Laser Polarization Direction Research Articles

Anisotropic effects in the nondipole relativistic photoionization of hydrogen

In the nonrelativistic and dipole regime of multiphoton ionization, spherical symmetry in all but the polarization direction of the laser pulse ensures that directional dependency in the photoelectron spectra is limited to the laser polarization direction, with the final distribution exhibiting no asymmetry along the propagation direction of the laser. When relativistic effects and spatial dependency in the external potential are accounted for however, the addition of time dilation and radiation pressure both impose anisotropic effects. Previously we have found that nondipole effects induce a redshift in the photoelectron energy distribution, while conversely relativistic effects induce a blueshift, with the net effect of an apparent near-cancellation of the two. In this work we study these effects further. By examining photoelectron momentum distributions acquired from simulations with the time-dependent Dirac equation we propose explanatory models for both phenomena and present a simplified model of the shifts as a function of the angle relative to the propagation direction of the laser pulse. It is found that both nondipole and relativistic effects must be accounted for on an equal footing in order to correctly describe the photoelectron momentum distribution in the high-intensity regime.

Investigating the transition from high to low spatial frequency periodic nanoripples on TiO2 (0 0 1) oriented surface upon irradiation with UV-femtosecond pulses

The formation of laser-induced periodic surface structures (LIPSS) on a TiO2 (001) oriented surface upon irradiation with UV-fs laser pulses was analyzed. Two systems of low spatial frequency LIPSS (LSFL) perpendicular to the direction of laser polarization are observed after N = 5 laser pulses. One was developed on the edges of the crater created by the first laser pulse, while the other appeared in the center of crater and spread towards the edges. When N was greater than 10, the two LIPSS systems merge. Both LSFL systems have a spatial period of approximately 200 nm and are due to the coherent superposition of surface waves scattered on inhomogeneities of a transient metallic surface. However they differ by the type of defects involved in the excitation process of surface waves. While a pronounced and localized rim triggered LSFL on the edge of the laser spot, LSFL in the central part nucleated at the shallow high spatial frequency LIPSS (HSFL), which are parallel to the laser polarization. A mechanism is proposed to explain the role of the HSFL in the formation of LSFL. Numerical simulations of the absorbed laser power density support the evolution from HSFL to LSFL as the number of laser pulses increased.

High-efficiency fourth harmonic generation and polarization smoothing based on an orthogonal cascade of DKDP crystals.

Because of the high efficiency of frequency conversion and beam-target coupling, a fourth harmonic (4ω) laser has a splendid application prospect in a high-power laser facility. The polarization smoothing (PS) crystal is preferably after the frequency conversion crystal to flexibly obtain the best uniformity illumination of the target. However, as a high irradiance 4ω laser beam propagates through the PS crystal, the transverse stimulated Raman scattering (TSRS) effect of the PS crystal will be stronger, resulting in significant energy dissipation and crystal damage. This paper proposes a novel, to the best of our knowledge, fourth harmonic generation (FHG) scheme based on an orthogonal cascade of the DKDP crystals. This orthogonal cascaded FHG (OC-FHG) scheme employs two cascaded FHG crystals with orthogonal optical axes, and the PS crystal is in the middle. The PS crystal can rotate the polarization direction of the 2ω laser by 90°, while the polarization direction of the 4ω laser is maintained to a great extent. This OC-FHG scheme realizes the FHG by two steps, and the laser intensity at the PS crystal cuts down nearly 50%. The output intensity of the 4ω laser can be increased from 1.8G W/c m 2 to about 3.6G W/c m 2 under the condition of effectively inhibiting the TSRS effect. Meanwhile, the output 4ω laser contains two orthogonal polarized beams realizing in-beam polarization smoothing instantaneously. In addition, the novel FHG scheme can also have a high conversion efficiency and bandwidth tolerance.

