Micropillar-based reconfigurable anti-counterfeiting holograms prepared by femtosecond laser two-photon polymerization

Abstract

This invention discloses a reconfigurable anti-counterfeiting hologram based on femtosecond laser two-photon polymerization, belonging to the field of laser application technology. This anti-counterfeiting computer hologram utilizes femtosecond laser two-photon polymerization technology. Through laser direct writing, micropillar structures, the smallest units constituting the reconfigurable hologram, can be fabricated. As a micro / nano structure, the micropillars can effectively modulate the incident laser in an upright state, and their height can be flexibly adjusted. As deformation units, the micropillars recover and deform by adding and evaporating alcohol, switching between upright and tilted states. Under laser irradiation, the pattern is decrypted and encrypted. Therefore, the prepared holographic pattern only appears when both laser irradiation and alcohol immersion are simultaneously met, achieving variable holographic anti-counterfeiting functionality and significantly increasing the difficulty of counterfeiting. The imaging transformation principle of this invention is simple and does not rely on a complex imaging system to achieve holographic imaging transformation.

Description

Technical Field

[0001] This invention relates to a micropillar reconfigurable anti-counterfeiting hologram prepared by femtosecond laser two-photon polymerization, belonging to the field of laser application technology. Technical Background

[0002] Product anti-counterfeiting refers to preventing the counterfeiting of branded products. With the development of society and the economy, anti-counterfeiting has become a field of great concern. It can protect the reputation of trademarks and brands, while also helping to safeguard consumer rights and public safety, and increasing consumer goodwill and trust in the consumer market. Holograms, because they can record the phase and amplitude information of light waves, present a realistic three-dimensional visual effect under laser illumination, giving people an immersive viewing experience. Therefore, they are widely used in 3D display, information storage, medical imaging and other fields. Computer-generated holograms (CGHs) are generally on the order of millimeters in size and contain tens of thousands of irregularly arranged pixels. In addition, computer-generated holograms can also form optical patterns visible to the naked eye under laser illumination. These optical patterns can contain information such as product trademarks and serial numbers, providing consumers with information to distinguish between genuine and counterfeit products.

[0003] However, with the development of science and technology, the emergence of various data algorithms has lowered the barriers to computer hologram design technology, and the development of manufacturing technology has also reduced the difficulty of replicating holograms. Currently, computer-generated holograms, when processed, can generally only present one optical pattern image under laser irradiation. The computer-generated hologram can be deduced from the optical pattern image. Although the deduced computer-generated hologram is very likely to be different from the original hologram, the optical images they form are roughly the same. Consumers rely on the information of the optical pattern for anti-counterfeiting authentication, posing a risk of counterfeit anti-counterfeiting marks being forged. Most holograms currently on the market are two-dimensional planar structures, which store less information compared to three-dimensional structures. Simple holograms can no longer meet the growing anti-counterfeiting and security demands. To improve security and reduce the difficulty of counterfeiting, researchers have studied the fabrication of variable holograms. However, currently, variable holograms mainly rely on metasurfaces, and their processing methods often employ electron beam lithography and ion beam etching, which involve complex processes and environmental pollution, hindering the practical application of variable holograms in the anti-counterfeiting field.

[0004] Therefore, in the field of advanced anti-counterfeiting, it is particularly important to use more environmentally friendly and simple processing methods to prepare three-dimensional variable anti-counterfeiting holograms that can store more encrypted information. Inspired by the characteristic that aquatic plants can stand upright in water but fall over when out of water, this invention prepares a micropillar reconfigurable hologram based on femtosecond laser two-photon polymerization. This invention is expected to be applied in fields such as anti-counterfeiting and information encryption. Summary of the Invention

[0005] The purpose of this invention is to provide a reconfigurable anti-counterfeiting hologram based on femtosecond laser two-photon polymerization, which can be used in advanced anti-counterfeiting fields. This anti-counterfeiting computer hologram uses femtosecond laser two-photon polymerization technology. Through laser direct writing, micropillar structures, the smallest units constituting the reconfigurable hologram, can be fabricated. As a micro / nano structure, the micropillars can effectively modulate the incident laser in an upright state, and their height can be flexibly adjusted. Inspired by the characteristic of aquatic plants that can stand upright in water but fall over when out of water, the micropillars, as deformation units, can recover and deform by adding and evaporating alcohol, switching between upright and tilted states. Under laser irradiation, the pattern is decrypted and encrypted. Therefore, the prepared holographic pattern only appears when both laser irradiation and alcohol immersion are simultaneously met, achieving variable holographic anti-counterfeiting functionality and significantly increasing the difficulty of counterfeiting.

