A bifocal achromatic superlens and a design and preparation method thereof and an imaging device

Abstract

The application discloses a bifocal achromatic superlens, which comprises a dielectric substrate and a dielectric nanometer column structure unit array, the dielectric nanometer column structure unit array is arranged on the dielectric substrate, and the dielectric nanometer column structure unit array comprises a plurality of dielectric nanometer column structure units with different shapes. The application obtains an achromatic superlens array with orthogonal polarization double channels through design of phase distribution of the superlens array and structure matching optimization, realizes light field imaging with three different focusing modes through adjustment of linear polarization state of incident light, can perform light field imaging on different depth of field ranges, and thus realizes depth of field extension.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical lens, in particular to a super lens for realizing broadband achromatic imaging, especially a bifocal achromatic super lens and a design and preparation method and an imaging device of application thereof. BACKGROUND

[0002] The traditional light field imaging system is obtained by adding a microlens array in the optical path to obtain two-dimensional intensity information and two-dimensional direction information. The traditional microlens array has limitations, for example, the microlens diameter cannot be too small due to the influence of the diffraction limit, the depth of field range close to the natural compound eye of insects cannot be obtained, and the spherical aberration exists to affect the imaging quality. These limitations result in that the traditional light field imaging optical system is complex, difficult to process, bulky and difficult to integrate.

[0003] With the development of intelligent devices, various devices of optical systems are developing towards miniaturization, integration, multifunction and high performance. The super surface is a new type of planar optical control element based on the generalized Snell's law. By adjusting the shape, size, position and direction of the scatterer, any electromagnetic wave parameters can be controlled, including the phase, amplitude, polarization and frequency of light. The super surface is used for imaging to realize a new type of element that is thin, planar and multifunctional integrated, called super lens, which is expected to provide a new solution to reduce the complexity of optical systems and has broad application prospects.

[0004] In addition, the depth of field of the traditional light field imaging is limited. The traditional method is to use microlenses with different focal lengths staggered to achieve extended depth of field, but this will cause the spatial resolution to decrease, so a trade-off between space, angle and depth of field is needed. The focal length of the microlens cannot be adjusted in real time after being designed for a specific occasion, while the polarization control function of the super lens can switch the focal length under different polarization channels, and can expand the depth of field without reducing the spatial resolution and viewing angle. At present, most of the polarization multiplexing super lenses can only realize imaging at a single wavelength or discrete wavelengths, which cannot meet the requirements of broadband achromatic imaging. Therefore, it is urgent to develop a super lens that can realize broadband achromatic imaging in multiple polarization channels to broaden its application. SUMMARY

[0005] The purpose of the present application is to design a bifocal achromatic super lens and a design and preparation method and an imaging device of application thereof to solve the problems proposed in the background art. By adjusting the in-plane angle of the nanostructure at each position, the coding of the bifocal super lens in three independent polarization channels can be realized. By adjusting the polarization and wavelength range of the incident light and the outgoing light, the light field imaging depth resolution under different focal length modes is enhanced, and the bifocal achromatic super lens has the advantages of tunability, high integration, multifunction and the like.

[0006] To achieve the above object, the present application provides the following technical solutions: a bifocal achromatic superlens, comprising a light source, a main lens, a linear polarizer, a polarization conversion sheet, a first objective lens, a bifocal achromatic superlens array, a second objective lens, and an optical sensor, wherein the light source, the main lens, the linear polarizer, the polarization conversion sheet, the first objective lens, the bifocal achromatic superlens array, the second objective lens, and the optical sensor are sequentially connected in an optical path, and the bifocal achromatic superlens array is composed of a plurality of superlens arrangements. The superlens comprises a dielectric substrate and a dielectric nanocolumn structure unit array, the dielectric nanocolumn structure unit array is arranged on the dielectric substrate, and the dielectric nanocolumn structure unit array comprises a plurality of dielectric nanocolumn structure units of different shapes. The dielectric nanocolumn unit array is represented by a Jones matrix, the Jones matrix contains two independent phase information, and the two independent phase information correspond to spherical focusing phase profiles of different polarization states, respectively. The phase profiles are designed to be independently reconstructed by two different orthogonal linear polarizations.

