高阶径向拉盖尔-高斯型飞秒脉冲激光场中级联三能级原子所受光学偶极力
When atoms are exposed to a non-uniform laser field, there will be gradient potential due to the electric dipole interactions between the light field and the atoms, and therefore atoms will be subjected to the action of optical dipole forces. With the recent development of ultra-short laser technology and light manipulation techniques, new types of light fields with complex spatial structures are possible to be constructed. Meanwhile, the application of these new laser fields in optical tweezers to achieve special and accurate control of micro-particles becomes a hotspot in light-matter interactions. Compared with the fundamental transverse mode Gaussian beams, these laser fields, such as hollow Gaussian beam, Laguerre-Gaussian beam, Bessel-Gaussian beam, and Airy beam, have more complex field structures and special optical characteristics. Additionally, they can provide extensive controllable degrees of freedom and more atomic beam guidance pathways for laser manipulation. Our paper studies the electric dipole interactions between the femtosecond Laguerre-Gaussian laser pulses of high radial modes and the three-level atomic systems. The spatiotemporal distribution characteristics of optical potential traps and optical dipole forces exerted by different radial modes of the femtosecond Laguerre-Gaussian laser beams are analyzed. We also reveal the advantages of Laguerre-Gaussian laser beams with high radial modes in atom trapping and manipulation. This theoretical study is expected to give insight into the optical manipulation of micro-particles with structured optical laser fields and provide guidance for possible experimental studies.
The semi-classical theory is employed to study the interactions of optical dipole forces between the femtosecond Laguerre-Gaussian laser pulses with high radial modes and the cascade three-level atoms. The laser field is treated with the classical Maxwell's theory and the atoms are treated with the quantum mechanical density matrix theory. Based on the density matrix theory, the optical Bloch equations for a cascade three-level system are derived without rotating wave approximation, and the coupled optical Bloch equation is solved numerically by utilizing the self-consistent numerical scheme. The induced electric dipole moments are then calculated from the product matrix trace of the density matrix operator and the electric dipole moment operator. The optical potentials and optical dipole forces are simulated for different radial modes of the femtosecond Laguerre-Gaussian lasers. Without generality loss, the atomic sodium is taken as the prototype for the cascade three-level atomic model, and the transitions from the ground state 3s to 3p and from 3p to 4s excited states of the sodium atom in the visible and infrared light bands respectively are chosen.
When the three-level atomic systems are exposed to a Laguerre-Gaussian beam of the n-th radial mode, there will be n+1 optical potential wells/barriers formed for negative/positive laser field detuning. With the same peak intensity of the laser field, by increasing the radial mode number n, the depth of the main potential well/barrier remains constant, but the spatial range of the main potential well/barrier becomes narrower, and the optical dipole force becomes stronger due to the increasing radial gradient of the potential (Figs. 1 and 2). Therefore, the particles are bound in much narrower optical potentials induced by the Laguerre-Gaussian laser beams of the higher radial modes, which is more conducive to the accurate manipulation and capture of particles. The transverse dipole force exerted by a Laguerre-Gaussian beam of the n-th radial mode has 2nnodal circles in the radial direction, and the direction of the dipole force is opposite on both sides of a nodal circle due to the changing electric field gradient. Therefore, the atomic beam will be split and trapped in the optical potential wells at different positions (Figs. 3 and 4). When atoms are exposed to an ultra-short femtosecond laser field, the carrier-wave effect becomes important and the induced optical dipole force oscillates with a frequency two times the carrier-wave frequency of the laser field.
