CN101335425A - A device and method for generating ultra-strong terahertz radiation - Google Patents
A device and method for generating ultra-strong terahertz radiationDownload PDFInfo
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Abstract
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Description
Translated fromChinese技术领域technical field
本发明属于强场和超快物理技术领域，具体地说，本发明涉及一种产生高功率太赫兹辐射的装置及方法。The invention belongs to the technical field of strong field and ultrafast physics, in particular, the invention relates to a device and method for generating high-power terahertz radiation.
背景技术Background technique
目前有关太赫兹(THz，1012赫兹)电磁辐射的产生和应用受到物理学界广泛关注。THz辐射的频率介于可见光和微波之间。凝聚态物质的声子频率、大分子的振转频率在THz波段有很多特征指纹谱，并包含着非常丰富的物理和化学信息。因此，在物理、材料、生物、信息等领域，用THz辐射对研究对象进行扫描成像和光谱检测有着广泛的应用前景。但是，由于基于激光与固体材料作用产生的THz辐射受转换效率低和材料破坏阈值的限制，目前的技术难以获得高功率的辐射，无法满足在某些重要应用方面的需求。譬如快速二维空间实时成像，强场凝聚态物理等研究需要场强达到100MV/m的THz辐射。为此，人们一直在寻找可以产生超强THz辐射的新方案。国际上一些重要研究机构都在开展用等离子体和高能电子加速器来产生超强THz辐射的研究。譬如，德国柏林电子储存环同步辐射公司通过利用超短脉冲电子束获得超强的相干THz辐射；美国劳伦斯伯克利实验室的科学家利用强激光产生的超短脉冲电子束穿过介质表面，产生超强的THz辐射；美国的几个国家实验室联合研究把超短脉冲激光辐照砷化镓产生超短电子束，经过加速后通过同步辐射产生超强的THz辐射。上述方案一般需要电子加速器装置，成本高昂，体积过大，一般小型实验室难以获得，同时THz辐射产生的效率不够高，相干性较难控制。At present, the generation and application of terahertz (THz, 1012 Hz) electromagnetic radiation has attracted extensive attention from the physics community. THz radiation has a frequency between visible light and microwaves. The phonon frequency of condensed matter and the vibrational frequency of macromolecules have many characteristic fingerprints in the THz band, and contain very rich physical and chemical information. Therefore, in the fields of physics, materials, biology, information, etc., scanning imaging and spectral detection of research objects with THz radiation has broad application prospects. However, because the THz radiation generated based on the interaction between laser and solid materials is limited by low conversion efficiency and material damage threshold, it is difficult to obtain high-power radiation with current technology, which cannot meet the needs of some important applications. For example, rapid two-dimensional space real-time imaging, strong-field condensed matter physics and other research require THz radiation with a field strength of 100MV/m. For this reason, people have been looking for new solutions that can generate ultra-strong THz radiation. Some important research institutions in the world are carrying out research on the use of plasma and high-energy electron accelerators to generate ultra-intense THz radiation. For example, the German Berlin Electronic Storage Ring Synchrotron Radiation Company obtains super-intensive coherent THz radiation by using ultra-short pulse electron beams; scientists at the Lawrence Berkeley Laboratory in the United States use ultra-short pulse electron beams generated by strong lasers to pass through the surface of the medium to generate super-intense THz radiation. THz radiation; Several national laboratories in the United States have jointly studied the ultrashort pulse laser irradiation gallium arsenide to generate ultrashort electron beams, which are accelerated to produce super strong THz radiation through synchrotron radiation. The above schemes generally require electron accelerator devices, which are expensive and bulky, and are difficult to obtain in small laboratories. At the same time, the efficiency of THz radiation is not high enough, and the coherence is difficult to control.
