Inhomogeneous Laser Field Research Articles

Nonsequential double ionization of Ar in two-color inhomogeneous laser fields

With the classical ensemble method, we investigate the correlated electron dynamics in nonsequential double ionization (NSDI) of Ar in parallel two-color inhomogeneous laser fields with different inhomogeneity parameter ϵ. Numerical results show that at low laser intensities, the probability of NSDI is higher for two-color laser fields with a higher inhomogeneity parameter ϵ. At high laser intensities, however, the probability of NSDI decreases as ϵ increases. It is also shown that under low laser intensities, the two-color inhomogeneous laser fields drives the NSDI predominantly dominated by the RII channel, and the ratio of the RII channel increases as ϵ increases. More interestingly, an arc-like structure is found in the correlated electron momentum distributions for the two-color inhomogeneous laser fields. The underlying dynamics are revealed by tracing back to classical trajectories.

Efficient spatial separation for chiral molecules via optically induced forces.

We investigate an efficient spatial enantioseparation method of chiral molecules in cyclic three-level systems coupled with three optical fields using optically induced forces. When the overall phase differs by π between two enantiomers, significant variations in the magnitude and direction of the optically induced forces are observed. The manipulation of the center of mass of chiral molecules in optical fields can be achieved through the induced gauge force, primarily generated from the variations in the chirality-dependent scalar potentials created by the three inhomogeneous laser fields. By appropriately configuring the system, we can completely separate the slow spatial and fast inner dynamics, making instantaneous eigenstates of the inner Hamiltonian independent of the transverse profiles of the laser beams. Compared to previous methods, which required adiabatic conditions to be satisfied, the proposed method overcomes the limitations of the adiabatic approximation by utilizing a specific system configuration. This allows for increased flexibility in the transverse profiles of the laser beams and relaxes the constraints on the velocity of chiral molecules, leading to significantly greater spatial separations achievable across a broader range of parameters.

High-order harmonic generation in a doped semiconductor by inhomogeneous laser field

High-order harmonic generation in a doped semiconductor by inhomogeneous laser field

Atomic Rydberg-state excitation in strong spatially inhomogeneous fields

We investigated the Rydberg-state excitation process of a hydrogen atom subjected to spatially homogeneous and inhomogeneous laser fields by means of ab initio calculations. It is found that, comparing with atoms exposed to spatially homogeneous laser fields, the excitation probability decreases and the electron tends to occupy the states with lower principal quantum numbers and angular quantum numbers for atoms in spatially inhomogeneous laser pulses. Furthermore, calculations of a quantum model without taking into account ionization of the electron after it is coherently captured by the Rydberg state are inconsistent with the above-stated findings by ab initio calculations. Analysis indicates that the aforementioned intriguing features can be attributed to the enhanced ionization of the Rydberg states by inhomogeneous laser fields since the distributions of Rydberg states of high principal quantum numbers and angular quantum numbers locate far away from the core where the inhomogeneous electric fields become significant.

Nonsequential double ionization of atoms driven by spatially inhomogeneous laser fields

<sec>Using a three-dimensional classical ensemble method, we investigate the nonsequential double ionization (NSDI) of xenon atoms from the near infrared wavelength to the mid-infrared wavelength in spatially inhomogeneous laser fields, and compare the results with those from spatially homogeneous laser fields. The results show that the NSDI probability curves from spatially inhomogeneous laser field and spatially homogeneous laser field at short wavelength are similar to each other. With the laser wavelength increasing, NSDI at the high intensities is more and more suppressed for spatially inhomogeneous laser field.</sec><sec>Compared with the result from the spatially homogeneous laser field, the final emission angle of two electrons from the NSDI exhibits a very strongly correlated characteristic in the spatially inhomogeneous field, especially at a longer laser wavelength, the final emission angles of two electrons are almost both concentrated around <inline-formula><tex-math id="M2">\begin{document}${0^\circ }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230548_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20230548_M2.png"/></alternatives></inline-formula>, meaning that the two electrons are always emitted into the same direction parallelly. Moreover, effective recollision of the NSDI is always dominated by first return of the first electron from the near infrared to the mid-infrared inhomogeneous laser fields, however, the transition from the first return dominance to the second return dominance occurs in the spatially homogeneous laser fields. Further, we reveal the more details of the ultrafast dynamics of the correlated electrons in the spatially inhomogeneous laser field by back-tracing the classical trajectories of NSDI.</sec>

Dynamics of frustrated tunneling ionization driven by inhomogeneous laser fields

We theoretically investigated frustrated tunneling ionization (FTI) driven by spatially inhomogeneous strong laser fields induced by surface plasmon resonance within a bow-tie metal nanostructure. The results show that the FTI probability and the principal quantum number distribution exhibit similar oscillatory behavior as a function of the pulse duration. Our analysis reveals that the periodic defocusing and refocusing of the electron spatial distribution due to the inhomogeneous laser field is responsible for the oscillatory structures. In addition, the initial tunneling coordinates and the angular momentum distributions of the FTI events and theirs pulse duration dependence are also explored. Moreover, our results show that the frequency of the oscillatory structures depends sensitively on the electron quiver amplitude and the inhomogeneity strength. Thus, the electron quiver amplitude and the size of the gap between bow-tie nanostructure are useful and efficient knobs for controlling the yield and properties of exited Rydberg states.

