CN115542437A - Embedded structure adjustable short-waveband confocal longitudinal bifocal superlens - Google Patents

Abstract

The invention provides an embedded structure adjustable short-waveband confocal longitudinal bifocal superlens, which is characterized in that under the condition of realizing high sampling precision and small required period, a structural unit adopts the following formula 1:1, the limitation of the aspect ratio can be reduced. And solving phases of different spatial positions according to the generalized Snell's law to construct the superlens. Numerical analysis shows that the focus deviation is about 2.2% when λ =600nm linearly polarized light, and the full width at half maximum (FWHM) of the focus breaks the diffraction limit, with high precision requirements. Under 650-550 nm, the maximum error of the focal length is 2.39um, and the error can be ignored due to the existence of the focal depth; a bifocal point with obvious depth intensity equivalent appears at the wavelength of 550 nm; an intensity-tunable focal spot with a larger numerical aperture and a FWHM of only 0.5um appears at 550-490 nm. Therefore, the designed super lens can realize the characteristics of aspect ratio elimination, longitudinal bifocal focus, controllable intensity and confocal short wave band, and can be applied to color imaging, optical path multiplexing and lens focusing integrated devices.

Description

Embedded structure adjustable short-waveband confocal longitudinal bifocal superlens

Technical Field

The invention relates to a longitudinal bifocal, short-waveband focusing and color imaging neighborhood, in particular to an adjustable short-waveband confocal longitudinal bifocal superlens design with an embedded structure.

Background

Conventional focusing lenses rely on phase accumulation along the optical path for light manipulation and are therefore limited by the refractive index of the natural material. In addition, the requirements for the manufacturing process are high, and it is difficult to process a lens with high precision.

Since the last 90 s of the 20 th century, as the precision of semiconductor manufacturing processes has become smaller than that of wavelengths, the interaction of light and substances at sub-wavelengths has become a further focus of research, and two-dimensional planar sub-wavelength modulation techniques, represented by super-surfaces, have gradually come into the field of vision. The optical super surface mainly refers to a two-dimensional metamaterial composed of ultra-thin nano structures, and the nano structures composing the super surface can be holes, slits or protrusions. The optical super-surface can regulate and control the amplitude, phase, polarization and transmission spectrum of light in a sub-wavelength range through the interaction of the nano-structure units and the light. The advantages of ultra-thin thickness, convenient manufacture, easy integration and global light field control enable the optical super-surface to be widely applied in the aspects of color filtering, polarization conversion, wave front regulation, abnormal transmission and reflection and the like.

The superior characteristics of the super-surface attract the great interest of scholars at home and abroad, the concept of the super-surface is firstly proposed by Yu and the like, the scholars propose the super-surface consisting of the V-shaped nano antenna, the regulation and control of circularly polarized light can be realized by changing the opening direction of the antenna, and the generalized Snell's law is proposed for explanation. In 2020, zhao Pengjiu proposes a polarization response-based bifocal super-surface lens design, a group of orthogonal polarization state incident lights are independently controlled, different longitudinal bifocal lights are generated by utilizing left-handed and right-handed polarized lights, and a 1/4 wave plate is required to be added in front of the linearly polarized lights for generation of light source circularly polarized lights, so that the practical complexity is generated; in 2021, xu Bijie provided a design and simulation of a near-infrared wavelength superlens, which uses time domain finite difference FDTD software to design and simulate a silicon-based polarization insensitive superlens with a working wavelength of 800nm, wherein the thickness of a lens is less than 0.5mm, and the collection efficiency is 75% when the numerical aperture is 0.41; in 2022, luo Wenfeng et al proposed the design of a dual-wavelength polarization control super-surface lens, a transmission type super-surface lens for polarization multiplexing was designed at 690nm visible light and 880nm near infrared light using propagation phases, and the generation of coaxial bifocal points required the incidence of two different wavelengths of light; in the same year, sun Ti proposes a broadband full stokes polarization spectrum imaging device based on a monolithic full-medium spatial multiplexing superlens, which is a design of a transverse spatial multiplexing superlens and can realize conversion of various polarized lights.