[0006] The realization of reconfigurable dynamic holograms typically requires two conditions: each unit of the hologram must have the ability to modulate the wavefront of light, and the modulation capability must be programmable or switchable. This invention proposes a method for realizing reconfigurable holograms using the deformation and recovery characteristics of micropillars in response to liquids. In this reconfigurable hologram, the micropillars have two functions. On one hand, the micropillars act as diffraction units to modulate the incident laser; on the other hand, the micropillars deform and recover under the influence of alcohol. The switching between the upright and tilted states of the micropillars is controlled by the wetting and evaporation of alcohol, resulting from the combined effects of capillary forces generated during liquid evaporation, elastic recovery forces generated when the micropillars deform, and van der Waals forces generated when the micropillars come into contact with each other and with the substrate after deformation to a certain extent. Because a specific amplitude modulation distribution is designed during the processing, after laser irradiation, in the upright state, the micropillars can effectively modulate the incident laser, presenting a pre-designed holographic pattern. However, after the liquid evaporates, the micropillars become bent, which weakens their modulation ability to the incident laser. Furthermore, due to the characteristics of amplitude-based holography, the distribution of micropillars around each micropillar is somewhat uncertain. This results in different directions of the resultant force of the capillary forces caused by alcohol evaporation acting on the micropillars, leading to different tilting directions for each micropillar. This situation disrupts the original amplitude modulation distribution, preventing the formation of a clear holographic pattern after laser irradiation. This simple stimulus-response method gives micropillars fabricated using femtosecond laser two-photon polymerization technology exceptional flexibility in reversible shape changes.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A reconfigurable micropillar anti-counterfeiting hologram based on femtosecond laser two-photon polymerization is proposed. Inspired by the property that aquatic plants can stand upright in water but fall over when out of water, this anti-counterfeiting computer hologram achieves its purpose by dripping and evaporating alcohol onto the prepared pattern. The micropillar structural units that make up the hologram will recover and deform, switching between an upright state and a tilted state. Under laser irradiation, the pattern is decrypted and encrypted. Therefore, the prepared holographic pattern only appears when both laser irradiation and alcohol immersion are met simultaneously, realizing the function of variable holographic anti-counterfeiting and significantly increasing the difficulty of counterfeiting.

[0009] This holographic imaging is based on the principle of amplitude holography. The transmittance of light differs between the positions of the processed and unprocessed micropillars, and this difference in transmittance leads to amplitude modulation of the incident laser beam. Phase holograms are designed using micropillars with a height difference ΔH, which is determined by the desired phase difference: ΔH = ΔΦλ / [2π(n-1)], where ΔΦ is the designed phase difference, λ is the wavelength of the laser, and n is the refractive index of the photoresist at the target wavelength.

[0010] The micropillar-based reconfigurable anti-counterfeiting hologram is fabricated using femtosecond laser two-photon polymerization technology. It is a reconfigurable hologram composed of the smallest unit micropillar structure. The holographic anti-counterfeiting encryption is achieved by utilizing the wavefront modulation capability of the micropillar unit and its deformation and recovery characteristics in response to liquid.

[0011] The switching between the upright and tilted states of the unit microcolumns is controlled by the wetting and evaporation of alcohol.

[0012] The specific steps for preparing the above-mentioned variable anti-counterfeiting computer hologram are as follows:

[0013] Step 1: Hologram Calculation

[0014] 1.1 Input the target amplitude distribution u{x,y} of the focal plane light field corresponding to the target image, and set the corresponding initial phase distribution as exp{i*rand{x,y}}, and temporarily set the weighting factor as the identity matrix;

[0015] 1.2. Using the product of the spatial distribution of the target image and the weighting factor as new amplitude information, and using the initial phase distribution or the phase distribution of the focal plane light field obtained in the previous iteration as phase information, the corrected focal plane light field distribution X is obtained by combining them. After normalization, the expression for the corrected complex amplitude distribution of the focal plane light field is:

[0016] u{x,y}w{x,y}exp{i*phase1} (1)

[0017] Where w{x,y} is a weighting factor used to adjust the spatial distribution of the target image; phase1 represents the phase angle of the light field at the focal plane.