This study provides insight into the optical manipulation of micro-particles in structured optical beam fields. Our attention is paid to the influence of the radial mode of the Laguerre-Gaussian beams on the optical potential and optical dipole force. The higher radial mode leads to steeper optical potential and larger optical dipole force. The Laguerre-Gaussian beam of the n-th radial mode will generate n+1 optical potential wells/barriers under negative/positive laser field detuning, and the corresponding optical dipole force has 2n nodal circles in the radial direction with opposite signs on both sides of any nodal circle. Therefore, atoms can be split and bound in much narrower optical potentials induced by the Laguerre-Gaussian laser beams of higher radial modes, and the Laguerre-Gaussian laser beams of higher radial modes are beneficial to the accurate manipulation, capture, and steering of particles. In the time regime, the carrier-wave effect induced by the femtosecond laser pulse is important and the optical dipole force oscillates with a frequency two times the carrier-wave frequency of the laser field.
1 引 言
1970年,Ashkin[1]首次通过激光辐射压力形成的稳定光学势,成功实现了对微小粒子的俘获和移动,开启了人们对光学俘获和光力操纵微粒的研究。1975年,Hänsch和Schawlow[2]基于多普勒频移效应提出了利用各向同性光源进行激光冷却原子的想法,此后各种激光冷却、捕获和操纵方案应运而生。1986年,Ashkin等[3]发现利用单束激光的光场梯度力可以实现对电介质微粒进行俘获的光镊效应。利用光镊进行微尺度物质捕获、移动和操纵的技术在物理学、化学、生物等领域都有重要的应用价值[4-10]。随着可调谐高强度超短脉冲激光技术的发展,利用激光进行原子冷却、囚禁、导引等激光操控原子的理论和实验研究取得了一系列重要进展,使得原子激光操控技术成功应用于原子漏斗、原子透镜、原子波导、原子分束器、原子干涉仪、玻色-爱因斯坦凝聚、原子光学晶格、原子芯片等原子光学元器件的研发和制作。
当原子在非均匀光场中运动时,光场与原子间存在电偶极相互作用的梯度势,原子将受到光学偶极力(ODF)的作用。在超短激光与原子相互作用的ODF研究中,激光场大都采用高斯型光束或者驻波场,通过调节激光脉冲宽度、拉比频率、光场失谐量等激光场参数,实现光场对原子的俘获和操控[11-13]。例如:Kumar等[11]研究了少周期高斯型脉冲激光场对两能级原子体系的ODF作用,发现通过调节光场失谐量的正负可以实现对原子束的聚焦和散焦;刘纪彩等[12]对飞秒高斯型激光场中两能级原子所受ODF与拉比振荡频率的关系进行了研究,发现光力大小随着拉比频率的变化呈现出周期性振荡分布特性;Chakraborty等[13]研究了啁啾飞秒高斯脉冲对N型四能级原子的光学偶极俘获作用,发现通过调节高斯光束的束腰半径、频率失谐量和啁啾率等,可以实现对光学势阱空间尺度的调控;Gong等[14]研究了高重复频率飞秒高斯型脉冲激光对瑞利粒子的光力作用,发现光束的自聚焦作用可以增强光俘获效率和稳定性。
近年来,随着激光技术和光场调控技术的发展,人们可以利用多种方式来获得具有复杂空间结构的新型光场。具有径向节线圆和角向节线分布的高阶拉盖尔-高斯(LG)激光束的产生方法有腔内法和腔外法[15-21]。直接由激光谐振腔获得高阶LG激光束的腔内生成法产生的光束阶次不可调且转换效率低,因此通常采用腔外转化方法来生成不同阶次的LG激光束,主要的腔外转化方法有模式转换法、螺旋相位板法、叉型光栅法等。对于高阶径向模式的LG激光束,可以采用阶梯相位板法和具有环状位错的叉型光栅获得。结合新型光场来实现特殊或复杂操控功能的光镊成为一个重要的发展趋势[22-25]。例如:Simpson等[22]研究了LG光束对直径大于光波长的球形微粒的光镊和光学扳手行为,发现高阶LG光束对该粒子的轴向光力作用远大于基横模高斯光束的作用;Bougouffa等[23]研究了在电偶极禁戒情况下,连续场LG光束及贝塞尔-高斯(BG)光束与铯原子的电四极相互作用,发现角向高阶LG光束能够显著增强电四极相互作用强度,并且将原子俘获在光学电四极势阱中;Lu等[24]研究了圆对称艾里光束的突然自聚焦性质及其对不同尺度微粒的光学操控性能,发现相较于尺寸小于光波长的瑞利粒子,圆对称艾里光束对尺寸大于光波长的米氏粒子具有更好的光力操控稳定性。
