CN103984051A - Electric control terahertz antireflection film based on graphene, manufacturing method and using method - Google Patents

Abstract

The invention discloses an electric control terahertz antireflection film based on graphene, a manufacturing method and a using method. The antireflection film is of a multilayer structure and comprises at least two graphene layers and a dielectric layer which completely separates adjacent graphene. The first graphene layer and the second graphene layer are connected with electrodes, and voltage is added, so that electric field adjusting is carried out conveniently. During using, the film can be machined or attached on a substrate or a terahertz element. According to the antireflection film made of a multilayer film structure, the refractive index range of the terahertz element or the substrate for achieving antireflection performance can be changed through different layer numbers of the graphene, and antireflection of any refractive index materials is achieved theoretically. By voltage adjusting and controlling, antireflection performance can be adjusted, and accuracy is high.

Description

Based on automatically controlled Terahertz antireflecting film, preparation method and the using method of Graphene

Technical field

The invention belongs to terahertz wave band device technology field, specifically a kind of THz wave antireflecting film, preparation method and using method based on Graphene.

Background technology

Terahertz (Terahertz is called for short THz) is electromagnetic cps, and 1 Terahertz equals 1012 hertz.THz wave refer to the electromagnetic wave of 0.3THz to 10THz scope, corresponding wavelength scope is from 0.03mm to 1mm.Terahertz wave band has higher spatial resolution (high-frequency) and temporal resolution (picopulse), and energy is less can not destroy material, and is the vibration of biomacromolecule, the resonance band of rotation.These characteristics make it have potential application prospect in fields such as such as broadband connections, radar, electronic countermeasure, ELECTROMAGNETIC WEAPON, astronomy, Non-Destructive Testing, medical imaging, safety inspections.Before the eighties, be subject to the restriction of THz wave generation source and detector, relate to the research of this wave band and apply considerably less.Along with after new technology (ultrafast technology), the development of new material, is developed rapidly Terahertz Technology.At present, the research of Terahertz Technology and application and development are just becoming the focus of optical field research.For example, but high performance novel THz devices (emissive source, detector, modulator, beam splitter, dispersing optics element, broadband transmission window etc.) is urgently design still, to improve the application efficiency of Terahertz Technology.Due to Fresnel effect, on the contact interface of these components and parts, can produce multiple reflections, thereby cause parasitic light, the terahertz pulse in interferometry, causes producing and interfering at frequency domain, reduces the resolution of spectrum, causes useful information to be covered.

Antireflecting film (antireflection coating) is a kind of functional film, and target is to reduce or elimination photovalve surface or inner reflected light.At visible, infrared band, antireflecting film is a kind of most widely used, and the optical thin film of demand maximum, also referred to as anti-reflection film.Major function is the light transmission capacity that increases optical element (for example lens, prism, level crossing etc.), the parasitic light of minimizing or the system of elimination.Typical optics antireflecting film is one deck low-index material (refractive index is made as) that is plated in optical element surface, the interference effect of action principle based on light wave.When the optical thickness (product of refractive index and actual (real) thickness) of rete is for seeing through 1/4th of optical wavelength, the light reflecting on film and the optical path difference of the light of film internal reflection meet the interference condition that disappears mutually, at this moment realize anti-reflective effect.The top condition of thin-film refractive index is (with the refractive index that is respectively air and optical element), and at this moment the amplitude of the two-beam of inside and outside reflection can reach equal, eliminates reflection completely so can realize.

At terahertz wave band, if use the principle of interference of optical region to make antireflecting film, so corresponding 0.03 to 1mm wavelength coverage, the thickness range of antireflecting film should be from 0.025 to 0.25mm.This has two shortcomings, first, because the wavelength of this wave band is long a lot of with respect to visible waveband, so the demand thickness of antireflecting film is too thick, is unfavorable for preparation and the application of film.Second, at terahertz wave band, we usually need broadband terahertz signal, and (for example pulse of terahertz time-domain spectroscopy system is broadband, conventionally can cover several Terahertzs from several Terahertzs at zero point), but the thickness of the antireflecting film needing corresponding to different-waveband differs too large, so this film can not be realized broadband antireflection.