[0018] Based on the Fresnel diffraction integral method, the incident light field distribution X is calculated in reverse, and its complex amplitude expression is:

[0019] U{x,y}exp{i*phase2} (2)

[0020] Where U{x,y} represents the complex amplitude distribution of the incident light field; phase2 represents the phase angle of the incident light field.

[0021] 1.3. Only the amplitude information of the incident light field after binarization is retained. The phase distribution of the actual incident laser is used to replace the original phase information. After normalization, the corrected incident light field A*exp{i*0} is obtained. Then, the focal plane light field is obtained by forward propagation transformation of the incident light field. The calculated light field of the focal plane is obtained by forward diffraction of the incident light field using the Fresnel diffraction integral method.

[0022] a{x,y}exp{i*phase1}(3)

[0023] Where a{x,y} represents the amplitude information of the incident light field after binarization.

[0024] 1.4, use w{x,y}=u{x,y} / a{x,y}*w prev The corrected weighting factor is calculated, and the focal plane light field is constrained during the iteration process. The mean square error (RMS) is used as the evaluation function to determine whether the calculation results meet the requirements.

[0025]

[0026] Among them, w prev E represents the weighting factor calculated in the previous iteration. out (x2, y2) represents the actual output light field result obtained in the current iteration step of the algorithm; This indicates the expected output light field result.

[0027] 1.5 Repeat steps 1.2-1.4 until the evaluation function is less than the set value; when the iterative algorithm stops calculating, the amplitude information of the incident light field is corrected to obtain the required amplitude information.

[0028] Step 2: Pattern Preparation

[0029] Using photopolymers as the processing material and near-infrared long-wavelength femtosecond lasers as the light source, holograms are processed through femtosecond laser two-photon polymerization technology. First, the light field is adjusted to ensure a Gaussian intensity distribution. Then, the processing translation stage and objective lens are leveled to ensure vertical laser emission, guaranteeing the perpendicularity of the processed micropillars. Finally, an attenuator is placed; by adjusting the laser energy, this ensures that optical components are not damaged and that the roundness of the processed micropillars is adjusted.

[0030] After completing the above operations, hold the sample with tweezers and gently place it on the processing translation stage. Then, ensure processing stability by setting the stage scanning speed; determine the number of processing pulses by appropriately setting the exposure time and laser repetition frequency; focus the laser beam inside the processing material through the processing objective lens, processing pillars where the amplitude in the calculated hologram is 0, and leaving areas with an amplitude of 1 unprocessed, as shown in the attached diagram. Figure 1 As shown, the movement of the translation stage is controlled by written G-code, thereby producing a large-area holographic processing. Subsequently, post-processing operations such as alcohol development and silanization are used to complete the preparation of the hologram.

[0031] This method for preparing holograms is simple to operate, convenient to process, and highly flexible. During the preparation process, the size, height, and spacing of the micropillars in the hologram can be varied by controlling parameters such as laser energy and the movement of the translation stage. Through this process, a usable amplitude hologram is finally prepared.

[0032] Step 3: Encryption and Decryption of the Pattern

[0033] After development, the micropillars in the hologram will collapse due to capillary force caused by alcohol evaporation, resulting in a disordered state and thus preventing the display of patterns, thereby achieving an encryption effect. After adding alcohol to wet the micropillars, the elastic recovery force generated by the bending deformation will be greater than the capillary force, thus restoring the micropillars to an upright state. At this point, laser irradiation of the hologram can be used to decrypt the pre-processed pattern.

