CN103984125A - Grapheme based electronically controlled terahertz attenuation piece, preparation method and utilization method - Google Patents

Abstract

The invention discloses a grapheme based electronically controlled terahertz attenuation piece, a preparation method and a utilization method. The attenuation piece comprises a substrate; at least two layers of grapheme layers are paved on the substrate; a layer of medium layer is paved between every two adjacent layers of grapheme layers; the mediums layers separate the two layers of grapheme layers. According to the grapheme based electronically controlled terahertz attenuation piece, the regulation range of device attenuation degrees can be changed through different layers of the grapheme, a high attenuation degree is achieved theatrically, the attenuation degree parameters can be controlled due to control of the attenuation degree of the attenuation piece through voltage, and the application accuracy of the attenuation piece products is high and flexible.

Description

Automatically controlled Terahertz attenuator, preparation method and using method based on Graphene

Technical field

The invention belongs to terahertz wave band device technology field, specifically a kind of automatically controlled Terahertz attenuator, preparation method and control method based on Graphene.

Background technology

Attenuator can utilize the absorption characteristic of material to light wave, thereby place it in light path, light intensity attenuation can be reached to the effect of control inputs light intensity.Light is relevant with the material category of attenuator by the ability after attenuator, also with the relating to parameters such as thickness of material.At present, the attenuator of optical region is mainly the metallic diaphragm that utilizes thickness linear gradient, thereby it is adjustable to reach machinery, and the attenuator that electricity is adjusted is more rare.On the other hand, when light beam be not monochromatic light but contain different wave length polychromatic light time, we wish that light passes through after this attenuator conventionally, different wave length is decay at equal pace all, namely will reach the effect of broadband work.

In recent years, Terahertz Technology showed wide application prospect in fields such as broadband connections, radar, electronic countermeasure, ELECTROMAGNETIC WEAPON, astronomy, Non-Destructive Testing, medical imaging, safety inspections.From its definition, Terahertz (Terahertz is called for short THz) is an electromagnetic cps, and 1 Terahertz equals 10 ¹²hertz.Conventionally we become THz wave the electromagnetic wave of 0.3THz～10THz (0.03mm is to 1mm).THz wave has higher spatial resolution (high-frequency) and temporal resolution (picopulse), energy is less can not destroy material, and the vibration of some biomacromolecules, rotate resonance level just in time at terahertz wave band, these characteristics have been established the basis of Terahertz as following spectrum and imaging applications.Before the eighties, be subject to the restriction that THz wave produces source and detector, relate to the research of this wave band and apply considerably less.Along with the development of new technology (ultrafast technology), new material, Terahertz Technology is developed rapidly.At present, the research of Terahertz Technology and application and development are just becoming the focus of optical field research.For example, yet high performance novel THz devices (emissive source, detector, modulator, beam splitter, dispersing optics element, attenuator etc.) is urgently design still, to improve the application efficiency of Terahertz Technology.

At present, attenuator is as the very important device of optical region, and its major function is carried out control necessary, special ratios to strong beam.Yet in Terahertz Technology field, a difficult problem is the acquisition of high-intensity Terahertz wave source, this makes Terahertz attenuator at present not yet obtain more application and enough attention.But along with developing rapidly of Terahertz Technology, the particularly development of thz laser device in recent years, Terahertz attenuator element has demand widely.

The material require of making high performance adjustable Terahertz attenuator meets following conditions: first, the conductivity of material must be to the sensitive response of THz wave, like this, for example, by changing device parameters (thickness, the number of plies), we could obtain different attenuation degrees.Particularly, for the requirement of the high dough softening, only have the Terahertz conductivity of material enough large, could be in the situation that guarantee that the very thin thickness of attenuator realizes.The second, must in wide THz wave segment limit, there is more consistent conductivity.This is because in the application of a lot of terahertz wave bands, terahertz signal is broadband, terahertz time-domain spectroscopy system for example, and this just requires Terahertz attenuator preferably can be operated in wider terahertz wave band.The 3rd, the material of making attenuator should possess adjustable conductivity.Because at this moment the dough softening of attenuating elements can regulate and control, range of application and the efficiency of a device will be expanded.Particularly, by automatically controlled attenuator, by being conducive to, better combine with existing Terahertz system.The 4th, the necessary calorifics of this material, stable chemical nature, do not vary with temperature, and being difficult to by number of chemical material corrosion (as commonly used chemical reagent etc.) is to guarantee the long-term non-maintaining condition of antireflection film; Finally, mechanical property is good, in proof strength, can have pliability.At present, existing traditional metal and oxide semiconductor material etc., be still not enough to realize completely above-mentioned requirements.

As a kind of emerging Two-dimensional Carbon nano material, Graphene has good mechanical strength and pliability, high coefficient of heat conductivity, stable chemical property, and excellent, unique photoelectric properties.Aspect photoelectric property, due to the linear dirac taper dispersion relation of Graphene uniqueness, it possesses high carrier mobility, unique ambipolar electric field effect.Ambipolar electric field effect refers to that carrier concentration and the Fermi level in Graphene can change under the effect of grid voltage, thereby changes the conductivity of the terahertz wave band of Graphene.At terahertz wave band, the conductivity of Graphene is obeyed De Lude (Drude) model.Study and show in Terahertz frequency range, the photoconductive dispersivity of Graphene is very little, and the variation with frequency changes hardly.Therefore with regard to photoelectric characteristic, Graphene has the Terahertz conductivity that broadband is stable, grid voltage is adjustable.

More than introduced the Principle and application present situation of attenuator, the element application present situation of THz wave, the requirement of Terahertz attenuator to material, and the characteristic of Graphene.Can find out, current effectively terahertz wave band attenuator shortcoming very also, existing method and material in various degree faced that working frequency range is narrow, poor stability, the defect such as untunable.And grapheme material possesses good power, heat, photoelectric characteristic, particularly possess broadband and tunable potential quality, therefore can be used for the preparation of the tunable attenuator of terahertz wave band.