At terahertz wave band, can use the principle of impedance matching (Impedance matching) to realize antireflecting film.Impedance matching is the technical term in microwave electronics, refer to a kind of collocation mode between signal source or transmission line and load, can make in this way microwave signal all propagate in load, avoid being reflected back source point, thereby affect the duty of equipment itself.Use impedance matching principle to make antireflecting film at terahertz wave band, corresponding principle is: suppose that one deck conductivity is σ _film, to cover refractive index be n to the thickness film that is d _suboptical element surface on, the very thin thickness of this tunic, is far smaller than the wavelength of transmission THz wave.In air/film/component interface, the reflection coefficient of the THz wave of propagating from element internal to direction of air on interface is r so _sub/film/air=(n _sub-n _air-Z ₀σ _filmd)/(n _sub+ n _air+ Z ₀σ _filmd), Z here ₀for vacuum impedance.So, when the sheet conductance of this tunic (equals the product σ of conductivity and thickness _filmd) meet σ _filmd=(n _sub-n _air)/Z ₀time, can obtain reflection coefficient and equal 0, at this moment meet impedance matching relation, element internal reflection can be eliminated completely.In the time that the sheet conductance specific impedance coupling numerical value of film is little, reflection coefficient is positive number, obtains and the synchronous reflection wave of original ripple; And in the time that the sheet conductance specific impedance coupling numerical value of film is large, reflection coefficient is negative, means that π reversion has occurred the phase place of reflection wave.

The material requirements that utilizes above-mentioned impedance matching principle to make Terahertz antireflecting film is: first very thin on thickness, otherwise can be because self thickness of film causes the interference of interference effect; Its less important conduction, because will be at the situation regulation and control sheet conductance of thinner thickness, only have the conductance of material itself to reach necessary numerical value, and if this value can be such as, with other parameter (carrier concentration etc.) adjustable, will improve antireflecting film performance of control in actual applications; The 3rd, must in wide THz wave segment limit, there is more consistent conductivity, because the dispersion properties of conductivity is less, just more can realize the antireflection effect of broadband scope; The 4th, the necessary calorifics of this material, stable chemical nature, do not vary with temperature, and being for example difficult to, by number of chemical material corrosion (alcohol, the acetone etc. of conventional wiping mirror) is to ensure the long-term non-maintaining condition of antireflection film; Finally, mechanical property is good, if can have pliability in proof strength, just can possess the potentiality that are applied on flexible device.At present, the material of studying comprises metal (for example gold, chromium), oxide semiconductor material (zinc paste, vanadium dioxide, tin indium oxide), and the super material of metal (artificial structure's material) etc., do not have a kind of material can realize above-mentioned requirement completely.

Graphene is the former molecular two-dimensional material of a kind of monolayer carbon, and this material has good mechanical strength and pliability, high coefficient of heat conductivity, stable chemical property, and excellent, unique photoelectric properties.The carrier mobility of Graphene is very high, and has unique ambipolar electric field effect, and this is the linear dirac taper dispersion relation due to its uniqueness.Ambipolar electric field effect refers to that carrier concentration in Graphene (to square being directly proportional of Fermi level) can change under the effect of grid voltage, thereby changes the sheet conductance 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, changes hardly with the variation of frequency.So with regard to photoelectric characteristic, Graphene has the Terahertz conductivity that broadband is stable, grid voltage is adjustable.

Introduce the characteristic of principle, Terahertz impedance matching film and the Graphene of principle, the electromagnetic wave impedance matching of element application present situation, the optics antireflecting film of THz wave above.Can find out, at present effectively also shortcoming very of terahertz wave band antireflecting film, existing method and material in various degree faced that working frequency range is narrow, poor stability and the defect such as untunable.And grapheme material possesses good power, heat, photoelectric characteristic, particularly possess broadband and tunable potential quality.

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 new electronic control antireflecting film based on Graphene is provided, and adopts following technical scheme:

Based on an automatically controlled Terahertz antireflecting film for Graphene, comprise 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.