[0034] The specific decryption principle is as follows:

[0035] The stress on the micropillar unit in a reconfigurable hologram is analyzed at different stages of deformation and recovery. During liquid evaporation, when the liquid level drops below the height of the micropillar, the micropillar will bend due to capillary forces. If the capillary force is greater than the force resisting deformation of the micropillar itself, the micropillar will bend. At this point, the micropillar will generate an elastic restoring force, and this deformation will occur accordingly. The magnitude of the capillary force is:

[0036] F C =2πγr 2 cos2 θ / d (5)

[0037] Where d is the distance between the tips of two adjacent micropillars, a value that changes dynamically as the micropillar bends; γ is the interfacial tension; r is the radius of the micropillar; and θ is the contact angle between the micropillar and the liquid surface. As the liquid level decreases, the capillary force gradually increases.

[0038] The magnitude of the elastic restoring force generated by the bending deformation of the micropillar is:

[0039] F S =3πEr 4 ν / 4h 3 (6)

[0040] Where ν is the horizontal displacement between the top of the micropillar and the substrate, therefore, when the micropillar is initially in an upright state, F S =0; E is the Young's modulus of the micropillar material; h is the height of the micropillar.

[0041] After the liquid evaporates, if the micropillar bends and comes into contact with adjacent micropillars or the substrate, the forces between the micropillars and between the micropillars and the substrate are van der Waals forces F, since the hydrogen bonding effect was eliminated during the silanization process. VDW (A weak intermolecular force). Considering the contact between intersecting micropillars (corresponding to the contact between pillars) and the sphere-ground contact (corresponding to the contact between the pillar head and the ground), F VDW They can be written as:

[0042]

[0043] F VDW =-Ar / 6D 2 (8)

[0044] Where A is the Hamark constant; r1 and r2 are the radii of the two micropillars, respectively; r is the radius of the micropillar tip; and D is the contact distance between the micropillars and between the micropillar and the substrate. Assuming the two micropillars and their tips have the same diameter, then F... VDW It can be uniformly represented as F VDW =-Ar / 6D 2 .

[0045] Therefore, when alcohol is dropped onto the sample surface, due to the F based on the "point contact" of the micropillars... VDW The force is relatively weak, when the elastic restoring force F of the micro-column is... S Greater than van der Waals force F VDW When the alcohol evaporates, the microcolumn will become upright; after that, the microcolumn will deform again under the action of capillary force.

[0046] Beneficial effects

[0047] 1. Inspired by the characteristic that aquatic plants can stand upright in water but bend over when out of water, this invention utilizes the deformation and recovery properties of micropillars in response to liquids to modulate incident light, thereby achieving the transformation of holographic imaging patterns. Its imaging transformation principle is simple, highly operable, and does not require a complex imaging system to achieve holographic imaging transformation. Because the pre-formed information only appears under the conditions of simultaneous laser irradiation and alcohol immersion, its counterfeiting difficulty is significantly increased, making it suitable for advanced anti-counterfeiting applications and possessing strong practicality and high confidentiality.

[0048] 2. This invention utilizes femtosecond laser two-photon polymerization technology to fabricate holograms. Micropillars, as a common micro / nano structure, have a wide range of applications. Compared to traditional micro / nano manufacturing methods such as photolithography and electron beam lithography, femtosecond laser two-photon polymerization technology offers advantages such as no need for masks, true 3D processing, ultra-precision manufacturing, and greater environmental friendliness. It allows for precise control of the position of each micropillar. By controlling the laser energy and the number of laser pulses, micropillars of different heights and spacings can be fabricated simply by using the displacement of a translation stage. This method is simple to implement, easy to use, highly flexible, and convenient, and has broad potential in anti-counterfeiting and optical security. Attached Figure Description

[0049] Figure 1 The binary hologram of the target pattern and its corresponding processing position are calculated using the GS algorithm; Figure a is the designed hologram; Figure b is a magnified view of a portion of Figure a; Figure c is the micropillar array corresponding to b.

