雙圓場中原子的高次諧波發(fā)射
發(fā)布時間：2019-05-10 07:17
【摘要】：原子、分子的高次諧波發(fā)射(HHG)是一個高度非線性的過程,即原子、分子在強(qiáng)紅外線偏振激光脈沖作用下產(chǎn)生極紫外(XUV)甚至軟X射線波段的相干光輻射。這種輻射可作為低成本XUV光源,也是當(dāng)前獲得阿秒(10-18s)持續(xù)時間光脈沖的最有效手段,也可作為復(fù)雜的分子結(jié)構(gòu)及動力學(xué)過程的探測手段等。因此關(guān)于諧波特性的研究一直是強(qiáng)場領(lǐng)域的熱門課題。尤其是近幾年,隨著飛秒激光技術(shù)的進(jìn)步,各種先進(jìn)的時間、空間、偏振等整形技術(shù)的出現(xiàn),使得這些復(fù)雜激光脈沖驅(qū)動下的高次諧波產(chǎn)生一些新的特性。如雙圓脈沖場驅(qū)動原子可以產(chǎn)生圓偏振的高次諧波。本文通過精確數(shù)值求解雙圓驅(qū)動脈沖與原子相互作用的含時薛定諤方程,模擬原子在雙圓場驅(qū)動下的單原子響應(yīng)過程。在理論上系統(tǒng)地研究了原子在雙圓場驅(qū)動下高次諧波的產(chǎn)生機(jī)理、諧波階次的禁戒規(guī)律、諧波譜的截止規(guī)則、諧波場的偏振特點(diǎn),以及雙圓場中兩個圓偏振脈沖的相對強(qiáng)度比、相對相位差、旋轉(zhuǎn)方向等對高次諧波的影響。理論研究表明,由兩個共面反向旋轉(zhuǎn)、頻率比為1:2的圓極化脈沖組成的雙圓場,在這樣的雙圓場驅(qū)動下,原子產(chǎn)生的高次諧波頻率為低頻驅(qū)動場頻率的()qq=±3,2,113(42)倍,3q次諧波不存在。這一結(jié)論可以由量子躍遷選擇定則給出解釋,即原子的基態(tài)電子吸收多個驅(qū)動光光子躍遷到高能連續(xù)態(tài),再由高能連續(xù)態(tài)釋放一個諧波光子躍遷回基態(tài),在該過程中遵循角動量守恒。由經(jīng)典三步模型角度來分析,基態(tài)電子在驅(qū)動場作用下發(fā)生隧穿電離,之后的三分之一低頻周期時間內(nèi),在驅(qū)動場和母核庫侖場共同作用下,獲得一定動能并再次返回到母離子附近并與之復(fù)合發(fā)射高能諧波光子。最大光子能量約為pp2.12/3.17IU+。另外,各個單一階次的諧波場均是圓偏振的,左旋圓偏振和右旋圓偏振間隔分布,這也通過量子躍遷給出了明確解釋。我們還發(fā)現(xiàn),對應(yīng)于一段較寬頻帶的諧波場,表現(xiàn)出在特定方向線偏振,偏振方向每三分之一低頻周期旋轉(zhuǎn)120度。這一點(diǎn)可以從經(jīng)典三步模型給出的貢獻(xiàn)于諧波發(fā)射的電離電子的運(yùn)動給出解釋。基于上述雙圓場下原子高次諧波的機(jī)理、特點(diǎn)和規(guī)律,我們發(fā)現(xiàn)可以通過雙圓場的旋轉(zhuǎn)方向來控制特定頻率諧波場的左旋或右旋方向;也可以通過兩個圓偏振驅(qū)動場的相對相位差,調(diào)控一定頻帶的諧波場的極化方向;最后我們還發(fā)現(xiàn)通過兩個圓偏振驅(qū)動場的強(qiáng)度比的調(diào)控,可以增強(qiáng)或抑制特定階次諧波的相對強(qiáng)度達(dá)一個數(shù)量級以上。這篇雙圓場驅(qū)動下原子高次諧波的系統(tǒng)研究工作,深化了人們對復(fù)雜光場驅(qū)動下原子高次諧波的新的特性和規(guī)律的認(rèn)識。為人們通過驅(qū)動激光的整形調(diào)控手段,更加有效地操控和利用強(qiáng)場高次諧波積累了經(jīng)驗。
[Abstract]:The high-order harmonic emission (HHG) of atoms and molecules is a highly nonlinear process, in which atoms and molecules produce ultra-ultraviolet (XUV) and even coherent light radiation in soft X-ray band under the action of intense infrared polarization laser pulses. This radiation can be used as a low-cost XUV light source. It is also the most effective means to obtain the attosecond (10? 18s) continuous-time light pulse. It can also be used to detect the complex molecular structure and the kinetic process, and so on. Therefore, the study of harmonic characteristics has always been a hot topic in the field of strong field. Especially in recent years, with the progress of femtosecond laser technology and the emergence of various advanced time, space, polarization and other shaping techniques, these complex laser pulse-driven high-order harmonics produce some new characteristics. For example, the driving atom of double circular pulse field can produce high order harmonics of circular polarization. In this paper, the time-dependent Schrodinger equation of the interaction between double circular driving pulse and atom is solved accurately, and the single atomic response process driven by double circular field is simulated. The generation mechanism of high order harmonics, the forbidden law of harmonic order, the cut-off rule of harmonic spectrum, the polarization characteristics of harmonic field and the relative intensity ratio of two circular polarization pulse in double circular field are systematically studied theoretically. The influence of relative phase difference and rotation direction on high order harmonics. The theoretical study shows that the double circular field is composed of two coplanar reverse rotation and the frequency ratio is 1:2. Driven by such a double circular field, the high order harmonic frequency produced by the atom is () qq= 鹵3, 2 of the low frequency driving field frequency. 113 (42) times, 3Q harmonics do not exist. This conclusion can be explained by the selection rule of quantum transition, that is, the electron of the ground state of the atom absorbs a number of driven photons to the high energy continuous state, and then releases a harmonic photon transition back to the ground state from the high energy continuous state. In this process, the conservation of angular momentum is followed. From the point of view of the classical three-step model, the ground state electrons are tunneling ionized under the action of the driving field, and then under the interaction of the driving field and the parent nuclear Coulomb field in the 1/3 low frequency periodic time thereafter. A certain kinetic energy is obtained and then returned to the vicinity of the parent ion and combined with it to emit high-energy harmonic photons. The maximum photonic energy is about pp2.12/3.17IU. In addition, the harmonic fields of each single order are circularly polarized, and the distribution of left-handed circular polarization and right-handed circular polarization interval is given, which is also explained by quantum transition. We also find that the harmonic field corresponding to a wide band is polarized in a specific direction, and the polarization direction rotates 120 degrees per 1/3 low frequency period. This can be explained by the motion of ionized electrons contributed to harmonic emission by the classical three-step model. Based on the mechanism, characteristics and law of atomic high order harmonics in the double circular field, we find that the left or right rotation direction of the harmonic field at a specific frequency can be controlled by the rotation direction of the double circular field. The polarization direction of harmonic field in a certain frequency band can also be controlled by the relative phase difference of two circular polarization driving fields. Finally, it is found that the relative intensity of specific order harmonics can be enhanced or suppressed by adjusting the intensity ratio of two circular polarization driving fields. In this paper, the systematic research work of atomic high order harmonics driven by double circular field deepens the understanding of the new characteristics and laws of atomic high order harmonics driven by complex light field. It accumulates experience for people to control and utilize high order harmonics in strong field more effectively by shaping and controlling laser driving.