Summary of the invention

For the defect existing in above-mentioned prior art or deficiency, one object of the present invention is, a kind of automatically controlled Terahertz attenuator based on Graphene is provided.

The described automatically controlled Terahertz attenuator based on Graphene comprises substrate, is successively equipped with at least two-layer graphene layer in substrate, between every two-layer adjacent graphene layer, is covered with one deck dielectric layer, and described dielectric layer is separated this two-layer graphene layer.

Further, according to different service conditions, the position relationship of described graphene layer and substrate: (1), for transmission THz wave, graphene layer is positioned at the front of substrate; (2), for transmission THz wave, graphene layer is positioned at the reverse side of substrate; (3) for reflected terahertz ripple hereby, graphene layer is positioned at the front of substrate; The front that the face that first regulation light incident direction arrives is substrate.

Further, described graphene layer is 2～6 layers.

Further, the computing formula of the dullness of described attenuator is as follows:

$X=1-{|t\·t_{\mathrm{sa}}\·\frac{h_{\mathrm{sub}}(\mathrm{\ω},d_{\mathrm{sub}})}{h_{\mathrm{air}}(\mathrm{\ω},d_{\mathrm{sub}})}\·\frac{1}{1-r\·r_{\mathrm{sa}}{h_{\mathrm{sub}}}^2(\mathrm{\ω},d_{\mathrm{sub}})}|}^2;$

In formula, the transmission coefficient t of medium and air interface _sa=2n _sub/ (n _sub+ n _air), the reflection coefficient r of medium and air interface _sa=(n _sub-n _air)/(n _sub+ n _air), h _sub(ω, d _sub)=exp (i ω d _subn _sub/ c) for refractive index is n _sub, thickness is d _subsubstrate in THz wave transmission factor, h _air(ω, d _sub)=exp (i ω d _subn _air/ c) for refractive index is n _air, thickness is d _subairborne THz wave transmission factor; ω is angular frequency, and c is the light velocity; T and r are respectively transmission, the reflection coefficients of attenuator, are determined respectively by following formula:

t＝4n ₁n ₃/{[( _Y1+Y ₂)n ₃+(Y ₁-Y ₂)(n ₂+Z ₀σ)](n ₁+n ₃+Z ₀σ)exp(ik ₃d)

-[(Y ₃+Y ₄)n ₃+(Y ₃-Y ₄)(n ₂+Z ₀σ)](n ₃-n ₁-Z ₀σ)exp(-ik ₃d)}；

$\left(\begin{array}{cc}{Y}_1& Y_2\\ Y_3& Y_4\end{array}\right)={\left(\begin{array}{cc}(1+\frac{Z_0\mathrm{\σ}}{2n_3})\exp \left({\mathrm{ik}}_{3}d\right)& \frac{Z_0\mathrm{\σ}}{2n_3}\exp (-{\mathrm{ik}}_{3}d)\\ -\frac{Z_0\mathrm{\σ}}{2n_3}\exp \left({\mathrm{ik}}_{3}d\right)& (1-\frac{Z_0\mathrm{\σ}}{2n_3})\exp (-{\mathrm{ik}}_{3}d)\end{array}\right)}^{N-2};$

r＝1-t；

In formula, i represents the imaginary part of symbol, N>=2nd, the number of plies of graphene layer in attenuator; n ₁and n ₂it is respectively the medium refraction index on N layer graphene both sides; Z ₀vacuum impedance, n ₃with d is refractive index and the thickness of spacer dielectric layer between Graphene, k ₃=ω n ₃/ c is wave vector wherein; σ is the Terahertz sheet conductance of each single-layer graphene, by following formula, is calculated:

|N _c|＝7.5×10 ¹⁰·|V _g-V _CNP|cm ^-2V ^-1；

In formula, ω is angular frequency, v _fit is the Fermi velocity of charge carrier in Graphene; E and respectively elementary charge, Planck's constant; Γ and N _crespectively carrier scattering rate and the carrier concentration in Graphene; V _gthe actual voltage adding, V _cNPrefer to the voltage that reaches Graphene sample electric neutrality point, V _g-V _cNPby being added grid voltage.

Further, the several adjustable extents according to needed attenuator dullness of described graphene layer are determined: the dullness of 2 layer graphene attenuators is adjustable continuously in 0.14～0.65 scope, the dullness of 3 layer graphene attenuators is adjustable continuously in 0.19～0.76 scope, the dullness of 4 layer graphene attenuators is adjustable continuously in 0.25～0.82 scope, the dullness of 5 layer graphene attenuators is adjustable continuously in 0.30～0.86 scope, and the dullness of 6 layer graphene attenuators is adjustable continuously in 0.34～0.90 scope.

Further, the area of described graphene layer is greater than the area of Terahertz hot spot, is not less than 1cm ².

Further, described substrate adopts quartzy, and described dielectric layer adopts silicon dioxide, boron nitride or alundum (Al2O3); Described electrode adopts metal or alloy electrode.

Another object of the present invention is, a kind of preparation method of the automatically controlled Terahertz attenuator based on Graphene is provided, comprise the steps: according to order from the bottom to top, successively to prepare each layer of structure in substrate, an electrode that have at least two-layer graphene layer in substrate, the Graphene of the common electrode being connected and odd-level is connected jointly on the dielectric layer between every two adjacent graphene layers, even level graphene layer, by on-load voltage on two electrodes; Each dielectric layer is separated adjacent graphite linings.

