CN115509047B - Circular polarization switching device and optimization method thereof - Google Patents

Abstract

The invention discloses a circular polarization switching device and an optimization method thereof, the basic structure of the circular polarization switching device is a symmetrical structure about a poincare sphere, the device comprises an uniaxial upper delay film layer, a half-wave switching layer and a uniaxial lower delay film layer which are sequentially distributed along the incident direction. The polarization switching device mainly aims at the problems of low switching speed, limited working wave band and view field and the like of a polarization switching device in the prior art. The circularly polarized switching device optimized by the method provided by the invention can provide rapid broadband orthogonal polarization response for incident circularly polarized light under a large viewing angle. And analyzing the view field characteristics of the circular polarization switching structure by using the Poincare sphere model, and giving out a view field optimization thought, so as to further improve the view field effect of the device in a short wave band. The method can also optimize the circular polarization switching device conforming to different application scenes, and solves the practical application problem that the polarization switching device is not matched with the wave band and the field of view of the liquid crystal geometric phase optical element.

Description

Circular polarization switching device and optimization method thereof

Technical Field

The present invention relates to the technical field of non-mechanical light beam control, and in particular to a circular polarization switching device and an optimization method thereof.

Background Art

Liquid crystal optoelectronic modes with high contrast, fast response, wide viewing area and low energy consumption have always been the pursuit of display and photonic devices. Improving the optical performance of optical systems and realizing rapid dynamic regulation of optical systems rely on the series combination of liquid crystal polarization elements and polarization conversion elements.

Therefore, it is urgently required to study and design polarization conversion devices whose band and field of view match those of liquid crystal optical elements. Circularly polarized light has certain advantages in display systems due to the high degree of symmetry of its projection, so we shifted our research focus to circular polarization conversion elements with large field of view, wide band and switchability.

At present, the commonly used devices with polarization conversion function mainly include wave plates, liquid crystal retarders, Fresnel prisms, etc. Among them, liquid crystal retarders have become the first choice for dynamic polarization control devices due to their excellent dynamic photoelectric response characteristics. They can provide polarization response while maintaining the overall flatness and ultra-thin profile of the device. In addition, active liquid crystal retarders can realize dynamic modulation of the polarization state of the light beam. At present, the liquid crystal materials used to achieve dynamic switching are mainly nematic liquid crystals and ferroelectric liquid crystals. However, nematic liquid crystals need to meet the Mauguin condition, which requires a thicker liquid crystal layer, so the response time is very slow, and it is usually difficult to meet the application requirements of high-speed switching. Ferroelectric liquid crystals can be used as the first choice for fast photoelectric switching because they have a photoelectric response speed 2-3 orders of magnitude faster than traditional nematic liquid crystals (tens to hundreds of microseconds), and can obtain better viewing angles and dynamic characteristics. However, ferroelectric liquid crystals have problems such as slow switching speed of polarization switching devices and limited working bands and fields of view.

Summary of the invention

The purpose of the present invention is to overcome the defects of the prior art and to propose a circular polarization switching device and an optimization method thereof.

To achieve the above object, the present invention adopts the following specific technical solutions:

The present invention provides a circular polarization switching device, the basic structure of which is a symmetrical structure about the Poincare sphere, comprising a uniaxial upper retardation film layer, a half-wave switching layer and a uniaxial lower retardation film layer sequentially distributed along the incident direction;

The upper retardation film layer and the lower retardation film layer are symmetrically distributed relative to the half-wave switching layer;

The half-wave switching layer is a ferroelectric liquid crystal cell;

The upper retardation film layer and the lower retardation film layer are both double-layer structures, and the double-layer structure is a viewing angle compensation film and a liquid crystal polymer film.

Furthermore, the liquid crystal polymer film is a uniaxial uniform parallel liquid crystal polymer film or a twisted liquid crystal polymer film.

Furthermore, the upper retardation film layer and the lower retardation film layer are arranged in the following manner:

The viewing angle compensation film, the uniaxial uniform parallel liquid crystal polymer film;

The uniaxial uniform parallel liquid crystal polymer film and the viewing angle compensation film;

The viewing angle compensation film, the uniformly twisted liquid crystal polymer film;

The uniformly twisted liquid crystal polymer film, the viewing angle compensation film;

Any of these.

Furthermore, the viewing angle compensation film is a positive C film, and the optical axis of the C film is perpendicular to the surface of the viewing angle compensation film.

The present invention also provides a method for optimizing a circular polarization switching device, comprising the following steps:

S1, performing band compensation on the circular polarization switching device to be optimized by controlling the optical axis angles among the half-wave switching layer, the uniaxial upper retardation film layer and the uniaxial lower retardation film layer of the circular polarization switching device to be optimized;

S2, calculating the polarization state evolution curve of the circular polarization switching device to be optimized and the polarization state output curve of each film layer, analyzing the viewing angle characteristics of the structure using the Poincare sphere model, and using the viewing angle compensation film to narrow the range of the polarization state output curve at each azimuth angle to compensate for the viewing angle of the circular polarization switching device to be optimized;

S3. Use the extended Jones matrix to simulate and analyze the band and viewing angle characteristics of the circular polarization switching device to be optimized, and use a nonlinear optimization method to globally optimize the structural parameters of the circular polarization switching device to be optimized to obtain an optimized circular polarization switching device.