The dielectric-based super lens needs a sub-wavelength scale high-aspect-ratio structure to realize the regulation and control of an optical field so as to achieve the purpose of changing the equivalent refractive index of a dielectric material, so that the processing of the high-aspect-ratio structure can be realized only by using an advanced micro-nano processing technology, the practical application is difficult to realize in a short period, and the processing technical requirement is very strict. Because the phase modulation of the superlens is discrete, smaller discrete phase modulation at nyquist sampling is more beneficial to achieve precise wavefront control of the superlens, and therefore requires a smaller lattice constant. Based on the theorem of equivalent refractive index, under a small lattice constant, a larger depth-to-width ratio is required for realizing the modulation of the transmission phase within the range of 2 pi. Based on this, a 1:1 embedded architectural unit design. In the design, because the structural units are 50% embedded in the substrate, the equivalent refractive index is larger than that of the structural units on the substrate, and a smaller depth-to-width ratio can be adopted; moreover, because 1:1, the depth-to-width ratio is reduced by half, the requirements on processing and manufacturing are reduced, and large-scale production is facilitated; and because of embedding, the thickness of the whole super lens is thinner and lighter than that of the super lens which combines the structural unit on the substrate, thereby facilitating the integration and planarization. Under the three-ring design, the incidence lambda =600nm, the lens transmittance is 95%, the focusing efficiency is 68.28%, the full width at half maximum is 0.477 lambda/NA, the Rayleigh criterion of 0.61 lambda/NA diffraction limit is broken, and the resolution is higher. The single-focus superlens under the single super surface designed in the past can only realize the focusing of a single wavelength due to material dispersion, structural dispersion and space wave vector, the maximum error of the focal length of the designed superlens is 2.39um (based on the actual focal length under 600 nm) under the irradiation of the linearly polarized light of 550-650 nm, the error can be ignored due to the existence of the focal depth (under the 600nm plane light, the electric field intensity is 300V 2/m 2, and 8um is more than 78% of the intensity), and in addition, the FWHM under the focal depth still has good resolution characteristic. When the light with the wavelength of 550nm is used for numerical analysis, the double focuses with the same axis and different distances on the longitudinal axis are found due to the embedding of the structural unit, and the control of the single focus intensity or the double focus display on the longitudinal axis can be realized according to the practical application condition at different wavelengths of 490nm to 650 nm. The embedded type of the design has good focusing in a short wave band range, and can be applied to color imaging and filtering display.

Disclosure of Invention

The confocal longitudinal bifocal superlens with the embedded structure and the adjustable short wave band has a simple structure and is convenient to use and manufacture.

The invention is realized by the following technical scheme:

an embedded structured short-waveband adjustable confocal longitudinal bifocal superlens is shown in fig. 1, and is characterized in that: the superlens body is constructed by arranging and constructing phase matching radiuses at different spatial positions solved by SiO2 and TiO2 unit columns of a substrate according to a generalized Snell's law.

The lattice constant of the structural unit column is U, the radius of the TiO2 unit column is R, the height of the structural unit column is H, and the total lens is about 6000 unit columns under the three-ring design.

The TiO2 unit column is prepared according to the following steps of 1: the embedded proportion of 1 is embedded into the substrate SiO2, 50% of TiO2 columns on the substrate can construct a convergent wavefront spherical wave, and 50% of TiO2 columns in the substrate can also construct a convergent wavefront spherical wave, so that the super lens can realize longitudinal bifocal confocal.

The invention utilizes FDTD to carry out numerical analysis on the design of the superlens, after the whole lens is constructed, all boundary conditions are set as a Perfect Matching Layer (PML), the used light source is a full-field scattered field (TFSF) plane light source, and the scattered field caused by the direct projection part (including substrate reflection) of the light source is subtracted, so that the numerical analysis is more accurate.

Compared with the prior art, the invention has the advantages that:

1. under the condition of realizing high sampling precision and small required period, the structural unit adopts the following steps of 1:1, the depth-to-width ratio due to the limitation of the manufacturing process can be reduced.

2. When the distance between the two lenses is lambda =600nm, the focal length deviation is about 2.2%, the full width at half maximum (FWHM) of the focus is 0.477 lambda/NA, the diffraction limit of the conventional lens is broken, and the high-precision requirement is met.

