CN109459805A - A kind of periodical media grating and THz wave condenser lens - Google Patents
A kind of periodical media grating and THz wave condenser lens Download PDFInfo
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- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 8
- 229920000306 polymethylpentene Polymers 0.000 claims description 5
- 239000011116 polymethylpentene Substances 0.000 claims description 5
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- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims description 3
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- G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
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Abstract
The present invention relates to a kind of periodical media grating and THz wave condenser lens, an embodiment of the periodical media grating includes the dielectric substrate with plane and multiple media units that the plane is arranged in；Wherein, the multiple media units are in the horizontal and vertical all periodic arrangements of the plane；For any medium unit in the multiple media units: it is described horizontal and vertical with equal wide；Each media units have double altitudes in the direction of the vertical plane.The embodiment have higher transmission transmissivity, can to incident THz wave carry out phase regulation and to polarize it is insensitive.
Description
Technical Field
The invention relates to the technical field of terahertz metamaterials, in particular to a periodic medium grating and a terahertz wave focusing lens.
Background
Terahertz is a section of electromagnetic spectrum between microwave and infrared radiation, and is a section of precious frequency resource to be fully developed and utilized, the frequency range of the terahertz is 0.3 THz-3 THz, and the wavelength range of the terahertz is 0.1 mm-1 mm. With the rapid development of the current terahertz scientific technology, the important value of terahertz waves gets more and more attention, and a great amount of applications are brought in the fields of terahertz communication, imaging, material detection, medical imaging and the like.
The traditional method for collimating and focusing terahertz waves is to adopt a quasi-optical device such as an optical prism or a parabolic reflector, but the size of the device is usually hundreds of times of the wavelength of the optical device, the structure size is larger, and the device is mostly of a three-dimensional structure, so that the miniaturization and the planarization of the terahertz device and the system are difficult to realize. One method of reducing the volume of the prism is to use a Fresnel prism, but the controllability of the structure is poor. In recent years, with the continuous development and application of terahertz materials, a lot of research and application have been made on terahertz lenses with planar metamaterial structures (i.e., super-surfaces), and metamaterials are a generic name of materials which are formed by periodic unit structures to form arrays in specific shapes to realize certain functions and are generally formed by sub-wavelength size units. In a terahertz frequency band, a device designed by a super-surface structure has the advantages of planarization, light material and low loss.
The current terahertz plane lens is realized by adopting a mode of adjusting the size of a periodic metal structure, and the principle of the terahertz plane lens can be mainly divided into two categories: one is realized by interference or diffraction of a periodic structure surface electric field, for example, terahertz wave focusing is realized by exciting surface wave diffraction on two sides by a periodic grating slit with gradually changed irradiation depth; through designing a periodic metal grating structure with a defective unit, terahertz wave focusing can be realized through the interference of the defective unit structure and a uniform periodic structure surface wave, but the structures adopted by the methods are still large in size, poor in tunability and sensitive to polarization directions.
In addition, a planar Gradient-index metamaterial or a super-surface structure (also called a phase Gradient super-surface structure) is adopted to realize terahertz radiation focusing, the planar Gradient-index metamaterial structure regulates and controls the phase of incident terahertz waves by changing the size of a unit structure, so that the phase changes with the position in a parabolic manner to realize terahertz wave focusing, and the currently adopted phase Gradient structure is mainly a periodic metal structure, such as a circular slotted periodic metal structure, a square slotted periodic metal structure, a V-shaped periodic metal structure, an arc-shaped periodic metal structure and the like. The terahertz lens adopting the phase gradient change super-surface of the periodic metal structure realizes phase control of transmission terahertz waves by adjusting the size of the metal unit structure, but the transmission coefficient is generally small, and because the metal is similar to an ideal conductor in the terahertz waveband, the terahertz lens reflects the terahertz waves greatly, so that the focused terahertz wave energy is small. Meanwhile, the metal structure has a small phase change range for the transmission of the terahertz wave, and although there is a method of increasing the transmission phase of the terahertz wave by using a multilayer metal structure, the structure complexity is increased. In addition, there are metal structures that are sensitive to polarization of incident terahertz waves, such as the V-shaped or arc-shaped metal structures mentioned above.