Furthermore, the optical axis angles between the half-wave switching layer, the uniaxial upper retardation film layer and the uniaxial lower retardation film layer of the circular polarization switching device to be optimized in step S1 are calculated by the following Jones matrix, which is shown in formula (1):

Wherein, Γ ₁ , Γ ₂ , and Γ ₃ represent the retardation values of the uniaxial upper retardation film layer, the half-wave switching layer, and the uniaxial lower retardation film layer, respectively. is the angle between the optical axis and the x-axis, Γ _e represents the optical delay of the equivalent wide-band wave plate of the structure of the circular polarization switching device to be optimized, is the equivalent optical axis orientation.

Furthermore, in step S2, the viewing angle compensation film is used to reduce the range of polarization state output curves at four azimuth angles of 0°, 90°, 45°, and 135°, so as to compensate for the viewing angle of the circular polarization switching device to be optimized.

Furthermore, the step S3 is specifically the following steps:

S301, select conversion efficiency as an evaluation function, the evaluation function is shown in formula (2):

Where η represents the conversion efficiency, S ₃ represents the circular polarization Stokes parameter;

S302. Rules for selecting positive and negative symbols:

When the circular polarization switching device to be optimized realizes the function of the switching state, the sign is a negative sign; when the circular polarization switching device to be optimized realizes the holding state, the sign is a positive sign;

S303, calculating the degree of deviation between the actual output and the expected output, so that the conversion efficiency in the entire field of view tends to be flat, thereby constraining the omnidirectional viewing angle of the circular polarization switching device to be optimized;

S304, comprehensively optimizing the waveband and field of view of the circular polarization switching device to be optimized by using an evaluation function, wherein the evaluation function is shown in formula (3):

Wherein, f _k (λ) represents the function of wavelength, k represents the corresponding central wavelength, θ and ψ represent the corresponding incident angles and azimuths, N _θ and N _ψ represent the number of grids, and S ₃ (i, j) represents the circular polarization Stokes parameter corresponding to a certain incident angle and azimuth.

S305, selecting a wavelength to be optimized according to an application scenario, setting upper and lower limits of optimization according to optical delay and dispersion relations, and obtaining an optimized circular polarization switching device;

The optical delay is shown in formula (4), the dispersion relation is shown in formula (5), and the optimization upper and lower limits are shown in formula (6):

Wherein, Γ represents light delay, d represents the thickness of each corresponding uniaxial retardation film layer or uniaxial retardation film layer, λ∈[λ _min ,λ _max ] represents the wavelength to be optimized, and λ _min and λ _max represent the boundaries of the wavelength to be optimized respectively;

Δn＝0.08423+5961/(1000·λ) ² (5)

Where Δn represents the dispersion relation;

Wherein, Δn _min and Δn _max represent the birefringence parameters at the corresponding wavelengths λ _min and λ _max , respectively.

The present invention can achieve the following technical effects:

The present invention relates to a circular polarization switching device and an optimization method thereof, which can provide a broadband orthogonal polarization response to incident circularly polarized light under a wide viewing angle, and can selectively switch the incident left-handed circular polarization state (or right-handed circular polarization state) to a left-handed circular polarization state or a right-handed circular polarization state, thereby solving practical application problems such as the mismatch between the band and field of view of polarization conversion devices and liquid crystal geometric phase optical elements, as well as slow response speed, and thus has broad application prospects in the fields of laser communication, laser countermeasures, laser radar, non-mechanical beam control, VR/AR display, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the overall structure of a circular polarization switching device according to an embodiment of the present invention;

2(a)-(d) are schematic diagrams of the structures of various layers of a circular polarization switching device according to an embodiment of the present invention;

3(a) and (b) are schematic diagrams showing how to make the circular polarization switching device equivalent to a wide-band wave plate according to an optimization method of the circular polarization switching device according to an embodiment of the present invention;

4(a)-(e) are schematic diagrams of a Poincare sphere model of a circular polarization switching device optimized according to an optimization method for a circular polarization switching device according to an embodiment of the present invention;

5(a)-(d) are schematic diagrams of the evolution trajectory of the Poincare sphere of the oblique-incident wide-band switching structure optimized by the optimization method of the circular polarization switching device according to an embodiment of the present invention;

6(a)-(d) are schematic diagrams of polarization state evolution trajectories in a switching state of an oblique-incident, large-field-of-view, wide-band circular polarization switching structure of a device optimized according to an optimization method for a circular polarization switching device arranged in a first arrangement manner according to an embodiment of the present invention;

7(a)-(e) are schematic diagrams of polarization state evolution trajectories in a holding state of an oblique-incident, large-field-of-view, wide-band circular polarization switching structure of a device optimized by an optimization method for a circular polarization switching device arranged in a first arrangement according to an embodiment of the present invention;

8(a)-(f) are schematic diagrams of viewing angle compensation results of a circular polarization switching device with a non-twisted structure optimized by an optimization method for a circular polarization switching device arranged in a first arrangement according to an embodiment of the present invention;

9(a)-(f) are schematic diagrams of viewing angle compensation results of a circular polarization switching device with a rear-twisted structure optimized by an optimization method for a circular polarization switching device arranged in a third arrangement according to an embodiment of the present invention;

FIG. 10 is a schematic diagram showing a comparison of conversion efficiencies of non-twisted and twisted structures after wide-band compensation optimized by the optimization method of a circular polarization switching device according to an embodiment of the present invention.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same modules are represented by the same reference numerals. In the case of the same reference numerals, their names and functions are also the same. Therefore, the detailed description thereof will not be repeated.

In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation of the present invention.