[0007] As a preferred embodiment of the present application, the material for manufacturing the dielectric substrate is indium tin oxide conductive glass, quartz substrate, silicon oxide substrate, silicon substrate, or diamond substrate.

[0008] As a preferred embodiment of the present application, the material for manufacturing the dielectric nanocolumn unit is TiO2, HfO2, ZrO2, GaN, Si2N3, Si, GaAs, ZnS, or AlN.

[0009] As a preferred embodiment of the present application, the shape of the dielectric nanocolumn structure unit is rectangular, v-shaped, asymmetric cross-shaped, or H-shaped.

[0010] As a preferred embodiment of the present application, the height of the dielectric nanocolumn structure unit ranges from 200 nm to 1500 nm, the size of the dielectric nanocolumn structure unit on the surface of the dielectric substrate ranges from 20 nm to 1000 nm, the distribution size of the dielectric nanocolumn structure unit on the dielectric substrate can be arbitrarily controlled, and the dielectric nanocolumn structure unit is arranged on the surface of the dielectric substrate.

[0011] As a preferred embodiment of the present application, the bifocal achromatic superlens is prepared by the following steps:

[0012] Designing the phase;

[0013] Then, the propagation phase response of the nanocolumn unit of different sizes and structures at different wavelengths is obtained by numerical simulation, and a database is constructed;

[0014] The dielectric nanocolumn unit is periodically distributed on the dielectric substrate according to a specific size to match the phase response and the lens phase distribution.

[0015] Generating a layout unit;

[0016] Generating a layout array;

[0017] Generating the superlens;

[0018] Selecting a transparent medium substrate, coating photoresist on the surface;

[0019] Baking, exposing and developing using electron beam lithography technology;

[0020] Making a hole structure complementary to the dielectric nanocolumn structure array on the transparent medium substrate, and then using atomic layer deposition technology to deposit dielectric material to fill the holes to obtain a dielectric nanostructure array and an attached dielectric film layer;

[0021] Then use ion beam etching technology to remove the dielectric film layer;

[0022] Finally, use reactive ion etching technology to remove the photoresist, release the dielectric nanostructure array, remove the metal mask, and complete the preparation of the superlens.

[0023] As a preferred embodiment of the present application, the dielectric nanocolumn unit array comprises a plurality of nanoscale dielectric nanocolumn structure units of the same or different sizes, which is represented by a Jones matrix comprising two independent phase information corresponding to spherical focusing phase profiles of different polarization states, and the phase profiles are independently reconstructed by two different orthogonal linear polarizations.

[0024] Compared with the prior art, the present application provides a bifocal achromatic superlens, a design and preparation method thereof, and an imaging device using the same, which has the following beneficial effects:

[0025] The present application can realize the coding of a bifocal superlens of three independent polarization channels by adjusting the in-plane angle of the nanostructure at each position.

[0026] The present application realizes light field imaging with three different focusing modes by adjusting the polarization and wavelength range of incident light and outgoing light, can perform light field imaging on different depth of field ranges, and thus realizes depth of field extension.

[0027] The present application realizes a polarization dual-channel achromatic superlens by reasonably designing the phase distribution of the superlens through structural phase matching, can perform achromatic focusing at different focal lengths, constitutes a bifocal achromatic superlens array, and the filling rate can reach 100%.

[0028] The imaging device of the present application adopts the bifocal achromatic superlens to perform light field imaging on different depth of field ranges, solves the restriction problem of the depth of field range, spatial resolution and viewing angle in light field imaging, and expands the depth of field range of light field imaging without reducing the spatial resolution and viewing angle range.

[0029] The bifocal achromatic superlens of the present application can be used in a visible light wideband range, can be used in a wider waveband by selecting different materials and structures, and can be used in other wavebands such as ultraviolet waveband and infrared waveband.

