CN114408912B - Preparation method of graphene deflection terahertz zone plate with low reflection - Google Patents

Preparation method of graphene deflection terahertz zone plate with low reflection Download PDF

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CN114408912B
CN114408912B CN202111645894.6A CN202111645894A CN114408912B CN 114408912 B CN114408912 B CN 114408912B CN 202111645894 A CN202111645894 A CN 202111645894A CN 114408912 B CN114408912 B CN 114408912B
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terahertz
zone plate
graphene oxide
deflection
ring structure
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文岐业
锁易昕
李懿航
武毓
黄浩铭
杨青慧
张怀武
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University of Electronic Science and Technology of China
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    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/182—Graphene
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    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
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    • C01B32/182—Graphene
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    • H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
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Abstract

A preparation method of a graphene deflection type terahertz wave zone plate with low reflection belongs to the technical field of terahertz waves. According to the invention, the reduced graphene oxide similar to a Fresnel ring is formed on graphene oxide by adopting a laser direct writing technology, and the obtained terahertz zone plate is a structure formed by alternately forming graphene oxide and reduced graphene oxide, and the thicknesses of the reduced graphene oxide and the reduced graphene oxide are similar. When the terahertz zone plate is applied, an incident terahertz wave beam passes through the zone plate, the incident wave beam is shielded by the reduced graphene oxide pattern part, and the graphene oxide pattern part diffracts, so that the propagation direction of the wave beam is changed, and the functions of focusing and deflecting the terahertz wave are realized; the thickness of the obtained terahertz zone plate is only 14-20 mu m, which is beneficial to integration of terahertz communication and an imaging system; the reflectivity is as low as 10%, the return loss is small, and the stability of an applied terahertz system can be effectively improved.