Therefore, in view of the above disadvantages, it is desirable to provide a structure having a high transmission transmittance, capable of phase-modulating an incident terahertz wave, and insensitive to polarization.
Disclosure of Invention
The technical problem to be solved by the invention is how to provide a structure which has higher transmission transmissivity, can perform phase control on incident terahertz waves and is insensitive to polarization.
To solve the above technical problem, in one aspect, the present invention provides a periodic dielectric grating.
The periodic medium grating of the embodiment of the invention comprises: a dielectric substrate having a planar surface and a plurality of dielectric elements disposed in the planar surface; wherein the plurality of media units are arranged periodically in both the transverse and longitudinal directions of the plane; for any of the plurality of media units: having equal widths in the transverse and longitudinal directions; each media unit has an equal height in a direction perpendicular to the plane.
Preferably, the plurality of media units are arranged periodically in both the transverse direction and the longitudinal direction of the plane, and specifically include: the plurality of medium units are arranged in the transverse direction with a preset transverse direction period; the plurality of media units are arranged in the transverse direction with a preset longitudinal period.
Preferably, the width of any media unit in the lateral or longitudinal direction is related to its lateral and longitudinal position.
Preferably, the dielectric units are made of the same dielectric material, and the relative dielectric constant of the dielectric units is larger than that of the dielectric substrate material.
Preferably, the lateral and longitudinal periods are both less than the height of the dielectric element, and the lateral and longitudinal periods are both less than the operating wavelength.
In another aspect, the invention further provides a terahertz wave focusing lens implemented by using the periodic dielectric grating
In the plurality of dielectric units of the terahertz wave focusing lens of the embodiment of the present invention: the media units in the center of the plane have the greatest lateral or longitudinal width; the width of the media element decreases monotonically in a direction from the center position to either edge position of the plane.
Preferably, the plurality of media units are centered symmetrically about the media unit at the center of the plane.
Preferably, the phase compensation of the incident terahertz wave by the dielectric unit monotonically increases along a direction from the center position to either edge position of the plane.
Preferably, the phase compensation of the medium unit for the incident terahertz wave increases in a parabolic manner from the center position in the lateral or longitudinal direction.
Preferably, the dielectric element material comprises silicon or silicon dioxide, and the dielectric substrate material comprises polymethylpentene.
The technical scheme of the invention has the following advantages: in the embodiment of the invention, the planar terahertz wave focusing lens made of all dielectric materials is provided, the advantage that the dielectric materials are transparent to terahertz waves is utilized, the transmittance of incident terahertz waves is increased, and the defects that the transmittance of a metal unit structure adopted by the conventional planar terahertz wave focusing lens to terahertz waves is small and the change range of transmission phases is small are overcome. Compared with a metal structure, the equivalent refractive index of the unit structure can be increased in a wider frequency band by adopting the all-dielectric material, so that the phase control of the transmitted terahertz waves is increased. Meanwhile, the designed planar dielectric lens can increase the focusing energy of the terahertz waves and has the advantage of high efficiency. In addition, a symmetrical periodic medium structure adopted by the planar terahertz wave focusing lens is insensitive to polarization of incident terahertz waves, and the defect that the conventional terahertz lens is sensitive to polarization can be overcome. The lens adopts a plane structure and has the advantages of easy processing and plane integration.
Drawings
FIG. 1 is a schematic diagram of a periodic dielectric grating in an x-z plane according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a periodic dielectric grating in an x-y plane according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of transmission coefficient and phase compensation of a terahertz wave focusing lens according to an embodiment of the present invention;
FIG. 4 is a schematic structural diagram of a terahertz wave focusing lens in an x-y plane according to an embodiment of the present invention;
fig. 5 is a schematic view of a terahertz wave focus generated using a terahertz wave focusing lens according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.
