CN113946034B - Broadband chiral spectrum analysis and large-view-field imaging system and design method - Google Patents
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Abstract
The invention discloses a broadband chiral spectrum analysis and large-view-field imaging system based on a micro-nano structure, which comprises two modules, wherein the two modules are medium materials, and the two modules are sequentially arranged along the optical axis direction: a tunable geometric phase polarization grating module and a broadband large-view-field imaging module; the tunable geometric phase polarization grating module is used for realizing the switching of two functions of broadband chiral spectrum analysis and broadband large-field imaging by separating left-handed circular polarization components and right-handed circular polarization components of incident light into different directions or directly transmitting the incident light; and the broadband large-view-field imaging module is used for focusing the light in different emergent directions at different positions of the same image plane and focusing the emergent light in different wavelengths in the same direction at the same position. The imaging system designed by the invention has the advantages of simple structure, flexible function and the like, and can realize object detection and analysis at different distances.
Description
Technical Field
The invention relates to the technical field of electromagnetic wave phase regulation, in particular to a compatible broadband chiral spectrum analysis and broadband large-view-field imaging system based on a micro-nano structure and a design method thereof.
Background
The infrared optical detection system is applied to society and life everywhere, but the performance of the infrared optical detection system is often limited by large volume, small view field, high heat sensitivity, high manufacturing cost and the like. In order to compress the volume of the infrared optical detection system, increase the field of view of incident light and reduce the manufacturing cost, the infrared optical detection system needs to be developed on a road with large field of view, light weight and integration. The total length of a typical infrared detection system is large, the number of lenses is large, the mechanical structure is complex, and portability, integration and easy use are difficult to realize. More importantly, the infrared optical detection system has single detection function when detecting within a certain bandwidth, is limited by the detection distance, and can only independently detect objects at a short distance or a long distance. Taking a traditional infrared optical detection system as an example, an infrared spectrum analysis system is carried out on a remote object, firstly, light incident by the object needs to be decomposed into different frequencies, then information of the different frequencies is detected, and the whole system comprises a very complex light path; the infrared broadband large-view-field optical system for detecting the short-distance object needs to perform achromatism treatment on broadband light incident from different view fields, and a large-view-field fisheye lens formed by a plurality of complex surface-shaped elements can be used to enable the different view fields to form complete and clear images, so that the whole system becomes heavy and complicated. Based on the background of the requirements and the current state of the art, it can be seen that the current infrared optical detection system still stays under the design paradigm of the traditional geometrical optical system, and there are many limit problems such as a large number of lenses, complex optical paths, limited functions, etc.
Disclosure of Invention
In order to solve the technical problems, the invention provides a broadband chiral spectrum analysis and broadband large-view-field imaging system based on a micro-nano structure, which can realize switching of detection functions with different distances. Objects at close distances can be imaged directly and thus can be characterized by wide-band large-field imaging. The object at a long distance can be imaged as a point on the image plane, specific contour information of the object can not be obtained, and the object attribute can be judged by adopting broadband chiral spectrum analysis for the object at a long distance, namely, as different objects have different spectrograms, the attribute of the object at a long distance can be judged by the spectrograms generated by the light splitting of the imaging system. The tunable geometric phase polarization grating module and the broadband large-view-field imaging module adopted by the invention get rid of the dependence on the technology such as complex fisheye lens and the like, greatly compress the space and the length of an optical system and are easy to integrate. The invention designs a micro-nano structural aberration correction module with specific phase distribution and a tunable geometric phase polarization grating module based on geometric phase. According to the preset radial phase distribution of the micro-nano structure phase difference correction module, the effect of eliminating chromatic aberration on broadband light incident in different view fields can be realized by adjusting the micro-nano structure phase difference correction module structure. And for the tunable geometric phase polarization grating module, the left circularly polarized light component and the right circularly polarized light component of the incident light are separated into different directions or the incident light is directly transmitted, so that the switching of the two functions of broadband chiral spectrum analysis and broadband large-field imaging is realized.
The technical scheme adopted for solving the technical problems is as follows:
a broadband chiral spectrum analysis and large-view-field imaging system based on a micro-nano structure comprises two modules, wherein the two modules are all dielectric materials, and the two modules are sequentially arranged along the optical axis direction: a tunable geometric phase polarization grating module and a broadband large-view-field imaging module; the tunable geometric phase polarization grating module is used for realizing the switching of two functions of broadband chiral spectrum analysis and broadband large-field imaging by separating left-handed circularly polarized light components and right-handed circularly polarized light components of incident light into different directions or directly transmitting the incident light; and the broadband large-view-field imaging module is used for focusing the light in different emergent directions at different positions of the same image plane and focusing the emergent light in different wavelengths in the same direction at the same position.
Further, the tunable geometric phase polarization grating module comprises two micro-nano structure polarization gratings which are used for separating left-handed circularly polarized light components and right-handed circularly polarized light components of incident light into different directions; at least one micro-nano structure polarization grating can rotate around the center of the micro-nano structure polarization grating and is used for realizing the switching of two functions of broadband chiral spectrum analysis and broadband large-field imaging.
Further, the tunable geometric phase polarization grating module comprises a first micro-nano structure polarization grating and a second micro-nano structure polarization grating; the first micro-nano structure polarization grating comprises a first medium substrate and a first grating unit structure which is arranged on the first medium substrate and has a plurality of periods; the second micro-nano structure polarization grating comprises a second medium substrate and a plurality of periodic second grating unit structures arranged on the second medium substrate; the first grating unit structure and the second grating unit structure are oppositely arranged.
