Laser Polarization Direction Research Articles

Ultrafast molecular orbital imaging based on attosecond photoelectron diffraction

We present ab initio numerical study of ultrafast ionization dynamics of H(2)(+) as well as CO(2) and N(2) exposed to linearly polarized attosecond extreme ultraviolet pulses. When the molecules are aligned perpendicular to laser polarization direction, photonionization of these molecules show clear and distinguishing diffraction patterns in molecular attosecond photoelectron momentum distributions. The internuclear distances of the molecules are related to the position of the associated diffraction patterns, which can be determined with high accuracy. Moreover, the relative heights of the diffraction fringes contain fruitful information of the molecular orbital structures. We show that the diffraction spectra can be well produced using the two-center interference model. By adopting a simple inversion algorithm which takes into account the symmetry of the initial molecular orbital, we can retrieve the molecular orbital from which the electron is ionized. Our results offer possibility for imaging of molecular structure and orbitals by performing molecular attosecond photoelectron diffraction.

Near-infrared femtosecond laser ionization of the acetic acid dimer

Fragment ions produced after the ionization of the acetic acid dimer (CH3COOH)2 in a near-infrared intense femtosecond laser field were measured using time-of-flight mass spectrometry. The two-body Coulomb explosion (CE) process (CH3COOH)22+→2CH3COOH+ with a kinetic energy release of 3.4eV, similar to the case of (HCOOH)22+ (Hoshina et al., 2012), was identified. The two-body CE of (CH3COOH)22+ was induced when the laser polarization direction was perpendicular to the C⋯C intermolecular axis in contrast to the case of (HCOOH)22+. The preferential configurations may be those that deform dimer structures close to dimer dications during the double-ionization process.

Crystal face identification by Raman microscopy for assessment of crystal habit of a drug

Crystal habit is one of the key crystallographic characteristics of active pharmaceutical ingredients (APIs), especially those that are poorly soluble. X-ray powder diffraction has commonly been used to assess crystal habit; however, it can only provide macro-information regarding crystal habit for a whole powder sample, not for individual crystals. We describe an approach that uses Raman microscopy for the identification of crystal faces to assess crystal habit at the individual particle level. An antiepileptic agent, phenytoin, was used as the model substance. Phenytoin crystals form a primitive orthorhombic cell. Raman microscopy was used to identify three different patterns of Raman spectra, corresponding to the crystallographic axis that was parallel to the polarization direction of the excitation laser. Thus, a combination of Raman spectra, in which the polarization direction was horizontal and vertical to the morphologically long axis of the crystal, characterized the crystal face. Phenytoin crystals were prepared under various conditions, and the horizontal/vertical combinations of Raman spectra were recorded for individual crystals. The dominantly exposed crystal faces for each condition were identified. This analytical method enables micro-view assessments of crystal habit, which are helpful for identifying the habits of APIs alone and in formulations.

Molecular dynamics simulations of anisotropic explosions of small hydrogen clusters in intense laser pulses

The explosion dynamics of small hydrogen clusters irradiated by intense femtosecond infrared laser pulses is investigated by classical molecular dynamics simulations. We find a spatial anisotropy in these explosions with proton energies enhanced along the laser polarization direction. Our simulations can identify the origin of this anisotropy: the interplay between the space charge separation in the early stage of cluster ionization and the Coulomb attraction between the rescattered electrons and protons during cluster explosion.

Deep-subwavelength nanohole arrays embedded in nanoripples fabricated by femtosecond laser irradiation.

We report on the formation of deep-subwavelength nanohole arrays embedded in silicon carbide nanoripples fabricated by an 800nm femtosecond laser in an underwater environment. The period of the nanoripples is about 500nm. The ripples are perpendicular to the polarization direction of the incident laser. The diameter of the holes is about 30nm, and the period of the hole array is about 60nm. Nanoripple formation is attributed to interference of the incident laser and a laser-induced plasma wave. Nanohole array formation is attributed to the formation of channel plasmon polaritons in the laser-induced nanogrooves associated with the nanoripples.

The impact of laser polarization direction on local dose rate effects simulation for modern integrated circuits

The effect of the direction of laser polarization on the transient radiation response of an IC memory device implemented using CMOS technology 0.18 μm is estimated. The effect of laser polarization is investigated both for local exposure and exposure on the entire crystal. It is shown that the direction of laser polarization should be taken into account when simulating local dose rate effects.