The specific working mode of the present invention is described in detail below in conjunction with Figures 1 to 10:

An embodiment of the present invention provides a circular polarization switching device, as shown in Figure 1(a), the basic structure of the circular polarization switching device is a symmetrical structure about the Poincare sphere, including a uniaxial upper delay film layer, a half-wave switching layer and a uniaxial lower delay film layer distributed in sequence along the incident direction; the upper delay film layer and the lower delay film layer are symmetrically distributed relative to the half-wave switching layer; the half-wave switching layer is a ferroelectric liquid crystal box; the upper delay film layer and the lower delay film layer are both double-layer structures, and the double-layer structure is a viewing angle compensation film and a liquid crystal polymer film.

The circular polarization switching device in the embodiment of the present invention is a symmetrical structure about the Poincare sphere, and is composed of three parts, namely, a uniaxial upper retardation film layer, a half-wave switching layer, and a uniaxial lower retardation film layer sequentially distributed along the incident direction, which respectively realize the polarization state conversion between "circle-line", "line-line", and "line-circle". The physical model that meets the above requirements is a λ/4 wave plate-λ/2 wave plate-λ/4 wave plate structure (hereinafter referred to as QHQ structure). This combined structure is the basic structure of the optimized structure, and the viewing angle compensation film is introduced on this basic structure for optimization. Therefore, two working modes, a switching state and a holding state, can be realized according to the switching of the optical axis orientation angle of the half-wave retardation layer, wherein the switching state represents the mutual conversion between the left-handed circular polarization state and the right-handed circular polarization state, and the holding state represents that the left-handed circular polarization state or the right-handed circular polarization state remains unchanged, and the band compensation is realized by the combination of the viewing angle compensation film and the liquid crystal polymer film and the optical axis angle design between each layer of the retardation film.

In the embodiment of the present invention, an electro-detangle type ferroelectric liquid crystal cell is used as a half-wave switching layer, as shown in FIG1(b), the working principle of the electro-detangle type ferroelectric liquid crystal, d is the cell thickness, _P0 is the pitch, and E is the applied electric field. As shown in FIG1(c), the electro-optical response characteristics of the ferroelectric liquid crystal molecules, the applied electric field E is greater than the helical expansion field threshold, the ferroelectric liquid crystal molecules rotate around the helical axis, and the rotation angle is 2θ. _{n o} refers to the refractive index for ordinary light whose photoelectric vector vibration direction is perpendicular to the crystal optical axis. _{n e} refers to the refractive index for extraordinary light whose photoelectric vector vibration direction is parallel to the crystal optical axis. The electro-detangle type ferroelectric liquid crystal is detangled under low voltage drive, and the in-plane switching is rapidly performed by applying a voltage higher than the helical expansion threshold. After the electric field is applied, the ferroelectric liquid crystal molecules rotate in unison due to their own polarization characteristics under the action of the electric field torque, and rapidly perform "in-plane switching", and the rotation trajectory is a cone. The ferroelectric liquid crystal cell in the embodiment of the present invention can be considered as a wave plate with a fixed phase delay, and the phase delay amount depends on the birefringence of the ferroelectric liquid crystal cell material and the thickness of the ferroelectric liquid crystal cell.

In the embodiment of the present invention, a viewing angle compensation film of a positive C film is used, and the optical characteristics are rotationally symmetric along the optical axis. The light delay is only related to the incident field angle and has nothing to do with the incident azimuth angle.

In the embodiment of the present invention, the liquid crystal polymer film is a uniaxial uniform parallel liquid crystal polymer film or a twisted liquid crystal polymer film, wherein the uniaxial uniform parallel liquid crystal polymer film is treated as a uniaxial a film, wherein the optical axis of the a film is parallel to the surface of the uniaxial uniform parallel liquid crystal polymer film; the optical axis of the uniform twisted liquid crystal polymer film changes uniformly along the direction of light propagation, and the twist angle is Φ.

There are four arrangements of the film layers of the circular polarization switching device in the embodiment of the present invention, as shown in FIG. 2( a ) to FIG. 2( d ):

The first film layer arrangement method: along the direction from the incident end to the exit end, there are respectively a viewing angle compensation film, a uniaxial uniform parallel liquid crystal polymer film, a ferroelectric liquid crystal box, a viewing angle compensation film, and a uniaxial uniform parallel liquid crystal polymer film.

The second film layer arrangement: along the direction from the incident end to the exit end, there are uniaxial uniform parallel liquid crystal polymer film, viewing angle compensation film, ferroelectric liquid crystal box, uniaxial uniform parallel liquid crystal polymer film, and viewing angle compensation film.

The third film layer arrangement: along the direction from the incident end to the exit end, there are respectively a viewing angle compensation film, a uniformly twisted liquid crystal polymer film, a ferroelectric liquid crystal box, a viewing angle compensation film, and a uniformly twisted liquid crystal polymer film.

The fourth film layer arrangement: along the direction from the incident end to the exit end, there are respectively a uniform twisted liquid crystal polymer film, a viewing angle compensation film, a ferroelectric liquid crystal box, a uniform twisted liquid crystal polymer film, and a viewing angle compensation film.

The four arrangements of the film layers of the circular polarization switching device in the embodiment of the present invention are divided into two categories, namely twisted and non-twisted structures, according to whether a uniform twisted liquid crystal polymer film is introduced. The twisted structure is more difficult to manufacture than the non-twisted structure, and the preparation process involves the control of the chiral doping concentration. After the twisted structure is introduced, the field angle characteristics at short waves are better than those of the non-twisted structure. However, considering the complexity of the manufacturing process, the non-twisted structure or the non-twisted structure can be selected according to the actual application situation, and the present invention does not limit this.

