WO2020210973A1 - 一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源及其制备方法 - Google Patents

一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源及其制备方法 Download PDF

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WO2020210973A1
WO2020210973A1 PCT/CN2019/082835 CN2019082835W WO2020210973A1 WO 2020210973 A1 WO2020210973 A1 WO 2020210973A1 CN 2019082835 W CN2019082835 W CN 2019082835W WO 2020210973 A1 WO2020210973 A1 WO 2020210973A1
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fused silica
silica substrate
layer
liquid crystal
thz
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PCT/CN2019/082835
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English (en)
French (fr)
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王磊
邱红松
肖芮文
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南京邮电大学
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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1343—Electrodes
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1347—Arrangement of liquid crystal layers or cells in which the final condition of one light beam is achieved by the addition of the effects of two or more layers or cells

Definitions

  • the invention relates to a multifunctional adjustable THz source of integrated ferromagnetic material and large birefringent liquid crystal material driven by a magnetic field and an electric field and a preparation method thereof, and can be used in the technical field of terahertz optoelectronics.
  • THz terahertz
  • THz emission source based on the spin current in a ferromagnetic heterojunction.
  • the frequency range of the compact heterojunction emission source is about 30THz.
  • the polarization of the THz radiation wave perpendicular to the spin polarization can be conveniently controlled in a non-contact manner by applying an external magnetic field.
  • its polarization tunability is only Limited to the direction change of linear THz waves. So far, the lack of broadband and easily adjustable THz sources with comprehensive polarization tunability has greatly restricted the development of polarization-sensitive applications.
  • LCs Liquid crystals
  • NJU-LDn-4 an electrically adjustable broadband THz wave plate
  • the broadband THz wave plate has the advantages of flexible and compact unit structure, and can be installed on a broadband THz emission source, making it a Complete THz active device, therefore, ferromagnetic heterojunction is a very promising choice.
  • Both the heterojunction emission source and the LC wave plate have the characteristics of compact structure and bandwidth.
  • the metal layer of the emission source can be used as the electrode of the LC cell.
  • the close combination of LC wave plate and ferromagnetic heterojunction can provide THz waves with different polarization states. As far as those skilled in the art know, similar highly integrated tunable THz emission sources have never been reported.
  • the purpose of the present invention is to solve the above-mentioned problems in the prior art, and propose a multifunctional adjustable THz source integrated ferromagnetic material and large birefringent liquid crystal material driven by a magnetic field and an electric field and a preparation method thereof.
  • a multifunctional adjustable THz source integrated ferromagnetic material and large birefringent liquid crystal material driven by a magnetic field and an electric field including a first fused silica substrate and a first fused silica substrate with a gap.
  • Two fused silica substrates, the cobalt layer and the platinum layer are respectively deposited as ferromagnetic and non-magnetic layers on the first fused silica substrate, and the few layers of porous graphene are transferred to the second fused silica substrate through a transfer process to transfer the azo
  • the dye SD1 is spin-coated and photo-oriented as the material of the orientation layer above the platinum layer and the graphene layer.
  • a polyester film is also arranged between the first fused silica substrate and the second fused silica substrate to form a unit of uniform thickness, filled with liquid crystal material, and sealed with epoxy resin to form a terahertz electrically controlled large birefringence LC layer.
  • the first fused silica substrate is a front fused silica substrate, the thickness of the front fused silica substrate is 0.5 mm, and the second fused silica substrate is a rear fused silica substrate, and the thickness of the rear fused silica substrate is 0.8 mm .
  • the cobalt layer and the platinum layer are cleaned in advance with acetone and isopropanol in an ultrasonic machine to remove any contaminants, and the cobalt layer and the platinum layer are both grown under ultra-high vacuum conditions by room temperature electron beam evaporation .
  • the thickness of the cobalt layer is 10 nm, and the thickness of the platinum layer is 7 nm.
  • the few-layer porous graphene is grown by chemical vapor deposition, and the porous structure is introduced into the graphene film by UV ozone treatment, and the porous structure has a high transmittance in the THz region.
  • the liquid crystal material is a terahertz electrically controlled large birefringent liquid crystal material NJU-LDn-4, and the average birefringence of the liquid crystal material NJU-LDN-4 in the range of 0.4-1.6 THz is 0.306, which is at 1.6 THz The maximum value is 0.314.
  • the thickness of the polyester film is 250 ⁇ m.
  • the invention also explains the preparation method of a multifunctional adjustable THz source of integrated ferromagnetic material and large birefringent liquid crystal material driven by magnetic field and electric field,
  • the method includes the following steps:
  • the first fused silica substrate is the front fused silica substrate
  • the second fused silica substrate is the rear fused silica substrate
  • the cobalt layer and the platinum layer Deposited on the front fused silica substrate as a ferromagnetic and non-magnetic layer
  • the few layers of porous graphene are transferred to the back fused silica substrate by a transfer process; among them, the Co layer and the Pt layer are both evaporated under the ultra-high vacuum condition with room temperature electron beam Growth, pre-cleaned with acetone and isopropanol in an ultrasonic machine, few layers of graphene are grown by chemical vapor deposition, and the porous structure is introduced by UV ozone treatment;
  • the present invention adopts the above technical solutions and has the following technical effects:
  • the polarization of the THz radiation wave perpendicular to the spin polarization can be conveniently controlled in a non-contact manner by applying an external magnetic field.
  • the transmittance of the few-layer porous graphene film grown by chemical vapor deposition (CVD) and UV ozone treatment is as high as 98% in the THz region, which can be used as a good THz electrode.
