CN117270241A - Geometric phase element based on nano bubble structure and preparation method thereof - Google Patents
Geometric phase element based on nano bubble structure and preparation method thereof Download PDFInfo
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- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 21
- 230000005540 biological transmission Effects 0.000 claims description 19
- 239000005350 fused silica glass Substances 0.000 claims description 16
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- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
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- G02F1/0136—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour for the control of polarisation, e.g. state of polarisation [SOP] control, polarisation scrambling, TE-TM mode conversion or separation
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Abstract
The invention relates to the field of femtosecond micromachining, and discloses a geometric phase element based on a nanobubble structure and a preparation method thereof, so as to realize batch production and ensure performance. The method comprises the following steps: an optical path system is deployed, and the unified partial parameters of the processing nanobubbles in each polarization direction, the layer distance and the layer number corresponding to half-wave retardation, and the pulse density and the single pulse energy respectively corresponding to each polarization direction are calibrated; processing the nano bubble structures of each layer one by one in a sequence from bottom to top, and downwards moving the nano bubble structures by one unit in the Z-axis direction of the triaxial displacement table in the process of switching the next layer to the previous layer after processing; in the process of processing the single-layer nano bubble structure, the single nano bubble structure is processed one by one through displacement in the X-axis and Y-axis directions of the triaxial displacement table; in the process of processing a single nano bubble structure, the power attenuation module, the polarization control module and the triaxial displacement table are controlled to execute corresponding linkage operation according to the calibration relation corresponding to the target polarization direction.
Description
Technical Field
The invention relates to the technical field of femtosecond micromachining, in particular to a geometric phase element based on a nano bubble structure and a preparation method thereof.
Background
In 1996, scientists found that irradiating transparent materials with a femtosecond laser changed the refractive index of the material, and they further used this property to process waveguides in quartz glass with a femtosecond laser. At this stage, the modification of the fused silica by the femtoseconds stays in the isotropic stage. In 1999, it was found that the application of femtosecond laser to fused silica can cause the fused silica to generate anisotropic refractive index change to realize birefringence. This discovery enables more applications such as phase regulation and data storage. Since 2003, many subject groups have developed applications and principle studies of this technology after more discovery of the birefringence modification resulting from the nanolayered structure formed in fused silica and called nanograting. For example, the geometric phase element is manufactured by utilizing a nano-grating structure, and the high-density data storage is realized by utilizing a multi-layer nano-grating.
The exploration of the formation principle of the nano-grating also enables people to further understand the formation mechanism of the nano-grating. Taylor studies revealed that the formation process of the nano-grating is mainly as follows:
first, the laser is absorbed by defects (Nonbridging oxygen hole centers) in the fused silica to form spherical vesicles, which grow on a plane along the perpendicular polarization direction under the laser polarization electric field to form oblate vesicles, and as the vesicles grow on the perpendicular polarization plane, the vesicles gradually link and form a lamellar structure, i.e., a nanograting. This process was also confirmed under an electron scanning microscope.
Therefore, intermediate state products in the process of forming the nano grating also arouse the interest of scientific researchers, and the spherical vesicle modified zone formed in the initial stage has the anisotropic structure to enable the fused quartz to have the birefringent property when being elongated to form an oblate spheroid structure. And the nano-grating has the high transmission characteristic which is not possessed by the nano-grating, and can keep high transmission rate especially in the ultraviolet wavelength range. This makes it of great application potential in the ultraviolet wavelength range.
The nanobubble (nanopourus) structure is an intermediate state of the femtosecond laser before the fused silica forms the nano grating structure, meaning that the processing parameter window of the modified fused silica is much smaller than the nano grating. Too low a laser power density means that the quartz with a forbidden band of 9eV (electron volt) is completely transparent, and a sufficiently high power density (focusing, high pulse energy) is necessary to trigger the multiple electron absorption process to modify the defect absorbing laser in the fused quartz, whereas too high a power density may cause the fused quartz to be damaged directly on the one hand, and also may cause the modified structure to transition to the nano-grating structure quickly on the other hand. In addition, the repetition rate of the femtosecond laser also determines the time that the fused silica can cool between adjacent laser pulses and the corresponding heat integration effect. Numerous processing parameters are interrelated, making nanobubble processing quite challenging.
Thus, how to manufacture high-performance optical components based on the nanobubble structure becomes a difficulty in technical research.