Light-induced nonlinear spin Hall current in single-layer WTe2

In this theoretical investigation, we analyze light-induced nonlinear spin Hall currents in a gated single-layer 1T ′ -WTe2, flowing transversely to the incident laser polarization direction. Our study encompasses the exploration of the second and third-order rectified spin Hall currents using an effective low-energy Hamiltonian and employing the Kubo’s formalism. We extend our analysis to a wide frequency range spanning both transparent and absorbing regimes, investigating the influence of light frequency below and above the optical band gap. Additionally, we investigate the influence of an out-of-plane gate potential on the system, disrupting inversion symmetry and effectively manipulating both the strength and sign of nonlinear spin Hall responses. We predict a pronounced third-order spin Hall current relative to its second-order counterpart. The predicted nonlinear spin currents show strong anisotropic dependence on the laser polarization angle. The outcomes of our study contribute to a generalized framework for nonlinear response theory within the spin channel will impact the development of emerging field of opto-spintronic.

Dissociative ionization and post-ionization alignment of aligned O2 in an intense femtosecond laser field.

We examined dissociative ionization of O2 in an intense femtosecond laser field (782 nm, 120 fs, 4 × 1014 W cm-2) by recording the kinetic energy distribution of O+ emitted along the laser polarization direction as a function of the delay time between the pump pulse (9 × 1013 W cm-2) for the molecular alignment and the probe pulse for the dissociative ionization. We found the two distinct rotational revival patterns which are out-of-phase by π with each other in the kinetic energy distribution of O+. One of the patterns shows the dissociative ionization is enhanced when the O2 axis is parallel to the laser polarization direction, suggesting that the ionization is induced by the electron emission from the 3σg orbital. On the other hand, the other pattern shows that the dissociative ionization is enhanced when the O2 axis is perpendicular to the laser polarization direction, suggesting that the ionization is induced by the electron emission from the 1πu orbital. Because of the collection efficiency of the time-of-flight mass spectrometer, the enhancement of the O+ yield at the anti-alignment time delay indicates that the electron emission from the 1πu orbital is followed by the molecular alignment of O2+ in the course of the dissociation. We performed classical trajectory Monte-Carlo simulation of O2+ with the dissociation and rotational coordinates in the light-dressed potential to evaluate the effect of the post-ionization alignment by the probe pulse.

Effects of polarization direction, amplitude, and photon energy of linearly polarized laser on high order harmonic generation of C60

The study of high-order harmonic generation (HHG) in confined quantum systems is essential for developing a comprehensive physical description of harmonic generation from atoms to bulk solids. Using the time-dependent density-functional theory, we demonstrate how the symmetry of the system modulates the generation of high-order harmonic in fullerene C60 molecules along different orthogonal directions, as well as the effects of amplitude and photon energy of a linearly polarized laser on high-order harmonics generation. We found that the generation of high-order harmonics perpendicular to the laser polarization direction (LPD) is related to the symmetry of molecules along the LPD and the symmetry of molecules perpendicular to the LPD. Within a certain parameter range, the cut-off energy is linearly proportional to the laser amplitude and the laser photon energy.

Formation of two-dimensional laser-induced periodic surface structures on titanium by GHz burst mode femtosecond laser pulses

GHz burst mode femtosecond (fs) laser pulses, which consist of a series of pulse trains with ultra-fast intervals of several hundred picoseconds, have offered distinct features for material processing compared to conventional irradiation of laser pulses (single-pulse mode). We apply GHz burst mode processing to fabricate laser-induced periodic surface structures (LIPSS) on the material surfaces. In our previous work for silicon (Si), we have found that GHz burst mode can create unique two-dimensional (2D) LIPSS composed of both parallel and perpendicular to the laser polarization direction. We proposed that the formation of 2D-LIPSS is attributed to the synergetic contributions of electromagnetic and hydrodynamic mechanisms. To further investigate more detailed formation mechanisms and explore practical applications, we employ titanium (Ti), whose properties are significantly different from Si. We demonstrate that GHz burst mode fs laser pulses (central wavelength: 1,030 nm, intra-pulse width: 230 fs, intra-pulse repetition rate (an intra-pulse interval): 4.88 GHz (205 ps) and burst pulse repetition rate: 10 kHz) can also fabricate 2D-LIPSS on Ti surfaces. We attribute the dominant formation mechanism of 2D-LIPSS to the generation of hot spots with highly enhanced electric fields due to transient change of material properties during GHz burst pulse irradiation. Based on this speculation, properly tailoring the shapes of the burst pulse with an optimum intra-pulse number enables the creation of well-defined 2D-LIPSS. Furthermore, essentially homogeneous 2D-LIPSS can be formed in a large area by laser scanning of a focused fs laser beam with a stage scanning speed of 5 mm/s.

