterized in that, The linearly polarized laser pulse has a pulse width of 10~50 fs and a repetition frequency of 1~2 kHz.

34. The ultrafast circularly polarized light modulation method according to claim 32, characterized in that, The wavelength of the white light emitted by the probe is 440~675 nm.

35. The ultrafast circularly polarized light modulation method according to claim 32, characterized in that, The delay time is -0.7 to 6 ps.

36. The ultrafast circularly polarized light modulation method according to claim 35, characterized in that, The delay time is 300 fs.

Images