[0030] The bifocal achromatic superlens array of the present application has the advantages of being super light and thin and easy to integrate, can be integrated with a sensor chip and the like, and realizes an ultra-compact light field imaging device. BRIEF DESCRIPTION OF DRAWINGS

[0031] Other features, objects and advantages of the present application will become more apparent from the following detailed description of non-limiting embodiments with reference to the following drawings:

[0032] Figure 1 It is a light path structure diagram of the imaging device of the present application using the bifocal achromatic superlens.

[0033] Among them, 1 is a light source, 2 is a main lens, 3 is a linear polarizer, 4 is a polarization conversion sheet, 5 is a first objective lens, 6 is a bifocal achromatic superlens array, 7 is a second objective lens, and 8 is an optical sensor.

[0034] Figure 2 It is an array arrangement schematic diagram of the bifocal achromatic superlens array of the present application.

[0035] Figure 3 It is a light ray focusing schematic diagram of the bifocal achromatic superlens array of the present application under three modes.

[0036] Figure 4 It is a focal length optical characterization diagram of the bifocal achromatic superlens of the present application under different polarization states.

[0037] Figure 5 It is a design process and preparation process schematic diagram of the bifocal achromatic superlens of the present application.

[0038] Figure 6 It is a light field depth of field expansion function schematic diagram in the embodiment of the present application. DETAILED DESCRIPTION

[0039] In order to better understand the purpose, structure and function of the present application, the bifocal achromatic superlens of the present application and the design and preparation method and the imaging device using the same are further described in detail below with reference to the drawings.

[0040] The bifocal achromatic superlens comprises a medium substrate and an array of different-shaped dielectric nanocolumn structure units arranged on the medium substrate.

[0041] Further, the medium substrate is made of indium tin oxide conductive glass, quartz substrate, silicon oxide substrate, silicon substrate or diamond substrate.

[0042] Further, the dielectric nanocolumn unit is made of TiO2, HfO2, ZrO2, GaN, Si2N3, Si, GaAs, ZnS or AlN.

[0043] Further, the dielectric nanocolumn structure unit has a shape of a rectangle, a v-shape, an asymmetric cross shape or an I-shape.

[0044] Further, the dielectric nanocolumn structure unit has a height ranging from 200 nm to 1500 nm, a size on the surface of the medium substrate ranging from 20 nm to 1000 nm, and an arbitrary distribution size on the medium substrate.

[0045] The bifocal achromatic superlens is prepared by the following steps.

[0046] Phase design is performed.

[0047] Then, the propagation phase response of the nanometer unit with different sizes and structures at different wavelengths is obtained by numerical simulation, and a database is constructed.

[0048] The dielectric nanocolumn unit is periodically distributed on the medium substrate according to a specific size to match the phase response and the lens phase distribution.

[0049] A layout unit is generated.

[0050] A layout array is generated.

[0051] The superlens is generated.

[0052] A transparent medium substrate is selected, and polymethyl methacrylate (PMMA) is coated on the surface.

[0053] Baking, exposure and development are performed using electron beam lithography technology.

[0054] A hole structure complementary to the dielectric nanocolumn structure array is made on the transparent medium substrate, and then an atomic layer deposition technology is used to deposit a dielectric material to fill the holes to obtain a dielectric nanometer structure array and a layer of attached dielectric film.

[0055] Then the dielectric film layer is removed by ion beam etching technology;

[0056] Finally, the PMMA is removed by reactive ion etching technology, the dielectric nanostructure array is released, the metal mask is removed, and the preparation of the superlens is completed.

[0057] Further, the dielectric nano pillar unit array comprises a plurality of nano-sized dielectric nano pillar structure units of the same or different sizes, can be represented by a Jones matrix, the Jones matrix comprises two independent phase information, respectively corresponding to spherical focusing phase profiles of different polarization states, and the design of the phase profiles can be independently reconstructed by two different orthogonal linear polarizations.