【學(xué)位授予單位】：吉林大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：O562
本文編號：2473448
[Abstract]:The high-order harmonic emission (HHG) of atoms and molecules is a highly nonlinear process, in which atoms and molecules produce ultra-ultraviolet (XUV) and even coherent light radiation in soft X-ray band under the action of intense infrared polarization laser pulses. This radiation can be used as a low-cost XUV light source. It is also the most effective means to obtain the attosecond (10? 18s) continuous-time light pulse. It can also be used to detect the complex molecular structure and the kinetic process, and so on. Therefore, the study of harmonic characteristics has always been a hot topic in the field of strong field. Especially in recent years, with the progress of femtosecond laser technology and the emergence of various advanced time, space, polarization and other shaping techniques, these complex laser pulse-driven high-order harmonics produce some new characteristics. For example, the driving atom of double circular pulse field can produce high order harmonics of circular polarization. In this paper, the time-dependent Schrodinger equation of the interaction between double circular driving pulse and atom is solved accurately, and the single atomic response process driven by double circular field is simulated. The generation mechanism of high order harmonics, the forbidden law of harmonic order, the cut-off rule of harmonic spectrum, the polarization characteristics of harmonic field and the relative intensity ratio of two circular polarization pulse in double circular field are systematically studied theoretically. The influence of relative phase difference and rotation direction on high order harmonics. The theoretical study shows that the double circular field is composed of two coplanar reverse rotation and the frequency ratio is 1:2. Driven by such a double circular field, the high order harmonic frequency produced by the atom is () qq= 鹵3, 2 of the low frequency driving field frequency. 113 (42) times, 3Q harmonics do not exist. This conclusion can be explained by the selection rule of quantum transition, that is, the electron of the ground state of the atom absorbs a number of driven photons to the high energy continuous state, and then releases a harmonic photon transition back to the ground state from the high energy continuous state. In this process, the conservation of angular momentum is followed. From the point of view of the classical three-step model, the ground state electrons are tunneling ionized under the action of the driving field, and then under the interaction of the driving field and the parent nuclear Coulomb field in the 1/3 low frequency periodic time thereafter. A certain kinetic energy is obtained and then returned to the vicinity of the parent ion and combined with it to emit high-energy harmonic photons. The maximum photonic energy is about pp2.12/3.17IU. In addition, the harmonic fields of each single order are circularly polarized, and the distribution of left-handed circular polarization and right-handed circular polarization interval is given, which is also explained by quantum transition. We also find that the harmonic field corresponding to a wide band is polarized in a specific direction, and the polarization direction rotates 120 degrees per 1/3 low frequency period. This can be explained by the motion of ionized electrons contributed to harmonic emission by the classical three-step model. Based on the mechanism, characteristics and law of atomic high order harmonics in the double circular field, we find that the left or right rotation direction of the harmonic field at a specific frequency can be controlled by the rotation direction of the double circular field. The polarization direction of harmonic field in a certain frequency band can also be controlled by the relative phase difference of two circular polarization driving fields. Finally, it is found that the relative intensity of specific order harmonics can be enhanced or suppressed by adjusting the intensity ratio of two circular polarization driving fields. In this paper, the systematic research work of atomic high order harmonics driven by double circular field deepens the understanding of the new characteristics and laws of atomic high order harmonics driven by complex light field. It accumulates experience for people to control and utilize high order harmonics in strong field more effectively by shaping and controlling laser driving.
【學(xué)位授予單位】：吉林大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：O562
【參考文獻(xiàn)】
相關(guān)期刊論文 前1條
1 張杰;強(qiáng)場物理—— 一門嶄新的學(xué)科[J];物理;1997年11期
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