Further, described graphene layer is 2～6 layers.

Further, the reflection coefficient of described antireflecting film is to realize antireflection at 0 o'clock, and the calculating formula of reflection coefficient is:

r _film＝1-4n ₁n ₃/{[(M ₁+M ₂)n ₃+(M ₁-M ₂)(n ₂+Z ₀σ _gra)](n ₁+n ₃+Z ₀σ _gra)exp(ik ₃d)

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

$\left(\begin{array}{cc}{M}_1& M_2\\ M_3& M_4\end{array}\right)={\left(\begin{array}{cc}(1+\frac{Z_0{\mathrm{\σ}}_{\mathrm{gra}}}{2n_3})\exp \left({\mathrm{ik}}_{3}d\right)& \frac{Z_0{\mathrm{\σ}}_{\mathrm{gra}}}{2n_3}\exp (-{\mathrm{ik}}_{3}d)\\ -\frac{Z_0{\mathrm{\σ}}_{\mathrm{gra}}}{2n_3}\exp \left({\mathrm{ik}}_{3}d\right)& (1-\frac{Z_0{\mathrm{\σ}}_{\mathrm{gra}}}{{2n}_3})\exp (-{\mathrm{ik}}_{3}d)\end{array}\right)}^{N-2};$

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 refractive index of the medium 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, and ω is angular frequency, and c is the light velocity; σ _grabe the Terahertz sheet conductance of each single-layer graphene, calculated by following formula:

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

In formula, e, k _bwith respectively elementary charge, Boltzmann constant and Planck's constant, T, Γ and E _frespectively temperature, scattered power and Fermi level, v _fthe Fermi velocity of charge carrier in Graphene, N _cit is 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 adjustable extent of the several refractive indexes according to needed element or substrate of the graphene layer in described antireflecting film is determined: the ranges of indices of refraction that 2 layer graphenes are realized antireflecting element or substrate is 1.2～2.4; The ranges of indices of refraction that 4 layer graphenes are realized antireflecting element or substrate is 1.3～3.8; The ranges of indices of refraction that 6 layer graphenes are realized antireflecting element or substrate is 1.5～5.2.

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

Further, 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 antireflecting film based on Graphene is provided, before being used, described antireflecting film is machined on substrate or Terahertz element, in Terahertz element or substrate, successively prepare each layer of structure according to order from the bottom to top, 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; Ensure that each dielectric layer separates adjacent graphite linings completely.

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 preparation method of the automatically controlled Terahertz antireflecting film based on Graphene is provided, it is characterized in that, for the antireflecting film of the fixing number of plies, add constant voltage by being positioned at electrode that even level connects and being positioned at the electrode that odd-level graphene layer is connected, voltage is converted to 60V from 10V, along with alive rising, observe the amplitude of reflected impulse, until the amplitude of reflected impulse disappears, to be now defined as meeting the antireflection coefficient of antireflecting film be the voltage of 1 o'clock to added magnitude of voltage; According to service condition difference, the use of described antireflecting film mainly contains three kinds of situations: (1), for transmitted light, antireflecting film is positioned at the front of substrate or element; (2), for transmitted light, Graphene antireflecting film is positioned at the reverse side of substrate or element; (3), for reflected light, Graphene antireflecting film is attached to the reverse side of substrate or element; The face that first incident of definition terahertz light passed through is substrate or Terahertz element front, and another side is reverse side.

The present invention has the following advantages:

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

A, possesses good mechanical strength.If Terahertz element (or substrate) is flexible material, we,, in the time selecting flexible media layer (as two-dimentional boron nitride etc.), can make device possess flexible application so.

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

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

The ambipolar electric field effect of D, Graphene, makes device can be subject to regulating and controlling voltage.First, expanded the range of application of device, dirigibility, sensitivity etc.Secondly, automatically controlled characteristic makes device be easy to be combined with prevailing system.

