March 7, 2022 feature
All-optical attoclock for imaging tunnelling wavepackets
Physicists can study the possible time delays of light-induced tunneling of an electron from an atom after conducting measurements of time delays when cold atoms tunnel through an optically created potential barrier. In a new report now published in Nature Physics, Ihar Babushkin and a research team in Germany, complemented photo-electron detection in laser-induced tunneling by measuring light emitted by the tunneling electron, known as Brunel radiation. Based on combined single and two-color driving fields, they identified all-optical signatures of reshaping tunneling wave-packets as they emerged from the tunneling barrier and moved away from the core. This reshaping led to an effective time-delay and time-reversal symmetry of the ionization process, described in theory, for experimental observation. The all-optical detection method can facilitate time-resolved measurements of optical-tunneling in condensed matter systems at the attosecond time-scale.
Attosecond science is a revolutionary technology, which combines optical and collision science to greatly extend the reach of each. The possibility of tunneling an electron through the potential barrier created by an oscillating electric field and the binding potential of the core is a fundamental resource in attosecond science. The phenomenon is at the heart of high harmonic generation and high harmonic spectroscopy. High harmonic generation is associated with radiative recombination based on the return of the laser-driven electron to the parent ion. But even when the electron does not return to the core, the setup emitted high harmonic radiation, referred to as the Brunel radiation or Brunel harmonics. The process is associated with bursts of current triggered by laser-induced tunneling, ubiquitous in atoms, molecules and solids. In this work, Babushkin et al. showed how Brunel harmonics generated in elliptically polarized single- and two-color laser fields provided a detailed picture of light induced tunneling of an electron. The described approach to imaging ionization dynamics distinctly differed from existing attoclock approaches based on photo-electron detection. The method allowed the introduction of a complementary, all-optical measurement protocol to establish extended measurements of tunneling dynamics in bulk solids.
The scientists validated the central idea behind the all-optical attoclock by determining vectorial properties of the emitted light, determined by the vectorial properties of the current generated by the tunneling electron to reflect the tunneling dynamics. The team considered two field arrangements, in the first they combined an intense circularly polarized infra-red pump with its co-rotating second harmonic to generate a total electric field with a reference direction for the optical attoclock. In the second arrangement the reference direction was provided by the major axis of the single-color elliptically polarized driving field. The team began with the first arrangement where the nonlinear response contained even and odd harmonics; with a signal dominated by Brunel radiation. For instance, the team injected a classical free electron by strong-field ionization into the atomic continuum with some velocity to accelerate in the laser field and potential of the core. Babushkin et al. verified the outcomes using the ab initio time-dependent Schrödinger equation (TDSE) simulations to compute the radiated field.
Imaging ionization dynamics and outlook
During the experiments, the team confirmed the predicted rotation of the polarization ellipse of the nonlinear response using experimental measurements with the setup. Babushkin et al. accomplished this using an 800-nm, 43-femtosecond-long, elliptically polarized pump pulse focused into a plasma spot for third harmonic generation to carefully separate and detect polarization components. The scientists compared the experimentally measured intensity-dependent parameters of the polarization ellipse with TDSE (time-dependent Schrödinger equation) simulation results to show good agreement between the experiment and simulation.
In this way, Ihar Babushkin and colleagues established a firm quantitative link between photo-electron spectra in strong-field ionization. They measured Brunel radiation generated by electrons on their way to the continuum to reveal the reshaping of electron wave packets during laser-induced tunneling. Based on Brunel harmonics imaging, the team reshaped mapping onto effective ionization delays, where Brunel harmonics in the terahertz and ultraviolet regions contained signatures of attosecond and sub-angstrom-scale electron dynamics. The researchers credited the origin of ionization asymmetry to the dynamics of the electron wave packet during and after tunneling for high intensities or saturation effects. The study provides promising capability to image tunneling and explore attosecond-scale wave packet reshaping in systems where photo-electron detection wasn't readily available. Such systems include bulk solids, where the detection of light is much simpler compared to the detection of electrons. Babushkin et al. expect the Brunel harmonics of yet higher order to allow the resolution of electron dynamics even closer to the core. The outcome will have impact beyond physics, to influence chemistry, biology and future technologies.
R. E. F. Silva et al, Topological strong-field physics on sub-laser-cycle timescale, Nature Photonics (2019). DOI: 10.1038/s41566-019-0516-1
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