Disclosure of Invention
The invention aims to disclose a preparation method and a preparation system of a geometric phase element based on a nano bubble structure, so as to realize batch production and ensure the performance of the prepared product.
In order to achieve the above object, the preparation method of the geometric phase element based on the nano bubble structure disclosed by the invention comprises the following steps:
step S1, deploying an optical path system, wherein the optical path system comprises: the device comprises a femtosecond laser source capable of adjusting pulse width, a power attenuation module capable of automatically switching single pulse energy through program control, a polarization control module, an objective lens and a triaxial displacement table for fixing a sample; the power attenuation module is arranged between the laser light source and the polarization control module, the polarization control module is used for adjusting the polarization direction of the linearly polarized light output to the objective lens by the attenuated light beam, and the triaxial displacement platform is used for switching the three-dimensional coordinates of the light spot focused by the objective lens in the sample one by one; calibrating the uniform light spot size, linear distance size, repetition frequency, pulse width, peak power density, layer distance and layer number corresponding to half-wave delay amount of the laser light source for processing the nanobubble in each polarization direction, and pulse density and single pulse energy respectively corresponding to each polarization direction; the pulse density = femtosecond laser repetition frequency/displacement table scan speed;
step S2, processing the nano bubble structures of all layers one by one in a sequence from bottom to top, and downwards moving the nano bubble structures by one unit in the Z-axis direction of the triaxial displacement table in the process of switching the next layer to the previous layer after processing; in the process of processing the single-layer nano bubble structure, the single nano bubble structure is processed one by one through displacement in the X-axis and Y-axis directions of the triaxial displacement table; in the process of processing a single nano bubble structure, the power attenuation module, the polarization control module and the triaxial displacement table are controlled to execute corresponding linkage operation according to the calibration relation corresponding to the target polarization direction.
Preferably, the obtaining of the calibration relation according to the present invention includes:
step S11, setting a first polarization direction of the linearly polarized light emitted by the objective lens based on the light path system;
s12, adjusting a light path to determine the polarization direction to be calibrated of the linear polarized light emitted by the objective lens and enabling the extinction ratio of the linear polarized light emitted by the objective lens to meet the requirement; then measuring the diameter D of a light spot below the objective lens, and determining the line distance generated by the displacement of the triaxial displacement table in the horizontal direction according to the diameter of the light spot, wherein the line distance comprises a line distance and a column distance, and the value range of the line distance is D/5 to D/4;
step S13, measuring the average power of the light spot, and determining the single pulse energy and the peak power density of the laser according to the average power of the light spot and the repetition frequency of the femtosecond laser;
s14, adjusting the laser source and the power attenuation module to enable the peak power density of the focusing light spot of the objective lens to be 3-18TW/cm 2 Is a section of (2);
s15, adjusting pulse density to be 50p/um-300p/um;
step S16, changing specific values of parameters corresponding to the steps S12 to S15 in corresponding ranges, and screening out various parameter combinations of single-layer nanobubbles with diffraction efficiency more than 95% and polarization transmission difference between P polarization and S polarization direction less than 5%, wherein the diffraction efficiency=the transmittance of a modified region/the transmittance of an unmodified region; polarization transmission difference between P polarization and S polarization direction = transmittance of P polarized light-transmittance of S polarized light; each of the parameter combinations includes: spot size, line spacing size, repetition frequency, single pulse energy, peak power density, pulse density, and pulse width;
step S17, determining a parameter combination with the largest retardation corresponding to the first polarization direction from the at least two parameter combinations screened in the step S16; determining the layer distance and the layer number of the nanobubbles for constructing the half-wave retardation corresponding to the laser light source according to the parameter combination with the maximum retardation;
and S18, changing the first polarization direction in the step S11 into other polarization directions one by one, and then in the process of executing the series of steps from the step S12 to the step S16, keeping the line distance size, the light spot size, the pulse width and the repetition frequency in the parameter combination with the maximum target wavelength and the maximum delay amount determined in the step S17 unchanged, adjusting the pulse density and the single pulse energy, and screening out at least one parameter combination of single-layer nanobubbles with the diffraction efficiency more than 95% and the polarization transmission difference between the P polarization direction and the S polarization direction less than 5%.
Further, in the process of constructing the calibration relation, when at least two parameter combinations of single-layer nanobubbles with diffraction efficiency more than 95% and polarization transmission differences of P polarization and S polarization directions less than 5% exist in any other polarization directions, the single-layer nanobubbles are subjected to weighted operation according to the detected diffraction efficiency and the polarization transmission differences of the P polarization and S polarization directions and then are sequenced, and the optimal parameter combination is determined according to the sequencing result.