2, the present invention utilizes antireflecting film prepared by multi-layer film structure, can change Terahertz element (or substrate) ranges of indices of refraction that realizes reflection preventing ability by the different numbers of plies of Graphene, can realize in theory the antireflection of arbitrary refractive index material.By regulating and controlling voltage, can make antireflecting performance adjustable, degree of accuracy is higher.

3, the grapheme material that the present invention uses, cost is lower, and preparation is easier to, and repeatability is higher, 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 antireflecting film has simply, is easy to the advantage of processing equally.

Brief description of the drawings

Fig. 1 is the structural representation of the antireflecting film of 2 graphene layers formations.

Fig. 2 is the structural representation of the antireflecting film of 4 graphene layers formations.

Fig. 3 is Graphene antireflecting film while being positioned at Terahertz element or substrate positive, and the light of THz wave normal incidence is propagated schematic diagram (face that first passes through of definition light incident is for positive, and another side is reverse side).

Fig. 4 is the THz wave Transflective principle schematic of the antireflecting film of N graphene layer formation.

Fig. 5 leads according to the De Lude model electricity of Graphene, and the Terahertz electricity of the single-layer graphene calculating is led the curve changing with institute's making alive.

Fig. 6 is the automatically controlled antireflecting film that 4 graphene layers form, and is positioned on High Resistivity Si front, and in transmission situation, the relative amplitude transmission coefficient of the 1st secondary reflection light wave (seeing Fig. 3) is with the curve of change in voltage.

Fig. 7 is that graphene layer number is respectively (a) 2, (b) 4, and (c) 6 o'clock, the reflection coefficient interval of antireflecting film, with the variation of target Terahertz element or substrate refractive index.In figure, reflection coefficient is to realize antireflection at 0 o'clock.

Fig. 8 is the automatically controlled antireflecting film that 4 graphene layers form, and is positioned on High Resistivity Si front, 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 layer of separating with dielectric layer, as the basic structure of antireflecting film, utilizes regulating and controlling voltage to change the photoconductivity of Graphene, thereby regulates and controls transmission, reflected light wave amplitude and phase place.Wherein, lead and meet under impedance matching condition at electricity, realize antireflection function.In antireflection application, need to select the Graphene number of plies according to the refractive index of target Terahertz element or substrate, and accurately realize antireflection condition by voltage-regulation.

One, the selection of parts

Graphene layer: the parameter of graphene layer is considered area and the number of plies.The area of graphene layer must be greater than the area of Terahertz hot spot, considers the installation of the material such as tunability and electrode in application, is preferably greater than 1cm ².The refractive index of Terahertz element (or substrate) material adhering to when the selection of the Graphene number of plies is depended on to application, and should take into full account the limit that the carrier concentration (Fermi level) under grapheme material regulating and controlling voltage can reach, 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, its principle theoretical analysis part that sees below.

Dielectric layer: the physical parameter of dielectric layer is mainly considered 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, and boron nitride or alundum (Al2O3) etc.

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

We provide in conjunction with experiment the number of plies that Graphene Terahertz antireflecting film determines by Terahertz element or substrate refractive index by theoretical analysis and select below.

Two, principle analysis

The use of Graphene antireflecting film mainly contains three kinds of situations: 1, for transmitted light, Graphene antireflecting film is positioned at the front of substrate or element; 2,, for transmitted light, Graphene antireflecting film is positioned at the reverse side of substrate or element; 3,, for reflected light, Graphene antireflecting film is attached to the reverse side of substrate or element.

As shown in Figure 3, be positioned at transmission and the internal reflection signal of the THz wave of the transmission situation of substrate front surface for antireflecting film.It is main transmitted wave that incident THz wave adjusts and reduce through Graphene electricity the light wave directly seeing through after reflectance coating 8 and Terahertz element or substrate 6, can be obtained by Fresnel law and formula (1) and (2), and the transmission coefficient of main transmitted wave is:

t ⁽⁰⁾＝t _filmt _sap _air(ω,-d _sub)p _sub(ω,d _sub) (1)