等离子体作为一个非线性介质，有望可以用来产生超强太赫兹辐射。由激光脉冲有质动力在稀薄等离子体中激发的电子等离子体波，其电场振幅可达100GV/m，这种大振幅等离子体波的典型振动频率在太赫兹附近。理论和数值模拟表明，在一定条件下，这种大振幅的电子等离子体波，可以通过模式转换，部分地转换成THz电磁辐射。这种方案采用了大尺度不均匀等离子体作为THz辐射产生介质。Plasma, as a nonlinear medium, is expected to be used to generate ultra-intense terahertz radiation. The electric field amplitude of the electron plasma wave excited by the qualitative force of the laser pulse in the thin plasma can reach 100GV/m, and the typical vibration frequency of this large-amplitude plasma wave is around terahertz. Theoretical and numerical simulations show that under certain conditions, this large-amplitude electron plasma wave can be partially converted into THz electromagnetic radiation through mode conversion. This scheme uses large-scale inhomogeneous plasma as the THz radiation generation medium.
另一种将电子等离子体振荡转换成电磁辐射的办法是采用激光照射很薄的均匀等离子体层，如图1所示。如果等离子体层2的厚度D趋近于THz辐射的波长λTH，等离子体密度在1016～1018/cm-3，那么激光驱动的电子等离子体振荡可以部分地转换成THz频段的电磁辐射。数值模拟研究也表明，当激光1以入射角α入射到稀薄等离子体层2时，当D趋近于λTH时产生两个强度相当的THz脉冲3、4，一个沿着激光入射方向，另一个在入射激光对于等离子体薄层的镜面反射方向。此外，THz辐射的振幅与入射角的正弦值成正比。这个方案可以产生单周期的THz辐射，并且其辐射强度与前一种方案相当。Another way to convert electron plasma oscillations into electromagnetic radiation is to irradiate a thin, uniform layer of plasma with a laser, as shown in Figure 1. If the thickness D of the
本发明主要采用了后一种THz辐射的产生原理，通过强激光的作用产生高强度的THz辐射。The present invention mainly adopts the latter generation principle of THz radiation, and generates high-intensity THz radiation through the action of strong laser.
发明内容Contents of the invention
本发明的目的是，克服现有技术的不足，提供一种尺寸小、且具有高转换效率的产生高强度THz辐射的装置和方法。The object of the present invention is to overcome the deficiencies of the prior art and provide a device and method for generating high-intensity THz radiation with small size and high conversion efficiency.
为了达到上述目的，本发明采取如下技术方案：In order to achieve the above object, the present invention takes the following technical solutions:
一种产生超强太赫兹辐射的装置，包括A device for generating ultra-intense terahertz radiation comprising
一气体填充管；a gas-filled tube;
一电离激光器，用于输出电离激光脉冲，以在所述气体填充管中形成真空层和电离气体层；以及an ionization laser for outputting ionization laser pulses to form a vacuum layer and an ionized gas layer in said gas-filled tube; and
一泵浦激光器；用于输出泵浦激光脉冲，所述泵浦激光脉冲通过所述气体填充管的侧壁多次反射，反复作用到所述电离气体层。A pumping laser; used to output pumping laser pulses, the pumping laser pulses are reflected multiple times by the sidewall of the gas-filled tube, and repeatedly act on the ionized gas layer.
上述技术方案中，所述真空层位于所述气体填充管的中心，所述真空层周围形成电离气体层。In the above technical solution, the vacuum layer is located at the center of the gas-filled tube, and an ionized gas layer is formed around the vacuum layer.
上述技术方案中，所述气体填充管的侧壁采用对泵陠激光反射率大于90％的材料制作；所述气体填充管的两端为对激光和太赫兹辐射透明的材料制作的镜面。In the above technical solution, the side wall of the gas-filled tube is made of a material with a reflectivity of more than 90% for the pumping laser; both ends of the gas-filled tube are mirrors made of a material transparent to laser and terahertz radiation.
上述技术方案中，所述气体填充管的轴线方向与所述电离激光器的激光出射方向相同；所述气体填充管的轴线方向与所述泵浦激光器的激光出射方向的夹角大于0度，小于30度。In the above technical solution, the axis direction of the gas-filled tube is the same as the laser emission direction of the ionization laser; the included angle between the axis direction of the gas-filled tube and the laser emission direction of the pump laser is greater than 0 degrees and less than 30 degrees.