Preferably, the value range of the light spot size D is 6-8um.
In order to achieve the above purpose, the invention also discloses a geometric phase element based on a nano bubble structure, which is prepared by the method.
Alternatively, the sample is a fused silica material. Alternatively, a hard and brittle material such as N-BK7 or sapphire may be used.
Preferably, the prepared geometric phase element is composed of an integral number of unit groups, and the unit groups are nano bubble structures combined by the number of layers corresponding to half-wave retardation; and the fast axis clamping angles among the unit groups are customized according to target requirements. For example: the geometric phase element is in particular an achromatic wave plate, an achromatic vortex wave plate, an achromatic lens, an achromatic conical lens, an achromatic polarization grating, an achromatic dammann grating, an achromatic homogenizer, a cascade grating, a beam deflector or an achromatic multifocal DOE, which are composed of at least two unit groups.
The invention has the following beneficial effects:
1. the system components are simple, the precision requirements among linkage equipment are high, and mass production can be performed according to the calibration relation after calibration; thereby ensuring the overall performance and preparation efficiency of the product.
2. In the calibrated parameters, the light spot size, the line distance size, the repetition frequency, the pulse width and the peak power density are kept uniform for each polarization direction, different polarization information is adapted by adjusting the pulse density and/or the single pulse energy, so that the differentiated internal structure of each layer of nanobubbles can be conveniently designed, the flexibility and the expansibility of the system are enriched, and the cooperative complexity in the switching process along with the polarization direction is reduced.
3. In the calibration relation, the layer distance and the layer number of the half-wave delay amount are calibrated, so that more complex geometric phase component design is conveniently carried out in the sample by taking the half-wave delay amount as a unit, the combination function of a plurality of existing independent optical components can be integrated in a single sample, and the overall cost performance of the product can be improved in a step-shaped manner.
The invention will be described in further detail with reference to the accompanying drawings.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention. In the drawings:
fig. 1 is a block diagram of an optical path system according to an embodiment of the present invention.
Detailed Description
Embodiments of the invention are described in detail below with reference to the attached drawings, but the invention can be implemented in a number of different ways, which are defined and covered by the claims.
Example 1
The embodiment discloses a preparation method of a geometric phase element based on a nano bubble structure, which comprises the following steps:
step S1, deploying an optical path system, wherein the optical path system comprises: the device comprises a femtosecond laser source capable of adjusting pulse width, a power attenuation module capable of automatically switching single pulse energy through program control, a polarization control module, an objective lens and a triaxial displacement table for fixing a sample; the power attenuation module is arranged between the laser light source and the polarization control module, the polarization control module is used for adjusting the polarization direction of the linearly polarized light output to the objective lens by the attenuated light beam, and the triaxial displacement platform is used for switching the three-dimensional coordinates of the light spot focused by the objective lens in the sample one by one; calibrating the uniform light spot size, linear distance size, repetition frequency, pulse width, peak power density, layer distance and layer number corresponding to half-wave delay amount of the laser light source for processing the nanobubble in each polarization direction, and pulse density and single pulse energy respectively corresponding to each polarization direction; pulse density = femtosecond laser repetition frequency/displacement table scan speed.
Femtosecond (fs) is a unit of measure of the length of time. 1 femtosecond is one hundred parts per trillion of 1 second. The femtosecond laser light source adopted in the embodiment generally has the following characteristics: firstly, the duration of the femtosecond laser and the short duration thereof are only a few femtoseconds, which are thousands of times shorter than the shortest pulse obtained by an electronic method, thus being the shortest pulse which can be obtained by human under the experimental condition; the femtosecond laser has very high instantaneous power which can reach hundreds of trillion watts, and the total power of the power generation is hundreds of times more than that of the power generation worldwide; again, the femtosecond laser energy is focused into a spatial region smaller than the diameter of the hair, making the electromagnetic field several times stronger than the force of an atomic pair of electrons around it, many of which extreme physical conditions are not present on earth, and not available by other methods.