  • the full-covered electrode on the entire surface provides a stronger and more uniform electric field than the wire grid electrode at the same voltage, resulting in higher modulation sensitivity
  • the linear THz emission mechanism of the Co/Pt layer is used to realize a high-efficiency and polarization-sensitive THz emission source.
  • the technical solution realizes a THz emission source with compact structure, wide frequency bandwidth, good economy, and easy adjustment, which can be widely used in polarization-sensitive research and engineering applications, and has good application prospects.
  • Figure 1 is a schematic diagram of the structure of the polarization-adjustable THz emission source of the present invention.
  • Figure 2 is a diagram of the linear THz emission mechanism of the Co/Pt layer of the present invention.
  • Fig. 3 is a magnetic control diagram of the polarization state of the present invention, the polarization state is right-handed circularly polarized light.
  • Fig. 4 is a magnetic control diagram of the polarization state of the present invention, and the polarization state is linearly polarized light along the e-axis.
  • Figure 5 is a magnetic control diagram of the polarization state of the present invention, and the polarization state is left-handed circularly polarized light.
  • FIG. 6 is a magnetic control diagram of the polarization state of the present invention, the polarization state is linearly polarized light along the O axis.
  • Fig. 7 is an ellipticity spectrum diagram of the waveforms of Figs.
  • FIG. 8 is an electrical modulation time-domain spectrum diagram of the phase delay of the present invention. As a voltage is applied between the metal layer electrode and the graphene electrode, the phase delay between the o light and the e light becomes smaller.
  • Fig. 9 is an electrical modulation frequency domain spectrum diagram of the phase delay of the present invention.
  • the voltage is about 8V, the frequency dependence of the phase difference disappears with the application of different voltages.
  • Fig. 10 is an electrical modulation diagram of the phase delay of the present invention, a diagram of the relationship between voltage and ellipticity at 1 THz and 1.5 THz.
  • the present invention discloses a multifunctional adjustable THz source integrated ferromagnetic material and large birefringent liquid crystal material driven by a magnetic field and an electric field. As shown in FIG. 1, it includes a first piece of fused silica substrate 1 and a second piece of fused silica substrate 1 arranged in a gap. Block fused silica substrate 2. Cobalt (Co) layer and platinum (Pt) layer 3 are respectively deposited as ferromagnetic (FM) and non-magnetic (NM) layers on the first fused silica substrate 1. The few layers of porous graphene 4 are transferred to On the second fused silica substrate 2, the azo dye SD1 5 is spin-coated and photo-oriented as the material for the orientation layer above the platinum layer and the graphene layer.
  • a polyester film is also arranged between the first fused silica substrate 1 and the second fused silica substrate 2 to form a unit of uniform thickness, filled with liquid crystal material, and sealed with epoxy resin to form a terahertz electronically controlled double Refractive LC layer.
  • the liquid crystal material is preferably a terahertz electrically controlled large birefringent liquid crystal material NJU-LDn-4.
  • the SD1 layer also ensures uniform pre-alignment of the liquid crystal molecules.
  • a polyester film is stacked between the two substrates to form a unit of uniform thickness and infiltrated with NJU-LDn-4 to form an LC layer 6 with a THz electrically controlled large birefringence.
  • a polyester film is also arranged between the first fused silica substrate 1 and the second fused silica substrate 2 to form a unit of uniform thickness.
  • the liquid crystal material NJU-LDn-4 is poured into it and sealed with epoxy resin to form a thin film. Hertz electrically controlled LC layer with large birefringence.
  • the first fused silica substrate 1 is a front fused silica substrate
  • the thickness of the front fused silica substrate is 0.5 mm
  • the second fused silica substrate 2 is a rear fused silica substrate.
  • the thickness of the fused silica substrate is 0.8 mm.
  • the cobalt layer and the platinum layer are cleaned in advance using acetone and isopropanol in an ultrasonic machine to remove any contaminants.
  • the cobalt layer and the platinum layer are both grown under the ultra-high vacuum condition by room temperature electron beam evaporation.
  • the thickness of the cobalt layer is 10 nm, and the thickness of the platinum layer is 7 nm.
  • the few-layer porous graphene is grown by chemical vapor deposition, and the porous structure is introduced into the graphene film by UV ozone treatment, and the porous structure has a high transmittance in the THz region.
  • the average birefringence of the liquid crystal material NJU-LDN-4 in the range of 0.4-1.6 THz is 0.306, and the maximum value is 0.314 at 1.6 THz.
  • the thickness of the polyester film is 250 ⁇ m.
  • the present invention also discloses a preparation method of a multifunctional adjustable THz source integrated ferromagnetic material and large birefringent liquid crystal material driven by a magnetic field and an electric field, and the method includes the following steps:
  • Co layer and the Pt layer are both grown under ultra-high vacuum conditions by room temperature electron beam evaporation. They are cleaned in advance with acetone and isopropanol in an ultrasonic machine. The few layers of graphene are grown by chemical vapor deposition, and the porous structure is treated by UV ozone.
  • Figure 2 is a diagram of the linear THz emission mechanism of the Co/Pt layer of the present invention.
  • the edges of the first fused silica substrate 1 and the second fused silica substrate 2 use polyester film as spacers to form a uniform liquid crystal layer with a thickness of 250 ⁇ m.
  • the liquid crystal material NJU-LDn-4 penetrates through capillary action and uses epoxy resin glue Sealing to form a large birefringent LC layer, the arrangement of LC molecules is parallel to the XY plane and is in the -45° direction to the X axis. The directions parallel to and perpendicular to the director are expressed as extraordinary light and ordinary light respectively;
  • the present invention also discloses a measurement method of a multifunctional adjustable THz source integrated ferromagnetic material and a large birefringent liquid crystal material driven by a magnetic field and an electric field.