Further, the preparation of described graphene layer adopts chemical gas-phase method, mechanical stripping method, epitaxial growth method or oxidation-reduction method; The preparation of described dielectric layer adopts vacuum vapour deposition, spin coating method or chemical vapour deposition technique; The preparation of described electrode adopts vacuum vapour deposition, vacuum sputtering, chemical vapour deposition technique or electrochemical deposition method.

Another object of the present invention is, a kind of using method of the automatically controlled Terahertz attenuator based on Graphene is provided, for the fixing attenuator of the number of plies, by on being positioned at electrode that even level connects and being positioned at the electrode that odd-level graphene layer is connected, add constant voltage, voltage is converted to 80V from 0V, along with alive rising, observe the amplitude of main pulse, until the amplitude of main pulse disappears, now added magnitude of voltage is defined as meeting the voltage of the dough softening demand of attenuator; Different according to service condition, the position relationship of described graphene layer and substrate: (1), for transmission THz wave, graphene layer is positioned at the front of substrate; (2), for transmission THz wave, graphene layer is positioned at the reverse side of substrate; (3) for reflected terahertz ripple hereby, graphene layer is positioned at the front of substrate; The front that the face that first regulation light incident direction arrives is substrate.

The present invention has the following advantages:

1, the present invention is based on grapheme material and make Terahertz attenuator, due to grapheme material, have the characteristics such as good power, heat, optical, electrical, so attenuator possesses following advantage:

A, possesses good mechanical strength.If choice of the substrates flexible material, when selecting flexible media layer (as two-dimentional boron nitride etc.), can make attenuator possess flexible application so.

The thermal conductivity of B, Graphene is good, and chemical stability is high, has guaranteed serviceable life and the scope of application of attenuator.

C, Graphene have broadband conductivity at terahertz wave band, guarantee the broadband working range (same attenuator is applicable to different Terahertz communication windows) of attenuator, and can be directly used in broadband terahertz pulse (as for terahertz time-domain spectroscopy).

The ambipolar electric field effect of D, Graphene, makes attenuator can be subject to regulating and controlling voltage.Expanded the range of application of attenuator, application has dirigibility and sensitivity etc.In addition, automatically controlled characteristic makes attenuator be easy to be combined with prevailing system.

2, the present invention is sandwich construction, can by the different numbers of plies of Graphene, change the modification scope of the device dough softening, has realized in theory high attenuation degree.By regulating and controlling voltage, can change arbitrarily attenuation parameter, application degree of accuracy is higher, more flexible.

3, the grapheme material preparation that the present invention uses is easier to, and cost is lower, and repeatability is high, and can realize by optimizing preparation scheme the production of large area high yield.The range of choice of dielectric layer is wider, and cost and processing scheme are easily controlled.The method of preparing Terahertz attenuator has simply, is easy to the advantage of processing equally.

Accompanying drawing explanation

Fig. 1 is the structural representation of the Terahertz attenuator of 2 graphene layers formations.

Fig. 2 is the structural representation of the Terahertz attenuator of 4 graphene layers formations.

Fig. 3 is the principle schematic after adjacent two graphene layer making alives.Wherein, Fig. 3 (a) is the principle schematic after adjacent two graphene layer making alives, and Fig. 3 (b) and Fig. 3 (c) are respectively electron adulterated and energy level variations schematic diagram hole doping Graphene.

Fig. 4 draws according to the De Lude model diagometer of Graphene: (a) single-layer graphene Fermi level is with the curve of change in voltage; (b) curve that Terahertz conductivity changes with institute's making alive.

Fig. 5 is the Transflective principle schematic of the Terahertz attenuator of N graphene layer formation.

Fig. 6 is the variation range of Graphene attenuator THz wave dullness when the different Graphene number of plies.

Fig. 7 is respectively 2,4,6 when graphene layer number, and it is in 1.5 substrate time that attenuator is positioned at refractive index, and THz wave dullness is with the curve of change in voltage.

Fig. 8 is the automatically controlled attenuator consisting of 6 graphene layers, is positioned at refractive index and is in 1.5 substrate front surface, and in transmission situation, terahertz time-domain spectroscopy is with the variation of regulation and control voltage.

Below in conjunction with the drawings and specific embodiments, the present invention is further explained.

Embodiment

Main thought of the present invention is: the Graphene of separating with dielectric layer is as the basic structure of attenuator, the photoconductivity of utilizing the different Graphene numbers of plies and being carried in the regulating and controlling voltage change Graphene at two ends, thus realize regulation and control transmitted light wave, reflected light wave amplitude.In decay application, the number of plies of Graphene, voltage have determined the highest THz wave dough softening that can realize, therefore to the selection of the Graphene number of plies and fixedly the voltage-regulation of the attenuator of the Graphene number of plies study.

One, the selection of parts

Substrate: first, substrate should have enough mechanical strengths, and the support of Graphene can be provided, and evenly smooth; Secondly, substrate self should be the material that does not absorb THz wave, to reduce extra loss; Finally, preferably the little material of terahertz wave band refractive index, reduces reflection loss.To sum up, substrate can be selected quartz or partially flexible polymkeric substance etc.

Graphene layer: area and the number of plies are mainly considered in the selection of graphene layer.The area of graphene layer is greater than the area of Terahertz hot spot.The THz wave that is 0.3THz for minimum frequency, wavelength is 1mm, considers the installation of the materials such as tunability in application and electrode, the best area of graphene layer is greater than 1cm ².The selection of the Graphene number of plies is decided by the maximum attenuation degree of depth that in application, hope reaches.For the lower situation of required fading depth, the preferred less but enough situations of the number of plies, the sensitivity in being conducive to regulate and control and the raising of precision, the number of plies is at least 2; The situation of having relatively high expectations for fading depth, the number of plies must be abundant, determines that the parameter of the device regulating and controlling voltage upper limit is carrier concentration (Fermi level) limit in Graphene, for example, in current a lot of researchs, the carrier concentration of Graphene can reach 1 * 10 ¹³cm ^-2, exceed this limit, then improve total electricity that voltage can not further improve device and lead, at this moment must use more multi-layered Graphene.

