CN117855857A - Device for realizing dynamic adjustable geometric phase on heterogeneous integrated terahertz metasurface - Google Patents
Device for realizing dynamic adjustable geometric phase on heterogeneous integrated terahertz metasurface Download PDFInfo
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- XHCLAFWTIXFWPH-UHFFFAOYSA-N [O-2].[O-2].[O-2].[O-2].[O-2].[V+5].[V+5] Chemical compound [O-2].[O-2].[O-2].[O-2].[O-2].[V+5].[V+5] XHCLAFWTIXFWPH-UHFFFAOYSA-N 0.000 claims abstract description 47
- 229910001935 vanadium oxide Inorganic materials 0.000 claims abstract description 46
- 238000010438 heat treatment Methods 0.000 claims abstract description 40
- 238000000034 method Methods 0.000 claims abstract description 38
- 239000002131 composite material Substances 0.000 claims abstract description 37
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 claims abstract description 27
- 239000010931 gold Substances 0.000 claims abstract description 26
- 229910052737 gold Inorganic materials 0.000 claims abstract description 19
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- 229910001922 gold oxide Inorganic materials 0.000 claims description 7
- 239000012782 phase change material Substances 0.000 claims description 6
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- 238000004364 calculation method Methods 0.000 claims description 4
- 238000005516 engineering process Methods 0.000 claims description 4
- 229910052594 sapphire Inorganic materials 0.000 claims description 4
- 239000010980 sapphire Substances 0.000 claims description 4
- 238000000609 electron-beam lithography Methods 0.000 claims description 3
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- CSCPPACGZOOCGX-UHFFFAOYSA-N Acetone Chemical compound CC(C)=O CSCPPACGZOOCGX-UHFFFAOYSA-N 0.000 description 2
- KFZMGEQAYNKOFK-UHFFFAOYSA-N Isopropanol Chemical compound CC(C)O KFZMGEQAYNKOFK-UHFFFAOYSA-N 0.000 description 2
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 2
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q15/00—Devices for reflection, refraction, diffraction or polarisation of waves radiated from an antenna, e.g. quasi-optical devices
- H01Q15/0006—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices
- H01Q15/0086—Devices acting selectively as reflecting surface, as diffracting or as refracting device, e.g. frequency filtering or angular spatial filtering devices said selective devices having materials with a synthesized negative refractive index, e.g. metamaterials or left-handed materials
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q3/00—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system
- H01Q3/26—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture
- H01Q3/30—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture varying the relative phase between the radiating elements of an array
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Abstract
The invention discloses a device for realizing dynamic adjustable geometric phase on a heterogeneous integrated terahertz metasurface, and provides a method for realizing dynamic adjustable geometric phase on the basis of the device, belonging to the field of micro-nano optics. The composite antenna on the metasurface consists of a double-layer split ring formed by heterogeneous integration of gold and a tunable material vanadium oxide, and the specific complex amplitude of a transmitted light field can be endowed on a pixel level by adjusting the relative rotation position between the gold and the tunable material vanadium oxide, so that customizable dynamic function combination at the temperature higher than the vanadium oxide threshold temperature is realized according to holographic coding. The invention does not need huge workload of traversing all structural parameters of the antenna in the early stage, and the switching of dynamic functions does not need any mechanical movement, and the dynamic adjustable metasurface can be realized only by globally heating the metasurface. The invention benefits from the characteristics of high efficiency, light weight and compactness of the transmission type metasurface modulation, can flexibly modulate various complex amplitude wave fronts, and is particularly suitable for the fields of dynamic display and projection, temperature change optical encryption, anti-counterfeiting and the like.
Description
Technical Field
The invention relates to a device for realizing dynamic adjustable geometric phase on a heterogeneous integrated terahertz metasurface, which endows a transmission light field with preset complex amplitude at a pixel level by adjusting a sub-wavelength composite antenna structure and belongs to the field of micro-nano optics.
Background
The metasurface is an artificial two-dimensional material composed of sub-wavelength antennas, and singular electromagnetic characteristics which are not possessed by nature can be obtained by exquisite design of geometric dimensions, materials and arrangement modes of the antennas, so that physical quantities such as amplitude, phase, wavelength and polarization of electromagnetic waves are flexibly regulated and controlled. Compared with a static metasurface, the dynamic metasurface has a wider application prospect, and is a common method for realizing the dynamic property of the metasurface by combining a micro-electromechanical system or a tunable material, wherein the tunable material refers to the phase state and optical parameters of the material can be changed along with external electric/optical/thermal stimulation excitation, so that the dynamic function is realized by combining the tunable material with the metasurface, and the tunable material comprises liquid crystal, phase change material and the like.