  • the measurement method includes the following steps:
  • S10 Use the linearly polarized laser pulse emitted by the Ti:Sapphire amplifying laser to pump and pump the THz emission source with a duration of 100fs, a repetition frequency of 1kHz and a center wavelength of 800nm;
  • S20 Use three parabolic mirrors to collect and focus THz radiation, and use (110)-oriented ZnTe crystals to detect focused THz pulses through electro-optical sampling;
  • S30 Use the front wire grid polarizer and the rear wire grid polarizer to independently measure the e and o components of the THz waveform.
  • the transmission axis of the front polarizer is aligned along the e axis or the o axis to align the front wire grid polarizer and the rear
  • the wire grid polarizers are separated from each other, and the transmission axis of the rear polarizer is always aligned along the X axis;
  • the diameter of the laser spot of the Ti:Sapphire amplification laser is 8mm, and the laser pulse energy is 0.15mJ; in the S20 step, the thickness of the ZnTe crystal is 1mm, and in the S30 step Where the e and o components are in an LC cell with a thickness of d, and the phase delay between them is determined by the equation Given that, in the step S40, the magnitude of the external magnetic field H is 150 mT.
  • the linear THz emission mechanism of the Co/Pt layer is used, as shown in Fig. 2 is the linear THz emission mechanism of the Co/Pt layer.
  • the ultrafast laser pulse excites Fermi level electrons in the Co layer to a higher energy band.
  • Most of the spin electrons excited by light are in the sp energy band, and a few are in the d energy band.
  • Most of the spin electrons in the SP energy band have a higher speed than the few spin electrons in the d band. Therefore, the light in the Co layer
  • the current is spin-polarized, the so-called spin current.
  • the e component of the THz wave has a certain phase delay relative to the o component.
  • the phase delay between them is determined by the equation It is given, where, [lambda] is the wavelength of the THz waves, n-e and n o are the refractive indices of e-wave and o-wave.
  • the linearly polarized laser pulse emitted by the Ti:Sapphire amplification laser is used to pump and pump the THz emission source with a duration of 100fs, a repetition frequency of 1kHz and a center wavelength of 800nm.
  • the diameter of the laser spot is 8mm.
  • Three parabolic mirrors are used to collect and focus the THz radiation.
  • the focused THz pulse is detected by electro-optical (EO) sampling.
  • an external electrostatic magnetic field H ( ⁇ 150mT) was applied to keep the sample in a saturated magnetization state.
  • H an external electrostatic magnetic field
  • two front wire grid polarizers and a rear wire grid polarizer are used.
  • the transmission axis of the front wire grid polarizer is aligned along the e-axis or the o-axis to separate the two components from each other, and the transmission axis of the rear wire grid polarizer is always aligned along the X axis.
  • the energy of each laser pulse is 0.15mJ.
  • the amplitude of the THz emission source is about 100V/cm, which is an order of magnitude smaller than the emission amplitude of 0.5mm thick (110) oriented ZnTe crystal measured under the same device. After integrating the liquid crystal cell, the attenuation of the THz radiation amplitude is less than 30% .
  • a magnetic field H is applied in various directions without voltage, and the influence of the external magnetic field on the polarization state of the THz emission is studied.
  • Figure 3, Figure 4, Figure 5 and Figure 6 show the detected THz waveforms when the magnetic field H is along the y-, o-, x- and e-axes, respectively, and the coordinates are shown in Figure 1.
  • the magnetic field H is applied along the (a)y-, (b)o-, (c)x- and (d)e axes, and the polarization states of the emitted THz waves are (a) right-handed circularly polarized light, (b) along Linearly polarized light along the e-axis, (c) left-handed circularly polarized light, and (d) linearly polarized light along the O-axis.
  • the external magnetic field controls the magnetization direction of the Co layer, thereby changing the relative angle between the initial THz emission and the director of the LC layer.
  • H acts in the y-direction
  • the THz wave generated by Co/Pt is polarized along x according to ISHE.
  • the e component has a phase delay with respect to the o component.
  • the right-handed circularly polarized THz pulse is obtained from Co/Pt/LC.
  • the THz polarization of the Co/Pt emission is equal to the e of LC -
  • the axes are parallel, so that Co/Pt/LC produces delayed linearly polarized e-waves.
  • left-handed circularly polarized pulses and linearly polarized pulses can also be obtained by applying H along the x-axis and e-axis. H rotates 45° around the Z axis, and the polarization state alternates between linear and circular.
  • Figure 7 shows the ellipticity spectrum of the THz wave shown in Figures 3-6. When a magnetic field is applied along the x and y directions, nearly perfect circular polarization can be obtained at 1 THz. For frequencies above and below 1 THz, the waves are elliptically polarized.
  • This embodiment studies the electrical response of the Co/Pt/LC emitter when the magnetic field is along the y direction, as shown in FIG. 8.
  • the metal layer also serves as the front electrode, opposite to the graphene layer behind, generating a uniform electric field in the LC layer, which is driven by a 1kHz square wave AC voltage source.
  • a voltage is applied to the LC, the molecules tend to align in the direction of the electric field.
  • the director continuously rotates from in-plane to out-of-plane, so that the e-wave and o-wave will not have a phase difference when they reach the saturation voltage. .
  • Figure 8 shows the electrical modulation of the phase delay between the o component and the e component.
  • the phase delay between the o and e light becomes smaller.
  • the o wave leads the e wave by ⁇ 0.21ps; when a voltage is applied, the o wave does not change significantly, while the e wave moves to the o wave in the time domain.