[0058] The imaging device using the bifocal achromatic superlens adopts the bifocal achromatic superlens, comprising a light source 1, a main lens 2, a linear polarizer 3, a polarization conversion sheet 4, a first objective lens 5, a bifocal achromatic superlens array 6, a second objective lens 7, and an optical sensor 8, wherein the light source 1, the main lens 2, the linear polarizer 3, the polarization conversion sheet 4, the first objective lens 5, the bifocal achromatic superlens array 6, the second objective lens 7, and the optical sensor 8 are sequentially connected in optical path.

[0059] In use, linearly polarized light is generated by the linear polarizer 3, when the x linearly polarized light is incident, the focal length of the bifocal achromatic superlens is fx, in the light field imaging device, the near range is imaged by light field, and the light field depth is DOFx; when the y linearly polarized light is incident, the focal length of the bifocal achromatic superlens is fy, the far range is imaged by light field, and the light field depth is DOFy; when the 45° polarized light is incident, the near range and the far range are simultaneously imaged by light field, the light field depth range is DOFx+DOFy, and the depth of field extension light field imaging in different modes is realized.

[0060] Wherein each dielectric nano pillar structure unit is regarded as a linear birefringence unit, and the dielectric nano pillar structure unit can be expressed by a Jones matrix:

[0061]

[0062] Wherein, And The phase retardation of the linearly polarized incident light along the long axis and the short axis of the nanostructure unit is determined by the structure parameters of the long axis and the short axis of the nanostructure unit, and by appropriately adjusting the size of the structure parameters, the phase of the outgoing light can cover the range of 0-2π.

[0063] To realize the lens focusing function, the phase distribution of the superstructure surface should follow the spherical lens phase formula:

[0064]

[0065] where λ is the design wavelength, x, y are the position coordinates of the nanostructure unit, and f is the focal length of the focusing.

[0066] To realize the polarization bifocal superlens, the switching of the focal length of the superlens is realized by changing the polarization state of the incident light. For the phase of the superlens of different polarization channels, the phase profile is designed to control the phase distribution of the x-polarized light incident, focusing at a distance f1 in the z direction; the phase profile is designed to control the phase distribution when the y-polarized light is incident, focusing at a distance f2 in the z direction.

[0067] The design of the phase control of the superstructure surface adopts the transmission phase, and the phase response of the waveguide effect propagated by each nano-pillar structure can be expressed as:

[0068]

[0069] where n eff is the effective refractive index of the nano-pillar, which is related to the intrinsic refractive index of the nano-pillar and the cross-sectional shape; λ is the wavelength, and H is the height of the nano-pillar structure.

[0070] Four different cross-section structures are selected as the structural units of the superstructure surface, as shown in Figure 2 , and the phase response database of different size parameters under different wavelengths when the x-polarized light and the y-polarized light are incident is obtained by FDTD numerical simulation.

[0071] In order to realize the wideband achromatic focusing function of the spherical lens, the phase of the superstructure surface should satisfy the formula:

[0072]

[0073] where λ is the design wavelength, x, y are the position coordinates of the nanostructure unit, f is the focal length of the focusing, and r λ is a value related to the wavelength introduced. When the distance between the position of the nanostructure unit and the center is less than r λ , the relationship between the phase response of the lens and the wave number (2π / λ) is converted into a positive correlation, so that the phase matching can be provided by the nanostructure unit, and r λ is related to the wavelength, and then the phase dispersion relationship at any position is constructed to accurately match the structure dispersion. The particle swarm optimization algorithm PSO is used to optimize the r λ value at different wavelengths, and the most suitable r λThe value is determined to minimize the phase matching error between the lens phase and the structure in both the x and y polarization channels. Ultimately, the phase profiles of the bifocal broadband achromatic lenses in the x and y polarization channels are encoded onto a single metasurface, switching between left-handed x-polarized and y-polarized light incidence, respectively. Figure 4 The figure shown is a focal length optical characterization diagram of the bifocal achromatic metalens of the present invention under different polarization incident conditions.