Fig. 1 is a schematic structural diagram of a periodic dielectric grating in an x-z plane according to an embodiment of the present invention, and fig. 2 is a schematic structural diagram of a periodic dielectric grating in an x-y plane according to an embodiment of the present invention. As shown in fig. 1 and 2, a periodic dielectric grating according to an embodiment of the present invention includes: a dielectric substrate 2 having a planar surface and a plurality of dielectric elements 1 disposed in the planar surface. In this context, the upward direction perpendicular to the plane may be referred to as the z-axis forward direction, and the two mutually perpendicular directions in which the in-plane medium units are arranged in sequence may be referred to as the x-and y-axes. As can be seen in fig. 1 and 2, the plurality of media units are arranged periodically in both the transverse (i.e., x-direction) and longitudinal (i.e., y-direction) directions of the plane. It is understood that the periodic arrangement refers to an equally spaced arrangement. For any of the plurality of media units: having equal widths in the transverse and longitudinal directions; each media unit has an equal height in a direction perpendicular to the plane (i.e., the z-direction). Since each dielectric element disposed in the plane of the dielectric substrate has an equal height, each dielectric element is in the same plane with respect to the incident electromagnetic wave. It should be noted that the medium may include any substance other than metal, as opposed to metal.
In an embodiment of the present invention, the plurality of medium units are arranged in the transverse direction with a predetermined transverse direction period, and arranged in the transverse direction with a predetermined longitudinal direction period. The transverse periods can be set to be equal or different according to actual requirements. Meanwhile, the width of any medium unit in the transverse direction or the longitudinal direction can be determined according to specific requirements, and the width of the medium unit is related to the transverse position and the longitudinal position of the medium unit.
In practical application, the same dielectric material (such as silicon or silicon dioxide) is adopted for each dielectric unit, and the relative dielectric constant of the dielectric unit is larger than that of the dielectric substrate material (such as polymethylpentene), so that the electromagnetic wave can be transmitted at a larger transmittance. Preferably, the lateral and longitudinal periods are both less than the height of the dielectric element, and both the lateral and longitudinal periods are less than the operating wavelength.
Please continue to refer to fig. 1 and 2. The periodic medium grating of the embodiment of the invention is composed of all media and comprises a planar medium substrate structure (namely a medium substrate) and a periodic medium structure (namely a structure composed of medium units). The planar medium substrate structures are uniformly distributed in an x-z plane, are rectangular, have the height H along the z direction and are symmetrically distributed in the x-y plane. The periodic dielectric structure is placed in z next to the planar dielectric substrate structure>0, and are periodically distributed along the x and y directions, and the periods are p respectivelyx(i.e., lateral period) and py(i.e., longitudinal period), the x-direction and y-direction period sizes of the periodic dielectric grating structure can be adjusted to be the same or different according to the needs in practical application. The periodic medium structure adopts a rectangular grating structure, the height along the z direction is h, and the widths of the periodic medium structure along the x direction and the y direction are d respectivelyxAnd dyIn practical application, the width of the periodic medium grating can be adjusted to be the same or different according to requirements.
The periodic medium structure of the embodiment of the invention adopts a rectangular grating, and the structural size of the rectangular grating is in a sub-wavelength order, namely the depth (namely the height of the medium unit) h of the periodic grating<λ (λ is working wavelength), and period p is arranged along x directionx<<λ, period p arranged in y-directiony<<λ，pxAnd pyTypically a fraction of a wavelength. Width 0 in x-direction of periodic dielectric grating<dx<pxWidth 0 in y direction<dy<pyIn practical application, the transmission amplitude and phase of the incident electromagnetic wave can be regulated and controlled by adjusting the width of the periodic medium grating (namely the width of the medium unit). The depth H of the periodic dielectric structure of the present invention is generally greater than or close to the depth H of the planar dielectric substrate structure. The relative dielectric constant of the plane medium substrate structure material of the periodic medium grating structure is epsilonsThe equivalent dielectric constant of the periodic dielectric structure material is epsilon, and epsilon is usually set for ensuring that incident terahertz waves are transmitted with larger transmittance>εsThat is, the periodic medium grating is made of material with larger refractive index, such as silicon, silicon dioxide, etc., and the linerThe substrate material is made of material with smaller refractive index, such as polymethylpentene. The plane substrate structure of the periodic medium grating structure simultaneously plays a role of supporting the periodic medium structure, and epsilon can be set under the condition of ensuring the transmission of incident terahertz waves with larger transmissivitys1, the planar dielectric substrate is air.