与基横模高斯光束相比,空心高斯光束、LG光束、BG光束、艾里光束等新型光场通常具有较复杂的光场结构和一些特殊的光学性质,可以形成复杂的光力场结构,能够为激光操控提供更加灵活的可控自由度和更多的原子束导引路径[25]。本文将对不同径向阶次的飞秒柱对称LG激光束与原子体系的电偶极相互作用进行研究,通过数值求解基于密度矩阵理论的三能级体系布洛赫方程,计算不同径向阶次的飞秒LG激光束对原子的ODF作用,对光势阱和光力的分布特性进行分析。
2 理论方法
在研究光场与原子、分子等微观粒子相互作用时,通常采用半经典理论,即将激光场作为经典麦克斯韦电磁场,对微观粒子体系进行量子力学处理,并采用薛定谔方程或者密度矩阵方程对粒子体系进行描述。在电偶极近似条件下,原子和激光场相互作用的哈密顿量可表示为
式中:
式中:
基于密度矩阵理论,将级联三能级原子体系各密度矩阵元的实部和虚部进行分离,可以得到如下光学布洛赫方程[27-28]:
式中:
LG光束是亥姆霍兹方程在柱坐标系下的解,对具有柱对称结构的n阶径向模式的LG光束(LG0n),其光场振幅可表示为
式中:
式中:
在柱对称径向LG0n飞秒激光脉冲场中,原子受到的光学势作用为
原子气中的跃迁偶极矩分布是各向同性的,故原子在光学势场[
3 数值计算模型
采用级联三能级原子模型来研究具有柱对称结构的高阶径向LG0n飞秒激光脉冲场与原子的光力学相互作用,其相互作用特性主要取决于所采用的LG激光束的径向阶次和原子体系的能级结构特点,对具体的原子和激光场的参数取值具有一定的普适性。通常情况下,原子、分子的较低几个激发态能级处于红外和可见光波长范围内。为不失一般性,选取钠原子的3s价电子基态、3p价电子激发态和4s价电子激发态作为级联三能级原子模型的能级结构参量[29-30]。选取的第一激发态共振跃迁频率
定义激光场失谐量
4 计算结果与讨论
对不同径向阶次和不同失谐量情况下柱对称LG0n飞秒激光脉冲的光力场作用
图 1. 当激光场为负失谐时,不同径向阶次的飞秒光束LG0n脉冲作用下的光学势 的时空分布。(a) ;(b) ;(c) ;(d)在 的脉冲峰值时刻,各阶次LG0n光场中光学势 关于 的径向分布情况
Fig. 1. Optical potential induced by different orders of femtosecond pulsed LG0n fields with negative detunings. (a) ; (b) ; (c) ; (d) radial distribution of optical potential at when the field intensity takes its maximum
在
图 2. 当激光场正失谐时,不同阶次的飞秒脉冲LG光束LG0n作用下的光学势 的时空分布。(a) ;(b) ;(c) ;(d)在 的脉冲峰值时刻,各阶次LG0n光场中光学势 关于 的径向分布情况,插图给出了相应的感生电偶极矩 的分布情况
Fig. 2. Optical potential induced by different orders of femtosecond pulsed LG0n fields with positive detunings. (a) ; (b) ; (c) ; (d) radial distribution of optical potential at when the field intensity takes its maximum, the inset shows the induced electric dipole moment
为了进一步研究原子在径向高阶LG0n脉冲激光场中的光力学行为,对激光场中原子所受的径向ODF
图 3. 在 的激光场负失谐情况下,不同径向阶次的LG0n飞秒脉冲激光场中的ODF分布。(a) 和(b) 的LG0n飞秒脉冲激光场产生的 的时空分布;(c) 和(d) 的LG0n场中, 在不同时刻沿径向r的分布情况;(e) 和(f) 的LG0n场中,不同径向位置r处 的时间演化情况
Fig. 3. Optical dipole force induced by negatively detuned femtosecond pulsed LG0n fields. Spatial-temporal distribution of with (a) and (b) ; radial distribution of the optical dipole force with (c) and (d) at different time instants; temporal evolution of the optical dipole force with (e) and (f) at different radial positions
在激光场正失谐情况下ODF的时空分布如