[0074] like Figure 3 The diagram shows the light focusing of the bifocal achromatic metalens array of the present invention in three modes. Under x-polarized light incidence, the bifocal achromatic metalens focuses light at a distance f. x Focusing is performed at the focal length of f, which is mode 1; under y-polarized light incident, the bifocal achromatic metalens focuses at a distance f. y Focusing is performed at the focal length of f, which is mode 2; under 45° linearly polarized light incident, the bifocal achromatic metalens focuses at a distance f. x and distance f y Simultaneous focusing at the focal length is mode 3.

[0075] like Figure 6 The diagram shows the light field depth extension function of the present invention. In mode 1, light field imaging is performed on the near scene at a distance of ax, and the depth of field range is DOFx. In mode 2, light field imaging is performed on the distant scene at a distance of ay, and the depth of field range is DOFy. In mode 3, light field imaging is performed on both the near scene at a distance of ax and the distant scene at a distance of ay, and the depth of field range is DOFx+DOFy.

[0076] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A bifocal achromatic metalens, characterized in that: The system includes a light source, a main lens, a linear polarizer, a polarization converter, a first objective lens, a bifocal achromatic metalens array, a second objective lens, and an optical sensor. These components are sequentially connected to form an optical path. The bifocal achromatic metalens array is composed of multiple metalenses. Each metalens includes a dielectric substrate and an array of dielectric nanopillar structures. The dielectric nanopillar structure array is disposed on the dielectric substrate and includes several dielectric nanopillar structures of different shapes. The dielectric nanopillar structure array is represented using a Jones matrix. The matrix contains two independent phase information, corresponding to spherical focusing phase profiles of different polarization states. The phase profiles are designed to be independently reconstructed through two different orthogonal linear polarizations. The dielectric nanopillar structure unit is an anisotropic structure. The shape of the dielectric nanopillar structure unit is rectangular, V-shaped, asymmetric cross-shaped, or I-shaped. The height of the dielectric nanopillar structure unit ranges from 200nm to 1500nm, and the size of the dielectric nanopillar structure unit on the dielectric substrate surface ranges from 20nm to 1000nm. The distribution size of the dielectric nanopillar structure unit on the dielectric substrate can be arbitrarily adjusted, and the dielectric nanopillar structure units can be arbitrarily set on the dielectric substrate surface.

2. The bifocal achromatic metalens according to claim 1, characterized in that: The dielectric substrate is made of indium tin oxide conductive glass, quartz substrate, silicon substrate or diamond substrate.

3. The bifocal achromatic metalens according to claim 2, characterized in that: The dielectric nanopillar structure unit is made of TiO2, HfO2, ZrO2, GaN, Si2N3, Si, GaAs, ZnS or AlN.

4. A method for designing and fabricating a bifocal achromatic metalens, comprising fabricating a bifocal achromatic metalens as described in any one of claims 1-3, characterized in that: Includes the following steps: design: Phase design; Then, the propagation phase response of nanounits of different sizes and structures at different wavelengths was obtained through numerical simulation and constructed into a database; Dielectric nanopillar units are periodically distributed on the dielectric substrate according to a specific size to form a phase response that matches the phase distribution of the lens; Generate layout units; Generate layout array; Generate the aforementioned metalens; preparation: Choose a transparent dielectric substrate and coat the surface with photoresist; Baking, followed by exposure and development using electron beam lithography; A porous structure complementary to the dielectric nanopillar structure array was fabricated on a transparent dielectric substrate, and then a dielectric material was deposited using atomic layer deposition technology to fill the pores to obtain a dielectric nanostructure array and an attached dielectric film layer. Then, the dielectric film layer is removed using ion beam etching. Finally, reactive ion etching is used to remove the photoresist, release the dielectric nanostructure array, remove the metal mask, and complete the fabrication of the metalens.

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