上述技术方案中，所述气体填充管的内侧壁至少具有两个平行面；所述电离气体层和与真空层的交界面也至少具有两个平行面，且该两个平行面与所述气体填充管的内侧壁的两个平行面平行。In the above technical solution, the inner wall of the gas-filled tube has at least two parallel surfaces; the interface between the ionized gas layer and the vacuum layer also has at least two parallel surfaces, and the two parallel surfaces are connected to the gas The two parallel faces of the inner side walls of the fill tube are parallel.
上述技术方案中，在所述各平面的法线方向上，所述真空层的厚度与位于其两侧的电离气体层的厚度均相等。In the above technical solution, in the normal direction of each plane, the thickness of the vacuum layer is equal to the thickness of the ionized gas layer on both sides thereof.
上述技术方案中，所述真空层的厚度与两侧的电离气体层的厚度也可以不等，电离气体层的密度密度分布可以不均匀，由此产生的THz辐射不是准单周期的，但仍可以产生高功率的THz辐射。In the above technical scheme, the thickness of the vacuum layer and the thickness of the ionized gas layer on both sides can also be different, and the density distribution of the ionized gas layer can be uneven, and the resulting THz radiation is not quasi-single-period, but still High power THz radiation can be produced.
上述技术方案中，所述气体填充管的激光入射端面与所述泵浦激光器的激光出射方向垂直。In the above technical solution, the laser incident end face of the gas-filled tube is perpendicular to the laser emission direction of the pump laser.
一种产生超强太赫兹辐射的方法，包括如下步骤：A method for producing ultra-strong terahertz radiation, comprising the steps of:
1)电离激光器发出中等强度的激光脉冲；该激光脉冲沿着气体填充管的轴向传播，在其经过的路线上形成真空层，两侧形成电离气体层；1) The ionization laser emits a medium-intensity laser pulse; the laser pulse propagates along the axial direction of the gas-filled tube, forming a vacuum layer on the route it passes, and forming an ionized gas layer on both sides;
2)经过纳秒量级的时间延迟后，泵陠激光器发出强激光脉冲，该脉冲以一定角度射入气体填充管，在气体填充管内经多次反射激发出超强的准单周期太赫兹相干辐射。2) After a time delay on the order of nanoseconds, the pump laser emits a strong laser pulse, which is injected into the gas-filled tube at a certain angle, and the super-strong quasi-single-cycle terahertz coherence is excited by multiple reflections in the gas-filled tube radiation.
上述技术方案中，所述步骤1)中的中等强度的激光脉冲的强度低于1016W/cm2，高于1013W/cm2；其脉宽在皮秒量级。In the above technical solution, the intensity of the medium-intensity laser pulse in step 1) is lower than 1016 W/cm2 and higher than 1013 W/cm2 ; the pulse width is on the order of picoseconds.
上述技术方案中，所述步骤2)中的高强度激光脉冲的强度大于1017W/cm2，脉宽在皮秒(10-12秒)量级，其入射方向与气体填充管的轴向的夹角大于0度，小于30度。In the above technical solution, the intensity of the high-intensity laser pulse in step 2) is greater than 1017 W/cm2 , the pulse width is on the order of picoseconds (10-12 seconds), and the incident direction is in the same direction as the axial direction of the gas-filled tube. The included angle is greater than 0 degrees and less than 30 degrees.
本发明具有如下技术效果：本发明采用的等离子体本身是完全电离的准中性介质，不存在被入射激光损坏的阈值(所谓材料的损坏阈值是指当入射激光强度高到一定程度，将材料损坏对应的激光强度)，可以产生100MW量级的THz辐射；通过两个入射激光脉冲间适当时间延迟控制，来控制腔管内气体密度的分布，使腔管内产生的单周期太赫兹脉冲实现相干叠加放大，产生准单周期的THz脉冲。另外，通常采用电光晶体、半导体和光导天线，通过光整流等方法产生THz辐射的能量转换效率为10-6，而本发明的方案则有望将转换效率提高到10-4以上。再者，本发明还能够在台面尺度产生超强THz辐射。The present invention has the following technical effects: the plasma itself used in the present invention is a completely ionized quasi-neutral medium, and there is no threshold value damaged by incident laser light (the so-called material damage threshold value refers to that when the incident laser intensity is high enough to a certain extent, the material Corresponding laser intensity damage), can generate 100MW level of THz radiation; through the appropriate time delay control between two incident laser pulses, to control the distribution of gas density in the cavity, so that the single-cycle terahertz pulses generated in the cavity can achieve coherent superposition Amplified to produce a quasi-monocyclic THz pulse. In addition, electro-optic crystals, semiconductors and photoconductive antennas are usually used to generate THz radiation with an energy conversion efficiency of 10-6 through optical rectification and other methods, but the solution of the present invention is expected to increase the conversion efficiency to above 10-4 . Furthermore, the present invention can also generate ultra-strong THz radiation at the mesa scale.