In this step, an optional optical path system structure is shown in fig. 1, and the femtosecond laser passes through the polarizer P1, and the polarization state is converted into linearly polarized light; the power attenuation module consists of a half-wave plate (HWP), a polarization beam splitting cube (PBS) and a light Block (Block) for absorbing S light, the polarization azimuth angle of laser light can be changed by rotating the angle of the HWP so as to realize attenuation, and finally the power of femtosecond laser light can be changed after the power attenuation module passes through the PBS; the light emitted by the PBS is P light (namely, the azimuth angle is 0 DEG with the polarization direction of the P1 emergent light beam), and the original polarization ellipticity can be maintained to the greatest extent after passing through a reflector (Mirror); then, the optical rotation is carried out through a polarization control module, the optical rotation consists of an electro-optical device such as an electro-optical crystal (EOM)/liquid crystal device and a Quarter Wave Plate (QWP), and the 0-degree fast axis of the QWP and the 0-degree fast axis of the EOM form 45 degrees, so that the optical rotation angle range is 0-180 degrees; focusing to the inside of a Sample (Sample) by an objective lens (OBJ), wherein the Sample is made of hard and brittle materials such as N-BK7, sapphire, fused silica glass and the like; the sample was placed on an XYZ Stage (XYZ Stage), and motion control was performed by the XYZ Stage.
Wherein, in fig. 1, the power attenuation module is used to fine tune the monopulse energy and peak power density; in actual control, the polarization direction of the light beam emitted by the objective lens is regulated mainly in an electric control mode of the polarization control module, and the influence on the polarization direction generated by the power attenuation module can be compensated; therefore, the influence of attenuation on the parameter and the adjustment of the polarization direction are effectively avoided, and a foundation is provided for improving the integral calibration efficiency by adopting partial calibration results for switching the polarization direction in the following step S18.
Preferably, referring to the optical path system shown in fig. 1, the step of obtaining the calibration relation includes:
and S11, setting a first polarization direction of the linearly polarized light emitted by the objective lens based on the optical path system.
This step can be specifically regulated by the polarization control module in fig. 1, and if necessary, can also be measured by a polarization measuring instrument placed under the objective lens.
S12, adjusting a light path to determine the polarization direction to be calibrated of the linear polarized light emitted by the objective lens and enabling the extinction ratio of the linear polarized light emitted by the objective lens to meet the requirement; and then measuring the diameter D of a light spot below the objective lens, and determining the line distance generated by the displacement of the triaxial displacement table in the horizontal direction according to the diameter of the light spot, wherein the line distance comprises a line distance and a column distance, and the value range of the line distance is D/5 to D/4.
Alternatively, this step may measure the spot diameter by a beam quality analyzer placed under the objective lens. In this embodiment, d=4/pi×λ×f/Din, where λ is the laser wavelength, F is the focal length of the objective lens, and Din is the spot size incident on the objective lens. Preferably, the value range of the spot size D is 6-8um.
In the calibration process, the nanobubbles can be in a uniform distribution state. The row spacing is the distance between two adjacent rows of nano bubble structures and is related to the period and the speed of the Y-axis direction of the triaxial displacement table; the column spacing is the distance between two adjacent nano bubble structures in the same row, namely the distance between two adjacent nano bubble structures, and is related to the period and the speed of the triaxial displacement table in the X-axis direction. Notably, are: too small a line spacing can easily cause the nanobubble structure to become a nano grating structure; the effective range is provided for the calibration of the nano bubble structure through the selection of the reasonable line distance range.
And S13, measuring the average power of the light spot, and determining the single pulse energy and the peak power density of the laser according to the average power of the light spot and the repetition frequency of the femtosecond laser.
In this step, the average power of the spot can be measured by a power meter placed under the objective lens. Wherein the average power of the laser is denoted as P avg The single pulse energy s=p of the laser avg R, R is the repetition frequency of the femtosecond laser; peak power density P of laser density ＝P peak /(π*(D/2)2)＝S/PW/(π*(D/2)2)＝P avg R/PW/(pi (D/2) 2) where P peak Peak power; PW is the pulse width of femtosecond laser (called pulse width for short), and the laser pulse width range is usually 200-600fs.
S14, adjusting the laser source and the power attenuation module to enable the peak power density of the focusing light spot of the objective lens to be 3-18TW/cm 2 Is defined in the above-described specification.
In this step, after the interval range of the peak power density is determined, a reasonable value range of the single pulse energy can be obtained based on the relationship between the parameters of step S13.
And S15, adjusting pulse density to be in a range from 50p/um to 300p/um.
In this step, the pulse density is preferably also adjusted by the displacement table scan speed only, based on the fact that the laser repetition frequency is usually kept constant during actual mass production.