An embodiment of the present invention also provides an optimization method for a circular polarization switching device, which broadens the operating band of the circular polarization switching device by controlling the QHQ structure, and realizes dynamic switching of the device by controlling the optical axis angle of the half-wave delay layer.

The optimization method of the circular polarization switching device comprises the following steps:

S1. By controlling the optical axis angle between the half-wave switching layer, the uniaxial upper delay film layer and the uniaxial lower delay film layer of the circular polarization switching device to be optimized, the working band of the circular polarization switching device is broadened, and the dynamic switching control of the half-wave and full-wave modulation of the combined structure is realized, as shown in Figure 3(a) and Figure 3(b). In Figure 3(a), the circular polarization switching device to be optimized is equivalent to a wide-band half-wave plate, and in Figure 3(b), the circular polarization switching device to be optimized is equivalent to a wide-band full-wave plate. The optical axis angle satisfies the Jones matrix, and the Jones matrix is shown in formula (1):

Wherein, Γ ₁ , Γ ₂ , and Γ ₃ represent the retardation values of the uniaxial upper retardation film layer, the half-wave switching layer, and the uniaxial lower retardation film layer, respectively. is the angle between the optical axis and the x-axis, Γ _e represents the optical delay of the equivalent wide-band wave plate of the structure of the circular polarization switching device to be optimized, is the equivalent optical axis orientation.

The wide-band wave plate needs to satisfy the partial derivative formula (2):

Let Γ ₁ = aΓ, Γ ₂ = bΓ, Γ ₃ = cΓ, and a, b, c are not 0, Γ is a wavelength-related quantity, and the following formula (3) is satisfied:

The following results are obtained:

When the whole structure is equivalent to a half-wave plate or a full-wave plate, Γ ₂ =π and the quantitative relationship between a, b, and c is a=c=0.5b. The structure in which the uniaxial upper retardation film layer, the half-wave switching layer, and the uniaxial lower retardation film layer are equivalent to each other corresponds to a QHQ structure.

When the angle between the optical axis orientation angle of the middle half-wave switching layer and the x-axis is When the angle is It is equivalent to a wide-band full-wave plate.

The circular polarization switching device with QHQ structure can provide wide-band circular polarization conversion function. The dynamic control function is realized by adopting an electro-detwist ferroelectric liquid crystal box as a half-wave switching layer, wherein the electro-detwist ferroelectric liquid crystal box is detwist under low voltage drive, and is rapidly switched in-plane by applying a voltage higher than the spiral unfolding threshold. After the electric field is applied, the ferroelectric liquid crystal molecules rotate consistently due to the electric field torque due to their own polarization characteristics, and are rapidly "switched in-plane". The rotation trajectory is a cone surface, and the ferroelectric liquid crystal box can be considered as a wave plate with a fixed phase delay (depending on the birefringence of the ferroelectric liquid crystal material and the thickness of the liquid crystal box). Most importantly, no residual phase delay similar to the TN box is generated.

Next, the band of the circular polarization switching device optimized in step S1 is analyzed by the Poincare sphere model to verify the wide band and dynamic switching function of the QHQ structure of the optimized circular polarization switching device. The circular polarization switching device is symmetrical about the equatorial plane of the Poincare sphere, so the half-wave switching layer is placed on the equatorial plane of the Poincare sphere to ensure the symmetry of the overall structure. The half-wave switching layer needs to satisfy that the polarization state of the incident light can remain unchanged or switch to its orthogonal polarization state after passing through the uniaxial upper delay film layer. After the polarization conversion of the half-wave switching layer, the output polarization state can be returned to the original circular polarization state or switched to the orthogonal circular polarization state along a certain meridian of the Poincare sphere after passing through the uniaxial lower delay film layer. For the wide-band normal incident circular polarization conversion, taking the "maintaining state" right-handed circular polarization to right-handed circular polarization and the "switching state" right-handed circular polarization-left-handed circular polarization as examples, the working principle of the wide-band circular polarization conversion based on the QHQ structure is explained. As shown in Figure 4, Figure 4 (a), (b), (c) corresponds to the process of maintaining the state (right-hand circular polarization state-right-hand circular polarization state), and Figure 4 (a), (d), (e) corresponds to the process of switching the state (right-hand circular polarization state-left-hand circular polarization state). The south pole and north pole of the Poincare sphere represent left-hand circular polarization and right-hand circular polarization, respectively. The north pole of the Poincare sphere is set to the right-hand circular polarization state, first of all, the reduction state, corresponding to the QHQ structure equivalent to a full-wave plate. When a beam of right-hand circularly polarized light passes through the uniaxial retardation film layer, the polarization state rotates clockwise around the positive direction of the _S1 axis. After the rotation, the polarization state distribution on the Poincare sphere due to dispersion can be represented by arc A (Arc A), as shown in Figure 4 (a). The half-wave switching layer is regarded as a uniform uniaxial a-film retardation plate whose optical axis orientation angle changes with the positive and negative changes of the external voltage, and the optical delay is equal to the half-wave delay. In order to restore the polarization state, the optical axis orientation of the half-wave switching layer is at an angle of -90° with the x-axis. At this time, the polarization state rotates clockwise from Arc A (Arc A) around the negative direction of the _S1 axis to Arc B (Arc B), as shown in Figure 4(b). After passing through the uniaxial lower delay film layer, the polarization state can return to the vicinity of the North Pole area, as shown in Figure 4(c). For the right-hand-left polarization conversion, the optical axis of the half-wave switching layer as the intermediate layer is at an angle of -45° with the x-axis, and the polarization state distribution is from Arc A (Arc A), as shown in Figure 4(d), and rotates clockwise around the negative direction of the _S2 axis to Arc C (Arc C), and the distribution of the polarization state with the wavelength is shown in Figure 4(e). Finally, after passing through the uniaxial lower delay film layer, the polarization state rotates around the _S1 axis to the South Pole, realizing the right-hand-left circular polarization conversion. The folding of the evolution trajectory of the Poincare sphere is achieved by controlling the optical axis angle between the half-wave switching layer, the uniaxial upper retardation film layer and the uniaxial lower retardation film layer.