Further, the diameter of the aperture diaphragm of the imaging system is Dk, and Dk is more than or equal to 0.3BFL and less than or equal to 0.6BFL; the half diameter of the first micro-nano structure polarization grating is R 1 And Dk is more than or equal to 0.5 and less than or equal to R 1 Less than or equal to 0.8Dk; the half diameter of the second micro-nano structure polarization grating is R 3 And Dk is more than or equal to 0.5 and less than or equal to R 3 BFL is the back focal length of the imaging system, which is less than or equal to 0.8Dk; the thickness of the first dielectric substrate is t 1 And t is 1 >10λ 0 The method comprises the steps of carrying out a first treatment on the surface of the The thickness of the second dielectric substrate is t 2 And t is 2 >10λ 0 ，λ 0 Is the center wavelength.
Further, the first grating unit structure and the second grating unit structure are catenary curves or discrete N-order catenary unit structures, N is an integer and is more than or equal to 4; the transverse period length of the first grating unit structure and the second grating unit structure is Px and 2lambda 0 ≤Px≤6λ 0 The method comprises the steps of carrying out a first treatment on the surface of the Longitudinal period is Py and 0.2λ 0 ≤Py≤0.8λ 0 The method comprises the steps of carrying out a first treatment on the surface of the The height of the grating structure is h and 0.2lambda 0 ≤h≤λ 0 The width of the grating structure is wt and 0.1λ 0 ≤wt≤0.4λ 0 The method comprises the steps of carrying out a first treatment on the surface of the When divided into N areas, each area has a lateral width lx and is 0.5 Px/N.ltoreq.lx.ltoreq.Px/N, lambda 0 Is the center wavelength.
Further, the tunable geometric phase polarization grating module comprises a tunable liquid crystal polarization grating module, and liquid crystal molecules of the tunable liquid crystal polarization grating module can deflect in a plane, so that switching of two functions of broadband chiral spectrum analysis and broadband large-field imaging is realized.
Further, the broadband large-view-field imaging module comprises a micro-nano structural aberration correction module and a lens module, wherein the micro-nano structural aberration correction module is a sub-wavelength structure with specific phase distribution and is used for correcting the aberration of the lens module; the lens module, which may be a single lens or a lens group, is used to focus the outgoing light on the image plane.
Further, the micro-nano structural aberration correction module is arranged at the downstream of the tunable geometric phase polarization grating module along the optical axis, and can be integrated on the tunable geometric phase polarization grating module or arranged separately from the tunable geometric phase polarization grating module.
Further, the micro-nano structural aberration correction module is of a step structure or a super-surface nano column structure with discrete height; the half diameter of the micro-nano structural aberration correction module is R 2 And Dk is more than or equal to 0.5 and less than or equal to R 2 Dk is less than or equal to 0.8Dk, and Dk is the diameter of the aperture diaphragm; the lens module is a spherical lens, the center thickness of the spherical lens is Dc and Dc is more than or equal to 0.2BFL, the half diameter of the spherical lens is R and R is more than or equal to 0.5BFL, and the curvature radius of the first surface is Rc 1 And Rc 1 BFL less than or equal to, the second surface has a radius of curvature Rc 2 And Rc 2 And (2) not more than-BFL, wherein BFL is the back focal length of the imaging system.
In another aspect of the present invention, a method for designing a broadband chiral spectrum analysis and large-field imaging system based on a micro-nano structure is provided, comprising the steps of:
determining the broadband range of incident light (lambda) min ，λ max ) And a polarization grating period Px of the tunable geometric phase polarization grating module;
the simulation method is adopted to carry out simulation calculation on the tunable geometric phase polarization grating module, the polarization grating structure of the tunable geometric phase polarization grating module is adjusted according to the geometric phase principle, and the polarization grating structure can separate left-handed and right-handed circularly polarized light components of incident light into +/-1 level;
according to the incident light broadband range (lambda min ，λ max ) And polarization grating period Px, calculating maximum diffraction angle theta max ，
The wide-band wide-field imaging module is designed to cover the wide-band range (lambda) of incident light min ，λ max ) The imaging field of view is greater than or equal to the maximum diffraction angle theta max 。
Further, the method for adjusting the polarization grating structure of the tunable geometric phase polarization grating module comprises the following steps: the tunable geometric phase polarization grating module transversely has a grating unit structure with M periods, and the phase distribution in one period is as follows:
the grating unit structure curve in one period is as follows:
where Px is the transverse period length, and x is the intra-period coordinate.
Further, the grating unit structure curve in one period is discretized into N-order unit structures, the deflection angle of the N-order unit structures relative to the transverse direction is 0 degree,
further, selecting a micro-nano structure phase difference correction module of the broadband large-view-field imaging module to be a step structure or a super-surface nano column structure with discrete height;
if the micro-nano phase difference correction module is selected to be a step structure with discrete height, the plane of phase distribution is known according to
Wherein Deltad is the height difference of any two steps processed on a plane, lambda 0 ΔΦ is a phase difference required for achieving an imaging effect and caused by a height difference, which is a center wavelength; n is the plane refractive index; step heights at different positions on a plane are obtained according to the delta d;
if the micro-nano structure phase difference correction module is selected to be a super-surface nano column structure, the plane of phase distribution is known according to
Wherein delta n is the equivalent refractive index difference of any two nano-pillars processed on a plane, H is the height of the nano-pillars, delta phi is the phase difference required for realizing the imaging effect and caused by different nano-pillar plane sizes; the planar dimensions of the nano-pillars at different positions on the plane can be optimized according to delta n.
The invention has the beneficial effects that:
firstly, the whole imaging system adopts an all-dielectric structure, and compared with a metal structure, the loss of light reflection, scattering and absorption is low;
secondly, the medium structure used for the micro-nano structure aberration corrector and the micro-nano structure polarization grating design can carry out high-efficiency and low-scattering wavefront regulation and control on the light wave, and has high performance even under oblique incidence;
finally, by adjusting the diffraction orders of the micro-nano structure polarization grating, the switching of the near-distance object broadband large-view-field imaging function and the remote-distance object broadband chiral spectrum analysis function can be conveniently realized, and the problems of a plurality of limitations of a traditional geometric optical system such as a large number of lenses, complex optical paths, limited functions and the like are solved.