The pulse density in this step is also divided into the pulse density in the X-axis direction and the pulse density in the Y-axis direction, corresponding to the line pitch being subdivided into the row pitch and the column pitch in step S11.
Step S16, changing specific values of parameters corresponding to the steps S12 to S15 in corresponding ranges, and screening out various parameter combinations of single-layer nanobubbles with diffraction efficiency more than 95% and polarization transmission difference between P polarization and S polarization direction less than 5%, wherein the diffraction efficiency=the transmittance of a modified region/the transmittance of an unmodified region; polarization transmission difference between P polarization and S polarization direction = transmittance of P polarized light-transmittance of S polarized light; each of the parameter combinations includes: spot size, line spacing size, repetition frequency, single pulse energy, peak power density, pulse density, and pulse width.
Step S17, determining a parameter combination with the largest retardation corresponding to the first polarization direction from the at least two parameter combinations screened in the step S16; and determining the layer distance and the layer number of the nanobubbles for constructing the half-wave retardation corresponding to the laser light source according to the parameter combination with the maximum retardation.
And S18, changing the first polarization direction in the step S11 into other polarization directions one by one, and then in the process of executing the series of steps from the step S12 to the step S16, keeping the line distance size, the light spot size, the pulse width and the repetition frequency in the parameter combination with the maximum target wavelength and the maximum delay amount determined in the step S17 unchanged, adjusting the pulse density and the single pulse energy, and screening out at least one parameter combination of single-layer nanobubbles with the diffraction efficiency more than 95% and the polarization transmission difference between the P polarization direction and the S polarization direction less than 5%.
Further, in the process of constructing the calibration relation, when at least two parameter combinations of single-layer nanobubbles with diffraction efficiency more than 95% and polarization transmission differences of P polarization and S polarization directions less than 5% exist in any other polarization directions, the single-layer nanobubbles are subjected to weighted operation according to the detected diffraction efficiency and the polarization transmission differences of the P polarization and S polarization directions and then are sequenced, and the optimal parameter combination is determined according to the sequencing result.
Step S2, processing the nano bubble structures of all layers one by one in a sequence from bottom to top, and downwards moving the nano bubble structures by one unit in the Z-axis direction of the triaxial displacement table in the process of switching the next layer to the previous layer after processing; in the process of processing the single-layer nano bubble structure, the single nano bubble structure is processed one by one through displacement in the X-axis and Y-axis directions of the triaxial displacement table; in the process of processing a single nano bubble structure, the power attenuation module, the polarization control module and the triaxial displacement table are controlled to execute corresponding linkage operation according to the calibration relation corresponding to the target polarization direction.
Therefore, the calibration process of the embodiment has at least the following beneficial effects:
1. in the detection process of the prepared sample, the polarization transmission difference of P light and S light is increased, and the P light and S light can be converted into a nano grating structure from a nano bubble structure; the invention can simply and efficiently screen out the reliable parameters of the processing nanobubble by making the polarization transmission difference between the P polarization and the S polarization less than 5%.
2. The diffraction efficiency is closely related to the overall performance of the nanobubble structure, and the parameter combination for stable processing performance of the nanobubbles can be reliably obtained by screening out the prepared samples with the diffraction efficiency of more than 95% in the prepared samples.
3. The method has the advantages that the parameter ranges of pulse density, peak power density, line distance and the like are determined, the specific values of the parameters of pulse density, pulse width, peak power density, line distance and the like can be flexibly determined and changed according to the configuration attributes of the laser light source, the power attenuation module and the triaxial displacement table, so that the parameter screening method can screen within an effective range, and the screening efficiency is improved.
Example 2
The embodiment discloses a geometric phase element based on a nano bubble structure, which is prepared by the method disclosed in the embodiment.
Alternatively, the sample is a fused silica material. Alternatively, a hard and brittle material such as N-BK7 or sapphire may be used.
Preferably, the prepared geometric phase element is composed of an integral number of unit groups, and the unit groups are nano bubble structures combined by the number of layers corresponding to half-wave retardation; and the fast axis clamping angles among the unit groups are customized according to target requirements. For example: the geometric phase element is in particular an achromatic wave plate, an achromatic vortex wave plate, an achromatic lens, an achromatic conical lens, an achromatic polarization grating, an achromatic dammann grating, an achromatic homogenizer, a cascade grating, a beam deflector or an achromatic multifocal DOE (Diffractive Optical Elements, diffractive optical element) which is composed of at least two unit groups. The specific cases are as follows:
[ multiple sets of achromatic elements ]: in the specific structure, the single group is a multi-layer element with half-wave retardation, the phase retardation of the single group element is set to be half wave of designed wavelength, and a series of achromatic devices such as an achromatic wave plate, an achromatic vortex wave plate, an achromatic lens, an achromatic polarization grating, an achromatic daman grating, an achromatic homogenizer, an achromatic multifocal DOE and the like can be processed according to the half-wave element structure of the 2N+1 group; one group, i.e., one unit group, generally includes multiple layers, and will not be described in detail later.