S2. Use the Poincare sphere model to analyze the viewing angle characteristics of the circular polarization switching device to be optimized after structural optimization.

When the light beam is obliquely incident into the circular polarization switching device to be optimized, different viewing angles and azimuth angles will affect the polarization state of the outgoing light, as shown in Figure 5(a). In the embodiment of the present invention, a uniaxial uniform parallel liquid crystal polymer film regarded as a uniaxial a film is used as an example for explanation, and other film layers can be analyzed in the same way. As shown in Figure 5(b), it represents a wavelength of 460nm; as shown in Figure 5(c), it represents a wavelength of 530nm; as shown in Figure 5(d), it represents a wavelength of 630nm.

Observe the Poincare sphere model of the uniaxial a film, i.e., the uniaxial uniform parallel liquid crystal polymer film, under oblique incidence, where Track 1 and Track 2 represent two different azimuth angles under the same field of view angle θ _2. and The polarization state evolution path under θ ₁ and θ ₂ , and the trajectory length corresponds to the light delay of film a. The solid closed curve and the dashed closed curve correspond to the polarization state output of all angles under two field angles θ 1 and θ 2, respectively, and θ ₁ <θ _2. According to the above theoretical model, the polarization state evolution process of the wide-band circular polarization conversion structure in the switching state of the Poincare sphere is shown in Figure 5(b)-Figure 5(d), which correspond to the polarization evolution of different wavelengths. The polarization state of obliquely incident circularly polarized light at different azimuth angles after passing through the uniaxial medium is distributed on the closed curve on the surface of the Poincare sphere. After passing through the uniaxial delay film layer, the polarization state evolves along path 1 to the closed curve L-1Q (representing the uniaxial delay film layer). After passing through the half-wave switching layer, the closed curve evolves to L-2H (representing the half-wave cutting) The closed curve L-3Q (representing the uniaxial delay film layer) evolves to the south pole (left-handed circular polarization state) after passing through the uniaxial delay film layer. In this structure, the polarization state evolution paths of circularly polarized light at different azimuths after passing through the uniaxial medium are basically the same. The maximum scale of the closed curve along the longitude and latitude directions of the Poincare sphere can be determined by four azimuths of 0°, 90°, 45°, and 135°, respectively. The field of view compensation film is required to reduce the polarization state output at the above four azimuths while maintaining the normal incidence wide-band characteristics. The viewing angle compensation requires the realization of high conversion efficiency at a large field of view angle and all-round angles, and the output polarization state of the delay film layer combination at different incident azimuths is distributed on a closed curve on the Poincare sphere. Based on the symmetry of the incident azimuth, in the embodiment of the present invention, only the angles with incident azimuths between 0° and 180° need to be optimized.

In the embodiment of the present invention, the viewing angle compensation film is used to compensate for the viewing field effect of the circular polarization switching device to be optimized, so that the circular polarization switching device maintains wide-band characteristics under normal incidence while having a large field of view. In the embodiment of the present invention, the viewing angle compensation film is a positive C film, the optical axis of the C film is perpendicular to the film surface and the optical characteristics are rotationally symmetric along the optical axis. The positive C film is used to compensate for the viewing angle of the wide-band circular polarization conversion device. According to the position change of the viewing angle compensation film, the polarization trajectory of the Poincare sphere along the longitude and latitude directions can be controlled. Based on the symmetry of the viewing angle compensation of the Poincare sphere, the C film is distributed on both sides of the half-wave switching layer. By optimizing the film layer, a twisted structure is introduced to improve the band characteristics of the device. Based on the above conclusions, four large viewing angle and wide-band circular polarization switching devices as shown in Figure 2 are designed. Figures 2 (a) to 2 (d) correspond to four film layer arrangements, respectively. The Poincare sphere trajectory of the compensated circular polarization switching device under oblique incidence at different azimuth angles of the wide-band circular polarization conversion structure is shown in FIG6. The polarization state of the right-handed circularly polarized light after passing through the viewing angle compensation film of the uniaxial medium c film and the uniaxial uniform parallel liquid crystal polymer film of the a film is distributed on a closed curve of the Poincare sphere. The solid closed curve and the dashed closed curve correspond to two viewing angles θ ₁ and θ ₂ , respectively, and θ ₁ <θ ₂ , as shown in FIG6(a). Each of the two figures in FIG6(b)-FIG6(d) corresponds to the evolution of the polarization state at all azimuth angles of three wavelengths of 460nm, 530nm, and 630nm. When the right-handed circularly polarized light passes through the viewing angle compensation film + c film, the polarization state evolves along path 1 to the closed curve L-1C (representing the first layer of viewing angle compensation film + c film). After passing through the second layer of liquid crystal polymer film whose optical axis is parallel to the film surface, the closed curve L-1C evolves along path 2 to the closed curve L-2A (representing the second layer of uniaxial uniform parallel liquid crystal polymer film). After passing through the half-wave switching layer, the closed curve L-2A evolves along path 3 to the closed curve L-3H (representing the third half-wave delay switching layer), and after passing through a layer of viewing angle compensation film + c film, the closed curve L-3H evolves along path 4 to the closed curve L-4C (representing the fourth viewing angle compensation film + c film), and finally passes through a layer of liquid crystal polymer film, and the closed curve L-4C evolves along path 5 to the closed curve L-5A (representing the fifth uniaxial uniform parallel liquid crystal polymer film) near the South Pole (left-handed circular polarization state). As shown in Figure 6, the introduction of the viewing angle compensation film + c film makes the output polarization state distribution after passing through each layer of film at different azimuth angles converge to different degrees, so the final output polarization state is close to the South Pole.