Laser Writing of Multilayer Structural Colors for Full‐Color Marking on Steel

Laser‐induced periodic surface structure (LIPSS) can form structural color on the metal surfaces with high production efficiency and thermal stability, and has been used in various industrial applications such as unfaded color marking and anti‐counterfeiting. Herein, a novel fabrication scheme of multilayer LIPSS is proposed by multiple writing in situ with changing the laser polarization direction, to exhibit an effect of color superposition. To verify this approach, the color formation mechanism of LIPSS on the stainless‐steel surface is analyzed by finite‐element numerical calculation and reveals that the high‐angular dispersion of LIPSS is mainly the result of optical diffraction occurring on the surface of periodic structures. The relationship between the angular dispersion of multilayer LIPSS and laser‐processing parameters is established. Through the proposed multilayer LIPSS coloring technology, vivid full‐color patterns on the steel surface are demonstrated, and the in situ superposition of three‐layer graphs is realized, which can greatly enrich the color levels and be competitive in industrial applications.

High quality stealth dicing of sapphire with a picosecond Bessel beam by controlling the polarization direction.

Sapphire is an important substrate material in optoelectronic devices, and it is also widely used as a touch screen panel. In order to achieve high quality cutting of sapphire, the stealth dicing of 500µm thick sapphire by a picosecond Bessel beam is studied in this paper. The influences of laser polarization direction and process parameters on cutting section roughness were studied. By controlling the laser polarization direction, different crack propagation morphologies were obtained. When the polarization direction was vertical to the cutting path, the crack propagation path was straighter, and the sapphire had better cutting quality. The laser processing parameters, including burst mode, hole spacing, and pulse energy, had a significant impact on the cutting section roughness. When the polarization direction was vertical to the cutting path under the optimal process parameters, the cutting section was uniform and flat, with no recondensable particles, no ripples, and no chamfer, and an 89.7nm average roughness of the cutting section could be obtained.

Rotational wave packet of NO+ created upon strong-field ionization of NO

We theoretically investigate the creation of the rotational wave packet in molecular ions upon the strong-field ionization of diatomic molecules by a femtosecond laser pulse. The rotational excitation of molecular cation is ascribed to the dependence of the strong-field ionization probability on the orientation angle of the molecular axis with respect to the laser polarization direction. By extending the molecular strong-field approximation theory, we calculate the rovibrational state distribution of the ground electronic X state of NO+ after the ionization of NO in the electronic ground X state. We also show that the extent of the rotational excitation of NO+ is enhanced by the stimulated impulsive Raman processes both in neutral NO and in NO+ cation. The resultant time evolution of the rotational wave packet of NO+ is in good agreement with the delay dependent NO2+ yield recorded experimentally in our previous pump–probe study.

Minimum structure of high-order harmonic spectrum from molecular multi-orbital effects involving inner-shell orbitals.

The spectral features of high-order harmonic spectra can provide rich information for probing the structure and dynamics of molecules in intense laser fields. We theoretically study the high harmonic spectrum with the laser polarization direction perpendicular to the N2O molecule and find a minimum structure in the plateau region of the harmonic spectrum. Through analyzing the time-dependent survival probability of different electronic orbitals and the time-dependent wave packet evolution, it is found that this minimum position is caused by the harmonic interference of HOMO a, HOMO-1, and HOMO-3 a orbitals. Moreover, this interference minimum is discovered over a wide frequency range of 0.087a.u. to 0.093a.u., as well as a range of driving laser intensities with peak amplitudes between 0.056 a.u. and 0.059 a.u.. This study sheds light on the multi-electron effects and ultrafast dynamics of inner-shell electrons in intense laser pulses, which are crucial for understanding and controlling chemical reactions in molecules.

Control of high-harmonic generation from periodic asymmetric lattices

Periodic asymmetric lattices, viewed from one side to the other, have different spatial potential energies. This difference affects the electronic structure of valence electrons. Our work shows that pronounced even harmonic signals are observed from periodic asymmetric lattices driven by a multi-cycle pulse field. The phases of the odd and even harmonics driven by parallel and anti-parallel laser polarization directions are compared and show different dependences on laser polarization direction. Moreover, it is found that each burst in the synthesized attosecond pulse trains in a periodic asymmetric lattice shows the same carrier-envelope phase. We also show that the even-order harmonic efficiency in periodic asymmetric lattices can be enhanced (reduced) by using a multi-cycle driving laser in the presence of a weak terahertz pulse field.