In attenuator, part graphene layer is used other semiconductor materials as alternative in gallium arsenide, Aluminum gallium arsenide, tin indium oxide etc., or increases barrier layer and transition bed etc., but can not change attenuator structural principle.

If while making the Terahertz attenuator of fixed attenuation degree, graphene layer can be used Graphene derivant material, for example, graphene oxide after reducing.At this moment, the thickness of material determines the size of conductivity.

Dielectric layer: the physical parameter of dielectric layer is mainly considered its thickness and specific inductive capacity.For fear of interference effect, the thickness of dielectric layer must be far smaller than the wavelength (0.03mm～1mm) of THz wave, and thickness is recommended to be less than 1 μ m, is preferably less than 100nm.The material of dielectric layer is insulating material or the wide bandgap semiconductor materials of high-k, as silicon dioxide, boron nitride or alundum (Al2O3) etc.

If while making the Terahertz attenuator of fixed attenuation degree, dielectric layer is as optional layer, i.e. working medium layer not.

Electrode: electrode material selects height to conduct electricity, adhesion is high, the membrane material of good stability.Metal and alloy electrode material that preferred inoxidizability and corrosion resistivity are good.

If make the Terahertz attenuator of fixed attenuation degree, need to be on graphene layer connecting electrode making alive.

Below we by theoretical analysis in conjunction with testing number of plies selection principle and the alive adjusting parameter area of determining Graphene Terahertz attenuator.

Two, theoretical analysis

As shown in Figure 3, adjacent graphene layer is after making alive, an accumulation electronics becomes electron adulterated, another loses electronics and becomes hole doping, because grapheme material has the excellent photoelectric property of uniqueness that is different from conventional semiconductor material, its electron mobility and hole mobility are approximately uniform, so two kinds of doping behaviors can improve the carrier concentration in Graphene, further make the conductivity values of terahertz wave band promote, thereby affect the attenuating of resulting devices.What Fig. 3 (b) and Fig. 3 (c) showed respectively is electron adulterated and energy level variations schematic diagram hole doping Graphene.In Fig. 3 (b), Fermi level is greater than Fermi level in 0, Fig. 3 (c) and is less than 0, yet in the ideal case, both Fermi level absolute values equate, be positive and negative contrary, so carrier concentration are also identical.Therefore, as long as we derive to send out from single-layer graphene electricity, reasonably control, the final performance parameter that just can realize attenuator is the control of the dough softening.Net result is relevant by the number of the conductivity with every layer graphene in attenuator and graphene layer, and the former is because alive relation is therefore relevant with voltage.Therefore, next will carry out the analysis of single-layer graphene Terahertz conductivity and voltage relationship, and the analysis of the total transmission coefficient of multi-layer graphene.Based on this, can draw the final relation of the dough softening and voltage, the Graphene number of plies.

Fig. 4 has provided the Terahertz electricity of the single-layer graphene calculating and has led the theoretical curve changing with institute's making alive.The Terahertz conductivity of single-layer graphene has been proved to be obeys De Lude model:

Wherein, ω is angular frequency, v _fthe Fermi velocity of charge carrier in Graphene, e and respectively elementary charge, Planck's constant, Γ and N _crespectively carrier scattering rate and the carrier concentration in Graphene.V _gthe actual voltage adding, V _cNPrefer to the voltage that reaches Graphene sample electric neutrality point, V _g-V _cNPby being added grid voltage.Note Fermi level E _fwith carrier concentration N _cthere is following relation: in formula, positive sign represents electron adulteratedly, and negative sign represents hole doping.Fermi velocity in Graphene is generally v _f=1.1 * 10 ⁶m/s.Under common service condition, ambient temperature T=300K.Here in order to do the example of a result of calculation, in above-mentioned parameter, Γ is made as not the constant 100cm with change in voltage ^-1(span of CVD sample conventionally).Like this, for the conductivity of calculating single-layer graphene, only need get different Fermi level (or carrier concentration) can calculate.For the impact of voltage is discussed, provide a carrier concentration and modulation voltage V below _g-V _cNPrelation: | N _c|=7.5 * 10 ¹⁰| V _g-V _cNP| cm ^-2v ^-1, note V in formula _gthe actual voltage adding, V _cNPrefer to the voltage that reaches Graphene electric neutrality point (Fermi surface is in dirac point).Above-mentioned parameter is only for setting forth result trend of the present invention, and in actual sample, the value of parameter should be subject to many-sided impact such as preparation method, jump condition, device fabrication method, device formation, and actual value should be obeyed the actual conditions of sample.The conductivity of Graphene is one, and with the less value of frequency change, we take 1THz as example, from Fig. 4 (a), can see, are applying a modulation voltage V from 0V to 80V _g-V _cNPtime, the Fermi level of Graphene declines from 0eV to-0.315eV (if electron adulterated, corresponding to 0eV, to 0.315eV, rising), and at this moment carrier concentration is corresponding to rising to 6 * 10 from 0 ¹²cm ^-1v ^-1.From Fig. 4 (b), the Terahertz conductivity of single-layer graphene is from 0.2 * 10 ^-3to 1.8 * 10 ^-3Ω ^-1adjustable continuously.