附图说明Description of drawings
图1是激光脉冲作用于单个薄层等离子体层产生的太赫兹辐射原理图；Figure 1 is a schematic diagram of the terahertz radiation generated by laser pulses acting on a single thin plasma layer;
图2a是激光脉冲作用于两个薄层等离子体层产生的太赫兹辐射原理图，其中稀薄等离子体层的厚度D1与两等离子体层的间距D2不同，两等离子体层的外侧是可以对入射激光有高反射率的金属；Figure 2a is a schematic diagram of the terahertz radiation generated by laser pulses acting on two thin plasma layers, where the thickness D1 of the thin plasma layer is different from the distance D2 between the two plasma layers, and the outer sides of the two plasma layers can be Metals with high reflectivity to incident laser light;
图2b是与图2a类似，其中等离子体层的厚度与两等离子体层的间距相同，均为D；Figure 2b is similar to Figure 2a, wherein the thickness of the plasma layer is the same as the distance between the two plasma layers, both are D;
图3a是本发明的气体填充管结构设计图和原理图；Fig. 3 a is the structural design diagram and schematic diagram of the gas filling tube of the present invention;
图3b气体填充管内经过电离脉冲作用后形成的电离气体密度分布的一种状态；Figure 3b shows a state of the ionized gas density distribution formed in the gas-filled tube after the action of the ionization pulse;
图3c气体填充管内经过电离脉冲作用后形成的电离气体密度分布的另一种状态；这是最理想情况，可以产生准单周期THz辐射；Fig. 3c Another state of the ionized gas density distribution formed after the ionization pulse in the gas-filled tube; this is the most ideal situation, which can produce quasi-single-cycle THz radiation;
图3d气体填充管内经过电离脉冲作用后形成的电离气体密度分布的又一种状态；Another state of the ionized gas density distribution formed after the ionization pulse is applied in the gas-filled tube in Fig. 3d;
图3e气体填充管内经过电离脉冲作用后形成的电离气体密度分布的第四种状态；Figure 3e is the fourth state of the ionized gas density distribution formed after the ionization pulse in the gas-filled tube;
图4a是本发明的工作示意图，其中采用的气体填充管两端平行；Fig. 4 a is the working schematic diagram of the present invention, wherein the two ends of the gas-filled tubes adopted are parallel;
图4b是本发明的工作示意图，其中采用的气体填充管两端面不平行；这种设计可以避免THz辐射的出射方向与入射激光的出射方向沿同方向出射。Fig. 4b is a working diagram of the present invention, in which the two ends of the gas-filled tube are not parallel; this design can prevent the THz radiation from emitting in the same direction as the incident laser.
具体实施方式Detailed ways
本发明是通过多次反射使得同一泵浦激光脉冲反复作用到等离子体薄层上，从而激发出多个相干的太赫兹辐射，通过调整各反射点的位置，即可将各相干的太赫兹辐射叠加而形成一个超强的太赫兹辐射。In the present invention, the same pump laser pulse repeatedly acts on the plasma thin layer through multiple reflections, thereby exciting a plurality of coherent terahertz radiations, and by adjusting the position of each reflection point, each coherent terahertz radiation can be superimposed to form a super strong terahertz radiation.