[ multiple sets of achromatic vortex waveplates ]: taking an achromatic range of 400nm-700nm, and taking an N=1, namely 2N+1=3 groups of structures as an example, the three groups of modified areas are consistent with retardation, and 0-degree fast axis angles of the three groups of modified areas are different; the 0 degree fast axis angles of the first group and the third group are consistent, and an included angle (such as 57 degrees) is formed between the fast axis directions of the second group and the first group according to the calculation of the Jones matrix; taking a vortex wave plate with m=1 as an example, the fast axis direction of one circle of the vortex wave plate is 0-180 degrees, so that the relative included angle between the second group and the first group is 57 degrees×2=114 degrees.
Multiple sets of achromats & multiple sets of achromats ]: taking an achromatic range of 400nm-700nm, and taking a 2n=1, namely 2n+1=3 groups of structures as an example, the modified areas and the retardation of the three groups are consistent, the initial positions of 0-degree fast axis angles of the three groups are different, the 0-degree fast axis angles of the first group and the third group are consistent, and an included angle (such as 57 degrees) is formed between the fast axis directions of the second group and the first group according to the calculation of a Jones matrix; the fast axis directions of the achromats are uniform over the same radius, starting with 57 ° for the first and third groups and 0 ° for the second group. Distinction between achromatic and achromatic axicon: the fast axis orientation of the achromatic lens is in a continuous gradual change distribution which gradually decreases periodically along the radial direction (the periodic interval in the radial direction is set as sqrt (2 npi), n refers to the nth period, the fast axis direction in a single period satisfies 0-180 degrees), and the fast axis orientation of the achromatic lens is in an equal periodic gradual change distribution along the radial direction of the substrate.
[ multiple sets of achromatic polarization gratings ]: taking achromatic range 400nm-700nm, and taking an N=1, namely 3 groups of structures as an example, wherein each group is an independent half-wave retardation polarization grating, the three groups of modified areas are consistent with retardation, the initial positions of 0-degree fast axis angles are different, the first group and the third group are consistent with each other, and an included angle (such as 57 degrees) is formed between the fast axis directions of the second group and the first group according to the calculation of a Jones matrix; the polarization grating fast axis orientation is periodically and continuously graded along the x direction, three groups of polarization gratings are aligned in a staggered way, the 0-degree fast axis angles of the first group and the third group are consistent (completely parallel and different in height), the second group is arranged between the first group and the third group, the fast axis angle of the second group and the first group is 57 degrees, namely the polarization gratings are staggered in the x direction, and the 57 degrees of the fast axis of the second group are aligned with the 0-degree fast axis of the first group.
[ multiple superimposed elements ]: taking a cascade grating as an example, setting half waves of designed wavelengths for phase delay, processing the cascade grating (consisting of two groups of polarization gratings), taking a cascade grating beam splitter of a 1×4 beam splitting mode as an example, and on the premise that the beam splitting angle (the included angle between adjacent beams) of each cascade grating phase period and the cascade grating beam splitter is +/-1% of beam splitting angle error, approximately meeting p1=2p2=λ/sin (θ/2), wherein p1 and p2 are the phase periods of a first-stage grating and a second-stage grating of the cascade grating beam splitter respectively, λ is the working wavelength of the cascade grating beam splitter, and θ is the beam splitting angle (full angle) of the cascade grating beam splitter; for the cascade grating beam splitter with the 2×2 beam splitting mode, the p1=p2=λ/sin (θ/2) is approximately satisfied on the premise that the beam splitting angle (the included angle between two beams of light on the square side) of each stage of sub-grating phase period and the cascade grating beam splitter is within ±1% of the beam splitting angle error.