The geometric phase principle, also called Bei Li phase principle, involves many fields of basic physics and plays a key role in the research field of metasurfaces. The geometrical phase can be directly encoded into the rotation angle of the metasurface antenna, is irrelevant to the optical path size of light passing through the optical element and has the characteristic of no color, so that the metasurface design is simpler and quicker, and a large number of application cases, such as a superlens, a beam splitter, a wave plate and the like, are realized on the basis of the geometrical phase principle design at present. However, this simple phase modulation approach is hindered from being used to achieve a dynamic metasurface because the angular distribution of rotation of the antenna array on the metasurface is fixed after processing. Meanwhile, the resonance phase is a common phase modulation principle for realizing the dynamic metasurface nowadays, but the application of the method involves a large amount of antenna structure parameter scanning in the early stage, and huge parameter traversing workload is sometimes difficult to ensure that a proper structure can be selected from the antenna structure, because the target structure needs to simultaneously meet complex amplitude parameters of the dynamic metasurface in at least two states, the design method of the dynamic metasurface with simple geometric phase and load phase delay in the rotation angle of the antenna is to be proposed aiming at the dynamic metasurface.
Disclosure of Invention
Aiming at the technical problems that the current dynamic adjustable metasurface implementation principle has limitation, micro-nano processing and testing conditions are severe, a simple coding mode and flexible adjustability cannot be achieved by the traditional geometric phase principle, and the like, the invention aims to provide a device for realizing dynamic adjustable geometric phase of a heterogeneous integrated terahertz metasurface, and provides a method for realizing dynamic adjustable geometric phase on the basis. Compared with the method for realizing the dynamic adjustable metasurface by utilizing the resonance phase principle and combining the tunable materials in the prior art, the method does not need huge workload of traversing all structural parameters of the antenna in the prior art, does not need any mechanical movement in the switching of the dynamic function, and can realize the dynamic adjustable metasurface by globally heating the metasurface. The invention benefits from the characteristics of high modulation efficiency, light weight and compactness of the transmission type metasurface, and can be used in the fields of dynamic display and projection, temperature change optical encryption, anti-counterfeiting and the like.
The aim of the invention is achieved by the following technical scheme.
The invention discloses a device for realizing dynamic adjustable geometric phase on a heterogeneous integrated terahertz metasurface, which is required to form a composite antenna of the metasurface so as to not only meet the simple angle coding characteristic of the geometric phase principle, but also realize pixel-level coding and temperature control adjustability of complex amplitude of an emergent light field by utilizing the heterogeneous integrated characteristic. The device can realize dynamic adjustable geometric phase by globally changing the temperature of the metasurface, and obtain a customizable reproduction function combination with switchable temperature control.
The device for realizing dynamic adjustable geometric phase on the heterogenous integrated terahertz metasurface takes sapphire as a substrate, and a heterogenous integrated composite antenna with gold and vanadium oxide split rings overlapped up and down is an antenna unit on the metasurface; the antenna unit is fixed on the substrate.
The golden split ring consists of a long arc and n equal-length short arcs which are split uniformly, the split ring is symmetrical along the central extension line of the long arc, and the extension line is called the symmetry axis of the golden split ring.
The included angle between the symmetry axis of the golden split ring and the X axis in the XY plane is theta 1 。
The symmetry axis of the golden split ring rotates in the XY plane at equal intervals around the Z axis within 180 degrees in the center of the ring to form an angle rotation order m of the golden split ring within 180 degrees, and meanwhile, m is the phase gradient number of the golden split ring within 2 pi.
The vanadium oxide split ring consists of a long arc and a short arc, the split ring is symmetrical along the central extension line of the short arc, and the extension line is called the symmetry axis of the vanadium oxide split ring.
The included angle between the symmetry axis of the vanadium oxide split ring and the X axis in the XY plane is theta 2 。
The short arc of the vanadium oxide split ring always overlaps with a certain short arc of the gold split ring in space up and down.
The double-layer composite antenna is equivalent to an integral structure, and the direction of the equivalent symmetry axis of the whole antenna is changed by introducing the phase change material vanadium oxide so as to realize the dynamically adjustable geometric phase.
Further, the dimensional parameters and the structure of the metasurface composite antenna are determined, and an antenna structure-phase matching library corresponding to the antenna before and after heating can be generated, and the method is concretely implemented as follows:
the main dimension parameter of the composite antenna comprises the outer diameter R of the split ring of gold and vanadium oxide 1 An inner diameter R 2 Layer thickness h, and period P of the pixel cell.
Rotation angle theta of gold-split ring contained in structure-phase matching library 1 Rotation angle theta of vanadia split ring 2 . According to the principle of geometric phase: the rotation angle of the antenna with the phase value of two times is obtainedThe split ring being at a certain rotation angle theta 1 Lower corresponding phase valueThe vanadia split ring at a certain rotation angle theta 2 Lower corresponding phase value +.>The horizontal data in the structure-phase matching library are all rotation angle values of the gold split ring within a range of 180 degrees and corresponding phase values covering 2 pi under the angle rotation order m, and the vertical data are all possible rotation angle values and corresponding phase values of the vanadium oxide split ring under the arrangement of an antenna structure with n equal-length short arcs of the gold split ring split uniformly, so that the antenna structure-phase matching library with capacity of m multiplied by n is formed.