Dynamic behavior of hydrogen clusters under intense femtosecond laser

The molecular dynamics model is adopted to investigate the dynamical behavior of hydrogen cluster irradiated by an intense femtosecond laser. Being contrary to the predictions from the Coulomb explosion model, this paper points out that the explosion of hydrogen cluster is anisotropic. The component of proton energy along the laser polarization direction is much larger than the component perpendicular to the polarization direction. This paper discusses the mechanism responsible for the anisotropy explosion. In the process of the interaction of femtosecond laser with cluster, the electrons undergo the inner ionization and then oscillate along the direction of laser polarization. During the oscillation of electrons, part of them will escape from cluster. The escaping of the electrons would lead to two correlation effects. First, the anisotropic distribution of the electric field caused by the oscillation of electrons would not be neutralized. For one thing, during the oscillating of electrons, they will be pulled to one pole of cluster so the electric field of the opposite pole would be larger, the electrons in this region will experience larger Coulomb repulsive force and gain lager acceleration. For another thing, the electron number contained in the cluster will decline during each laser cycle. So the proton in this region will gain a pure acceleration. Second, during the oscillation of electrons, part of electrons will escape from cluster. During their escaping they will pull the protons at the pole and the protons move toward the direction of electron escaping direction. These two correlation effects cause the anisotropic explosion of hydrogen cluster. This paper also discusses the influences of cluster and laser parameters on the degree of anisotropy.

Visualization of Primary Particles in a Tablet Based on Raman Crystal Orientation Mapping

Objective: The morphology of the primary particles in the active pharmaceutical ingredient (API) is one of the most important determinants for formulation function. However, it has not been possible to visualize primary particles in a tablet using various imaging methods, including spectroscopic mappings, because these particles usually exist as aggregated clusters in tablets. We revealed that the Raman spectrum of crystalline particles is determined depending upon the angle between the crystallographic axis and the polarization direction of the excitation laser. In this paper, we report a method to visualize primary particles within the aggregated cluster based on the Raman spectral change on the boundary of primary particles. Method: Metformin hydrochloride was chosen as a model API for this study. The crystal structure of metformin hydrochloride was solved using X-ray crystal structure analysis. The Raman spectra of metformin hydrochloride crystal along the xyz axes were recorded and resolved into components along the abc axes. Using the abc components, Raman mapping of metformin hydrochloride in tablets was performed to visualize the crystal orientation at each data point. Results: Metformin hydrochloride crystals recrystallized from water/ethanol formed a primitive monoclinic cell. Datasets of five distinct peak areas from the metformin hydrochloride Raman spectrum were used for analyses. Raman crystal orientation mapping (RCOM) from the tablet cross-section provided an image of primary particles within the aggregation cluster of metformin hydrochloride in the tablet. Conclusion: Based on the RCOM, we developed a visualization method for primary API particles in tablets. Because the morphology of primary particles is the key factor of formulation function, this method would contribute to better formulation development and quality control.

Hanle detection for optical clocks.

Considering the strong inhomogeneous spatial polarization and intensity distribution of spontaneous decay fluorescence due to the Hanle effect, we propose and demonstrate a universe Hanle detection configuration of electron-shelving method for optical clocks. Experimental results from Ca atomic beam optical frequency standard with electron-shelving method show that a designed Hanle detection geometry with optimized magnetic field direction, detection laser beam propagation and polarization direction, and detector position can improve the fluorescence collection rate by more than one order of magnitude comparing with that of inefficient geometry. With the fixed 423 nm fluorescence, the improved 657 nm optical frequency standard signal intensity is presented. The potential application of the Hanle detection geometry designed for facilitating the fluorescence collection for optical lattice clock with a limited solid angle of the fluorescence collection has been discussed. The Hanle detection geometry is also effective for ion detection in ion optical clock and quantum information experiments. Besides, a cylinder fluorescence collection structure is designed to increase the solid angle of the fluorescence collection in Ca atomic beam optical frequency standard.

Generation and elimination of polarization-dependent ablation of cubic crystals by femtosecond laser radiation.

We experimentally showed that the π/2-period oscillation of an ablation area with laser polarization direction can be observed in GaAs, ZnSe, MgO and LiF with cubic crystal by a femtosecond laser (800 nm, 100 fs) and that the modulation in the ablation area can be controlled by the laser fluence. While the polarization dependence is sustained in a wide range of laser fluences for a narrow band-gap crystal, it is strongly suppressed with a slight augmentation of laser fluence in a wide band-gap material. The polarization-dependent ablation is explained by the crystal's orientation-dependent reduced-electron mass and the resultant contrasting nonlinear absorptions with slightly different reduced electron mass. The interplay between photoionization and avalanche ionization is discussed to interpret the influence of laser fluence on polarization-dependent ablation. Based on Keldysh's theory, polarization-dependent ablation occurs in a mixed regime between tunneling and multiphoton ionization.

