Channel-closing Effects Research Articles

Intensity modulation of the population of atomic excited states by using near-infrared femtosecond laser pulses

We have theoretically investigated the modulation structures of the excited-state population of an argon atom exposed to a linearly polarized 800-nm laser pulse in the tunneling region. The numerical result identifies a nearly opposite phase modulation structure between the low-lying n = 4–7 states and over all higher $$n>7$$ states. This phase-dependent alternating population phenomenon is found to be related to the periodic channel-closing effect that results from ac-Stark-shifted multiphoton resonances. Moreover, the angular momentum states attributed to different parities have a similar alternating effect, which confirms a multiphoton process in the tunneling region. We introduce a mechanism of two-photon $$\Lambda $$ -type Raman transitions involving Rydberg and continuum states with a simultaneous redistribution of states to account successfully for this observed out-of-phase angular momentum state population.

Channel-closing effects of electronic excitation in solids.

We investigate the electronic excitation of solids in strong fields by solving the time-dependent Schrödinger equation. The excitation probability exhibits a strong modulation as a function of laser intensity when the initial states fill in the whole valence band. To have a clear insight into the modulation, we further study the electronic excitation from a single eigenstate in solids. A series of resonance-like enhancements of excitation probability are produced by changing the laser intensity and wavelength. We attribute the resonance-like enhancements to the channel-closing effects in solids. It is shown that the excitation probability exhibits enhancements when the value of channel is odd for intracycle interference and an integer for intercycle interference. This is different from the atom that the enhancement occur in the integer channels. We also reveal that the channel-closing effects can be observed by solid high-order harmonic generation.

Contribution of Floquet-Bloch states to high-order harmonic generation in solids

We identify the contribution of Floquet-Bloch states to the high-order harmonic generation (HHG) in solids by numerically solving the time-dependent Schr\odinger equation for both a one-dimensional and a two-dimensional model. Results from the single $k$ point and the full Brillouin zone are compared to each other and the symmetry- breaking effect is discussed. We show that the observed phenomena can be explained under the framework of the Floquet-Bloch theory and the strong-field approximation, respectively. Our results indicate that the total yield of the harmonic radiation increases nonmonotonically with the intensity of the driving pulse. After a rough consideration of the focusing volume effects, we find that the yield of the harmonics shows a steplike structure, which is similar to several recent experimental observations. The present work can contribute to a better understanding of the channel-closing effect in the HHG of solids and may provide a way to detect the Floquet-Bloch bands in a laser field.

Channel-closing effects in strong-field ionization by a bicircular field

Channel-closing effects, such as threshold anomalies and resonantlike intensity-dependent enhancements in strong-field ionization by a bicircular laser field are analyzed. A bicircular field consists of two coplanar corotating or counter-rotating circularly polarized fields having different frequencies. For the total detachment rate of a negative ion by a bicircular field we observe threshold anomalies and explain them using the Wigner threshold law and energy and angular momentum conservation. For the corotating bicircular case, these effects are negligible, while for the counter-rotating case they are pronounced and their position depends on the magnetic quantum number of the initial state. For high-order above-threshold ionization of rare-gas atoms by a counter-rotating bicircular laser field we observe very pronounced intensity-dependent enhancements. We find all four types of threshold anomalies known from collision theory. Contrary to the case of linear polarization, channel-closing effects for a bicircular field are visible also in the cutoff region of the electron energy spectrum, which is explained using quantum-orbit theory.

Below-threshold harmonic generation from strong non-uniform fields

Strong-field photoemission below the ionization threshold is a rich/complex region where atomic emission and harmonic generation may coexist. We studied the mechanism of below-threshold harmonics (BTH) from spatially non-uniform local fields near the metallic nanostructures. Discrete harmonics are generated due to the broken inversion symmetry, suggesting enriched coherent emission in the vuv frequency range. Through the numerical solution of the time-dependent Schrödinger equation, we investigate wavelength and intensity dependence of BTH. Wavelength dependence identifies counter-regular resonances; individual contributions from the multi-photon emission and channel-closing effects due to quantum path interferences. In order to understand the underlying mechanism of BTH, we devised a generalized semi-classical model, including the influence of Coulomb and non-uniform field interactions. As in uniform fields, Coulomb potential in non-uniform fields is the determinant of BTH; we observed that the generation of BTH are due to returning trajectories with negative energies. Due to large distance effectiveness of the non-uniformity, only long trajectories are noticeably affected.