The film layers in the embodiment of the present invention are arranged as follows: along the direction from the incident end to the exit end, there are respectively a viewing angle compensation film, a uniaxial uniform parallel liquid crystal polymer film, a ferroelectric liquid crystal box, a viewing angle compensation film, and a uniaxial uniform parallel liquid crystal polymer film. The Poincare sphere model is used to verify the band compensation effect of the circular polarization switch after field compensation in the "switching state". As shown in Figure 7, the obliquely incident wide-band right-handed circularly polarized light first passes through the viewing angle compensation film + c film. The polarization trajectory of the Poincare sphere at a fixed azimuth angle is clockwise rotated around the negative half axis of the _S1 axis to arc A (Arc A) as shown in Figure 7(a). Then, it passes through a layer of uniaxial uniform parallel liquid crystal polymer film with 1/4λ light retardation, and the polarization state is rotated to the vicinity of the equatorial plane to form the polarization state distribution of arc B (Arc B) as shown in Figure 7(b). It passes through a half-wave switching layer and rotates to arc C (Arc C) as shown in Figure 7(c). Then, it passes through a layer of viewing angle compensation film + c film, and the arc C (Arc C) is approximately translated along the meridian direction on the spherical surface to arc D (Arc D) As shown in FIG7(d), the polarization state is finally rotated to the vicinity of the South Pole after passing through a layer of uniaxial uniform parallel liquid crystal polymer film with 1/4λ light delay, thereby realizing the switching from right-handed circular polarization state to left-handed circular polarization state, as shown in FIG7(e).

S3. In the embodiment of the present invention, an extended Jones matrix is used to simulate and analyze the wide-band and viewing angle characteristics of the circular polarization switching device to be optimized, and a nonlinear optimization method is used to globally optimize the structural parameters of the circular polarization switching device to be optimized to obtain an optimized circular polarization switching device.

S301, select conversion efficiency as the evaluation function, the evaluation function is shown in formula (4):

Where η represents the conversion efficiency, S ₃ represents the circular polarization Stokes parameter;

S302. Rules for selecting positive and negative symbols:

When the circular polarization switching device to be optimized realizes the switching state, the sign is a negative sign; when the circular polarization switching device to be optimized realizes the holding state, the sign is positive.

S303, calculating the degree of deviation between the actual output and the expected output, so that the conversion efficiency in the entire field of view tends to be flat, thereby constraining the omnidirectional viewing angle of the circular polarization switching device to be optimized;

S304, comprehensively optimizing the waveband and field of view of the circular polarization switching device to be optimized by using an evaluation function, wherein the evaluation function is shown in formula (5):

Wherein, f _k (λ) represents the function of wavelength, k represents the corresponding central wavelength, θ and ψ represent the corresponding incident angles and azimuths, N _θ and N _ψ represent the number of grid divisions, and S ₃ (i, j) represents the circular polarization Stokes parameter corresponding to a certain incident angle and azimuth.

Formula (5) is used as the evaluation function to optimize the wavelength band and field of view of the circular polarization switching device in all directions.

S305, selecting a wavelength to be optimized according to an application scenario. The wavelength is selected according to the application scenario. Multiple wavelengths may be selected for optimization to broaden the working band of the circular polarization switching device.

Set the optimization variables: thickness variable, representing the thickness of each retardation layer; angle variable, optical axis orientation and twist angle variable (all angles are referenced to the x-axis), where the liquid crystal polymer layer can be either uniaxially uniformly oriented (in this case, it can be regarded as a film treatment) or a twisted layer. Set the upper and lower limits of optimization according to the optical delay and dispersion relationship to obtain the optimized circular polarization switching device.

The optical delay is shown in formula (6), the dispersion relation is shown in formula (7), and the optimization upper and lower limits are shown in formula (8):

Wherein, Γ represents light delay, d represents the thickness of each corresponding uniaxial retardation film layer or uniaxial retardation film layer, λ∈[λ _min ,λ _max ] represents the wavelength to be optimized, and λ _min and λ _max represent the boundaries of the wavelength to be optimized respectively;

Δn＝0.08423+5961/(1000·λ) ² (8)

Where Δn represents the dispersion relation;

Wherein, Δ _nmin and Δ _nmax represent the birefringence parameters at the corresponding wavelengths λ _min and λ _max , respectively.

In the embodiment of the present invention, the optimization is performed within the optimization boundary to minimize the evaluation function, which is the optimal structure of the circular polarization switching device. In the embodiment of the present invention, for the visible light band, the optimization range of the delay value of the uniaxial upper delay film layer and the uniaxial lower delay film layer is 1.0229 to 1.6029 μm, and the optimization range of the delay value using the ferroelectric liquid crystal layer as the half-wave switching layer is between 2.0458 and 3.2058 μm.