Description

Preparation method of graphene deflection terahertz zone plate with low reflection
Technical Field
The invention belongs to the technical field of terahertz waves, and particularly relates to a preparation method of an ultrathin flexible low-reflection graphene deflection zone plate applied to terahertz wave bands.
Background
In recent years, terahertz technology has been successfully applied to a plurality of fields such as security inspection imaging, security monitoring, biological physiotherapy detection, nondestructive detection, high-frequency communication and the like. With the rapid development of terahertz technology in these fields, particularly in the fields of terahertz imaging and communication, the demand for terahertz beam deflection and focusing devices is increasing. The terahertz wave beam deflection and focusing device is an important component for realizing functions such as wave beam shaping deflection in a terahertz imaging and communication system, and the traditional terahertz imaging and communication system usually uses a lens and an off-axis parabolic mirror to realize wave beam deflection and focusing, but is limited by thickness and hardness, and cannot meet the current requirements on integration and miniaturization of the terahertz system.
The zone plate is a common optical device for realizing deflection and focusing functions, is often applied to a plurality of fields such as space optics, image recognition, optical communication and the like, and is applied to microwave, millimeter wave and terahertz wave bands in many researches in recent years. Compared with a lens, the thickness of the zone plate is reduced, and the zone plate is more suitable for an integrated system. However, the existing zone plate is generally a focusing zone plate, the deflection function cannot be realized, and the pattern part of the zone plate reflects incident waves highly. The stronger reflected wave is reflected back to the source of the emission and can have an impact on the system. In addition, the existing zone plate often adopts a hard substrate, and is not suitable for various application scenes. The Japanese scientific research team proposed a metallic Fresnel zone plate (document Han, ZL. Terahertz Fresnel-zone-plate film lens based on double-layer metamaterial phase shifter) combined with a super surface in 2020, realizing the function of beam focusing, with a thickness of 80 μm and being a hard substrate. The thickness is larger, the integration is not suitable, the bending is not easy, the metal material has a complete reflection effect on terahertz waves, the performance is not improved, only the focusing effect is realized, and the deflection effect is not realized. The university of Beijing university Hu Bin in 2020 proposed the use of polyimide-induced graphene (Wang ZY. Patterned laser-induced graphene for terahertz wave modulation), which is also about 80 μm thick, to induce complete reflection of the incident beam and generally requires fabrication and processing on substrate materials such as silicon, quartz, and the like. The ultra-thin flexible low-reflection substrate-free zone plate for the terahertz wave band is to be researched and developed in combination with the problems of high reflection, large thickness, high manufacturing cost, inconvenience for integration based on a hard substrate and the like of the existing terahertz zone plate.
Disclosure of Invention
The invention aims to provide a preparation method of a terahertz zone plate with low reflection, small thickness, no substrate and flexible deflection aiming at the problems of high reflection, large thickness, high manufacturing cost, inconvenience for integration based on a hard substrate and the like of the existing zone plate in the background technology.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
the preparation method of the graphene deflection terahertz zone plate with low reflection is characterized by comprising the following steps of:
step 1, preparing a graphene oxide film;
step 2, designing a square Fresnel-like ring structure;
2.1, assuming that the working frequency of the terahertz wave zone plate is f, the deflection angle is theta, the focal length is l, and the radius of the cross section of the incident terahertz wave is r;
2.2 determining the side length d of a square Fresnel-like ring structure:
Figure BDA0003445126430000021
2.3 designing N eccentric circles with centers on a straight line, namely 1 st, 2 nd, … th, N, … th and N eccentric circles from inside to outside in sequence, and assuming that the coordinates of the N eccentric circles are
Figure BDA0003445126430000022
Wherein (1)>
Figure BDA0003445126430000023
Figure BDA0003445126430000024
For the phase difference of two adjacent eccentric circles, it is usually pi +.>
Figure BDA0003445126430000025
c is the speed of light; assume that the radius of the nth eccentric circle is
Figure BDA0003445126430000026
2.4, regarding the N eccentric circles with the centers on the same straight line obtained in the step 2.3, taking the eccentric circles at the odd positions from inside to outside as the inner side of the endless belt and taking the eccentric circles at the even positions as the outer side of the endless belt to obtain an endless belt structure; intercepting the annular belt structure by adopting a square with the side length of d to obtain a square Fresnel-like ring structure, wherein the distance between the center of the square Fresnel-like ring structure and the center of the 1 st eccentric circle is
Figure BDA0003445126430000027
One side of the square Fresnel-like ring structure is parallel to the straight line where the circle centers of the N eccentric circles are positioned;
and 3, reducing the graphene oxide film obtained in the step 1 by adopting a laser direct writing technology to obtain reduced graphene oxide with a Fresnel ring structure of the step 2, and thus the preparation of the terahertz zone plate can be completed.
Further, the graphene oxide film prepared in the step 1 can be prepared by adopting a modified Hummer method, a Tang-Lau method and the like, and the thickness of the prepared graphene oxide film is 14-20 mu m.
Further, in the laser direct writing process in step 3, CO with a wavelength of 350nm is adopted 2 The laser has scanning speed of 700-800 mm/s, current of 1A, scanning frequency of 30-35 KHz and pulse width of 15-20 mu s.
According to the preparation method of the low-reflection graphene deflection type terahertz zone plate, the terahertz zone plate is of a structure formed by graphene oxide and reduced graphene oxide alternately, and the thickness of Graphene Oxide (GO) is similar to that of Reduced Graphene Oxide (RGO). When the incident terahertz beam reaches the zone plate, the wave acting on the graphene oxide pattern portion is almost fully transmitted, and the wave acting on the reduced graphene oxide pattern portion is largely absorbed. Thus, the reflectivity of the entire zone plate is very low, whether due to high transmittance or strong absorptivity. The terahertz wave passing through the graphene oxide pattern part is diffracted, so that the propagation direction of the wave beam is changed, and the functions of focusing and deflecting the terahertz wave are realized. Meanwhile, the alternating structure of the oxidation part and the reduction part also ensures that the device obtains good self-supporting capability under the ultra-thin condition, and gets rid of the limitation that the traditional zone plate needs a hard supporting material.
Compared with the prior art, the invention has the beneficial effects that:
1. according to the preparation method of the graphene deflection type terahertz zone plate with low reflection, provided by the invention, the reduced graphene oxide similar to a Fresnel ring is formed on graphene oxide by adopting a laser direct writing technology, and the thickness of the obtained terahertz zone plate is only 14-20 mu m, so that the integration of terahertz communication and an imaging system is facilitated; the reflectivity is as low as 10%, the return loss is small, and the stability of an applied terahertz system can be effectively improved; and a hard substrate is not needed, so that a favorable support is provided for the research and development of the flexible terahertz zone plate.
2. According to the preparation method of the graphene deflection type terahertz zone plate with low reflection, the reduced graphene oxide with the square Fresnel-like ring structure is designed, and the deflection effect on the terahertz wave beam propagation direction is achieved by utilizing the transmissivity difference between the reduced graphene oxide and the graphene.