Shown in Fig. 5 is the THz wave propagation principle schematic diagram of the attenuator that consists of N graphene layer 1.The number of plies of dielectric layer 2 is N-1, and multi-layer graphene is positioned between two kinds of different mediums 6 (substrate) and 7 (air).Medium 7 and 6 refractive index be corresponding n respectively ₁with n ₂, and n ₁< n ₂.The thickness of graphene layer is 0.335nm, can be left in the basket and also be regarded as the conductive layer of one deck zero thickness.Dielectric thickness is made as d, and refractive index is made as n ₃.The position of the graphene layer adjacent with medium 7 is made as 0 point on z direction of principal axis.Light transmission and the reflection coefficient of Graphene are expressed as:

$t_{\mathrm{gra}}=\frac{2n_j}{n_i+n_j+Z_0\mathrm{\&sigma;}},r_{\mathrm{gra}}=\frac{n_i-n_j-Z_0\mathrm{\&sigma;}}{n_i+n_j+Z_0\mathrm{\&sigma;}}---\left(2\right)$

In formula, n _iand n _jrepresent respectively the refractive index of graphene layer media of both sides, Z ₀=377 Ω are vacuum impedances, and σ is that the Terahertz electricity of each single-layer graphene is led (referring to formula (1)).

In Fig. 5, we suppose E _i, E _rand E _tbe respectively the electric field of incident, reflection and transmission THz wave, and attenuator at least forms (N>=2) by two layer graphenes.The equation of the ripple in medium 7 (z < 0) and medium 6 (z > (N-1) d, d is dielectric thickness) can be made as E so _i+ E _r=exp (ik ₁z)+rexp (ik ₁z) and E _t=texp{-ik ₂[z-(N-1) d] }, wherein r and t are reflection and transmission coefficient, k ₁=ω n ₁/ c and k ₂=ω n ₂/ c is respectively the wave vector of THz wave in medium 1 and 2.Dielectric layer between graphene layer (n-1) d < z < nd (n=1,2 ..., N-1) in, wave equation is $E_n+E_n^{\&prime;}=A_n\exp [-{\mathrm{ik}}_3(z-\mathrm{nd})]+A_n^{\&prime;}\exp \left[{\mathrm{ik}}_3(z-\mathrm{nd})\right],$ K wherein ₃=ω n ₃/ c is the wave vector in dielectric layer, A _nwith it is corresponding wave amplitude.By separating the transmission matrix equation on interface, can obtain when light wave incident from medium 7, the transmission coefficient to medium 6 after the attenuator consisting of N layer graphene is:

t＝4n ₁n ₃/{[(Y ₁+Y ₂)n ₃+(Y ₁-Y ₂)(n ₂+Z ₀σ)](n ₁+n ₃+Z ₀σ)exp(ik ₃d)

-[(Y ₃+Y ₄)n ₃+(Y ₃-Y ₄)(n ₂+Z ₀σ)](n ₃-n ₁-Z ₀σ)exp(-ik ₃d)} (3)

$\left(\begin{array}{cc}{Y}_1& Y_2\\ Y_3& Y_4\end{array}\right)={\left(\begin{array}{cc}(1+\frac{Z_0\mathrm{\&sigma;}}{2n_3})\exp \left({\mathrm{ik}}_{3}d\right)& \frac{Z_0\mathrm{\&sigma;}}{2n_3}\exp (-{\mathrm{ik}}_{3}d)\\ -\frac{Z_0\mathrm{\&sigma;}}{2n_3}\exp \left({\mathrm{ik}}_{3}d\right)& (1-\frac{Z_0\mathrm{\&sigma;}}{2n_3})\exp (-{\mathrm{ik}}_{3}d)\end{array}\right)}^{N-2}---\left(4\right)$

r＝1-t (5)

In formula, i represents the imaginary part of symbol, n ₁and n ₂it is respectively the medium refraction index on N layer graphene both sides; Z ₀vacuum impedance, n ₃with d be refractive index and the thickness of dielectric layer, k ₃=ω n ₃/ c is the wave vector in dielectric layer, N>=2nd, and the number of plies of graphene layer in attenuator, σ is the Terahertz sheet conductance (referring to formula (1)) of each single-layer graphene.

Accordingly, light wave incides on device from medium 7, and the reflection coefficient in reflected back into medium 7 can be calculated by r=1-t.Simultaneous formula (1), (3), (4), can obtain saturating, the reflection coefficient on any number of plies Graphene both sides, thereby instruct attenuator design.

From above formula, thoroughly, the number of plies of reflection coefficient Graphene in the conductivity of graphene layer, attenuator, medium 6 and 7 refractive index and dielectric thickness and refractive index determine.Except the conductivity of Graphene, other value is all determined in advance.For example, medium 6 and 7 refractive index are fixed values, and dielectric thickness and refractive index are variable, and at terahertz wave band, the requirement that its thickness is very little has guaranteed that it causes that at terahertz wave band the variation of transmission refraction is less, can ignore.The number of plies of Graphene sample is (the referring to the below analysis of embodiment) designed in advance according to the dough softening requirement of practical application.So for the attenuator of a definite number of plies, the control of its dough softening realizes by handling Graphene conductivity.In our invention, the method for controlling conductivity is exactly grid voltage.

According to above-mentioned analysis, can calculate the Transflective coefficient of attenuator, thereby calculate the dough softening in practical application.Below, we take transmission situation as example, provide the dough softening calculating formula of Graphene attenuator.

Attenuator of the present invention is because service condition is different, and graphene layer also has different from the position relationship of substrate: the front that the face that first regulation light incident direction arrives is substrate.(1), for transmission THz wave, graphene layer is positioned at the front of substrate; (2), for transmission THz wave, graphene layer is positioned at the reverse side of substrate; (3) for reflected terahertz ripple hereby, graphene layer can only be attached to the front of substrate.