[0050] Figure 2 Schematic diagram of the process for fabricating reconfigurable holograms using femtosecond laser two-photon polymerization;

[0051] Figure 3 Schematic diagram of the optical path for reconfigurable holographic imaging;

[0052] Figure 4 Figure 1 shows the stress analysis of the micropillar element at different stages of deformation and recovery; Figure 2a shows the capillary force F acting on the micropillar when the liquid level drops below the height of the micropillar during liquid evaporation. C The micropillar bends due to the influence of the deformation, and at the same time, it generates an elastic restoring force F to resist the deformation force. S Figure b shows that after the liquid evaporates, if the microcolumns bend, van der Waals forces F will be generated between the microcolumns. VDW Figure c shows that after adding alcohol, the microcolumns that were initially attracted to each other will return to their upright position; Figure d shows that during the evaporation of the liquid, when the liquid level drops below the height of the microcolumns, a single microcolumn will be subjected to a capillary force F. CFigure e illustrates that after liquid evaporation, if a single micropillar bends, a van der Waals force F may be generated between it and the substrate. VDW Figure f shows that after adding alcohol, the originally bent microcolumn will return to an upright position. Detailed Implementation

[0053] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0054] This example uses the letter "A" as the pattern, SZ2080 photoresist (provided by IESL-FORTH) as the processing material, and a Ti:sapphire laser with a center wavelength of 800nm ​​and a repetition frequency of 1kHz for processing. Figure 3 The pattern is restored and encrypted by wetting and evaporating alcohol, followed by irradiation with a 633nm red continuous helium-neon laser. The specific steps of this example are as follows:

[0055] 1. Select the pattern as the letter "A" and use the GS amplitude-type spatial shaping algorithm to calculate the computer-generated hologram corresponding to the pattern.

[0056] 1.1 Input the target amplitude distribution u{x,y} of the focal plane light field corresponding to the target image, and set the corresponding initial phase distribution as exp{i*rand{x,y}}, and temporarily set the weighting factor as the identity matrix;

[0057] 1.2. Using the product of the spatial distribution of the target image and the weighting factor as new amplitude information, and using the initial phase distribution or the phase distribution of the focal plane light field obtained in the previous iteration as phase information, the corrected focal plane light field distribution X is obtained by combining them. After normalization, the expression for the corrected complex amplitude distribution of the focal plane light field is:

[0058] u{x,y}w{x,y}exp{i*phase1} (1)

[0059] Where w{x,y} is a weighting factor used to adjust the spatial distribution of the target image; phase1 represents the phase angle of the light field at the focal plane.

[0060] Based on the Fresnel diffraction integral method, the incident light field distribution X is calculated in reverse, and its complex amplitude expression is:

[0061] U{x,y}exp{i*phase2} (2)

[0062] Where U{x,y} represents the complex amplitude distribution of the incident light field; phase2 represents the phase angle of the incident light field;

[0063] 1.3. Only the amplitude information of the incident light field after binarization is retained. The phase distribution of the actual incident laser is used to replace the original phase information. After normalization, the corrected incident light field A*exp{i*0} is obtained. Then, the focal plane light field is obtained by forward propagation transformation of the incident light field. The calculated light field of the focal plane is obtained by forward diffraction of the incident light field using the Fresnel diffraction integral method.

[0064] a{x,y}exp{i*phase1} (3)

[0065] Where a{x,y} represents the amplitude information of the incident light field after binarization;

[0066] 1.4, use w{x,y}=u{x,y} / a{x,y}*w prev The corrected weighting factor is calculated, and the focal plane light field is constrained during the iteration process. The mean square error (RMS) is used as the evaluation function to determine whether the calculation results meet the requirements.

[0067]

[0068] Among them, w prev E represents the weighting factor calculated in the previous iteration. out (x2, y2) represents the actual output light field result obtained in the current iteration step of the algorithm; This indicates the expected output light field result.

[0069] 1.5 Repeat steps 1.2-1.4 until the evaluation function is less than the set value; when the iterative algorithm stops calculating, the amplitude information of the incident light field is corrected to obtain the required amplitude information.

[0070] Through the above process, the holographic matrix of the pattern can be obtained, and the amplitude distribution information of the pre-processed pattern can be obtained (such as...). Figure 1 a and enlarged view Figure 1 (As shown in b). Based on the obtained holographic amplitude distribution information, write G-code to distribute the micropillars at the corresponding positions in the hologram, such as... Figure 1 As shown in c, this is to facilitate subsequent processing operations.