3. According to the preparation method of the graphene deflection type terahertz zone plate with low reflection, the graphene oxide film is directly reduced to the reduced graphene oxide by adopting a laser direct writing technology, the reduced graphene oxide has a good absorption and shielding effect on terahertz waves, a mask plate is not needed in the process, the preparation process is simple, the processing time is short, and the preparation method is suitable for large-area high-flux device quantitative processing production.
Drawings
Fig. 1 is a schematic structural diagram of a graphene deflection terahertz zone plate with low reflection provided by the invention;
fig. 2 is a design pattern (upper left) and sample real image (lower left) of a zone plate deflected by 12 degrees obtained in example 2, and a design pattern (upper right) and sample real image (lower right) of a zone plate deflected by 7 degrees obtained in example 1;
FIG. 3 is a graph showing the reflectivity of a 7 degree deflected zone plate obtained in example 1, a 12 degree deflected zone plate obtained in example 2, and a conventional PCB zone plate;
fig. 4 is a graph showing the electric field intensity distribution of terahertz waves in the horizontal direction of the graphene oxide film of example 1;
FIG. 5 is a graph showing the electric field intensity distribution of a 7 degree deflection zone plate obtained in example 1;
fig. 6 is a plot of the horizontal electric field intensity profile of a zone plate deflected 12 degrees from example 2.
Detailed Description
The invention will be described in further detail with reference to the drawings and examples.
Example 1
A preparation method of a graphene deflection terahertz zone plate with low reflection specifically comprises the following steps:
step 1, preparing a graphene oxide film: preparing a graphene oxide film with the thickness of 20 mu m by adopting an improved Hummer method;
step 2, designing a square Fresnel-like ring structure;
2.1, assuming that the working frequency of the terahertz zone plate is f=0.34 THz, the deflection angle is θ=7°, the focal length is l=50 mm, the radius of the cross section of the incident terahertz wave is r=7 mm, and the side length d=20 mm of the square fresnel-like ring structure;
2.2, designing 7 eccentric circles with centers on a straight line, wherein the eccentric circles are sequentially recorded as 1 st, 2 nd, … th and 7 th eccentric circles from inside to outside, and the coordinates and the radius of the centers of the 7 eccentric circles are shown in the following table:
Figure BDA0003445126430000041
Figure BDA0003445126430000051
/>
2.3, regarding the 7 eccentric circles with the centers on the same straight line obtained in the step 2.2, taking the eccentric circles at the odd positions from inside to outside as the inner side of the endless belt and taking the eccentric circles at the even positions as the outer side of the endless belt to obtain an endless belt structure; intercepting the ring belt structure by adopting a square with the side length d=20mm to obtain a square Fresnel-like ring structure, wherein the distance between the center of the square Fresnel-like ring structure and the circle center of the 1 st eccentric circle is 6.19mm, and one side of the square Fresnel-like ring structure is parallel to the straight line where the circle centers of the 7 eccentric circles are located;
and 3, reducing the graphene oxide film obtained in the step 1 by adopting a laser direct writing technology to obtain reduced graphene oxide with a Fresnel ring structure of the step 2, and thus the preparation of the terahertz zone plate can be completed. In the laser direct writing process, CO with the wavelength of 350nm is adopted 2 The laser has a scanning speed of 750mm/s, a current of 1A, a scanning frequency of 30KHz and a pulse width of 20 mu s.
Fig. 1 is a schematic structural diagram of a graphene deflection terahertz zone plate with low reflection provided by the invention; as can be seen from fig. 1, the terahertz zone plate obtained in the invention has a structure formed by graphene oxide and reduced graphene oxide alternately, and the reduced graphene oxide and the graphene oxide have similar thicknesses. The incident terahertz wave beam passes through the zone plate, the incident wave beam is shielded by the reduced graphene oxide pattern part, and the oxidized graphene pattern part is diffracted, so that the propagation direction of the wave beam is changed, and the functions of focusing and deflecting the terahertz wave are realized.
Example 2
A preparation method of a graphene deflection terahertz zone plate with low reflection specifically comprises the following steps:
step 1, preparing a graphene oxide film: preparing a graphene oxide film with the thickness of 20 mu m by adopting an improved Hummer method;
step 2, designing a square Fresnel-like ring structure;
2.1, assuming that the working frequency of the terahertz zone plate is f=0.34 THz, the deflection angle is θ=12°, the focal length is l=50 mm, the radius of the cross section of the incident terahertz wave is r=7 mm, and the side length d=20 mm of the square fresnel-like ring structure;
2.2, 10 eccentric circles with centers on the same straight line are designed, namely 1 st, 2 nd, … th, 10 eccentric circles are sequentially recorded from inside to outside, and the coordinates and the radius of the centers of the 10 eccentric circles are shown in the following table:
Figure BDA0003445126430000052
Figure BDA0003445126430000061
2.3, regarding the 10 eccentric circles with the centers on the same straight line obtained in the step 2.2, taking the eccentric circles at the odd positions from inside to outside as the inner side of the endless belt and taking the eccentric circles at the even positions as the outer side of the endless belt to obtain an endless belt structure; intercepting the annular belt structure by adopting a square with the side length d=20mm to obtain a square Fresnel-like ring structure, wherein the distance between the center of the square Fresnel-like ring structure and the circle center of the 1 st eccentric circle is 6.88mm, and one side of the square Fresnel-like ring structure is parallel to the straight line where the circle centers of the 7 eccentric circles are located;
and 3, reducing the graphene oxide film obtained in the step 1 by adopting a laser direct writing technology to obtain reduced graphene oxide with a Fresnel-like ring structure, and thus the preparation of the terahertz zone plate can be completed. In the laser direct writing process, CO with the wavelength of 350nm is adopted 2 The laser has a scanning speed of 750mm/s, a current of 1A, a scanning frequency of 30KHz and a pulse width of 20 mu s.
FIG. 3 is a graph showing the reflectance contrast curves of a 7 degree deflected zone plate obtained in example 1, a 12 degree deflected zone plate obtained in example 2, and a conventional PCB zone plate (conventional PCB copper-clad plate having the same pattern as in example 1); the zone plate obtained in example 1, the zone plate obtained in example 2, and the conventional PCB zone plate (the conventional PCB copper-clad plate having the same pattern as in example 1) were respectively detected using a terahertz time-domain spectrometer, and the results are shown in fig. 3. As can be seen from fig. 3, the reflectivity of the terahertz zone plates obtained in embodiments 1 and 2 of the present invention is only about 10%, which is one fifth of the reflectivity of the conventional PCB zone plate with copper coated surface.
The graphene oxide film of example 1, the zone plate obtained in example 1 and the zone plate obtained in example 2 were tested by using a terahertz imaging system based on a 0.34THz terahertz emission source, and an electric field intensity distribution diagram was obtained by scanning images with a step length of 1mm, as shown in fig. 4 to 6, in which the white region is a portion with a strong field intensity and the black region is a portion with a weak field intensity. As can be seen from fig. 4 to 6, the terahertz zone plate of the 7 degree deflection obtained in example 1, the beam center of the scanned image is shifted to the left of 5mm, the deflection angle is 5.7 ° (θ=arctan (offset/focal distance) =5.7 °); the beam center of the scanned image of the zone plate deflected by 12 degrees obtained in example 2 was shifted to the left by 9mm and deflected by 10.2 °. The experimental data and the simulation data are basically consistent, and the requirement of calculating the deflection angle is met.