Bubble structure evolution and electron injection controlled by optical cycles in wakefields

The evolution of bubble structure and electron injection in laser wakefield acceleration with different optical cycles is investigated through three-dimensional particle-in-cell simulations. Under fixed transverse and longitudinal ponderomotive force, the effect of optical cycles on the evolution of bubble structure and electron injection is studied by changing the laser wavelength. For a multi-cycle laser, electron acceleration is dominated by the ponderomotive force that produces symmetrical bubble and continuous injection. As the optical cycles decrease, the dominant effect of the electron acceleration can transition from the ponderomotive force to the carrier wave, and the carrier envelope phase shift can cause transverse oscillation of the bubble and periodic electron injection in the direction of laser polarization. The criterion for the dominant acceleration mechanism and the dependence of transition distance on the optical cycles and pulse width are obtained. The results are beneficial for manipulating electron acceleration and betatron radiation generation.

Role of topological charges in the nonlinear optical response from Weyl semimetals

The successful realization of the topological Weyl semimetals has revolutionized contemporary physics. In recent years, multi-Weyl semimetals, a class of topological Weyl semimetals, has attracted broad interest in condensed-matter physics. Multi-Weyl semimetals are emerging topological semimetals with nonlinear anisotropic energy dispersion, which is characterized by higher topological charges. In this study, we investigate how the topological charge affects the nonlinear optical response from multi-Weyl semimetals. It has been observed that the laser-driven electronic current is characteristic of the topological charge, and the laser polarization's direction influences the current's direction and amplitude. In addition, the anomalous current, perpendicular to the laser's polarization, carries a distinct signature of the topological charges and encodes the information about the parity and amplitude of the nontrivial Berry curvature. We show that the anomalous current associated with the anomalous Hall effect remains no longer proportional to the topological charge at higher laser intensity -- a significant deviation from the linear response theory. High-harmonic spectroscopy is employed to capture the distinct and interesting features of the currents in multi-Weyl semimetals where the topological charge drastically impacts the harmonics' yield and energy cutoff.

Effect of sharp vacuum–plasma boundary on the electron injection and acceleration in a few-cycle laser driven wakefield

The electron injection and acceleration driven by a few-cycle laser with a sharp vacuum–plasma boundary have been investigated through three-dimensional (3D) particle-in-cell simulations. It is found that an isotropic boundary impact injection (BII) first occurs at the vacuum–plasma boundary, and then carrier-envelope-phase (CEP) shift causes the transverse oscillation of the plasma bubble, resulting in a periodic electron self-injection (SI) in the laser polarization direction. It shows that the electron charge of the BII only accounts for a small part of the total charge, and the CEP can effectively tune the quality of the injected electron beam. The dependences of laser intensity and electron density on the total charge and the ratio of BII charge to the total charge are studied. The results are beneficial to electron acceleration and its applications, such as betatron radiation source.

Orientation-angle-resolved photoelectron angular distribution in dissociative ionization of methanol induced by an intense ultraviolet laser pulse

We investigate dissociative ionization of methanol, $\mathrm{C}{\mathrm{H}}_{3}\mathrm{OH}\ensuremath{\rightarrow}{\mathrm{CH}}_{3}{}^{+}+\mathrm{OH}+{e}^{\ensuremath{-}}$, in a linearly polarized intense ultraviolet laser field (400 nm, 67 fs, $3.1\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$) by photoelectron-photoion coincidence three-dimensional momentum imaging. The photoelectron angular distributions (PADs) are recorded as a function of the orientation angle between the molecular C-O bond axis and the laser polarization direction. Photoelectrons are emitted dominantly along the laser polarization direction when ${\mathrm{CH}}_{3}{\mathrm{OH}}^{+}$ is produced in the $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{X}$ state via the four-photon absorption. By contrast, the photoelectron emission becomes prominent along the direction perpendicular to the laser polarization direction when ${\mathrm{CH}}_{3}{\mathrm{OH}}^{+}$ is produced in the $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{B}$ state via the five-photon absorption. The orientation-angle-resolved PADs are expanded in terms of spherical harmonics in the laboratory frame, and then the anisotropy of the PADs varying as a function of the orientation angle of the C-O bond axis with respect to the laser polarization direction is examined quantitatively. We discuss how the shape of the molecular orbital from which an electron is emitted is reflected in the recorded PADs.

