Vector vortex light field modulation method based on birefringent material

Abstract

The invention discloses a vector vortex light field modulation method based on a birefringent material. The method comprises the following steps: irradiating an incident light field on an incident surface of an electro-optical modulator based on the birefringent material; and under a set condition, adjusting the output voltage of the electro-optical modulator to obtain a modulated emergent light field. According to the invention, under the condition of not changing any parameter of a light path, accurate and efficient modulation of the vector vortex light field is realized only by changing the voltage signal applied to the electro-optical modulator based on the birefringent crystal.

Description

Technical Field

[0001] The present invention belongs to the technical field of light modulation, and in particular relates to a vector vortex light field modulation method based on birefringence materials. Background Art

[0002] A vector vortex light field refers to a complex light field that has both a non-uniform distribution of spatial polarization states and a spiral phase distribution. Its core meaning lies in the realization of multi-dimensional design of the spatial characteristics of the light field through the coordinated regulation of the polarization state and phase field. This light field has a non-uniform polarization state distribution in the cross section and carries orbital angular momentum. The polarization state of the vector vortex light beam in the beam cross section is not uniform, but presents a certain specific distribution pattern. The modulation technology of the vector vortex light field not only deepens the understanding of the topological structure of the light field and the interaction between light and matter, but also promotes the innovation of optical technology through the precise control of its polarization state, intensity distribution and center of gravity of the beam. How to accurately and efficiently modulate the vector vortex light field is a technical problem that needs to be solved urgently. Summary of the Invention

[0003] In order to address the deficiencies in the prior art, the present invention provides a vector vortex light field modulation method based on birefringent materials, which achieves accurate and efficient modulation of the vector vortex light field without changing any parameters of the optical path.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is: In a first aspect, a vector vortex light field modulation method based on a birefringent material is provided, comprising: irradiating an incident light field onto an incident surface of an electro-optical modulator based on a birefringent material; and adjusting the output voltage of the electro-optical modulator under set conditions to obtain a modulated output light field.

[0005] Furthermore, the electro-optic modulator includes: a waveguide layer, wherein the crystal axis direction of the waveguide layer is perpendicular to the surface of the waveguide layer; a metal substrate layer and a metal covering layer serving as an incident surface are respectively arranged on a group of opposite side surfaces of the waveguide layer along the crystal axis direction of the waveguide layer; a first electrode and a second electrode arranged on a group of opposite side surfaces of the waveguide layer along a direction perpendicular to the crystal axis of the waveguide layer; and a voltage source electrically connected to the first electrode and the second electrode, the voltage source being configured to output a set voltage to the waveguide layer through the first electrode and the second electrode along a direction perpendicular to the crystal axis of the waveguide layer, thereby converting the waveguide layer from a polarization-independent state to a birefringent crystal state.

[0006] Furthermore, the material of the waveguide layer includes but is not limited to lithium niobate.

[0007] Furthermore, the metal substrate layer and the metal covering layer are both precious metal films, and the precious metal films are gold films or silver films.

[0008] Furthermore, the thickness t0 of the birefringent material serving as the waveguide layer satisfies: 1 mm ≤ t0 < 10 mm.

[0009] Furthermore, the thickness of the birefringent material used as the waveguide layer is 1 mm, the thickness of the silver film used as the metal substrate layer is 200 nm, and the thickness of the silver film used as the metal cover layer is 40 nm.

[0010] Furthermore, the first electrode and the second electrode are both transparent conductive adhesive films.

[0011] Furthermore, assuming that the crystal axis direction of the waveguide layer is the Z axis direction, and a set voltage is output to the waveguide layer along the Y axis direction, the refractive index of the waveguide layer along the X, Y, and Z axis directions in the birefringent crystal state is n x 、 n y 、 n z They are: , , , in, n 0 is the ordinary refractive index of the birefringent material before voltage is applied, n e is the extraordinary refractive index of the birefringent material with no applied voltage, is the linear electro-optic coefficient of the birefringent material, is the electric field applied along the Y axis; the refractive index along the y axis n y It corresponds to the TM mode of the waveguide structure of birefringent material; the refractive index along the x-axis is n x It corresponds to the TE mode of the waveguide structure of birefringent materials.

