image: a Layout showing the preparation of the two-color co-rotating circular light fields with the 800-nm light pulses (red) and 400-nm light pulses (blue). A Mach–Zehnder interferometer scheme is used. BBO, barium borate crystal; DM, dichroic mirror; QWP, quarter-wave plate; SPP, spiral phase plate. b Illustration of the strong-field ionization detection experiment. The generated two-color co-rotating circular field interacts with the supersonic argon atom gas jet inside of COLTRIMS apparatus. c Diagram of the electric field vectors of the 800-nm circular field (red), 400-nm circular field (blue), and two-color co-rotating circular field (purple), respectively. For each laser field, a 3D polarization profile (left) and a 2D Lissajous figure of its electric field in transverse plane (right) are given. The deflected angle of peak electric field of the two-color field, ξm, is related with that in Fig. 2a. d The simulated light spatial profiles in the focal plane. The top panels show the normalized intensity distributions and the bottom panels show the phase structure of x-component electric field. The white circles enclose the regions (exclude the singularities in b4 and b5), where the light intensity is larger than 0.5Im (Im, the peak intensity of focal light field). view more
Credit: by Yiqi Fang, Zhenning Guo, Peipei Ge, Yankun Dou, Yongkai Deng, Qihuang Gong, and Yunquan Liu
Optical vortex plays an important role in a variety of fields, such as particle tweezing, quantum communication, astrophysics, and microscopy. Especially, the application of optical vortex has been recently extended into strong-field regime, where the unique helical phase structure provides a robust toolbox for the spatio-temporal manipulation of extreme ultraviolet radiations and photoelectron states. Since the dynamic signature of vortex beams is determined by the OAM mode, the OAM mode measurement is one of the crucial tasks which is prior to the applications of vortex beams. Hitherto, the measurement of OAM has been less studied in the intense-light-matter interaction processes.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Yunquan Liu from State Key Laboratory for Mesoscopic Physics and Frontiers Science Center for Nano-optoelectronics, School of Physics, Peking University, China, and co-workers have developed an approach for in-situ probing the OAM of intense vortex pulses with photoelectron momentum imaging via strong-field photoionization experiment. They employed a synthesized two-color co-rotating circular laser field configuration, in which a spatialy sculptured plane wave is used as a probing light to detect the helical phase structure of the other vortex pulses. The core of this method is to link the relative-phase structure of two-color light to the photoelectron momentum distributions or angle-resolved yields. Experimentally, they successfully characterized the high-power femtosecond vortex pulses carrying three different OAM modes (0, 1, and 2). Based on the experimental observation and theoretical prediction, they further proposed a universal scheme for the detection of intense optical vortex pulses with higher OAM modes. Interestingly, taking the advantage of the intrinsic nature of multiphoton ionization, the measurement process has almost no influence on the OAM states of vortex pulses, so that the measured optical vortex beams can then be utilized in subsequent applications.
In photoionization, because of the large difference between the focal spot and the electron ponderomotive motion in the intense light field, the electrons cannot “see” the global geometric structure of the driving laser field. So, it is considered to be hard to embody optical OAM in the dynamics of electrons in photoionization experiments. To overcome this difficulty, here they have used the synthesized two-color laser field configuration. These scientists summarize the operational principle of their detection method:
“We design a pioneering OAM measurement scenario with synthesized two-color co-rotating circular laser fields. First, we employ a non-spatially sculpted probing field to discriminate optical vortices (OAM≠0) from plane waves (OAM=0). Then, the spatially sculpted probing pulses are utilized to further characterize the topological charge of the optical vortices. In this geometry, the value of optical OAM plays an important role in strong-field ionization. With the support of the classical-trajectory Monte Carlo (CTMC) model and our analytical model, the OAM of optical vortices is well detected.”
“Besides, we can speculate that the number of photons absorbed from the laser pulses is approximately ~10, which is very trivial compared to the total number of photons per laser pulse. That is, the probe process has almost no effect on the laser OAM state. Such a unique feature makes the photoionization method an ideal tool for measuring the quantum states of optical systems, such as the OAM or spin angular momentum of photons. The measured optical systems can be used in further broad applications.” they added.
“The methodology used in this work can be extended to the control of EUV radiations in high-order harmonic generation using the intense vortex pulse, which may largely facilitate the spatio-temporal control over the radiation of high-energy photons. Moreover, this work has also implications for generating and probing the structured electron beams, such as electron vortices.” the scientists forecast.
Journal
Light Science & Applications