3. Under the irradiation of 650-550 nm plane light, the maximum error of the focal length is 2.39um, and the error can be ignored due to the existence of the focal depth; a bifocal point with obvious depth intensity equivalent appears at the wavelength of 550 nm; an intensity-tunable focal spot with a larger numerical aperture and a FWHM of only 0.5um appears at 550-490 nm.

4. The designed superlens can realize the characteristics of aspect ratio elimination, longitudinal bifocal focus, controllable strength and confocal short wave band, and can be applied to color imaging, optical path multiplexing and lens focusing integrated devices.

Drawings

FIG. 1 is a schematic diagram of a superlens and a cell structure of the present invention.

Fig. 2 (a) is a transmission phase curve and transmittance of the structural unit of the present invention, and fig. 2 (b) is a comparison graph of the target phase and the actual phase of the superlens constructed by the present invention.

FIG. 3 (a) is the whole superlens constructed by the present invention, FIG. 3 (b) is the intensity distribution on Z-axis under different ring numbers of the present invention, and FIG. 3 (c) is the Z-X electric field intensity distribution diagram of the substrate-based and embedded superlens of the present invention.

FIG. 4 shows the focal length distribution at different wavelengths according to the present invention.

FIG. 5 is a graph of focal length, intensity, FWHM and focusing efficiency at different wavelengths according to the present invention.

Detailed Description

For a better understanding of the objects, aspects and advantages of the invention, reference will now be made to the following detailed description of the invention, taken in conjunction with the accompanying drawings, in which various aspects of the invention are shown, and in which, unless otherwise indicated, the drawings are not necessarily drawn to scale.

As shown in fig. 1, the superlens body is constructed by arranging phase-matched radii of different spatial positions solved by SiO2 and TiO2 unit columns of a substrate according to the generalized snell's law. The phase at different positions of the superlens can be obtained by the following formula:

the lattice constant of the structural unit column is U =330nm, the radius of the TiO2 unit column is R between 0.043 and 0.104um, the height is H =120nm, and the whole lens is composed of about 6000 unit columns in total under the design of a three-ring. The TiO2 unit column is prepared according to the following steps of 1: the embedded proportion of 1 is embedded into the substrate SiO2, 50% of TiO2 columns on the substrate can construct a convergent wavefront spherical wave, and 50% of TiO2 columns in the substrate can also construct a convergent wavefront spherical wave.

As shown in fig. 2 (a), at a radius of 0.043 to 0.104um, a phase coverage of 2 × pi can be achieved, while at a radius of 95nm, the transmittance abruptly changes to 68%, resonance occurs, the average transmittance is 95% or more, and the maximum aspect ratio is 13.9. In the actual process, the maximum aspect ratio of the prepared nano structure is about 10, and the lower the ratio of the height to the minimum diameter of the nano column is, the easier the nano column is to process, namely, the inclination angle error in the nano column etching process is reduced, so that the actual performance of the device is improved. With the design proposed 1:1 embedded structure, can reduce the aspect ratio by 50%, so the actual maximum aspect ratio is about 7, and meets the actual process requirements.

As shown in fig. 2 (b), the target phase is interpolated with the phase at different positions of the radius of the superlens, and the red point is the phase of the superlens actually designed according to the database information, which varies with the radius, and it can be seen from the figure that the actual phase points of the designed superlens are mostly on the target phase curve, and only a few scattered points have sampling information not on the real line, because the whole superlens is constructed from discrete phases, the wavefront phase spherical wave of the superlens is constructed from discrete phases, and due to the limitation of the size of the lattice constant, it is impossible to make the wavefront phase contributed by all the discrete structural units have a difference on the designed target phase, which is one of the reasons for designing the focal length target and the actual numerical analysis target.

As shown in fig. 3 (a), the rectangular frame is an enlarged view of the superlens ring and at the edge of the ring.

