arXiv subjects

Yizhu Zhang

Publications and source records attributed to Yizhu Zhang.

17 recordsLinked to original sources

arXiv2026

Symmetry-Breaking Electron Dynamics Enable Ultrabroadband Optical-Field Sampling via Second-Harmonic Generation

Optical-field sampling using second-harmonic generation (SHG) from strong-field ionization enables ultrabroadband terahertz detection, but the microscopic origin of the SHG signal and its ultrabroadband response have been unclear. Here we show that the target field lifts the half-cycle cancellation of photoelectron dipole emission, generating the SHG signal used for field sampling. Time-dependent Schrodinger-equation simulations, supported by classical-trajectory Monte Carlo analysis, demonstrate that the SHG yield directly encodes the instantaneous target electric field at the ionization time, enabling waveform retrieval by scanning the probe-target delay. Because the SHG response is gated by a subcycle ionization window rather than the probe envelope, the detection bandwidth can extend far beyond the probe duration. We further quantify practical constraints on retrieval, including intrinsic probe asymmetry and SHG back-action, providing a predictive framework to optimize sensitivity, temporal resolution, and fidelity through controlled electron dynamics.

physics.optics↗

arXiv2026

Experimental Realization of All-Optical Terahertz Attoclock

The attoclock is a powerful tool for probing ultrafast electron dynamics with attosecond precision.Here, we demonstrate an all-optical terahertz (THz) attoclock that reconstructs photoionization dynamics by detecting the THz radiation emitted from Ar atoms ionized by two-color (800 nm/400 nm) laser fields. In this approach, the polarization direction of the emitted THz field reflects the direction of the photoelectron drift velocity and thus serves as a direct observable that encodes the effective ionization delay, analogous to the angular deflection of photoelectrons in conventional attoclocks. By precisely tailoring the relative phase and ellipticity of the driving fields, we observe intensity-dependent rotations of the THz polarization. These rotations, which reveal changes of the effective delay, are consistent with both conventional attoclock measurements and time-dependent Schrödinger equation simulations. Our experiment establishes the feasibility of the THz attoclock as a vacuum-free and contactless probe of tunneling dynamics, offering a transformative alternative for investigating condensed-matter systems where photoelectron detection is challenging.

physics.optics↗

arXiv2024

Probing Electronic Motion and Core Potential by Coulomb-reshaped Terahertz Radiation

The nature of electronic motion and structural information of atoms and molecules is encoded into strong-field induced radiations ranging from terahertz (THz) to extreme ultraviolet wavelength. The dependence of THz yields in bi-chromatic laser fields on ellipticity and interpulse phase delay were experimentally measured, and the trajectory calculations establish the link between the THz emission and the motion of the photoelectron wave packet. The interaction between the photoelectron and parent core transforms from soft collision to recollision as the laser field tuned from elliptical to linear polarization, which can be reflected in THz emission. The soft collision is found to be more effective in reconstructing electron dynamics through THz polarization, which enables to construct the effective core potential of the generating medium with the Coulomb-reshaped THz radiation in an elliptically polarized laser field. Our work allows designing innovative all-optical THz measurements of electronic and structural dynamics.

physics.atom-ph↗

arXiv2024

Temporal-Spatial Manipulation of Bi-Focal Bi-Chromatic Fields for Terahertz Radiations

Mixing the fundamental ($ω$) and the second harmonic (2$ω$) waves in gas phase is a widely employed technique for emitting terahertz (THz) pulses. The THz generation driven by bi-chromatic fields can be described by the photocurrent model, where the THz generation is attributed to free electrons ionized by the $ω$ field, and the 2$ω$ field provides a perturbation to break the symmetry of the asymptotic momentum of free electrons. However, we find that the THz radiation is amplified by one order of magnitude when driven by bi-focal bi-chromatic fields, contradicting the common understanding of the photocurrent model. Meanwhile, present measurements demonstrate that the THz radiation mainly originates from the plasma created by the 2$ω$ pulses instead of the $ω$ pulses. Energy transfer from the 2$ω$ beam to the THz beam during the THz generation has been observed, validating the major contribution of the 2$ω$ beam. Furthermore, the THz bandwidth has been observed to extensively exceed the bandwidth of the pump pulse, not be explained by the photocurrent model as well. These counterintuitive results indicate that undiscovered physical mechanisms are involved in bi-chromatic THz generation in plasma, presenting a significant challenge for understanding strong-field nonlinear optics and simultaneously expanding various applications.