  • the voltage is higher than 8V, the phase delay disappears .
  • Figure 9 shows the frequency-dependent phase delay under different voltages.
  • the voltage is around 8V, with the application of different voltages, the frequency dependence of the phase difference disappears. Due to the low dispersion of LC in a wide frequency range, the linear relationship between phase delay and frequency can be clearly seen in the figure.
  • the frequency is higher than 1THz, a phase delay greater than ⁇ /2 can be achieved.
  • a wave of any ellipticity can be obtained in the frequency range of 1 to 1.5 THz, which is consistent with the result shown in Fig. 8; when a saturation voltage of about 8V is applied, there is almost no wave in the entire frequency range in Fig. 9 Phase difference.
  • the sensitivity is higher than the results reported in the previous work, which requires a voltage of ⁇ 20V.
  • the LC material NJU-LDN-4LC
  • the thickness of the LC layer 250 ⁇ m
  • the preparation and transfer methods of the back electrode (small-layer porous graphene) in the two processes are the same.
  • a metal wire mesh was used instead of the metal layer as the front electrode. Compared with the wire grid electrode, the full-covered electrode on the entire surface in this work provides a stronger and more uniform electric field under the same voltage, thereby producing a higher modulation sensitivity.
  • the evolution of the ellipticity when the working voltage is 1THz and 1.5THz is shown in Figure 10.
  • the ellipticity is sensitively dependent on the working voltage. When a high working voltage is applied, the ellipticity tends to zero. When no voltage is applied, an approximately circularly polarized wave is obtained at 1THz; on the contrary, at 1.5THz, a working voltage of about 1.3V is required to obtain a circularly polarized wave. This value is estimated by fitting and extrapolation Yes, this means that even if circular polarization is only achieved within a limited bandwidth, the frequency range can be adjusted by applying an appropriate operating voltage.
  • the minimum value of the tunable frequency is 1 THz. If a thicker LC layer, such as a double-stacked LC, is used, the tunable range can be extended to a lower frequency range.
  • the small deviation of the ellipticity at 0V relative to that at 1V is mainly due to the difference between the losses in the LC along the e-axis and the o-axis. The amplitudes of the two components are slightly different.
  • One possible solution is to compensate for the amplitude difference by precisely changing the direction of the external magnetic field H; considering the thickness of the LC layer in the experiment, the electrical response time is in seconds, on the contrary , The magnetic response time is determined by the magnetization process of the Co layer, which is about 10ns.