As shown in fig. 3 (b), after the superlens with a radius of 7.7um 1 ring and a radius of 13.5um three rings is calculated by the script difference, the intensity distribution of the superlens on the Z plane of the propagation axis is calculated, the inset is the electric field intensity diagram of the X-Y plane at the focal point under the three ring design, it can be seen from the diagram that the distance between the numerical analysis result of the superlens with 1 ring and the design focal length is about 10um, and the deviation is too large to be used for design and manufacture. This is because too few structural units reduce the control of light propagation, and lattice discrete arrangement of each surface does not allow gapless full-phase sampling. Based on this, the design is followed by numerical analysis of the superlens based on a three-ring design. For numerical analysis of the three-ring superlens, the maximum electric field intensity of a Z-X plane is 48.878um, namely the focus, the difference between the focus and a design target is 2.2%, the design of the superlens is met, and the analysis reason is that FDTD incident waves are not ideal plane waves when the aperture is small. The intensity can be seen to be centered in the insert of the X-Y plane at the focus, the edge has no diffracted light intensity, the FWHM at the focus is 1.1um (0.477 lambda/NA) through calculation, the Rayleigh criterion diffraction limit defines 0.61 lambda/NA and is 1.4um, and the super lens breaks the diffraction limit, has better resolution and can be applied to large-scale integrated lenses.

As shown in fig. 3 (c), at the conventional structure (1) λ =600nm/500nm, the lens is focused at about 60nm, and the focal length increases as the incident wavelength decreases because the structural dispersion and the spatial wavevector are different; whereas in the immersion configuration (2) it can be seen that at λ =600nm/500nm, in addition to the first focus, an additional second focus appears, shown in white circles.

The effect of this lens on dispersion was tested using discrete light with wavelengths 490-650 nm as shown in figure 4. Between 650nm and 550nm, the focus is at 48.878um on the Z axis, and the electric field intensity distribution at the focus is concentrated, because of the existence of the focal depth (at 600nm plane, the electric field intensity is 300V 2/m 2, i.e. the field intensity exceeds 78%, the focal depth is 8 um), all the focuses in the short band of 650nm to 550nm are at the focus position of the super lens structure design of the design at 600 nm. And the super-surface lens can better prolong the focal length and can maintain better transverse resolution. An interesting point was found in numerical analysis, introducing an embedded structure, with decreasing incident wavelength, the intensity at the main focus decreases, while the peak at the side lobe evolves to the focus with maximum intensity. Therefore, the super lens of the design can realize the longitudinal bifocal function and can be used on optical path multiplexing functional devices. Under the irradiation of light with wavelength of 550nm, two longitudinal bifocal lenses are appeared, and the focal point of the other lens is about 28.929um, which is about the same as the intensity of the main focal point. This is because the embedded structure proposed by the present design not only provides a convergent spherical wave according to the original design, but also constructs a convergent wavefront spherical wave to control the emergent field because the lower 50% of the structure and the substrate SiO2 form a system. The second focus is at the focus of 28.929um at 550 nm-490 nm, and the lens has a focal spot with a larger numerical aperture and a full width at half maximum of 0.413 lambda/NA. While super-diffraction-limited focusing is achieved, it can be seen from the figure that there are focus side lobes of considerable intensity inevitably associated around the main focus lobe, because there is some error in the calculation of the phase and optimization of the structure size, which results in the presence of the side lobes, but the influence on the whole is small and negligible.

As shown in fig. 5 (a), in the wavelength range of 650 to 550nm, the maximum deviation of the Z-axis peak focal point from the focal point at the wavelength of 600nm (numerical analysis 48.878 um) is 2.39um, and the maximum of the electric field intensity is shown as a white line, and at 510nm, the maximum appears in the entire wavelength band because the smaller the wavelength, i.e., the higher the frequency, the higher the energy of light and thus the larger the field intensity. The lens has slow field intensity change in a 590nm to 650nm wave band and has better confocal characteristic of a short wave band. According to the principle of dispersion, the long wavelength light is focused closer to the lens, while the short wavelength light is focused farther from the lens, consistent with the numerical analysis of the present design. Super-surface lenses present non-negligible dispersive effects. In general, hypersurface lenses cause dispersion primarily for two reasons. One aspect is dispersion caused by the different response of the sub-wavelength structures on the meta-surface for different wavelength complex amplitudes. Another aspect is the dispersion in space of the light waves leaving the hypersurface due to the propagating wave vectors that vary with wavelength. The principle of complementary dispersion of the structure and the material is generally utilized to eliminate the dispersion caused by the structure and the material, but because wave vectors k of different wavelengths propagating in space are different, the dispersion of the superlens cannot be absolutely eliminated, and only at a focal point, due to the existence of focal depth, the superlens focuses at the same focal point position.