physics.optics↗

arXiv2023

Coulomb potential determining terahertz polarization in two-color laser field

The orientation and ellipticity of terahertz (THz) polarization generated by two-color strong field not only cast light on underlying mechanisms of laser-matter interaction, but also play an important role for various applications. We develop the Coulomb-corrected classical trajectory Monte Carlo (CTMC) method to well reproduce the joint measurements, that the THz polarization generated by the linearly-polarized 800 nm and circularly-polarized 400 nm fields is independent on two-color phase delay. The trajectory analysis shows that the Coulomb potential twists the THz polarization by deflecting the orientation of asymptotic momentum of electron trajectories. Further, the CTMC calculations predict that, the two-color mid-infrared field can effectively accelerate the electron rapidly away from the parent core to relieve the disturbance of Coulomb potential, and simultaneously create large transverse acceleration of trajectories, leading to the circularly-polarized THz radiation.

physics.optics↗

arXiv2022

Intensity-surged and Bandwidth-extended Terahertz Radiation in Two-foci Cascading Plasmas

The two-color strong-field mixing in gas medium is a widely-used approach to generate bright broadband terahertz (THz) radiation. Here, we present a new and counterintuitive method to promote THz performance in two-color scheme. Beyond our knowledge that the maximum THz generation occurs with two-color foci overlapped, we found that, when the foci of two-color beams are noticeably separated along the propagation axis resulting in cascading plasmas, the THz conversion efficiency is surged by one order of magnitude and the bandwidth is stretched by more than 2 times, achieving $10^{-3}$ conversion efficiency and $>$100 THz bandwidth under the condition of 800/400 nm, $\sim$35 fs driving lasers. With the help of the pulse propagation equation and photocurrent model, the observations can be partially understood by the compromise between THz generation and absorption due to the spatial redistribution of laser energy in cascading plasmas. Present method can be extended to mid-infrared driving laser, and the new records of THz peak power and conversion efficiency are expected.

physics.optics↗

arXiv2020

Terahertz field induced near-cutoff even-order harmonics in femtosecond laser

High-order harmonic generation by femtosecond laser pulse in the presence of a moderately strong terahertz (THz) field is studied under the strong field approximation, showing a simple proportionality of near-cutoff even-order harmonic (NCEH) amplitude to the THz electric field. The formation of the THz induced-NCEHs is analytically shown for both continuous wave and Gaussian pulse. The perturbation analysis with regard to the frequency ratio of the THz field to the femtosecond pulse shows the THz-induced NCEHs originates from its first-order correction, and the available parametric conditions for the phenomenon is also clarified. As the complete characterization of the time-domain waveform of broadband THz field is essential for a wide variety of applications, the work provides an alternative time-resolved field-detection technique, allowing for a robust broadband characterization of pulses in THz spectral range.

physics.optics↗

arXiv2020

Ellipticity-dependent sequential over-barrier ionization of cold rubidium

We perform high-resolution measurements of momentum distribution on Rb$^{n+}$ recoil ions up to charge state $n=4$, where laser-cooled rubidium atoms are ionized by femtosecond elliptically polarized lasers with the pulse duration of 35 fs and the intensity of 3.3$\times$10$^{15}$ W/cm$^2$ in the over-barrier ionization (OBI) regime. The momentum distributions of the recoil ions are found to exhibit multi-band structures as the ellipticity varies from the linear to circular polarizations. The origin of these band structures can be explained quantitatively by the classical OBI model and dedicated classical trajectory Monte Carlo simulations with Heisenberg potential. Specifically, with back analysis of the classical trajectories, we reveal the ionization time and the OBI geometry of the sequentially released electrons, disentangling the mechanisms behind the tilted angle of the band structures. These results indicate that the classical treatment can describe the strong-field multiple ionization processes of alkali atoms.