When Graphene Terahertz attenuator is positioned at the transmission situation of substrate front surface, as shown in Figure 5.In application, we weigh the dough softening of attenuator with dullness.Dullness X is defined as the 1 relative transmitance that deducts Graphene attenuator.We are expressed as follows the final calculation result of dullness:

$X=1-{|t\&CenterDot;t_{\mathrm{sa}}\&CenterDot;\frac{h_{\mathrm{sub}}(\mathrm{\&omega;},d_{\mathrm{sub}})}{h_{\mathrm{air}}(\mathrm{\&omega;},d_{\mathrm{sub}})}\&CenterDot;\frac{1}{1-r\&CenterDot;r_{\mathrm{sa}}{h_{\mathrm{sub}}}^2(\mathrm{\&omega;},d_{\mathrm{sub}})}|}^2---\left(5\right)$

In formula, the transmission coefficient t of medium and air interface _sa=2n _sub/ (n _sub+ n _air), the reflection coefficient r of medium and air interface _sa=(n _sub-n _air)/(n _sub+ n _air), h _sub(ω, d _sub)=exp (i ω d _subn _sub/ c) for refractive index is n _sub, thickness is d _subsubstrate in THz wave transmission factor, h _air(ω, d _sub)=exp (i ω d _subn _air/ c) for refractive index is n _air, thickness is d _subairborne THz wave transmission factor, ω is angular frequency, c is the light velocity, t and r are respectively transmission, the reflection coefficients of attenuator, by formula (3), (4), are determined.

To sum up, we utilize formula (5) to calculate the final dough softening of attenuator, by formula (1), can be determined that the Terahertz electricity of each independent graphene layer leads the variation relation with voltage, by formula (3), (4), determine the THz wave Transflective coefficient of multi-layer graphene and the relation of the number of plies, so convolution (1), (3), (4), (5), we can obtain the relation of the dough softening of attenuator and voltage, the Graphene number of plies.

Three, embodiment

For design of the present invention is further explained; below provide some embodiment; it should be noted that; invention which is intended to be protected is not limited to following examples; the interpolation of making on the basis of the technical scheme that those skilled in the art provides at these embodiment and equivalence are replaced, and all should belong to protection scope of the present invention.

Embodiment 1:

Shown in Fig. 1 is the automatically controlled Terahertz attenuator that two graphene layers form, and this figure be take side as visual angle.This attenuator comprises substrate 6, three-decker is successively laid in substrate 6 tops, comprising two-layer graphene layer 1 and 3, and by these adjacent two graphene layers 1 and 3 dielectric layers 2 of separating completely, connecting electrode 4 making alive 5 between graphene layer 1 and graphene layer 3.

Embodiment 2:

Shown in Fig. 2 is the attenuator consisting of 4 graphene layers, comprises substrate 6, and in substrate 6, successively having laid is 7 layers of structure, comprising 4 graphene layers and the dielectric layer 2 between every two adjacent graphene layers.In figure from left to right, first is connected respectively an electrode with the 3rd graphene layer, second is connected respectively another electrode with the 4th graphene layer, on these two electrodes, be voltage 5, odd-level and even level graphene layer have identical current potential separately, can carry out the adjusting of every layer graphene layer conductivity by making alive 5.In Fig. 2, electrode do not illustrated out, in actual applications, and being connected and realizing by making electrode between conductor wire and Graphene.

In actual design, the number of plies of graphene layer is not limited to 2 layers and 4 layers, but changes according to the range of application of attenuator.

Embodiment 3:

In this embodiment, the attenuator that we select following device preparation to comprise the different Graphene numbers of plies:

Graphene sample (fixing) parameter: area 1cm ², carrier scattering probability 100cm ^-1, Fermi velocity v _f=1.1 * 10 ⁶m/s, probe temperature: 300K, preparation method: CVD.

(on-fixed) parameter of Graphene: the number of plies: 2-6, voltage: 0-80V.

Dielectric layer: SiO ₂, thickness: 50nm; Method: magnetron sputtering, refractive index: 1.5.

Substrate: quartz, thickness 500 μ m, refractive index 1.5.

Electrode: copper, magnetron sputtering.

Incident Terahertz wave frequency is with 1THz.The Fermi level of Graphene be 0eV and-0.315eV, corresponding voltage regulates 0V to 80V, as the bound of Graphene Fermi level variation.

The variation range of the THz wave dullness that obtains the Graphene attenuator shown in Fig. 6 when the different Graphene number of plies.From the result of Fig. 6, can see, when the Graphene number of plies increases, dullness rises, and attenuator depth of modulation maximal value increases.Yet along with the increase of the number of plies, the minimum value of the dough softening is also increasing.Therefore,, for the needs of differential declines degree, we should adopt the Graphene attenuator of the different numbers of plies.From figure, the maximum attenuation degree of 6 layer devices can approach 90%, is enough to meet the requirement of the high dough softening.

Above-mentioned conclusion can be used as the reference standard of selecting the Graphene number of plies in attenuator.For example, if we need the attenuator of 80% incident wave of can decaying, result from Fig. 6, just must select the device of 4 layers of above Graphene, considers the sensitive of adjusting and the simplification of preparing, the preferably device of 5 layers.The attenuator of 20% incident wave if our needs can be decayed, needs 3 layers of device of Graphenes below, and preferably 2 layers possess larger tuning range.

Fig. 7 is when graphene layer number is respectively 2,4,6, and the THz wave dullness of transmission is with the curve of change in voltage.What be different from Fig. 6 is, in the figure, the Fermi level of Graphene is subject to the regulation and control of voltage and changes, the dullness of 2 layer graphene attenuators is adjustable continuously in 0.14～0.65 scope, the dullness of 4 layer graphene attenuators is adjustable continuously in 0.25～0.82 scope, and the dullness of 6 layer graphene attenuators is adjustable continuously in 0.34～0.90 scope.Through test, obtain in addition: the dullness of 3 layer graphene attenuators is adjustable continuously in 0.19～0.76 scope, and the dullness of 5 layer graphene attenuators is adjustable continuously in 0.30～0.86 scope.