The periodic dielectric grating structure adopts all-dielectric materials, and as the dielectric materials have higher transparency to terahertz waves compared with metal structures, the terahertz waves transmitted along the-z direction have higher transmittance under the condition of selecting the structural material parameters and the structural parameters, and the transmittance is greater than 90% in a wider frequency range under the condition of meeting the structural parameters. The phase of incident terahertz waves can be regulated and controlled by changing the width of the periodic medium structure in one period, and the phase regulation amplitude range of the medium grating is close to 360 degrees due to the fact that the medium grating is made of a material with a large refractive index. In the periodic dielectric grating structure, under the condition of ensuring that the transmission phase of the transmission terahertz wave is changed to be close to 360 degrees, in order to ensure that the transmittance of the incident terahertz wave is as large as possible, the size d of the periodic unit structure is generally setx<0.7px，dy<0.7py. When the rectangular grating width of the periodic dielectric structure is large or close to the period (i.e., d)x≈pxOr dy≈py) The transmission transmittance is reduced. The rectangular grating of the periodic medium structure adopts a symmetrical structure, namely, the p is satisfiedx＝py，dx＝dyThe transmission rate and the phase change of the terahertz wave are insensitive to the polarization direction of the incident terahertz wave, namely, the x-polarization or y-polarization electromagnetic wave transmitted along the z direction has the same transmission effect.
In another aspect, the invention further provides a terahertz wave focusing lens implemented by using the periodic dielectric grating.
Most of existing terahertz planar lenses adopt periodic metal structures, the terahertz wave transmission phase is regulated and controlled by adjusting the size of a periodic metal unit structure, planar incident wave focusing is realized by designing a phase gradient array structure, the equivalent refractive index of the unit structure can be increased by adopting an all-dielectric periodic structure, and then the regulation and control of incident terahertz waves are increased, so that a new idea can be provided for the research and design of novel planar terahertz lenses.
The structure of the terahertz wave focusing lens in the embodiment of the invention on the x-y plane is shown in FIG. 4. In a plurality of media units of the lens: the media units in the center of the plane have the greatest lateral or longitudinal width; the width of the media element decreases monotonically in a direction from the center position to either edge position of the plane.
Preferably, the plurality of dielectric units are centered on the dielectric unit located at the center of the plane on which the dielectric unit is placed on the dielectric substrate. Based on the above structure, the phase compensation of the medium unit for the incident terahertz wave can be monotonically increased along the direction from the center position to either edge position of the plane; in the lateral or longitudinal direction, the phase compensation of the incident terahertz wave by the dielectric unit can be increased in a parabolic manner. Fig. 3 is a schematic diagram of transmission coefficients and phase compensation of a terahertz wave focusing lens according to an embodiment of the present invention. In FIG. 3, the abscissa d/p may be dx/pxOr dy/pyThe left ordinate is the transmission coefficient and the right ordinate is the phase compensation. In the approximate range of the abscissa 0 to 0.7, as viewed from the center position of the plane toward the edge (i.e., the abscissa is reversed), the phase compensation for the incident terahertz wave increases monotonously, the phase change of the transmitted wave approaches 360 degrees, and the transmission coefficient remains at 0.95 or more.