Taking a beam deflector as an example, the traditional structure of the beam deflector consists of two components of a 1/4 wave plate and a polarization grating, wherein linear polarized light and a 0-degree fast axis of the 1/4 wave plate form an angle of 45 degrees or-45 degrees, and left-handed circularly polarized light or right-handed circularly polarized light is generated; the left circularly polarized light or right circularly polarized light enters the polarization grating, generates circularly polarized light of opposite handedness, and has only-1 order or +1 order, and is therefore called a grating deflector. And based on the present embodiment, it is possible to integrate on the same sample in a unit group and layer-to-layer design.
Thus, in the present embodiment, the main differences between the femtosecond laser modification technique and the liquid crystal photo-alignment technique include:
1. material
The liquid crystal photo-alignment material and the LCP polymer material (the applicable wave band range is more than 400 nm), the femtosecond laser modification material is fused quartz (more than 200 nm), the applicable wave band is wider, other materials are not needed to be added, the damage threshold is determined by the glass, so that the damage threshold is higher, and the application range is wider; the femtosecond laser modified quartz device also does not need to use optical glue, so that the quartz device is more convenient for storage, has higher durability and longer service life.
2. Processing technology
The liquid crystal photo-alignment needs to be performed with alignment on alignment materials first, and the polarization azimuth angle of the liquid crystal is determined; the LCP is spin-coated, namely, the LCP is arranged according to the azimuth angle of the orientation material, and the thickness of the LCP determines the retardation (namely, the orientation and the retardation are two steps); the femtosecond laser modification is to process in the fused quartz, and combine two processes of orientation and delay, wherein the orientation is determined by the polarization direction of the laser, and the delay is determined by pulse density, single pulse energy, line distance, layer number and the like.
3. Easy integration
The femtosecond modification can be processed in the fused quartz glass, and different structures are all processed in one glass substrate in a multilayer mode; and the liquid crystal photo-alignment needs to be made into different structures, and then the liquid crystal photo-alignment is bonded together, so that the bonding can increase the process steps, reduce the yield and increase the cost.
In summary, the preparation method and the system of the geometric phase element based on the nano bubble structure disclosed by the embodiment of the invention have at least the following beneficial effects:
1. the system components are simple, the precision requirements among linkage equipment are high, and mass production can be performed according to the calibration relation after calibration; thereby ensuring the overall performance and preparation efficiency of the product.
2. In the calibrated parameters, the light spot size, the line distance size, the repetition frequency, the pulse width and the peak power density are kept uniform for each polarization direction, different polarization information is adapted by adjusting the pulse density and/or the single pulse energy, so that the differentiated internal structure of each layer of nanobubbles can be conveniently designed, the flexibility and the expansibility of the system are enriched, and the cooperative complexity in the switching process along with the polarization direction is reduced.
3. In the calibration relation, the layer distance and the layer number of the half-wave delay amount are calibrated, so that more complex geometric phase component design is conveniently carried out in the sample by taking the half-wave delay amount as a unit, the combination function of a plurality of existing independent optical components can be integrated in a single sample, and the overall cost performance of the product can be improved in a step-shaped manner.
The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (8)
1. A method for preparing a geometric phase element based on a nanobubble structure, comprising:
step S1, deploying an optical path system, wherein the optical path system comprises: the device comprises a femtosecond laser source capable of adjusting pulse width, a power attenuation module capable of automatically switching single pulse energy through program control, a polarization control module, an objective lens and a triaxial displacement table for fixing a sample; the power attenuation module is arranged between the laser light source and the polarization control module, the polarization control module is used for adjusting the polarization direction of the linearly polarized light output to the objective lens by the attenuated light beam, and the triaxial displacement platform is used for switching the three-dimensional coordinates of the light spot focused by the objective lens in the sample one by one; calibrating the uniform light spot size, linear distance size, repetition frequency, pulse width, peak power density, layer distance and layer number corresponding to half-wave delay amount of the laser light source for processing the nanobubble in each polarization direction, and pulse density and single pulse energy corresponding to each polarization direction respectively; the pulse density = femtosecond laser repetition frequency/displacement table scan speed;
step S2, processing the nano bubble structures of all layers one by one in a sequence from bottom to top, and downwards moving the nano bubble structures by one unit in the Z-axis direction of the triaxial displacement table in the process of switching the next layer to the previous layer after processing; in the process of processing the single-layer nano bubble structure, the single nano bubble structure is processed one by one through displacement in the X-axis and Y-axis directions of the triaxial displacement table; in the process of processing a single nano bubble structure, the power attenuation module, the polarization control module and the triaxial displacement table are controlled to execute corresponding linkage operation according to the calibration relation corresponding to the target polarization direction.