Drawings
Fig. 1 is a schematic diagram of an optical system for implementing two functions of broadband chiral spectrum analysis and broadband large-field imaging according to a first embodiment of the present invention, where fig. 1 (a) is a schematic plan view of the optical system for implementing broadband chiral spectrum analysis, and fig. 1 (b) is a schematic plan view of the optical system for implementing broadband large-field imaging.
Fig. 2 (a) is a top view of a grating unit structure of a micro-nano structure polarization grating according to the first embodiment of the present invention, fig. 2 (b) is a schematic diagram of a cross section of the grating unit structure of the micro-nano structure polarization grating according to the first embodiment, fig. 2 (c) is a graph of absolute efficiency and relative efficiency of polarization conversion simulated at 8-14 μm by the grating unit structure CST, and fig. 2 (d) is a schematic diagram of a principle of realizing function switching by two micro-nano structure polarization gratings.
Fig. 3 is a schematic plane arrangement diagram of a liquid crystal molecule of a tunable liquid crystal polarization grating module according to a second embodiment of the present invention, in which fig. 3 (a) is a diagram showing that when long axes of the liquid crystal molecule are in catenary distribution in a plane, left and right circular polarization components of incident light are separated to ±1 stages, and fig. 3 (b) is a diagram showing that when directions of the long axes of the liquid crystal molecule in the plane are identical, geometric phase modulation is lost, and the incident light directly passes through the tunable liquid crystal polarization grating module.
Fig. 4 is a schematic diagram of a tunable lc polarization grating module according to a second embodiment of the present invention for realizing function switching, where fig. 4 (a) is a schematic diagram showing that when the long axes of the lc molecules are in catenary distribution in a plane, incident light with the same wavelength exits at different diffraction angles, so as to realize a broadband chiral spectrum analysis function of a remote object, and fig. 4 (b) is a schematic diagram showing that when the long axes of the lc molecules in the plane are consistent, the exiting light with different wavelengths at the same direction at the same incident angle exits at the same direction, so as to realize a broadband large-field imaging function of a near object.
Fig. 5 is a schematic diagram of an optical system for implementing two functions of broadband chiral spectrum analysis and broadband large-field imaging in the second embodiment of the present invention, where fig. 5 (a) is a schematic plan view of the optical system for implementing broadband chiral spectrum analysis, and fig. 5 (b) is a schematic plan view of the optical system for implementing broadband large-field imaging.
Fig. 6 (a) is a phase distribution diagram of the micro-nano structural aberration correction module according to the first embodiment of the present invention along the diameter direction, fig. 6 (b) is a phase distribution generated in the radial direction by the micro-nano structural aberration correction module according to the first embodiment of the present invention for different incident wavelengths, and fig. 6 (c) is a schematic diagram of the nano pillar structure and its transmission coefficient when the micro-nano structural aberration correction module 5 according to the first embodiment of the present invention is a super-surface nano pillar structure.
Fig. 7 (a) -7 (d) are simulation results of the transmission function (MTF) of the ZEMAX optical software for wideband chiral spectrum analysis at different incident wavelengths according to the first embodiment of the present invention. Fig. 7 (e) is a graph showing spectral resolution at different incident wavelengths according to the first embodiment of the present invention.
Fig. 8 (a) -8 (d) are simulation results of MTF of ZEMAX optical software in broadband large-field imaging with different incident wavelengths and different incident angles according to an embodiment of the present invention.
Reference numerals illustrate:
the optical system comprises a first medium substrate, a second medium substrate, a 3-first grating unit structure, a 4-second grating unit structure, a 5-micro-nano structural aberration correction module, a 6-lens module, a 7-tunable liquid crystal polarization grating module, an 8-first negative meniscus lens (the left spherical surface of the lens is an aperture diaphragm of a system), a 9-biconvex lens and a 10-second negative meniscus lens.
Detailed Description
The present invention will be described in detail with reference to the drawings and the detailed description, but the scope of the invention is not limited to the following examples, which should be construed as including the full scope of the claims. And one skilled in the art will realize that the claims are fully enabled from the following one embodiment.
As shown in fig. 1, the broadband chiral spectrum analysis and large-view-field imaging system based on the micro-nano structure comprises two modules, wherein the two modules are made of dielectric materials, and the two modules are sequentially arranged along the optical axis direction: a tunable geometric phase polarization grating module and a broadband large-view-field imaging module; the tunable geometric phase polarization grating module is used for realizing the switching of two functions of broadband chiral spectrum analysis and broadband large-field imaging by separating left-handed circular polarization components and right-handed circular polarization components of incident light into different directions or directly transmitting the incident light; and the broadband large-view-field imaging module is used for focusing the light in different emergent directions at different positions of the same image plane and focusing the emergent light in different wavelengths in the same direction at the same position.
The tunable geometric phase polarization grating module comprises two composition modes, namely two micro-nano structure polarization gratings or a tunable liquid crystal polarization grating module. When the tunable geometric phase polarization grating module consists of two micro-nano structure polarization gratings, the micro-nano structure polarization gratings are used for separating left-handed circular polarization components and right-handed circular polarization components of incident light into different directions or enabling the incident light to directly penetrate; at least one micro-nano structure polarization grating can rotate around the center of the micro-nano structure polarization grating and is used for realizing the switching of two functions of broadband chiral spectrum analysis and broadband large-field imaging. The first micro-nano structure polarization grating comprises a first medium substrate 1 and a plurality of first grating unit structures 3 arranged on the first medium substrate, wherein the first grating unit structures 3 are periodically arranged; the second micro-nano structure polarization grating comprises a second dielectric substrate 2 and a plurality of second dielectric substrates arranged on the second dielectric substrateThe grating unit structures 4, and the second grating unit structures 4 are also periodically arranged; the first grating unit structure and the second grating unit structure are oppositely arranged, and the left-handed circular polarization component and the right-handed circular polarization component of the incident light can be separated into different directions or the incident light can be directly transmitted through the two grating unit structures by changing the relative positions of the first grating unit structure and the second grating unit structure. The diameter of an aperture diaphragm of the imaging system is Dk, and Dk is more than or equal to 0.3BFL and less than or equal to 0.6BFL; the half diameter of the first micro-nano structure polarization grating is R 1 And Dk is more than or equal to 0.5 and less than or equal to R 1 Less than or equal to 0.8Dk; the half diameter of the second micro-nano structure polarization grating is R 3 And Dk is more than or equal to 0.5 and less than or equal to R 3 BFL is the back focal length of the imaging system, which is less than or equal to 0.8Dk; the first dielectric substrate 1 has a thickness t 1 And t is 1 >10λ 0 The method comprises the steps of carrying out a first treatment on the surface of the The thickness of the second dielectric substrate 2 is t 2 And t is 2 >10λ 0 ，λ 0 Is the center wavelength. The first grating cell structure and the second grating cell structure may be catenary curves or discrete N-order catenary cell structures.