[0071] 2. Before processing, place the cover glass slides in beakers containing water and alcohol and ultrasonically vibrate for 5 minutes to clean the glass surface. Add a surfactant to the water or treat the glass surface with oxygen plasma to enhance the adhesion between the photoresist and the glass. After treating the substrate, apply SZ2080 photoresist to a 170nm thick fused silica substrate. Before exposure, to remove excess solvent and better complete the preparation, bake the processing material at 95℃ for one hour.

[0072] like Figure 2 As shown in the processing flowchart, a Ti:sapphire laser is used as the light source, with a laser repetition frequency of 1 kHz and a laser energy of 25 μW. The laser beam is focused onto the photoresist using an objective lens with 50x magnification and a numerical aperture of 0.8. Femtosecond laser two-photon polymerization is employed, with the translation stage controlled by G-code. Pillars are fabricated at locations with amplitudes of 0 in the calculated hologram, while areas with amplitudes of 1 are left unprocessed. After polymerization, silanization is performed to eliminate the influence of hydrogen bonds during the contact between micropillars, ensuring complete contact between the reagent and the micropillars. To ensure complete contact between the reagent and the micropillars during silanization, the micropillars are temporarily kept upright after development using a critical point dryer. An immersion-type silanization method is used, where the sample is immersed in heptadecafluoro-1,1,2,2-tetrahydrodecayl at room temperature for 1.5 hours, followed by heating to 120°C and holding for 1 hour. Figure 4 Through the above process, a usable amplitude hologram is finally prepared.

[0073] 3. A red continuous-wave helium-neon laser with a wavelength of 633nm is used as the irradiation source. The laser energy is adjusted by an attenuator to avoid excessive brightness due to excessive laser energy, which would affect the imaging effect. Patterns can be created or removed by adding or evaporating alcohol. Figure 3 As shown, because a transparent cover glass is used as the processing substrate, laser light can pass through the micropillar hologram for imaging. During the imaging process, the transmittance of the laser light is affected by the processed micropillar areas, resulting in different transmittance compared to the unprocessed micropillar areas, thus causing amplitude modulation of the laser. The light spot after passing through the sample is focused by a lens onto an industrial camera CCD for imaging. The CCD is connected to a computer, allowing for the observation and capture of a clear holographic pattern. It should be noted that since the hologram imaging method prepared in this invention is a far-field holographic projection, the CCD can also be replaced with a white light-blocking plate for imaging, a feature that greatly enhances the practicality of this invention. In use, by reasonably adjusting parameters such as laser energy and lens focusing, more accurate holographic imaging results can be obtained.

[0074] The specific principle is as follows: Figure 4 As shown, the stress situation of the micropillar unit in the reconfigurable holography is analyzed at different stages of deformation and recovery. During the liquid evaporation process, when the liquid level drops below the height of the micropillar, the micropillar will be subjected to capillary forces (such as...). Figure 4 a, Figure 4 As shown in diagram d), under the influence of capillary force, the microcolumn will bend. If the capillary force is greater than the force resisting deformation of the microcolumn itself, the microcolumn will bend. At this time, the microcolumn will generate an elastic restoring force, and this deformation will occur accordingly. The magnitude of the capillary force is:

[0075] F C =2πγr 2 cos 2 θ / d (5)

[0076] Where d is the distance between the tips of two adjacent micropillars, a value that changes dynamically as the micropillar bends; γ is the interfacial tension; r is the radius of the micropillar; and θ is the contact angle between the micropillar and the liquid surface. As the liquid level decreases, the capillary force gradually increases.

[0077] The magnitude of the elastic restoring force generated by the bending deformation of the micropillar is:

[0078] F S =3πEr 4 ν / 4h 3 (6)

[0079] Where ν is the horizontal displacement between the top of the micropillar and the substrate, therefore, when the micropillar is initially in an upright state, F S =0; E is the Young's modulus of the micropillar material; h is the height of the micropillar.