本发明的基本构思如下：Basic idea of the present invention is as follows:
如图1所示，如果只有单个的等离子体薄层，激光脉冲只作用一次，产生的太赫兹辐射效率比较低。如图2a，如果有两个等离子体层，并且在等离子体层的外侧用高发射率的金属来发射激光脉冲，那么每反射一次，就产生两个单周期的太赫兹脉冲，能量转换效率提高一倍。如果有N次反射，其转换效率就是单次作用的N倍。其次，如果等离子体层之间的间隔D2与等离子体层的厚度D1相当，即厚度均为D，如图2b所示，此时激光各次反射产生的太赫兹辐射实现同步，从而产生相干叠加。其经过叠加后的电场振幅是单次反射产生的电场的N倍，功率是N2倍。因此，本发明能够产生功率达到100MW的超强太赫兹辐射。As shown in Figure 1, if there is only a single plasma thin layer, the laser pulse is only applied once, and the efficiency of the generated THz radiation is relatively low. As shown in Figure 2a, if there are two plasma layers, and a high-emissivity metal is used to emit laser pulses on the outside of the plasma layers, two single-period terahertz pulses will be generated for each reflection, and the energy conversion efficiency will be improved. double. If there are N reflections, the conversion efficiency is N times that of a single action. Secondly, if the distance D2 between the plasma layers is equivalent to the thickness D1 of the plasma layer, that is, the thickness is D, as shown in Figure 2b, at this time, the terahertz radiation generated by each reflection of the laser is synchronized, resulting in coherent superposition . The amplitude of the electric field after superposition is N times that of the electric field generated by a single reflection, and the power isN2 times. Therefore, the present invention can generate ultra-strong terahertz radiation with a power up to 100MW.
下面结合附图和具体实施例对本发明作进一步地描述。The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.
实施例1Example 1
本实施例的产生超强太赫兹辐射的装置，包括气体填充管、泵浦激光器和电离激光器。其中，气体填充管的侧壁采用对泵浦激光反射率很高(如反射率大于90％)的金属材料制作(本实施例中采用铜，也可采用其他金属或合金；另外如在上述材料表面镀金可提高反射率)。两个端面(即气体填充管的入射端面和出射端面)均为镜面，采用石英玻璃等对入射激光和THz辐射高透射的材料制作。所述气体填充管的轴线为直线，该轴线与气体填充管的入射端面呈夹角α。所述入射激光器的激光输出方向垂直于所述入射端面。所述电离激光器的激光输出方向与所述气体填充管的轴线方向一致且与该轴线共线。The device for generating ultra-strong terahertz radiation in this embodiment includes a gas-filled tube, a pump laser and an ionization laser. Wherein, the sidewall of the gas-filled tube is made of a metal material with high reflectivity (such as reflectivity greater than 90%) to the pump laser (copper is used in this embodiment, other metals or alloys can also be used; in addition, as in the above-mentioned materials Surface gold plating can improve reflectivity). The two end faces (that is, the incident end face and the exit end face of the gas-filled tube) are both mirror faces, made of quartz glass and other materials with high transmittance to incident laser and THz radiation. The axis of the gas-filled tube is a straight line, and the axis forms an angle α with the incident end face of the gas-filled tube. The laser output direction of the incident laser is perpendicular to the incident end face. The laser output direction of the ionization laser is consistent with and collinear with the axis of the gas-filled tube.
所述气体填充管的横截面为矩形，长、宽分别为2λTH、λTH的整数倍(这里的λTH是所希望产生的太赫兹辐射的波长)。本实施例中，长、宽取值分别为600μm和300μm(假设λTH＝100μm，对应的THz辐射频率为3THz)。而气体填充管的长度一般大于10λTH，由于辐射能量正比于管子的长度，所以在一定范围内，越长越好。但随着管子的长度加长，激光反射次数的增加，产生的THz辐射的相干叠加变得越来越难以实现，最后产生的THz辐射的相干性变差。所以气体填充管的长度也不能太长，譬如一般小于30λTH。所填充气体为一般气体(如氢气)，其密度取决于所希望产生的太赫兹频率，一般为1016～1019cm-3。(THz频率与密度关系为f[THz]＝9×10-9n1/2[cm-3]，譬如密度n＝1016cm-3，辐射频率为0.9THz)。The cross-section of the gas-filled tube is rectangular, and the length and width are integer multiples of 2λTH and λTH respectively (where λTH is the wavelength of the terahertz radiation to be generated). In this embodiment, the length and width are respectively 600 μm and 300 μm (assuming λTH =100 μm, the corresponding THz radiation frequency is 3 THz). The length of the gas-filled tube is generally greater than 10λTH . Since the radiation energy is proportional to the length of the tube, within a certain range, the longer the better. However, as the length of the tube increases and the number of laser reflections increases, the coherent superposition of the generated THz radiation becomes more and more difficult to achieve, and the coherence of the finally generated THz radiation becomes worse. Therefore, the length of the gas filling tube should not be too long, for example generally less than 30λTH . The filled gas is a general gas (such as hydrogen), and its density depends on the desired terahertz frequency, generally 1016 -1019 cm-3 . (The relationship between THz frequency and density is f[THz]=9×10-9 n1/2 [cm-3 ], for example, density n=1016 cm-3 , and the radiation frequency is 0.9 THz).