Nonsequential double ionization driven by inhomogeneous laser fields.

With a three-dimensional classical ensemble method, we theoretically investigated the correlated electron dynamics in nonsequential double ionization (NSDI) driven by the spatially inhomogeneous fields. Our results show that NSDI in the spatially inhomogeneous fields is more efficient than that in the spatially homogeneous fields at the low laser intensities, while at the high intensities NSDI is suppressed as compared to the homogeneous fields. More interestingly, our results show that the electron pairs from NSDI exhibit a much stronger angular correlation in the spatially inhomogeneous fields, especially at the higher laser intensities. The correlated electron momentum distribution shows that in the inhomogeneous fields the electron pairs favor to achieve the same final momentum, and the distributions dominantly are clustered in the more compact regions. It is shown that the electron's momentum is focused by the inhomogeneous fields. The underlying dynamics is revealed by back-tracing the classical trajectories.

Effects of inhomogeneous laser field in time and space on the generation of broad spectral continuum and ultrashort attosecond pulse

The inhomogeneous effects of laser field in time and space on the generations of high-order harmonic spectra and attosecond pulses from He atom have been investigated with the assistance of the frequency-chirping technique and the metallic nanostructure. We find that in different combinations of chirps and spatial inhomogeneous effects, not only can the harmonic cutoff be extended, but the single harmonic emission peak (HEP) can also be selected to contribute to the spectral continuum. In detail, with the combinations of (i) symmetric chirp in frequency and positive inhomogeneous effect in space or (ii) asymmetric down chirp in frequency and negative inhomogeneous effect in space, two broad spectral continua with bandwidths of 496 eV and 480 eV can be obtained. Further, by properly adding IR or UV controlling pulse, the harmonic emission efficiency can be enhanced by several orders of magnitude. The enhancement of harmonic yield from adding UV pulse is higher than that from adding IR pulse. Moreover, the difference of harmonic enhancement between UV combined field and IR combined field is much more obvious at lower controlling laser intensity. Through analyzing the ionization probability, we find that the higher enhancement of harmonic yield from UV combined field is because of UV-resonance-enhancement-ionization between the ground state and the excited state of He atom. Finally, by properly superposing some harmonics, a number of attosecond pulses with pulse durations of 45 as can be obtained.

Above-threshold ionization driven by few-cycle spatially bounded inhomogeneous laser fields

In this work, we study the main features of the photoelectrons generated when noble gas atoms are driven by spatially bounded inhomogeneous strong laser fields. These spatial inhomogeneous oscillating fields, employed to ionize and accelerate the electrons, result from the interaction between a pulsed low intensity laser and bow-tie shaped gold nanostructures. Under this excitation scheme, energy-resolved above-threshold ionization (ATI) photoelectron spectra have been simulated by solving the one-dimensional (1D) time-dependent Schrödinger equation (TDSE) within the single active electron (SAE) approximation. These quantum mechanical results are supported by their classical counterparts, obtained by the numerical integration of the Newton–Lorentz equation. By using near-infrared wavelengths (0.8–3 μm) sources, our results show that very high energetic electrons (with kinetic energies in the keV domain) can be generated, far exceeding the limits obtained by using conventional, spatially homogeneous fields. This new characteristic can be supported considering the non-recombining electrons trajectories, already reported by Neyra and coworkers (Neyra E, et al 2018 J. Opt. 20, 034002). In order to build a real representation of the spatial dependence of the plasmonic-enhanced field in an analytic function, we fit the generated ’actual’ field using two Gaussian functions. We have further analyzed and explored this plasmonic-modified ATI phenomenon in a model argon atom by using several driven wavelengths at intensities in the order of 1014 W cm−2. Throughout our contribution we carefully scrutinize the differences between the ATI obtained using spatially homogeneous and inhomogeneous laser fields. We present the various physical origins, or correspondingly distinct physical mechanisms, for the ATI generation driven by spatially bounded inhomogeneous fields.