The following describes the switching principle of the broadband large-field imaging function of the near-distance object and the broadband chiral spectrum analysis function of the far-distance object:
as shown in the left diagram of fig. 2 (d), at this time, the first micro-nano structure polarization grating and the first micro-nano structure polarization grating are oppositely arranged in a central symmetry manner, and in an initial state, incident light is incident from the first dielectric substrate 1, and the left-hand circular polarization component and the right-hand circular polarization component are separated into +1 and-1 stages, according to the grating equation:
Pxsin(θ)＝±λ 0 (1)
theta is the diffraction angle lambda 0 For the center wavelength, px is the transverse period length, and the +1 and-1 diffraction angles are respectively:
when the light beam acts on the second micro-nano structure polarization grating, according to the grating equation:
wherein θ' is the diffraction angle of the second micro-nano structure polarization grating, in the initial state, the light beam continuously passes through the first micro-nano structure polarization grating and the second micro-nano structure polarization grating, the diffraction of the two emergent light beams is the same, and is +1 level and +1 level, or-1 level and-1 level, formula (2) and formula (3) are substituted into formula (4), and the diffraction angles of the emergent light beam +1 and-1 level of the second micro-nano structure polarization grating can be respectively:
the light beam can be seen to pass through the micro-nano structure polarization grating with the same diffraction order twice, the diffraction angle is increased, the left circularly polarized light and the right circularly polarized light are emitted in different orders, and the formulas (5) and (6) can show that the diffraction angle and the wavelength are in positive correlation, so that the light with different wavelengths under the condition of normal incidence is emitted in different diffraction angles, and the broadband chiral spectrum analysis function of the remote object can be realized.
At this time, as shown in the right diagram of fig. 2 (d), at least one of the micro-nano structure polarization gratings is rotated, so that the two micro-nano structure polarization gratings are rotated 180 ° relative to each other, and at this time, the two micro-nano structure polarization gratings are arranged in a mirror image. In the first embodiment, the first micro-nano structure polarization grating rotates 180 ° around the center of the first micro-nano structure polarization grating, the second micro-nano structure polarization grating is fixed and does not rotate, the light beam continuously passes through the first micro-nano structure polarization grating and the second micro-nano structure polarization grating, the diffraction of the two emergent light beams is different and is +1 level and-1 level, or-1 level and +1 level, formula (2) and formula (3) are substituted into formula (4), and the emergent diffraction angles of +1 level and-1 level of the second micro-nano structure polarization grating are respectively:
the light beam passes through the micro-nano structure polarization grating with two different diffraction orders, the two diffraction angles are counteracted to be 0 degrees, the incident light and the emergent light have the same direction, which is equivalent to counteraction of the circular polarization by the two gratings, namely, the emergent light with different wavelengths under the same incidence angle is emergent in the same direction, and the broadband large-view-field imaging function of the short-distance object can be realized.
When the tunable geometric phase polarization grating module is composed of the tunable liquid crystal polarization grating module 7, liquid crystal molecules of the tunable liquid crystal polarization grating module 7 can deflect in a plane, and switching of two functions of broadband chiral spectrum analysis and broadband large-field imaging is realized. As shown in fig. 3, the horizontal direction is the transverse direction, and the vertical direction is the longitudinal direction. In fig. 3 (a), when the long axes of the liquid crystal molecules are in catenary distribution in the plane, the left and right circular polarization components of the incident light are separated to ±1 level, so that the incident light with different wavelengths is emitted at different diffraction angles, and the broadband chiral spectrum analysis function of the remote object can be realized at this time, see fig. 4 (a). In fig. 3 (b), when the long axis directions of the liquid crystal molecules in the plane are consistent, the geometric phase modulation effect is lost, and the incident light directly passes through the tunable liquid crystal polarization grating module, that is, the emergent light with different wavelengths under the same incident angle is emergent in the same direction, so that the broadband wide-field imaging function of the close-range object can be realized, as shown in fig. 4 (b).