所述电离激光器产生的激光射入气体填充管后，由于热膨胀作用，气体填充管中形成中空密度分布的电离气体。即在气体填充管中心产生-真空层，其周围形成电离气体层(即等离子体层)。电离激光脉冲的理想的横截面分布是正方形。这可以将入射电离激光通过一个正方形光栏来实现。由此形成的真空层的横截面为正方形，而且其边缘与气体填充管的金属包层(即金属侧壁)平行。如图3a所示，在所述入射激光的入射平面上，所述真空层上下两侧均为电离气体层。After the laser light generated by the ionization laser is injected into the gas-filled tube, ionized gas with a hollow density distribution is formed in the gas-filled tube due to thermal expansion. That is, a vacuum layer is generated in the center of the gas-filled tube, and an ionized gas layer (ie, a plasma layer) is formed around it. The ideal cross-sectional distribution of ionizing laser pulses is square. This is achieved by passing the incident ionizing laser light through a square aperture. The vacuum layer thus formed has a square cross-section and its edges are parallel to the metal cladding (ie the metal side walls) of the gas-filled tube. As shown in FIG. 3 a , on the incident plane of the incident laser light, the upper and lower sides of the vacuum layer are ionized gas layers.
本发明的气体填充管两个端面可以是平行(如图4a所示，该图中0为电离激光脉冲、1为泵浦激光器脉冲、3为沿泵浦激光入射方向产生的太赫兹辐射、4为沿泵浦激光镜面反射方向产生的太赫兹辐射、5为电离激光器、6为泵浦激光器)，也可存在一定的夹角(如图4b所示，该图中0为电离激光脉冲、1为泵浦激光器脉冲、3为沿泵浦激光入射方向产生的太赫兹辐射、4为沿泵浦激光镜面反射方向产生的太赫兹辐射、5为电离激光器、6为泵浦激光器)，此时太赫兹辐射的出射方向与激光脉冲不同。The two end faces of the gas-filled tube of the present invention can be parallel (as shown in Figure 4a, 0 is the ionization laser pulse in this figure, 1 is the pump laser pulse, 3 is the terahertz radiation produced along the incident direction of the pump laser, 4 is the terahertz radiation generated along the specular reflection direction of the pump laser, 5 is the ionization laser, and 6 is the pump laser), and there may also be a certain included angle (as shown in Figure 4b, 0 in this figure is the ionization laser pulse, 1 is the pulse of the pump laser, 3 is the terahertz radiation generated along the incident direction of the pump laser, 4 is the terahertz radiation generated along the mirror reflection direction of the pump laser, 5 is the ionization laser, and 6 is the pump laser), at this time Hertzian radiation exits in a different direction than laser pulses.
利用本实施例的装置产生超强太赫兹辐射的方法包括如下步骤：The method for generating ultra-strong terahertz radiation using the device of this embodiment includes the following steps:
1)电离激光器发出中等强度的激光脉冲；该激光脉冲沿着气体填充管的轴向传播，在其经过的路线上形成真空层，两侧形成电离气体层。本步骤中的中等强度的激光脉冲的强度低于1016W/cm2，高于1013W/cm2；其脉宽在皮秒(10-12秒)量级。1) The ionization laser emits a medium-intensity laser pulse; the laser pulse propagates along the axial direction of the gas-filled tube, forming a vacuum layer on the route it passes, and forming an ionized gas layer on both sides. The intensity of the medium-intensity laser pulse in this step is lower than 1016 W/cm2 and higher than 1013 W/cm2 ; its pulse width is on the order of picoseconds (10-12 seconds).