Further, by adjusting the structural parameters of the composite antenna, the capacity of the antenna structure-phase matching structure library can be directly adjusted to provide finer phase distribution and phase selection for the metasurface.
The structural parameters comprise the short arc number n of the golden ring or the angular rotation order m of the golden ring within the range of 180 degrees. In theory, m and n can be increased continuously if processing levels allow.
The invention also discloses a method for realizing dynamic adjustment of the geometric phase, which is realized based on the heterogeneous integrated terahertz metasurface. The invention discloses a method for realizing dynamic adjustment of geometric phase, which realizes customized temperature control function combination before and after heating of a metasurface. The light field distribution combination of the device on the final observation plane before and after heating is determined in advance, and two corresponding holograms are obtained by means of a holographic algorithm or a theoretical formula; and selecting an antenna structure which can be matched with each pixel in two phases before and after heating from the antenna structure-phase matching library, thereby obtaining the final metasurface composite antenna array. The specific implementation method of the antenna pixel-by-pixel matching is as follows:
according to the principle of geometric phase, the phase value corresponding to each pixel before and after heating on the metasurface is twice the rotation angle of the equivalent symmetry axis of the double-layer composite antenna. Before heating, vanadium oxide is in an insulating stateAt this time, the equivalent symmetry axis of the double-layer composite antenna coincides with the symmetry axis of the upper layer gold split ring, and the corresponding phase value of the antenna isAfter heating, the vanadium oxide is converted into a metal state, at the moment, the equivalent symmetry axis of the double-layer composite antenna coincides with the symmetry axis of the split ring of the lower layer vanadium oxide, and the corresponding phase value of the antenna is +.>Thereby realizing the temperature control dynamic adjustment of the geometric phase value of each local pixel.
Further, the holographic algorithm uses an iterative calculation holographic algorithm or a non-iterative algorithm. Iterative calculation holographic algorithms include fresnel holographic algorithm and Gerchberg-Saxton (GS) holographic algorithm. Non-iterative algorithms, including point source algorithms.
Further, two holograms corresponding to the light field distribution combination on the final observation plane before and after the preset heating are obtained by means of a holographic algorithm or a calculation formula. The specific implementation mode is as follows: the holographic algorithm is a Fresnel holographic algorithm, and is characterized in that light field forward and backward propagation is repeatedly carried out between two planes through Fresnel transformation until the generated phase-only hologram meets the reproduction requirement.
Step one: determining and inputting a target reproduction image I, changing the size of the target reproduction image I into the size of a target hologram, and extracting the amplitude distribution of the I as amp;
step two: randomly generating a complex amplitude distribution H with the number of pixels of the target hologram and the like, wherein the amplitude distribution value is 1;
step three: carrying out Fresnel transformation on the H to obtain a complex amplitude reproduction image R of an observation plane;
step four: judging whether the complex amplitude reproduction image R is similar to the target reproduction image I or not, and if the reproduction quality requirement is met, directly jumping to the step seven; if the reproduction quality requirement is not satisfied, the phase distribution of R is reserved, the amplitude distribution is replaced by the amplitude distribution amp of the target reproduction image I in the first step, and a new reproduction image R' is formed. Performing inverse Fresnel transformation on the reproduction image R' to obtain complex amplitude holographic distribution X;
step five: the phase distribution of X is reserved, and the amplitude distribution value is completely replaced by 1, so that a new complex amplitude distribution H' is formed;
step six: replacing H with H', and repeating the fourth step and the fifth step until the reproduction quality requirement is met;
step seven: the phase distribution in the complex amplitude distribution H at this time is output as a final phase-only hologram for matching the antenna structure.
Further, according to the final metasurface composite antenna array, a micro-nano processing technology mainly comprising a two-step electron beam lithography method is used for preparing the metasurface, then an optical path is built for experiment, data are collected, and the test data are processed by using a polarization synthesis method, so that a final light field reproduction result is obtained.
Further, the test data is processed by using a polarization synthesis method to realize the final light field reproduction, and the specific implementation method is as follows:
combining a polarizer and an analyzer in a fixed light path at a predetermined angle, respectively making terahertz light waves of a target working frequency omega incident on the metasurface and the reference surface, and receiving a corresponding time domain signal E at a position where the light waves penetrate a certain antenna at a transmission end sam (t) and E ref (t) repeating the process for 30 times to obtain average value, and performing Fourier transform to obtain corresponding frequency domain complex amplitudeAnd->Both can be unwrapped written as:
where i is an imaginary unit, phi sam And phi ref For passing throughPhase values of the sample and reference surface waves. The light field transmission coefficient through a particular polarizer angle combination is then expressed as:
the ± 45 ° orthogonal linear polarization basis is written as:
wherein, the angle sign + -1 respectively represents + -45 DEG linear polarization, and the pair of orthogonal bases can form arbitrary circular polarized light. Under the pair of orthogonal bases, the complex transmission matrix of the total emergent light field is written as:
wherein, the angle mark l represents left rotation, r represents right rotation, the angle mark is in front of the polarization direction of the emergent component, and the angle mark is in back of the incident component. The invention only considers the incidence of right-handed circularly polarized light, and the light field component t of left-handed circularly polarized emergent light lr The polarizer combination in the light path is required to be adjusted, and the corresponding light field transmission coefficient t is obtained according to the formula (3) +1-1 、t -1+1 、t +1+1 T -1-1 Then according to formula (6), calculating to obtain the reproduction complex amplitude component t of the emergent end corresponding to the local pixel lr After the above-mentioned test and data processing are performed on all pixels on the metasurface, a two-dimensional complex amplitude distribution of the final emergent light field can be obtained.