Control of threshold enhancements in harmonic generation by atoms in a two-color laser field with orthogonal polarizations

Threshold phenomena (or channel-closing effects) are analyzed in high-order harmonic generation (HHG) by atoms in a two-color laser field with orthogonal linearly polarized components of a fundamental field and its second harmonic. We show that the threshold behavior of HHG rates for the case of a weak second harmonic component is sensitive to the parity of a closing multiphoton ionization channel and the spatial symmetry of the initial bound state of the target atom, while for the case of comparable intensities of both components, suppression of threshold phenomena is observed as the relative phase between the components of a two-color field varies. A quantum orbit analysis as well as phenomenological considerations in terms of Baz' theory of threshold phenomena [Zh. Eksp. Teor. Fiz. 33, 923 (1957)] are presented in order to describe and explain the major features of threshold phenomena in HHG by a two-color field.

Phase-dependent interference fringes in the wavelength scaling of harmonic efficiency

We describe phase-dependent wavelength scaling of high-order-harmonic generation efficiency driven by ultrashort laser fields in the midinfrared. We employ both numerical solution of the time-dependent Schr\"odinger equation and the strong field approximation to analyze the fine-scale oscillations in the harmonic yield in the context of channel-closing effects. We show, by varying the carrier-envelope phase, that the amplitude of these oscillations depends strongly on the number of returning electron trajectories. Furthermore, the peak positions of the oscillations vary significantly as a function of the carrier-envelope phase. Owing to its practical applications, we also study the wavelength dependence of harmonic yield in the ``single-cycle'' limit, and observe a smooth variation in the wavelength scaling originating from the vanishing fine-scale oscillations.

Resonancelike enhancement in high-order above-threshold ionization of molecules

Rresonancelike enhancement of groups of adjacent peaks in the photoelectron spectrum of high-order above-threshold ionization has been well documented for noble-gas atoms subjected to intense infrared laser pulses. However, its physical origin is still under debate. In this Rapid Communication, we investigate experimentally and theoretically high-order above-threshold ionization of diatomic nitrogen and oxygen molecules in order to shed more light on the underlying mechanism. The resonancelike enhancement is experimentally observed for N-2 but is absent for O-2 molecules. A simulation on the basis of S-matrix theory and the strong-field approximation reproduces the experimental observations. This implies that the resonancelike enhancement can be attributed to the channel-closing effect. The specific molecular structure plays a decisive role for the presence or absence of this enhancement in molecular systems.

High-order above-threshold ionisation of atoms and negative ions: channel-closing effects and the low-frequency approximation

The so-called low-frequency approximation (LFA) is an improved version of the strong-field approximation. The LFA describes the rescattering step of high-order above-threshold ionisation by utilising the exact field-free scattering amplitude, which has to be calculated separately. We apply the LFA to high-order above-threshold detachment of the fluorine negative ion. By comparing the so-obtained angle- and energy-resolved spectra with the exact results obtained as solutions of the time-dependent Schrödinger equation we show that the LFA is a more adequate approximation than the so-called improved strong-field approximation used previously. The LFA is also superior to the recently introduced quantitative rescattering (QRS) model. We show this by applying the LFA to the analysis of the intensity-dependent enhancements in above-threshold ionisation spectra of argon atoms. These enhancements can be explained as channel-closing-induced effects. In particular, we have observed and explained the behaviour of these channel-closing enhancements for higher values of the electron emission angle.