T in formula _sa=2n _sub/ (n _sub+ n _air), n _suband n _airrespectively the refractive index of Terahertz element or substrate and air, p _air(ω ,-d _sub)=exp (i ω d _subn _air/ c) and p _sub(ω, d _sub)=exp (i ω d _subn _sub/ c) be the THz wave transmission factor in air and element or substrate, d _subbe the thickness of Terahertz element or substrate, ω and c are respectively angular frequency and the light velocity; t _filmit is the transmission coefficient of antireflecting film.In the drawings, the ripple after internal reflection is defined as reflection wave, is defined as m secondary reflection ripple again on inner anti-reflection face according to the number of times reflecting.Therefore the transmission coefficient of m secondary reflection ripple is:

$t^{\left(m\right)}=t_{\mathrm{film}}{t}_{\mathrm{sa}}{r}_{\mathrm{film}}^m{r}_{\mathrm{sa}}^m{p}_{\mathrm{air}}(\mathrm{\ω},-d_{\mathrm{sub}}){\left[p_{\mathrm{sub}}(\mathrm{\ω},d_{\mathrm{sub}})\right]}^{2m+1}---\left(2\right)$

R in formula _sa=(n _sub-n _air)/(n _sub+ n _air), r _filmit is the internal reflection coefficient of antireflecting film.Formula (2) can be used for weighing the size of reflected impulse, and in the time realizing antireflection, its value is 0.And be easy to get, work as r _filmwhen being 0, the value of formula (2) is 0.

Next calculate the transmission coefficient t of antireflecting film _filmwith reflection coefficient r _film.Shown in Fig. 4 is the THz wave Transflective principle schematic of the antireflecting film that is made up of N graphene layer.The number of plies of dielectric 2 is N-1, and device is positioned between two kinds of different mediums 6 (element or substrate) and medium 7 (air).The refractive index of this two media 6 and 7 is corresponding n respectively ₁with n ₂, and n ₁> n ₂.The thickness of Graphene is 0.335nm, can be left in the basket and also be regarded as the conductive layer of one deck zero thickness.The thickness of dielectric 2 is made as d, and refractive index is made as n ₃.The position of the graphene layer adjacent with medium 6 is made as 0 point on z direction of principal axis, and the optical transmission coefficient of Graphene and reflection coefficient can be expressed as:

$t_{\mathrm{gra}}=\frac{{2n}_i}{n_i+n_j+Z_0{\mathrm{\σ}}_{\mathrm{gra}}},---\left(3\right)$

$r_{\mathrm{gra}}=\frac{n_i-n_j-Z_0{\mathrm{\σ}}_{\mathrm{gra}}}{n_i+n_j+Z_0{\mathrm{\σ}}_{\mathrm{gra}}}.---\left(4\right)$

N in formula _iand n _jformula represents respectively the refractive index of graphene layer media of both sides, Z ₀=377 Ω are vacuum impedances, σ _grathe Terahertz electricity that is each single-layer graphene is led.

We suppose E _i, E _rand E _tbe respectively the electric field of incident, reflection and transmission THz wave, and antireflecting film at least forms (N>=2) by two layer graphenes.At medium 6 (z < 0) and medium 7, (equation of the ripple of z > (N-1) in d) can be made as E so _i+ E _r=exp (ik ₁z)+r _filmexp (ik ₁and E z) _t=t _filmexp{-ik ₂[z-(N-1) d] }, wherein k ₁=ω n ₁/ c and k ₂=ω n ₂/ c is respectively the wave vector of THz wave in medium 6 and 7.Dielectric layer (n-1) d < z < nd between graphene layer (n=1,2 ..., N-1) in, wave equation is E _n+ E _n'=A _nexp[-ik ₃(z-nd)]+A _n' exp[ik ₃(z-nd)], k wherein ₃=ω n ₃/ c is the wave vector in dielectric layer.By separating the transmission matrix equation on interface, the THz wave reflection coefficient that can obtain N layer Terahertz antireflecting film is:

r _film＝1-4n ₁n ₃/{[(M ₁+M ₂)n ₃+(M ₁-M ₂)(n ₂+Z ₀σ _gra)](n ₁+n ₃+Z ₀σ _gra)exp(ik ₃d)