The beneficial effects are that:
1. the device and the method for realizing dynamic adjustable geometric phase on the heterogeneous integrated terahertz metasurface disclosed by the invention endow the traditional geometric phase principle with adjustability and keep the simple angle coding characteristic by virtue of a composite antenna structure formed by double-layer split circular rings of gold and phase-change material vanadium oxide heterogeneous integration.
2. The device and the method for realizing dynamic adjustable geometric phase on the heterogeneous integrated terahertz metasurface disclosed by the invention combine the Fresnel holographic algorithm and the temperature change material characteristic of the phase change material vanadium oxide to realize customizable temperature control function combinations, such as temperature change angle deflection, temperature change vortex light generation, temperature change holographic dynamic display and the like, and realize flexible modulation of optical field complex amplitude.
3. The device and the method for realizing the dynamic adjustable geometric phase of the heterogeneous integrated terahertz metasurface disclosed by the invention adopt a global heating mode to carry out temperature modulation on the dynamic composite antenna on the metasurface, have a faster modulation speed and a simpler executing device compared with point-by-point regulation, do not need to consider thermal crosstalk among pixels, and have more remarkable advantages for a large array metasurface.
4. The invention discloses a device and a method for realizing dynamic adjustable geometric phase on a heterogeneous integrated terahertz metasurface, which are characterized in that the novel design structure of a composite antenna enables an equivalent symmetrical axis of the composite antenna to have temperature variation adjustability, and the precision degree of an antenna structure library can be directly adjusted by adjusting and controlling structural parameters (the number n of short arcs of a gold-split ring or the number m of degrees of angle rotation gradient within the range of 2 pi) of the composite antenna so as to adapt to finer phase distribution on the surface of Yu Chaoying.
5. The device and the method for realizing the dynamic adjustable geometric phase on the heterogeneous integrated terahertz metasurface disclosed by the invention can flexibly modulate various complex amplitude wave fronts on the basis of realizing the beneficial effects 1 to 4, thereby being applied to application occasions such as solid-state scanning devices, dynamic display and projection, optical fiber communication, temperature-changing optical encryption and anti-counterfeiting.
Drawings
FIG. 1 is a step diagram of a device for realizing dynamic adjustable geometric phase on a heterogeneous integrated terahertz metasurface for realizing dynamic holographic switching;
fig. 2 is a schematic structural diagram of a heterogeneous integrated dual-layer composite antenna (n=4) in the embodiment;
fig. 3 is an antenna structure-phase matching library (n=4, m=8) corresponding to the embodiment;
FIG. 4 is a phase hologram obtained by using the Fresnel hologram according to the embodiment, wherein a is a phase hologram corresponding to a reproduction letter A before heating and b is a phase hologram corresponding to a reproduction letter C after heating;
FIG. 5 is a process flow diagram of a heterogeneous integrated antenna used in an embodiment;
FIG. 6 is an experimental light path diagram used in the examples, in which P 1 、P 2 And P 3 Representing a linear polarizer;
fig. 7 is a diagram of the final hologram reproduction result of the present embodiment. Wherein a is a phase distribution diagram corresponding to a reproduction image letter A before heating, and b is a phase distribution diagram corresponding to a reproduction image letter C after heating; c is the intensity profile corresponding to the letter a reproduced before heating, and d is the intensity profile corresponding to the letter C reproduced after heating.
Detailed Description
For a better description of the objects and advantages of the present invention, the following description of the invention refers to the accompanying drawings and examples.
Examples: the device for realizing dynamic adjustable geometric phase by utilizing the heterogeneous integrated terahertz metasurface realizes temperature control reproduction switching of holographic images.
The specific implementation steps are shown in fig. 1.
Step one: the structural and dimensional parameters of the metasurface composite antenna are determined. The composite antenna on the metasurface takes sapphire as a substrate, and gold and vanadium oxide split rings are stacked up and down to be heterogeneous and integrated. Considering the actual machining precision, the thickness h of the split rings of gold and vanadium oxide is 300nm, and the outer diameter R of the rings 1 Are all 43 μm, and the inner diameter R 2 All 31 μm and the pixel period P was 100 μm. Wherein the golden split ring consists of a long arc and 4 equal-length short arcs which are uniformly split (n=4), the split ring is wholly symmetrical along the central extension line of the long arc, the extension line is called the symmetry axis of the golden split ring, and the included angle between the extension line and the X axis in the XY plane is theta 1 . The gold split ring can be in 180 DEG range around the Z axis in the XY plane with the center of the ringThe inner equidistant rotation 22.5 ° constitutes an 8 th order angular rotation in the range of 180 ° and a phase gradient number in the range of 2pi (m=8). The vanadium oxide split ring consists of a long arc and a short arc, the split ring is symmetrical along the central extension line of the short arc, the extension line is called the symmetry axis of the vanadium oxide split ring, and the included angle between the extension line and the X axis in the XY plane is theta 2 . It is noted that the short arc of the vanadia split ring always overlaps spatially one short arc of the golden split ring up and down, so its symmetry axis orientation is affected by both the rotation of the upper layer of the golden split ring and the corresponding positions of its short arc and the short arc of the golden split ring. Fig. 2 is a schematic structural diagram of a hetero-integrated dual-layer composite antenna in the present embodiment.
