Dejan B. Milosevic

We present a combined experimental and theoretical study of high-harmonic generation driven by an ω +4 ω two-color bicircular laser field. By exploiting the circularly polarized components of the driving field, we generate extreme-ultraviolet radiation consisting predominantly of harmonics with the same helicity, resulting in a strongly chiral harmonic spectrum. Individual harmonics exhibit ellipticities of up to 0.8, demonstrating the feasibility of generating elliptically polarized attosecond pulse trains. We further show that varying the ellipticity of the 4 ω driving field provides an effective means of controlling both the spectral distribution of the emitted radiation and the polarization state of the generated harmonics. Theoretical simulations are in good agreement with the experimental results, providing insight into the harmonic-generation mechanism and elucidating the role of contributions from the magnetic quantum numbers m =±1.

The generation of intense few-cycle laser pulses with a stabilized carrier-envelope phase (CEP) has enabled unprecedented control of strong-field laser–matter interactions on an attosecond time scale. Few-cycle laser fields allow direct manipulation of the instantaneous electric-field waveform, making the CEP and other waveform parameters fundamental tools for steering electron dynamics. In this review, we present a comprehensive overview of the role of waveform control in strong-field processes driven by intense few-cycle laser pulses. We discuss the generation and characterization of few-cycle pulses and summarize the influence of the CEP on above-threshold ionization, high-order above-threshold ionization, nonsequential double ionization, high-order harmonic generation, and laser-assisted scattering and radiative recombination. Particular attention is devoted to bichromatic laser fields, where waveform engineering is achieved not only through the CEP but also by controlling the relative phase, polarization, frequency ratio, and intensity of the field components. These additional degrees of freedom provide powerful control knobs for tailoring electron trajectories, photoelectron momentum distributions, harmonic spectra, cutoff energies, and the polarization properties of the emitted radiation. Representative theoretical results are used to illustrate how waveform engineering enables efficient control of strong-field phenomena. The developments reviewed here demonstrate that waveform-controlled few-cycle laser fields have become indispensable tools for attosecond science, ultrafast spectroscopy, and coherent control, while continuing to stimulate new theoretical and experimental advances in strong-field physics. Overall, this review highlights the evolution of strong-field physics from CEP-controlled interactions to comprehensive waveform engineering, where multiple independently tunable laser parameters enable precise control of ultrafast electron dynamics and light emission.

Phys. Rev. A 114, 049901 (2026)

We formulate a theory of laser-assisted radiative recombination of electron with diatomic homonuclear ion in a linearly polarized laser field using the S -matrix formalism. We investigate how the multicenter molecular structure modifies the differential power spectra and polarization properties of the emitted x-rays. The emitted spectra exhibit characteristic two-center interference minima whose positions depend on the molecular orientation, internuclear distance, symmetry of the molecular orbital, and the incident electron momentum. We show that the visibility of the interference minima is strongly influenced by the molecular orientation and by the component of the incident electron momentum perpendicular to the laser-field polarization vector. The minima remain comparatively robust against imperfect molecular alignment. In addition, the emitted x-rays remain linearly polarized, while their polarization direction depends on the emitted-photon energy and on the geometry of the recombination process. The obtained results demonstrate that laser-assisted radiative recombination provides direct access to structural and symmetry properties of molecular targets.

We theoretically investigate the physical principles underlying the attoclock method in strong-field ionization, with particular emphasis on the interpretation of the offset angle and the role of the Coulomb potential. Our analysis is based on the numerical solution of the time-dependent Schrödinger equation (TDSE) and several variants of the strong-field approximation (SFA) combined with quantum-orbit theory. Among the investigated models, the best agreement with the TDSE is obtained when the ionization probability and the initial conditions for the electron trajectory are determined using the SFA together with the saddle-point method. The corresponding final electron momentum is then calculated by classical propagation in the combined laser and Coulomb fields.