Study on morphology of high-aspect-ratio grooves fabricated by using femtosecond laser irradiation and wet etching

Morphologies of high-aspect-ratio silicon grooves fabricated by using femtosecond laser irradiation and selective chemical etching of hydrofluoric acid (HF) were studied. Oxygen was deeply doped into silicon under femtosecond laser irradiation in air, and then the oxygen-doped regions were removed by HF etching to form high-aspect-ratio grooves. After HF etching, periodic nano-ripples which were induced in silicon by femtosecond laser were observed on the groove sidewalls. The ripple orientation was perpendicular or parallel to the laser propagation direction (z direction), which depended on the relative direction between the laser polarization direction and the scanning direction. The formation of nano-ripples with orientations perpendicular to z direction could be attributed to the standing wave generated by the interference of the incident light and the reflected light in z direction. The formation of nano-ripples with orientations parallel to z direction could be attributed to the formation of self-organized periodic nanoplanes (bulk nanogratings) induced by femtosecond laser inside silicon. Materials in the tail portion of laser-induced oxygen doping (LIOD) regions were difficult to be etched by HF solution due to low oxygen concentration. The specimen was etched further in KOH solution to remove remaining materials in LIOD regions and all-silicon grooves were fabricated.

Alignment and pulse-duration effects in two-photon double ionization ofH2by femtosecond XUV laser pulses

We present calculations for the dependence of the two-photon double ionization (DI) of ${\mathrm{H}}_{2}$ on the relative orientation of the linear laser polarization to the internuclear axis and the length of the pulse. We use the fixed-nuclei approximation at the equilibrium distance of 1.4 ${a}_{0}$, where ${a}_{0}=0.529\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{m}$ is the Bohr radius. Central photon energies cover the entire direct DI domain from 26.5 to 34.0 eV. In contrast to the parallel geometry studied earlier [X. Guan, K. Bartschat, B. I. Schneider, and L. Koesterke, Phys. Rev. A 83, 043403 (2011)], the effect of the pulse duration is almost negligible for the case when the two axes are perpendicular to each other. This is a consequence of the symmetry rules for dipole excitation in the two cases. In the parallel geometry, doubly excited states of ${}^{1}{\ensuremath{\Sigma}}_{u}^{+}$ symmetry affect the cross section, while in the perpendicular geometry only much longer-lived ${}^{1}{\ensuremath{\Pi}}_{u}$ states are present. This accounts for the different convergence patterns observed in the calculated cross sections as a function of the pulse length. When the photon energy approaches the threshold of sequential DI, a sharp increase of the generalized total cross section (GTCS) with increasing pulse duration is also observed in the perpendicular geometry, very similar to the case of the molecular axis being oriented along the laser polarization direction. Our results differ from those of Colgan et al. [J. Colgan, M. S. Pindzola, and F. Robicheaux, J. Phys. B 41, 121002 (2008)] and Morales et al. [F. Morales, F. Mart\'{\i}n, D. A. Horner, T. N. Rescigno, and C. W. McCurdy, J. Phys. B 42, 134013 (2009)], but are in excellent agreement with the GTCSs of Simonsen et al. [A. S. Simonsen, S. A. S\o{}rng\aa{}rd, R. Nepstad, and M. F\o{}rre, Phys. Rev. A 85, 063404 (2012)] over the entire domain of direct DI.

Polarization-dependent elliptical crater morphologies formed on a silicon surface by single-shot femtosecond laser ablation.

Formation of the elliptical-shaped craters on a silicon surface is investigated comprehensively using a single shot of a femtosecond laser. It is observed that the ablation craters are elongated along the major axis of the polarization direction, while their orientation is parallel to the polarization direction. The ablation area grows and the morphology of the craters evolves from an ellipse to nearly a circle with increasing fluence. The underlying physical mechanism is revealed through numerical simulations that are based on the finite-difference time-domain technique. It is suggested that the initially formed craters or surface defects lead to the redistribution of the electric field on the silicon surface, which plays a crucial role in the creation of the elliptical-shaped craters. In addition, the field intensity becomes enhanced along the incident laser polarization direction, which determines the elliptical crater orientations.

Manipulation of the recombination channels and isolated attosecond pulse generation from HeH2+ with multicycle combined field

We present a method to manipulate the recombination channels in harmonic emission from HeH2+ via combing a multicycle trapezoidal laser field and a static electric field. By solving the time-dependent Schrödinger equation, we find that the positive peak intensity can steer the electron localization by controlling the electron transfer at the time of laser polarization direction antiparallel to the permanent dipole of HeH2+. With appropriate laser parameters, the recombination channels originating from the nucleus He2+ are effectively controlled by enhancing electron localization around the nucleus H+. Additionally, an isolated 48-as pulse can be obtained by superposing proper harmonics. Time-frequency maps, electronic wave packet probability density and a laser dressed picture of the Coulomb potential are adopted to provide the underlying physical mechanism.

