Orientation of linear molecules in two-color laser fields with perpendicularly crossed polarizations

© 2019 American Physical Society.Molecular orientation methods based on nonresonant two-color laser pulses having parallel polarizations have been reported theoretically and experimentally. In this work, we demonstrate that perpendicularly polarized two-color laser fields can be used to achieve stronger molecular orientation when nanosecond laser pulses are used. The two-color fields align the molecules to the two-dimensional plane parallel to the field polarization; at the same time, they orient the molecules in the direction of the 2ω polarization. We show that the interplay between the interactions due to the ω- and 2ω-laser fields provides stronger molecular orientation than the parallel field configuration. This is due to temporally synchronized generations of alignment and orientation, which reduce the nonadiabatic effect

Improving molecular orientation by optimizing relative delay and intensities of two-color laser pulses

We numerically explore molecular orientation dynamics with moderately intense nanosecond two-color laser pulses. It is believed that the nanosecond two-color pulse can adiabatically control the molecular orientation. However, in our simulation based on the time-dependent Schrodinger equation, which naturally includes nonadiabatic effects, the orientation dynamics shows clear deviations from the adiabatic approximation (AA) results, while the molecular alignment dynamics is in good agreement with the AA results. The nonadiabaticity is significantly influenced by three parameters, the intensities, and the relative delay of the two wavelengths. In this work, we clarify the reason behind the nonadiabaticity and provide the solutions for achieving higher degrees of orientatio

Field-free molecular orientation by delay- and polarization-optimized two fs pulses

Unless the molecular axis is fixed in the laboratory frame, intrinsic structural information of molecules can be averaged out over the various rotational states. The macroscopic directional properties of polar molecules have been controlled by two fs pulses with an optimized delay. In the method, the first one-color laser pulse provokes molecular alignment. Subsequently, the molecular sample is irradiated with the second two-color laser pulse, when the initial even-J states are aligned, and the odd-J states are anti-aligned in the thermal ensemble. The second pulse selectively orients only the aligned even-J states in the same direction, which results in significant enhancement of the net degree of orientation. This paper reports the results of simulations showing that the two-pulse technique can be even more powerful when the second pulse is cross-polarized. This study shows that the alignment and orientation can be very well synchronized temporally because the crossed field does not disturb the preformed alignment modulation significantly, suggesting that the molecules are very well confined in the laboratory frame. This cross-polarization method will serve as a promising technique for studying ultrafast molecular spectroscopy in a molecule-fixed frame.11Ysciescopu

Development of a plasma shutter applicable to 100-mJ-class, 10-ns laser pulses and the characterization of its performance

© 2019 Optical Society of America.For the purpose of preparing a sample of aligned and oriented molecules in the laser-field-free condition, we developed a plasma shutter, which enables laser pulses with 100-mJ-class, 10-ns pulse durations to be rapidly turned off within ∼150 fs. In this work, the residual field intensity after the rapid turn off is carefully examined by applying the shaped laser pulse to OCS molecules in the rotational ground state. Based on the comparison between the observation of alignment revivals of the OCS molecules and the results of numerical simulations, we demonstrate that the residual field intensity is actually negligible (below 0.4% of the peak intensity) and, if any, does not influence the alignment and orientation dynamics at al

Time-Dependent Unitary Transformation Method in the Strong-Field-Ionization Regime with the Kramers-Henneberger Picture

Time evolution operators of a strongly ionizing medium are calculated by a time-dependent unitary transformation (TDUT) method. The TDUT method has been employed in a quantum mechanical system composed of discrete states. This method is especially helpful for solving molecular rotational dynamics in quasi-adiabatic regimes because the strict unitary nature of the propagation operator allows us to set the temporal step size to large; a tight limitation on the temporal step size (delta t<<1) can be circumvented by the strict unitary nature. On the other hand, in a strongly ionizing system where the Hamiltonian is not Hermitian, the same approach cannot be directly applied because it is demanding to define a set of field-dressed eigenstates. In this study, the TDUT method was applied to the ionizing regime using the Kramers-Henneberger frame, in which the strong-field-dressed discrete eigenstates are given by the field-free discrete eigenstates in a moving frame. Although the present work verifies the method for a one-dimensional atom as a prototype, the method can be applied to three-dimensional atoms, and molecules exposed to strong laser fields.11Nsciescopu

Path integral formulation of light propagation in a static collisionless plasma, and its application to dynamic plasma

© 2020 OSA - The Optical Society. All rights reserved.In many studies on the laser impinging on a plasma surface, an assumption is made that the reflection of a laser pulse propagating to a plasma surface takes place only at the turning point, at which the plasma density exceeds the critical one. A general reflection amplitude of light R from an arbitrary inhomogeneous medium can be obtained by solving a Riccati-type integral equation, which can be solved analytically in low-reflection conditions, i.e., jRj2_1. In this work, we derive an intuitive analytic solution for the reflection amplitude of light R from a plasma surface by integrating all possible reflection paths given by the Fresnel equation. In the low-reflection condition, reflection paths having only one reflection event can be used. By considering the higher-order reflection paths, our analytic expression can describe reflection in the high-reflection condition. We show the results of a one-dimensional particle-in-cell simulation to support our discussions. Since our model derived for static plasmas is well corroborated by the simulation results, it can be a useful tool for analyzing light reflection from dynamically varying plasma

Few-Cycle, μJ-Class, Deep-UV Source from Gas Media

Energetic, few-fs pulses in the deep-UV region are highly desirable for exploring ultrafast processes on their natural time scales, especially in molecules. The deep-UV source can be generated from gas media irradiated with few-cycle near-infrared laser pulses via a third-order frequency conversion process, which is a perturbative mechanism in a relatively weak field regime. In this work, we demonstrate that the deep-UV generation process is significantly affected by also even higher nonlinear processes, such as the ionization depletion of gas and plasma-induced spatiotemporal distortion of propagating light. In the experiment, by optimizing the deep-UV (3.6–5.7 eV) generation efficiency, the highest deep-UV energy of 1 μJ was observed from a moderately ionized 0.8-bar Ar gas target. The observed UV spectra exhibited frequency shifts depending on the experimental conditions—gas type, gas pressure, and the gas cell location—supporting the importance of the highly nonlinear mechanisms. The experimental observations were well corroborated by numerical simulations

Coherent extreme-ultraviolet emission generated through frustrated tunnelling ionization

Coherent extreme-ultraviolet emission can be obtained through high-harmonic generation and multiphoton excitation from atoms exposed to a strong laser field. We report the generation of a new kind of coherent extreme-ultraviolet emission from He atoms excited by intense few-cycle laser pulses. An atom can be excited after tunnelling in a strong laser field, in the process known as frustrated tunnelling ionization (FTI). We find that excitation through FTI leads to coherent extreme-ultraviolet emission, and its intensity strongly depends on the ellipticity and carrier-envelope phase of the laser pulses. Additionally, the propagation direction of the emission can be coherently controlled by employing the attosecond lighthouse technique. This coherent control of tunnelling and recombination dynamics, which has provided the fundamental basis of attosecond physics, promises the utilization of FTI emission as a coherent light source and offers new opportunities in ultrafast spectroscopy. © 2018, The Author(s), under exclusive licence to Springer Nature Limite