physics.atom-ph↗

arXiv2020

Terahertz Emisssion from Quantum Interference of Electron Trajectories

The semiclassical electron trajectory, the so-called quantum orbits, is employed to explain the terahertz wave generation (TWG) in dual-color strong field, and the feasibility of the theory is validated by the measurement. We find that TWG stems from quantum path interference of partial electron wavepacket released at the neighbouring cycles of the dual-color electric field, manifesting the temporal Young's double-slit interference of single electron. The trajectories released from neighbouring cycles, creating TWG, also account for intercycle interference fringes in the photoelectron momentum distribution, whereas no signature of TWG is found when taking trajectories only from a single cycle.

physics.atom-ph↗

arXiv2020

Momentum spectroscopy for multiple ionization of cold rubidium in the elliptically polarized laser field

Employing recent developed magneto-optical trap recoil ion momentum spectroscopy (MOTRIMS) combining cold atom, strong laser pulse, and ultrafast technologies, we study momentum distributions of the multiply ionized cold rubidium (Rb) induced by the elliptically polarized laser pulses (35 fs, $1.3 \times 10^{15}$ W/cm$^2$). The complete vector momenta of Rbn+ ions up to charge state n = 4 are recorded with extremely high resolution (0.12 a.u. for Rb$^+$). Variations of characteristic multi-bands displayed in momentum distributions, as the ellipticity varies from the linear to circular polarization, are interpreted qualitatively with the classical over-barrier ionization model. Present momentum spectroscopy of cold heavy alkali atoms presents novel strong-field phenomena beyond the noble gases.

physics.atom-ph↗

arXiv2019

Dual-Color Laser Induced Terahertz Generation in Strong Field Approximation

The mechanism of the terahertz (THz) wave generation (TWG) in dual-color fields is elucidated within the theoretical framework of single-atom based strong field approximation (SFA). Evaluating the transition dipole moment, the continuum-continuum (CC) transition, rather than the continuum-bound recombination for the high-order harmonic generation, is confirmed to be the core mechanism of the TWG. The analytic form of the SFA-based CC description is consistent with the classical photoelectric current model, establishing the quantum-classical correspondence for the TWG. The theory is supported by parametric dependence of experimental THz yields calibrated by the joint measurement of the third-order harmonics. Present studies leave open the possibility of probing the ultrafast dynamics of continuum electron.

quant-ph↗

arXiv2018

Recoil-ion momentum spectroscopy of photoionization of cold rubidium atoms in a strong laser field

We study photoionization of cold rubidium atoms in a strong infrared laser field using a magneto-optical trap (MOT) recoil ion momentum spectrometer. Three types of cold rubidium target are provided, operating in two-dimension (2D) MOT, 2D molasses, and 3D MOT with densities in the orders of $10^7$ atoms/cm$^3$, $10^8$ atoms/cm$^3$, and $10^9$ atoms/cm$^3$, respectively. The density profile and the temperature of 3D MOT are characterized using the absorption imaging and photoionization. The momentum distributions of Rb$^+$ created by absorption of two- or three-photon illuminate a dipole-like double-peak structure, in good agreement with the results in the strong field approximation. The yielding momentum resolution of $0.12 \pm 0.03$ a.u. is achieved in comparison with theoretical calculations, exhibiting the great prospects for the study of electron correlations in alkali metal atoms through interaction with strong laser pulses.

quant-ph↗

arXiv2018

Ultrafast Mapping of Coherent Dynamics and Density Matrix Reconstruction in Terahertz-Assisted Laser Field

A time-resolved spectroscopic protocol exploiting terahertz-assisted photoionization is proposed to reconstruct transient density matrix. Population and coherence elements are effectively mapped onto spectrally separated peaks in photoionization spectra. The beatings of coherence dynamics can be temporally resolved beyond the pulse duration, and the relative phase between involved states is directly readable from the oscillatory spectral distribution. As demonstrated by a photo-excited multilevel open quantum system, the method shows potential applications for sub-femtosecond time-resolved measurements of coherent dynamics with free electron lasers and tabletop laser fields.