Mechanisms of strong-field double ionization of Xe.

We perform a fully differential measurement on strong-field double ionization of Xe by 25 fs, 790 nm laser pulses in intensity region (0.4-3)×10(14) W/cm2. We observe that the two-dimensional correlation momentum spectra along the laser polarization direction show a nonstructured distribution for double ionization of Xe when decreasing the laser intensity from 3×10(14) to 4×10(13) W/cm2. The electron correlation behavior is remarkably different with the low-Z rare gases, i.e., He, Ne, and Ar. We find that the electron energy cutoffs increase from 2.9Up to 7.8Up when decreasing the laser intensities from the sequential double ionization to the nonsequential double ionization regime. The experimental observation indicates that multiple rescatterings play an important role for the generation of high energy photoelectrons. We have further studied the shielding effect on the strong-field double ionization of high-Z atoms.

Azimuthal asymmetry in collective electron dynamics in relativistically transparent laser–foil interactions

Asymmetry in the collective dynamics of ponderomotively-driven electrons in the interaction of an ultraintense laser pulse with a relativistically transparent target is demonstrated experimentally. The 2D profile of the beam of accelerated electrons is shown to change from an ellipse aligned along the laser polarization direction in the case of limited transparency, to a double-lobe structure aligned perpendicular to it when a significant fraction of the laser pulse co-propagates with the electrons. The temporally-resolved dynamics of the interaction are investigated via particle-in-cell simulations. The results provide new insight into the collective response of charged particles to intense laser fields over an extended interaction volume, which is important for a wide range of applications, and in particular for the development of promising new ultraintense laser-driven ion acceleration mechanisms involving ultrathin target foils.

Femtosecond laser-induced nanoperiodic structures and simultaneous crystallization in amorphous indium-tin-oxide thin films

Abstract Fabrication of crystalline indium-tin-oxide (c-ITO) nanogratings and smooth line patterns on amorphous ITO (a-ITO) thin films by femtosecond laser-induced structuring and simultaneous crystallization followed by chemical etching is demonstrated. Three types of c-ITO structures are obtained merely by controlling the laser pulse energy of a high-repetition rate femtosecond laser at wavelength 532 nm: nanogratings perpendicular to laser polarization direction with period of 130 ± 11 nm, smooth c-ITO line patterns as well as nanogratings parallel to laser polarization direction with period of 390 ± 10 nm. Large area c-ITO patterns and nanograting structures are fabricated, which are expected to be used in optoelectronic and micro-electronic devices.

Asymmetric orientational writing dependence on polarization and direction in Li2O–Nb2O5–SiO2 glass with femtosecond laser irradiation

Strong birefringence photo-induced by the femtosecond laser irradiation and a sensitivity to the orientation of the laser movement versus the laboratory reference (i.e., difference between rightward and leftward or between upward and downward) have been demonstrated in 32.5Li2O–27.5Nb2O5–40SiO2 glass. An asymmetry with the orientation of the laser scanning was firstly observed by one of the authors in 2003 in pure silica (Poumellec et al. in Opt Express 11:1070, 2003). Here, we show in addition that this asymmetric orientational sensitivity is dependent on the direction of laser polarization and the laser scanning (i.e., when the direction is horizontal or vertical). In particular, no asymmetric writing is detected when both laser scanning and polarization directions are vertical. These observations are not explained by the available model.

Self-organizing microstructures orientation control in femtosecond laser patterning on silicon surface.

Self-organizing rippled microstructures are induced on silicon surface by linearly polarized femtosecond laser pulses. At a near threshold fluence, it is observed that ripple orientation is co-determined by the laser polarization direction and laser scanning parameters (scanning direction and scanning speed) in surface patterning process. Under fixed laser polarization, the ripple orientation can be controlled to rotate by about 40° through changing laser scanning parameters. In addition, it is also observed that the ripple morphology is sensitive to the laser scanning direction, and it is an optimal choice to obtain ordered ripple structures when the angle between laser scanning and laser polarization is less than 45°.

Polarization Dependent Nonlinear Absorption in Au Nanowire Array

The polarization dependent nonlinear absorption was observed in ordered Au nanowire array. The linear and nonlinear polarized absorption at 780 nm is investigated by Z-scan. When the angle between the polarization direction of input laser and the long axis of Au nanowires is 90°(perpendicular), the linear absorption reaches the maximum and the nonlinear absorption reaches the minimum. The nonlinear absorption coefficient β decreases from positive to negative as the angle increase from 0° to 90°. There may be two nonlinear absorption mechanisms resulting in this phenomenon: two-photon absorption and saturable absorption in nanoscale.