-[(M ₃+M ₄)n ₃+(M ₃-M ₄)(n ₂+Z ₀σ _gra)](n ₃-n ₁-Z ₀σ _gra)exp(-ik ₃d)} (5)

$\left(\begin{array}{cc}{M}_1& M_2\\ M_3& M_4\end{array}\right)={\left(\begin{array}{cc}(1+\frac{Z_0{\mathrm{\&sigma;}}_{\mathrm{gra}}}{2n_3})\exp \left({\mathrm{ik}}_{3}d\right)& \frac{Z_0{\mathrm{\&sigma;}}_{\mathrm{gra}}}{2n_3}\exp (-{\mathrm{ik}}_{3}d)\\ -\frac{Z_0{\mathrm{\&sigma;}}_{\mathrm{gra}}}{2n_3}\exp \left({\mathrm{ik}}_{3}d\right)& (1-\frac{Z_0{\mathrm{\&sigma;}}_{\mathrm{gra}}}{{2n}_3})\exp (-{\mathrm{ik}}_{3}d)\end{array}\right)}^{N-2}---\left(6\right)$

Transmission coefficient is:

t _film＝1-r _film (7)

In the parameter of decision formula (5), (6), (7), medium 7 is generally air, and dielectric thickness and refractive index are generally constants.Set the goal after Terahertz element (or substrate), the refractive index of medium 6 is constant.At this moment most important variable is that the electricity of graphene layer is led σ _grawith number of plies N, by regulating this two values, make r in formula (5) _filmbecome 0, can realize antireflection.

Finally, provide single-layer graphene electricity and lead σ _grarelation with voltage.Fig. 5 is that the Terahertz electricity of the single-layer graphene that calculates is 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, i is the imaginary part of symbol, e, k _bwith respectively elementary charge, Boltzmann constant and Planck's constant, T, Γ and E _frespectively temperature, scattered power and Fermi level, ω is angular frequency.Note Fermi level and 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.In above-mentioned parameter, Γ can establish it for not with the constant 100cm of change in voltage ^-1(span of CVD sample conventionally).Like this, only need get different Fermi level (or carrier concentration) for the conductivity of calculating single-layer graphene 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 (9)

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 layer be one with the less value of frequency change, we,, taking 1THz as example, as seen from Figure 5, are applying a modulation voltage V from 0V to 80V here _g-V _cNPtime (corresponding to from 0 to 6 × 10 ¹²cm ^-1v ^-1carrier concentration, and Fermi level from 0eV to ± 0.315eV), the e that the Terahertz conductivity of single-layer graphene is from 3.3 to 29 times ²/ adjustable (e2/ continuously the constant photoconduction of Graphene at visible waveband).

To sum up, formula (2) is that 0 interval scale inner counter ejected wave is 0, thereby realizes antireflection.The condition that formula (2) is 0 is the reflection coefficient r in formula (5) _filmbe 0, can realize by regulating the electricity of Graphene to lead with the number of plies; The former is relevant to voltage, meets formula (8), (9).In conjunction with formula (5)～(9), can obtain the reflection coefficient of antireflecting film and the relation of voltage and the Graphene number of plies, for instructing the selection of the antireflecting film 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 schematic diagram of the basic structure of the automatically controlled antireflecting film of 2 graphene layers formations, and this figure is the structural representation as visual angle taking side.This device is three-decker, comprises two-layer graphene layer 1 and 3, and the dielectric layer 2 that adjacent graphene layer is separated completely, and between graphene layer 1 and graphene layer 3, connecting electrode 4 making alive 5 are for regulating.

Embodiment 2:

Shown in Fig. 2 is the basic structure schematic diagram of the automatically controlled antireflecting film of 4 graphene layers formations, and this figure is the structural representation as visual angle taking side.This antireflecting film is 7 layers of structure, comprises 7 layer graphenes, and adjacent Graphene is separated to dielectric layer 2 completely.In figure from top to bottom, 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 is not illustrated out, in actual applications, being connected between conductor wire and Graphene should pass through to make electrode and realize.