  • the broadband polarization tunable THz source can be used as a powerful tool for sensing polarization-sensitive samples (such as spiral biomolecules).
  • polarization-sensitive samples such as spiral biomolecules
  • a compact and economical polarization tunable THz source can also be implemented on a machining machine to perform in-situ topographic measurements on surface samples.
  • the invention utilizes the electronically controlled birefringence characteristics of liquid crystals and the anti-spin Hall effect (ISHE) of ferromagnetic heterojunctions, and by separately adjusting the applied electric field and magnetic field to conveniently control the phase delay and polarization state, linear polarization and circularity are realized.
  • the non-contact conversion of polarization can also perform highly efficient electrical tuning of the phase delay.
  • the terahertz emission source with compact structure, wide working frequency bandwidth, good economy and easy adjustment can be widely used in polarization-sensitive terahertz research and engineering applications, and has good application prospects.

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Abstract

一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源及其制备方法，THz源包括间隙设置的第一块熔融石英基板（1）和第二块熔融石英基板（2），钴层和铂层（3）分别作为铁磁和非磁性层沉积在第一块熔融石英基板（1）上，少层多孔石墨烯（4）通过转移工艺转移到第二块熔融石英基板（2）上，将偶氮染料SD1（5）旋涂于两块熔融石英基板并光取向，填充入THz大双折射的LC材料形成均匀取向LC层（6），第一块熔融石英基板（1）和第二块熔融石英基板（2）之间还间隔设置有聚酯薄膜用以形成厚度均匀的单元。利用LC的电控双折射特性和铁磁异质结的反自旋霍尔效应，通过分别调整外加电场和磁场从而方便地控制THz相位延迟和偏振状态，实现了THz波线性偏振和圆偏振的非接触转换。

Description

一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源及其制备方法 技术领域
本发明涉及一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源及其制备方法，可用于太赫兹光电子技术领域。
背景技术
在过去的几十年里，太赫兹(THz)辐射的产生和应用已经取得了很大的进展，它的偏振信息为螺旋生物聚合物传感、高分辨率地形探测和自旋动力学研究等各种应用提供了更深入的见解，因此，宽带偏振可调THz发射源在现代THz系统中有着潜在的应用前景。调整THz波偏振状态的一种可能方法是调节泵浦光束的相位延迟或方位角；另一种方法是在线性偏振THz系统中插入一个额外的波片，但是，这两种方法都需要进行复杂的调整，以实现在较宽的THz范围内对偏振状态的控制，不利于实际应用。
近年来，研究学者们提出了一种基于铁磁异质结中自旋电流的新型THz发射源。紧凑型异质结发射源的频率范围约为30THz，垂直于自旋极化的THz辐射波的极化可以通过外加磁场以非接触的方式方便地加以控制，然而，它的偏振可调性仅限于线性THz波的方向变化。迄今为止，由于缺乏宽带和易于调节的具有综合偏振可调性的THz源，极大地限制了偏振敏感应用的发展。
液晶(LCs)作为一种连续可变的延迟器受到广泛关注，因其在较宽的频率范围内具有相对大的双折射和易于调控的特性等。利用大双折射LC材料NJU-LDn-4开发出了电可调宽带THz波片，而且该宽带THz波片具有灵活和紧凑的单元结构优势，可安装在宽带THz发射源上，使其成为一个完整的THz有源器件，因此，铁磁异质结是一种很有前景的选择。异质结发射源和LC波片都具有结构紧凑、频带宽的特点。此外，发射源的金属层又可作为LC单元的电极。LC波片与铁磁异质结的紧密结合可以提供具有 不同偏振状态的THz波。据本领域技术人员所知，类似的高度集成可调谐THz发射源从未报道过。
发明内容
本发明的目的就是为了解决现有技术中存在的上述问题，提出一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源及其制备方法。
本发明的目的将通过以下技术方案得以实现：一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源，包括间隙设置的第一块熔融石英基板和第二块熔融石英基板，钴层和铂层分别作为铁磁和非磁性层沉积在第一块熔融石英基板上，少层多孔石墨烯通过转移工艺转移到第二块熔融石英基板上，将偶氮染料SD1旋涂并光取向，作为铂层和石墨烯层上方的取向层材料，
第一块熔融石英基板和第二块熔融石英基板之间还间隔设置有聚酯薄膜用以形成厚度均匀的单元，填充入液晶材料，用环氧树脂胶密封形成太赫兹电控大双折射的LC层。
