Two-photon absorption of ultrashort electromagnetic pulses by negative halogen ions

Abstract

Within the framework of the perturbation theory, two-photon absorption of ultrashort electromagnetic pulses of a corrected Gaussian shape by negative halogen ions during the action of a pulse is calculated and analysed. Photoabsorption with excitation of an electron to a state with a specified energy is also considered. Particular attention is given to the dependence of the probability of these processes on the pulse duration. The features of two-photon absorption that are characteristic of sufficiently short pulses and do not take place in the monochromatic limit are revealed.     

Absorption of ultrashort electromagnetic pulses by metal nanospheres in a dielectric medium

The absorption of ultrashort electromagnetic pulses on silver nanosphere embedded into glass in vicinity of plasmon resonance is studied theoretically in the frame of perturbation theory. The calculations are made for corrected Gaussian shape of incident pulse which enables us to consider both the short duration and the long duration regimes. Analysis based on numerical calculations reveals the specific features of considered process so as the change of absorption spectra for different pulse length and nonlinear dependence of absorbed energy upon pulse duration.     

Electron transitions and radiation of atoms interacting with ultrashort electromagnetic pulses

Excitation and ionization of an atom interacting with a short electromagnetic pulse is studied within the framework of the sudden perturbation approximation. The excitation and ionization probabilities and the spectra and cross sections of reradiation of the pulse by the atom are calculated. It is suggested that the process of reradiation of ultrashort electromagnetic pulses by multielectron atoms possesses a coherent character.     

超声的电磁脉冲激发方式的探讨

主要研究了一种新的电磁超声激发方法．脉冲电磁超声的激发,旨在提高电磁超声换能器的磁声转换效率,解决其灵敏度低以及热效应的不良影响等问题。设计制作了脉冲电磁超声的激发装置。通过实验验证,该方式超声信号强度较强,激发装置发热量小。最后分析了超声波信号的激发强度与初始电压、电容容值等因素的关系,认为线圈中铁芯的加入在一定程度上可以提高磁声转换的效率。但目前产生的超声波信号频率仍在低频范围内,在今后的研究中还将进一步考虑采用信号压缩等方法来提高频率。     

Generation of a train of ultrashort pulses using periodic waves in tapered photonic crystal fibres

We consider a light wave propagation in tapered photonic crystal fibres (PCFs) wherein the wave propagation is described by the variable coefficient nonlinear Schrödinger equation. We solve it directly by means of the theta function identities and Hirota bilinear method in order to obtain the exact periodic waves of sn, cn and dn types. These chirped period waves demand exponential variations in both dispersion and nonlinearity. Besides, we analytically demonstrate the generation of a train of ultrashort pulses using the periodic waves by exploiting the exponentially varying optical properties of the tapered PCFs. As a special case, we discuss the chirped solitary pulses under long wave limit of these periodic waves. In addition, we derive these types of periodic waves using the self-similar analysis and compare the results.     

Electron spectrum of atoms ionized by an ultrashort laser pulse of relativistic intensity

Energy transfer to electrons in atoms ionized by an ultrashort laser pulse of relativistic intensity (I > 10¹⁹ W/cm²) is considered. The ionization process is taken into account using the mechanism of direct suppression of the Coulomb barrier in a laser field. The acceleration of electrons in the laser wave field is described in terms of the classical relativistic equation of motion. Electron energies in the final state are determined as dependent on the average intensity of a laser pulse with a given shape. A distribution of the final energies of electrons upon multiple ionization of a complex atom is found.     

