Search

Refraction is a basic beam bending effect at two media's interface. While traditional studies focus on stationary boundaries, moving boundaries or potentials could enable new laws of refractions. Meanwhile, media's discretization plays a pivotal role in refraction owing to Galilean invariance breaking principle in discrete-wave mechanics, making refraction highly moving-speed dependent. Here, by harnessing a synthetic temporal lattice in a fiber-loop circuit, we observe discrete time refraction by a moving gauge-potential barrier. We unveil the selection rules for the potential moving speed, which can only take an integer v = 1 or fractional v = 1/q (odd q) value to guarantee a well-defined refraction. We observe reflectionless/reflective refractions for v = 1 and v = 1/3 speeds, transparent potentials with vanishing refraction/reflection, refraction of dynamic moving potential and refraction for relativistic Zitterbewegung effect. Our findings may feature applications in versatile time control and measurement for optical communications and signal processing. Here, authors construct a moving potential barrier in a synthetic temporal lattice. They unveil the selection rules for choosing potential moving speed as well as the conditions to achieve transparent moving potentials and refraction. [ABSTRACT FROM AUTHOR]

Light–matter interactions are usually described within the electric-dipole approximation, where the magnetic-field component and the spatial variation of the light electric field over the relevant electronic length scales are both ignored. Non-dipole effects in photoionization were revealed to be tiny from the infrared to the soft X-ray domains, and all non-dipole observations reported so far were limited to single-pulse, single-colour measurements. Here we advance attosecond time-resolved spectroscopy into the non-dipole interaction regime. Using a self-referenced attosecond photoelectron interferometry on helium atoms, we resolve the electron subcycle motion along the light propagation direction in the 15 pm range driven by the magnetic component of a near-infrared laser field. Furthermore, we measure a time delay of 15 ± 10 as between the electric-dipole and electric-quadrupole transitions by resolving the asymmetry of the photoelectron forward–backward yields with attosecond resolution. These fundamental findings are supported by ab initio calculations based on the non-dipole time-dependent Schrödinger equation. Using a self-referenced attosecond photoelectron interferometry on helium atoms, the electron subcycle motion along the light propagation direction is observed in the 15 pm range. A time delay of 15 ± 10 as between the electric-dipole and electric-quadrupole transitions is also revealed. [ABSTRACT FROM AUTHOR]

An ultrafast fiber laser system comprising two coherently combined amplifier channels is reported. Within this system, each channel incorporates a rod-type fiber power amplifier, with individual operations reaching approximately 233 W. The active-locking of these coherently combined channels, followed by compression using gratings, yields an output with a pulse energy of 504 μJ and an average power of 403 W. Exceptional stability is maintained, with a 0.3% root mean square (RMS) deviation and a beam quality factor M2 < 1.2. Notably, precise dispersion management of the front-end seed light effectively compensates for the accumulated high-order dispersion in subsequent amplification stages. This strategic approach results in a significant reduction in the final output pulse duration for the coherently combined laser beam, reducing it from 488 to 260 fs after the gratings compressor, while concurrently enhancing the energy of the primary peak from 65% to 92%. [ABSTRACT FROM AUTHOR]

Dynamic localization (DL) of photons, i.e., the light-motion cancellation effect arising from lattice's quasi-energy band collapse under a synthetic ac-electric-field, provides a powerful and alternative mechanism to Anderson localization for coherent light confinement. So far only low-order DLs, corresponding to weak ac-fields, have been demonstrated using curved-waveguide lattices where the waveguide's bending curvature plays the role of ac-field as required in original Dunlap-Kenkre model of DL. However, the inevitable bending losses pose a severe limitation for the observation of high-order DL. Here, we break the weak-field limitation by transferring lattice concepts from spatial to synthetic time dimensions using fiber-loop circuits and observe up to fifth-order DL. We find that high-order DLs possess superior localization and robustness against random noise over lower-order ones. As an exciting application, by judiciously combining low- and high-order DLs, we demonstrate a temporal cloaking scheme with flexible tunability both for cloak's window size and opening time. Our work pushes DL towards high-order regimes using synthetic-lattice schemes, which may find potential applications in robust signal transmission, protection, processing, and cloaking. Dynamic localization is a method of confining light. Here the authors demonstrate higher-order dynamic localization of photons in a synthetic temporal mesh lattice and discuss the idea of tunable temporal cloaking by combining different-order localizations. [ABSTRACT FROM AUTHOR]

Electron migration in molecules is the progenitor of chemical reactions and biological functions after light-matter interaction. Following this ultrafast dynamics, however, has been an enduring endeavor. Here we demonstrate that, by using machine learning algorithm to analyze high-order harmonics generated by two-color laser pulses, we are able to retrieve the complex amplitudes and phases of harmonics of single fixed-in-space molecules. These complex dipoles enable us to construct movies of laser-driven electron migration after tunnel ionization of N2 and CO2 molecules at time steps of 50 attoseconds. Moreover, the angular dependence of the migration dynamics is fully resolved. By examining the movies, we observe that electron holes do not just migrate along the laser polarization direction, but may swirl around the atom centers. Our result establishes a general scheme for studying ultrafast electron dynamics in molecules, paving a way for further advance in tracing and controlling photochemical reactions by femtosecond lasers. Seeing molecules in action under the influence of very short pulses is interesting. Here the authors demonstrate charge migration dynamics in molecules by using high harmonic spectroscopy and machine learning algorithm. [ABSTRACT FROM AUTHOR]