[0012] Compared with the existing technology, the present invention has the following beneficial effects: the present invention irradiates the incident light field onto the incident surface of the electro-optic modulator based on the birefringent crystal; adjusts the output voltage of the electro-optic modulator to obtain the modulated output light field, and without changing any parameters of the optical path, only by changing the voltage signal applied to the metal planar waveguide structure, the vector vortex light field is accurately and efficiently modulated. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 1 is a schematic diagram of the principle of an electro-optic modulator based on birefringent materials in an embodiment of the present invention; Figure 2 is a reflectivity curve of the metal slab waveguide when no voltage is applied in an embodiment of the present invention; Figure 3 FIG1 is a schematic diagram showing a mode of a metal slab waveguide in an embodiment of the present invention splitting under an applied electric field; Figure 4 : is the modulation effect of the metal slab waveguide on the vector vortex light field in an embodiment of the present invention, wherein (a) is a schematic diagram of the integral of the spot intensity of the radial vortex along the x-direction under different voltages, (b) is a schematic diagram of the integral of the spot intensity of the angular vortex along the x-direction under different voltages, (c) is a schematic diagram of the spot state of the radial vortex under different voltages, and (d) is a schematic diagram of the spot state of the angular vortex under different voltages; Figure 5 2 is a schematic diagram of the polarization state distribution of the vector vortex light field after voltage is applied in an embodiment of the present invention, wherein (a) is a schematic diagram of the integral of the spot intensity along the x-direction for a radial vortex under different polarization directions, (b) is a schematic diagram of the integral of the spot intensity along the x-direction for an angular vortex under different polarization directions, (c) is a schematic diagram of the spot state of a radial vortex under different polarization directions, and (d) is a schematic diagram of the spot state of an angular vortex under different polarization directions; Figure 1 Middle: 1. Metal covering layer; 2. Waveguide layer; 3. Metal substrate layer; 41. Electrode 1; 42. Electrode 2; 5. Voltage source; 6. Incident light field; 7. Outgoing light field. DETAILED DESCRIPTION

[0014] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention.

[0015] like Figure 1 As shown, a vector vortex light field modulation method based on birefringent material includes: irradiating an incident light field 6 onto the incident surface of an electro-optic modulator based on birefringent material; under set conditions, adjusting the output voltage of the electro-optic modulator to obtain a modulated output light field 7.

[0016] The principle of the electro-optic modulator based on birefringence material in the present invention is as follows Figure 1 As shown, it includes: a metal covering layer 1, a waveguide layer 2, a metal substrate layer 3, an electrode 1 41, an electrode 2 42 and a voltage source 5.

[0017] The crystal axis direction of the waveguide layer 2 is perpendicular to the surface of the waveguide layer 2; the metal substrate layer 3 and the metal covering layer 1 for receiving the incident light field 6 are respectively arranged on a group of opposite side surfaces of the waveguide layer 2 along the crystal axis direction of the waveguide layer 2; the electrode 1 41 and the electrode 2 42 are arranged on a group of opposite side surfaces of the waveguide layer 2 along a direction perpendicular to the crystal axis of the waveguide layer 2; and the voltage source 5 is electrically connected to the electrode 1 41 and the electrode 2 42.

[0018] A metal slab waveguide structure is formed by a metal cover layer 1, a waveguide layer 2, and a metal substrate layer 3. Both the metal substrate layer 3 and the metal cover layer 1 are noble metal films, primarily gold or silver films. In the present invention, the metal cover layer 1 is composed of a 40nm thick silver film; the waveguide layer 2 is a 1mm thick birefringent material with a crystal axis oriented along the z-axis, perpendicular to the waveguide surface (the thickness t0 of the waveguide layer made of birefringent material is in the millimeter range, 1mm≤t0<10mm); and the metal substrate layer 3 is composed of a 200nm thick silver film. Electrodes 1 41 and 42 are thin films of transparent conductive adhesive (ITO) fabricated on either side of the birefringent material (in the y-axis direction), acting as electrodes to apply a voltage in the y-axis direction to the waveguide layer. A voltage source 5 outputs an adjustable external voltage to the waveguide layer 2 via electrodes 1 41 and 42. An incident light field 6 impinges on the surface of the metal cover layer 1 and generates an outgoing light field 7.