优选地，所述第一块熔融石英基板为前熔融石英基板，前熔融石英基板的厚度为0.5mm，所述第二块熔融石英基板为后熔融石英基板，后熔融石英基板的厚度为0.8mm。
优选地，所述钴层和铂层预先在超声波机中使用丙酮和异丙醇进行清洁，用于去除任何污染物，所述钴层和铂层均在超高真空条件下室温电子束蒸发生长。
优选地，所述钴层厚度为10nm，铂层厚度为7nm。
优选地，所述少层多孔石墨烯通过化学气相沉积生长，并通过UV臭氧处理将多孔结构引入石墨烯薄膜，该多孔结构在THz区域的透射率高。
优选地，所述液晶材料为太赫兹电控大双折射率液晶材料NJU-LDn-4，所述液晶材料NJU-LDN-4在0.4-1.6THz范围内的平均双折射为0.306，在1.6THz时最大值为0.314。
优选地，所述聚酯薄膜厚度为250μm。
本发明还解释了一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源的制备方法，
该方法包括以下步骤：
S1：取第一块熔融石英基板和第二块熔融石英基板，所述第一块熔融石英基板为前熔融石英基板，所述第二块熔融石英基板为后熔融石英基板，钴层和铂层分别作为铁磁和非磁性层沉积在前熔融石英基板上，少层多孔石墨烯通过转移工艺转移到后熔融石英基板上；其中，Co层和Pt层均在超高真空条件下室温电子束蒸发生长，在超声波机中用丙酮和异丙醇预先进行清洁，少层石墨烯通过化学气相沉积生长，多孔结构由UV臭氧处理引入；
S2：在铂层和石墨烯层表面分别旋偶氮染料作为取向层材料，确保液晶分子的均匀预取向；
S3：第一块熔融石英基板和第二块熔融石英基板边缘用聚酯薄膜作为间隔物以形成250μm厚均匀的液晶层，液晶材料NJU-LDn-4通过毛细作用渗入并用环氧树脂胶密封以形成大双折射的LC层，LC分子的排列平行于X-Y平面，并与X轴成-45°方向，与指向矢平行和垂直的方向分别表示为非常光和寻常光；
S4：利用铁磁异质结的反自旋霍尔效应和液晶的电控双折射特性，通过改变外磁场的方向控制THz发射的偏振状态，e波和o波之间的相位延迟也能够高效率地进行电调谐。
本发明采用以上技术方案与现有技术相比，具有以下技术效果：
(1)利用铁磁异质结中的反自旋霍尔效应(ISHE)，垂直于自旋极化的THz辐射波的极化可以通过外加磁场以非接触的方式方便地加以控制。
(2)利用大双折射LC材料NJU-LDN-4作为连续可变的延迟器，该材料在较宽的频率范围内具有相对大的双折射和易于调控的特性，在0.4～1.6THz范围内的平均双折射为0.306，在1.6THz时有最大值为0.314，且在研究的频率范围内没有明显的吸收。
(3)大双折射的LC材料与铁磁异质结的紧密结合可以提供具有不同偏振状态的THz波。
(4)利用化学气相沉积(CVD)生长和UV臭氧处理的少层多孔石墨烯薄膜在THz 区域的透射率高达98％，可作为很好的THz电极。
(5)本工作中整个表面的全覆盖电极在相同电压下比线栅电极提供了更强大、更均匀的电场，从而产生更高的调制灵敏度
(6)利用Co/Pt层的线性THz发射机制实现了高效率、偏振敏感的THz发射源。
(7)通过改变外磁场的方向，可以实现线性偏振和圆偏振的非接触转换，e波和o波之间的相位延迟也可以高效率地进行电调谐。
本技术方案实现了一种结构紧凑、频带宽、经济性好、易于调节的THz发射源，可广泛应用于偏振敏感的研究和工程应用中，具有很好的应用前景。
附图说明
图1为本发明的偏振可调的THz发射源结构图示意图。
图2为本发明的Co/Pt层的线性THz发射机制图。
图3为本发明偏振状态的磁控制图，偏振状态为右旋圆偏振光。
图4为本发明偏振状态的磁控制图，偏振状态为沿e轴的线性偏振光。
图5为本发明偏振状态的磁控制图，偏振状态为左旋圆偏振光。
图6为本发明偏振状态的磁控制图，偏振状态为沿O轴的线性偏振光。
图7为本发明图3-图6波形的椭圆率谱图。
图8为本发明的相位延迟的电调制时域谱图，随着在金属层电极和石墨烯电极之间施加电压，o光和e光之间的相位延迟变小。
图9为本发明的相位延迟的电调制频域谱图，当电压在8V左右时，随着不同电压的施加，相位差的频率依赖性消失。
图10为本发明的相位延迟的电调制图，1THz和1.5THz时电压和椭圆率之间的关系图。
具体实施方式
本发明的目的、优点和特点，将通过下面优选实施例的非限制性说明进行图示和解 释。这些实施例仅是应用本发明技术方案的典型范例，凡采取等同替换或者等效变换而形成的技术方案，均落在本发明要求保护的范围之内。
本发明揭示了一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源，如图1所示，包括间隙设置的第一块熔融石英基板1和第二块熔融石英基板2。钴(Co)层和铂(Pt)层3分别作为铁磁(FM)和非磁性(NM)层沉积在第一块熔融石英基板1上，少层多孔石墨烯4通过先进的转移工艺转移到第二块熔融石英基板2上，将偶氮染料SD1 5旋涂并光取向，作为铂层和石墨烯层上方的取向层材料。
第一块熔融石英基板1和第二块熔融石英基板2之间还间隔设置有聚酯薄膜用以形成厚度均匀的单元，填充入液晶材料，用环氧树脂胶密封形成太赫兹电控大双折射的LC层。在本技术方案中，所述液晶材料优选为太赫兹电控大双折射率液晶材料NJU-LDn-4。
作为Pt层和石墨烯层上方的对准层材料，SD1层同时还确保了液晶分子的均匀预对准。两个基板之间堆叠了聚酯薄膜以形成厚度均匀的单元并用NJU-LDn-4浸润，形成THz电控大双折射的LC层6。
第一块熔融石英基板1和第二块熔融石英基板2之间还间隔设置有聚酯薄膜用以形成厚度均匀的单元，灌入液晶材料NJU-LDn-4，用环氧树脂胶密封形成太赫兹电控大双折射的LC层。具体地，在本技术方案中，所述第一块熔融石英基板1为前熔融石英基板，前熔融石英基板的厚度为0.5mm，所述第二块熔融石英基板2为后熔融石英基板，后熔融石英基板的厚度为0.8mm。
所述钴层和铂层预先在超声波机中使用丙酮和异丙醇进行清洁，用于去除任何污染物，所述钴层和铂层均在超高真空条件下室温电子束蒸发生长。所述钴层厚度为10nm，铂层厚度为7nm。
所述少层多孔石墨烯通过化学气相沉积生长，并通过UV臭氧处理将多孔结构引入石墨烯薄膜，该多孔结构在THz区域的透射率高。所述液晶材料NJU-LDN-4在 0.4-1.6THz范围内的平均双折射为0.306，在1.6THz时最大值为0.314。所述聚酯薄膜厚度为250μm。
本发明还揭示了一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源的制备方法，该方法包括以下步骤：