Step two: and generating a structure-phase matching library corresponding to the composite antenna before and after heating. The capacity of the corresponding antenna structure-phase matching library in this embodiment is m×n=8×4=32, and the horizontal data of the matching library is that under the angular rotation order m=8, all rotation angle values of the golden split ring in the 180 ° range and corresponding phase values covering 2pi, that is, the 8-order rotation angle of the symmetrical axis of the golden split ring is θ 1 ＝[0，22.5°，45°，67.5°，90°，102.5°，145°，167.5°]The corresponding geometric phase value is twice the rotation angle and isπ/4，π/2，3π/4，π，5π/4，3π/2，7π/4]. The vertical row data of the matching library is all possible rotation angle values and corresponding phase values of the vanadia split ring under the antenna structure setting that the equal length short arcs of the golden split ring are n=4, and fig. 3 is an antenna structure-phase matching library (n=4, m=8) corresponding to the embodiment, wherein all possible symmetry axis angle orientation values and corresponding geometric phase values of the vanadia split ring under different rotation angles of the upper layer golden split ring are given.
Step three: and determining a switchable function combination corresponding to the heating device before and after heating, and generating two holographic phase distribution diagrams of the corresponding functions before and after heating by utilizing a Fresnel holographic algorithm. The embodiment is to realize the switching of the temperature control holographic image, the selected working wavelength is 0.6THz, the letter A is reproduced before heating, and the letter C is reproduced after heating. The designed metasurface consists of 100 x 100 antennas, and the corresponding hologram contains 100 x 100 pixels. Taking the generation of the hologram corresponding to the letter A as an example, the specific implementation process is as follows: (1) Inputting an image to be reproduced, namely a letter A, changing the size of the image to contain 100 multiplied by 100 pixels, and extracting the amplitude distribution amp of the image; (2) Randomly generating a complex amplitude distribution H with the number of pixels of the target hologram and the like, wherein the amplitude distribution value is 1; (3) Carrying out Fresnel transformation on the H to obtain a complex amplitude reproduction image R of an observation plane; (4) Judging whether the complex amplitude reproduction image R is similar to the target reproduction image letter C or not, and if the reproduction quality requirement is met, directly jumping to the step (7); if the reproduction quality requirement is not satisfied, the phase distribution of R is reserved, the amplitude distribution is replaced by the amplitude distribution amp of the target reproduction image letter A, and a new reproduction image R' is formed. Performing inverse Fresnel transformation on the reproduction image R' to obtain complex amplitude holographic distribution X; (5) The phase distribution of X is reserved, and the amplitude distribution value is completely replaced by 1, so that a new complex amplitude distribution H' is formed; (6) Replacing H with H', repeating (4) and (5) until the reproduction quality requirement is met; (7) And outputting the phase distribution in the complex amplitude distribution H at the moment as a final pure phase hologram, namely, a phase hologram corresponding to the reproduction letter A. And similarly, generating a phase hologram corresponding to the letter C. FIG. 4 is a phase hologram obtained by using the Fresnel hologram according to the embodiment, wherein a is a phase distribution diagram corresponding to a reproduction letter A before heating, and b is a phase distribution diagram corresponding to a reproduction letter C after heating;
step four: according to the phase distribution of the metasurface before and after heating, selecting the optimal antenna structure from the antenna structure-phase matching library pixel by pixel to form a final metasurface array, and generating a corresponding processing file. The specific process of structure matching is as follows: the metasurface is composed of a double-layer composite antenna formed by vertically superposing gold and vanadium oxide split rings, the vanadium oxide is in an insulating state at room temperature, and at the moment, the equivalent symmetry axis of the double-layer composite antenna is coincident with the symmetry axis of an upper gold structure and corresponds to a certain rotation angle theta 1 And phase valueAfter heating, the vanadium oxide is converted into a metal state, and at the moment, the equivalent symmetry axis of the double-layer composite antenna is coincident with the symmetry axis of the lower-layer vanadium oxide structure and corresponds to another rotation angle theta 2 And phase value->Thereby achieving a temperature controlled geometric phase. Therefore, when the structures are matched, the horizontal data in the antenna structure-phase matching library corresponds to the phase value before heating, namely the phase value on the hologram corresponding to the letter A determines the rotation angle distribution of the upper layer gold of the antenna, and the vertical data in the antenna structure-phase matching library corresponds to the phase value after heating, namely the phase value on the hologram corresponding to the letter C determines the rotation angle distribution of the lower layer vanadium oxide of the antenna under the condition that the rotation angle of the upper layer gold splitting ring is fixed. After pixel-by-pixel matching of the antenna structures, a final metasurface array and corresponding processing file may be generated.