In order to achieve circular polarization separation to ±1 level by the tunable geometric phase polarization grating module through geometric phase, a design method of the first grating unit structure and the second grating unit structure or the liquid crystal molecule deflection angle will be described:
as shown in fig. 2 (a), the horizontal direction is the transverse direction and the vertical direction is the longitudinal direction. The tunable geometric phase polarization grating module transversely has a grating unit structure with M periods, and as a diffraction device, the geometric phase distribution generated by one grating unit structure (or liquid crystal molecular unit structure) can be expressed as:
wherein θ is the diffraction angle (see FIG. 2 (d)), λ 0 As the center wavelength, px is the transverse period length, and x is the coordinates in the period; the wave front regulation is carried out due to the geometric phase generated by the transverse deflection of the grating unit structure (or the liquid crystal molecular unit structure), namely the grating unit structure (or the liquid crystal molecular unit structure) with the geometric phase value equal to 2 times is deflected longitudinally in the transverse direction by an angle beta. Thus, for a grating cell structure (or liquid crystal molecular cell structure), its deflection angle distributionFurther, the slope distribution of a curve composed of grating unit structures (or liquid crystal molecular unit structures) of different deflection angles can be expressed as:
k(x)＝tan(β(x))(10)
and (3) carrying out integral operation on the formula (10) to obtain the distribution of the grating unit structure (or liquid crystal molecule unit structure) curve:
it is apparent that equation (11) is a standard iso-intensity catenary equation. When x is close to +/-0.5 Px, the slope k of the curve approaches infinity or infinity, and the process difficulty is increased. In order to reduce the processing difficulty of the micro-nano structure polarization grating, the grating unit structure curve can be discretized into N-order catenary unit structures, the included angle between the discretized N-order catenary unit structures and the transverse direction is in the range of 0-180 degrees, the deflection angles of the N-order catenary unit structures relative to the transverse direction are respectively 0 degrees,n is an integer and is more than or equal to 4; grating sheetThe transverse period length of the meta structure (or liquid crystal molecular unit structure) is Px and 2lambda 0 ≤Px≤6λ 0 The method comprises the steps of carrying out a first treatment on the surface of the Longitudinal period is Py and 0.2λ 0 ≤Py≤0.8λ 0 When divided into N areas, the lateral width of each area is lx and is 0.5 Px/N.ltoreq.lx.ltoreq.Px/N. The grating structure height of the grating unit structure is h and 0.2lambda 0 ≤h≤λ 0 The width of the grating structure is wt and 0.1λ 0 ≤wt≤0.4λ 0 ；λ 0 Is the center wavelength. The grating cell structure (or liquid crystal molecular cell structure) of the present invention is not limited to a catenary curve or an N-th order catenary cell structure, and other continuous or discrete structures or other constituent forms may be used to generate a specific geometric phase.
The broadband large-view-field imaging module comprises a micro-nano structural aberration correction module 5 and a lens module 6, wherein the micro-nano structural aberration correction module 5 is a sub-wavelength structure with specific phase distribution and is used for correcting the aberration of the lens module; a lens module 6, which may be a single lens or a lens group (see fig. 5. The lens module 6 is composed of 8-first negative meniscus lens, 9-biconvex lens, and 10-second negative meniscus lens together), for focusing the outgoing light on the image plane; when the lens module is a spherical lens, the center thickness of the spherical lens is Dc and Dc is more than or equal to 0.2BFL, the half diameter of the spherical lens is R and R is more than or equal to 0.5BFL, the radius of curvature of the first surface is Rc1 and Rc1 is less than or equal to-BFL, the radius of curvature of the second surface is Rc2 and Rc2 is less than or equal to-BFL, and the BFL is the back focal length of the imaging system. In practical application, the micro-nano structural aberration correction module and the lens module are different in position on the optical axis under different conditions, the micro-nano structural aberration correction module can be arranged at the upstream and downstream of the lens module or in the lens group, and the broadband large-view-field imaging module formed by the micro-nano structural aberration correction module and the lens module can focus emergent light on an image surface. In the first embodiment, the micro-nano structural aberration correction module is located upstream of the lens module (see fig. 1), and in the second embodiment, the micro-nano structural aberration correction module is located downstream of the lens module (see fig. 5). The micro-nano structured aberration correction module is disposed downstream of the tunable geometric phase polarization grating module along the optical axis, and may be integrally formed with the tunable geometric phase polarization grating module (see embodiment one, fig. 1),or separately from the tunable geometry phase polarization grating module (see embodiment two, fig. 5). The micro-nano structured aberration correction module 5 has a specific phase distribution, and according to the principle of phase delay caused by the light beam in the propagation process, the micro-nano structured aberration correction module 5 of the present invention is not limited to use of a discrete height step structure and a super surface nano column structure, and other continuous or discrete structures can be used to generate the required phase distribution. The half diameter of the micro-nano structural aberration correction module 5 is R 2 And Dk is more than or equal to 0.5 and less than or equal to R 2 Dk is less than or equal to 0.8Dk, and Dk is the diameter of the aperture diaphragm.
The micro-nano structured aberration correction module 5 can generate a specific phase distribution. The incident light beam is a large-field light beam after passing through the tunable geometric phase polarization grating module, the micro-nano structural aberration correction module 5 can realize aberration correction of the lens module 6 under the condition of large-field incidence, and the broadband range (lambda) of the incident light is determined min ，λ max ) And the polarization grating period of the tunable geometric phase polarization grating module is Px; according to the incident light broadband range (lambda min ，λ max ) And polarization grating period Px, calculating maximum diffraction angle theta max ，
Designing a broadband large field of view imaging module such that the imaging bandwidth of the broadband large field of view imaging module covers the broadband range (lambda) of incident light min ，λ max ) The imaging field of view is greater than or equal to the maximum diffraction angle theta max 。
The method for designing the micro-nano structured aberration correction module 5 will now be described:
the phase delay caused by the light beam during propagation can be expressed as:
when the micro-nano structural aberration correction module 5 adopts a step structure with discrete height, a plane with known phase distribution is selected, and according to the known phase distribution diagram of the micro-nano structural aberration correction module 5 along the diameter direction, the height difference delta d of steps at different positions on the plane can be calculated:
wherein Deltad is the height difference of any two steps processed on a plane, lambda 0 For the center wavelength, ΔΦ is the known phase difference required for imaging due to the height difference, n is the refractive index of the plane; a step structure in a plane is obtained from Δd.
When the micro-nano structural aberration correction module 5 adopts a super-surface nano column structure, according to the known phase distribution diagram of the micro-nano structural aberration correction module 5 along the diameter direction, the equivalent refractive indexes n at different positions are obtained, and the side lengths of the nano columns at different positions are obtained. And then the phase regulation is carried out under the condition of unchanged plane thickness. According to
Wherein Δn is the equivalent refractive index difference of any two nano-pillars processed on the plane, ΔΦ is the phase difference caused by different planar dimensions of different nano-pillars, and the planar dimensions of the nano-pillars at different positions on the plane can be obtained according to Δn.