S1：取第一块熔融石英基板1和第二块熔融石英基板2，所述第一块熔融石英基板1为前熔融石英基板，所述第二块熔融石英基板2为后熔融石英基板，钴层和铂层分别作为铁磁和非磁性层沉积在前熔融石英基板上，少层多孔石墨烯通过转移工艺转移到后熔融石英基板上。
其中，Co层和Pt层均在超高真空条件下室温电子束蒸发生长，在超声波机中用丙酮和异丙醇预先进行清洁，少层石墨烯通过化学气相沉积生长，多孔结构由UV臭氧处理引入，图2为本发明的Co/Pt层的线性THz发射机制图。
S2：在铂层和石墨烯层表面分别旋偶氮染料作为取向层材料，确保液晶分子的均匀预取向；
S3：第一块熔融石英基板1和第二块熔融石英基板2边缘用聚酯薄膜作为间隔物以形成250μm厚均匀的液晶层，液晶材料NJU-LDn-4通过毛细作用渗入并用环氧树脂胶密封以形成大双折射的LC层，LC分子的排列平行于X-Y平面，并与X轴成-45°方向，与指向矢平行和垂直的方向分别表示为非常光和寻常光；
S4：利用铁磁异质结的反自旋霍尔效应和液晶的电控双折射特性，通过改变外磁场的方向控制THz发射的偏振状态，e波和o波之间的相位延迟也能够高效率地进行电调谐。
本发明还揭示了一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源的测量方法，该测量方法包括以下步骤：
S10：利用钛宝石放大激光器发出的线性极化激光脉冲，以100fs的持续时间、1kHz的重复频率和800nm的中心波长对THz发射源进行泵浦和抽运；
S20：用三个抛物面反射镜收集和聚焦THz辐射，利用(110)取向的ZnTe晶体，通过电光取样探测聚焦的THz脉冲；
S30：使用前线栅偏振器和后线栅偏振器，对THz波形的e分量和o分量进行独立 测量，前偏振器的传输轴沿着e轴或o轴对齐，以将前线栅偏振器和后线栅偏振器彼此分离，后偏振器的传输轴始终沿X轴对齐；
S40：研究无电压的情况下外加磁场H对THz发射偏振状态的影响和磁场沿y方向时Co/Pt/LC发射极的电响应。通过改变外磁场的方向，可以使偏振态在线偏振和圆偏振之间转换，频率高于1THz的相位延迟可通过施加低电压在π/2范围内连续调节。
在所述S10步骤中，所述钛宝石放大激光器的激光光斑的直径为8mm，激光脉冲的能量为0.15mJ；在所述S20步骤中，所述ZnTe晶体的厚度为1mm，在所述S30步骤中，所述e分量和o分量在厚度为d的LC单元中，它们之间的相位延迟由方程
Figure PCTCN2019082835-appb-000001
给出，在所述S40步骤中，所述外加磁场H的大小为150mT。
为实现构造偏振敏感的THz发射源，利用了Co/Pt层的线性THz发射机制，如图2所示为Co/Pt层的线性THz发射机制。在几百飞秒之内，超快激光脉冲将Co层中的费米能级电子激发到更高的能带。光激发的多数自旋电子位于sp能带，少数位于d能带，SP能带中的大多数自旋电子比d带中的少数自旋电子具有更高的速度，因此，Co层中的光电流是自旋极化的，即所谓的自旋电流。
当自旋电流J s流入Pt层时，由于铁磁异质结的反自旋霍尔效应(ISHE)，它被转换成电荷电流J c。自旋电流与电荷电流之间的转换由J c∝γJ s×H描述，其中γ是自旋霍尔角，H是外磁场，沿x轴的电荷电流J c充当电偶极子，使THz发射源在自由空间产生线性的E THz。由于J C的方向总是垂直于H，因此通过改变H的方向可以方便地控制E THz的极化，线性极化的THz波可以分解为e分量和o分量。THz波的e分量相对于o分量有一定的相位延迟，在厚度为d的LC单元中，它们之间的相位延迟由方程
Figure PCTCN2019082835-appb-000002
给出，其中，λ是THz波的波长，n e和n o分别是e波和o波的折射率。
利用钛宝石放大激光器发出的线性极化激光脉冲，以100fs的持续时间、1kHz的重复频率和800nm的中心波长对THz发射源进行泵浦和抽运。激光光斑的直径为8mm。用三个抛物面反射镜收集和聚焦THz辐射。利用1mm厚(110)取向的ZnTe晶体，通 过电光(EO)取样探测聚焦的THz脉冲。
在实验过程中，通过施加外部静电磁场H(～150mT)，使样品保持饱和磁化状态。为了独立测量THz波形的e分量和o分量，使用了两个前线栅偏振器和后线栅偏振器。前线栅偏振器的传输轴沿着e轴或o轴对齐，以将这两个组件彼此分离，后线栅偏振器的传输轴始终沿X轴对齐。在所有的测量中，每个激光脉冲的能量为0.15mJ。THz发射源的振幅约为100V/cm，它比在相同装置下测得的0.5mm厚(110)取向的ZnTe晶体的发射振幅小一个数量级，整合液晶盒后，THz辐射振幅的衰减小于30％。
实施例1：
本实施例为在无电压的情况下向各个方向施加磁场H，研究外加磁场对THz发射偏振状态的影响。图3、图4、图5和图6显示磁场H分别沿y-、o-、x-和e-轴时检测到的THz波形，坐标如图1所示。磁场H沿(a)y-、(b)o-、(c)x-和(d)e轴施加，发射的THz波的偏振状态依次为(a)右旋圆偏振光、(b)沿e轴的线性偏振光、(c)左旋圆偏振光和(d)沿O轴的线性偏振光。
外部磁场控制着Co层的磁化方向，从而改变初始THz发射和LC层指向矢之间的相对角度。当H作用于y-方向时，Co/Pt产生的THz波按照ISHE沿x极化。e分量相对于o分量存在相位延迟，从Co/Pt/LC获得右旋圆偏振的THz脉冲，相反地，当沿着o-轴施加H时，Co/Pt发射的THz极化与LC的e-轴平行，从而Co/Pt/LC产生延迟的线性偏振e波。类似地，也可以通过沿x轴和e轴施加H来获得左旋圆偏振脉冲和线性偏振脉冲。H围绕Z轴旋转45°，偏振状态在线性和圆之间交替。图7显示了图3-图6所示THz波的椭圆率谱。当沿x和y方向施加磁场时，在1THz时可以获得近乎完美的圆偏振。对高于和低于1THz的频率，波是椭圆偏振的。
实施例2：
本实施例研究的是当磁场沿y方向时Co/Pt/LC发射极的电响应，如图8所示。金属层也作为前电极，与后面的石墨烯层相对，在LC层中产生均匀的电场，LC层由1kHz 的方波交流电压源驱动。当电压施加在LC上时，分子倾向于沿电场方向排列，随着电压的增加，指向矢不断从平面内旋转到平面外，从而使e波和o波在达到饱和电压时不会产生相位差。
图8显示了o分量和e分量之间相位延迟的电调制，随着在金属层电极和石墨烯电极之间施加电压，o和e光之间的相位延迟变小。在没有施加电压的情况下，o波超前e波～0.21ps；当施加电压时，o波没有明显的变化，而e波在时域内向o波移动，当电压高于8V时，相位延迟消失。