High-Order Harmonic Enhancement in Nanowire by Inhomogeneous Field

We theoretically investigate high-order harmonic generation from nanowire in inhomogeneous laser field. By solving the time-dependent Schrodinger equation, the simulated harmonic spectrum exhibits a double-plateau structure. The cutoff of the second plateau is extended and the intensity of the high-order harmonics in the second plateau is increased by about two to three orders of magnitude compared with that from the homogeneous laser field. Moreover, we analyze the influence of the inhomogeneous laser field polarization orientation to the nanowire. It is shown that the intensity of the high-order harmonics in the second plateau can be effectively controlled by varying the inhomogeneity parameter and the laser field polarization orientation to the nanowire. These phenomena are attributed to the time-dependent population of the electron, which has a strong relevance with the intensity of the harmonic.

Generation of High Order Harmonics from H2+ Molecule Ion by Using Homogenous and Inhomogeneous Laser Fields

Generation of High Order Harmonics from H2+ Molecule Ion by Using Homogenous and Inhomogeneous Laser Fields

Nanoscale Imaging of Ultrafast Light Coupling to Self-Organized Nanostructures

In the course of laser-induced surface self-organization, an optical feedback mechanism is generally evoked as the main process driving the final rippled topography. To unravel the role of light and transient nanostructures in a multipulse irradiation regime, we use a high-resolved imaging technique, the photoemission electron microscopy. The pulse-to-pulse evolution of the inhomogeneous laser field distribution on a titanium surface nanostructured by a femtosecond laser is investigated at the nanoscale. Photoelectron imaging allows to separate the contributions of radiative and nonradiative scattered fields and to unveil the correlation between the nanocavity density and the quasi-periodic light absorption. The multipulse experimental observations are supported by electromagnetic calculations showing that the absorption of light on an evolving surface relief drives the selection of the final periodicity. The surface distribution of light absorption is influenced by surface topography that transiently ada...

Theoretical study of high-order harmonic generation from the hydrogen molecular ion with a dichromatic spatially inhomogeneous field

We theoretically investigate the enhancement of high-order harmonic generation by numerically solving the non-Born–Oppenheimer time-dependent Schrödinger equation from the hydrogen molecular ion in a dichromatic inhomogeneous laser field. An ultrabroad supercontinuum up to 300 orders spectral width is generated. It is found that not only the inhomogeneity, but also the dichromatic field contributes to the significant extension of the harmonic cutoff compared with a monochromatic inhomogeneous laser field. Meanwhile, the long quantum paths can be suppressed and short ones can be enhanced by selecting optimized inhomogeneous parameter [Formula: see text], intensity and carrier envelope phase of the dichromatic inhomogeneous laser field. Furthermore, by superposing a properly selected range of the harmonic spectrum in the continuum region, an isolated 29-as pulse is generated. Both the classical theory and quantum time–frequency analysis are adopted to explain the physical mechanism.

High-intensity attosecond pulse generation by using inhomogeneous laser field in frequency and space

High-order harmonic spectra and attosecond source generation from Ar atom driven by the inhomogeneous laser field in frequency and space have been theoretically investigated. The results showed that (i) for the case of the frequency modulation of the laser, the harmonic cutoff can be remarkably extended with the introduction of the chirp of the laser field, (ii) for the case of the space modulation of the laser, the harmonic cutoff can be further extended by using the positive spatial inhomogeneous field, (iii) by properly adding a terahertz controlling field, the intensity of the harmonic yield can be enhanced by 2 orders of magnitude, showing a 775[Formula: see text]eV supercontinuum. As a result, five isolated attosecond pulses (IAPs) from 60 as to 22 as can be obtained.

Controlling harmonic emission and attosecond pulse generation from H2+ by using an asymmetric few-cycle inhomogeneous laser field

The generation of the higher-order harmonics and attosecond pulses from H2+ driven by an asymmetric few-cycle inhomogeneous laser field has been theoretically investigated. It is found that (i) due to the asymmetric enhancement of the laser intensity in space, larger extensions of the harmonics with only one harmonic emission event from E(t) > 0 and E(t) < 0 can be found for the cases of positive and the negative asymmetric inhomogeneous fields, respectively. Moreover, due to the asymmetric inhomogeneous effect, the modulation of the harmonics, caused by the coupling of the two quantum paths can be decreased. (ii) As the polarization angle of the laser field increases, not only the intensities and the cutoffs of the harmonics are decreased, but also the modulation of the harmonics are enhanced. (iii) The contribution of the harmonic plateau can be controlled by changing the carrier-envelope phase of the laser field or by properly adding controlling pulses (e.g. adding a second controlling pulse with a longer wavelength or adding a half-cycle pulse). As a result, a supercontinuum with the bandwidth of 434 eV and a near-isolated 37 as pulse can be obtained.