Tight focusing induces pulse delay and pulse compression of double-ring-shaped radially polarized ultrashort light pulses

The focusing of double-ring-shaped radially polarized ultrashort light pulses by a high-numerical aperture objective is investigated using vectorial Debye theory. After focusing, the double-ring-shaped radially polarized ultrashort light pulses slow down near the focus, and this pulse delay induces pulse compression in the propagation direction. That is, without changing the pulse duration, the spatial pulse length of the ultrashort light pulse is decreased near the focus. The velocity of the longitudinal component of the light pulse near the focus is lower than that of the radial component. The simulation results demonstrate that control of the ratio of the pupil radius to the beam radius affects not only the longitudinal component of the light pulse, but also the velocity and spatial pulse length of the light pulse because of the destructive interference between the longitudinal components of the inner and outer rings of the light pulse.     

Spatial effects in two-photon excitation transient absorption spectroscopy

We investigate the influence of the transverse mode of the pump beam and the geometry of the pump–probe configuration on the femtosecond two-photon excitation transient absorption (TA) spectroscopy. Strong modulation of the transient TA dynamics may result from the multi-peak distribution of the transverse mode of the pump beam, the large pump beam size, and the large separation angle between the pump and the probe beams, which is sensitive to the pulse length. Quantitative characterization of this kind of spatial effect is of general importance for the correct interpretation of the photophysical mechanisms that can be revealed by the two-photon TA measurements.     

外电场中Yb原子多光子光电离光谱研究

在近零场至 15 0 0 0 V/ cm的电场范围内 ,采用多光子技术对 Yb原子多种高激发态共振谱作了测量 .对测得的各类高激发态作了标定 ,讨论了谱线强度及跃迁选择定则 ,并观察和分析了谱线在外电场中的演化规律 .本文结果直接表明了 4f壳层电子的激发在 Yb原子高激发光谱中的重要性     

一种共轭有机杂环分子的双光子吸收与电化学性质

实验研究了一种新的共轭杂环化合物9-乙基-3-{5-(4-叔丁基苯基)-[1,3,4]-噁二唑-2-苯乙烯基}-咔唑(VBPOEC)的双光子吸收性质和电化学性质.双光子吸收截面采用非线性透过率法测定,双光子最大吸收波长位于800nm,吸收截面为1350GM.采用紫外光谱的边带吸收法得到带隙为2.77eV,用循环伏安法对其电化学性质进行了初步研究,氧化电位为1.059V,其HOMO能级为-5.80eV,其LUMO轨道能级为-3.03eV.研究了光学、电化学性质与分子电子结构的关系.     

Propagation of ultrashort RF pulses in a polluted atmosphere

The effect of absorption and dispersion on the spectral characteristics and the shape of ultrashort RF pulses propagating in a polluted atmosphere is studied theoretically. The frequency band occupied by the pulses essentially covers the rotational spectrum of gas molecules encountered in a real atmosphere.     

The local-field corrective effect on Rabi oscillation of ultrashort pulse excitation in semiconductor GaAs

The Rabi oscillation of the thin bulk semiconductor GaAs, which takes into account the effect of the local-field correction induced by the interacting excitons, is investigated by numerically solving the semiconductor Bloch equations. It is found, for a 2π few-cycle pulse excitation, that two incomplete Rabi-floppings emerge due to the competition between the Rabi frequency of the incident pulse and the internal-field matrices. Furthermore, for a sub-cycle 2π pulse excitation a complete Rabi-flopping can occur because of the absolute phase effect. We ascribe these characteristics of the Rabi oscillation to the renormalized Rabi frequency.     

一种基于分数阶傅立叶变换的超声编码激励方法

将编码激励技术应用到医学超声成像系统中,选择和设计合适的发射编码和脉冲压缩方法非常重要.文中提出采用加Tukey窗的Chirp信号作为发射脉冲,同时采用分数阶傅立叶变换的进行脉冲压缩的编码激励方法.将该方法与传统方法进行了比较研究,实验结果表明:文章提出的基于分数阶傅立叶变换的编码激励方法在先验性方面优于传统方法.     