[0019] When the incident light field 6 meets the mode matching condition, it will excite the ultra-high-order guided mode in the waveguide, thereby forming an oscillating standing wave field in the waveguide layer 2. This mode is named ultra-high-order guided mode because the thickness of the waveguide layer 2 is on the millimeter scale, and the wavelength of the excitation is in the visible light band, so the mode order of the excited oscillation mode is in the range of 10 3 The ultrahigh-order guided modes are polarization-independent, meaning there is no difference between the TE and TM modes. This polarization-independence characteristic of metallic slab waveguides makes it impossible for conventional metallic slab waveguides to modulate vector vortex light fields.

[0020] To overcome the aforementioned difficulties and achieve modulation of the vector vortex light field, the present invention employs a birefringent crystal as the waveguide's waveguide layer. Furthermore, to achieve precise modulation of the vector vortex light field, the present invention also incorporates the electro-optic effect, altering the refractive index parameters of the birefringent crystal material by applying a voltage. In the present invention, the crystal axis of the birefringent crystal is along the z-axis, so when no voltage is applied, the metal slab waveguide disclosed herein is polarization-independent. However, when a voltage is applied along the y-axis, the electro-optic effect causes a difference in the refractive index of the birefringent crystal along the x- and y-axes, transforming it into a birefringent crystal.

[0021] Here it is necessary to explain that according to Figure 1 , the refractive index along the y-axis n y corresponds to the TM mode of the waveguide structure; the refractive index along the x-axis is n x It corresponds to the TE mode of the waveguide structure.

[0022] According to the photoelectric effect of birefringent crystals, when an electric field is applied along the y-axis, E y, and when no voltage is applied along the x-axis and z-axis directions, the refractive index of the three orthogonal coordinate axes is n x 、 n y 、 n z They are: , , , in, n 0 is the refractive index of o-light (ordinary light) of the birefringent crystal before voltage is applied, n e is the e-light (extraordinary light) refractive index of the birefringent crystal with no voltage applied, is the linear electro-optic coefficient of the birefringent crystal.

[0023] The birefringent crystals in the present invention include but are not limited to lithium niobate and other crystals.

[0024] In the following simulation cases of the present invention, the values ​​of the above parameters are obtained from experimental measurements, specifically: n 0 =2.297, n e =2.208, =6.8×10 -12 m / V .

[0025] Assuming that the width of the waveguide layer 2 in the y-axis direction is 0.5 cm, when the voltage of the voltage source 5 is set to 5 V, the difference in refractive index between the x-direction and the y-direction is: .

[0026] It can be seen that a voltage change of 5 volts can only cause a weak birefringence effect.

[0027] The present invention provides a vector vortex light field modulator. It can modulate the polarization distribution, intensity distribution, and center of gravity of a light beam. The present invention uses two basic vector vortex light fields as examples of incident light fields: radial and angular vector vortex fields.

[0028] The parameters are: incident light wavelength of 632nm, metal waveguide (metal cover layer 1 and metal substrate layer 3 are made of silver, with thicknesses of 40nm and 200nm, respectively; waveguide layer 2 is made of lithium niobate crystal, with the optical axis along the z-axis and the applied electric field along the y-axis, and a thickness of 1mm; the electro-optic effect model for the refractive index is described in the previous model). The voltage range is 5V to 50V.

[0029] 1.1. Changes in the reflectivity curve of a metal-clad waveguide when voltage is applied.

[0030] The following is the result of analytical calculation based on the above structural parameters using the analytical transfer matrix. Figure 2 As shown in Figure 1, it is the reflectivity curve of the metal slab waveguide when no voltage is applied. Two conclusions can be drawn from the figure: (1) Since the thickness of the waveguide layer is in the order of millimeters, the mode order number of the ultra-high-order guided mode is very high, and there are many guided modes within the range of 10 degrees; (2) When no voltage is applied, the reflectivity of the TE mode and the TM mode is degenerate, and the reflectivity of the two modes completely coincides. The metal slab waveguide exhibits polarization-independent characteristics and has no modulation effect on the polarization distribution of the vector vortex light field.