[0080] After the liquid evaporates, if the microcolumn bends and merges with adjacent microcolumns (such as...), Figure 4 b) or substrate (e.g. Figure 4 e) Contact occurs because the effects of hydrogen bonding are eliminated during the silanization process. At this point, the forces between micropillars and between the micropillars and the substrate are van der Waals forces F. VDW (A weak intermolecular force). Considering the contact between intersecting micropillars (corresponding to the contact between pillars) and the sphere-ground contact (corresponding to the contact between the pillar head and the ground), F VDW They can be written as:

[0081]

[0082] F VDW =-Ar / 6D 2 (8)

[0083] Where A is the Hamark constant; r1 and r2 are the radii of the two micropillars, respectively; r is the radius of the micropillar tip; and D is the contact distance between the micropillars and between the micropillar and the substrate. Assuming the two micropillars and their tips have the same diameter, then F... VDW It can be uniformly represented as F VDW =-Ar / 6D 2 .

[0084] Therefore, when alcohol is dropped onto the sample surface, due to the F based on the "point contact" of the micropillars... VDW The force is relatively weak, when the elastic restoring force F of the micro-column is... S Greater than van der Waals force F VDWAt that time, the microcolumn will become upright (as shown in the image). Figure 4 c. Figure 4 (as shown in f); after the alcohol evaporates, the microcolumn deforms again under the action of capillary force.

[0085] The above detailed description further illustrates the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing micropillar-based reconfigurable anti-counterfeiting holograms using femtosecond laser two-photon polymerization, characterized in that: The specific steps are as follows: Step 1: Hologram Calculation 1.1, Input the target amplitude distribution u{x,y} of the light field corresponding to the focal plane of the target image, and set the corresponding initial phase distribution as... The weighting factor is temporarily set to the identity matrix; 1.

2. Using the product of the spatial distribution of the target image and the weighting factor as new amplitude information, and using the initial phase distribution or the phase distribution of the focal plane light field obtained in the previous iteration as phase information, the corrected focal plane light field distribution X is obtained by combining them. After normalization, the expression for the corrected complex amplitude distribution of the focal plane light field is: (1) in, It is a weighting factor used to adjust the spatial distribution of the target image; The phase angle represents the light field at the focal plane. Based on the Fresnel diffraction integral method, the incident light field distribution X is calculated in reverse, and its complex amplitude expression is: (2) in, This represents the complex amplitude distribution of the incident light field; This represents the phase angle of the incident light field; 1.3 Only the amplitude information of the incident light field after binarization is retained. The phase distribution of the actual incident laser is used to replace the original phase information, and the corrected incident light field is obtained after normalization. Then, the focal plane light field is obtained by transforming the incident light field in the forward propagation direction. Using the Fresnel diffraction integral method, the calculated focal plane light field is obtained from the forward diffraction of the incident light field. (3) in, This represents the amplitude information of the incident light field after binarization. 1.4, use The corrected weighting factor is calculated, and the focal plane light field is constrained during the iteration process. The mean square error (RMS) is used as the evaluation function to determine whether the calculation results meet the requirements. (4) in, This represents the weighting factor calculated in the previous iteration; This represents the actual output light field result obtained in the current iteration step of the algorithm; This indicates the expected output light field result. 1.5 Repeat steps 1.2-1.4 until the evaluation function is less than the set value; when the iterative algorithm stops calculating, the amplitude information of the incident light field is corrected to obtain the required amplitude information. Step 2: Pattern Preparation Holograms are processed using photopolymers as the processing material and near-infrared long-wavelength femtosecond lasers as the light source through femtosecond laser two-photon polymerization technology. 2.1 Adjust the light field to ensure that the light field energy has a Gaussian intensity distribution; 2.2 Level the processing stage and objective lens to ensure vertical laser emission; place the attenuator and adjust the roundness of the micropillar by adjusting the laser energy; transfer the sample onto the processing stage. 2.3 By focusing the laser through the processing objective lens and controlling the movement of the translation stage, a column is processed at the location where the amplitude is 0 in the calculated hologram. After processing, the sample is obtained. 2.4 The micropillar reconfigurable anti-counterfeiting hologram was prepared by using alcohol development, critical point drying and silanization treatment; The hologram is a reconfigurable hologram made up of the smallest unit micropillar structure through femtosecond laser two-photon polymerization technology; the holographic anti-counterfeiting encryption is achieved by utilizing the wavefront modulation capability of the micropillar unit and its deformation and recovery characteristics in response to liquid. The switching between the upright and tilted states of the unit microcolumns is controlled by the wetting and evaporation of alcohol.

Images