Therefore, in actual applications, the scope that can modulate as required, first selects the number of plies of Graphene, then making alive regulates the realize target dough softening continuously on the attenuator of the fixing Graphene number of plies.Within the specific limits, the attenuator depth of modulation upper limit that more multi-layer graphene forms is larger, but the sensitivity from modulating, and the attenuator that the Graphene of few layer forms has superiority when compared with high permeability demand.Automatically controlled attenuator in use, adds constant voltage between two kinds of electrodes, the numerical value of regulation voltage, and the electricity of adjustable Graphene is led, thereby obtains the different doughs softening.

Fig. 8 is the automatically controlled Terahertz attenuator that 6 graphene layers form, and in transmission situation, terahertz time-domain spectroscopy is with the variation of regulation and control voltage.The signal of terahertz time-domain spectroscopy is the broadband signal of a 0-2THz.The main pulse of pulse in Fig. 8 for directly seeing through, by formula 5, it also can reflect total size that sees through light intensity.As we can see from the figure, along with alive rising, the amplitude of main pulse is more and more less, has embodied the effect of Terahertz attenuator.Meanwhile, this embodiment has also showed the broadband effect of Graphene Terahertz attenuator.

Four, the preparation method of attenuator

In substrate, according to order from the bottom to top, successively prepare each layer of structure, an electrode that comprise at least two-layer graphene layer, the Graphene of the common electrode being connected and odd-level is connected jointly on the dielectric layer between every two adjacent graphene layers, even level graphene layer, on-load voltage on two electrodes; Guarantee that each dielectric layer separates adjacent graphite linings completely.

The preparation of graphene layer preferably but to be not limited to various chemical gaseous phase (normal pressure, low pressure, plasma) legal system standby.Or use improved for example mechanical stripping, epitaxial growth, the means such as redox, are as the criterion to obtain the large-area graphene layer of above-mentioned requirements high-quality.In order to obtain the Graphene of select location in target substrate, and at the target location of dielectric layer placing graphite alkene, for the epitaxially grown Graphene of metal, can select but be not limited to the method that wet etching substrate recycling polymkeric substance shifts, and heat discharges the method that adhesive tape shifts Graphene.Take the former as example, for metal (copper, nickel, platinum etc.) Graphene of upper growth, first be cut into target size, then make (for example rotary coating) one deck suitable polymers (for example PMMA) on Graphene face, the chemical etchant of recycling respective metal substrate is removed substrate, afterwards the Graphene being attached on polymkeric substance is transferred to substrate target location, the solvent (for example acetone solution PMMA) that re-uses phase emergencing copolymer is removed Graphene.The method of above-mentioned transfer Graphene should consider dissolves the match condition with substrate and medium with solvent, heat release adhesive tape etc.For example take polymkeric substance during as substrate, should avoid using the lytic agent (such as acetone etc.) of injury substrate, can select heat to discharge adhesive tape method.To different media and substrate, prepared by preferred difference, transfer scheme in a word.

Dielectric layer can adopt but be not limited to the method for manufacturing thin film such as vacuum evaporation, rotary coating, chemical vapor deposition.

Electrode can adopt but be not limited to the preparation method of the metallic films such as vacuum evaporation, vacuum sputtering, chemical vapor deposition or electrochemical deposition.When shape, position, size are had to requirement, can adopt the application that is not limited to micro-processing technology etc.

To sum up, the number of plies of dielectric layer 2 changes with the number of plies of graphene layer, guarantees all the time adjacent graphene layer completely separated.The connected mode of electrode 4 and graphene layer is change structure, connected mode, material etc. in the situation that guaranteeing conducting.Electrode 4 also can adopt sandwich construction to guarantee the adhesiveness to sample and substrate 6.Voltage source 5 can pass through electric wire with the connected mode of electrode 4, also can be by the electrode wires on circuit board etc.It should be noted that in the design of fixing Terahertz attenuator simultaneously, voltage electric wire can, at this moment, dielectric layer 2 also can be used as optional layer.Those skilled in the art is to be understood that, the thickness of dielectric layer, refractive index can change, and the thickness of substrate and refractive index change with application background equally, the character of Graphene has the difference in parameter because of actual conditions, and data area in actual applications can change to some extent.

Claims (10)

1. the automatically controlled Terahertz attenuator based on Graphene, it is characterized in that, comprise substrate, in substrate, be successively equipped with at least two-layer graphene layer, between every two-layer adjacent graphene layer, be covered with one deck dielectric layer, described dielectric layer is separated this two-layer graphene layer.

2. the automatically controlled Terahertz attenuator based on Graphene as claimed in claim 1, is characterized in that, according to different service conditions, and the position relationship of described graphene layer and substrate: (1), for transmission THz wave, graphene layer is positioned at the front of substrate; (2), for transmission THz wave, graphene layer is positioned at the reverse side of substrate; (3) for reflected terahertz ripple hereby, graphene layer is positioned at the front of substrate; The front that the face that first regulation light incident direction arrives is substrate.

3. the automatically controlled Terahertz attenuator based on Graphene as claimed in claim 1, is characterized in that, described graphene layer is 2～6 layers.