Claims (3)

1. The preparation method of the graphene deflection terahertz zone plate with low reflection is characterized by comprising the following steps of:
step 1, preparing a graphene oxide film;
step 2, designing a square Fresnel-like ring structure;
2.1, assuming that the working frequency of the terahertz wave zone plate is f, the deflection angle is theta, the focal length is l, and the radius of the cross section of the incident terahertz wave is r;
2.2 determining the side length d of a square Fresnel-like ring structure:
Figure FDA0004146012130000011
2.3 designing N eccentric circles with centers on a straight line, namely 1 st, 2 nd, … th, N, … th and N eccentric circles from inside to outside in sequence, and assuming that the coordinates of the N eccentric circles are
Figure FDA0004146012130000012
Wherein (1)>
Figure FDA0004146012130000013
Figure FDA0004146012130000014
For the phase difference of two adjacent eccentric circles, +.>
Figure FDA0004146012130000015
c is the speed of light; assume that the radius of the nth eccentric circle is
Figure FDA0004146012130000016
2.4, regarding the N eccentric circles with the centers on the same straight line obtained in the step 2.3, taking the eccentric circles at the odd positions from inside to outside as the inner side of the endless belt and taking the eccentric circles at the even positions as the outer side of the endless belt to obtain an endless belt structure; intercepting the annular belt structure by adopting a square with the side length of d to obtain a square Fresnel-like ring structure, wherein the distance between the center of the square Fresnel-like ring structure and the center of the 1 st eccentric circle is
Figure FDA0004146012130000017
One side of the square Fresnel-like ring structure is parallel to the straight line where the circle centers of the N eccentric circles are positioned;
and 3, reducing the graphene oxide film obtained in the step 1 by adopting a laser direct writing technology to obtain reduced graphene oxide with a Fresnel ring structure of the step 2, and thus the preparation of the terahertz zone plate can be completed.
2. The method for preparing the low-reflection graphene deflection terahertz zone plate according to claim 1, wherein the thickness of the graphene oxide film prepared in the step 1 is 14-20 μm.
3. The method for preparing the low-reflection graphene deflection terahertz zone plate according to claim 1, wherein in the laser direct writing process in step 3, CO with the wavelength of 350nm is adopted 2 The laser has scanning speed of 700-800 mm/s, current of 1A, scanning frequency of 30-35 KHz and pulse width of 15-20 mu s.
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US20130146221A1 (en) * 2011-12-13 2013-06-13 Southern Illinois University Carbondale Graphene-based membranes as electron transparent windows for ambient pressure x-ray photoelectron spectroscopy
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