In specific application, the dielectric units in the terahertz planar lens (namely the terahertz wave focusing lens) based on the periodic dielectric grating structure are arranged on the planar dielectric substrate along the x-y direction, the depths of the planar dielectric substrate structure and the periodic dielectric structure along the z direction are respectively H and H, and the H is generally satisfied<H or H is approximately equal to H, and the arrangement period p of the periodic medium structure along the x direction and the y directionx＝pyWidth of each dielectric unit structure along x direction and y directiondx＝dyThe incident terahertz waves can be transmitted with larger transmittance by adjusting the width dx or dy of each unit dielectric grating, and the focusing of the incident terahertz waves can be realized by designing an array structure with the transmission phase gradually changed in a parabolic gradient along with the space coordinate (x, y). In practical application, the periodic medium structures are placed on the substrate structure and are symmetrically distributed along the central origin of an x-y plane, the width of the medium grating structure at the center is the largest, and the width is reduced from the center to the outside along the coordinate position, but the phase increase of the transmission terahertz waves is compensated.
The terahertz lens based on the phase gradient periodic medium grating focuses linearly polarized terahertz waves transmitted along the z direction, and due to the adoption of the all-dielectric material structure, incident terahertz waves have high transmittance, and when the incident terahertz waves reach the medium units of which the transmission phases gradually change along with the spatial gradient, the compensation phases of the medium units with different widths for the incident terahertz waves meet the parabolic change, so that the incident terahertz waves are focused on the periodic medium. Under the condition that the frequency of incident terahertz waves is given, the position of focused terahertz waves is determined by the gradient periodic medium array, the smaller the phase change difference of adjacent periodic medium structures to the transmission terahertz waves is, the farther the focal length position of the focused terahertz waves is away from the periodic medium structures, the larger the phase change difference of the adjacent periodic medium structures to the transmission terahertz waves is, and the closer the focal length position of the focused terahertz waves is to the periodic medium structures.
Because the medium unit structure has higher transmittance to incident terahertz waves, when the periodic medium array meets the gradient phase gradient distribution, the focused terahertz waves have higher energy which is 10 of the amplitude peak value of the electric field of the incident terahertz waves4～105And the order of magnitude is high, so that the focusing efficiency of the all-dielectric periodic medium structure on incident terahertz waves is high. Meanwhile, the all-dielectric material has high transmittance to terahertz waves in a wide frequency band range, so that the all-dielectric material also has the advantage of broadband focusing.
A specific embodiment of the above-described terahertz wave focusing lens is provided below.
In this embodiment, the operating frequency of the terahertz wave is 0.95THz, the polarization mode is polarization along the y direction, the height H of the dielectric unit is 400 μm, and the depth H of the planar dielectric substrate structure is 300 μm. Periodic dielectric structures arranged periodically in the x-y plane, px＝py＝61μm，dx＝dy. The periodic medium structure is made of silicon with high refractive index and relative dielectric constant epsilon is 11.7, the planar medium substrate structure is made of polymethylpentene TPX with low refractive index and relative dielectric constant epsilons2.1. The width of each unit can be changed to realize phase gradient change, the phase gradient array structure is symmetrically distributed by the unit at the center, the width of the medium grating of the unit at the center is the maximum, and the phase distribution of the medium grating gradually reduces along with the outward width of the position, and the phase distribution of the medium grating accords with the parabolic two-dimensional distribution relation along with the change of the position coordinate.
Fig. 5 is a schematic view of a terahertz wave focus generated using a terahertz wave focusing lens according to an embodiment of the present invention. As shown in FIG. 5, the terahertz plane wave is transmitted along the z direction, the incident wave is polarized along the y direction, the amplitude is 1V/m, the focusing position is 200 μm above the periodic medium grating, and the focusing electric field amplitude reaches 106V/m magnitude, it can be seen that the planar periodic structure made of all-dielectric material realizes high transmission of terahertz waves, and a high-efficiency focusing lens is obtained.