Step five: and (3) preparing the metasurface by using a micro-nano processing technology mainly comprising a two-step electron beam lithography method according to the processing file obtained in the step (IV). In the embodiment, the vanadium oxide and gold split ring structure are sequentially processed with high alignment precision by using a two-step photoetching method. FIG. 5 is a process flow diagram of a heterogeneous integrated antenna used in an embodiment;
the first step is to prepare a vanadium oxide split ring structure; depositing a 300nm vanadium oxide film on a 500 mu m sapphire substrate; spin coating photoresist and pre-baking at 95 ℃ for 120 seconds; exposing by using a mask plate to endow the photoresist with a specific shape; removing part of the photoresist on the sample by using a developing solution with the concentration of 2.38%, leaving the photoresist with the split ring pattern, then curing the sample for 120 seconds by using a heating plate at the temperature of 110 ℃, cleaning the sample for 120 seconds by using plasma in an oxygen environment, and then etching the vanadium oxide layer by using an ion beam to pattern the vanadium oxide layer; the photoresist was washed away with ultrasonic cleaning in acetone and isopropanol solutions, respectively, twice for 20 minutes each, followed by cleaning the sample with plasma in an oxygen atmosphere for 300s. The second step is to prepare a golden split ring structure; sputtering a 300nm thick gold layer on the vanadium oxide split ring structure by utilizing a magnetron sputtering technology, spin coating photoresist, and preparing the gold split ring by utilizing the same photoetching process as that for preparing the vanadium oxide split ring structure, wherein the steps comprise pre-baking, patterning, developing, curing, etching and finally removing the photoresist and cleaning.
Step six: and (3) constructing an experimental light path, carrying out experiments and collecting data. FIG. 6 is an experimental light path diagram used in the examples, in which P 1 、P 2 And P 3 A linear polarizer is shown. Terahertz detection is carried out pixel by pixel, terahertz light waves with target working frequency are incident to a certain pixel point on the metasurface, the time domain signal received by a transmission end of the terahertz light waves is taken as an average result of 30 repeated tests, the detection end in a light path is moved in a two-dimensional plane, so that the time domain signal of an emergent end received pixel by pixel is formed into a time domain signal array E corresponding to a sample sam (t). Then the sample is changed into a pure substrate area, and the test is repeated to obtain a corresponding time domain signal array E ref (t). The specific light path details and detection process are as follows: firstly, the linear polarization terahertz wave emitted by the terahertz emission end is collected and collimated by a lens with the focal length of 50mm, and then the light wave passes through a polaroid and a sample and is finally received by the detection end. The probe of the probe end is 12mm away from the sample, and the sample is scanned along the x and y directions by 0.2mm step length, and the scanning range is 10X 10mm 2 . In the light path, the linear polarizer P1 is used for further purifying the linear polarized light output from the emitting end, and the linear polarizers P2 and P3 are respectively set to be four different combinations of +45°/+45°, -45 °/+45°, and-45 °/-45 °. The experimental test procedure described above was repeated after heating.
Step seven: and processing the test data by using a polarization synthesis method to obtain a final light field reproduction result. According to the data processing method of polarization synthesis, under the condition of + -45 DEG orthogonal linear polarization, the total complex transmission matrix t of the circularly polarized emergent light field lr Can be written as:
the angle sign l indicates left-hand rotation, r indicates right-hand rotation, and + -1 corresponds to + -45 DEG linear polarization directions, respectively. This embodimentConsidering only right-handed circularly polarized light incidence, left-handed circularly polarized emergent light field t lr Therefore, in the fifth step of experimental test, the polarizer and analyzer in the fixed optical path are combined at a specific angle, and the time domain signal array E of the transmitted sample and the pure substrate under different polarizer combinations is tested sam (t) and E ref (t) Fourier transforming the two to obtain corresponding frequency domain complex amplitudeAndunfolding and writing:
the light field transmission coefficient of a light wave through a particular polarizer combination can then be expressed as:
step six, after the original data obtained by testing under the four different polarization combination settings formed by the polarizers P2 and P3 are processed, the original data respectively correspond to the light field complex transmission coefficients t in the formula (7) +1+1 、t +1-1 、t -1+1 And t -1-1 Then calculate the outgoing component t lr And the intensity and phase distribution of the reproduced light field is obtained by:
intensity= |t lr | 2 (11)
Phase=arg (t) lr ) (12)
The heated data processing method is the same as above. FIG. 7 is a diagram of the resulting holographic reconstruction results of this example, in which the patterned letter A and letter C were reconstructed before and after heating, respectively, verifying the temperature controlled dynamic tunable function of the metasurface.