The embodiment of the present invention takes the arrangement of the viewing angle compensation film, the uniaxial uniform parallel liquid crystal polymer film, the ferroelectric liquid crystal box, the viewing angle compensation film, and the uniaxial uniform parallel liquid crystal polymer film as an example to illustrate the optimized circular polarization switching device.

The viewing angle compensation film, uniaxial uniform parallel liquid crystal polymer film, ferroelectric liquid crystal box, viewing angle compensation film, and uniaxial uniform parallel liquid crystal polymer film are non-twisted five-layer structures. The optimized variables from top to bottom are: d _c1 , d _a1 , Φ _a1 , d _h , Φ _s , Φ _r , d _c2 , d _a2 , Φ _a2 (unit d/um; Φ/°), where d _c1 , d _a1 , d _h , d _c2 , d _a2 are thickness variables, representing the thickness of each layer respectively, Φ _a1 , Φ _a2 are orientation angle variables, representing the optical axis orientation of the two uniaxial uniform parallel liquid crystal polymer films respectively, Φ _s , Φ _r are orientation angle variables, representing the optical axis angles before and after the switching layer is switched respectively. The parameters of the circular polarization switching device with a non-twisted structure after optimization are shown in Table 1. The optimized parameters under the non-twisted structure show that the thickness of the liquid crystal polymer film is very close, and the optical axis orientations of the two layers of liquid crystal polymer films are approximately parallel, the thickness of the switching layer is nearly twice the thickness of the liquid crystal polymer film, which meets the basic structure of wide-band conversion.

Table 1 Optimization parameters of circular polarization switching device

The field of view and optical compensation effects of the circular polarization switching device of this parameter are represented by polar coordinates and two-dimensional coordinates respectively. The viewing angle compensation effects of the circular polarization switching device with a non-twisted structure at wavelengths of 460nm, 530nm, and 630nm and oblique incident angles of 0° to 45° are shown in Figure 8. Figures 8(a), (b), and (c) are the switching state compensation effects. Except for the conversion efficiency of incident light in the wavelength band of 460-530nm at large oblique incident angles at certain specific azimuths, which is approximately 97%, the conversion efficiency in the full field of view is in the range of 99% to 100%. Figures 8(d), (e), and (f) are the reduced state compensation effects. It is obvious that the conversion efficiency of the reduced state viewing angle compensation at each wavelength is close to 100%. This is because the evolution of the reduced state polarization trajectory only occurs in the northern hemisphere and the trajectory is more symmetrical.

In order to improve the viewing angle compensation effect of the switching state in the wavelength band of 460~530nm, we try to introduce a twisted structure to enhance the optimization freedom of the structure and thus improve the optical effect of the device. Taking the arrangement of the film layers of uniform twisted liquid crystal polymer film, viewing angle compensation film, ferroelectric liquid crystal box, uniform twisted liquid crystal polymer film, and viewing angle compensation film from the incident end direction to the exit end direction as an example, the optimization variables of the circular polarization switching device are as follows from top to bottom: d _c1 , d _a1 , Φ _a1 , Φ _t1 , d _h , Φ _s , Φ _r , d _c2 , d _a2 , Φ _a2 , Φ _t2 (unit d/um; Φ/°), where d _c1 , d _a1 , d _h , d _c2 , d _a2 are thickness variables, representing the thickness of each layer respectively, Φ _a1 , Φ _a2 are orientation angle variables, representing the optical axis orientation of the two uniaxial a films respectively, Φs , Φr are orientation angle variables, representing the optical axis angles before and after the switching layer is switched respectively, Φ _t1 , Φ _t2 are twist angle variables, the twist angles of the twisted liquid crystal polymer film (all angles are referenced to the x-axis). The optimization parameters after the twisted layer is introduced are shown in Table 2:

Table 2 Optimization parameters of circular polarization switching device

The optical compensation effect of the twisted layer on the field of view and band of the circular polarization switching device is represented by polar coordinate plotting and two-dimensional coordinate plotting, respectively. The viewing angle compensation effect of the twisted structure at wavelengths of 460nm, 530nm, and 630nm and an oblique incident angle of 45° is shown in Figure 9. Figures 9(a), (b), and (c) are the switching state compensation effects after the introduction of the twisted structure. The compensation effect of the switching state is significantly improved compared to the compensation effect under the non-twisted structure, especially in the short wave aspect. The conversion efficiency in the entire field of view at almost the optimized central wavelength is between 99% and 100%. Figures 9(d), (e), and (f) are the reduced state compensation effects. The conversion efficiency of the reduced state viewing angle compensation at each wavelength is close to 100%.

The above is the viewing angle effect of the circular polarization switching device. The wide-band compensation effect of the switching state under vertical incidence is shown in Figure 10. It can be seen that the wide-band effect of the twisted structure near the center band is slightly better than that of the non-twisted structure, and the improvement of the viewing angle compensation effect is more obvious. The wide-band characteristics can maintain a high conversion efficiency of more than 99% in both the restored state and the switched state, meeting the needs of actual applications.

The four structures in the embodiments of the present invention can realize the circular polarization switching function with high conversion rate for any application scenario. The design and optimization method of the wide-band circular polarization switching device provided in the embodiments of the present invention can be optimized for any working band. The present invention takes the visible light band of the display system as an example, 400-700nm, to optimize and give structural parameters. The method can also be used to optimize the design of the circular polarization conversion structure in the infrared band. The structural arrangement remains unchanged, and only the central wavelength of the optimized band needs to be manually modified.