2)经过纳秒(10-9秒)量级的时间延迟后，泵浦激光器发出强激光脉冲射入气体填充管，在气体填充管内经多次反射激发出超强的准单周期太赫兹相干辐射。本步骤中的高强度激光脉冲的强度大于1017W/cm2，脉宽在皮秒(10-12秒)量级，其入射方向与气体填充管的轴向有一夹角，通常小于30度(即入射激光在真空层与电离气体层的界面上的入射角大于60度)。泵浦激光的聚焦光斑尺寸应该接近或大于一个THz波长，对产生3THz辐射(对应等离子体电子密度为n＝1017cm-3)，聚焦光斑在100μm左右。2) After a time delay on the order of nanoseconds (10-9 seconds), the pump laser emits a strong laser pulse into the gas-filled tube, and the super-strong quasi-single-cycle terahertz coherence is excited by multiple reflections in the gas-filled tube radiation. The intensity of the high-intensity laser pulse in this step is greater than 1017 W/cm2 , the pulse width is on the order of picoseconds (10-12 seconds), and its incident direction has an included angle with the axial direction of the gas-filled tube, usually less than 30 degrees (That is, the incident angle of the incident laser light on the interface between the vacuum layer and the ionized gas layer is greater than 60 degrees). The focus spot size of the pump laser should be close to or larger than one THz wavelength, and for generating 3THz radiation (corresponding to plasma electron density n=1017 cm-3 ), the focus spot is about 100 μm.
在步骤1)发出的电离激光脉冲作用后，在激光传输的光轴附近通过光电离、碰撞加热等将气体部分电子并将其温度提高。由此形成一个与电离激光光轴垂直的横向激波。由于激波的作用，气体密度经过纳秒(10-9秒)量级时间演化后，形成如图3b，3c，3d，3e等不同的分布。不管是怎样的密度分布，泵浦激光的作用都能使电离气体辐射出高强度THz辐射。由于电离气体层的厚度在THz辐射波长量级，泵浦激光每与电离气体作用一次都产生一个准单周期的THz脉冲。在图3c这种密度下(主要通过控制电离激光与泵浦激光之间的时间延迟实现)，产生的THz辐射有可能产生相干叠加，产生准单周期的脉冲，否则产生多周期脉冲。其中准单周期THz辐射在时域频谱分析方面更有价值。After the ionizing laser pulse emitted in step 1), the electrons in the gas are partly electronized near the optical axis of the laser transmission and the temperature is increased by photoionization, collision heating, etc. Thus, a transverse shock wave perpendicular to the optical axis of the ionizing laser is formed. Due to the action of the shock wave, the gas density evolves in the order of nanoseconds (10-9 seconds), forming different distributions as shown in Figures 3b, 3c, 3d, and 3e. Regardless of the density distribution, the action of the pump laser can make the ionized gas radiate high-intensity THz radiation. Since the thickness of the ionized gas layer is on the order of the THz radiation wavelength, every time the pump laser interacts with the ionized gas, a quasi-single-period THz pulse is generated. At the density shown in Figure 3c (mainly realized by controlling the time delay between the ionizing laser and the pumping laser), the generated THz radiation may be coherently superimposed to produce quasi-single-cycle pulses, otherwise multiple-cycle pulses will be generated. Among them, quasi-single-cycle THz radiation is more valuable in time-domain spectrum analysis.
最后所应说明的是，以上实施例仅用以说明本发明的技术方案而非限制。尽管参照实施例对本发明进行了详细说明，本领域的普通技术人员应当理解，对本发明的技术方案进行修改或者等同替换，都不脱离本发明技术方案的精神和范围，其均应涵盖在本发明的权利要求范围当中。Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than limit them. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent replacements to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all of them should be included in the scope of the present invention. within the scope of the claims.
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