In actual design, the number of plies of graphene layer is not limited to 2 layers and 4 layers, but changes according to the target Terahertz element (or substrate) of antireflecting film.

Embodiment 3:

In this embodiment, we select following device to prepare the Terahertz antireflecting film that comprises 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: 2,4,6 voltage: 0～80V.

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

Checking reflection preventing ability adopts substrate: High Resistivity Si, thickness 500 μ m, refractive index 3.42.

Fig. 6 is under above-mentioned parameter, and the relative transmittance of the first reflection light wave of 4 layer graphene antireflecting films is with the variation of voltage.Relative transmittance is the transmission coefficient for the first time (formula (2) is shown in definition) while having antireflecting film, the ratio of transmission coefficient when thering is no reflectance coating.Be 0 to be to represent to realize antireflection state when it.From result, in the time that voltage approaches 57V, can obtain complete antireflecting effect.At this moment the corresponding carrier concentration of Graphene is 4.275 × 10 ¹²cm ^-1v ^-1, Fermi level is ± 0.266eV (the Fermi level symbol of odd-level and even level is contrary).

Fig. 7 is that graphene layer number is respectively 2,4,6 o'clock, and the reflection coefficient r of antireflecting film _filminterval, with the variation of element or substrate refractive index (variable).According to the principle of impedance matching, while realizing antireflection, reflection coefficient should be 0.Reflection coefficient is the phase reversal that negative value represents reflection wave.The parameter of Graphene antireflecting film is the same.

From the result of Fig. 7, the ranges of indices of refraction that 2 layer graphenes can be realized antireflecting element or substrate is 1.2～2.4 (Fig. 7 (a)); 4 layers be 1.3～3.8 (Fig. 7 (b)); 6 layers be 1.5～5.2 (Fig. 7 (c)).That is to say, in the time that the Graphene number of plies increases, the variation range of reflection coefficient is widened, and entirety moves to negative direction.According to this experiment, we can determine the number of plies that uses Graphene in application.For example, in the time that we use the refractive index of substrate to be 1.5 left and right (corresponding multiple polymers material), must select below 5 layers, the preferably device of 2 layers of composition, this is because need to promote sensitivity and reduce preparation difficulty in the time that modulation capability is enough.Just like, when we are when to use refractive index be 3.42 substrate (silicon materials), just must select 4 layers and above Graphene.

Fig. 8 is that while using the automatically controlled antireflecting film of 4 graphene layers formations of embodiment 2, the terahertz time-domain spectroscopy of transmitted wave is with the variation of regulation and control voltage.The signal of terahertz time-domain spectroscopy is the broadband signal of a 0～2THz.As we can see from the figure, do not using antireflection pattern, first reflection ripple (for time shaft on the pulse of second appearance) fairly obvious, this is to cause serious interference effect to whole spectrum.And when using after Graphene antireflecting film, along with the variation of regulation and control voltage from 10V to 60V, reflected impulse is obviously constantly suppressed.In the time of 60V, approach and disappear, this has just realized the antireflection effect to High Resistivity Si.Meanwhile, this embodiment shows that Graphene antireflecting film has broadband effect.

Four, the preparation method of antireflecting film

First determine the Terahertz element or the substrate that use antireflecting film, afterwards, in applied element or substrate, successively prepare each layer of structure according to order from the bottom to top, 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; Ensure 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 for adhesive tape transfer Graphene.Taking the former as example, for metal (copper, nickel, platinum etc.) upper Graphene of growing, first be cut into target size, on Graphene face, make again (for example rotary coating) one deck suitable polymers (for example PMMA), 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, during taking polymkeric substance 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.In the time that 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, ensures all the time adjacent graphene layer to separate completely.The connected mode of electrode 4 and graphene layer is change structure, connected mode, material etc. in the situation that ensureing conducting.Electrode 4 also can adopt sandwich construction to ensure 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 will be understood by those of skill in the art that the thickness of dielectric layer, refractive index can change, and 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 (9)

1. the automatically controlled Terahertz antireflecting film based on Graphene, is characterized in that, comprises at least two-layer graphene layer, 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.