High-order harmonic emission in bowtie-shaped nanostructure with few-cycle spatially inhomogeneous laser fields when the atom is placed at different spatial positions

In this study, we investigate the harmonic spectra in the vicinity of the center of the nanostructure gap when helium atom is placed at different spatial positions. We find that the high-order harmonic spectra is very sensitive to the atomic spatial positions in a inhomogeneous laser field. When the initial atomic spatial position is changed from −9.0 a.u. to 9.0 a.u., the cutoff of the high-order harmonic generation (HHG) can be extended, and the supercontinuum harmonic spectrum of the second plateau is extended to the high-order and the low-order simultaneously. The HHG process is demonstrated by using the time-frequency analysis, the semi-classical three-step model and the ionization probability.

Phototaxis of synthetic microswimmers in optical landscapes.

Many microorganisms, with phytoplankton and zooplankton as prominent examples, display phototactic behaviour, that is, the ability to perform directed motion within a light gradient. Here we experimentally demonstrate that sensing of light gradients can also be achieved in a system of synthetic photo-activated microparticles being exposed to an inhomogeneous laser field. We observe a strong orientational response of the particles because of diffusiophoretic torques, which in combination with an intensity-dependent particle motility eventually leads to phototaxis. Since the aligning torques saturate at high gradients, a strongly rectified particle motion is found even in periodic asymmetric intensity landscapes. Our results are in excellent agreement with numerical simulations of a minimal model and should similarly apply to other particle propulsion mechanisms. Because light fields can be easily adjusted in space and time, this also allows to extend our approach to dynamical environments.

Enhanced high-order harmonic generation from periodic potentials in inhomogeneous laser fields

We theoretically study high-order harmonic generation (HHG) from solid-phase systems in spatially inhomogeneous strong laser fields originated by resonant plasmons within a metallic nanostructure. The intensity of the second plateau in HHG may be enhanced by two-three orders and be comparable with the intensity of the first plateau. This is due to bigger transition probabilities to higher conduction bands. It provides us a practical way to increase the yields of HHG with laser intensity below the damage threshold. It presents a promising way to triple the range of HHG spectra in experimental measurements. It also allows to generate intense isolated attosecond pulse from solids driven by few-cycle laser fields.

High-order-harmonic generation from Rydberg atoms driven by plasmon-enhanced laser fields

We theoretically investigate high-order harmonic generation (HHG) in Rydberg atoms driven by spatially inhomogeneous laser fields, induced, for instance, by plasmonic enhancement. It is well known that the laser intensity should to exceed certain threshold in order to generate HHG, when noble gas atoms in their ground state are used as an active medium. One way to enhance the coherent light coming from a conventional laser oscillator is to take advantage of the amplification obtained by the so-called surface plasmon polaritons, created when a low intensity laser field is focused onto a metallic nanostructure. The main limitation of this scheme is the low damage threshold of the materials employed in the nanostructures engineering. In this work we propose to use Rydberg atoms, driven by spatially inhomogeneous, plasmonic-enhanced laser fields, for HHG. We exhaustively discuss the behaviour and efficiency of these systems in the generation of coherent harmonic emission. To this aim we numerically solve the time-dependent Schr\"{o}dinger equation for an atom with an electron initially in a highly excited $n$-th Rydberg state, located in the vicinity of a metallic nanostructure, where the electric field changes spatially on the scales relevant for the dynamics of the laser-ionized electron. We first use a one-dimensional model to investigate the phenomena systematically. We then employ a more realistic situation, when the interaction of a plasmonic-enhanced laser field with a three-dimensional Hydrogen atom is modelled. We discuss the scaling of the relevant input parameters with the principal quantum number $n$ of the Rydberg state in question, and demonstrate that harmonic emission could be achieved from Rydberg atoms well below the damage threshold, thus without deteriorating the geometry and properties of the metallic nanostructure.

Near-circularly polarized single attosecond pulse generation from nitrogen molecules in spatially inhomogeneous laser fields

The generation of an attosecond pulse in nitrogen molecules using spatially inhomogeneous laser fields is investigated by numerically solving the time-dependent Schrödinger equation. It is found that an isolated attosecond pulse with elliptical polarization can be generated using linearly polarized laser fields. By changing polarization direction with respect to the molecular axis, the ellipticity of the attosecond pulse can be easily controlled. At some specific angles, the intensities of the two mutually vertical harmonic components, parallel and perpendicular to the driving laser polarization direction, are comparable. Additionally, the relative phase between the two components is about π/2. As a result, it supports the generation of the isolated near-circularly polarized attosecond pulse with a duration of 155 as.