2. The method of claim 1, wherein the obtaining of the calibration relationship comprises:
step S11, setting a first polarization direction of the linearly polarized light emitted by the objective lens based on the light path system;
s12, adjusting a light path to determine the polarization direction to be calibrated of the linear polarized light emitted by the objective lens and enabling the extinction ratio of the linear polarized light emitted by the objective lens to meet the requirement; then measuring the diameter D of a light spot below the objective lens, and determining the line distance generated by the displacement of the triaxial displacement table in the horizontal direction according to the diameter of the light spot, wherein the line distance comprises a line distance and a column distance, and the value range of the line distance is D/5 to D/4;
step S13, measuring the average power of the light spot, and determining the single pulse energy and the peak power density of the laser according to the average power of the light spot and the repetition frequency of the femtosecond laser;
s14, adjusting the laser source and the power attenuation module to enable the peak power density of the focusing light spot of the objective lens to be 3-18TW/cm 2 Is a section of (2);
s15, adjusting pulse density to be 50p/um-300p/um;
step S16, changing specific values of parameters corresponding to the steps S12 to S15 in corresponding ranges, and screening out various parameter combinations of single-layer nanobubbles with diffraction efficiency more than 95% and polarization transmission difference between P polarization and S polarization direction less than 5%, wherein the diffraction efficiency=the transmittance of a modified region/the transmittance of an unmodified region; polarization transmission difference between P polarization and S polarization direction = transmittance of P polarized light-transmittance of S polarized light; each of the parameter combinations includes: spot size, line spacing size, repetition frequency, single pulse energy, peak power density, pulse density, and pulse width;
step S17, determining a parameter combination with the largest retardation corresponding to the first polarization direction from the at least two parameter combinations screened in the step S16; determining the layer distance and the layer number of the nanobubbles for constructing the half-wave retardation corresponding to the laser light source according to the parameter combination with the maximum retardation;
and S18, changing the first polarization direction in the step S11 into other polarization directions one by one, and then in the process of executing the series of steps from the step S12 to the step S16, keeping the line distance size, the light spot size, the pulse width and the repetition frequency in the parameter combination with the maximum target wavelength and the maximum delay amount determined in the step S17 unchanged, adjusting the pulse density and the single pulse energy, and screening out at least one parameter combination of single-layer nanobubbles with the diffraction efficiency more than 95% and the polarization transmission difference between the P polarization direction and the S polarization direction less than 5%.
3. The method as recited in claim 2, further comprising:
when at least two parameter combinations of single-layer nanobubbles with diffraction efficiency more than 95% and polarization transmission difference of P polarization and S polarization direction less than 5% exist in any other polarization direction, sorting after weighting operation according to the detected diffraction efficiency and the polarization transmission difference of P polarization and S polarization direction, and determining the optimal parameter combination according to the sorting result.
4. A method according to any one of claims 1 to 3, wherein the calculation formula for the spot size D is specifically: d=4/pi λ F/Din, where λ is the laser wavelength, F is the focal length of the objective lens, and Din is the spot size incident on the objective lens.
5. A geometric phase element based on a nanobubble structure, prepared by the method of any one of claims 1 to 4.
6. The geometric phase element of claim 5, wherein the sample is a fused silica material.
7. A geometric phase element according to claim 5 or 6, characterized in that the geometric phase element is composed of an integer number of unit groups, the unit groups being nanobubble structures combined with half-wave retardation corresponding to the number of layers; and the fast axis clamping angles among the unit groups are customized according to target requirements.
8. Geometric phase element according to claim 7, characterized in that it is in particular an achromatic wave plate, an achromatic vortex wave plate, an achromatic lens, an achromatic conical lens, an achromatic polarization grating, an achromatic dammann grating, an achromatic homogenizer, a cascade grating, a beam deflector or an achromatic multifocal DOE, which consists of at least two unit groups.
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| CN118023736A (en) * | 2024-03-01 | 2024-05-14 | 南方科技大学 | Preparation method of geometric phase optical element |
| CN118976980A (en) * | 2024-08-21 | 2024-11-19 | 清华大学 | A chiral structure processing device based on single position exposure |
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| CN118023736A (en) * | 2024-03-01 | 2024-05-14 | 南方科技大学 | Preparation method of geometric phase optical element |
| CN118976980A (en) * | 2024-08-21 | 2024-11-19 | 清华大学 | A chiral structure processing device based on single position exposure |
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