In summary, in the device for realizing dynamic adjustable geometric phase on the heterogeneous integrated terahertz metasurface disclosed in the embodiment, the composite antenna on the metasurface is composed of a double-layer split ring formed by heterogeneous integration of gold and a tunable material vanadium oxide, and a specific complex amplitude can be endowed to a transmitted light field at a pixel level by adjusting the relative rotation position between the gold and the tunable material vanadium oxide, so that customizable dynamic function combination at the temperature higher than the vanadium oxide threshold temperature is realized according to a holographic coding flow. Compared with the prior scheme of realizing the dynamic adjustable metasurface by utilizing the resonance phase principle and combining a tunable material, the invention does not need huge workload of traversing all structural parameters of the antenna in the early stage, and the switching of the dynamic function can not generate any mechanical movement and can be realized by only globally heating the metasurface. In addition, due to the characteristics of high transmission type metasurface modulation efficiency, light weight, thinness and compactness, the method can be used for dynamic display and projection in a miniature device, and the defect of heavy weight of the traditional display projection device can be effectively overcome by the ultra-thin size. The device is introduced with the phase-change material vanadium oxide, the temperature change threshold temperature is only 68 ℃, compared with the threshold temperature of hundreds of degrees of other tunable materials, the temperature is easy to realize, and the damage to other devices which do not endure high temperature in the application occasion is avoided, so that the device can be applied to temperature control optical encryption in daily life, and the temperature control anti-counterfeiting effect is realized.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
Claims (8)
1. The device for realizing dynamic adjustable geometric phase on the heterogeneous integrated terahertz metasurface is characterized in that: taking sapphire as a substrate, and enabling a heterogeneous integrated composite antenna with gold and vanadium oxide split rings overlapped up and down to be an antenna unit with a metasurface; the antenna unit is fixed on the substrate;
the golden split ring consists of a long arc and n equal-length short arcs which are uniformly split, the split ring is wholly symmetrical along the central extension line of the long arc, and the extension line is called the symmetry axis of the golden split ring;
the included angle between the symmetry axis of the golden split ring and the X axis in the XY plane is theta 1 ；
The symmetry axis of the golden split ring rotates in the XY plane at equal intervals around the Z axis within 180 degrees in the center of the ring to form an angle rotation order m of the golden split ring within 180 degrees, wherein m is the phase gradient number of the golden split ring within 2 pi;
the vanadium oxide split ring consists of a long arc and a short arc, the whole split ring is symmetrical along the central extension line of the short arc, and the extension line is called the symmetry axis of the vanadium oxide split ring;
the included angle between the symmetry axis of the vanadium oxide split ring and the X axis in the XY plane is theta 2 ；
The short arc of the vanadium oxide split ring always overlaps with a certain short arc of the gold split ring in space up and down;
the double-layer composite antenna is equivalent to an integral structure, and the direction of the equivalent symmetry axis of the whole antenna is changed by introducing the phase change material vanadium oxide so as to realize the dynamically adjustable geometric phase.
2. The apparatus for implementing dynamically adjustable geometric phase for a heterogeneous integrated terahertz metasurface of claim 1, wherein: the dimensional parameters and the structure of the composite antenna with the metasurface are determined, and an antenna structure-phase matching library corresponding to the antenna before and after heating is generated, and the method is concretely implemented as follows:
the main dimension parameter of the composite antenna comprises the outer diameter R of the split ring of gold and vanadium oxide 1 An inner diameter R 2 Layer thickness h, and period P of the pixel cell;
rotation angle theta of gold-split ring contained in structure-phase matching library 1 Rotation angle theta of vanadia split ring 2 The method comprises the steps of carrying out a first treatment on the surface of the According to the principle of geometric phase: the rotation angle of the antenna with the phase value of two times is obtained to obtain the golden split ring at a certain rotation angle theta 1 Lower corresponding phase valueThe vanadia split ring at a certain rotation angle theta 2 Lower corresponding phase value +.>The horizontal data in the structure-phase matching library are all rotation angle values of the gold split ring within a range of 180 degrees and corresponding phase values covering 2 pi under the angle rotation order m, and the vertical data are all possible rotation angle values and corresponding phase values of the vanadium oxide split ring under the arrangement of an antenna structure with n equal-length short arcs of the gold split ring split uniformly, so that the antenna structure-phase matching library with capacity of m multiplied by n is formed.
3. The apparatus for implementing dynamically adjustable geometric phase for a heterogeneous integrated terahertz metasurface of claim 1, wherein: the capacity of an antenna structure-phase matching structure library is directly regulated by regulating and controlling the structural parameters of the composite antenna so as to provide finer phase distribution and phase selection of the metasurface;
the structural parameters include: the short arc number n of the golden ring or the angular rotation order m of the golden ring within 180 degrees.