Generation of carrier-envelope phase-controlled isolated attosecond pulses using chirped femtosecond excitation lasers

ABSTRACT

We present an efficient approach for producing a carrier-envelope phase controlled isolated attosecond pulse by an optimized intense driving laser pulse. High-order harmonics are produced by numerically solving the time-dependent Schrödinger equation for the one-dimensional hydrogen atom in an ultrashort laser pulse. We define an efficient cost function to optimize the laser pulse by a genetic algorithm scheme. Our approach produces single attosecond pulses with desired properties, including the carrier-envelope phase, central frequency, and duration. Also, we analyze the time–frequency profiles of the attosecond emissions to gain a deeper insight into the underlying physical mechanism.     

Time dependence of Fresnel diffraction of ultrashort laser pulses by a circular aperture

We report on the time dependences of the Fresnel diffraction of ultrashort laser pulses by a circular aperture. The diffraction leads to a pulse delay, time shape change, pulse broadening, and peak power decrease. These effects may have to be taken into account whenever critical thresholds are encountered as, for example, in nonlinear optics or laser fusion.     

Experimental setup for X-ray spectroscopy of muonic atoms formed from implanted ions in solid hydrogen

An experimental setup comprising of a cryogenic target and an ion implantation system has been constructed to perform muonic X-ray spectroscopy with muonic atoms formed from implanted ions in solid hydrogen films. Gaseous mixtures of hydrogen (H₂) and deuterium (D₂) have been frozen onto a thin 60-mm diameter silver foil, and then irradiated with charged particles of energies up to 33 keV/q. Already, solid films of H₂ and D₂ mixtures with implanted helium ions have been successfully used in muon catalyzed fusion related experiments at RIKEN-RAL Muon Facility. This new method allows studies of the nuclear charge distribution of unstable atoms by means of the muonic X-ray method at facilities where both negative muon and radioactive nuclear beams would be available.     

Effect of material dispersion on the focusing of isodiffracting ultrashort pulses by a lens

Abstract

Based on the Fourier transform, the focusing of isodiffracting ultrashort pulses by a lens is studied, where the material dispersion of first, second and higher order is taken into account, respectively. Numerical calculation results for spatial and temporal intensity distributions, photon flux and energy density of focused isodiffracting ultrashort pulses are given and illustrated. It is shown, compared to the dispersion-free case, that the first-order dispersion leads to a broadening of the pulse form, photon flux and energy density, and a decrease of their peak values. The second-order dispersion results in a further broadening of the pulse form and photon flux, and a further decrease of their peak values, whereas the higher-order dispersion plays a relatively minor role.     

Frequency spectrum of focused broadband pulses of electromagnetic radiation generated by polarization currents with superluminally rotating distribution patterns

We investigate the spectral features of the emission from a superluminal polarization current whose distribution pattern rotates (with an angular frequency omega) and oscillates (with a frequency omega > omega differing from an integral multiple of omega) at the same time. This type of polarization current is found in recent practical machines designed to investigate superluminal emission. Although all of the processes involved are linear, we find that the broadband emission contains frequencies that are higher than omega by a factor of the order of (omega/omega)2. This generation of frequencies not required for the creation of the source stems from mathematically rigorous consequences of the familiar classical expression for the retarded potential. The results suggest practical applications for superluminal polarization currents as broadband radio-frequency and infrared sources.     

Phase-transitions in GeSb induced by customized ultrafast optical pulses

The use of ultrafast laser pulses to initiate solid-state phase-transitions in certain materials has shown promise in achieving sub-nanosecond phase changes with different optical properties. These phase changes have been well studied using pulse durations between femtoseconds and nanoseconds to determine the dynamics for the reversible phase changes on multiple time scales. In this study femtosecond pulse shaping techniques, driven by evolutionary algorithms, were used to obtain optimized temporally shaped ultrashort laser pulses to induce and control permanent phase changes in GeSb thin-films. Through monitoring the pulse effects it has been determined that the crystalline-to-amorphous phase transition is minimized using optical pulses with pulse widths less than the electron-phonon coupling time. It is maximized by using pulses longer than the time required for energy transfer from the excited carriers to the lattice.