4. the automatically controlled Terahertz attenuator based on Graphene as claimed in claim 1, is characterized in that, the computing formula of the dullness of described attenuator is as follows:

$X=1-{|t\&CenterDot;t_{\mathrm{sa}}\&CenterDot;\frac{h_{\mathrm{sub}}(\mathrm{\&omega;},d_{\mathrm{sub}})}{h_{\mathrm{air}}(\mathrm{\&omega;},d_{\mathrm{sub}})}\&CenterDot;\frac{1}{1-r\&CenterDot;r_{\mathrm{sa}}{h_{\mathrm{sub}}}^2(\mathrm{\&omega;},d_{\mathrm{sub}})}|}^2;$

In formula, the transmission coefficient t of medium and air interface _sa=2n _sub/ (n _sub+ n _air), the reflection coefficient r of medium and air interface _sa=(n _sub-n _air)/(n _sub+ n _air), h _sub(ω, d _sub)=exp (i ω d _subn _sub/ c) for refractive index is n _sub, thickness is d _subsubstrate in THz wave transmission factor, h _air(ω, d _sub)=exp (i ω d _subn _air/ c) for refractive index is n _air, thickness is d _subairborne THz wave transmission factor; ω is angular frequency, and c is the light velocity; T and r are respectively transmission, the reflection coefficients of attenuator, are determined respectively by following formula:

t＝4n ₁n ₃/{[(Y ₁+Y ₂)n ₃+(Y ₁-Y ₂)(n ₂+Z ₀σ)](n ₁+n ₃+Z ₀σ)exp(ik ₃d)

-[(Y ₃+Y ₄)n ₃+(Y ₃-Y ₄)(n ₂+Z ₀σ)](n ₃-n ₁-Z ₀σ)exp(-ik ₃d)}；

$\left(\begin{array}{cc}{Y}_1& Y_2\\ Y_3& Y_4\end{array}\right)={\left(\begin{array}{cc}(1+\frac{Z_0\mathrm{\&sigma;}}{2n_3})\exp \left({\mathrm{ik}}_{3}d\right)& \frac{Z_0\mathrm{\&sigma;}}{2n_3}\exp (-{\mathrm{ik}}_{3}d)\\ -\frac{Z_0\mathrm{\&sigma;}}{2n_3}\exp \left({\mathrm{ik}}_{3}d\right)& (1-\frac{Z_0\mathrm{\&sigma;}}{2n_3})\exp (-{\mathrm{ik}}_{3}d)\end{array}\right)}^{N-2};$

r＝1-t；

In formula, i represents the imaginary part of symbol, N>=2nd, the number of plies of graphene layer in attenuator; n ₁and n ₂it is respectively the medium refraction index on N layer graphene both sides; Z ₀vacuum impedance, n ₃with d is refractive index and the thickness of spacer dielectric layer between Graphene, k ₃=ω n ₃/ c is wave vector wherein; σ is the Terahertz sheet conductance of each single-layer graphene, by following formula, is calculated:

|N _c|＝7.5×10 ¹⁰·|V _g-V _CNP|cm ^-2V ^-1；

In formula, ω is angular frequency, v _fit is the Fermi velocity of charge carrier in Graphene; E and respectively elementary charge, Planck's constant; Γ and N _crespectively carrier scattering rate and the carrier concentration in Graphene; V _gthe actual voltage adding, V _cNPrefer to the voltage that reaches Graphene sample electric neutrality point, V _g-V _cNPby being added grid voltage.

5. the automatically controlled Terahertz attenuator based on Graphene as claimed in claim 1, it is characterized in that, the several adjustable extents according to needed attenuator dullness of described graphene layer are determined: the dullness of 2 layer graphene attenuators is adjustable continuously in 0.14～0.65 scope, the dullness of 3 layer graphene attenuators is adjustable continuously in 0.19～0.76 scope, the dullness of 4 layer graphene attenuators is adjustable continuously in 0.25～0.82 scope, the dullness of 5 layer graphene attenuators is adjustable continuously in 0.30～0.86 scope, the dullness of 6 layer graphene attenuators is adjustable continuously in 0.34～0.90 scope.

6. the automatically controlled Terahertz attenuator based on Graphene as claimed in claim 1, is characterized in that, the area of described graphene layer is greater than the area of Terahertz hot spot, is not less than 1cm ².

7. the automatically controlled Terahertz attenuator based on Graphene as claimed in claim 1, is characterized in that, described substrate adopts quartzy, and described dielectric layer adopts silicon dioxide, boron nitride or alundum (Al2O3); Described electrode adopts metal or alloy electrode.

8. the preparation method of the automatically controlled Terahertz attenuator based on Graphene claimed in claim 1, it is characterized in that, comprise the steps: according to order from the bottom to top, successively to prepare each layer of structure in substrate, an electrode that have at least two-layer graphene layer in substrate, the Graphene of the common electrode being connected and odd-level is connected jointly on the dielectric layer between every two adjacent graphene layers, even level graphene layer, by on-load voltage on two electrodes; Each dielectric layer is separated adjacent graphite linings.

9. the preparation method of the automatically controlled Terahertz attenuator based on Graphene claimed in claim 1, is characterized in that, the preparation of described graphene layer adopts chemical gas-phase method, mechanical stripping method, epitaxial growth method or oxidation-reduction method; The preparation of described dielectric layer adopts vacuum vapour deposition, spin coating method or chemical vapour deposition technique; The preparation of described electrode adopts vacuum vapour deposition, vacuum sputtering, chemical vapour deposition technique or electrochemical deposition method.

10. the using method of the automatically controlled Terahertz attenuator based on Graphene claimed in claim 1, it is characterized in that, for the fixing attenuator of the number of plies, by on being positioned at electrode that even level connects and being positioned at the electrode that odd-level graphene layer is connected, add constant voltage, voltage is converted to 80V from 0V, along with alive rising, observe the amplitude of main pulse, until the amplitude of main pulse disappears, now added magnitude of voltage is defined as meeting the voltage of the dough softening demand of attenuator; Different according to service condition, the position relationship of described graphene layer and substrate: (1), for transmission THz wave, graphene layer is positioned at the front of substrate; (2), for transmission THz wave, graphene layer is positioned at the reverse side of substrate; (3) for reflected terahertz ripple hereby, graphene layer is positioned at the front of substrate; The front that the face that first regulation light incident direction arrives is substrate.