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

3. the automatically controlled Terahertz antireflecting film based on Graphene as claimed in claim 1, is characterized in that, the reflection coefficient of described antireflecting film is to realize antireflection at 0 o'clock, and the calculating formula of reflection coefficient is:

r _film＝1-4n ₁n ₃/{[(M ₁+M ₂)n ₃+(M ₁-M ₂)(n ₂+Z ₀σ _gra)](n ₁+n ₃+Z ₀σ _gra)exp(ik ₃d)

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

$\left(\begin{array}{cc}{M}_1& M_2\\ M_3& M_4\end{array}\right)={\left(\begin{array}{cc}(1+\frac{Z_0{\mathrm{\&sigma;}}_{\mathrm{gra}}}{2n_3})\exp \left({\mathrm{ik}}_{3}d\right)& \frac{Z_0{\mathrm{\&sigma;}}_{\mathrm{gra}}}{2n_3}\exp (-{\mathrm{ik}}_{3}d)\\ -\frac{Z_0{\mathrm{\&sigma;}}_{\mathrm{gra}}}{2n_3}\exp \left({\mathrm{ik}}_{3}d\right)& (1-\frac{Z_0{\mathrm{\&sigma;}}_{\mathrm{gra}}}{{2n}_3})\exp (-{\mathrm{ik}}_{3}d)\end{array}\right)}^{N-2};$

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 refractive index of the medium 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, and ω is angular frequency, and c is the light velocity; σ _grabe the Terahertz sheet conductance of each single-layer graphene, calculated by following formula:

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

In formula, e, k _bwith respectively elementary charge, Boltzmann constant and Planck's constant, T, Γ and E _frespectively temperature, scattered power and Fermi level, v _fthe Fermi velocity of charge carrier in Graphene, N _cit is 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.

4. the automatically controlled Terahertz antireflecting film based on Graphene as claimed in claim 1, it is characterized in that, the adjustable extent of the several refractive indexes according to needed element or substrate of graphene layer in described antireflecting film is determined: the ranges of indices of refraction that 2 layer graphenes are realized antireflecting element or substrate is 1.2～2.4; The ranges of indices of refraction that 4 layer graphenes are realized antireflecting element or substrate is 1.3～3.8; The ranges of indices of refraction that 6 layer graphenes are realized antireflecting element or substrate is 1.5～5.2.

5. the automatically controlled Terahertz antireflecting film 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.

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

7. the preparation method of the automatically controlled Terahertz antireflecting film based on Graphene claimed in claim 1, it is characterized in that, before being used, described antireflecting film is machined on substrate or Terahertz element, in Terahertz element or substrate, successively prepare each layer of structure according to order from the bottom to top, 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; Ensure that each dielectric layer separates adjacent graphite linings completely.

8. the preparation method of the automatically controlled Terahertz antireflecting film based on Graphene as claimed in claim 7, 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.

9. the preparation method of the automatically controlled Terahertz antireflecting film based on Graphene claimed in claim 1, it is characterized in that, for the antireflecting film of the fixing number of plies, add constant voltage by being positioned at electrode that even level connects and being positioned at the electrode that odd-level graphene layer is connected, voltage is converted to 60V from 10V, along with alive rising, observe the amplitude of reflected impulse, until the amplitude of reflected impulse disappears, to be now defined as meeting the antireflection coefficient of antireflecting film be the voltage of 1 o'clock to added magnitude of voltage; According to service condition difference, described antireflecting film mainly contains three kinds of situations while use: (1), for transmitted light, antireflecting film is positioned at the front of substrate or Terahertz element; (2), for transmitted light, Graphene antireflecting film is positioned at the reverse side of substrate or Terahertz element; (3), for reflected light, Graphene antireflecting film is attached to the reverse side of substrate or Terahertz element; The face that first incident of definition terahertz light passed through is substrate or Terahertz element front, and another side is reverse side.