图 4. 在 的激光场正失谐情况下,LG0n飞秒脉冲激光场中的 分布。径向阶次为(a) 和(b) 的LG0n飞秒脉冲激光场产生的 的时空分布;(c) 和(d) 的LG0n场中, 在不同时刻沿径向r的分布情况;(e) 和(f) 的LG0n场中,不同径向位置r处 的时间演化情况
Fig. 4. Optical dipole force induced by positively detuned femtosecond pulsed LG0n fields. Spatial-temporal distribution of with (a) and (b) ; radial distribution of the optical dipole force with (c) and (d) at different time instants; temporal evolution of the optical dipole force with (e) and (f) at different radial positions
5 结 论
研究了飞秒径向高阶LG型脉冲激光束中,级联三能级原子所受的光学势和ODF分布,发现:在飞秒脉冲激光场中,ODF的大小以两倍于激光载波频率的频率作周期性振荡,但是同一空间位置处的偶极力方向并不随着时间改变;在
[1] Ashkin A. Acceleration and trapping of particles by radiation pressure[J]. Physical Review Letters, 1970, 24(4): 156-159.
[2] Hänsch T W, Schawlow A L. Cooling of gases by laser radiation[J]. Optics Communications, 1975, 13(1): 68-69.
[3] Ashkin A, Dziedzic J M, Bjorkholm J E, et al. Observation of a single-beam gradient force optical trap for dielectric particles[J]. Optics Letters, 1986, 11(5): 288-290.
[4] Ashkin A, Dziedzic J M. Optical trapping and manipulation of viruses and bacteria[J]. Science, 1987, 235(4795): 1517-1520.
[5] Zhu R X, Avsievich T, Popov A, et al. Optical tweezers in studies of red blood cells[J]. Cells, 2020, 9(3): 545.
[6] Zheng H X, Chen H J, Ng J, et al. Optical gradient force in the absence of light intensity gradient[J]. Physical Review B, 2021, 103(3): 035103.
[7] Descheemaeker L, Ginis V, Viaene S, et al. Optical force enhancement using an imaginary vector potential for photons[J]. Physical Review Letters, 2017, 119(13): 137402.
[8] 李宝军, 辛洪宝, 张垚, 等. 光捕获和光操控研究进展[J]. 光学学报, 2011, 31(9): 0900126.
[9] 荣升, 刘洪双, 钟莹, 等. 基于光力捕获金纳米立方体的拉曼光谱增强[J]. 光学学报, 2021, 41(17): 1730003.
[10] Gao D L, Ding W Q, Nieto-Vesperinas M, et al. Optical manipulation from the microscale to the nanoscale: fundamentals, advances and prospects[J]. Light: Science & Applications, 2017, 6(9): e17039.
[11] Kumar P, Sarma A K. Optical force on two-level atoms by few-cycle-pulse Gaussian laser fields beyond the rotating-wave approximation[J]. Physical Review A, 2011, 84(4): 043402.
[12] 刘纪彩, 成飞, 赵亚男, 等. 飞秒激光场中原子所受光学偶极力研究[J]. 物理学报, 2019, 68(3): 033701.
Liu J C, Cheng F, Zhao Y N, et al. Atom-subjected optical dipole force exerted by femtosecond laser field[J]. Acta Physica Sinica, 2019, 68(3): 033701.