4. The method for realizing dynamic geometrical phase adjustment by adopting the device for realizing dynamic adjustable geometrical phase of the heterogeneous integrated terahertz metasurface of claim 2 is characterized in that: the customized temperature control function combination is realized before and after the heating of the metasurfaces; the light field distribution combination of the device on the final observation plane before and after heating is determined in advance, and two corresponding holograms are obtained by means of a holographic algorithm or a theoretical formula; selecting an antenna structure capable of being matched with two phases before and after heating on each pixel from the antenna structure-phase matching library of claim 2, and obtaining a final metasurface composite antenna array; the specific implementation method of the antenna pixel-by-pixel matching is as follows:
according to the geometrical phase principle, the phase value corresponding to each pixel on the metasurface before and after heating is twice the rotation angle of the equivalent symmetry axis of the double-layer composite antenna; before heatingThe vanadium oxide is in an insulating state, at the moment, the equivalent symmetry axis of the double-layer composite antenna coincides with the symmetry axis of the upper-layer gold split ring, and the corresponding phase value of the antenna isAfter heating, the vanadium oxide is converted into a metal state, at the moment, the equivalent symmetry axis of the double-layer composite antenna coincides with the symmetry axis of the split ring of the lower layer vanadium oxide, and the corresponding phase value of the antenna is +.>Thereby realizing the temperature control dynamic adjustment of the geometric phase value of each local pixel.
5. The method of claim 4, wherein: the holographic algorithm uses a Fresnel holographic algorithm, a Gerchberg-Saxton holographic algorithm or a point source algorithm.
6. The method of claim 4, wherein: obtaining two holograms corresponding to the light field distribution combination on the final observation plane before and after preset heating by means of a holographic algorithm or a calculation formula; the specific implementation mode is as follows: the holographic algorithm is a Fresnel holographic algorithm, and is characterized in that light field forward and backward propagation is repeatedly carried out between two planes through Fresnel transformation until the generated phase-only hologram meets the reproduction requirement;
step one: determining and inputting a target reproduction image I, changing the size of the target reproduction image I into the size of a target hologram, and extracting the amplitude distribution of the I as amp;
step two: randomly generating a complex amplitude distribution H with the number of pixels of the target hologram and the like, wherein the amplitude distribution value is 1;
step three: carrying out Fresnel transformation on the H to obtain a complex amplitude reproduction image R of an observation plane;
step four: judging whether the complex amplitude reproduction image R is similar to the target reproduction image I or not, and if the reproduction quality requirement is met, directly jumping to the step seven; if the reproduction quality requirement is not met, reserving the phase distribution of R, and replacing the amplitude distribution with the amplitude distribution amp of the target reproduction image I in the first step to form a new reproduction image R'; performing inverse Fresnel transformation on the reproduction image R' to obtain complex amplitude holographic distribution X;
step five: the phase distribution of X is reserved, and the amplitude distribution value is completely replaced by 1, so that a new complex amplitude distribution H' is formed;
step six: replacing H with H', and repeating the fourth step and the fifth step until the reproduction quality requirement is met;
step seven: the phase distribution in the complex amplitude distribution H at this time is output as a final phase-only hologram for matching the antenna structure.
7. The method of processing according to claim 6, wherein: according to the composite antenna array of the metasurface, a micro-nano processing technology mainly comprising a two-step electron beam lithography method is used for preparing the metasurface, then an optical path is built for experiment, data are collected, and the test data are processed by using a polarization synthesis method, so that a final light field reproduction result is obtained.
8. The method of claim 7, wherein: processing the test data by using a polarization synthesis method to obtain a final light field reproduction result; the specific implementation method is as follows:
combining a polarizer and an analyzer in a fixed light path at a predetermined angle, respectively making terahertz light waves of a target working frequency omega incident on the metasurface and the reference surface, and receiving a corresponding time domain signal E at a position where the light waves penetrate a certain antenna at a transmission end sam (t) and E ref (t) repeating the preset times to obtain an average value, and performing Fourier transform to obtain a corresponding frequency domain complex amplitudeAnd->Both can be unwrapped written as:
where i is an imaginary unit, phi sam And phi ref Phase values for light waves transmitted through the sample and the reference surface; the light field transmission coefficient through a particular polarizer angle combination is then expressed as:
the ± 45 ° orthogonal linear polarization basis is written as:
wherein, the angle marks + -1 respectively represent + -45 DEG linear polarization, and the pair of orthogonal bases can form any circular polarization; under the pair of orthogonal bases, the complex transmission matrix of the total emergent light field is written as:
wherein the angle mark l represents left rotation, r represents right rotation, the angle mark is in front of the angle mark represents the polarization direction of the emergent component, and the angle mark is in back of the angle mark represents the polarization direction of the incident component; considering only right-handed circularly polarized light incidence, left-handed circularly polarized emergent light field component t lr The polarizer combination in the light path is required to be adjusted, and the corresponding light field transmission coefficient t is obtained according to the formula (3) +1-1 、t -1+1 、t +1+1 T -1-1 Then according to formula (6), calculating to obtain the reproduction complex amplitude component t of the emergent end corresponding to the local pixel lr For metasAnd after all pixels on the surface are subjected to the test and the data processing, obtaining the two-dimensional complex amplitude distribution of the final emergent light field.
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