图9显示了不同电压下的频率相关相位延迟，当电压在8V左右时，随着不同电压的施加，相位差的频率依赖性消失。由于LC在宽频带范围内的低色散，可以清楚地在图中看到相位延迟与频率之间的线性关系。当频率高于1THz时，可以实现大于π/2的相位延迟。通过施加适当的电压，可以在1～1.5THz的频率范围内获得任意椭圆率的波，与图8所示结果一致；当施加约8V的饱和电压时，图9中的整个频率范围内几乎没有相位差。
值得注意的是，在LC的电控中，灵敏度高于先前工作中报告的结果，其中需要电压～20V。在这两个工作中，LC材料(NJU-LDN-4LC)和LC层的厚度(250μm)是相同的，两种工艺中的后电极(少层多孔石墨烯)的制备和转移方法相同。然而，在先前的工作中，使用金属丝网代替金属层作为前电极。与线栅电极相比，本工作中整个表面的全覆盖电极在相同电压下提供了更强大、更均匀的电场，从而产生更高的调制灵敏度。
工作电压为1THz和1.5THz时的椭圆率演变如图10所示，椭圆率敏感地取决于工作电压，当施加高工作电压时，椭圆率趋于零。在不施加电压的情况下，在1THz获得一个近似圆极化波；相反，在1.5THz时，需要1.3V左右的工作电压来获得圆极化波，这个值是通过拟合和外推来估计的，这意味着即使圆极化只在有限的带宽内实现，也可以通过施加适当的工作电压来调整频率范围。
在实验中，由于LC的厚度很小，可调谐频率的最小值是1THz，如果使用较厚的LC层，例如双叠层LC，则可以将可调谐范围扩展到较低的频率范围。此外，0V时椭圆率相对于1V时的小偏差主要是由于LC中沿e轴和o轴的损耗之间的差异。两种分量的振幅略有不同，一种可能的解决办法是通过精确地改变外磁场H的方向来补偿振幅差；考虑到实验中LC层的厚度，电响应时间以秒为单位，与此相反，磁响应时间是由Co层的磁化过程决定的，Co层的磁化过程约为10ns。
该宽带极化可调谐THz源可以作为一种强有力的工具，用于对极化敏感样品(如螺旋状生物分子)进行传感。此外，在工程应用中，还可以在机械加工机上实现紧凑、经济的极化可调谐THz源，以对表面样品进行原位地形测量。
本发明利用液晶的电控双折射特性和铁磁异质结的反自旋霍尔效应(ISHE)，通过分别调整外加电场和磁场从而方便的控制相位延迟和偏振状态，实现了线性偏振和圆偏振的非接触转换，也可以对相位延迟进行高效率的电调谐。该结构紧凑、工作频带宽、经济性好、易于调节的太赫兹发射源可广泛应用于偏振敏感的太赫兹研究和工程应用中，具有很好的应用前景。
本发明尚有多种实施方式，凡采用等同变换或者等效变换而形成的所有技术方案，均落在本发明的保护范围之内。

Claims (8)

  1. 一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源，其特征在于：包括间隙设置的第一块熔融石英基板(1)和第二块熔融石英基板(2)，钴层和铂层(3)分别作为铁磁和非磁性层沉积在第一块熔融石英基板(1)上，少层多孔石墨烯(4)通过转移工艺转移到第二块熔融石英基板(2)上，将偶氮染料SD1(5)旋涂并光取向，作为铂层和石墨烯层上方的取向层材料，第一块熔融石英基板(1)和第二块熔融石英基板(2)之间还间隔设置有聚酯薄膜用以形成厚度均匀的单元，填充入液晶材料，用环氧树脂胶密封形成太赫兹电控大双折射的LC层。
  2. 根据权利要求1所述的一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源，其特征在于：所述第一块熔融石英基板(1)为前熔融石英基板，前熔融石英基板的厚度为0.5mm，所述第二块熔融石英基板(2)为后熔融石英基板，后熔融石英基板的厚度为0.8mm。
  3. 根据权利要求1所述的一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源，其特征在于：所述钴层和铂层预先在超声波机中使用丙酮和异丙醇进行清洁，用于去除任何污染物，所述钴层和铂层均在超高真空条件下室温电子束蒸发生长。
  4. 根据权利要求1所述的一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源，其特征在于：所述钴层厚度为10nm，铂层厚度为7nm。
  5. 根据权利要求1所述的一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源，其特征在于：所述少层多孔石墨烯通过化学气相沉积生长，并通过UV臭氧处理将多孔结构引入石墨烯薄膜，该多孔结构在THz区域的透射率高。
  6. 根据权利要求1所述的一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源，其特征在于：所述液晶材料为太赫兹电控大双折射率液 晶材料NJU-LDn-4，所述液晶材料NJU-LDN-4在0.4-1.6THz范围内的平均双折射为0.306，在1.6THz时最大值为0.314。
  7. 根据权利要求1所述的一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源，其特征在于：所述聚酯薄膜厚度为250μm。
  8. 一种磁场和电场共同驱动的集成铁磁材料和大双折射液晶材料的多功能可调THz源的制备方法，其特征在于：
    该方法包括以下步骤：
    S1：取第一块熔融石英基板(1)和第二块熔融石英基板(2)，所述第一块熔融石英基板(1)为前熔融石英基板，所述第二块熔融石英基板(2)为后熔融石英基板，钴层和铂层分别作为铁磁和非磁性层沉积在前熔融石英基板上，少层多孔石墨烯通过转移工艺转移到后熔融石英基板上；其中，Co层和Pt层均在超高真空条件下室温电子束蒸发生长，在超声波机中用丙酮和异丙醇预先进行清洁，少层石墨烯通过化学气相沉积生长，多孔结构由UV臭氧处理引入；
    S2：在铂层和石墨烯层表面分别旋偶氮染料作为取向层材料，确保液晶分子的均匀预取向；
    S3：第一块熔融石英基板(1)和第二块熔融石英基板(2)边缘用聚酯薄膜作为间隔物以形成250μm厚均匀的液晶层，液晶材料NJU-LDn-4通过毛细作用渗入并用环氧树脂胶密封以形成大双折射的LC层，LC分子的排列平行于X-Y平面，并与X轴成-45°方向，与指向矢平行和垂直的方向分别表示为非常光和寻常光；
    S4：利用铁磁异质结的反自旋霍尔效应和液晶的电控双折射特性，通过改变外磁场的方向控制THz发射的偏振状态，e波和o波之间的相位延迟也能够高效率地进行电调谐。
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