Science科学杂志：具有变轨道角动量紫外线光束的产生
Generation of extreme-ultraviolet beams with time-varying orbital angular momentum
这项研究20190628发表在科学杂志上
结构光束可以作为涡旋光束携带光角动量，并已被用于增强光通信和成像。Rego等人通过高次谐波产生过程中干扰两束不同轨道角动量的入射延时的涡旋光束来产生动态涡旋脉冲。脉冲之间的受控时间延迟允许高谐波极紫外涡旋光束表现出依赖于时间的角动量，称为自扭矩。这种动态涡旋脉冲可能被用于在超快时间尺度上操纵纳米结构和原子。
Structured light beams can serve as vortex beams carrying optical angular momentum and have been used to enhance optical communications and imaging. Rego et al. generated dynamic vortex pulses by interfering two incident time-delayed vortex beams with different orbital angular momenta through the process of high harmonic generation. A controlled time delay between the pulses allowed the high harmonic extreme-ultraviolet vortex beam to exhibit a time-dependent angular momentum, called self-torque. Such dynamic vortex pulses could potentially be used to manipulate nanostructures and atoms on ultrafast time scales.
Science, this issue p. eaaw9486
介绍
光束同时携带能量和动量，可以对它们照射的物体施加很小但可以检测到的压力。1992年，人们认识到当光束的空间形状绕其轴旋转(或扭转)时，光也可以具有轨道角动量(OAM)。虽然肉眼看不见，但当光束与物质相互作用时，OAM的存在是可以被发现的。OAM光束在光通信、显微术、量子光学和微粒子操纵等领域有了新的应用。然而，迄今为止，所有的OAM波束——也被称为涡旋波束——都是静态的;也就是说，OAM不随时间变化。本文介绍并实验验证了光束的一种新特性，即沿光脉冲方向的时变OAM；我们把这个性质称为光的自扭矩。
INTRODUCTION
Light beams carry both energy and momentum, which can exert a small but detectable pressure on objects they illuminate. In 1992, it was realized that light can also possess orbital angular momentum (OAM) when the spatial shape of the beam of light rotates (or twists) around its own axis. Although not visible to the naked eye, the presence of OAM can be revealed when the light beam interacts with matter. OAM beams are enabling new applications in optical communications, microscopy, quantum optics, and microparticle manipulation. To date, however, all OAM beams—also known as vortex beams—have been static; that is, the OAM does not vary in time. Here we introduce and experimentally validate a new property of light beams, manifested as a time-varying OAM along the light pulse; we term this property the self-torque of light.
基本原理
尽管自扭矩存在于不同的物理系统中(如电动力学和广义相对论)，但到目前为止，人们还没有意识到光在没有外力参与的情况下可以具有这样的性质。自扭矩是光的一种固有特性，有别于静态OAM光束对物体施加的机械扭矩。在两种不同OAM、时间相对滞后的超快激光脉冲驱动下，高次谐波产生的极端非线性过程中，极紫外(EUV)自扭光束自然产生。HHG沿着EUV脉冲刻印了一个时变的OAM，其中所有后续的OAM组件都是物理存在的。在未来，这种新型的动态OAM光束可以用于在纳米尺度上控制最快的磁、拓扑、分子和量子激发。
RATIONALE
Although self-torque is found in diverse physical systems (e.g., electrodynamics and general relativity), to date it was not realized that light could possess such a property, where no external forces are involved. Self-torque is an inherent property of light, distinguished from the mechanical torque exerted on matter by static-OAM beams. Extreme-ultraviolet (EUV) self-torqued beams naturally arise when the extreme nonlinear process of high harmonic generation (HHG) is driven by two ultrafast laser pulses with different OAM and time delayed with respect to each other. HHG imprints a time-varying OAM along the EUV pulses, where all subsequent OAM components are physically present. In the future, this new class of dynamic-OAM beams could be used for manipulating the fastest magnetic, topological, molecular, and quantum excitations at the nanoscale.
结果
HHG：高次谐波，EUV：极紫外线，OAM：轨道角动量
自扭矩光束是由HHG自然产生的，在这个过程中，一个超快的激光脉冲被连贯地向上转换成光谱中的EUV和x射线区域。通过两个时滞过程的HHG，流拥有不同的OAM的红外脉冲涡来驱动，ℓ1和ℓ2，生成的高次谐波出现如同自扭矩EUV光束，ℏξq≃ℏq(ℓ2−ℓ1)/ tdℏξq≃ℏq(ℓ2−ℓ1)/ td，这取决于驱动场的性能，OAM量和他们的相对时间延迟(td)——在谐波次数(q)。值得注意的是，光的自扭矩也表现为频率啁啾以及他们的方位坐标，这使其实验表征成为可能。这种从qℓ1到qℓ2的超快、连续、暂时的OAM变化比驱动激光脉冲持续时间要小得多，并且在飞秒（10-15 s）甚至亚秒时间尺度上对高自扭矩值的变化也要小得多。通过测量实验产生的EUV光束的方位频率啁啾，证实了EUV光束中存在自扭转，这种啁啾是通过调整驱动脉冲之间的时间延迟来控制的。此外，在周期量级的脉冲驱动下，超连续的EUV光谱产生了大量的频率啁啾。
RESULTS
Self-torqued beams are naturally produced by HHG, a process in which an ultrafast laser pulse is coherently upconverted into the EUV and x-ray regions of the spectrum. By driving the HHG process with two time-delayed, infrared vortex pulses possessing different OAM, ℓ1 and ℓ2, the generated high harmonics emerge as EUV beams with a self-torque, ℏξq≃ℏq(ℓ2−ℓ1)/tdℏξq≃ℏq(ℓ2−ℓ1)/td, that depends on the properties of the driving fields—that is, their OAM content and their relative time delay (td)—and on the harmonic order (q). Notably, the self-torque of light also manifests as a frequency chirp along their azimuthal coordinate, which enables its experimental characterization. This ultrafast, continuous, temporal OAM variation that spans from qℓ1 to qℓ2 is much smaller than the driving laser pulse duration and changes on femtosecond (10−15 s) and even subfemtosecond time scales for high values of self-torque. The presence of self-torque in the experimentally generated EUV beams is confirmed by measuring their azimuthal frequency chirp, which is controlled by adjusting the time delay between the driving pulses. In addition, if driven by few-cycle pulses, the large amount of frequency chirp results in a supercontinuum EUV spectrum.