In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

Although the embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and cannot be understood as limiting the present invention. Those skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

The above specific implementations of the present invention do not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the protection scope of the claims of the present invention.

Claims (8)

1. A circular polarization switching device, characterized in that the basic structure of the circular polarization switching device is a symmetrical structure about the Poincare sphere, including a uniaxial upper retardation film layer, a half-wave switching layer and a uniaxial lower retardation film layer sequentially distributed along the incident direction; The upper retardation film layer and the lower retardation film layer are symmetrically distributed relative to the half-wave switching layer; The half-wave switching layer is a ferroelectric liquid crystal cell; The upper retardation film layer and the lower retardation film layer are both double-layer structures, and the double-layer structure is a viewing angle compensation film and a liquid crystal polymer film. 2 . The circular polarization switching device according to claim 1 , wherein the liquid crystal polymer film is a uniaxial uniform parallel liquid crystal polymer film or a twisted liquid crystal polymer film. 3. The circular polarization switching device according to claim 2, characterized in that the upper retardation film layer and the lower retardation film layer are arranged in the following manner: The viewing angle compensation film, the uniaxial uniform parallel liquid crystal polymer film; The uniaxial uniform parallel liquid crystal polymer film and the viewing angle compensation film; The viewing angle compensation film, the uniformly twisted liquid crystal polymer film; The uniformly twisted liquid crystal polymer film, the viewing angle compensation film; Any of these. 4 . The circular polarization switching device according to claim 1 , wherein the viewing angle compensation film is a positive C film, and the optical axis of the C film is perpendicular to the surface of the viewing angle compensation film. 5. A method for optimizing a circular polarization switching device, comprising the following steps: S1, performing band compensation on the circular polarization switching device to be optimized by controlling the optical axis angles among the half-wave switching layer, the uniaxial upper retardation film layer and the uniaxial lower retardation film layer of the circular polarization switching device to be optimized; S2, calculating the polarization state evolution curve of the circular polarization switching device to be optimized and the polarization state output curve of each film layer, analyzing the viewing angle characteristics of the structure using the Poincare sphere model, and using the viewing angle compensation film to narrow the range of the polarization state output curve at each azimuth angle to compensate for the viewing angle of the circular polarization switching device to be optimized; S3. Use the extended Jones matrix to simulate and analyze the band and viewing angle characteristics of the circular polarization switching device to be optimized, and use a nonlinear optimization method to globally optimize the structural parameters of the circular polarization switching device to be optimized to obtain an optimized circular polarization switching device. 6. The optimization method according to claim 5, characterized in that the optical axis angles between the half-wave switching layer, the uniaxial upper retardation film layer and the uniaxial lower retardation film layer of the circular polarization switching device to be optimized in step S1 are calculated by the following Jones matrix, and the Jones matrix is shown in formula (1): Wherein, Γ ₁ , Γ ₂ , and Γ ₃ represent the retardation values of the uniaxial upper retardation film layer, the half-wave switching layer, and the uniaxial lower retardation film layer, respectively. is the angle between the optical axis and the x-axis, Γ _e represents the optical delay of the equivalent wide-band wave plate of the structure of the circular polarization switching device to be optimized, is the equivalent optical axis orientation. 7. The optimization method according to claim 5 is characterized in that in step S2, the viewing angle compensation film is used to narrow the range of the polarization state output curve at four azimuth angles of 0°, 90°, 45°, and 135° to compensate for the viewing angle of the circular polarization switching device to be optimized. 8. The optimization method according to claim 5, characterized in that the step S3 is specifically the following steps: S301, select conversion efficiency as an evaluation function, the evaluation function is shown in formula (2): Where η represents the conversion efficiency, S ₃ represents the circular polarization Stokes parameter; S302. Rules for selecting positive and negative symbols: When the circular polarization switching device to be optimized realizes the function of the switching state, the sign is a negative sign; when the circular polarization switching device to be optimized realizes the holding state, the sign is a positive sign; S303, calculating the degree of deviation between the actual output and the expected output, so that the conversion efficiency in the entire field of view tends to be flat, thereby constraining the omnidirectional viewing angle of the circular polarization switching device to be optimized; S304, comprehensively optimizing the waveband and field of view of the circular polarization switching device to be optimized by using an evaluation function, wherein the evaluation function is shown in formula (3): Wherein, f _k (λ) represents the function of wavelength, k represents the corresponding central wavelength, θ and ψ represent the corresponding incident angles and azimuths, N _θ and N _ψ represent the number of grids, and S ₃ (i, j) represents the circular polarization Stokes parameter corresponding to a certain incident angle and azimuth. S305, selecting a wavelength to be optimized according to an application scenario, setting upper and lower limits of optimization according to optical delay and dispersion relations, and obtaining an optimized circular polarization switching device; The optical delay is shown in formula (4), the dispersion relation is shown in formula (5), and the optimization upper and lower limits are shown in formula (6): Wherein, Γ represents light delay, d represents the thickness of each corresponding uniaxial retardation film layer or uniaxial retardation film layer, λ∈[λ _min ,λ _max ] represents the wavelength to be optimized, and λ _min and λ _max represent the boundaries of the wavelength to be optimized respectively; Δn＝0.08423+5961/(1000·λ) ² (5) Where Δn represents the dispersion relation; Wherein, Δn _min and Δn _max represent the birefringence parameters at the corresponding wavelengths λ _min and λ _max , respectively.