Emission of a photon during the process of above-threshold ionization is investigated. In previous work [D. B. Milošević, ] one of the present authors studied this process in the leading order of the strong-field approximation for the matrix element that describes direct emission of a photon during the transition from the bound state to the continuum. We now show that the correction that corresponds to photon emission while the electron is propagating in the continuum, very significantly modifies the process for all but some intermediate photon energies. In contrast to the lowest-order term of the previously considered expansion, the continuum correction is infrared divergent and resonant when the photon energy agrees with the energy of a driving photon. If the energy of the emitted photon is much larger than the energy of a driving photon, then the continuum correction is very close to the leading-order direct contribution and the two terms interfere constructively. These statements hold true for a variety of laser-field configurations; we investigated various bichromatic linearly and circularly polarized fields including bicircular fields.

We investigate directional asymmetry in strong-field ionization driven by a few-cycle near-infrared laser pulse assisted by a time-delayed terahertz (THz) pulse. Within the strong-field approximation, we show that the relative delay between the laser and THz pulses provides an efficient control parameter for steering photoelectron emission along the laser polarization axis. Energy-resolved photoelectron spectra, momentum distributions, and asymmetry parameter demonstrate a pronounced and tunable left-right asymmetry for both direct and rescattered electrons, governed by the value of the THz vector potential during the ionization window. This control is significantly stronger than that achievable by varying the carrier-envelope phase of the laser pulse alone and remains robust with respect to variations of the THz-pulse parameters and focal averaging. Our results build upon the theoretical concept introduced earlier [Milošević, ], whose predictions were subsequently confirmed by the experiment [Mitrofanov , ], establishing THz-assisted strong-field ionization as a reliable and experimentally validated approach for controlling photoelectron emission on ultrafast timescales.

We have extended the two-color bicircular approach for generating chiral XUV radiation via high-harmonic generation by combining femtosecond driving laser fields with 1:3 frequency ratio of wavelengths 1200 and 400 nm. With optimized driving-field intensities, we produced highly chiral spectra in which the vast majority of harmonics sharing the same helicity constitute the vast majority of spectral content. We support these experimental finding with a numerically comprehensive analytical model that describes the propensity (selection) rules for emission of circularly polarized high harmonics. We identify and confirm the rules responsible for the contrast between the allowed odd harmonics of orders 4 q + 1 and 4 q − 1 ( q an integer), which posses opposite helicities, as well as the complete prohibition of even-order harmonics in the high-harmonic spectra. We further apply these rules to explain the spectral shape and ellipticity of the emitted odd harmonics as functions of both the intensity and the ellipticity of the driving-field components. Using XUV polarimetry, we retrieved full Stokes vector, confirming achieved ellipticities higher than 0.9.

Squeezed coherent quantum light is described by the complex squeeze parameter ζ = s e i ϑ and the coherent-state displacement parameter β = | β | e i ϕ β . Our study of strong-field ionization [D. Habibović and D. B. Milošević, ], which focused on bright squeezed vacuum quantum light with β = 0 and s ≫ 1 , is extended to ionization by squeezed coherent light. It is shown that the amplitudes s and | β | strongly influence the ionization process, whereas phases ϑ and ϕ β play a less significant role. The present work provides a unified treatment of coherent, squeezed vacuum, and squeezed coherent light within a single theoretical framework. The quantum nature of the driving field is incorporated by averaging the strong-field ionization probability over the Husimi-Kano phase-space distribution. Using neon as a representative target, we analyze how the parameters characterizing squeezed coherent light modify photoelectron spectra and intensity-dependent enhancements. We show that nonclassical photon statistics can lead to a pronounced steepening of the rescattering plateau and, at low mean intensities, to an effective suppression of the nominal high-energy cutoff. These effects originate from the reduced weight of high-intensity field components inherent to squeezed states and have no analog in strong-field ionization driven by a coherent light. Our results highlight the potential for quantum-statistical control of strong-field ionization and provide a foundation for exploring laser-matter interactions driven by quantum light.