For a better understanding of the present invention, a further explanation will now be made in connection with the first embodiment.
In the present embodiment, the spectrum is divided into a plurality of wavelengths (λ min ，λ max ) A broadband large-field imaging system based on micro-nano structure is designed for 8-14 mu m, and the center wavelength lambda is at the moment 0 11 μm. The invention is also applicable to optical wave bands, terahertz wave bands and microwave bands, but the dielectric materials are required to be selected, different dielectric materials can be selected for different incident lights, such as TiO can be selected in the visible light wavelength range 2 Si, ge, znS, etc. can be selected in the infrared wavelength range, and the implementationThe material of the medium spherical lens 6 is Ge, and the material of the first micro-nano structure polarization grating and the material of the second micro-nano structure polarization grating are Si. The micro-nano structural aberration correction module 5 is integrally integrated on the tunable geometric phase polarization grating module, and in the first embodiment, the micro-nano structural aberration correction module 5 and the plurality of second grating unit structures 4 are respectively arranged on two side planes of the second medium substrate 2. Imaging system back focal length bfl= 12.514mm, aperture stop diameter dk=5 mm, spherical lens center thickness dc=5 mm, half diameter r=8 mm, first surface radius of curvature Rc 1 = -33.329mm, second face radius of curvature Rc 2 -19.059mm; the first dielectric substrate 1 has a thickness t 1 The second dielectric substrate 2 has a thickness t of =0.5 mm 2 =1mm, the half diameter of the first micro-nano structure polarization grating is R 1 = 3.046mm, second micro-nano structured polarization grating half diameter R 3 And micro-nano structural aberration correction module 5 half diameter R 2 All are R 3 ＝R 2 =2.835 mm. In the embodiment, CST electromagnetic simulation software and ZEMAX optical design software are adopted to carry out simulation test on the performance of the system.
As shown in fig. 1 (a), when a remote object is detected and imaged, the first micro-nano structure polarization grating and the second micro-nano structure polarization grating are oppositely arranged in a central symmetry mode, and in this case, in an initial state, incident light is incident from the first dielectric substrate 1. The light beams are emitted in different orders through the first micro-nano structure polarization grating 3 and the first micro-nano structure polarization grating 4 respectively, diffraction angles and wavelengths are in positive correlation, so that light with different wavelengths under the condition of normal incidence is emitted in different diffraction angles, and the broadband chiral spectrum analysis function of the remote object can be realized by combining spectrograms of different objects. As shown in fig. 1 (b), the first micro-nano structure polarization grating is rotated 180 ° around the center thereof, so that the first micro-nano structure polarization grating and the second micro-nano structure polarization grating are placed in mirror image opposition, at this time, the light beam continuously passes through different orders of the first grating unit structure 3 and the second grating unit structure 4, the diffraction angles of the two times are counteracted to 0 °, the directions of the incident light and the emergent light are the same, and the effect of the two micro-nano structure polarization gratings on circular polarization is counteracted, at this time, the broadband large-view field imaging function of the short-distance object can be realized.
As shown in fig. 2 (a) and 2 (b), the grating unit structure of each period is dispersed into a catenary eight-step unit structure, the included angle between the eight-step unit structure and the transverse direction is in the range of 0-180 degrees, the included angles relative to the transverse direction are respectively 0 °,22.5 °,45 °,67.5 °,90 °,112.5 °,135 ° and 157.5 °, the transverse period of the grating composed of eight-step grating unit structures is px= 58.8235 μm, the transverse width of each region is lx= 6.493 μm, the longitudinal period of the grating unit structure is py=1.62 μm, the height of the grating unit structure is h=6μm, and the width of the grating unit structure is wt=1.52 μm. For carrying out CST simulation, the designed eighth-order cell structure is artificially divided into a plurality of simulation cell structures, the structures of which are similar to the super-surface nano-pillars in fig. 6 (c); a simulation unit structure of the vertical bar grating was taken for CST simulation, and fig. 2 (c) shows the simulation result, wherein the average polarization conversion efficiency and the average relative efficiency in the range of 8-14 μm are about 89.3% and 99.3%, respectively, and the former is defined as the total energy of the incident light (transmitted light) over the energy of the polarization conversion portion occurring in the transmitted light. The micro-nano structure polarization grating based on geometric phase can separate left-handed and right-handed circular polarization components of incident light into different directions, high efficiency can ensure that stray light of an imaging system is less, and high spatial resolution and high spectral resolution are realized.
Fig. 6 (a) is a sectional phase distribution diagram of the micro-nano structural aberration correction module 5 along the diameter direction, the micro-nano structural aberration correction module 5 is set as a binary surface in the ZEMAX optical software, and the optimized phase distribution has rotational symmetry, so the structural design of the micro-nano structural aberration correction module 5 is performed by using the sectional phase distribution diagram of fig. 6 (a) along the diameter direction. The specific structure is designed at the center wavelength 11 μm with the micro-nano structured aberration correction module 5 phase distribution shown in fig. 6 (a), and fig. 6 (b) is the phase distribution at different wavelengths according to the embodiment. The parameters of the super-surface nano-pillar structure of the embodiment are as follows: the substrate period of the square column is p=3.3 μm, the thickness T=1 μm, the unit structure column is a square column, the column height H=10 μm, the side length D is from 1.96 μm to 2.28 μm, the step is 0.01 μm, and the total number of the unit structures is 33. Fig. 6 (c) is a schematic representation of the transmittance of the super surface unit nanopillar structure when d=1.96 μm,2.00 μm,2.10 μm,2.20 μm and 2.28 μm are selected.