In summary, in the technical solution of the embodiment of the present invention, a planar thz wave focusing lens made of all-dielectric materials is provided, where the lens is made of a double-layer all-dielectric material and includes a planar dielectric substrate structure and a periodic dielectric structure, the periodic dielectric structure is made of a square high refractive index material, and the planar dielectric substrate structure is made of a uniform low refractive index material. The size of the high-refractive-index periodic medium structure is adjusted to realize high-transmittance transmission of terahertz waves, the phase of the terahertz waves is regulated and controlled, the periodic medium planar array structure with gradually changed phase gradient is designed to realize focusing of incident terahertz waves, and the linear polarization direction can be selected at will. Compared with the terahertz plane lens adopting a periodic metal super-surface structure at present, the terahertz plane lens adopting the all-dielectric plane structure can increase the equivalent refractive index of the unit structure, so that the regulation and control range of the phase of the transmission terahertz wave can be enlarged, the terahertz plane lens has the advantage of high transmission, and meanwhile, the planar lens structure also has the advantages of easiness in processing and integration.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. A periodic dielectric grating, comprising: a dielectric substrate having a planar surface and a plurality of dielectric elements disposed in the planar surface; wherein,
the plurality of media units are arranged periodically in both the transverse direction and the longitudinal direction of the plane;
for any of the plurality of media units: having equal widths in the transverse and longitudinal directions; each media unit has an equal height in a direction perpendicular to the plane.
2. The grating of claim 1, wherein the plurality of dielectric elements are periodically arranged in both the transverse and longitudinal directions of the plane, specifically comprising:
the plurality of medium units are arranged in the transverse direction with a preset transverse direction period;
the plurality of media units are arranged in the transverse direction with a preset longitudinal period.
3. The grating of claim 2 wherein the width of any dielectric element in either the transverse or longitudinal direction is related to its transverse and longitudinal position.
4. The grating of claim 3 wherein the dielectric elements are formed from the same dielectric material and have a relative permittivity greater than a relative permittivity of the dielectric substrate material.
5. The grating of claim 4 wherein both the lateral and longitudinal periods are less than the height of the dielectric element and both the lateral and longitudinal periods are less than the operating wavelength.
6. A terahertz wave focusing lens realized using the periodic dielectric grating according to any one of claims 1 to 5, wherein, in a plurality of dielectric units of the lens:
the media units in the center of the plane have the greatest lateral or longitudinal width;
the width of the media element decreases monotonically in a direction from the center position to either edge position of the plane.
7. The lens of claim 6, wherein the plurality of media units are centered symmetrically about the media unit at a center of the plane in the plane.
8. The lens of claim 7, wherein the phase compensation of the incident terahertz wave by the dielectric unit monotonically increases along a direction from the center position to either edge position of the plane.
9. The lens of claim 8, wherein phase compensation of incident terahertz waves by the dielectric unit increases parabolically from the central location along the lateral or longitudinal direction.
10. A lens as claimed in any one of claims 6 to 9, characterized in that the dielectric element material comprises silicon or silicon dioxide and the dielectric substrate material comprises polymethylpentene.
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| CN111816535A (en) * | 2020-07-13 | 2020-10-23 | 东南大学 | A Coherent Electromagnetic Wave Radiation System Based on Dielectric Grating Smith-Purcell Effect |
| CN112764146A (en) * | 2019-10-21 | 2021-05-07 | 宁波舜宇车载光学技术有限公司 | Optical device and light source and application thereof |
| CN113009603A (en) * | 2019-12-20 | 2021-06-22 | 浜松光子学株式会社 | Optical element for terahertz wave and method for manufacturing optical element for terahertz wave |
| US20210194144A1 (en) * | 2019-12-20 | 2021-06-24 | Hamamatsu Photonics K.K. | Terahertz wave lens and method for producing terahertz wave lens |
| CN113267913A (en) * | 2021-05-29 | 2021-08-17 | 枣庄学院 | Metamaterial modulator |
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