Manipulating Parallel and Perpendicular Multiphoton Transitions in H2 Molecules

We demonstrate that dissociative ionization of H_{2} can be fully manipulated in an angle-time-resolved fashion, employing a polarization-skewed (PS) laser pulse in which the polarization vector rotates. The leading and falling edges of the PS laser pulse, characterized by unfolded field polarization, trigger, sequentially, parallel and perpendicular transitions of stretching H_{2} molecules, respectively. These transitions result in counterintuitive proton ejections that deviate significantly from the laser polarization directions. Our findings demonstrate that the reaction pathways can be controlled through fine-tuning the time-dependent polarization of the PS laser pulse. The experimental results are well reproduced using an intuitive wave-packet surface propagation simulation method. This research highlights the potential of PS laser pulses as powerful tweezers to resolve and manipulate complex laser-molecule interactions.

Alignment sensitivity of molecular-frame photoelectron momentum and angular distribution of by XUV attosecond pulses

In this work, we investigate the molecular-frame photoelectron momentum and angular distributions (MF-PMD and MF-PAD) of in the presence of XUV Attosecond Pulses. We present the alignment-sensitive effect of MF-PMDs and MF-PADs when the laser field polarization and the molecular axis are close to parallel geometry. Our results show that the MF-PADs and MF-PMDs of single-photon ionization of ion vary drastically when the alignment slightly shifts from parallel geometry to a small alignment angle (e.g. 4°) between the laser polarization direction and the molecular axis at the equilibrium inter-nuclear distance of R c = 2 atomic units. As R c increases to 4 atomic units, such sensitivity is decreased. The R c -dependent transition cross-sections in different channels are found to be the origin of such sensitivity.

Magic wavelengths for 6s1/2 → 5d3/2,5/2 transitions of Yb+ ions

The wave functions, energy levels and matrix elements of Yb+ ions are calculated using the relativistic configuration interaction plus core polarization (RCICP) method. The static and dynamic electric dipole polarizabilities of the ground state and low-lying excited states are determined. Then, the magic wavelengths of the magnetic sublevel 6s1/2,m = 1/2 → 5d3/2,m = ±3/2,±1/2 and 6s1/2,m = 1/2 → 5d5/2,m = ±5/2,±3/2,±1/2 transitions in the linearly, right-handed, and left-handed polarized light are further determined. The dependence of the magic wavelengths upon the angle between the direction of magnetic field and the direction of laser polarization is analyzed.

Two-dimensional laser-induced periodic surface structures formed on crystalline silicon by GHz burst mode femtosecond laser pulses

Femtosecond laser pulses with GHz burst mode that consist of a series of trains of ultrashort laser pulses with a pulse interval of several hundred picoseconds offer distinct features in material processing that cannot be obtained by the conventional irradiation scheme of femtosecond laser pulses (single-pulse mode). However, most studies using the GHz burst mode femtosecond laser pulses focus on ablation of materials to achieve high-efficiency and high-quality material removal. In this study, we explore the ability of the GHz burst mode femtosecond laser processing to form laser-induced periodic surface structures (LIPSS) on silicon. It is well known that the direction of LIPSS formed by the single-pulse mode with linearly polarized laser pulses is typically perpendicular to the laser polarization direction. In contrast, we find that the GHz burst mode femtosecond laser (wavelength: 1030 nm, intra-pulse duration: 220 fs, intra-pulse interval time (intra-pulse repetition rate): 205 ps (4.88 GHz), burst pulse repetition rate: 200 kHz) creates unique two-dimensional (2D) LIPSS. We regard the formation mechanism of 2D LIPSS as the synergetic contribution of the electromagnetic mechanism and the hydrodynamic mechanism. Specifically, generation of hot spots with highly enhanced electric fields by the localized surface plasmon resonance of subsequent pulses in the bursts within the nanogrooves of one-dimensional LIPSS formed by the preceding pulses creates 2D LIPSS. Additionally, hydrodynamic instability including convection flow determines the final structure of 2D LIPSS.