[0031] like Figure 3 As shown, a certain mode of the metal slab waveguide splits under the condition of an external electric field. Figure 3 Figure 3 shows that as the applied voltage increases, the TE mode shifts to larger angles and the TM mode shifts to smaller angles. The four voltages are 200V, 400V, 600V, and 800V. The higher the voltage, the more pronounced the mode splitting. When the voltage reaches 800V, the resonance peaks of the TE and TM modes are completely separated.

[0032] Figure 3 The paper demonstrates that by utilizing lithium niobate crystals and the electro-optic effect, the previously polarization-independent metal slab waveguide exhibits polarization-dependent properties. Furthermore, the present invention utilizes this property to modulate both radial and angular vector vortex light fields.

[0033] 1.2. Electro-optical modulation of vector vortex light field.

[0034] The present invention electro-optically controls the radial vector vortex light field and the angular vector vortex light field, that is, without changing any parameters of the optical path, the performance of the vector vortex light field is changed by simply changing the voltage signal applied to the waveguide. The above simulated guided mode is still used. When no voltage is applied, the TE and TM modes are degenerate, and the resonance angle is about 8.852 degrees. Therefore, the present invention fixes the incident angle of the incident light field at 8.85 degrees. This angle is Figure 3 The arbitrarily selected voltage is located near the TE and TM guided modes, which can simultaneously excite these modes. The voltage is then gradually changed. The present invention studies two fundamental incident vector vortex light fields: radial and angular. The topological charge of these vortices is positive 1.

[0035] 1.2.1. Radial vector vortex light field.

[0036] The research method of the present invention is to analyze the traditional light beam transmission method combined with the transfer matrix method of the metal clad waveguide.

[0037] like Figure 3 As shown in the figure, when the optical path is completely fixed, changing the voltage on both sides of the metal slab waveguide waveguide layer can modulate the vector vortex light field. First, when there is no voltage, the metal slab waveguide is polarization-independent, so Figure 4 As can be seen from the comparison diagram, the reflection field distribution of the radial vortex vector light and the angular vortex vector light field is exactly the same, and there is no difference due to the difference in polarization state. However, after the voltage is applied, as the voltage increases, the center of gravity of the radial vortex vector light rotates clockwise, and the energy is finally concentrated in the lower right corner of the light spot; while the energy of the angular vortex vector light field is concentrated in the upper right corner. From the center of gravity of the entire light spot, by integrating the light intensity on the x-axis, it can be seen that when the voltage is applied, the radial vortex vector light field produces a negative lateral displacement, while the angular vortex vector light field produces a positive lateral displacement. Therefore, the birefringent metal slab waveguide structure and electro-optical effect proposed in the present invention can produce different lateral displacements for vector vortex light fields of different polarizations.

[0038] 1.2.2. Polarization state distribution of the vector vortex light field after voltage application.

[0039] Under the condition of applying voltage of 800V, the spatial distribution of polarization state of vector vortex light field was studied respectively. The research method was to add polarizer in front of CCD and observe the transmission of two kinds of vector vortex light fields by rotating the polarization direction of polarizer. Obviously, after being scattered by metal slab waveguide, the two kinds of vector vortex light fields showed different polarization distribution, and the lateral distribution of beam center of gravity also showed different patterns, such as Figure 5 shown.

[0040] Due to the complexity of birefringent metal slab waveguides, the above two examples are just special cases. The actual situation is more complicated and diverse.

[0041] The metal slab waveguide structure itself is polarization-independent and cannot modulate the vector vortex light field. Therefore, the present invention introduces birefringence of birefringent materials as a waveguide layer. Secondly, the present invention sets the optical axis of the birefringent crystal to be perpendicular to the waveguide direction, so that when no external electric field is applied, the metal slab waveguide is still polarization-independent. Therefore, the metal slab waveguide can be transformed from polarization-independent to polarization-dependent by applying an external electric field, and this modulation is electrically controlled without changing the optical path at all. The above changes cause the coupling conditions of the TE mode and the TM mode of the metal slab waveguide to change, so that different polarization states in the original vector vortex light field are coupled to different degrees with the ultra-high-order guided mode, which ultimately causes the characteristics of the scattered light field of the waveguide to change. Finally, the waveguide with added polarization characteristics is used to regulate the vector vortex light field. The case shows that, according to the different initial structures of the vector vortex light field, the applied voltage can continuously regulate the polarization state distribution, light field intensity distribution and center of gravity of the vector vortex light field.