Low-energy rescattering effects in photoionization

The Coulomb-corrected KFR theory is applied to the analysis of the photoemission spectra of hydrogen atoms irradiated by a high intensity laser field. We report the observation of peaks in the ionization signals obtained for intensities in the tunnelling regime and show that their positions are intensity-independent. The interpretation of this result by means of the Coulomb-corrected KFR theory leads to the conclusion that the above features of the photoionization spectra are due to low-energy rescattering and channel closing effects. Our investigation of the electron momentum distributions suggests that low-energy photoelectrons are created in attosecond pulses.

Atom under an intense laser pulse: Stabilization effect and strong-field approximation

Direct numerical calculations of the single-photon ionization dynamics of the hydrogen atom were compared with the data obtained within the strong-field approximation (SFA). An analysis showed that the SFA model accurately determines the range of electromagnetic field intensities, upon reaching of which the ionization mode deviates from that described within perturbation theory; in particular, the ionization rate decreases with increasing intensity. It was demonstrated that the actual ionization mechanism under an intense pulse differs significantly from the SFA predictions. For example, an analysis of photoelectron angular distributions and energy spectra showed that the strong-field ionization features within the SFA model are primarily controlled by the ionization channel closing effect associated with the ponderomotive shift of the continuum boundary. At the same time, the results of direct numerical calculations of the ionization dynamics suggest that the Kramers-Henneberger atom is formed in a strong field, which is characterized by increased stability to strong-field ionization.

Interaction of KATPChannel Modulators with Sulfonylurea Receptor SUR2B: Implication for Tetramer Formation and Allosteric Coupling of Subunits

Sulfonylurea receptors (SURs) are subunits of ATP-sensitive K(+) channels (K(ATP) channels); they mediate the channel-closing effect of sulfonylureas such as glibenclamide and the channel-activating effect of K(ATP) channel openers such as the pinacidil analog P1075. We investigated the inhibition by MgATP and P1075 of glibenclamide binding to SUR2B, the SUR subtype in smooth muscle. To increase specific binding, experiments were also performed using SUR2B(Y1206S), a mutant with higher affinity for glibenclamide than for the wild-type (K(D )= 4 versus 22 nM, respectively) but otherwise exhibiting similar pharmacological properties. In the absence of MgATP, [(3)H]glibenclamide binding to both SURs was homogenous. MgATP inhibited [(3)H]glibenclamide binding to both SURs to 25% by reducing the apparent number of glibenclamide binding sites, leaving the affinity unchanged. In the absence of MgATP, P1075 inhibited [(3)H]glibenclamide binding in a monophasic manner with K(i) approximately 1 microM. In the presence of MgATP (1 mM), inhibition was biphasic with one K(i) value resembling the true affinity of P1075 for SUR2B (2-6 nM) and the other resembling K(i) in the absence of MgATP (approximately 1 microM). The data show that (1) MgATP induces heterogeneity in the glibenclamide sites; (2) the high-affinity glibenclamide sites remaining with MgATP are linked to two classes of P1075 sites; and (3) P1075 interacts specifically with SUR2B also in the absence of MgATP. The data are discussed with the assumption that SUR2B, expressed alone, forms tetramers; that MgATP induces allosteric interactions between the subunits; and that mixed SUR2B-glibenclamide-P1075 complexes can exist at equilibrium.

Channel-closing effects inhigh-order above-threshold ionization and high-order harmonicgeneration

The strong-field approximation, amended so as to allow forrescattering, is used to calculate high-order above-thresholdionization (ATI) spectra. The single-active-electron bindingpotential is modelled by a zero-range potential. The emphasis ison enhancements of groups of ATI peaks that occur for sharplydefined laser intensities. The enhancements are traced tomultiphoton resonance with the ponderomotively upshiftedcontinuum threshold. Good agreement both with experimental dataand with numerical simulations using the three-dimensionaltime-dependent Schrödinger equation for an optimizedone-electron binding potential is observed. The physical reasonfor the close agreement of the results of the two, apparently sodifferent, models is discussed. For quantitative agreement withthe experimentally observed positions of the resonances, an`effective' continuum threshold has to be introduced. Theeffects of focal averaging are evaluated and discussed. Resonantenhancement of high-order harmonic generation is alsoconsidered.