[13] Chakraborty S, Sarma A K. Optical trap potential control in N-type four-level atoms by femtosecond Gaussian pulses[J]. Journal of the Optical Society of America B, 2015, 32(2): 270-274.
[15] 朱向阳, 邱松, 丁友, 等. 多径向节次拉盖尔-高斯光束旋转多普勒效应分析[J]. 光学学报, 2023, 43(7): 0726003.
[16] 刘俊, 王健. 涡旋光激光器研究进展[J]. 中国激光, 2022, 49(12): 1201001.
[17] 蔡田, 张晓波, 叶芳伟, 等. 产生拉盖尔-高斯模的全息光栅实验研究[J]. 光学学报, 2005, 25(11): 1457-1460.
[20] Busleev N I, Kudryashov S I, Danilov P A, et al. Symmetric nanostructuring and plasmonic excitation of gold nanostructures by femtosecond Laguerre–Gaussian laser beams[J]. Quantum Electronics, 2019, 49(7): 666-671.
[21] Li J E, Guan W H, Yuan S, et al. Laser shaping and optical power limiting of pulsed Laguerre-Gaussian laser beams of high-order radial modes in fullerene C60[J]. Chinese Physics B, 2023, 32(2): 024203.
[22] Simpson N B, Allen L, Padgett M J. Optical tweezers and optical spanners with Laguerre-Gaussian modes[J]. Journal of Modern Optics, 1996, 43(12): 2485-2491.
[23] Bougouffa S, Babiker M. Atom trapping and dynamics in the interaction of optical vortices with quadrupole-active transitions[J]. Physical Review A, 2020, 101(4): 043403.
[24] Lu W L, Chen H J, Liu S Y, et al. Circular Airy beam with an arbitrary conical angle beyond the paraxial approximation[J]. Physical Review A, 2022, 105(4): 043516.
[25] Zhou L M, Shi Y Z, Zhu X Y, et al. Recent progress on optical micro/nanomanipulations: structured forces, structured particles, and synergetic applications[J]. ACS Nano, 2022, 16(9): 13264-13278.
[26] Kumar P, Kumar P, Sarma A K. Simultaneous control of optical dipole force and coherence creation by super-Gaussian femtosecond pulses in Λ‑like atomic systems[J]. Physical Review A, 2014, 89(3): 033422.
[27] Liu J C, Guo F F, Zhao Y N, et al. Time-frequency analysis of ultrafast dynamics in cascade three-level system driven by hyper-Gaussian pulses[J]. Optics Communications, 2019, 438: 25-33.
[28] Liu J C, Guo F F, Zhao Y N, et al. Optical power limiting of ultrashort hyper-Gaussian pulses in cascade three-level system[J]. Chinese Physics B, 2018, 27(10): 104209.
[29] Martin W C, Zalubas R. Energy levels of sodium Na I through Na XI[J]. Journal of Physical and Chemical Reference Data, 1981, 10(1): 153-196.
[30] Sansonetti J E. Wavelengths, transition probabilities, and energy levels for the spectra of sodium (Na I-Na XI)[J]. Journal of Physical and Chemical Reference Data, 2008, 37(4): 1659-1763.
管文慧, 王翦, 袁烁, RashidAbdul Gheyas Abdul, 郭芬芬, 刘纪彩. 高阶径向拉盖尔-高斯型飞秒脉冲激光场中级联三能级原子所受光学偶极力[J]. 光学学报, 2023, 43(19): 1927001. Wenhui Guan, Jian Wang, Shuo Yuan, Abdul Gheyas Abdul Rashid, Fenfen Guo, Jicai Liu. Optical Dipole Forces of Laguerre-Gaussian Femtosecond Laser Pulses with High Radial Modes on Cascade Three-Level Atoms[J]. Acta Optica Sinica, 2023, 43(19): 1927001.