[0042] The present invention is based on the guided mode excitation of a metal slab waveguide and utilizes the electro-optic effect to excite the polarization difference between the TE (transverse electric mode) and TM (transverse magnetic mode) of a birefringent waveguide structure to achieve electro-optical modulation of a vector vortex light field with a spatial polarization distribution. The purpose is to change the polarization state distribution, intensity distribution, and center of gravity position of the vector vortex light field by changing the voltage applied to the waveguide structure while keeping the optical path fixed.

[0043] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A vector vortex light field modulation method based on birefringent materials, characterized in that: include: The incident light field (6) is irradiated on the incident surface of the electro-optic modulator based on the birefringent material; Under set conditions, the output voltage of the electro-optical modulator is adjusted to obtain a modulated output light field (7).

2. The vector vortex light field modulation method based on birefringence material according to claim 1, characterized in that: The electro-optic modulator comprises: A waveguide layer (2), wherein the crystal axis direction of the waveguide layer (2) is perpendicular to the surface of the waveguide layer (2); The metal substrate layer (3) and the metal covering layer (1) serving as the incident surface are respectively arranged on a group of opposite side surfaces of the waveguide layer (2) along the crystal axis direction of the waveguide layer (2); An electrode 1 (41) and an electrode 2 (42) are arranged on a group of opposite sides of the waveguide layer (2) along a direction perpendicular to the crystal axis of the waveguide layer (2); A voltage source (5) electrically connected to the electrode 1 (41) and the electrode 2 (42), the voltage source (5) being used to output a set voltage to the waveguide layer (2) along a direction perpendicular to the crystal axis of the waveguide layer (2) through the electrode 1 (41) and the electrode 2 (42), so as to transform the waveguide layer (2) from a polarization-independent state to a birefringent crystal state.

3. The vector vortex light field modulation method based on birefringence material according to claim 2, characterized in that: The material of the waveguide layer (2) includes but is not limited to lithium niobate.

4. The vector vortex light field modulation method based on birefringence material according to claim 2, characterized in that: The metal substrate layer (3) and the metal covering layer (1) are both precious metal films, and the precious metal films are gold films or silver films.

5. The vector vortex light field modulation method based on birefringence material according to claim 2, characterized in that: The thickness t0 of the birefringent material serving as the waveguide layer (2) satisfies the following conditions: 1 mm ≤ t0 < 10 mm.

6. The vector vortex light field modulation method based on birefringence material according to claim 2, characterized in that: The thickness of the birefringent material as the waveguide layer (2) is 1 mm, the thickness of the silver film as the metal substrate layer (3) is 200 nm, and the thickness of the silver film as the metal covering layer (1) is 40 nm.

7. The vector vortex light field modulation method based on birefringence material according to claim 2, characterized in that: The electrode 1 (41) and the electrode 2 (42) are both transparent conductive adhesive films.

8. The vector vortex light field modulation method based on birefringence material according to claim 2, characterized in that: Assuming that the crystal axis direction of the waveguide layer (2) is the Z axis direction, and a set voltage is output to the waveguide layer (2) along the Y axis direction, the refractive index of the waveguide layer (2) along the X, Y, and Z axis directions in the birefringent crystal state is n x 、 n y 、 n z They are: ， ， ， in, n 0 is the ordinary refractive index of the birefringent material before voltage is applied, n e is the extraordinary refractive index of the birefringent material with no applied voltage, is the linear electro-optic coefficient of the birefringent material, is the electric field applied along the Y axis; the refractive index along the y axis n y It corresponds to the TM mode of the waveguide structure of birefringent material; the refractive index along the x-axis is n x It corresponds to the TE mode of the waveguide structure of birefringent materials.

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