结论
我们已经从理论上预测和实验上产生了一种新特性的光束，我们称之为光的自扭矩，在这个特性中，OAM含量沿脉冲本身的时间变化极快。这种光固有特性为创建结构化光束开辟了额外的途径。此外，由于OAM值在飞秒时间尺度上发生变化，在比可见光波长短得多的波长上，自扭矩HHG光束可以成为阿秒时间尺度和纳米空间尺度上激光物质操纵的特殊工具。
CONCLUSION
We have theoretically predicted and experimentally generated light beams with a new property that we call the self-torque of light, where the OAM content varies extremely rapidly in time, along the pulse itself. This inherent property of light opens additional routes for creating structured light beams. In addition, because the OAM value is changing on femtosecond time scales, at wavelengths much shorter than those of visible light, self-torqued HHG beams can be extraordinary tools for laser-matter manipulation on attosecond time and nanometer spatial scales.
产生具有自扭矩的EUV梁
(A) 将具有不同OAM的两个延时飞秒红外（IR）脉冲聚焦到气体靶中，通过HHG产生自扭矩的EUV光束。自扭矩光束的独特特征是其随时间变化的OAM，如(B) 所示，对于第17谐波(47 nm，自扭转ξ17ξ17 = 1.32 fs−1). (C)自力矩刻印一个方位频率啁啾，使其能够进行实验的测量。
Generation of EUV beams with self-torque.
(A) Two time-delayed, femtosecond infrared (IR) pulses with different OAM are focused into a gas target to produce self-torqued EUV beams through HHG. The distinctive signature of self-torqued beams is their time-dependent OAM, as shown in (B) for the 17th harmonic (47 nm, with self-torque ξ17ξ17 = 1.32 fs−1). (C) The self-torque imprints an azimuthal frequency chirp, which enables its experimental measurement.
摘要
携带轨道角动量（OAM）的光场为光通信、显微镜、量子光学和微粒操纵等领域的应用提供了强大的能力。我们介绍了光束的一个特性，表现为沿脉冲的时间OAM变化：光的自扭矩。虽然在不同的物理系统（即电动力学和广义相对论）中发现了自扭矩，但人们没有意识到光可以具有这样的性质。我们证明，在不同OAM的时间延迟脉冲驱动的高次谐波产生里出现了极紫外自扭转光束。我们通过极紫外光束的方位频率啁啾来监测其自扭转。这类动态OAM光束提供了控制磁性、拓扑和量子激发以及在其自然时间和长度尺度上操纵分子和纳米结构的能力。
Abstract
Light fields carrying orbital angular momentum (OAM) provide powerful capabilities for applications in optical communications, microscopy, quantum optics, and microparticle manipulation. We introduce a property of light beams, manifested as a temporal OAM variation along a pulse: the self-torque of light. Although self-torque is found in diverse physical systems (i.e., electrodynamics and general relativity), it was not realized that light could possess such a property. We demonstrate that extreme-ultraviolet self-torqued beams arise in high-harmonic generation driven by time-delayed pulses with different OAM. We monitor the self-torque of extreme-ultraviolet beams through their azimuthal frequency chirp. This class of dynamic-OAM beams provides the ability for controlling magnetic, topological, and quantum excitations and for manipulating molecules and nanostructures on their natural time and length scales.
- Laura Rego1,*,†, Kevin M. Dorney2,*,†, Nathan J. Brooks2, Quynh L. Nguyen2, Chen-Ting Liao2, Julio San Román1, David E. Couch2, Allison Liu2, Emilio Pisanty3, Maciej Lewenstein3,4, Luis Plaja1, Henry C. Kapteyn2,5, Margaret M. Murnane2, Carlos Hernández-García1
- 1Grupo de Investigación en Aplicaciones del Láser y Fotónica, Departamento de Física Aplicada, University of Salamanca, Salamanca E-37008, Spain.
- 2JILA, Department of Physics, University of Colorado and NIST, Boulder, CO 80309, USA.
- 3ICFO, Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Av. Carl Friedrich Gauss 3, 08860 Castelldefels (Barcelona), Spain.
- 4ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain.
- 5Kapteyn-Murnane Laboratories Inc. (KMLabs Inc.), 4775 Walnut Street no. 102, Boulder, CO 80301, USA.
- ↵*Corresponding author. Email: laura.rego@usal.es (L.R.); kevin.dorney@colorado.edu (K.M.D.)
- ↵† These authors contributed equally to this work.
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Science 28 Jun 2019:
Vol. 364, Issue 6447, eaaw9486
DOI: 10.1126/science.aaw9486
http://www.sciencemag.org/about/science-licenses-journal-article-reuse
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