Fig. 7 (a) -7 (d) are simulation results of ZEMAX optical software MTF when designing a reconfigurable broadband chiral spectrum analysis and broadband large field of view imaging system constructed using this method for broadband spectrum analysis at different incident wavelengths. In this example, simulation results were obtained for the optical systems at wavelengths of 8 μm,10 μm,12 μm and 14 μm, respectively, with an incident angle of 0 °.
The angle of incidence is 0℃and the broadband bands are 8-14. Mu.m, illustrated here with spacing of 2. Mu.m, namely 8. Mu.m, 10. Mu.m, 12. Mu.m, and 14. Mu.m. In the simulation process, two micro-nano structure polarization gratings in an optical system are oppositely placed in a central symmetry mode, the state is set as an initial state, and incident light is incident from the first medium substrate 1. The result shows that the right-left circular polarization component can be distinguished in different emergent directions due to the micro-nano structure polarization grating with geometric phase. In fig. 7 (a) and 7 (b), the system imaging effect is very close to the diffraction limit at 8 μm and 10 μm, and the system imaging effect is relatively close to the diffraction limit at 12 μm and 14 μm, so that the overall effect is better. Fig. 7 (e) is the spectral resolution of the imaging system at different wavelengths. Spectral resolution refers to the ability to decompose spectral features and bands into separate components, with higher spectral resolution the finer the separation of the bands. In the range of 8-14 μm, the resolution increases with increasing wavelength, but is higher than 50nm, and has good spectral separation capability.
FIGS. 8 (a) -8 (d) are ZEMAX optical software MTF simulation results when designing a reconfigurable broadband chiral spectrum analysis and broadband large field imaging system constructed by the method for broadband large field imaging at different incident angles, wherein the first micro-nano structure polarization grating 3 and the second micro-nano structure polarization grating 4 are rotated 180 degrees relative to each other so that the two are placed in mirror images, the incident wavelengths are 8 μm,9 μm,10 μm,11 μm,12 μm,13 μm and 14 μm, and the incident angles are 0 °,10 °,20 ° and 30 °.
The angles of incidence are 0 °,10 °,20 ° and 30 °, respectively, the broadband bands are 8-14 μm, here illustrated with a spacing of 1 μm, i.e. 8 μm,9 μm,10 μm,11 μm,12 μm,13 μm and 14 μm. In the simulation process, the second grating in the initial state is fixed, the first grating is rotated 180 degrees relative to the second grating, so that the first grating and the second grating are placed in mirror image opposition, and incident light is incident from the substrate of the first grating. In fig. 8, the imaging effect of the system is very close to the diffraction limit at 0 °,10 ° and 20 °, and is relatively close to the diffraction limit at 30 °, so the broadband achromatic focusing effect is better.
In addition, in the process of rotating the first micro-nano structure polarization grating 180 degrees relative to the second micro-nano structure polarization grating, the rotation angle can be changed from 0 degrees to 180 degrees, for spectrum analysis, as the rotation angle is increased, the spectrum resolution is gradually increased, and the maximum is reached at 180 degrees, and at the moment, the broadband chiral spectrum analysis function of the system is best. Under the condition that the rotation direction is unchanged, in the process that the rotation angle is continuously changed from 180 degrees to 360 degrees, as the rotation angle is increased, the spectrum resolution is gradually reduced, the system is restored to an initial state at 360 degrees, and at the moment, the system function is changed into broadband large-field imaging.
Thus, while the embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to the above-described embodiments, which are merely illustrative, and not restrictive. Those skilled in the art, having the benefit of this disclosure, may make numerous forms without departing from the spirit of the invention and the scope of the claims which follow. The present invention is not described in detail in part as being well known to those skilled in the art.
Claims (13)
1. A broadband chiral spectrum analysis and large-view-field imaging system based on a micro-nano structure comprises two modules, wherein the two modules are all made of dielectric materials, and the two modules are sequentially arranged along the optical axis direction:
a tunable geometric phase polarization grating module and a broadband large-view-field imaging module;
the tunable geometric phase polarization grating module is used for realizing the switching of two functions of broadband chiral spectrum analysis and broadband large-field imaging by separating left-handed and right-handed circular polarization components of incident light into different directions or directly transmitting the incident light, wherein the direction of the incident light is the same as that of the emergent light when the incident light is directly transmitted;
the tunable geometric phase polarization grating module comprises two micro-nano structure polarization gratings which are oppositely arranged or tunable liquid crystal polarization grating modules;
the broadband large-view-field imaging module is used for focusing light in different emergent directions at different positions of the same image plane and focusing emergent light with different wavelengths in the same direction at the same position;
the broadband large-view-field imaging module comprises a micro-nano structural aberration correction module and a lens module.
2. The micro-nano structure based broadband chiral spectrum analysis and large field of view imaging system according to claim 1, wherein the micro-nano structure polarization grating is used to separate the left and right circular polarization components of the incident light into different directions; at least one micro-nano structure polarization grating can rotate around the center of the micro-nano structure polarization grating and is used for realizing the switching of two functions of broadband chiral spectrum analysis and broadband large-field imaging.
3. The micro-nano structure based broadband chiral spectrum analysis and large field of view imaging system of claim 2, wherein the tunable geometric phase polarization grating module comprises a first micro-nano structure polarization grating and a second micro-nano structure polarization grating; the first micro-nano structure polarization grating comprises a first medium substrate and a first grating unit structure which is arranged on the first medium substrate and has a plurality of periods; the second micro-nano structure polarization grating comprises a second medium substrate and a plurality of periodic second grating unit structures arranged on the second medium substrate.
4. A micro-nano structure based broadband chiral spectrum analysis and large field imaging system according to claim 3, wherein the imaging system aperture stop diameter is Dk and 0.3BFL is equal to or less than Dk is equal to or less than 0.6BFL; the half diameter of the first micro-nano structure polarization grating is R 1 And Dk is more than or equal to 0.5 and less than or equal to R 1 Less than or equal to 0.8Dk; the half diameter of the second micro-nano structure polarization grating is R 3 And Dk is less than or equal to 0.5R 3 BFL is the back focal length of the imaging system, which is less than or equal to 0.8Dk; the thickness of the first dielectric substrate is t 1 And t is 1 >10λ 0 The method comprises the steps of carrying out a first treatment on the surface of the The thickness of the second dielectric substrate is t 2 And t is 2 >10λ 0 ，λ 0 Is the center wavelength.