As shown in fig. 5 (b), the FWHM at the respective focuses is constant in the short band, the FWHM at the focuses within 650nm to 550nm is 1.1um, the maximum focusing efficiency of the lens at 600nm is 68.28%, and the focusing efficiency is high in the whole short band; while the FWHM in the 550-490 nm range is 0.5um, with larger numerical aperture and higher resolution, the focusing efficiency of the lens is somewhat insufficient.

The superlens of the embedded structure unit based on the design shows that the focal length is in the range of 48.878um at 650-550 nm, has a certain achromatization function, and can realize confocal superlens of short wave band. The super lens has the performance of larger numerical aperture and higher resolution within the range of 550-500 nm, and various combinations of super lenses with different focuses and different intensities can be selected according to requirements.

The super lens can realize the characteristics of aspect ratio elimination, longitudinal bifocal focus, controllable intensity and confocal short wave band, and can be applied to color imaging, optical path multiplexing and lens focusing integrated devices.

It should be noted that, although the above-mentioned embodiments of the present invention are illustrative, the present invention is not limited thereto, and thus the present invention is not limited to the above-mentioned embodiments. Other embodiments, which can be made by those skilled in the art in light of the teachings of the present invention, are considered to be within the scope of the present invention without departing from its principles.

Claims (6)

1. An embedded structured short-waveband adjustable confocal longitudinal bifocal superlens is shown in fig. 1, and is characterized in that: firstly, under the condition of realizing high sampling precision and small required period, a structural unit adopts the following formula 1:1, can reduce the limitation of the depth-to-width ratio of the manufacturing process, and can realize the TiO covering the whole 2 pi under the conditions that the lattice constant is U =330nm and the height is H =120nm ₂ The radius interval of the structural unit is 0.043-0.104 um.

2. The embedded structured tunable short-waveband confocal longitudinal bifocal superlens of claim 1, wherein: the transmission of the lens at λ =600nm was as high as 95% with a focusing efficiency of 68.28%.

3. The embedded structured tunable short-waveband confocal longitudinal bifocal superlens of claim 1, wherein: in the wavelength range of 650nm to 550nm, the focus is approximately at 48.878um on the Z axis, and the electric field intensity distribution at the focus is concentrated, because of the existence of the focal depth (under 600nm plane light, the electric field intensity is 300V 2/m 2, namely the field intensity exceeds 78%, the focal depth is 8 um), all the focuses in the short wave band of 650nm to 550nm are at the focus position of the super lens structure design under 600nm, the FWHM at the focus is 0.477 lambda/NA, and the diffraction limit is broken.

4. The embedded structured tunable short-waveband confocal longitudinal bifocal superlens of claim 1, wherein: upon irradiation with light of 550nm wavelength, two distinct longitudinal bifocal lenses appeared, the other lens having a focal point of about 28.929um, about the same intensity as the primary focal point, since the embedded structure proposed by this design not only provided a convergent spherical wave as originally designed, but also because the lower 50% of the structure was in contact with the substrate SiO ₂ The system is formed, and a converged wavefront spherical wave is constructed to control the emergent field.

5. The embedded structured tunable short-waveband confocal longitudinal bifocal superlens of claim 1, wherein: the second focus is at the focus of 28.929um at 550nm to 490nm and the lens has a larger numerical aperture and a focal spot with a full width at half maximum of 0.413 x λ/NA.

6. The embedded structured tunable short-waveband confocal longitudinal bifocal superlens of claim 1, wherein: the super lens of the embedded structure unit based on the design shows that the focal length is in the range of 48.878um at 650-550 nm, has a certain achromatization function, and can realize the confocal super lens of a short wave band; the super lens has the performance of larger numerical aperture and higher resolution within the range of 550-500 nm, and various combinations of super lenses with different focuses and different intensities can be selected according to requirements.

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