quant-ph↗

arXiv2018

Terahertz generation in dual-color laser: continuum electron in strong-field single-atom ionization

The terahertz (THz) generation in a dual-color field was investigated experimentally by precisely controlling the relative time delay $τ$ and polarization $θ$ of dual-color lasers, where the accompanying third-harmonic generation (THG) is employed to determine $τ$ up to the sub-wavelength accuracy. An anticorrelation of the yields between THz and THG with $τ$ was displayed. Compared with the theoretical simulations, the experimental results reveal that the continuum-continuum transitions of the released electron after single-atom ionization are the dominating mechanism for THz generation, which is opposite to the high-harmonic generation (HHG) based on the recollision mechanism, or the radiation induced by perturbative bound-bound transitions.

physics.optics↗

arXiv2013

Terahertz Wave Guiding by Femtosecond Laser Filament in Air

Femtosecond laser filament generates strong terahertz (THz) pulse in air. In this paper, THz pulse waveform generated by femtosecond laser filament has been experimentally investigated as a function of the length of the filament. Superluminal propagation of THz pulse has been uncovered, indicating that the filament creates a THz waveguide in air. Numerical simulation has confirmed that the waveguide is formed because of the radially non-uniform refractive index distribution inside the filament. The underlying physical mechanisms and the control techniques of this type THz pulse generation method might be revisited based on our findings. It might also potentially open a new approach for long-distance propagation of THz wave in air.

physics.optics↗

arXiv2012

Quantum Interferometry and Correlated Two-Electron Wave-Packet Observation in Helium

The concerted motion of two or more bound electrons governs atomic and molecular non-equilibrium processes and chemical reactions. It is thus a long-standing scientific dream to measure the dynamics of two bound correlated electrons in the quantum regime. Quantum wave packets were previously observed for single-active electrons on their natural attosecond timescales. However, at least two active electrons and a nucleus are required to address the quantum three-body problem. This situation is realized in the helium atom, but direct time-resolved observation of two-electron wave-packet motion remained an unaccomplished challenge. Here, we measure a 1.2-femtosecond quantum beating among low-lying doubly-excited states in helium to evidence a correlated two-electron wave packet. Our experimental method combines attosecond transient-absorption spectroscopy at unprecedented high spectral resolution (20 meV near 60 eV) with an intensity-tuneable visible laser field to couple the quantum states from the perturbative to the strong-coupling regime. This multi-dimensional transient-coupling scheme reveals an inversion of the characteristic Fano line shapes for a range of doubly-excited states. Employing Fano-type autoionization as a natural quantum interferometer, a dynamical phase shift by laser coupling to the N=2 continuum is postulated and experimentally quantified. This phase maps a transition from effectively single-active-electron to two-electron dynamics as the electron-electron interaction increases in lower-lying quantum states. In the future, such experiments will provide benchmark data for testing dynamical few-body quantum theory. They will boost our understanding of chemically and biologically important metastable electronic transition states and their dynamics on attosecond time scales.

physics.atom-ph↗

arXiv2011

Two-Dimensional Spectral Interferometry using the Carrier-Envelope Phase

Two- and multi-dimensional spectroscopy is used in physics and chemistry to obtain structural and dynamical information that would otherwise be invisible by the projection into a one-dimensional data set such as a single emission or absorption spectrum. Here, we introduce a qualitatively new two-dimensional spectroscopy method by employing the carrier-envelope phase (CEP). Instead of measuring spectral vs. spectral information, the combined application of spectral interferometry and CEP control allows the measurement of otherwise inseparable temporal events on an attosecond time scale. As a specific example, we apply this general method to the case of attosecond pulse train generation, where it allows to separate contributions of three different sub-cycle electron quantum paths within one and the same laser pulse, resulting in a better physical understanding and quantification of the transition region between cutoff and plateau harmonics. The CEP-dependent separation in time between two full-cycle spaced attosecond pulses was determined to modulate by (54 +/- 16) attoseconds.

physics.chem-ph↗