5. A micro-nano structure based broadband chiral spectrum analysis and large field of view imaging system according to claim 3, wherein the first grating unit structure and the second grating unit structure are catenary curves or discrete N-order catenary unit structures, N is an integer and N is not less than 4; the transverse period length of the first grating unit structure and the second grating unit structure is Px and 2lambda 0 ≤Px≤6λ 0 The method comprises the steps of carrying out a first treatment on the surface of the Longitudinal period is Py and 0.2λ 0 ≤Py≤0.8λ 0 The method comprises the steps of carrying out a first treatment on the surface of the The height of the grating structure is h and 0.2lambda 0 ≤h≤λ 0 The width of the grating structure is wt and 0.1λ 0 ≤wt≤0.4λ 0 The method comprises the steps of carrying out a first treatment on the surface of the When divided into N areas, each area has a lateral width lx and is 0.5 Px/N.ltoreq.lx.ltoreq.Px/N, lambda 0 Is the center wavelength.
6. The micro-nano structure-based broadband chiral spectrum analysis and large-field imaging system according to claim 1, wherein the liquid crystal molecules of the tunable liquid crystal polarization grating module can deflect in a plane, so that the switching of the two functions of broadband chiral spectrum analysis and broadband large-field imaging is realized.
7. The micro-nano structure based broadband chiral spectrum analysis and large field of view imaging system according to claim 1, wherein the micro-nano structure aberration correction module is a sub-wavelength structure with a specific phase distribution for correcting the aberration of the lens module; the lens module, which may be a single lens or a lens group, is used to focus the outgoing light on the image plane.
8. The micro-nano structure based broadband chiral spectrum analysis and large field of view imaging system according to claim 7, wherein the micro-nano structure aberration correction module is disposed downstream of the tunable geometric phase polarization grating module along the optical axis, and can be integrally integrated on the tunable geometric phase polarization grating module or separately disposed from the tunable geometric phase polarization grating module.
9. The micro-nano structure-based broadband chiral spectrum analysis and large-field imaging system according to claim 7, wherein the micro-nano structure aberration correction module is a discrete height step structure or a super-surface nano-pillar structure; the half diameter of the micro-nano structural aberration correction module is R 2 And Dk is more than or equal to 0.5 and less than or equal to R 2 Dk is less than or equal to 0.8Dk, and Dk is the diameter of the aperture diaphragm; the lens module is a spherical lens, the center thickness of the spherical lens is Dc and Dc is more than or equal to 0.2BFL, the half diameter of the spherical lens is R and R is more than or equal to 0.5BFL, and the curvature radius of the first surface is Rc 1 And Rc 1 BFL less than or equal to, the second surface has a radius of curvature Rc 2 And Rc 2 And (2) not more than-BFL, wherein BFL is the back focal length of the imaging system.
10. A method for designing a micro-nano structure based broadband chiral spectrum analysis and large field of view imaging system for the micro-nano structure based broadband chiral spectrum analysis and large field of view imaging system according to any one of claims 1-9, comprising the steps of:
determining the broadband range of incident light (lambda) min ，λ max ) And a polarization grating period Px of the tunable geometric phase polarization grating module;
adopting a simulation method to perform simulation calculation on a tunable geometric phase polarization grating module, and adjusting the polarization grating structures of two micro-nano structure polarization gratings of the tunable geometric phase polarization grating module according to a geometric phase principle, wherein the polarization grating structures can separate left-handed and right-handed circular polarization components in incident light to +/-1 level;
according to the incident light broadband range (lambda min ，λ max ) And polarization grating period Px, calculating maximum diffraction angle theta max ，
，
Designing a broadband large field of view imaging module such that the imaging bandwidth of the broadband large field of view imaging module covers the broadband range (lambda) of incident light min ，λ max ) The imaging field of view is greater than or equal to the maximum diffraction angle theta max 。
11. The method for designing a broadband chiral spectrum analysis and large-field-of-view imaging system based on micro-nano structure according to claim 10, wherein the method for adjusting the polarization grating structure of the tunable geometric phase polarization grating module is as follows:
the tunable geometric phase polarization grating module transversely has a grating unit structure with M periods,
the grating unit structure curve in one period is as follows:
，
where Px is the transverse period length, and x is the intra-period coordinate.
12. The method for designing a micro-nano structure-based broadband chiral spectrum analysis and large-field imaging system according to claim 11, wherein the grating unit structure curve in one period can be discretized into N-order catenary unit structures, the deflection angles of the N-order catenary unit structures relative to the transverse direction are respectively 0 °,，/>，……，/>。
13. the method for designing a micro-nano structure based broadband chiral spectrum analysis and large field of view imaging system according to claim 10, wherein,
selecting a micro-nano structural aberration correction module of the broadband large-view-field imaging module to be a step structure or a super-surface nano column structure with discrete height; selecting a plane with known phase distribution;
if the micro-nano structure aberration correction module is of a step structure with discrete height, the micro-nano structure aberration correction module is based on
，
wherein ,for the difference in height, lambda, between any two steps machined in said plane 0 For the center wavelength +.>N is the planar refractive index, which is the required phase difference due to the height difference; according to->Obtaining step heights at different positions on the plane;
if the micro-nano structure aberration correction module is of a super-surface nano column structure, the micro-nano structure aberration correction module is based on the following
，
wherein ,for the equivalent refractive index difference of any two nano-pillars processed on the plane, H is the height of the nano-pillar, +.>For the desired phase difference due to different planar dimensions of the nanopillars, according to +.>The planar dimensions of the nanopillars at different locations on the plane can be obtained.
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